development of solution processed thin film …...stefan langner, leona wendt, yugal agarwal, varun...
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Development of Solution Processed Thin Film Barriers for Encapsulating
Thin Film Electronics
Entwicklung von lösungsprozessierten Dünnschichtbarrieren für die Verpackung von
Dünnschichtelektronik
Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg
zur
Erlangung des Grades
DOKTOR-INGENIEUR (Dr.-Ing.)
vorgelegt von
M.Eng. Iftikhar Ahmed Channa
aus Naushahro Feroze, Pakistan
Als Dissertation genehmigt
von der Technischen Fakultät der
Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 13 Dezember 2019
Vorsitzender des Promotionsorgans: Prof. Dr.-Ing. habil. Andreas Paul Fröba
Gutachter: Prof. Dr. Christoph J. Brabec
Prof. Dr. Josef Breu
i
This thesis is dedicated to my
Teachers, Family, Friends and Colleagues
ii
ACKNOWLEDGMENTS
Taking this opportunity, I wish to say thank Prof. Dr. Christoph J. Brabec for accepting and
providing me the great opportunity to perform a Ph.D. in his group and introducing me to
a nice world of photovoltaics. It has been a great pleasure to work under his supervision.
Secondly, I would like to address to my group leader and mentor Dr. Hans-Joachim
Egelhaaf, who has been a true inspiration throughout the time. His open-minded look to
my research activities, perfect suggestions, new ideas, and meaningful guidance,
encouraged me to make a nice and very interesting scientific work. I highly appreciate that
Dr. Egelhaaf always had a time for the discussions on my results.
I owe a great deal of appreciation for Dr. Andreas Distler for teaching me research
methodologies and healthy discussions on barrier performance and lifetime of organic solar
cells. I would also like to thank Dr. Edda Stern for her constant support and help during
early stages of my PhD that gave me a smooth start.
Special thanks to Mr. Benedikt Scharfe for providing glass flakes and sharing tricks for the
processing of the glass flake filled films. I am very grateful to Dr. Karen Forberich and Dr.
Benjamin Lipovsek for their support in performing optical simulations on the layers filled
with glass flakes.
I am particularly grateful to my colleague Eric Tam for performing SEM cross sections of
the layers which was a tricky and time consuming job, and he did that nicely for me. I am
also thankful to my other colleagues Atif Makhdoom, Taimoor Ahmed, Arne Riecke,
Dongju Jang, Sarmad Feroze, Philipp Maisch, Peter Kubis, Felix Hoga, Michael Wagner,
Stefan Langner, Leona Wendt, Yugal Agarwal, Varun Sharma, Frank Fecher and Fu Yang
for having a wonderful time during my PhD. I would also like to thank Dieter Schmidt for
helping me out with his technical skills in handling hardware. A special thanks to wonderful
ladies at ZAE and iMEET Astrid Kidzun, Nidia Gawehns, Anja Kottlowski, Irina Döhrer,
Madeleine Heyder and Claudia Koch for extending their support whenever I needed,
especially in handling formalities and document translations.
I thank my friends, Syed Qurban Ali, Hassan Sohaib, Jamal uddin, Ayaz Mahmood, Asmat
Soomro, Khalid Rasheed, Abdul Latif, Saleem Raza and Laraib Sarfraz for their constant
iii
support during my stay in Germany specially Yaseen Memon (your wonderful cooking
skills can never be forgotten).
Special thanks to the Higher Education commission Pakistan and German Academic
Exchange Service (DAAD) for their financial support and ZAE for allowing me to work in
its laboratories and providing me a nice research environment.
I take pride to express thanks and love for my family for their endless support throughout
my life that provided me confidence and courage. Whatever I have achieved, is due to their
care, prayers and untiring efforts. I also pay my special gratitude to my brothers specially
Muhammad Nawaz and Sister Noor Jehan for their unconditional help, prayers and love.
Later in the day after work, nothing was more jubilant than time spent with my children
(Aiza and Muhammad Yaqub). You both are most nearest to my heart.
My acknowledgement would be incomplete without thanking my wife. She has been the
biggest source of my strength. Her care and unwavering love provided me courage and
confidence to meet every challenge.
iv
SUMMARY
Recently, organic solar cells (OSCs) with efficiencies of 17% have been demonstrated,
which brings organic photovoltaics in the same league as inorganic thin film technologies.
OSCs require encapsulation by transparent and high quality barrier materials to achieve
decent lifetimes without compromising performance. The most common practice for
encapsulation of the OSCs is the lamination between barrier sheets using adhesives. This
lamination process adds extra processing steps and thus increases overall processing cost
and limits the throughput. Many attempts have thus been made to create coated barriers
with quality comparable to those processed from vacuum assisted techniques. Direct
application of coated barriers on top of OSCs will not only minimize cost but also maximize
throughput as direct coating processes can be performed with roll-to-roll methods.
Therefore, the goal of this work is the development of materials and processes for the
encapsulation of organic thin films electronics by direct coating.
The thesis is subdivided into six chapters. The main objective of Chapter 1 is to express
the motivation towards the research direction. In this chapter, also the challenges and
hurdles faced by coated barriers are discussed. Chapter 2 describes the theoretical
background of diffusion and permeability and introduces industrial units for measuring the
barrier quality. Various factors are also defined briefly which influence the barrier quality.
This chapter gives theoretical details of the barriers based on filler platelets and describes
various theoretical models for predicting the barrier quality from the platelet properties
such as size, shape, concentration and orientation. Finally basics about organic solar cells
and the working principle along with degradation mechanism are described in this chapter.
Chapter 3 describes the state of the art of coated barriers. Silica layers processed from the
polymer class of polysilazanes are also described in detail, along with the methods of
processing. Finally, miscellaneous materials like ORMOCERS and fluoropolymers are also
discussed in this chapter. Chapter 4 is devoted to experimental details describing all of the
raw materials and processing methods used in the work. In Chapter 5 results on
experimental data discussed. This chapter is further divided into three parts. Part I provides
results obtained from the investigations on filler based barriers using clay as filler. Clay
based barriers were prepared as a well characterized reference system. In Part II, the novel
concept of barriers based on PVB films filled with glass flakes was investigated. To this
end, barriers were prepared from glass flakes of different aspect ratios and different loading
v
concentrations to systematically study the effect of aspect ratio and loading concentration
on barrier quality and optical transmission. It was found that the glass flakes are distributed
homogeneously in the PVB film, with an almost perfect orientation of the platelets’ long
axes parallel to the film surface. In this way, barrier films with optical transmission values
of > 85% and moisture permeation values of ~0.14 g.m-2.day-1 were obtained with glass
flakes having an aspect ratio of 2000 at a loading concentration of 25 vol%.The barrier
properties persisted even after 20,000 cycles of bending at a radius of 3 cm. The WVTR
values measured for different aspect ratios and different loadings were shown to be in
reasonable accordance with the predictions of the Bharadwaj model. The haze of the glass
flake filled PVB films, which, according to optical simulations, is mainly due to surface
roughness of the films, was reduced by coating a smoothing layer on top. The lifetime of
organic solar cells (OSCs) increased from few hours to beyond 150 h under damp heat
conditions without any loss in efficiency, when the devices were encapsulated with the
glass flake based barrier films. Part III describes the results obtained on barrier films based
on perhydropolysilazane (PHPS). Two methods were used to cure PHPS, namely curing by
exposure to damp heat and curing by irradiation with deep UV. Curing with deep UV in
addition with heat is found to be the quickest way to cure PHPS completely. FTIR has been
used to find the end point of curing which can subsequently be used to predict the barrier
properties of cured PHPS layers. Prepared barrier films show water vapor transmission
rates (WVTR) of <10-2 g m-2day-1 (40oC / 85%RH) and oxygen transmission rates (OTR)
of <10-2 cm3m-2 day-1 bar-1 at ambient conditions maintaining optical transmission of >90%
in visible region. Flexibility of the resulting barrier films is improved by coating a barrier
stack of several thin PHPS layers alternating with organic polymer interlayers. These stacks
show an increase of WVTR values by less than 10% after 3000 bending cycles. Direct
coating of the PHPS films on top of organic solar cells enhances the device lifetime in damp
heat conditions from few hours to around 700 hours. Chapter 6 gives the conclusions and
provides an outlook on the possible impact of the developments of this thesis on the roll-
to-roll production of printed opto-electronics.
vi
ZUSAMMENFASSUNG
Kürzlich wurden organische Solarzellen (OSCs) mit Wirkungsgraden von 17%
demonstriert, was die organische Photovoltaik in die gleiche Liga wie anorganische
Dünnschichttechnologien bringt. OSCs benötigen eine Verkapselung mit transparenten und
hochwertigen Barrierematerialien, um eine lange Lebensdauer zu erreichen, ohne die
Leistung zu beeinträchtigen. Die gebräuchlichste Verkapselung von OSCs ist die
Laminierung zwischen zwei Barrierefolien mittels Klebstoff. Dieser Laminierprozess
erfordert zusätzliche Verarbeitungsschritte und erhöht so die Gesamtverarbeitungskosten
und begrenzt den Durchsatz. Es wurden daher viele Versuche unternommen, gedruckte
Barrieren mit einer Qualität zu schaffen, die mit der von vakuumunterstützten Techniken
vergleichbar ist. Die direkte Anwendung von gedruckten Barrieren auf OSCs minimiert
nicht nur die Kosten, sondern maximiert auch den Durchsatz, da direkte
Beschichtungsprozesse mit Rolle-zu-Rolle-Verfahren durchgeführt werden können.
Ziel dieser Arbeit ist daher die Entwicklung von Materialien und Verfahren zur
Verkapselung von organischer Dünnschichtelektronik durch Direktbeschichtung.
Die Arbeit ist in sechs Kapitel unterteilt. Das Hauptziel von Kapitel 1 ist es, die Motivation
und Zielsetzung dieser Forschungsarbeit zu beschreiben. In diesem Kapitel werden auch
die Herausforderungen für gedruckte Barrieren diskutiert. Kapitel 2 beschreibt den
theoretischen Hintergrund von Diffusion und Permeabilität und stellt industrielle Geräte
zur Messung der Barrierequalität vor. Darüber hinaus werden kurz verschiedene Faktoren
definiert, die die Barrierequalität beeinflussen. Dieses Kapitel enthält zudem theoretische
Details zu Barrieren auf Basis von Füllplättchen und beschreibt verschiedene theoretische
Modelle zur Vorhersage der Barrierequalität aus den Eigenschaften der Plättchen wie
Größe, Form, Konzentration und Ausrichtung. Schließlich werden in diesem Kapitel die
Grundlagen organischer Solarzellen und deren Funktionsprinzip sowie
Degradationsmechanismen beschrieben. Kapitel 3 beschreibt den Stand der Technik von
gedruckten Barrieren. Aus der Polymerklasse der Polysilazane prozessierte
Kieselsäureschichten werden ebenso wie die Verarbeitungsmethoden ausführlich
beschrieben. Schließlich werden in diesem Kapitel auch verschiedene Materialien wie
ORMOCERS und Fluorpolymere behandelt. Kapitel 4 widmet sich experimentellen
Details, die alle Materialien und Verarbeitungsmethoden beschreiben, die in der Arbeit
verwendet werden. In Kapitel 5 werden Ergebnisse zu experimentellen Daten diskutiert.
vii
Dieses Kapitel ist in drei Teile gegliedert. Teil I enthält Ergebnisse aus den Untersuchungen
zu Füllstoffbarrieren mit Ton als Füllstoff. Auf Ton basierende Barrieren wurden als gut
charakterisiertes Referenzsystem hergestellt. In Teil II wurde ein neuartiges Konzept mit
Barrieren auf Basis von PVB-Folien mit Glasflocken untersucht. Zu diesem Zweck wurden
Barrieren aus Glasflocken mit unterschiedlichen Seitenverhältnissen und Konzentrationen
hergestellt, um den Einfluss von Seitenverhältnis und Konzentration auf die
Barrierequalität und optische Transmission systematisch zu untersuchen. Es wurde
festgestellt, dass die Glasflocken homogen in der PVB-Folie verteilt sind, mit einer nahezu
perfekten Ausrichtung der Längsachse der Plättchen parallel zur Folienoberfläche. Auf
diese Weise wurden Barriereschichten mit optischen Transmissionswerten > 85% und
Feuchtepermeationswerten von ~0,14 g.m-2.day-1 mit Glasflocken mit einem
Aspektverhältnis von 2000 bei einer Beladungskonzentration von 25 Vol% erhalten, wobei
die Barriereeigenschaften auch nach 20.000 Biegezyklen bei einem Radius von 3 cm
erhalten blieben. Die WVTR-Werte, die für verschiedene Seitenverhältnisse und
unterschiedliche Belastungen gemessen wurden, stimmten gut mit den Vorhersagen des
Bharadwaj-Modells überein. Die Trübung der glaslamellengefüllten PVB-Folien, die nach
optischen Simulationen hauptsächlich auf die Oberflächenrauheit der Folien
zurückzuführen ist, wurde durch die Beschichtung einer Glättungsschicht reduziert. Die
Lebensdauer von organischen Solarzellen (OSCs) stieg von wenigen Stunden auf über 150
Stunden unter feuchten Wärmebedingungen ohne Effizienzverlust, wenn die Zellen mit
Barrierefolien auf Glasflockenbasis verkapselt wurden. Teil III beschreibt die Ergebnisse
von Barriereschichten auf Basis von Perhydropolysilazan (PHPS). Zwei Methoden wurden
zur Aushärtung von PHPS verwendet, nämlich die Aushärtung durch Behandlung mit
feuchter Hitze und die Aushärtung durch Bestrahlung mit tiefem UV. Die Aushärtung mit
tiefem UV und zusätzlich mit Wärme ist der schnellste Weg PHPS vollständig auszuhärten.
FTIR wurde verwendet, um den Zeitpunkt der vollständigen Aushärtung zu finden, mit
dem anschließend die Barriereeigenschaften der ausgehärteten PHPS-Schichten ermittelt
werden können. Die Barrierefolien zeigen Wasserdampfdurchlässigkeitsraten (WVTR)
von <10-2 g m-2day-1 (40oC / 85%RH) und Sauerstoffdurchlässigkeitsraten (OTR) von <10-
2 cm3m-2 day-1 bar-1 bei Umgebungsbedingungen, die eine optische Transmission von >90%
im sichtbaren Bereich aufweisen. Die Flexibilität der resultierenden Barrierefolien wird
verbessert, indem ein Stapel aus mehreren dünnen PHPS-Schichten im Wechsel mit
organischen Polymerfolien beschichtet wird. Diese Stapel zeigen eine Erhöhung der
WVTR-Werte um weniger als 10% nach 3000 Biegezyklen. Die direkte Beschichtung der
viii
PHPS-Schichten auf organischen Solarzellen erhöht die Lebensdauer der Zellen in feuchter
Hitze von wenigen Stunden auf etwa 700 Stunden. Kapitel 6 enthält die
Schlussfolgerungen und einen Ausblick auf die möglichen Auswirkungen der in dieser
Arbeit entwickelten Verkapselungstechnologien für die Rolle-zu-Rolle-Fertigung
gedruckter Opto-Elektronik.
TABLE OF CONTENTS
1
TABLE OF CONTENTS
ACKNOWLEDGMENTS ................................................................................................... ii
SUMMARY .................................................................................................................... iv
ZUSAMMENFASSUNG ................................................................................................ vi
TABLE OF CONTENTS ................................................................................................. 1
.............................................................................................................................. 5
MOTIVATION AND CONCEPT ................................................................................... 5
.............................................................................................................................. 8
THEORETICAL BACKGROUND ................................................................................. 8
2.1 Theoretical background of diffusion through barriers ....................................... 9
2.2 Permeation rates .............................................................................................. 10
2.2.1 Temperature dependence: ........................................................................ 11
2.3 Factors affecting Permeability ......................................................................... 13
2.3.1 Coefficient of diffusion (D) ..................................................................... 14
2.3.2 Coefficient of solubility (H) ..................................................................... 14
2.3.3 Surface coverage () ................................................................................ 14
2.3.4 Tortuosity (τ) ............................................................................................ 15
2.4 Modeling and simulation of barrier characteristics of filled polymers ........... 15
2.4.1 Overview of the various models............................................................... 17
2.5 Working principle and degradation of organic solar cells ............................... 24
2.5.1 Working principle .................................................................................... 24
2.5.2 Current density voltage characteristics .................................................... 26
2.6 Degradation mechanism of Organic Solar Cells ............................................. 27
............................................................................................................................ 32
STATE OF THE ART.................................................................................................... 32
3.1 Bulk Polymers ...................................................................................................... 33
TABLE OF CONTENTS
2
3.2 Increasing tortuous path ........................................................................................ 35
3.2.1 Clay based barriers ................................................................................... 35
3.2.2 Graphene based barriers ........................................................................... 38
3.2.3 Getter materials ........................................................................................ 42
3.3 Barriers based on impermeable coatings .............................................................. 43
3.3.1 Polysilazane .............................................................................................. 43
3.3.1.1 Thermal curing ........................................................................................ 45
3.3.1.2 Curing in the presence of catalyst ........................................................... 46
3.3.1.3 Deep UV curing ..................................................................................... 47
3.3.1.4 Combined methods .................................................................................. 48
3.3.1.5 Barrier performance ................................................................................ 49
3.3.2 ORMOCERS ............................................................................................ 53
3.4 Reducing solubility ............................................................................................... 56
............................................................................................................................ 60
EXPERIMENTAL ......................................................................................................... 60
4.1 Materials .......................................................................................................... 61
4.2 Processing ........................................................................................................ 62
4.3 Preparation of OSCs ........................................................................................ 64
4.3.1 Encapsulation of the OSC devices ........................................................... 64
4.4 Characterization ............................................................................................... 65
4.4.1 Barrier quality .......................................................................................... 65
4.4.1.1 Water vapor transmission rate (wvtr) ................................................... 65
4.4.1.2 Oxygen transmission rate (OTR) .......................................................... 67
4.4.2 Spectroscopic analysis.............................................................................. 68
4.4.3 Bending of the barrier layers .................................................................... 68
4.4.4 Degradation test........................................................................................ 68
4.4.4.1 Optical measurements ........................................................................... 68
TABLE OF CONTENTS
3
4.4.4.2 Damp heat degradation ......................................................................... 68
4.4.4.3 Degradation under sun .......................................................................... 68
4.4.4.4 Electrical measurements ....................................................................... 69
4.4.4.5 SEM images .......................................................................................... 69
4.4.4.6 Optical micrographs ............................................................................. 69
............................................................................................................................ 70
RESULTS AND DISCUSSION .................................................................................... 70
5.1 Filler based barriers: Clay and glass flakes ..................................................... 71
5.1.1 Clay based barriers ................................................................................... 71
5.1.1.1 IR analysis of nanocomposites ............................................................. 72
5.1.1.2 Surface morphology ............................................................................. 72
5.1.1.3 Transparency and haze of the Nanocomposites ................................... 73
5.1.1.4 Moisture permeability........................................................................... 74
5.1.1.5 Validation of the experimental data...................................................... 77
5.1.1.6 Bendability............................................................................................ 78
5.1.1.7 Conclusion ............................................................................................ 79
5.1.2 Glass flakes based barriers ....................................................................... 81
5.1.2.1 Surface roughness of the layers ............................................................ 83
5.1.2.2 Transparency of the layers .................................................................... 83
5.1.2.3 Influence of bulk scattering: ................................................................. 86
5.1.2.4 Influence of the Surface roughness: ..................................................... 89
5.1.2.5 Barrier performance of glass flakes ...................................................... 91
5.1.2.6 Oxygen permeability ............................................................................ 94
5.1.2.7 Bendability............................................................................................ 95
5.1.2.8 Encapsulation of organic solar cells ..................................................... 96
5.1.2.9 Photo bleaching of P3HT ..................................................................... 96
5.1.2.10 Lifetime under damp heat ................................................................... 97
TABLE OF CONTENTS
4
5.1.2.11 Lifetime under irradiation by sun simulator ....................................... 98
5.1.2.12 Conclusion .......................................................................................... 99
5.2 Polysilazane ................................................................................................... 101
5.2.1 Optimizing the curing method for PHPS ............................................... 101
5.2.2 Curing by the combination of heat and deep UV at distance of 5 mm: . 103
5.2.3 Correlation of WVTR with Infrared peak ratios .................................... 104
5.2.4 Correlation of WVTR with IR peak (damp heat) ................................... 107
5.2.5 Optimization of the PHPS conversion rate ............................................ 109
5.2.6 Hydrophobic nature of the PHPS film: .................................................. 111
5.2.7 Flexibility / bendability of PHPS-based barriers.................................... 111
5.2.8 Protection of organic electronic devices by PHPS-based barriers ......... 117
5.2.9 Encapsulation of OSCs by direct deposition of PHPS ........................... 121
5.2.10 Intermediate layer of ZnO to avoid delamination .................................. 124
5.2.11 Lifetime tests .......................................................................................... 125
5.2.12 Investigation on device failure in sun test: ............................................. 126
5.2.13 Conclusion .............................................................................................. 127
.......................................................................................................................... 129
CONCLUSION ............................................................................................................ 129
Biblography .................................................................................................................. 130
List of Tables .................................................................................................................... a
List of Figures .................................................................................................................. b
List of Abbreviations, symbols and constants ................................................................... i
MOTIVATION AND CONCEPT
5
MOTIVATION AND CONCEPT
Organic electronics, namely organic light emitting diodes (OLEDs) [1], [2], organic
solar cells (OSCs) [3], [4], and organic field effect transistors (OFETs) [5] have opened up
new chances, attributable to their intrinsic characteristic, for example, light weight,
mechanical adaptability and semitransparency [6]. This is especially true for organic solar
cells as these properties make them the perfect choice for mobile chargers and building
integration [7]. Very recently, organic photovoltaics (OPV) have experienced a boost as
power conversion efficiencies (PCEs) of ~17% have been reported, which brings OPV into
the same league as inorganic thin films technologies [8]. Another intriguing characteristic
of organic electronics is the printability, which makes high throughput roll-to-roll (R2R)
processing possible with the use flexible substrates and thus production at low cost [9]. So
as to be feasible in the market, such products should not only offer high efficiencies and
low cost but also adequately long lifetimes. As the degradation of unencapsulated organic
devices is mainly caused by moisture and oxygen, their lifetime can be extended
significantly by encapsulation with appropriate barrier materials [10].
Figure 1.1 shows an overview of the requirements to packaging in different fields of
application. Figure 1.2 serves to visualize the challenge of preparing adequate barriers by
showing the amounts of water diffusing through football ground sized barriers of different
WVTR values over the time period of one month.
While food and pharmaceutical products can be packaged in barriers having water vapor
transmission rate (WVTR) values between 100 -101 g.m-2.day-1 by utilizing common
polymers [11]–[14], OLEDs are highly sensitive to moisture and oxygen and hence require
ultra-high barriers (WVTR ~10-6 g.m-2.day-1) for their encapsulation [15], [16], which can
only be achieved with metal oxide coatings like SiOx [14], SiNx [17], and Al2O3 [18] etc.,
deposited from the gas phase (ALD, CVD, PVD).
In a study carried out by Hauch et al., it was shown that for OSCs barrier materials
having WVTRs of around 10-3 g m-2 day-1 @ 25oC/40%RH can protect the device to provide
lifetimes of 3-5 years [11]. These WVTR values do not require vacuum processed barriers,
but are within the reach of solution processed barriers [19]–[21]. From the point of view of
printed flexible opto-electronics as appealing products for textile or building integration, it
MOTIVATION AND CONCEPT
6
is important that the encapsulation does not affect the respective properties of the devices.
Thus, the encapsulating material should be optically transparent, flexible, light weight and
cost efficient [16]. As encapsulation of OSCs in barrier films adds an extra lamination step
to the manufacturing process and also makes the resulting modules much heavier and less
flexible, direct coating of the barrier on top of the devices would be very beneficial in terms
of both, costs and quality of the modules. Additionally, directly coated barrier layers can
also enhance the compatibility of encapsulation with roll-to-roll manufacturing, thus
increasing the throughput, reducing the processing steps and minimizing the overall
processing cost. Coatable barriers can thus be a promising alternative to vacuum assisted
vapor deposition techniques for obtaining medium quality barriers at reasonable cost.
Interesting applications for coated barriers are either short to medium life time devices, e.g.
mobile phone chargers, or robust products such as inorganic PV modules or luminescence
down shifting foils for retrofitting solar power plants. Finally, in some cases, e.g. for
devices of arbitrary 3-dimensional shape, coating of barriers is the only possible way of
applying barriers.
The goal of this thesis is thus the development of solution processed barriers for the
R2R compatible encapsulation of flexible OSCs, providing a combination of high barrier
quality (WVTR < 10-2 g.m-2.day-1), transparency (~90% in a range of 400 nm to 1000 nm)
and flexibility (several thousand bending cycles at a bending radius < 5 cm) [16]. Following
Fick’s 1st law of diffusion as a guideline, two approaches towards coatable barriers will be
used, namely enhancing tortuosity by filling glass flakes into PVB films and reducing
accessible area by silica coatings obtained by UV curing of perhydropolysilazanes.
MOTIVATION AND CONCEPT
7
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
10-5
10-4
10-3
10-2
10-1
100
101
102
103
Encapsulation
for OSCs
BetterExcellent Poor
OT
R (
cm
3/m
2.d
ay
.ba
r)
WVTR (g/m2.day)
Bulk
polymers
Food /
Pharmaceutical
packaging
Encapsulation
for OLEDs
PVD / CVD / ALD
single / multilayers
Solution processed
Filler based/
PHPS based
PET,
PEN
Good
Metal coated
polymers
Figure 1.1. Water vapor transmission rates (WVTR) and oxygen transmission rates (OTR)
of bulk polymers, food packaging, as well as of solution and vacuum processed high quality
barriers. (Reproduced from [22] with the permission from Elsevier, with modifications)
Figure 1.2. Illustration of the amounts of water transmitted through barrier films of the
size of a football field (5000 m2) over a period of 1 month at the WVTR values given (in
g.m-2.day-1). Data extracted from [23].
THEORETICAL BACKGROUND
8
THEORETICAL BACKGROUND
This chapter presents the theoretical background of diffusion through barriers. Later in this
chapter, the theoretical background of organic solar cells and their degradation due to
oxygen and humidity is given.
THEORETICAL BACKGROUND
9
2.1 Theoretical background of diffusion through barriers
Transfer of gases or vapors through membranes due to concentration gradients
usually occurs by the means of diffusion. Diffusion is defined as a flow of species from one
place to another as a result of difference in chemical potential of the flowing species [24].
In the field of packaging materials for organic electronics, we are mainly concerned with
the diffusion of oxygen and moisture, as these are the most detrimental substances to the
different layers of the devices.
The amount of substance n passing through unit area A per unit time t is termed flux
[24]–[27]:
J = 1
𝐴lim⧍t→0
∆𝑛
∆𝑡 =
𝑑𝑛
𝐴∙𝑑𝑡
Eq. 1
In the case of transport by diffusion, referring to Fick’s first law, the flux (J) is obtained as:
𝐽 =1
𝐴
𝑑𝑛
𝑑𝑡= −𝐷
𝜕𝑐
𝜕𝑥
Eq. 2
Where D is the coefficient of diffusion and ∂c
∂x is the concentration gradient normal to the
membrane surface.
In the steady state, i.e. at J=constant, the diffusion flux through a membrane of thickness l,
with C1 and C2 being the concentrations of the diffusant at opposite sides of the membrane,
becomes
J = 𝐷(𝐶2−𝐶1)
𝑙 Eq. 3
Assuming that the coefficient of diffusion is not a function of concentration, i.e., D≠f(c).
Substituting J from relation Eq. 2 we obtain
⧍𝑛
𝐴∆𝑡= 𝐷
(𝐶2 − 𝐶1)
𝑙
Eq. 4
And
⧍𝑛= D (𝐶2−𝐶1)
𝑙 𝐴∆𝑡
Eq. 5
If the partial pressure p of the diffusant outside of the membrane is sufficiently low, the
equilibrium concentration C of the diffusant inside the membrane is given by Henry’s law:
THEORETICAL BACKGROUND
10
C = H* ƿ Eq. 6
Where H is the solubility coefficient of the diffusing gas, called Henry’s constant, which is
defined as the concentration at a certain partial pressure of that gas. The SI unit of H is
mol/(m3 Pa).
By substituting C in relation Eq. 5 by the partial pressure of the diffusant, we obtain:
⧍𝑛 =D 𝐻(ƿ2−ƿ1)
𝑙 𝐴∆𝑡
Eq. 7
The permeability (P) of the diffusing species is given by the relation:
P = DH = ⧍𝑛∙ 𝑙
𝐴∙∆𝑡∙(ƿ2−ƿ1)
Eq. 8
Therefore, permeability depends only on material constants, namely the coefficient of
diffusion and the solubility of the permeant in the membrane material, respectively. These
properties vary as the function of various materials’ properties like morphology, cohesive
energy and volume etc. [28].
2.2 Permeation rates
Transmission rates are given as the amount of material diffusing through unit area
membrane in unit time, i.e., it depends on both material constants (D and H) and
experimentally variable parameters (l and p):
∆𝑛
𝐴∙∆𝑡=
𝐷∙𝐻
𝑙∙ ∆𝑝
Eq. 9
The water vapor transmission rate it is usually given in mass units (𝑔
𝑚2∙𝑑𝑎𝑦), rather than in
molar units:
𝑊𝑉𝑇𝑅 =∆𝑚
𝐴 ∙ ∆𝑡 =
𝐷 ∙ 𝑆𝐻2𝑂𝑚
𝑙∙ 𝑃𝐻
𝐻2𝑂
(𝑇) ∙ ∆𝑟ℎ Eq. 10
𝑆𝐻2𝑂𝑚 denotes the solubility of water in the membrane material in units of mass per volume
and pressure. 𝑃𝐻𝐻2𝑂
(T) denotes the saturation partial pressure of water at the given
THEORETICAL BACKGROUND
11
temperature T (Figure 2.1) and ∆𝑟ℎ denotes the difference of relative humidity across the
barrier.
Figure 2.1: Relation of water vapor pressure vs temperature (Data taken from Dortmund
data bank, licensed by CC BY 3.0).
The oxygen transmission rate (OTR), typical unit 𝑐𝑚3
𝑚2∙𝑏𝑎𝑟∙𝑑𝑎𝑦, is given as the volume ∆𝑉𝑂2
0 of
oxygen, reduced to normal conditions, passing through the unit area A of a membrane of
thickness l per unit time at a given oxygen partial pressure difference of ∆𝑃𝑂2
𝑂𝑇𝑅 =∆𝑉𝑂2
0
𝐴 ∙ ∆𝑃𝑂2∙ ∆𝑡
=𝐷 ∙ 𝑆𝑂2
𝑉
𝑙
Eq. 11
Where 𝑆𝑂2
𝑉 denotes the solubility of oxygen in the membrane material in units of volume
oxygen, reduced to normal conditions, per membrane volume and pressure.
2.2.1 Temperature dependence:
The Arrhenius equation is the most straight forward way for analyzing the effects of
temperature on gas permeability. In general, Arrhenius relation works decently well, over
moderate temperature ranges, to simulate the temperature dependences of diffusion,
solubility, saturation vapor pressure, and thus of permeation.
The relation of diffusion coefficients of permeating gases to temperature is given as Eq. 12,
where 𝐷0 is the diffusion coefficient of the permeating gas and 𝐸𝑑 its activation energy,
R is the universal gas constant and T is temperature. The diffusion coefficient D ideally
follows an Arrhenius relationship [29], [30].
THEORETICAL BACKGROUND
12
𝐷 = 𝐷0 𝑒𝑥𝑝 (−𝐸𝑑
𝑅𝑇) Eq. 12
This relation is illustrated in Figure 2.2(a, c), where the permeation (WVTR and OTR)
dependence of biaxially oriented polypropylene (BOPP) and biaxially oriented polyvinyl
alcohol (BOPVA) on temperature are shown [31]. Solubility coefficients of permeating
gases are also temperature dependent and usually described well by the Arrhenius relation
(Eq. 13) [30], [32].
𝑆 = 𝑆0 𝑒𝑥𝑝 (−∆𝐻𝑠
𝑅𝑇) Eq. 13
Where 𝑆 is the solubility, 𝑆0 is solubility coefficient of permeating gas and 𝐻𝑠 is apparent
heat of solution. For the solubility of gases in liquids and polymers around room
temperature, ∆𝐻𝑠 < 0, so that solubility decreases with increasing temperature.
It follows for the temperature dependence of permeability[33], [34].
𝑃 = 𝐷 ∙ 𝑆 = 𝐷0 ∙ 𝑆0 ∙ 𝑒−(𝐸𝑑+∆𝐻𝑠) 𝑅𝑇⁄ = 𝑃0 ∙ 𝑒−𝐸𝑝 𝑅𝑇⁄
Usually, |𝐸𝑑| > |∆𝐻𝑠|Figure 2.2
It should be noted that permeation of gases is generally affected by the presence of other
gases. Figure 2.2 (b,d) shows the dependence of permeation of oxygen and water on relative
humidity for BOPP and BOPVA membranes. PVA being a water soluble polymer, it shows
more moisture permeation with increasing relative humidity on one hand, in contrast to the
hydrophobic polymer BOPP, while on the other hand, it shows a reduced oxygen
permeation rate due to increasing –OH intermolecular forces and the resulting lower
oxygen solubility [31].
THEORETICAL BACKGROUND
13
a)
b)
c)
d)
Figure 2.2: Moisture permeation of biaxially oriented polypropylene and biaxially
oriented PVA, a) moisture permeation dependence on temperature at 50% RH, b)
moisture permeation dependence on relative humidity (RH%) at 23oC, c) OTR values of
biaxially oriented PVA at different temperatures at 50% RH and d) OTR values of
biaxially oriented PVA at different relative humidity (RH%) at 23˚C (Copied from [31]
licensed bb CC BY 4.0) .
2.3 Factors affecting Permeability
Rearranging Eq. 9 yields
∆𝑛
∆𝑡=
𝐷 ∙ 𝐻 ∙ 𝐴𝑒𝑓𝑓
𝑙𝑒𝑓𝑓∙ ∆𝑝 Eq. 14
Which states that permeation rates are proportional to diffusion coefficient D, solubility H,
the surface area of the membrane which is actually accessible to the permeant Aeff, and the
THEORETICAL BACKGROUND
14
actual length leff of the path the permeant has to take to arrive at the opposite end of the
membrane. Relating the actual surface area to the geometric one by the surface coverage
and relating the actual path length to the geometric one by the tortuosity factor τ, we obtain
∆𝑛
∆𝑡=
𝐷 ∙ 𝐻 ∙ 𝐴𝑔𝑒𝑜𝑚 ∙ (1 − 𝜃)
𝑙𝑔𝑒𝑜𝑚 ∙ 𝜏∙ ∆𝑝 Eq. 15
Which yields the equation for transmission rates when normalized to the geometric area of
the barrier
∆𝑛
𝐴𝑔𝑒𝑜𝑚 ∙ ∆𝑡=
𝑫 ∙ 𝑯 ∙ (1 − 𝜽)
𝑙𝑔𝑒𝑜𝑚 ∙ 𝝉∙ ∆𝑝 Eq. 16
Eq. 16 serves as a design rule of barriers. There are thus four levers which serve to control
barrier properties: D, H, , and τ.
2.3.1 Coefficient of diffusion (D)
The diffusion coefficient is usually decreased by decreasing the Free Volume of polymers,
e.g., by cross linking of the polymer chains, or by enhancing the crystallinity of a material,
e.g., by decreasing the defect density of the barrier material [35], [36].
2.3.2 Coefficient of solubility (H)
Solubility is defined as a measure of the amount of solute that is dissolved in the membrane
at equilibrium. Solubility of gas in rubbery polymers is well characterized in terms of
Henry’s law of solubility, Eq. 6. This expression is effective for gasses with low molecular
weight and at low pressures [36]. For glassy polymers, the solubility of gases is more
accurately described by the Langmuir isotherm [37]. The deployment of materials having
low oxygen and moisture solubilities can be used as effective packaging materials [31].
Solubility of oxygen decreases with increasing polarity of the membrane materials, the
solubility of water decreases with decreasing polarity and decreasing tendency for
hydrogen bonding.
2.3.3 Surface coverage ()
The effective surface of a membrane which is actually exposed to diffusing molecules is
reduced by covering the surface by an impermeable barrier, such as a (defect free) metal
oxide layer [18], [38], [39].
THEORETICAL BACKGROUND
15
2.3.4 Tortuosity (τ)
The effective path length that a diffusing molecule must take from one side of the barrier
to another relates to both, the thickness of the barrier film and its internal structure [40].
Increasing thickness would result in decreased permeability as the molecules will take long
time for the diffusion. But constant increase in the thickness is not the solution for
packaging. Increase in thickness may post certain disadvantages like additional weight and
relatively less mechanical flexibility. Furthermore, thicker films are also not compatible for
roll-to-roll production. Therefore, in order to not go beyond certain thickness requirement
for roll-to-roll processing, the effective path length is increased by creating a zig zag path
for diffusing molecules by adding impermeable platelets [41]. Due to zig zag (tortuous)
path diffusing molecules take longer time and overall permeability is decreased [42],
[43].This is further explained in the following section.
2.4 Modeling and simulation of barrier characteristics of filled polymers
As described earlier in Eq. 8, permeability is the product of the diffusion coefficient D and
solubility H. In the binary system, containing the polymer along with the nano-fillers, the
gas solubility in the nanocomposite is expressed as:
𝐻 = (1 − Φ)𝐻𝑜 Eq. 17
Where H0 is the solubility of the gas in the unfilled polymer and Φ is the filler volume
fraction. Eq. 17 assumes that the local properties of the matrix are not affected by the
presence of the nanostructures. In such a system, where fillers are assumed impermeable,
the diffusing molecules have to go through a more tortuous path to leave the coating [44].
Thus, the effective path is enhanced and the path length in the nanocomposite is given by:
𝑙𝑒𝑓𝑓 = 𝑙𝑔𝑒𝑜𝑚 ∙ 𝜏 Eq. 18
where lgeom is the thickness of the pure polymer film and 𝜏 is the tortuosity factor. Formally,
the increase of the effective diffusion path can also be expressed by the decrease of the
diffusion coefficient
𝐷 =𝐷0
𝜏
Eq. 19
THEORETICAL BACKGROUND
16
By combining Eq. 17 and Eq. 19, it is possible to define the relative permeability as a
function of the filler volume fraction and the tortuosity factor:
𝑃
𝑃0=
1 − Φ
𝜏
Eq. 20
where P0 is the permeability coefficient of the pure polymer and P that of the
nanocomposite.
Each of the different models that are presented in the following sections proposes an
expression for the tortuosity factor 𝜏 and thus for the relative permeability. The tortuosity
factor 𝜏 depends on several geometrical parameters such as the volume fraction of nano-
fillers Φ, their aspect ratio 𝛼 and the aspect ratio of pores and slits across adjacent fillers in
the similar horizontal plane σ. To define these parameters, we first consider a repeating unit
cell as seen in Figure 2.3(ii), where each of the rectangular plates, representing the filler
particles with dimensions w, t, and l, is filled into one of the polymer unit cells, having
finite width W, thickness T and length L [45], [46].
The volume fractions in two and three dimensions, respectively, are defined as follows
[46]:
Φ2𝐷 =𝑤𝑡
𝑊𝑇
Eq. 21
Φ3𝐷 =𝑤𝑡𝑙
𝑊𝑇𝐿
Eq. 22
THEORETICAL BACKGROUND
17
Figure 2.3: Schematic diagram a film (i) without fillers offering no hindrance, (ii) film
filled with regularly arranged platelets perpendicular to the direction of diffusion, creating
a tortuous path. (Reproduced from [45] with permission from Elsevier).
2.4.1 Overview of the various models
Modeling of the barrier characteristics depends on the distribution and arrangement of the
fillers within the matrix. These can further be classified in three different classes as
described below.
a) Regularly distributed and perpendicularly oriented fillers
Most of the models in literature developed earlier describing diffusion in a composite
material are based on 2D systems in which the filler particles have a rectangular shape,
resembling ribbons of infinite length with a finite width (w) and thickness (t) [47]. These
models assume that the fillers are arranged regularly in the polymer matrix and
perpendicularly with respect to the diffusion direction, as can be seen in Figure 2.3. Some
of these models are presented in Table 1. First, Nielsen [48] proposed a simple permeability
model (Eq. 23) for such a system which is based on the idea that diffusing molecules need
to pass by a longer path in order to exit the film as seen in Figure 2.3. This model is widely
used but is only applicable in the dilute regime (𝛼Φ ≪ 1).Wakeham and Mason [49]
suggested a new model (Eq. 24) that is based on perforated laminae in which they found
the resistance to diffusion was created by the need of the penetrant to enter into the narrow
perforations. Similarly, Aris [50] developed an analogous model (Eq. 25) where the
diffusion resistance is mainly due to aspect ratio of pores (σ) and interaction of molecules
THEORETICAL BACKGROUND
18
with pores which is termed as necking. Eq. 25 consists of four terms, where the first term
is 1 representing the case when the volume fraction of flakes is zero. The second term
represents the contribution due to the tortuous path taken by diffusing molecules around
the flakes. The third term characterizes the resistance due to the constraints between the
flakes. The last term is attributed to the diffusing of permeant through the pores and flakes
[51], [52]. Cussler et al., [43] neglected the third and fourth term and proposed a simpler
model (Eq. 26) for narrower pores (σ ≪ 1). This model predicts a quick decrease of the
relative permeability already at small values of Φ [43], [52]. Moggridge et al. [53] modified
Cussler et al.’s model by multiplying the second term by ½, which was their estimate for
the miss-alignment of the ribbon-like flakes. Additionally, they developed another model
for hexagonal flakes (Eq. 28). From this model, it can be noted that a change in the
geometry of the flakes affects the effectiveness of the barrier. In this case, hexagonal flakes
are less effective compared to ribbon-like flakes.
Table 1: Some characteristics of the models developed to study regularly distributed and
perpendicularly oriented fillers in polymer nanocomposites. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝜎 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑙𝑖𝑡𝑠.
Model
(Year)
Particle
type Tortuosity factor
Equation
#
Nielsen[48]
(1968) Ribbons 1 +
𝛼Φ
2 Eq. 23
Wakeham
and
Mason[49]
(1979)
Perforated
laminae 1 +
𝛼2Φ2
4(1 − Φ)+
𝛼Φ
2𝜎+ 2(1 − Φ) ln [
1 − Φ
2𝜎Φ] Eq. 24
Aris[50]
(1986) Ribbons
1 +𝛼2Φ2
4(1 − Φ)+
𝛼Φ
2𝜎
+2𝛼Φ
𝜋(1 − Φ)ln [
𝜋𝛼2Φ
4𝜎(1 − Φ)]
Eq. 25
Cussler et
al.[43]
(1988)
Ribbons 1 +𝛼2Φ2
4(1 − Φ) Eq. 26
Moggridge
et al.[53]
(2003)
Ribbons 1 +𝛼2Φ2
8(1 − Φ) Eq. 27
Hexagonal
flakes 1 +
𝛼2Φ2
54(1 − Φ) Eq. 28
THEORETICAL BACKGROUND
19
b) Randomly distributed and perpendicularly oriented fillers
To describe the random distribution of fillers that are oriented perpendicularly to the
diffusion direction, Brydges et al. [54] proposed a model (Eq. 30) where they use the
stacking parameter 𝛾 defined by:
𝛾 =𝑥
𝑙 Eq. 29
where x and l are represented in Figure 2.3. This parameter takes into account the deviation
from ideally positioned flakes by expressing the horizontal offset of each particle with
regard to the one below it [55].
When 𝛾 = 1 2⁄ , the ribbons in one layer are centered to the gaps of the layer under it.
Cussler et al.[43] derived a similar expression for such systems (Eq. 31), however instead
of the factor 𝛾(1 − 𝛾) , they introduced 𝜇 , a combined geometrical factor which
characterizes the randomness of the porous media. Models from the beginning of the 21st
century focused on more realistic representations of the geometry of the fillers; particles
are not any longer considered as infinite ribbons but instead they have finite width, length,
and thickness (3D systems)[46] . Such a model was presented in Fredrickson and Bicerano
[56], where they examined the effective diffusion in composites containing randomly
placed disks having high aspect ratios and derived Eq. 32. To calculate the aspect ratio of
the disks, they used the radius of the disk instead of the width. Gusev and Lusti [57]
obtained Eq. 33 for a similar 3D system. They obtained it using a finite-element method
and fitting their results to an exponential function. A more recent model was proposed by
Minelli et al.[58] in which computational fluid dynamics (CFD) was used to solve the mass
transport problem in the layers filled with flakes and evaluation of the reduction in
permeability. From their study, they were able to derive the models presented in Eq. 34 and
Eq. 35 where 𝑟 is described by Eq. 36. This model incorporates many filler geometries and
can be used to study the enhancement of barrier properties resulting from the addition of
disks as well as rectangular, hexagonal, and octagonal flakes to a polymer matrix.
THEORETICAL BACKGROUND
20
Table 2: Some characteristics of the models developed to study randomly distributed and
perpendicularly oriented fillers in polymer nanocomposites. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝛾 = 𝑠𝑡𝑎𝑐𝑘𝑖𝑛𝑔 𝑝𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟, 𝜇 = 𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐𝑎𝑙 𝑓𝑎𝑐𝑡𝑜𝑟 .
Model
(Year)
Particle
type Tortuosity factor
Equation
#
Brydges et al. [54]
(1975)
Rectangular
flakes 1 +
𝛼2Φ2
(1 − Φ)𝛾(1 − 𝛾) Eq. 30
Cussler et al. [43]
(1988) Ribbons 1 +
𝜇 𝛼2Φ2
(1 − Φ) Eq. 31
Fredrickson and
Bicerano [56]
(1999)
Disks
(1
2 + [(2 − √2)𝜋𝛼Φ/4 ln(𝛼2)]
+1
2 + [(2 + √2)𝜋𝛼Φ/4 ln(𝛼2)]
)
−2
Eq. 32
Gusev and Lusti
[57]
(2001)
Disks 𝑒𝑥𝑝 [−𝛼Φ
3.47]0.71
Eq. 33
Minelli et al. [58]
(2011)
Disks and
rectangular,
hexagonal
or
octagonal
flakes
𝑟 ≤ 1
∶ (𝛼 + 2)2Φ
2𝛼+
(𝛼 + 2)4Φ2
4(𝛼2 − 𝛼Φ(𝛼 + 2))
+2(𝛼 + 2)2Φ
𝜋𝛼ln (
4
𝜋(
𝛼
Φ(𝛼 + 2)− 1))
Eq. 34
𝑟 ≥ 1
∶ 1 +Φ(𝛼 + 2)
2
+2(𝛼 + 2)2Φ
𝜋𝛼ln (
𝛼 + 2
𝜋)
Eq. 35
Minelli et al. [58]
(2011) 𝑟 =
2(𝛼 − Φ(𝛼 + 2))
(𝛼 + 2)2Φ Eq. 36
THEORETICAL BACKGROUND
21
c) Randomly oriented fillers
The barrier properties are best, i.e., the tortuosity factor 𝜏 is at its highest, when the nano-
fillers are oriented perpendicularly with respect to the diffusion direction [55]. However,
in reality, this cannot always be achieved and, thus, the need for models that take the filler
orientation into account is imperative. Bharadwaj [42] introduced an order parameter S in
order to quantify the non-uniform orientation of the particles. This parameter is expressed
by:
𝑆 =1
2⟨3 cos2 𝜃 − 1⟩
Eq. 37
where 𝜃 represents the angle between the diffusion direction, usually identical with the
normal vector to the membrane surface, and S is calculated by averaging over all particles.
Figure 2.4: The order parameter S for three different cases; when all filler particles are
parallel to the diffusion direction (S=-1/2), when they are perpendicularly oriented (S=1)
and when they are randomly oriented (S=0). (Reproduced with modifications from [42]
with permission from American Chemical Society (ACS))
This parameter was used by Bharadwaj to modify Nielsen’s equation in order to take into
account the orientation of the flakes and the new model that he proposed is given by Eq.
38.
𝑃𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒
𝑃𝑝𝑜𝑙𝑦𝑚𝑒𝑟=
1 − Φ
1 + (𝛼2 ∙ Φ ∙ (
23) ∙ (𝑆 +
12))
Eq. 38
Lu and Mai [41], [47] developed a model that uses Bharadwaj’s order parameter to
approximate critical volume fraction Φ𝑐 of platelets/ ribbons of clay corresponding to
lowest gas permeability in exfoliated nanocomposites. They determined that the
permeation of gas molecules in clay based nanocomposites is an aspect ratio-controlled
mechanism and it is at its lowest when the volume fraction satisfies Eq. 39. In this equation,
THEORETICAL BACKGROUND
22
p𝑐 takes the value of 0.38 and 0.72 for two-dimensional and three-dimensional models
respectively [45], [47].
Φ𝑐 = (3
2𝑆 + 1) (
1
𝛼) p𝑐
Eq. 39
Maksimov et al. [59] developed an empirical relation given by Eq. 40 to predict the
permeation in nanocomposites with randomly oriented rectangular flakes in 3D. In their
equation, 𝑃∥ refers to the permeability of the composite calculated by Nielsen’s relation.
𝑃 =1
3(𝑃∥ + 2𝑃0(1 − Φ))
Eq. 40
Recently, Greco and Maffezzoli [60] derived a new geometrical model for arbitrarily
oriented lamellae based on 2D and 3D simulation results. This model predicts the
permeability in polymers filled with impermeable fillers by calculating normalized path
length and probability of collision between the diffusing particle and the lamellar surface
[60]. For simulation of the diffusion, a finite-element (FE) method was used and the
simulations were performed for different filler concentrations, aspect ratios as well as
different orientation angles. It was found that all the data fit on a single curve indicating
that the normalized diffusion coefficient 𝐷𝑛𝑜𝑟𝑚 depends on the normalized path length
𝐿𝑛𝑜𝑟𝑚, which is a function of the nanocomposite morphology that is a combination of
aspect ratio, volume fraction and orientation) [60]; as given in Eq. 41 and Eq. 42.
𝐷𝑛𝑜𝑟𝑚 = (1
𝐿𝑛𝑜𝑟𝑚)4
Eq. 41
𝐿𝑛𝑜𝑟𝑚 = 1 +𝛼Φ
2cos 𝜃 (1 − sin 𝜃)
Eq. 42
Where, 𝐷𝑛𝑜𝑟𝑚 is the ratio between diffusion coefficients of simulated nanocomposite and
polymeric matrix. Additionally, in their paper, they showed that their new 3D model fits
better to the finite element calculations than the 2D Bharadwaj model.
THEORETICAL BACKGROUND
23
d) Accounting for additional influencing parameters
So far, the presented models have considered mainly some of the fillers parameters such as
their aspect ratio, volume fraction, stacking position, and orientation. In this section,
additional models are discussed that include further influencing parameters like the
polydispersity of the fillers and their thickness, [44], [53] the polymer chain immobility,
the existence of an interfacial region [40], and aggregation of the fillers in the matrix [61],
[62].
Table 3: Some characteristics of the models presented in this sub-section.
Model
(Year)
Particle
type Tortuosity factor
Equation
#
Lape et al.
[62]
(2004)
Ribbons [1 +2
3(Φ
𝑡∑𝑛𝑖𝑤𝑖)∑𝑛𝑖𝑤𝑖
2]2
Eq. 43
Xu et al.
[63]
(2006)
Ribbons 𝜉 [1 +𝛼𝑡
2√Φ(t + b)−3/2] Eq. 44
Sorrentino
et al. [64]
(2006)
Ribbons (1 − Φ) + Φ(
𝑤 + 2𝑡𝑤 𝑠𝑖𝑛𝜃 + 2𝑡 𝑐𝑜𝑠𝜃)
2
1 + 𝛽Φ
Eq. 45
Picard et
al.[44]
(2007)
Ribbons (1 +Φ
3
∑𝑛𝑖 (𝑤𝑖
𝑡𝑖)2
∑𝑛𝑖𝑤𝑖
𝑡𝑖
)
2
Eq. 46
Nazarenko
et al.[61]
(2007)
Disks 1 +Φ𝛼
2𝑁 Eq. 47
𝑛𝑖 is number of flakes in size category, 𝑤𝑖 is half of the flake length, 𝜉 is polymer chain segment
immobility, b is face to face distance between ribbons, 𝛽 is volume and diffusion function of
ribbons as described in Eq. 48 and N refers to number of layers in layer stack.
Lape et al. [62] proposed a model (Eq. 43) that deals with flakes having a size distribution
in a system where they are randomly dispersed with an infinite length. They assumed the
flakes to have a discrete distribution of widths and a constant thickness t. In Eq. 43, 𝑛𝑖
THEORETICAL BACKGROUND
24
represents the number of flakes having a specific width and 𝑤𝑖 represents half of the flake
width[45], [65]. In their study, Lape et al. [62] deduced an unexpected result; they found
that the barrier properties of polydispersed flakes are superior to that of monodispersed
ones with the same average size. Picard et al. [44] modified Lape et al.’s model in order to
also take into account the polydispersity of the thickness of the flakes. The relation
proposed by them is described in Eq. 46. Consequently, Picard et al. [44] also account for
the presence of aggregation in the system since it includes the polydispersity of the filler’s
thickness. Similarly, Nazarenko et al. [61] modified Nielsen’s model in order to consider
the effect of layer aggregation on the barrier properties. This model is presented in Eq. 47
where N represents the number of layers stacked together [55]. When N=1, the layers are
well dispersed and thus they are in an exfoliated state. However, when N increases, the
barrier properties are less efficient, leading to a bad quality barrier [55].
Another parameter influencing barrier properties was analysed by Sorrentino et al. [40].
They proposed a model (Eq. 45) that includes the effect of the presence of an interfacial
region between the polymer matrix and the inorganic flakes. This was done by the
introduction of the 𝛽 parameter, which is calculated by:
𝛽 =𝑉𝑠𝐷𝑠
Φ𝐷0−
𝑉𝑠 + Φ
Φ
Eq. 48
where 𝐷𝑠 and 𝑉𝑠 are the diffusion coefficient and the volume fraction of the interface
region, respectively, and 𝐷0 is the diffusion coefficient of the unfilled matrix.
2.5 Working principle and degradation of organic solar cells
2.5.1 Working principle
As shown in Figure 2.5, a typical organic solar cell has two electrodes, at least one of which
is semitransparent. This permits light radiations to interact with active layer. The active
layer uses the energy of the radiations (photons) to create charge carriers. A layer between
electrode and active layer i.e. usually PEDOT:PSS (Poly(3,4-ethylenedioxythiophene)
polystyrene sulfonate) to achieve selectivity of the contacts and to support charge extraction
[66], [67]. Two cell architectures are possible by making variations in the sequence of the
layers. One is termed as ‘normal structure’ and other is called ‘inverted structure’. In normal
structure holes (positive charges) are obtained through the bottom electrode, while negative
THEORETICAL BACKGROUND
25
charges leave the device through the top electrode. In the case of inverted structure, the
device polarity is reversed [3], [4], [68].
Figure 2.5: a) Schematic structure of a typical organic solar cell showing a glass or PET:
Polyethylene terephthalate (substrate), Indium tin oxide: ITO (bottom electrode), ZnO:
Zinc oxide (electron extraction layer), blend of P3HT:PCBM (active layer), PEDOT:PSS:
Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (hole extraction layer) and
Silver:Ag (top electrode) b) Energy band diagram of a normal cell structure and c) Energy
band diagram of an inverted cell structure. Data extracted from [69].
In contrast to a typical inorganic silicon based photovoltaic, a typical organic solar cell has
an active layer of semiconductors which is of organic nature. In such organic
semiconductors, carbon atoms having single and double alternating bonds are the core
responsible for initiation of the conductivity [70]. Because of the sp2 hybridization, the
overlapping pz electron wave functions allow delocalization of the corresponding electrons,
which enables transportation of the charge. HOMO (bonding π orbital) is defined as highest
occupied molecular orbital, LUMO (antibonding π* orbital) is referred as lowest
unoccupied molecular orbital. Opening between bonding (π) and antibonding (π*) states
pertaining to Peierls distortion is called as band gap (𝐸𝑔) [71]. By irradiation with light in
the range of visible region, electronic conversion between HOMO and LUMO can be
stimulated [72].
THEORETICAL BACKGROUND
26
The working principle of an OSC device involves six basic steps [67]. These basis steps
are briefly described below.
Absorption of light and exciton formation: A light ray transmits from the semitransparent
electrode and incites the shift of an electron from HOMO to LUMO.
This shift results in a stimulated state, which is described as an electron-hole pair on the
same molecule (‘exciton’). Pertaining to their relatively petite distance and the feeble
relative permittivity of compounds of 𝜀 ≈ 2-4 of organic nature, electron and hole possess
sturdily binding of Coulomb [73].
Exciton diffusion: The exciton of neutral charge exciton distribute throughout the layer
until it decays or bumps into an interface. When exciton reaches at interfaces, it detaches
and gets into separate charges.
Charge carrier separation: To incapacitate the binding of electron and hole, a material
having inferior LUMO is employed. This low LUMO material forms a second phase and
delivers a propitious energy level for the electron. On the boundary of donor and acceptor
within photoactive layer, the electron is shifted between phases and as a result, holes and
electrons are separated. If the event of absorption takes place at donor / acceptor phase
boundaries, charge carriers are generated at once which is within the femtosecond time
scale, thus excluding the earlier mentioned exciton diffusion process.
Movement of carriers to the electrodes: In general, holes and free electrons move by
means of hopping from molecule to molecule unless encountered by electrodes. Electrons
travel in the phase formed by the acceptor molecules, while holes travel in the phase formed
by the donor molecules. This requires phase separation to an extent that continuous
percolation paths are formed.
Charge collection: After charges have crossed the phase boundary barriers, they are
collected at an electrode and hence photocurrent is generated.
2.5.2 Current density voltage characteristics
For the characterization of a solar cell, current density-voltage (J-V) curves are measured
under illumination. The jV-curve can be obtained by treating the solar cell in terms of the
so called one diode model, which corresponds to the equivalent circuit described in Figure
2.6. From a typical curve, the useful information is extracted in terms of power conversion
efficiency (PCE), short circuit current density (Jsc), open circuit current (Voc) and fill factor
(FF).
THEORETICAL BACKGROUND
27
Figure 2.6: Equivalent circuit of an organic solar cell (one diode model) (Reproduced from
[74] with permission from Elsevier)
• The power conversion efficiency (𝑷𝑪𝑬) is the most important parameter and defines the
actual maximum electric power obtained divided by the radiation power which the device
is exposed to. It is calculated from the jV curve by
𝑃𝐶𝐸 = 𝑗𝑠𝑐 ∙ 𝑉𝑜𝑐 ∙ 𝐹𝐹
𝑃𝑙𝑖𝑔ℎ𝑡,𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡
• The short circuit current density (𝑱𝑺𝑪) is the current across the solar cell when the voltage
through the cell is nil. It corresponds to the generation and collection of light-generated
carriers at electrode and thus depends on the number of incoming light rays as well as on
the results of each of the six basic steps described earlier in the Working principle.
• The open circuit voltage (𝑽𝐎𝐂)
The open-circuit voltage, 𝑉OC, is the highest potential difference obtained from a solar cell,
and this occurs at zero current [75]. In a heterojunction solar cell, 𝑉OC is described as a
formation of a charge-transfer complex between the blend of materials in active layer. Each
phase goes through charge transfer when irradiated with light. The charge transfer of
excitonic state is lower than pristine state and hence is occupied at the interface between
the donor and acceptor[75], [76]. The energy of the charge transfer in excitonic state is
equal to the difference of the donor’s HOMO and the acceptor’s LUMO level [77].
• The fill factor (𝑭𝑭) is the ratio of the maximum obtainable power, divided by the product
of 𝑉OC and compared to 𝑉𝑂𝐶 and the 𝐹𝐹 is a more delicate sign for voltage dependent
recombination processes within the solar cell device.
2.6 Degradation mechanism of Organic Solar Cells
As compared to materials used in inorganic solar cells, most organic matter utilized in
organic solar cells, have the susceptibility to degrade chemically by oxygen, moisture and
THEORETICAL BACKGROUND
28
ultraviolet rays interactions (as shown in Figure 2.7) [6], [78]–[80]. Such materials are also
prone to morphological instabilities at relatively higher temperatures [81]–[83]. The
degradation within OSC can start in a localized manner or at the interfaces of individual
layers and thus leading to loss of overall cell performance [84], [85].
Figure 2.7: A schematic diagram of few processes responsible for degradation in OSC with
P3HT:PCBM as photoactive layer, (Reproduced from [80] with permission from Elsevier).
Degradation of solar cell in chemical or physical manner may affect different phenomenon
such as absorption of photons, dissociation and transportation of charge towards electrode.
The degradation leaves a negative impact on aforementioned phenomenon and as a result
number of charges collected at electrode decrease, which ultimately deteriorates short
circuit current (𝐽𝑆𝐶) [73], [80]. Changes in the work function of electrodes or levels of
HOMO and LUMO can leave a significant negative impact on open circuit current (𝑉𝑂𝐶).
[67]. Since, the fill factor (FF) is responsible for provides information on the quality of the
charge extraction, therefore any changes in recombination losses / formation of space
charges due to instable transport could results in FF loses [86]. Therefore, the fill factor is
mainly impelled by parallel and series resistances, thus can be co-related with the
modulation in the JV-curve [73], [80], [86].
Some of the possible degradation mechanisms of organic solar cells reported in literature
are mentioned below:
THEORETICAL BACKGROUND
29
The main channels for diffusion of oxygen and moisture in to encapsulated OSC devices
are either microscopic pinholes present in the encapsulation or through edges of lamination
via glues [87], [88]. Molecules continue diffusing within the layer until they reach electrode
[89]. These diffused moisture and oxygen molecules modify inner surface of the electrode
by chemical reactions such as voids formation or patches of insulation, and ultimately
causing reduction in the electrode/photoactive layer charge transfer which is usually
referred as photo-bleaching [6], [76], [87]. This rate of photo-bleaching and permeability
of oxygen of a barrier film of thickness d to oxygen at partial pressure p(O2) can be
calculated from the rate of photobleaching of a P3HT film underneath the barrier when the
photobleaching reaction is diffusion controlled. The reaction rate is defined as the number
of moles, n, of thiophene rings being oxidized per unit time t and unit area A, ∆𝑛
𝐴∙∆𝑡, of the
P3HT film underneath the barrier. Assuming the consumption of three to five moles of
molecular oxygen per mole of thiophene rings, depending on the stage of the reaction [90],
we obtain from the resulting oxygen flux J(O2) Eq. 49. The reaction rate is obtained from
the rate of absorbance loss, inserting 𝜀 = 8000 𝑐𝑚2 𝑚𝑚𝑜𝑙−1 thiophene rings for the molar
extinction coefficient at the absorption maximum. This provides the permeability and the
OTR of the barrier Eq. 50 & Eq. 51):
𝑃(𝑂2) = 𝐽(𝑂2) ∙𝑑
∆𝑝(𝑂2)=
5∙∆𝑛
𝐴∙∆𝑡∙
𝑑
∆𝑝(𝑂2) Eq. 49
𝑃(𝑂2) =5 ∙ ∆𝐸
𝜀 ∙ ∆𝑡∙
𝑑
∆𝑝(𝑂2)
Eq. 50
𝑂𝑇𝑅 =5 ∙ ∆𝐸
𝜀 ∙ ∆𝑡∙
1
∆𝑝(𝑂2)
Eq. 51
Mechanical delamination can also take place if the device goes through extended exposure
to diffusing gasses and simultaneous mechanical stresses [91].
Metal electrodes used for collecting electrons having low work function are susceptible to
oxidation as compared to hole collecting metal electrodes which usually have high work
function [3], [66], [89]. Like OLEDs, there is always a chance of diffusion of silver
electrode into the active layer which may create shorting issues [66]. As mentioned earlier
organic materials of OSC are sensitive to moisture and oxygen especially under
illumination and elevated temperature which are termed as photo and thermal oxidation
[79], [92]. In one of the study it was stated that degradation in OSC can either be reversible
THEORETICAL BACKGROUND
30
or irreversible depending on the types of degradation i.e. limited exposure to either oxygen
or moisture [79]. However, long term device exposure to oxygen / water under illumination
can generate defects and can cause loss of absorption density which will lead to irreversible
degradation. [79]. Doping of active layer with p or n type dopants can significantly alter
the device performance [87], [93]. Aggregation within the blend of active layer may block
the path of charge carriers which may result in substantial decrease in in device
performance [94]. Additional hindrances to charge carriers may also come from the
impurities, dopants or during handling / processing of the active layer .[87], [93].
Furthermore, the use of highly hydrophilic materials like PEDOT:PSS can also
significantly accelerate degradation under humid conditions [6], [66], [95]. Additionally,
PEDOT:PSS loses conductivity when exposed to UV radiation which ultimately causes
degradation in OSC device [95].
Table 3: Degradation of the OSC parameters and their possible effect on the device
performance as described by Grossiord et al,. [86]
Parameters affecting PCE and key factors
determining them
Possible causes
Fill factor (FF) –
- Charge transportation and
recombination process
- Weakening of charge transportation in
active layers (P3HT;PCBM) or hole
transport layer (PEDOT:PSS)
- Modification in charge recombination
mechanism
- Generation of shorts or shunts.
Open circuit current (VOC) –
- Differences in levels of HOMO (donor)
LUMO (acceptor)
- recombination process
- Active layer or electrode interface
reduction
- Modification of effective band gap in
blend of photoactive materials.
- Modification in work function of
electrode
- generation of shorts
Short circuit current (JSC) – - Degradation of the conjugation of the
photoactive polymer
THEORETICAL BACKGROUND
31
- Efficient absorption of light (thickness
of active layer, band gap, molecular
architecture),
- Efficient dissociation of exciton
(matching level of HOMO (acceptor)
and LUMO (Donor), morphology of
blend in active layer),
- Efficient carrier transportation as well
as collection (path towards electrodes
i.e., donor goes to electrode that collects
hole and acceptor goes to electron that
collects electron,
- Crystallinity of active layer,
- Architecture of the device
- Deterioration of the optical transparency
of the layers laying between light
illumination and active layer
- Deterioration of interface between donor
and acceptor or increase in blend
domains above the diffusion path length
of the excitons
- Loss of percolating paths due to blend
reorganization
- Deterioration of interface between active
layer and electrode
- Formation of crack in active layer
- Delamination of electrode or active layer
- Deterioration of the mobility of charge
carriers due to degradation of materials.
STATE OF THE ART
32
STATE OF THE ART
This chapter gives the state of the art in the field of coated barriers. It is categorized
according to the parameters on which permeability depends, i.e., coefficient of solubility,
thickness (increasing path of diffusion), coefficient of diffusion and effective area.
To control permeability, the use of polymers having low solubility of diffusing gases can
be beneficial. Therefore in this chapter, different polymers are described which are cost
effective and abundantly available.
Later in the chapter filler barriers are discussed which are based on increasing the diffusion
path called as tortuosity factor, this is done by filling polymers with impermeable inorganic
platelets (nanoclay, graphene oxide, glass flakes etc.,) having some aspect ratio. The use of
platelets will increase the diffusing path, thereby referred as tortuous path. Various
theoretical models are discussed which predict the barrier quality of filler polymers.
In order to decrease permeation, control over the coefficient of diffusion is necessary, hence
polymers like SiO2 are effective materials and can decrease permeability by exhibiting low
coefficient of diffusion. The perhydropolysilaze simply termed as polysilazane or PHPS is
an inorganic polymer which yields SiO2 network after curing. The resulting SiO2 layer acts
as an excellent diffusion barrier against water and oxygen. Various curing mechanism of
PHPS are discussed in this chapter.
ORMOCERS (Organically modified ceramics) are a class of materials having tailored
properties. The choice of organic or inorganic parts depends on the application.
ORMOCERS with tailored properties can effectively be used in packaging industry not
only as barrier in itself but also as the planarization layer. Such type of materials have
effectively been utilized to decouple surface defects and hence decreasing effective area.
Fluoropolymers due to their hydrophobic nature, can be effectively used in the packaging
industry. CYTOP, a class of fluoropolymer is described in this chapter, has not only the
barrier effect but also leave a smoother surface finish.
The use of scavengers has also been beneficial for the packaging industry as they can absorb
moisture and can keep the product safe. Such kinds of barriers are also discussed in this
chapter.
STATE OF THE ART
33
3.1 Bulk Polymers
Polymers are one of the most important classes of packaging materials, along with metallic,
ceramic (glass), and cellulosic materials (paper and cardboard) [96]. Because of the light
weight and cost effectiveness, along with a variety of other favorable physical and chemical
characteristics, polymers are the most commonly used materials in the field of packaging
[97]. As per an estimation, polymers carry around 40% of the market share in the food
packaging industry [65]. One of the basic beneficial factor of the polymers is their simple
processing by roll-to-roll techniques. Although polymeric materials have revolutionized
the packaging sector and exhibit many merits over their counterparts, their permeability to
small molecules and environmental gases is a serious disadvantage, which limits their
applicability in encapsulating organic electronic devices (OEDs) [98]. Figure 3.1 compares
the permeability to oxygen and water vapor for different polymeric materials.
The barrier quality in terms of OTR of most of the common bulk polymers like PET, PAN
etc. ranges between 10 𝑐𝑚3 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 to 1 𝑐𝑚3 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 and
are usually considered sufficient for packaging food [24], [35], [99].
100010010
1
0.10.01
10000
1000
100
10
1
0.1
OT
R (
cm
3/m
2.d
ay.b
ar)
@ 2
3oC
/ 5
0 %
RH
WVTR (g/m2.day)
@ 23oC / 85% RH
0.01
LCP
CelluloseEVOH
PVDC
PEN PAN
PET
PVC-UPLA
BOPP
PE-HD
PVB
PA 6
PS
COC
PC
PVC-P
PP
PE-LD
Figure 3.1: OTR and WVTR of different bulk polymers normalized to 100 µm thickness.
[56] PE-LD =polyethylene low density, PE-HD= polyethylene high density,
PP=polypropylene, PS=polystyrene, bopp=biaxially oriented polypropylene,
PLA=polylactic acid, PVC=polyvinyl chloride, PA6=polyamide 6, LCP=liquid crystalline
polymer, EVOH=ethyl vinyl alcohol, PAN= polyacrylonitrile, PEN= polyethylene
napthalene, PET= polyethylene terephthalate, PVDC= polyvinylidene chloride, PC=
polycarbonate, PVC-P=polyvinyl chloride-plasticized, PVC-U= polyvinyl chloride-
unplasticized, PVB= Polyvinhyla butyral. (Reproduced from [100] licensed by CC BY 3.0).
STATE OF THE ART
34
The permeation to a particular molecule of gas can be influenced by the existence of others.
For example, ethylene vinyl alcohol (EVOH) offers high hindrance to the permeation of
oxygen at relatively low humidity conditions. However, in exceptionally wet conditions
(e.g., above 75% RH), its oxygen permeation rate is enhanced by almost one order of
magnitude. The reason for this is the swelling of polymer which happens due to the
presence of water [101].
Polymers in combination with other materials or multilayer stack systems are commonly
employed. For instance, to obtain a good quality oxygen barrier in a highly wet
environment, materials like EVOH or PVA, which show a high sensitivity to water, but
exhibit a very low permeation to oxygen, can be placed between two layers of a
hydrophobic polymer like polyethylene (PE) [100] [102]. Mixing of one polymer into
another is also an effective approach to get desired barrier characteristics that cannot be
obtained with a single polymer [103]–[105]. With polymer mixing [104] and creating
multilayer by lamination of different polymeric films [106], packaging materials with
relatively low gas permeations can be produced, however, these methods unfortunately
have high production costs because of the use of special and expensive glues that
complicate the manufacturing process. Recycling of such barriers films is also a
challenging work. Therefore, there is still a great interest in the polymer industry to generate
monolayer coatings with improved mechanical and barrier properties [24], [106].
The barrier properties required for organic electronic devices (OEDs) are clearly
substantially beyond those bulk polymers [107] and there is no pristine polymer that shows
all the required gas barrier and mechanical characteristics for such applications [7].
Nevertheless, as a temporary protection of inorganic PV against moisture and oxygen; poly
(vinyl butyral), ethylene vinyl acetate, polyolefins, ionomers and thermoplastic
polyurethanes have been reported in the literature [31]. Therefore, in order to fulfil the
barrier requirements for OED encapsulation different strategies based on Eq. 16 are used
to improve the barrier properties of polymers, which include used of fillers and creating
impermeable coatings [108]. The later are usually deposited by sputtering or ALD, often in
multilayer structures with polymers. These achieve excellent values e.g. Barix [13].
However, these barriers are expensive and have other disadvantages like the processing of
such barrier require special vacuum systems which are not only complex and require high
level of maintenance, moreover super flexibility is still a challenge for such barriers.
Therefore, solution processable barriers based on polymer films have been developed and
are described in the following sections.
STATE OF THE ART
35
3.2 Increasing tortuous path
As pointed out above, barrier layers consisting of neat polymer films do not have the
capacity to hinder the permeability of unwanted substances to the extent required for
protecting OEDs [109], [110]. According to Eq. 16 a possible strategy of decreasing the
permeability consists in increasing the path that permeant molecules have to follow through
the coating in order to reach the other side of the membrane [110], [111]. The addition of
obstacles (Figure 2.3) is thought to create a tortuous path for diffusing molecules [45], [65].
By virtue of their improved barrier characteristics, polymers filled with particles have
gained much importance and have been studied extensively in literature [45], [110]. The
addition of the particles usually does not affect coatability and thus processing remains
simple. The type of materials used for this application are two-phase systems consisting of
a polymeric matrix with dispersed inorganic nanoparticles such as clay minerals, metal
oxides, graphene etc., which are impermeable to molecular species [45], [46], [58], [112].
The nature of the fillers and their degree of compatibility govern the permeation
characteristics. If the filler is compatible with the polymer, it can easily fill the voids present
inside the polymer, thus resulting in a tortuous path that makes the diffusion path for
permeants longer, which in turn improves the permeation resistance [113]. According to
Eq. 16, the degree of tortuosity depends on the aspect ratio and orientation of the filler
particles as well as on the volume fraction of the filler [45], [65], [111]. If the filler is
incompatible with the polymeric system, instead of filling up the free volume sites, the filler
will agglomerate and result in the formation of voids, which increases the free volume sites
and reduces the permeation resistance [114], [115]. Several material systems have been
developed which will be described in the following.
3.2.1 Clay based barriers
The most widely used class of inorganic fillers in polymers for the production of packaging
is clay. [70], [94] This frequent use of clay as the filler is favored by its availability in
abundance, easy processing, non-toxic nature and relatively high aspect ratios. The clays
belong to a 2:1 phyllosilicates family,[70] which have sheet like structures.
Montmorillonite (MMT) [93], hectorite, and vermiculite are some of the examples of the
family and most commonly used fillers in polymer clay composites [70], [92], [95]. The
clay being hydrophilic in nature usually requires water as a dispersing agent. The
performance of MMT clay as a filler in polymer matrix usually depends on its aspect ratio
STATE OF THE ART
36
along with its surface compatibility [96]. In order to increase the compatibility of the clay
with the polymer matrix, quaternary ammonium salts can be used as surfactants, which
develop strong interactions between clay and the polymer chains of the matrix. This
interaction plays a vital role in improving barrier characteristics [97], [98]. In general, clays
exhibit three types of morphologies in the composites which are based on the degree of the
dispersion of the clay, these are: aggregated, intercalated and exfoliated [99]. When clay is
in the aggregated structure its tactoids are nicely dispersed in the matrix, but the single
platelets within the tactoids are not delaminated and still remain at their original positions
as shown in Figure 3.2.
Figure 3.2 Schematic representation of clay morphology when mixed with polymers.
(Reproduced from [116] with permission from Elsevier)
In case of the intercalated structure, the clay platelets to some extent are delaminated from
the tactoids, and thus these platelets can diffuse and distribute through the polymer chains.
In the exfoliated condition, the clay platelets are completely delaminated from the tactoids
and single layered platelets disperse homogeneously within the matrix. The most desired
state is the completely exfoliated structure because it has the capability to offer excellent
barrier characteristics and also imparts maximum resistance against mechanical and
STATE OF THE ART
37
thermal deteriorations at relatively very low clay contents [117]. However, complete
exfoliation state is very hard to achieve and hence polymer nanocomposites have clays in
states either intercalated or semi exfoliated [45], [118]. Using MMT clay, Gaume et al.,
[119] produced a barrier film comprising of PVA and clay. OSCs based on P3HT:PCBM
photoactive layers were laminated between the barrier films, and their lifetimes were
compared to those obtained for OSCs packaged in PET (a common polymer used as a
substrate) and PVA coated PET.
Figure 3.3: Lifetime of organic solar cells tested under irradiation with a solar simulator
(AM 1.5G, 30 oC, ambient RH 30-40%): normalized power conversion efficiency (PCE)
of OSCs encapsulated with PET film, PVA coated PET film, and PET coated with PVA-
MMt 5 wt% nanocomposite (Copied from [119] with permission from Elsevier).
Encapsulated solar cells were irradiated with a solar simulator. The use of clay-based
barriers seems to result in slightly improved lifetime as compared to PET. Laminated OSCs
lost 20% of the initial power conversion efficiency (PCE) within 20 hours of testing time
and 80% of its initial performance in around 70 h (as shown in Figure 3.3).
This improved lifetime of the OSC was due to improved barrier performance of the
PVA/clay composite and exhibited an improvement factor for OTR and WVTR of 6.9 and
2.6, respectively as compared with pristine PVA. This is not an ideal lifetime performance,
but clearly shows the potential of the clay based barriers.
STATE OF THE ART
38
In one of the studies on clay, Yano et al., fabricated a comoposite of clay and polyimide
with four different clays which included synthetic mica, montmorillonite, hectorite and
saponite [120]. It was noticed that the improvements in permeation characteristics rely on
the size (aspect ratio) of the platelets. Mica showed the most impressive effect and
enhanced barrier properties by the factor of ~10 with clay concentration of just 2 wt% in
nanocomposites. In other studies carrier out by Messersmith and Giannelis [121] reported
79% reduction in moisture permeation with 4.8 vol% of mica /poly (Ɛ- caprolactone) (PCL)
nanocomposites. In another study, Zhou et al. in 2016, studied the permeability of
poly(butelene succinate) (PBS)/clay nanocomposites [122]. They reported that if the
content of clay exceeds a certain amount, it exhibits a greater aspect ratio and more regular
dispersion in PBS matrix which eventually results in a large decrease in gas permeation
properties. Recently, modified synthetic clay (smectites) is reported to have ultra high
barrier characteristics [123]. The modification of the clay was performed by osmotic
swelling method, which yielded ultra-high aspect ratio platelets of around ~20,000. Such
clay platelets were used with PVA matrix [123], [124] and resulting composite barrier
exhibited the OTR and WVTR of 0.11 cm3 m−2 day−1 bar−1 (23oC/ 90%RH) and 0.18 g m−2
day−1 (23oC/ 90%RH), respectively, for a coating of 0.42 μm. In another study regarding
such a clay, a thin layer polymer/clay composite having thickness of 21.4 μm showed
extremely high barrier characteristics i.e. OTR < 0.0005 cm3 m−2 day−1 bar−1 and WVTR
of 0.0007 g m−2 day−1 at testing conditions of 23 °C and 50% RH. Even in the most
challenging environments (38 °C and 90% RH), values as low as 0.24 cm3 m−2 day−1 bar−1
and 0.003 g m−2 day−1 were found for OTR and WVTR, respectively [110], [124]. These
recent developments in clay modification generating very high aspect ratio of over 20000
and producing extremely high gas barrier from solutions make clay a wonder material and
ideally suited for encapsulation of organic electronic devices.
3.2.2 Graphene based barriers
Graphene is a single layered sp2 hybridised atom of carbon arranged in a two dimensional
lattice [45], [125]. It is usually fabricated by the exfoliation of graphite nano-sheets [126].
The theoretical specific surface area of graphene sheet is 2630-2956 m2/g with a large
aspect ratio exceeding = 2000 [125], [127]. One of the most beneficial use of graphene
is its application in producing polymer nanocomposite where graphene is used as a filling
agent. However, this application has some limitations like solubility and uniform dispersion
STATE OF THE ART
39
in polymers [45], [127]. This is because of graphene has insignificant solubility in most of
the conventional solvents [116], [128]. Additionally, existence of wan der Walls interaction
between large surface area of graphene platelets causes substantial aggregations when
incorporated in polymeric matrix [129]. Tseng et al., [130] fabricated polyimide/graphene
oxide (GO) nanocomposite with various GO loading. It was observed that the WVTR is
reduced by 83% with the addition of only 0.001 wt%. It was reported that the increase in
filler content from 0.001 wt% to 0.01 wt% linearly reduced WVTR. This result is in contrast
to polyimide/montmorillonite nanocomposites, in which an amount of around 8 wt.% was
needed to reduce WVTR to same extent [120] . In Figure 3.4, the effects of contents of
graphene and nanoclay on water vapour transmission rate of polyimide nanocomposite are
shown. Nanocomposite of polyimide containing only 0.5 wt.% of graphene reduced water
vapour transmission rate by 88% and nanocomposite with clay of the same content reduced
WVTR by 63% as competed to pristine polyimide. The different of barrier performance of
the nanocomposite can be attributed to significantly higher aspect ratio of graphene offering
large resistance to permeating molecules as compared to clay [131].
Figure 3.4: WVTR (g/m2.h) of composites of polyaniline / graphene and polyaniline / clay
as function of graphene loading. (Copied from [45] with permission from Elsevier).
Chen et al. in 2014 prepared a gas barrier nanocomposite comprising of poly(vinyl alcohol)
(PVA) and graphene oxide and reported reduced gas permeabilities. The reported OTR was
<0.005 cm3/m2.day.bar at graphene oxide loadings of only 0.07 vol%. Isothermal
recrystallization of the composite introduced PVA crystals, which acted as barriers along
STATE OF THE ART
40
with graphene oxide sheets. The resulting barrier properties of the composite were 1-8
orders of magnitude better compared with polymer/inorganic composite coatings [132].
The O2 relative permeability of the film showed almost the exact fitting to the Cussler
model Figure 3.5. At lower GO contents in PVA (referred as bled of PVA and GO), the
experimental data are in close accordance with the Cussler curve with ɑ =500, whereas the
data approach the curve with ɑ= 2000 at higher concentrations. The deviation of the data
from Cussler’s model for ɑ=2000 is due to GO agglomeration because of higher loading
concentrations which reduces the surface area and effective aspect ratio of graphene oxide
(GO) platelets. In contrast to blend of PVA/graphene oxide, hybrid PVA/graphene oxide
(0.07 vol% of GO and PVA crystallization), effectively reduced oxygen permeability. This
reduction in relative permeability fits the curve for ɑ=5000 [132].
Figure 3.5: Relative oxygen permeability for PVA, mixture of PVA/GO coating and hybrid
PVA/GO for 0.07 vol% layer in comparison to predictive permeation curves proposed by
three models ( i.e. Nielsen, modified Nielsen and Cussler) for different aspect ratios (𝛼)
(Copied from [132] with permission from Elsevier,
Kim et al. (2014) developed solution processed barriers for encapsulating OSC using
opaque layers of reduced graphene oxide (rGO) [133]. Inverted OSC devices based on poly
[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-
benzothiadiazole)]:[6,6]-phenyl-C71 butyric acid methyl ester ( PCDTBT:PC70BM) active
layers were encapsulated by two methods, i.e. direct coating rGO (without polymer matrix)
STATE OF THE ART
41
on device by spin coating and lamination of barrier (rGO without polymer matrix coated
on PEN substrate) using an adhesive. By coating devices directly, the lifetime was
improved by a factor of 50, while for lamination, devices maintained 43% efficiency even
after exposure to 100% humidity (at room temperature) for 240 h [133]. These observations
suggest that graphene oxide based nanocomposites have a great potential for developing
coated barriers but still are not suitable for encapsulation OSC because of lacking of
transparency.
Figure 3.6 gives summarized permeation reduction (%) of polymeric composites (Pc) as
compared to pristine polymer (Pp) which is calculated as a function of various filler types
(Eq. 52) against transparency:
Permeation reduction (%) = 𝑃𝑝−𝑃𝑐
𝑃𝑝 x 100
Eq. 52
It is observed that the graphene based barriers have the potential to be used a quality barrier
and can also maintain high transparency. Clay as discussed before is the best available filler
that not only can fulfil barrier requirements but can also exhibit high transparency. Boron
nitride, Molybdenum(IV)sulphide (MoS2) and glass flakes have also been reported in the
literate as a potential barrier fillers but no significant barrier improvements are reported
[115], [134], [135],
30405060708090100
10
20
30
40
50
60
70
80
90
100
Tra
nsp
are
ncy (
%)
(GO + recrystallization, 0.07 vol%)
(Expanded graphite, 4 wt%)
Boron Nitride, 0.01 vol%
(RGO, 4 wt%)
(Cloisite B30, 5 wt%)
(Cloisite Na+, 5 wt%)
(GO, 1 wt%)
(Al2O
3 grafted GO
(Synthetic clay (Smectic)
Reduction in permeation (%)
Carbon based
Clay based
Boron Nitride
Figure 3.6: Transparency vs reduction in permeation for different filler types and loadings
in polymer matrices.
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3.2.3 Getter materials
Getters are usually termed as scavengers. Such materials are used to attract and capture the
unwanted substances. Getters are used to capture substances like solid, liquid and gases,
especially oxygen, hydrogen and moisture [136]. The incorporation of getters can be highly
beneficial for the field of packaging and can help keeping the product safe from unwanted
contaminations [137]. The getters are generally used as fillers within a permeable
polymeric material, however in this case the fillers will not generate a tortuous path but
will capture the unwanted species like moisture and prevent their diffusion to the other side
of the membrane, hence decreasing the permeation rate.
Water getters are usually inorganic compounds that form hydrates when exposed to
moisture. Zeolites and other minerals are most commonly employed getter materials [12],
[80]. The getters are uniformly distributed in a polymeric matrix and films from such
materials can be used as effective moisture barriers. Getters may require thermal activation,
as they absorb water during storage or transportation. This thermal activation will dehydrate
the getters and bring back their full capacity to absorb water. Wu et al (2010) used beta
type of zeolite nanoparticles in the packaging of OLED to keep the environment dry [12].
Beta zeolite nanoparticles were synthesized from a clear solution of tetraethyl orthosilicate
(TEOS), aluminum iso-propoxide (AIP), tetraethylammonium hydroxide (TEAOH), and
water at a molar ratio of 1 TEOS : 0.04 AIP : 0.36 TEAOH : 25 H2O. A semi-transparent
solution was prepared which was diluted and centrifuged at high rotational speeds (23000
rpm). With this process nanoparticles of zeolite having crystalline nature were separated
from amorphous part. The nano-zelolite particles were mixed with acrylic resins and a
nanocomposite was thus prepared. The coated layer exhibited a good transparency of
around 85% in the visible region. The water absorption of acrylic/zeolite films was
analyzed and the performance was compared with films of acrylic without nanoparticles. It
was observed that layers with ~10 wt% zeolite nanoparticles show improved water
adoption capacity as compared to films of pristine acrylic. It was reported that the specific
capacity of the embedded zeolite was slightly lower than that of silane modified zeolite
powder preheated at 200 oC. Thus, the acrylic matrix reduces the moisture adsorption
capacity of the zeolite only to a small extent. Based on these results, the authors claim that
a WVTR of about 10-3 g/m2.day can be achieved with a film thickness of 130 µm having
40 wt% loading of zeolite without compromising the transparency [12].
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3.3 Barriers based on impermeable coatings
Barriers based on impermeable coatings deposited on polymeric substrates rely on the
reduction of unprotected polymer surface according to Eq. 2.14. These coatings generally
consist of metal or semimetal oxides, namely Al2O3 and SiO2. Especially silica coatings
with a small coefficient of diffusion have attained much importance in many fields,
including corrosion [138]–[140], erosion [141], [142], wear [143], electrical insulation
[144], anti-oxidation [140], packaging [145]–[147] etc. These types of thin coatings are
usually prepared by vacuum assisted methods such as chemical vapor deposition [148],
[149], ion beam assisted deposition [150], or flame hydrolysis deposition [151]. In order to
enhance barrier quality, multilayer stacks are formed, decoupling defect growth by
depositing organic polymer layers between the metal oxide layers. In this way, barriers with
WVTR values below 10-5 gm-2day-1 have been created and such barrier coatings are referred
as Barix. Ultrathin multilayers of metal oxides (SiO2, SiNx, Al2O3) are deposited on plastic
substrate as the main barrier layers. Polymers are used as alternating interlayers between
metal oxides layers [13], [152]–[155]. The polymeric interlayers not only serve as
planarization layer allowing defect free growth of metal oxide on them but also enhance
overall flexibility [13]. Many efforts have been made to create coatings having barrier
effects comparable to Barix. Methods like sol-gel [140], [142], liquid phase deposition
[156], and electrophoretic deposition [157], [158] have been reported. The relatively low
density of the films obtained through these coating techniques causes high permeability
due to the availability of free volume within the film. In order to obtain dense silica
coatings, sol gel reactions are followed by thermal treatments at several hundred degrees
to remove the organic parts [159], [160]. Therefore, due to the boundary condition of
temperature, it is very hard to prepare dense silica coatings on polymeric substrates
applicable to packaging materials [161]. One of the solutions to this matter of the concern
is the use of liquid precursor avoiding high temperature to produce dense silica coatings.
3.3.1 Polysilazane
Recently, polysilazanes have been reported in the literature as an alternative route for the
production of dense, homogeneous, and defect free silica films [162].
Perhydropolysilazanes (PHPS) are inorganic polymers that consist of silicon and nitrogen
atoms in their backbone (-SiH2-NH-) and have been used extensively to produce silica
coatings [163]. With specified conditions PHPS yields a dense and homogenous SiO2
STATE OF THE ART
44
structure, which is the main reason for their good barrier properties [164]. This makes
PHPS unique materials that can be used in a variety of applications, especially in the
semiconductor industry, OLED displays, and packaging [143], [158], [165]. One of the
advantages of PHPS based coatings is their lower susceptibility to crack formation and
shrinkage. This is because its molecular weight rises during the conversion of PHPS to
silica, owing to its reaction with air and moisture [21], [164], [166], which results in volume
expansion. During the curing process of sol-gel layers, in contrast, alcohol or water is
released, resulting in a lower molecular weight and consequent shrinkage [167].
The concept of polysilazanes was first introduced by Krüger and Rochow in 1964 [168].
They formed polysilazane by a reaction of chlorosilanes with ammonia which generated
tetrameric cyclosilazanes. This product was further treated at high temperature with a
catalyst and formed a polysilazane with high molecular weight. Complete cured PHPS
remains optically clear and transparent and yield a very smooth surface as can be seen in
Figure 3.7 [169].
Figure 3.7: Transmission and appearance of cured PHPS films, a) showing the transparent
appearance, b) bendable transparent cured PHPS coating and c) transmission spectra of
PET film and different types of SiO2 coatings.(Copied from [169] licensed by CC BY 4.0)
There are different methods used for complete curing of Polysilazane to yield silica. These
methods can be summarized as:
STATE OF THE ART
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a) Thermal curing
b) Curing in the presence of catalyst
c) Deep UV curing
d) Combination of the methods.
3.3.1.1 Thermal curing
The most common method to obtain a dense and stable SiOx network from PHPS is the
hydrolysis of Si-NH bonds and the subsequent formation of Si-O bonds at high
temperatures [170]–[174]. At low temperatures this process proceeds slowly. Complete
conversion to SiO2 requires treatment at temperatures of ~1000oC for several hours [175].
Matsuo et al. were the first to report that PHPS can be converted to silica thin films with a
density of 2.1 g cm-3 and a refractive index of 1.45, similar to silica glass, at relatively lower
temperatures (300 oC – 350 oC) by adding catalysts that promote oxidation or hydrolysis
[172]. Bauer et al., worked on the curing kinetics of PHPS at low temperature and
concluded that moisture and temperature have significant effects on the PHPS curing
mechanism [174]. At relatively low temperature or nearly ambient conditions the curing of
PHPS happens in two steps as shown in scheme 1. At first hydrolysis takes place and
silanols are generated by eliminating hydrogen and ammonia. . In the next step silanols go
through a polycondensation process, resulting in silica and water. It should also be noted
that the crosslinking of the silanols starts before all the Si-N bonds are hydrolyzed [173].
Scheme 1. Perhydropolysilazane (PHPS) curing mechanism at low temperature in the
presence of moisture:. a) Hydrolysis, b) polycondensation [173].
Zhang et al. 2015 conducted a work to explain the conversion of PHPS to silica via
thermogravimetric analysis (TGA) [166], [176]. It was concluded that PHPS exhibits a
minor loss of weight of 1.2% between 70 oC and 180 oC. This loss in weight is due to
evaporation of entrapped solvent and to some extent to the deterioration of chemical
compounds including N-H and Si-H bonds. From temperatures of around 200 oC and above
STATE OF THE ART
46
PHPS weight gain reaches 105.4% at 510 oC, followed by loss in weight. At the end the
weight gain reaches 103.8% at 1000oC. The weight gain below 510 oC can be assigned to
the oxidization of Si-H bonds as well as to the replacement of residual N atoms with O
atoms. Loss of weight above 510 oC is mostly due to the condensation reaction of the Si-
OH groups. This suggests that the volume variation of PHPS is negligible while heating
which results in a defect free and highly compact coating [166].
3.3.1.2 Curing in the presence of catalyst
For various applications, the use of high temperature is not always recommended for curing
PHPS. Therefore, different alternative methods for curing PHPS have been reported. These
include; exposure to ammonia atmosphere, reaction with water by catalytic action of amine
in the baking step, and exposure to hydrogen peroxide vapor.
a) Exposure to ammonia
Some authors have reported that the curing of PHPS can be promoted by exposing it to
gaseous ammonia. Kubo et al., [139] reported that the exposure of PHPS to ammonia gas
for 6 hours resulted in fully converted silica which resembles the amorphous silica
structure. Morlier et al., [173] reported that the curing of PHPS can be carried out by
exposing PHPS to either aqueous or gaseous ammonia. Both, aqueous and gaseous
ammonia have significant influence on the transformation of PHPS in terms of conversion
rate. However, immersion in ammonia solution seems to have a dominant effect and leads
to faster conversion. Immersing PHPS in water alone is not a sufficient condition to
significantly accelerate conversion. Exposing the PHPS coating to ammonia vapors for
elongated time duration, i.e. 24 h, does not promote total cross linking of silanols. Ammonia
catalyst makes the conversion rate faster but still leaves an incomplete conversion because
it promotes exclusively the first reaction step, i.e. hydrolysis. Morlier et al., 2012 [173]
assumed that the ammonia acts either as nucleophilic agent or as a base, in the latter case,
the catalyst effect could be attributed to the pH of the solution.SiOH polycondensation is
only promoted by elevated temperatures; therefore, post curing of 30 min at 150 oC is
preferred.
b) Exposure to peroxide catalyst
A further method for curing PHPS is its exposure to aqueous hydrogen peroxide [139].
Hydrogen peroxide acts as a catalyst and strongly accelerates SiH and SiN bond scission
[139] [173]. A study performed by Morlier et al., shows that the conversion of SiN and
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SiH is achieved 10 minutes after immersion in a hydrogen peroxide solution. However, the
layers obtained via this method showed highly hydroxylated silica. While curing with
peroxide catalyst, two competitive reactions are assumed to occur in the presence of HO.
radicals and water: SiN and SiH bonds can either be gradually hydrolyzed by moisture or
rather be quickly dissociated with the formation of radicals by HO. species, as can be
observed from Figure 15(c). The radicals formed as result of this bond scission can
recombine with other hydroxyl radicals and lead to the formation of silanol species [165].
a)
b)
c)
Figure 3.8: FTIR spectra of uncured PHPS and PHPS cured with different methods.
a) IR spectra of uncured PHPS (solid line) and IR spectra of PHPS curing at 180 °C
under moisturized atmosphere for 300 min (dashed line), b) IR spectra of uncured PHPS
(solid line), PHPS cured after exposure to ammonia vapor for 60 minutes (dashed line),
c) IR spectra of uncured PHPS (solid line) and PHPS cured by submerging into 20%
aqueous hydrogen peroxide solution for 10 minutes. (Copied from [165] with
permission from Elsevier)
3.3.1.3 Deep UV curing
Prager et al., [164], reported for the first time a water free vacuum ultra violet triggered
process of converting PHPS into SiOx in the presence of O2. This UV curing process was
a break through because by the use of this method conversion of silazanes was possible
without using harsh conditions like pyrolysis or hydrolysis [164]. This opened a vast range
of applications for polysilazanes, including OSC encapsulation [165]. Since then, this
method has been adopted by various investigators [19], [155], [175], [177]. Xenon excimer
lamps are generally used as high power vacuum ultraviolet light sources that can efficiently
emit radiation with a wavelength of 172 nm [164], [174][164], [174]. Exposure of oxygen
to deep UV light generates excited atomic oxygen (O(1D)) and ozone (O3) [164]. Since
oxygen has a high absorption coefficient with respect to VUV light with a wavelength of
STATE OF THE ART
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172 nm, VUV irradiation can generate reactive oxygen species very efficiently [178], [179].
Prager et al. in his study reported deep UV irradiation from various sources at wavelengths
ranging between 172 and 222 nm. The study was carried out under controlled atmosphere
because oxygen absorbs such high energy photos to form free radicals and ozone [165],
[178]. Free radicals of oxygen (generated as a result of interaction with deep UV light) react
with PHPS and conversion to SiO2 starts from top surface and progresses inwards [19].
Additionally, water generates hydroxyl radicals upon absorbing VUV light. Due to the
extremely highly oxidizing nature of atomic oxygen and hydroxyl radicals, they react with
polysilazane, thus forming silica [164], [174], [180].
−(SiH2 − NH)𝑛 − + 𝑛𝑥O. → 𝑛SiO𝑥 + 𝑛NH3 Eq. 53
The conversion of PHPS to silica under VUV light may in principle proceed via two
different mechanisms. One mechanism involves the dissociation of bonds in PHPS, which
is directly caused by VUV irradiation, while the other process involves oxidative reaction
involving O(1D) and O3. Nagnuma et al., [180] reported that oxidation reactions were
dominant as compared with dissociation of bonds by VUV irradiation. Prager et al., [164]
and Kobayashi et al., [161] strongly suggested that the dissociation of bonds is much faster
than the oxidation reaction. Therefore, VUV light should effectively reach the coating in
order to efficiently convert PHPS to silica. The following Eq. 54 has been used by
Kobayashi (2013) to study the effect of irradiation dose on the conversion mechanism of
PHPS to silica.
𝐼
𝐼0= 𝑒𝑥𝑝 (−Ɛ ×
𝐶𝑂2
100 ×
𝑑
10)
Eq. 54
Where 𝑐𝑂2 , d, and I/I0 are the oxygen concentration of the atmosphere (%), irradiation
distance, and VUV light transmittance, respectively. The absorption coefficient of O2 for
172 nm VUV light is = 15 cm-1. This equation clearly demonstrates that the transmittance
of VUV light increases as 𝐶𝑂2 and d become smaller.
3.3.1.4 Combined methods
In order to convert PHPS not only at milder conditions but also at faster pace, various
studies have been carried out. One study used dipping of PHPS into water or aqueous H2O2
and simultaneous irradiation with light of 405 nm wavelength and observed that PHPS
cures relatively fast and at lower temperature [174]. Similar studies also used dipping of
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PHPS into NH4OH and observed conversion of PHPS into dense silica [138], [158]. Jung
et al., in 2010 used dipping in combination with heating the bath to 80 oC for ~60 minutes
and observed a faster way of conversion, which yielded PHPS of better quality in terms of
density and hydrophobicity [172]. It was observed that the conversion rate of PHPS was
still low. In order to further reduce the curing time, Ohisi et al., [169] reported the heating
of the PHPS coating at simultaneous irradation with deep UV, thus successfully reducing
the curing time down to ~20 minutes. In this study, it was reported that emission at 172 nm
has a photon energy of 166 kcal/mol and the polysilazane bonds like Si-N, Si-H, and N-H
have bond energies 105, 71 and 92 kcal/mol, respectively. The energy of the deep UV light
breaks the bonds and SiO2 can be formed by singlet oxygen (O(1D)) reaction with PHPS.
Introducing slightly higher temperature during vaccum UV exposure accelerating the
diffusion of oxygen radicals into the coated film, which promotes the reaction of singlet
oxygen with PHPS and enhances the conversion rate. In order to further investigate this
phenomenon, a PHPS film was treated in different ways which included heating it with
different temperatures like 80 oC, 100 oC, 200 oC and 300 oC. Simultaneously, other PHPS
films were treated with deep UV irradiation and heated at different temperatures. It was
observed via FTIR analysis that increasing temperature during UV irradiation, enhances
curing rate of polysilazane. Uncured polysilazane shows characteristic peaks at 830 cm-1,
2200 cm-1, and 3400 cm-1 which refer to Si-N, Si-H and N-H, respectively Figure 3.8. PHPS
cured with vacuum UV irradiation and simultaneous heating shows rapid decrease in the
peak intensities of Si-N, Si-H and N-H and formation of new peaks of Si-O-Si at 450 cm-1
and 1050 cm-1. This gave evidence that the combination of heat and irradiation is the fastest
way of curing PHPS. In comparison to other curing methods like pyrolysis, which not only
depends on the film thickness but also on the range of temperature and may still have
incomplete transformation. In contrast, the combination of temperature and VUV
irradiation is the fastest and have high chances of complete transformation of PHPS into
SiO2[169].
3.3.1.5 Barrier performance
Various coating techniques, curing methods, and strategies have been reported in the
literature to improve the barrier characteristics of Polysilazane derived coatings. It is
reported that the barrier performance of Polysilazane against moisture and oxygen mainly
depends on the completeness of the curing process of the PHPS coating, thickness of the
layer, number of defects present in the layer and number of coating layers.
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Prager et al. reported an oxygen barrier improvement factor of 400 with a single 150 nm
deposit of PHPS on a PET substrate.
Ohishi 2003 et al., [181] developed a thin PHPS layer by curing it at 140 oC for 20 min in
90 % RH atmosphere on PET already coated with ITO and reduced its moisture and oxygen
barrier properties down to 0.15 g/m2.day and 0.02 cm3/m2.day.bar, respectively. In another
study in 2014, PHPS layers were cured with low pressure mercury lamps (HgLP) at a
radiation of 185 nm wavelength under nitrogen having low concentration of O2 [165]. It
was reported that the complete transformation of PHPS to silica depends on the thickness
of the film. As the films gets thicker, the curing with deep UV becomes difficult. The curing
starts from the surface and proceeds into the depth of the film. As soon as the surface is
cured, it becomes harder for the oxygen radicals to diffuse and hence leave uncured PHPS
underneath. This uncured PHPS is a poor barrier and offers channels of diffusion and thus
results in poorer barrier properties. Thick PHPS layers are not only difficult to cure but also
exhibit brittle nature and are highly susceptible to cracking which leads to deteriorate the
barrier properties [182].
Ohishi et al., 2017 [169] reported that photo-heat treated films at 150oC exhibit WVTR of
<0.02 g.m-2.day-1. Kobayashi et al., 2013 [161] studied silica coatings fabricated by VUV
irradiation of PHPS films under various conditions. It was reported that silica coatings
prepared with lower oxygen concentration and shorter irradiation distance show the lowest
WVTR values (Table 4), indicating that the direct dissociation of bonds by the VUV light
plays a key role in the formation of dense silica coating exhibiting high gas barrier
properties with respect to water vapor.
Table 4: Irradiation condition and WVTR of each sample
Samples Oxygen
concentration %
Irradiation
distance (mm)
WVTR
g. m-2.day-1
1 <5 2.6 0.721
2 19 2.6 1.88
3 <5 13.4 1.91
4 19 13.4 2.74
In order to develop superior gas barrier coatings, it is very important to reduce the size and
number of defects on and beneath the surface. It was reported that the silica coatings
prepared from PHPS via hydrolysis by water possessed a relatively small number of SiOH
groups, which might ultimately serve as defects. Since curing with VUV does not involve
water, the number of SiOH in VUV cured coatings can be minimum and, thus, it can be
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expected that the siloxane network would be fully developed with dense and homogenous
silica [164], [183]. Therefore, it is suggested that the silica coating obtained via VUV
irradiation in combination with temperature is advantageous in fast curing along with layer
quality as this method promotes defect free growth and eliminates the chances of both
macro and molecular size defects. Table 5 below gives barrier quality of PHPS coated on
both sides of polyimide substrate cured with temperature and combination of simultaneous
VUV irradiation at different temperatures. The barrier quality obtained with combined
method (VUV+temperature) is superior than curing at higher temperatures without VUV
irradiation.
Preparing multilayers of PHPS is another effective method to enhance barrier performance.
Barrier characteristics against oxygen <0.1 cm3.m2.day.bar and <0.02 g.m-2.day-1 for double
layers have been reported [169]. Ohishi et al. 2017 prepared a stack of two PHPS layers
and subsequently cured each layer by deep UV irradiation. The value of water vapor
permeability for a two layered stack (Figure 3.9) at 25oC calculated from the Arrhenius plot
showed an extremely low value of 4.9 x 10-4 g/m2.day and the activation energy was 236
kJ/mol.
Table 5: WVTR of PHPS coated on polyimide substrate on both sides via spin coating and
cured via VUV irradiation at different temperatures for 20 minutes (data extracted from
[162]).
Treatment type Thickness
(nm)
WVTR
(g/m2.day)
Polyimide (substrate) without PHPS ~300 143
heat treatment at 150oC ~300 92.8
Heat treatment at 200 oC ~300 40.9
Heat treatment at 250 oC ~300 13.8
Heat treatment at 300 oC ~300 1.36
VUV+ Heat treatment at 80 oC ~300 15.3
VUV + Heat treatment at 100 oC ~300 3.42
VUV+ Heat treatment at 120 oC ~300 1.84
VUV + Heat treatment at 150 oC ~300 0.17
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On one hand, increasing the number of layers may enhance barrier properties but, on the
other hand induce brittleness and chances of defects within the film [184]. Adding an
organic layer between SiO2 layers seems to be advantageous. It does not only enhance the
flexibility of the coating but also decouples the fracture growth and surface defects [19],
[21], [184]. Such barrier materials may fulfill OPV requirements as shown by Burrows et
al., [13]. However, Graff et al., [19], [185] studied multilayer structures and stated that
molecules need to permeate through the defects of inorganic layer and diffuse horizontally
in the organic layers until they encounter another defect in the next inorganic layer, hence
generating a tortuous path and decreasing permeability. Permeation below 10-4 g/m2.day
cannot be obtained with such multilayers stacks as the presence of nano-sized defects is
unavoidable [13], [185]. Morlier et al., [182] achieved moisture permeation rates of 2x10-2
g/m2.day for a multilayer structure.
Figure 3.9: Temperature dependence of water vapor transmission rates of the polysilazane
derived SiO2 coatings (2 coates) (Copied from [169] licensed by CC BY 4.0).
Morlier et al., for the protection of organic solar cells, modified the structure of the barrier
and produced stack of 5 barrier layers on PET, consisting of one PVA layer sandwiched
between two PHPS layers on each side (PET/PHPS/PHPS/PVA/PHPS/PHPS) and
laminated OSC (P3HT:PCBM) device with this barrier. The performance of this
encapsulation film was compared with bare PET and a commercial barrier (Figure 3.10). It
was observed that devices encapsulated with PET degraded faster because PET is a
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53
relatively poor barrier against moisture and oxygen. In contrast, the devices encapsulated
with PHPS or the commercial barrier remained stable. The loss of ~30% in PCE is observed
for devices encapsulated with the commercial barrier and PHPS based barrier coated on
both sides of PET, whereas devices encapsulated with PHPS based barrier having a 5-layer
structure remained stable and only a minor degradation was observed over a period of ~400
h. This degradation is exclusively due to the side ingression of oxygen through the
adhesive. This suggests that PHPS has a potential to be used as an encapsulation for the
OSCs [182].
Figure 3.10:Performance of organic solar cells in terms of normalized power conversion
efficiency (PCE) and normalized short circuit current (Jsc) during the exposure to AM 1.5,
10 0 0 W m –2 light soaking, encapsulated with (a) PET having thickness of 50 μ m, (b) a
commercial barrier, (c) PHPS based barrier having one PHPS (250 nm) coat on both sides
of PET, (d) PHPS based barrier, having 5 layer structure (PET/PHPS250 nm/PHPS250
nm/PVA1 μ m /PHPS250 nm/PHPS250 nm) on one side of PET and (e) un-encapsulated OSC
device degraded under irradiation in glovebox. (Copied from [182] with permission from
Elsevier).
3.3.2 ORMOCERS
ORMOCER system (organically modified ceramics) are hybrid type composite materials
and mainly consist of three parts i.e. organic, inorganic and polysiloxane [186]. All three
STATE OF THE ART
54
parts have different roles to play to create certain functionality of an ORMOCER. The
organic part is responsible for cross linking, polarity and optical behavior. Inorganic part
withstands against thermal, mechanical and chemical deteriorations and elasticity is
generated by the polysiloxane parts [108], [186]. Variety of properties can be obtained by
tailoring ORMOCER structure [187], [188]. Unlike conventional composites,
ORMOCERS are processed from alkoxysilanes and hence have inorganic center having
silicon and oxygen [189], [190]. In order to not to destroy the organics, ORMOCER
structures are usually processed at low temperatures. That means the potential of molecular
chemistry to tailor structures can be used advantageously for ORMOCER processing.
Generally the inorganic backbone is synthesized by the sol-gel process and the organic part
(-R) having reasonable molar mass are added by crosslinking polymerization [188]–[191].
The type functional group “-R” of course determines the material properties to a great deal,
different groups as alkyl or unsubstituted aryl (e.g. –C4H2n+1, – C6H5) functional groups
(e.g. –NH2, -COOH, other chelate ligands, -CH, -SH, -CN) along with polymerizing groups
(e.g. epoxy, methacryl, vinyl and other olefins) can be used as organic part in ORMOCERS.
Therefore, a wide range of materials can be developed according to the need of specific
applications.
Table 6) [30], [159]. E.g., the ligands can impart hydrophilic or hydrophobic properties to
the matrix or crosslink it, which changes the permeation properties of small molecules [16],
[192], [193] [190],[194].
Table 6: Bifunctional silanes R’ (CH2)nSi(OR)3, few functional organic groups R’ for
producing an organic network and functionalization of the matrix. Data extracted from
[192].
n R’ effect
3
Network former
Density
Elasticity
Rigidity
Thermally or
Photochemically curable
0
3
3
3
STATE OF THE ART
55
3
2 -CH3 Network modifier
Density
Hydrophobic
Hydrophilic
Oleo-phobic
Better adhesion
3 -SH
3 -NH2 + -NR3
3 -(CF2)5CF3
As the barrier properties of ORMOCER layers are almost as same as for commercial PET
i.e. around 4 g.m-2.day-1 [195], which is not sufficient for packaging opto-electronic
devices. Thus Fraunhofer ISC developed a new class of barrier called POLO using
ORMOCER as interlayers in combination with vapor deposited SiOx [196]. POLO barrier
has the same structure as that of Barix i.e. several metal oxide layers having organic
interlayers as shown in Figure 3.11. In POLO, ORMOCERS are used as interlayers to
smoothen the surface to allow homogenous growth of evaporated layer. Moisture
permeation values as low as 10-4 g.m-2.day-1 have been reported [39], [197].
Figure 3.11: Schematic diagram for roll-to-roll production of ORMOCER/inorganic oxide
hybrid barrier films. (Re drawn from [167]).
Miesbauer et al., [170] conducted a study on the role of substrate on the performance of
Fraunhofer’s developed POLO barrier and concluded that PET may not be suitable as
encapsulation material for long term outdoor applications, since polyester films like PET
are degraded by UV radiation and humidity in combination with high temperature.
Therefore, they suggested the use of fluoropolymers, e.g. ethylene tetrafluoroethylene
(ETFE) [2] [171] that show excellent resistance against UV radiation and also against
STATE OF THE ART
56
weathering and, therefore, qualify well as flexible substrates instead of PET for
encapsulation of photovoltaic devices [170].
The use of ORMOCER lacquer is not limited to create planarization layers but can also be
used as an adhesive and sealant [198] [192]. A combination of smoothening and sealing
effect of ORMOCER can be a suitable choice for cost effective short term encapsulation of
solar cells avoiding classical lamination processes
3.4 Reducing solubility
One way for enhancing barrier properties of the coatings is to incorporate hydrophobicity.
Hydrophobicity may not exactly be an intrinsic property of a polymer but is more of its
surface property. One study concluded that although hydrophobic surfaces are water-
repelling, they do not repel water vapor [199]. Condensation of water vapor will take place,
thus resulting in higher permeation followed by internal wetting. For example
Polydimethylsiloxane (PDMS) is a hydrophobic material but still has high moisture
permeability [199], [200]. This effect is not suitable for OPV’s. However, roughening of
the polymer substrate by lithography followed by coating with an encapsulant will help
improve its barrier values. Hence, efficient methods and techniques must be found out to
improve these limitations [16], [200]. In an study, in order to fabricate a highly hydrophobic
surface, a modified fluoroalkylsilanes were applied on a Poly(methyl methacrylate) (
PMMA) substrate and cured, which resulted in a hydrophobic PMMA surface with contact
angles increased from 60° to 110° [201].
Fluorinated polymers which are commonly called as fluoropolymers. By using
fluoropolymers in organic electronic device packaging, lifetime performance can be
substantially enhanced [16]. The characteristics of fluoropolymers make them a suitable
solution for the applications that require barrier against the diffusion of moisture, oxygen,
bases, acids, and most importantly an ability to significantly reduce mechanical wear and
friction. Additionally, the easy processing and coatability of the fluoropolymers is also
advantageous for their use in the packaging industry. For creating commercially viable
encapsulation structures for flexible OPV devices, a commercial polymer CytopTM was
reported [16].
CytopTM is colorless, transparent, amorphous, and can be deposited using conventional thin
film deposition techniques such as spin coating, doctor blading etc. CytopTM is
STATE OF THE ART
57
commercially available as a high viscosity resin and its chemical structure is shown in
Figure 3.12.
Figure 3.12:(a) Chemical structure for Cytop TM (b) Spin-coated Cytop film on glass
substrate under atomic force microscope (Copied from [202] with permission from AIP
Publishing).
J.B. Chae, et al. (2014) [203]) studied the hydrophobic nature of the Cytop materials. The
contact angles of diionized water droplets on Cytop layers thicker than 3 nm maintained an
angle of approximately 110°; the contact angles of the droplets on Cytop layers thinner than
3 nm abruptly dropped and decreased as the layer thickness decreased Figure 3.13. On the
basis of the result the author optimized the layer thickness, suitable for having a highly
hydrophobic nature.
STATE OF THE ART
58
Figure 3.13: Characterization of hydrophobicity in terms of water droplet contact angles
and thickness values of films with respect to weight percentages (wt%) of Cytop in solution.
Images below droplets are measured by atomic force microscopy (AFM) (Copied from
[203] with permission from Elsevier).
In order to achieve a coatable barrier materials [204] carried out the study on Cytop and
used it as an encapsulation for an OPV device. AFM (Atomic Force Microscopy) analysis
shows that CytopTM yields very smooth films using spin coating with a root mean square
roughness of 3.8 Å [204].
Work done by Jimmy Granstrom et al., [202] showed that Cytop can be used an organic
interlayer in multilayer structure to produce ultra-high barrier to sufficiently increase the
lifetime of organic light emitting diodes (OLEDs), where Cytop provides the part of the
barrier. Results of calcium degradation indicated that the multilayer of metal oxides having
CytopTM interlayer (see inset of Figure 3.14 type B) are more likely to reach the oxygen
and moisture transmission rates (<10-2 g.m-2.day-1) needed for 10,000 hour lifetimes [202],
[205]. The authors concluded that the thicker Cytop has better planarization effect but is
likely to cause cracking in ALD processed Al2O3 layer due to elastic mis-match. This
cracking can either be decrease with inserting additional layer in the stack or precisely
controlling the parameters to alter the Cytop thickness. Jimmy Granstrom et al., altered
Cytop thickness and also deposited a compressively stressed SiNx layer between Cytop and
Al2O3. With the use of SiNx in between cytop and Al2O3 crack generation in ALD layer
can be avoided completely. This enables the free choice of cytop thickness ranging from
STATE OF THE ART
59
40 nm to 4300 nm without affecting the barrier performance. Figure 3.14 shows
degradation mechanism of calcium encapsulated with multilayers of Al2O3 and Cytop a)
without SiNx and b) with SiNx which are referred as type a and type b respectively in the
Figure 3.14.
Figure 3.14: Calcium degradation mechanism, Type A Ca films as a function of time for
varying CYTOP film thicknesses and Ttype-B Ca films as a function of time for varying
CYTOP film thicknesses having SiNx interlayer (Copied from [205] with permission from
AIP Publishing).
In order to make use of Cytop without using evaporated metal oxide, Grandstrom et al.,
[206] carried out a work in which they used Cytop as a coated barrier and encapsulated
Calcium directly with it. The coated calcium was exposed to water vapors and degradation
was analyzed by optical calcium degradation test. Lifetime of Ca film was found to be
around 200 minutes when coated with 200 nm of Cytop layer. The Ca lifetime of 200
minutes is equivalent to WVTR of a commercial PET film having thickness of 100 µm
thick in the same conditions. A coated Cytop layer 50 times thinner than PET yielded
almost equivalent Ca lifetime as compared to 100 µm thick layer. Ca degradation indicated
that the deterioration started from the defects (pin holes) within the coated layer.
Controlling surface and internal defects and increasing layer thickness may increase the
overall lifetime of Ca [206].
This indicates the potential of Cytop as an intermediate barrier and can be used a temporary
protection of OSCs.
EXPERIMENTAL
60
EXPERIMENTAL
This chapter introduces the details of all of the raw materials used in the work, all materials
used for producing barrier layers including organic and inorganic polymers, inorganic
fillers and substrates.
This chapter also gives details on the methods adopted for the processing of the films
including solution preparation, film coating and drying. Pristine polymers and their
composites with fillers were coated on substrates (PET and glass) and later peeled off for
getting a free standing layers. Uniform distribution of the fillers within matrix is very
essential for creating effective gas barriers. It was a real challenge to prepare uniform layer
from glass flakes filled polymer. The main reason was a high viscosity of the solution. The
coating parameters, processing and coating optimization of the polysilazane based layers is
discussed later in the chapter. This chapter also includes the details of OSC structure,
materials and processing.
Finally, short descriptions of the characterization techniques are given.
Part of this chapter has been published in:
I.A Channa, A. Distler, M. Zaiser, C.J. Brabec, H.-J. Egelhaaf, Thin Film Encapsulation of
Organic Solar Cells by Direct Deposition of Polysilazanes from Solution, Advanced Energy
Materials (2019) In this section parts authored by I.A. Channa are reproduced in subchapter 2.5.2.
Additionally, through author ownership declaration, a permission was granted from all co-authors for
utilization of the whole content of publication as part of this thesis.
EXPERIMENTAL
61
4.1 Materials
All of the materials used in this work were commercially purchased and their details are
mentioned in the table 8 below:
Table 7: Materials used in the experiments
Materials Trade name Supplier Used for
Materials for Barrier preparation
Organic polymers
Polyvinyl alcohol (PVA) - Sigma Aldrich
Chemie GmbH
Polymeric matrix
for fillers
Polyvinyl butyral (PVB) Butvar-98 Sigma Aldrich
Chemie GmbH
Polymeric matrix
for fillers
Polyvinylidene fluoride
(PVDF)
- Thermo Fisher
GmbH
Interlayer for
barrier
Ethylene vinyl acetate
(EVA)
- Honeywell
International Inc.
Interlayer for
barrier
Inorganic polymer
Perhydropolysilazane
(PHPS)
Polysilazane durXtreme GmbH Barrier layer
Fillers
MMT Na+ clay
CLOISITE-
Na+
BYK ltd.
Glass flakes - Eckart GmbH
Solvents
De-ionized water - Solvent for PVA
Dimethyl sulfoxide
(DMSO)
- Thermo Fisher
GmbH
Solvent for
PVA/PVDF
Benzyl alcohol (BA) - Solvent for PVB
Di-n-butyl ether - For dilution of
PHPS
Adhesives
DELO Katiobond
LP655
DELO Industrie
Klebstoffe GmbH &
Co. KGaA
Barrier lamination
and interlayer
barrier
Rolic RPL-521 Rolic Technologies
Ltd. Switzerland
Interlayer for
barrier
Materials for Organic solar cells
ZnO N10 Avantama,
Switzerland
Hole Inject layer
P3HT - OPVIUS GmbH Semiconductor
EXPERIMENTAL
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PC60BM - OPVIUS GmbH Semiconductor
PEDOT:PSS HTL Solar HERAEUS Hole transport
layers
Substrates
Polyethylene (PE) Common
Commercial grade
packaging
Substrate for FTIR
of PHPS coatings
PET Melinex
ST504
DuPont Teijin
Films UK Ltd
Substrate for barrier
coating
ITO sputtered glass 15-20
Ω/sq
- Weidner Glas
GmbH
Substrate for OSC
Commercial barrier
Mitsubishi - VIEW-BARRIER,
VD-K3DA
Reference barrier
4.2 Processing
All of the solutions were processed with doctor blade (ZAA 2300- manufactured by
Zehntner Testing Instruments, Switzerland). The details of the layers are given below.
4.2.1 Filler based barrier films
Mainly two types of filler materials were used that include, MMT nanoclay and glass flakes
having different aspect ratios.
4.2.1.1 Clay based barriers
Dissolution of 10 wt% PVA in de-ionized water was carried out and the solution was stirred
continuously on a hot plate at 90oC for 3-4 hours (until the solution became homogenous).
After the complete dissolution of PVA, clay (Cloisite MMT-Na+ nanoclay powder) was
mixed to the solution with concentrations of 2 vol% to 10 vol% and stirred overnight at
60oC. This mixture was ultra-sonicated for 10 minutes right before the coating process.
For coatings glass substrates (5 minutes ultrasonically cleaned by each isopropanol and
acetone bath) were used. The layers were prepared by means of the doctor blade. The
coating speed was set to 5 𝑚𝑚. 𝑠−1, and the temperature on the doctor blade surface was
maintained at 30oC. As soon as the layers were processed with doctor blade, they were
positioned in an oven at 80oC for a few hours drying. The films were peeled off from the
substrate and free standing were obtained. Further characterizations including
measurements of barrier properties were carried out on free standing films.
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4.2.1.2 Glass flakes based
Polyvinyl butyral (PVB) was dissolved in benzyal alcohol with 30 wt% concentrtion. The
solution was stirrered at 80oC on hot plate for few hours. As sson as the solution became
homogenized glass flakes were blended in the solution at different volume fractions (0
vol%, 5 vol%, 10 vol%, 15 vol%, 20 vol%, 25 vol%, 30 vol%). Dispersion of glass flakes
in this mixture is very important, but due to 30wt % solution of PVB, the viscosity is very
high, therefore; continous gentle mixing is very essential because vigorious or hard mixing
can break the glass flakes that will change the aspect ratios. Therefore, manual and gentle
mixing procudure was adopted. Once, the glass flakes were properly dispersed in the PVB
matrix, the solution was exposed to vacuum for at least 15-30 mins to extract entrapped air
from the solution. The coatings on PET substrate were processed with low coating speeds,
i.e., 5 𝑚𝑚 ∙ 𝑠−1, and the blade gap was varied between 500 – 2000 µm. After the coating,
the samples were positioned in an oven at 80 oC for complete evaporation of the solvent.
The layers were peeled off from the subrates and characterizations were carried out on free
standing layers.
4.2.2 Polysilazane based barriers films
A cleaned PET was used as a substrate for Perhydropolysilazane (PHPS 20 wt% in di-butyl
ether) based barriers films. The cleaning of the substrate plays a vital role in PHPS based
thin films. A films processed on an improper cleaned substrate may result in bad barrier.
Therefore, extra care has to be take while cleaning PET substrates.
The PHPS solution was diluted in di-butyl ether before the processing of the barrier films.
Parameter related to PHPS coatings are summarized in Table 8.
Table 8: Coating Parameters for coating PHPS layers from an amount of 70 µL on PET
substrate, subsequently cured with deep UV irradiation in combination with temperature.
Parameters
Substrate PET (125 µm)
Amount of materials (PHPS) 50 – 70 µm
Dilution ratio 1:1 – 1:6 (PHPS: solvent)
Coating speed 1 – 30 mm/s
Blade gap 20 – 400 µm
Curing method Heat / vacuum VU / combination
Curing time Depends on curing method
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4.3 Preparation of OSCs
Bulk heterojunction organic photovoltaic cells having inverted structure,
glass/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag were produced and subsequently either
laminated with either polymer filled barrier and PET coated PHPS based barrier films or
directly coated with solution of PHPS onto the top electrode. For preparation of the devices
glass substrates coated with laser structured indium tin oxide (ITO) (sheet resistance of
21 Ω/) were used. The processing of all layers was carried out by blade coating in ambient
conditions except the top silver (Ag) electrode. After coating, the ZnO layer was annealed
for 5 min at 120 oC in air. The blend of P3HT:PCBM [with a ratio of 1:0.8 (wt/wt)] was
coated via doctor blading from o-xylene:1-methylnaphthalene (19:1, v/v) solution. A layer
of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was prepared
from the HTL Solar diluted with 1:1 ratio in water. After deposition of all layers, the silver
top electrode was deposited by thermal evaporation in ultrahigh vacuum (10-4 Pa) through
a mask to define an OSC active area of 0.1 cm2. The layout of the solar cell device and
schematic diagram of working cell is shown in Figure 4.1. The thickness of the evaporated
Ag film was kept as small as 50 nm in order to minimize its gas barrier effect. Initial current
voltage characteristics and power conversion efficiencies were measured in inert
atmosphere.
Figure 4.1: Details of the OSC device, a) Layout of the complete OSC device, b) schematic
diagram of the working cell
4.3.1 Encapsulation of the OSC devices
After the initial measurement, the OSC device was encapsulated by two methods: a)
lamination with polymer filled barriers and PET coated with PHPS films by means of a
commercial epoxy based adhesive (DELO Katiobond LP655) which was subsequently
EXPERIMENTAL
65
cured with UVA ~400 nm light for 2 minutes (a shown in Figure 4.2a, b) direct coating of
PHPS based barrier from solution via doctor blading on top of solar cell (as shown in Figure
4.2b).
Figure 4.2: Schematic diagrams of the encapsulated solar cells, a) solar cells encapsulated
with traditional lamination of the barrier films using epoxy as an adhesive, b) directly
coated solar cell.
4.4 Characterization
Prepared films were characterized via various different techniques mainly, barrier quality,
spectroscopy, bendability etc., and applied to solar cells as barrier layers and lifetime of the
encapsulated solar cells were monitored. The descriptions of these characterization
techniques are given in the following sections.
4.4.1 Barrier quality
The films were characterized in terms of barrier quality and for that water vapor
transmission rate (WVTR) and oxygen transmission rate (OTR) were measured.
4.4.1.1 Water vapor transmission rate (wvtr)
Measurement of water vapor transmission rate was carried out with two methods, one is
using cup in compliance with ASTM E-96 standard and other is using commercial testing
equipment called as SYSTECH 7002.
a) Cup method
Standard Aluminum cup as shown in Figure 4.3 having diameter of 6.35 cm which
compiles with ASTM standard E-96 [3] purchased from Thwing-Albert Instrument
Company (Germany) was used. This test can be carried out in two approaches; the first
approach is filling the cup with water up to ¾ of its capacity with distilled water and then
sealing the cup with the barrier layer as in Figure 4.3a. The cup filled with water is placed
EXPERIMENTAL
66
in the controlled conditions and weight loss of the water is monitored with time. Leakage
is considered as the primary error while performing this kind of experiments. In this case,
there will be 100 %RH inside the cup and controlled atmosphere of set %RH will be
outside. This change will cause the moisture to transmit from inside to outside hence loss
of water can be observed. In second approach the cup is filled with a desiccant (Calcium
chloride) Figure 4.3b. The cup was then place inside a humidity chamber with controlled
temperature and relative humidity conditions. In this case, there will be nearly 0 %RH
inside the cup because of calcium chloride (a desiccant material) and controlled atmosphere
of set %RH will be outside. This change will cause the moisture to transmit from outside
of the cup to inside, hence weight gain can be observed. The weight of whole assembly
(cup plus barrier layer) was measured before placing it inside the controlled environment
and every 24 hours afterwards, with first approach i.e. water method the weight of the
assembly decreased and in second approach using desiccant, the weight of the assembly
increased. The straight slope of change of the rate of the weight was then used to calculate
water vapor transmission rate and permeability.
Figure 4.3: a) Schematic diagram showing cup test using water b) cup test using
desiccants, c) Aluminum cup according to ASTM standard E96, b) SYSTECH 7002 method
Moisture permeability measurements were also performed using an M7002 water vapor
permeation analyzer (SYSTECH Illinois, UK) as shown in Figure 4.4, having lower
detection limits of 0.02 g m-2day-1 or 0.002 g m-2day-1, depending on the size of the sample
with temperature and humidity range of 5 oC to 50 oC and 20 to 90 %RH respectively. This
device is equipped with a sensitive P2O5 sensor for accurate measurement of vapor
transmission with exact temperature and humidity. The measurement method complies
with ASTM F-1249*, ISO 15105-2, ISO 15106-3 and DIN 53122-2.
EXPERIMENTAL
67
a)
b)
Figure 4.4: a) photograph of the WVTR device (SYSTECH 7002), b) schematic view of
the permeation cell of SYSTECH 7002 device showing the flow of the dry and wet
nitrogen through the cell chamber.
4.4.1.2 Oxygen transmission rate (OTR)
Oxygen permeation rate was analyzed by using a permeation chamber fitted with an optical
oxygen sensing spot PSt9 (Manufactured by PreSens Precision Sensing GmbH) with a
detection limit of 0.1 cm3 m-2 day-1 bar-1. Samples were carefully mounted between the two
chambers of the device and nitrogen gas was flushed inside both of the permeation cells for
15 minutes and leakage rate was measured. Then oxygen was flushed in bottom chamber
(as shown in Figure 4.5) for half a minute and then increase in oxygen fraction in upper
chamber was measured constantly during a time interval for a few days, this data was
further used to calculate oxygen transmission rate (OTR) and permeability.
Gas outletGas inlet
Gas outletFilm sample
Gas inlet
Optical fiberOxygen sensor
Oxygen
Nitrogen
Figure 4.5: Schematic view of the oxygen permeation cell
EXPERIMENTAL
68
4.4.2 Spectroscopic analysis
A Perkin Elmer Lambda 950 double beam spectrometer including a 150 mm integrating
sphere with a photomultiplier and an InGaAs detector was used for the measurements
including total and diffuse transmittance and reflectance of pristine polymer and polymers
containing fillers. For measurement, the samples were positioned in the transmission and
reflection ports of the sphere in a way that the rougher side faced the illumination.
IR spectra were recorded in ATR mode with a Fourier transform infrared (FTIR)
spectrophotometer (Bruker ALPHA-P) FTIR operating with OPUS 7.2 software. Spectra
were obtained using 128 scan summations at 4 cm-1 resolution.
4.4.3 Bending of the barrier layers
Bending tests were performed by using an in-house made cyclic bend tester having one end
fixed, other end moves linearly back and forth, thus cycling the barrier film in a customized
bending radius. For each test at least three samples having size of 3 x 10 cm2 were used.
Bent films were cut from the middle for further characterizations.
4.4.4 Degradation test
The degradation tests were perform by below mentioned methods.
4.4.4.1 Optical measurements
UV/vis absorption spectra of single photoactive layers (P3HT) were recorded with
Shimadzu UV-1800 spectrophotometer. A customized sample holder was used to make
sure that the beam hits the same spot when remounting the sample after various time
interval during degradation test.
4.4.4.2 Damp heat degradation
The coated and laminated OSC devices were placed in an artificial weathering chamber
(ESPEC LHL-114), with the pre-set condition of 40oC and 85%RH. Accordingly,
degradation tests were performed under the same conditions that were used to measure the
barrier characteristics of the films.
4.4.4.3 Degradation under sun
To check degradation induced by light, coated and laminated devices were placed under
constant illumination in ambient air in the compartment of a SUNTEST XXL+ sun
simulator (Atlas Materials Testing Technology GmbH) equipped with daylight filter. The
EXPERIMENTAL
69
light source is a Xenon lamp with an illumination intensity pre-set to 60 W m-² in the range
of 300-400 nm. The compartment temperature was controlled at 65 °C.
4.4.4.4 Electrical measurements
Current-voltage characteristics and power conversion efficiencies of the solar cells were
measured during the ageing experiments by an LOT solar simulator (Class AAA) at 1000
W m-². For this purpose, the solar cells were taken out of the respective ageing chambers
and put back after the measurement.
4.4.4.5 SEM images
Images of the cross section were obtained by a JEOL scanning electron microscopy (SEM)
JSM-7610F using a secondary electron image detector. For this purpose, SEM was operated
at 2 kV accelerating voltage, in a low probe current mode of ~65 nA, maintaining a distance
of 6 mm. Backscattered as well as secondary electrons and a combination of both was
detected for the creation of an image.
IB-19500CP polisher was used to prepare samples for SEM cross section analysis.
4.4.4.6 Optical micrographs
For microscopic analysis, two optical microscopes were utilized. One of them is an MX51
manufactured by Olympus Co. which can perform imaging in various modes including
bright filed, dark field and polarized. Other microscope is a μsurf confocal manufactured
by NanoFocus AG. This microscope was used for the surface characterization such as
roughness as well as thickness measurements.
RESULTS AND DISCUSSION
70
RESULTS AND DISCUSSION
This chapter presents the development of solution-based barrier coatings for the protection
of organic solar cells against oxygen and moisture. It is divided into two subchapters. The
first one describes the development of barriers based on clay and glass flakes. The second
one describes the development of barriers based on impermeable silica coatings from
perhydropolysilazanes.
Part of this chapter has been published in:
I.A Channa, A. Distler, M. Zaiser, C.J. Brabec, H.-J. Egelhaaf, Thin Film Encapsulation of
Organic Solar Cells by Direct Deposition of Polysilazanes from Solution, Advanced Energy
Materials (2019)
In this chapter only parts authored by I.A. Channa are reproduced in section 5.2.
Additionally, through author ownership declaration, a permission was granted from all co-authors for
utilization of the whole content of publication as part of this thesis.
RESULTS AND DISCUSSION
71
5.1 Filler based barriers: Clay and glass flakes
In this chapter barriers based on impermeable fillers (i.e. increasing the tortuous path) are
discussed. The addition of fillers with high aspect ratios (α) incorporated at sufficient
concentrations creates hurdles for diffusing molecules, thus compelling them to take longer
routes. This results in extra time for diffusion and hence improved barrier characteristics.
This chapter is mainly divided into two sections, first section is related to the use of clay as
a filler and the second section deals with fillers based on glass flakes.
In the first section, work on the clay filler particles is described. The clay as filler was used
to start the work to develop decent barriers. For this purpose, polyvinyl alcohol (PVA)
polymer (a water soluble) matrix was used. As clay is of hydrophilic nature, it requires
water for its uniform distribution. The barrier characteristics of PVA against moisture were
improved with addition of clay and WVTR of 2.8 g.m-2.day-1 was achieved which
corresponds to an improvement of 86% as compared to PVA without clay. The barrier layer
maintained transparency in the visible region.
In the second section, barriers based on glass flakes fillers are discussed. In order to develop
high quality barriers, glass flakes having different aspect ratios of α = 200, α = 400 and α
= 2000 were incorporated into the polymer matrix. The use of glass flakes showed
significant improvement in the barrier quality and moisture permeation rates as low as 0.04
g.m-2.day-1 were achieved. OSCs were encapsulated with such barrier films and accelerated
lifetime tests in damp heat (40oC/ 85%RH) were performed. The results showed significant
stability improvement of OSCs encapsulated with glass flakes based barrier as compared
to barriers prepared from polymers without glass flakes. The OSC showed almost no loss
in efficiency for a period of 150 h, whereas the OSC with neat polymer encapsulation died
within less than 24 h. This suggests that the glass flakes have a high potential in the
packaging industry.
5.1.1 Clay based barriers
In order to start the development of barrier films which comply with all of the requirements
of OSC encapsulation, various materials were screened for their applicability, including
boron nitride, graphene, montmorillonite (MMT) and mica clay (See appendix). Due to its
RESULTS AND DISCUSSION
72
attractive aspect ratio, uniform dispensability, and transparency along with sufficient
barrier quality, MMT-Na+ clay was chosen as potential filler material.
5.1.1.1 IR analysis of nanocomposites
FTIR spectra were recorded on free standing films of PVA/MMT Na+ clay composite as a
supporting data to prove the existence of clay in the matrix. The FT-IR spectra of pure PVA
and its nanocomposite in the 1500–500 cm-1 spectral range are shown in Figure 5.1.
500600700800900100011001200130014001500
500600700800900100011001200130014001500
Tra
nsm
ittance
[%
]
Wavenumber (cm-1)
PVOH
6 vol% clay
Figure 5.1: FT-IR transmission spectra of PVA films and its composites with clay
concentration of 6 vol. % in the range of 1500–500 cm-1.
Pristine PVA shows no significant characteristic peak near 1030 cm-1 and with the addition
of the MMT clay, a peak near to 1030 cm-1 appears. This peak can be associated to the
stretching vibration of the Si–O bond, representing the clay, which contains silicates as the
main component. Therefore, the FTIR spectra confirm the presence of the nano-clay within
the PVA matrix. This result is in accordance with the work done by Gaume et al. [119].
5.1.1.2 Surface morphology
Figure 5.2 (a, b and c) shows the surface morphologies of PVA/MMT-NA+ clay layers
having 2 vol%, 4 vol% and 10 vol%, respectively. The micrographs reveal no detrimental
or damaging surface defects. Addition of the clay to the PVA matrix does not create any
surface defects that could affect the overall barrier quality of the layers. The small black
dots represent agglomerated clay particles. It will be shown later that the agglomeration of
the particles has almost negligible effect on the barrier quality.
RESULTS AND DISCUSSION
73
a): 2 vol. % clay
b): vol. % clay
c): 6 vol. % clay
Figure 5.2: Optical micrographs of the PVA-clay nanocomposite, where PVA contains
a) 2 vol%, b) 4 vol%, and c) 6 vol% MMT-Na+ nanoclay, respectively.
5.1.1.3 Transparency and haze of the Nanocomposites
UV-vis spectroscopic analysis reveals that pristine PVA exhibits a transparency of 92.5%
(Figure 5.3a). However, a slight decrease in total transmittance to ~90% is observed when
MMT-Na+ clay is added to PVA ranging from 2 to 6 vol%. The negligible decrease in
transparency at a wavelength of 600 nm is shown in Figure 5.3b. PVA has a refractive
index of 1.5 and MMT-Na+ clay has a refractive index of 1.52, which explains the small
decrease in total transmittance. In order to characterize the optical properties of the barrier
further, diffuse transmission measurements of the layers were carried out. Diffuse
transmission and the increase in diffuse transmission at 600 nm is shown in Figure 5.3 (c,d).
The pure PVA shows a diffuse transmittance of around 2% and PVA with 6 vol. % clay
exhibits diffuse transmittance of around 8% at 600 nm wavelength. Haze can be due to
different reasons. Surface roughness or agglomeration of the clay particles at the surface or
within the film can cause scattering of the light. In this case, the increase in haze is
associated to surface roughness, as the layer with 6 vol% clay shows a relatively rough
surface as compared to layers with low concentration clay (Figure 5.2).
a) b)
RESULTS AND DISCUSSION
74
300 450 600 750 9000
20
40
60
80
100T
ota
l tr
ansm
isttance (
%)
Wavelength (nm)
Pristine PVA
2 wt% clay in PVA
4 wt% clay in PVA
6 wt% clay in PVA
0 2 4 686
88
90
92
94
96
98
100
To
tal T
ran
sm
itta
nce
(%
) @
60
0 n
m
Clay in PVA (wt%) c)
Pristine PVA
2 wt% clay in PVA
4 wt% clay in PVA
6 wt% clay in PVA
300 450 600 750 9000
20
40
60
80
100
Diffu
se T
ransm
itta
nce (
%)
Wavelength (nm)
d)
0 2 4 6
2
4
6
8
10
Diffu
sed tra
nsm
itta
nce (
%)
@ 6
00 n
m
Clay wt % in PVA
Figure 5.3: UV-vis spectra of PVA and its composites with different clay concentrations
coated on glass substrates, a) Total transmittance spectra of PVA and its composites
with clay, b) total transmittance at 600 nm as a function of clay content, c) diffuse
transmittance spectra of PVA and its composites with clay, d) diffuse transmittance at
600 nm as a function of clay content.
5.1.1.4 Moisture permeability
Being water soluble, PVA still shows a decent barrier to water vapors and maintains low
moisture permeability. The PVA layer with a thickness of about 25 µm shows moisture
permeation of 90 g.m-2.day-1. This permeation rate is further decreased by increasing the
thickness of the layers. The effect of layer thickness and its effect on moisture permeation
is shown in Figure 5.4 and subsequent values are mentioned in Table 9.
RESULTS AND DISCUSSION
75
0 1 2 3 4 5 6 7 8 9 10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
We
igh
t lo
ss (
g)
Time (days)
PVA (25 µm)
PVA (50 µm)
PVA (75 µm)
PVA (100 µm)
Figure 5.4: Weight loss of water from a cup sealed with PVA barriers of different
thicknesses vs time at 40oC / 65%RH (moisture permeation calculated from the slope
according to ASTM-E96).
Table 9: Moisture permeation of the PVA layer having different thickness values.
Thickness of the layers
(µm)
Moisture permeation
(g.m-2.day-1)
25 90 ± 5.2
50 47 ± 4.2
75 29 ± 3.4
100 20.5 ± 2.5
The increase in thickness creates longer paths and moisture molecule take longer time to
diffuse from one side to another, hence moisture permeation is decreased. The thickest layer
with thickness of 100 µm exhibited moisture permeation rate of 20 g.m-2.day-1. This infers
to ~77% improvement in barrier quality. According to Eq. 13, the blocking effect of the
layers is linearly proportional to the thickness of the PVA layer; as can be seen in Figure
5.5.
RESULTS AND DISCUSSION
76
0 10 20 30 40 50 60 70 80 90 1000.00
0.01
0.02
0.03
0.04
0.05
1/W
VT
R (
m2.d
ay/g
)
Thickness (µm)
1/WVTR
Linear fit
Figure 5.5: Blocking effect of PVOH layers having different thicknesses.
The WVTR values of PVA films are significantly reduced upon adding MMT-NA+
nanoclay. Table 10 gives the values of moisture transmission rate and permeability which
are obtained from the cup measurements of water vapor transmission rate for PVA films
containing different volume fractions of nanoclay particles, as shown in Figure 5.6. At 40oC
and 65 %RH, the 10 vol% clay layer shows the lowest moisture permeability, which is 2.8
g.m-2.day-1. This means that the addition of clay of up to 10 vol% reduces the permeation
of moisture through PVOH down by 86%, whereas reduction by 84%, 80%, 71% and 49%
was observed for 8 vol%, 6 vol%, 4 vol% and 2 vol% clay, respectively (Table 10).
Table 10: Calculated moisture permeation values of PVOH and its composites at
conditions 40oC / 65%RH; film thickness of 100 µm in all cases except PET (125 µm).
Material WVTR [g.m-2.day-1] Permeability [g.cm.m-2.day-1]
PET 5 0.062
PVA 20.5 0.205
2 vol % clay 10 0.1
4 vol % clay 6 0.06
6 vol % clay 4 0.04
8 vol % clay 3.2 0.032
10 vol % clay 2.8 0.028
RESULTS AND DISCUSSION
77
0 2 4 6 8 100.0
0.2
0.4
0.6
0.8
1.0W
eig
ht lo
ss (
g)
Time (days)
Pristine PVA
2 wt % clay
4 wt % clay
6 wt % clay
8 wt % clay
10 wt % clay
Figure 5.6: Water weight loss vs time from cups sealed with films of PVA and its composites
with MMt clay at 40oC / 65%RH.
5.1.1.5 Validation of the experimental data
In order to validate the permeation data for the nanocomposite, the experimental values
were compared to the theoretical permeation model proposed by Bharadwaj et al. (Eq. 41).
[42] This model calculates the tortuosity factor considering volume fraction of the
particles, their aspect ratio along with the order parameter based on the orientation of the
particles within the polymer matrix as well as the thickness of the layers. The aspect ratio
of α = 500 for MMT-Na+ clay is used in this work as suggested by Manias and Gaume
[119], [207] for semi-exfoliated structures. The calculations based on this aspect ratio fit
quite well with Bharadwaj’s model. The order parameter of S = 0 as suggested by Gaume
[119] corresponds to random orientation of the clay particles. The comparison of
experimental data to Bhardwaj’s theoretical model is shown in
Figure 5.7. The experimental data match with the theoretical calculations for S = 0 and
hence confirm that the nanoclay particles in the PVOH films do not have a preferential
orientation.
RESULTS AND DISCUSSION
78
0.00 0.02 0.04 0.06 0.08 0.100.0
0.2
0.4
0.6
0.8
1.0R
ela
tive
Pe
rme
ab
ility (
Pc / P
p)
Clay content (wt %)
Bharadwaj's model (S=0)
Experimental
Figure 5.7: Evolution of relative permeability of PVA films with increasing content of
nanoclay particles. (Triangles – experimental data. DDotted line - calculated data)
according to the Bharadwaj model for an aspect ratio of α = 500 and order parameters of
S = 0.
5.1.1.6 Bendability
The PVA/ clay nanocomposites were subjected to bending cycles at a bending radius of 5
cm to check if they lose barrier quality (
Figure 5.8). The nanocomposite films show a decrease of the water barrier properties by
only 10% of the initial value even after 10K bending cycles. This suggests that after 10K
bending cycles the nano-clay platelets are still firmly connected to the PVA matrix.
RESULTS AND DISCUSSION
79
0 2000 4000 6000 8000 100000.0
0.2
0.4
0.6
0.8
1.01
/WV
TR
[N
orm
alize
d]
Bending cycles (No.)
PVA
2 %
4 %
6 %
8 %
10 %
Figure 5.8: Reciprocal WVTR of PVA and its MMT-Na+ clay composite with loading
concentrations of 0-10 volume %.vs number of bending cycles with bending radius of 5 cm.
Each layer has a thickness of 100 µm.
5.1.1.7 Conclusion
A promising decrease in moisture permeability has been achieved by adding nanoclay into
the matrix of PVA. The moisture barrier has been deposited from solution. Processing of
barrier layers from solution is an easy, environmentally friendly and economical method.
The nanocomposites were tested for barrier quality using ASTM E96 standard.
Nanocomposites having nanoclay contents of 2 vol%, 4 vol%, 6 vol%, 8 vol% and 10 vol%
show reduced moisture permeation by 49%, 71%, 80%, 84% and 86% respectively, as
compared to the pure polymer. They exhibit high transparencies of ~92% in the white light
region. The addition of nanoclay seems to have an almost negligible effect on transparency
except for light scattering by agglomerated nanoclay particles at the surface. The haze
caused by this scattering was found to be not more than 8% in the visible region, even for
the highest loading of 10 vol%. The resulting polymer/nanoclay composites also show
RESULTS AND DISCUSSION
80
excellent flexibility, maintaining their barrier quality even after 10,000 bending cycles.
These properties make PVA/clay nano-composites good candidates for packaging
materials. However, even composites with 10 vol% nanoclay show WVTR values no
smaller than 2 𝑔 ∙ 𝑚−2 ∙ 𝑑−1. These values are appropriate for food packaging, but are not
sufficient for the application to organic solar cells encapsulation, where WVTR values of
less than 10-2 g.m-2.day-1 are required. According to the Bharadwaj model, the barrier
properties can be enhanced by two parameters. Increasing the orientation factor from S = 0
to S = 1 will enhance the barrier effect by a factor of almost three (for high aspect ratios).
However, due to the small lateral extension of the Montmorillonite particles of few
nanometers, the particles are not aligned by the coating process. An increase of the aspect
ratio from 500 to, e.g., 2000 will increase the barrier effect by another factor of around four.
Thus, WVTR values of an order of magnitude less than for Montmorillonite clay particles
can potentially be achieved. Enhancing the filler loading and increasing the thickness of
the barrier films will provide another order of magnitude. In the following chapter, this
approach will be demonstrated by using glass flakes as filler particles with high aspect
ratios.
RESULTS AND DISCUSSION
81
5.1.2 Glass flakes based barriers
In this section, glass flakes of different aspect ratios are used as filler particles, instead of
clay to produce flexible barriers with enhanced barrier properties. Glass flakes have been
chosen as candidates for filler particles because they offer several favorable properties with
respect to their use in transparent and flexible barriers. They are transparent in the visible
range of the spectrum. They offer large aspect ratios (AR) and they are easily aligned
parallel to the film surface during the coating process, due to their large lateral extension
which is on the same order as the film thickness. Due to their particulate nature, the
resulting barrier should be resilient towards bending.
Three different types of glass flakes are investigated. The different types of glass flakes
have almost the same lateral extension of around 50 – 300 µm, but different thicknesses of
about 1 µm down to 0.1 µm, hence the aspect ratio (α ) ranges from 200 to 2000 (Figure
5.9). The barrier films are prepared by inserting the glass flakes into PVB matrices as
described in chapter 4. PVB is chosen as the matrix polymer due to its flexible nature and
compatibility with glass flakes in terms of good adhesion of the filler particles with the
matrix and relatively good refractive index matching (n(550 nm) = 1.525) and PVB (n(550
nm) = 1.49) [208], [209].
a)
b)
c)
Figure 5.9: Optical micrographs of glass flakes. (a) glass flakes with thickness ~ 1 µm,
(b) glass flakes with thickness ~0.5 µm, (c) glass flakes with thickness of ~0.1 µm
RESULTS AND DISCUSSION
82
The top view of the coated layers is shown in Figure 5.10a. The film contain 15 vol% of
glass flakes. The top view of the layers suggests that the flakes are oriented preferentially
parallel to the film surface. SEM cross section image and EDX image mapping of Si of
corresponding PVB films are shown in Figure 5.10(b,c). The SEM cross section clearly
shows that the flakes are preferentially oriented parallel to the film and this is further
confirmed by the EDX mapping of SiO2.
a)
b)
c)
Figure 5.10: Micrographs of PVB films containing 15 vol% glass flakes of AR = 400. a)
Optical micrograph (top view) (b) SEM cross section of a semi-polished PVB/glass
flakes composite filmc) EDX image of the highlighted section (fully polished) of the
PVB/glass flakes layer with Si mapping.
RESULTS AND DISCUSSION
83
5.1.2.1 Surface roughness of the layers
Confocal images of the layer of PVB containing 25 vol% of glass flakes are shown in Figure
5.11 (a-c). The figures show the surface morphological scan of the layers having areas of
1.5 x 1.5 mm2. The surface of the layer containing flakes of A.R~200, shows a highly rough
surface (Ra ~ 120 nm). Similarly, the layers with flakes of A.R~400 and A.R~2000 also
exhibited rough surfaces i.e. Ra ~100 nm and Ra ~120 nm respectively (Figure 5.11 (a-c)).
This surface roughness in the layers can be attributed to the large lateral extension of the
glass flakes (200-300 µm), together with the high volume fraction of glass flakes. As soon
as the solvent starts to evaporate, the PVB matrix contracts around the flakes and as a result
leaves behind substantial surface roughness.
a)
b)
c)
Figure 5.11: Confocal micrograph showing the surface of the PVB filled with 25 vol%
of the glass flakes, a) glass flakes with A.R~200, b) glass flakes with A.R~400, and c)
glass flakes with A.R~2000.
5.1.2.2 Transparency of the layers
Figure 5.12 shows the optical measurement of a pristine PVB layer having a thickness of
~70 µm. PVB shows a total transmission of ~93% throughout the visible range of the
spectrum, whereas the diffuse transmission increases continuously from 5% at 800 nm to
12% at 400 nm, due to scattering at crystalline domains (Figure 5.12 a). Total reflectance
of ~7% is observed in the visible region of the spectrum (Figure 5.12 b). The diffuse
reflectance shown by the PVB layer increases steadily from 3% at 800 nm to 7% at 250
nm. The sum of total transmittance and total reflectance of PVB layer is 100%. It means
that there is no measurable absorption by the PVB film above 400 nm.
0.0 0.5 1.0 1.5 mm
mm
0.0
0.5
1.0
0
50
100
150
200
250
0.0 0.5 1.0 1.5 mm
mm
0.0
0.5
1.0
0
50
100
150
200
250 0.0 0.5 1.0 1.5 mm
mm
0.0
0.5
1.0
0
50
100
150
200
250
nm nm nm
RESULTS AND DISCUSSION
84
a)
300 400 500 600 700 8000
20
40
60
80
100
TotalTransmittance
Tra
nsm
itta
nce (
%)
Wavelentgth (nm)
Diffused Transmission
b)
300 400 500 600 700 8000
20
40
60
80
100
Diffused reflectanceReflecta
nce (
%)
Wavelentgth (nm)
Total reflectance
Figure 5.12: Spectra of a pristine PVB film having a thickness of ~ 70 µm, a) Total
transmission (open triangles in black), diffuse transmission (full triangles in red), b)
Total reflectance (full square in black) and diffuse reflectance (black squares).
Subsequently, transmission (total and diffuse) along with reflection (total and diffuse)
measurements were carried out for the PVB layers containing glass flakes. The addition of
glass flakes leads to cutoff around 330 nm. The results are shown in Figure 5.13 a-f. PVB
layers filled with glass flakes having aspect ratios of 200 and 400, at concentrations of 5
vol%, 15 vol% and 25 vol%, respectively, show total transmittance values of around 90%,
whereas for AR = 2000, the total transmission decreases from 89% to 85% with increasing
volume fraction of glass flakes. Total reflection is between 8% and 9% for all samples, so
that absorption in the visible range is smaller than 6% for all samples. Only below 350 nm,
there is significant absorption by the glass flakes.
In contrast to total transmission, diffuse transmittance values vary significantly at different
concentrations of the flakes. For glass flakes of α = 200, the diffuse transmittance remains
around 5% and 10% for concentrations of 5 vol% and 15 vol%, respectively, but for 25
vol% the diffuse transmittance inreases to around 30% at 550 nm (Figure 5.13a). Nearly
the same behaviour is observed for glass flakes of α = 400, i.e., diffuse transmittance
remains around 10% for 5 vol% and 15 vol% and increases to ~30% for 25 vol% (Figure
5.13b). For glass flakes of α = 2000, almost all of the glass flake loadings show a high
diffuse transmittance of around 33% (Figure 5.13c). Diffuse reflectance in all cases remains
around 6%.
RESULTS AND DISCUSSION
85
a)
300 400 500 600 700 8000
20
40
60
80
100T
ransm
itta
nce (
%)
Wavelength (%)
Total transmission
5 vol. %
15 vol. %
25 vol. %
Diffused transmission
5 vol. %
15 vol. %
25 vol. %
b)
300 400 500 600 700 8000
20
40
60
80
100
Diffuse transmission
5 vol. %
15 vol. %
25 vol. %
Tra
nsm
itta
nce (
%)
Wavelength (nm)
Total transmission
5 vol. %
15 vol. %
25 vol. %
c)
300 400 500 600 700 8000
20
40
60
80
100
Tra
nsm
itta
nce (
%)
Wavelength (nm)
Total transmission
5 vol. %
15 vol. %
25 vol. %
Diffuse transmission
5 vol. %
15 vol. %
25 vol. %
d)
300 400 500 600 700 8000
20
40
60
80
100
Reflecta
nce (
%)
Wavelentgth (nm)
Total Reflectance
5 vol. %
15 vol. %
25 vol. %
Diffuse Reflectance
5 vol. %
15 vol. %
25 vol. %
e)
300 400 500 600 700 8000
20
40
60
80
100R
eflecta
nce (
%)
Wavelentgth (nm)
Total Reflectance
5 vol. %
15 vol. %
25 vol. %
Diffuse Reflectance
5 vol. %
15 vol. %
25 vol. %
f)
300 400 500 600 700 8000
20
40
60
80
100
Re
fle
cta
nce
(%
)
Wavelentgth (nm)
Total Reflectance
5 vol. %
15 vol. %
25 vol. %
Diffuse Reflectance
5 vol. %
15 vol. %
25 vol. %
Figure 5.13: Transmittance and reflectance spectra of PVB containing glass flakes. a)
Total transmittance and diffuse transmittance spectra of PVB filled with 5-15 vol% of
glass flakes (A.R = 200), b) Total transmittance and diffuse transmittance spectra of PVB
filled with 5-15 vol% of glass flakes (A.R = 400), c) Total transmittance and diffuse
transmittance spectra of PVB filled with 5-15 vol% of glass flakes (A.R = 2000), d) Total
reflectance and diffuse reflectance spectra of PVB filled with 5-15 vol% of glass flakes
(A.R = 200), e) Total reflectance and diffuse reflectance spectra of PVB filled with 5-15
vol% of glass flakes of A.R = 400, and f) Total reflectance and diffuse reflectance spectra
of PVB filled with 5-15 vol% of glass flakes of A.R = 2000.
In summary, glass flakes are ideally suited as filler particles for PVB films, as the total
transmittance of the film remains around 90% even at high glass loading and thus is hardly
reduced with respect to pristine PVB. This is mainly due to the almost perfect refractive
index matching of sold lime glass (n(550 nm) = 1.52) and PVB (n(550 nm) = 1.48). Both,
total and diffuse reflectance hardly vary with the type of glass flake or the glass loading.
However, diffuse transmittance is significant, even for low glass loadings and increases
further with increasing glass loading of the film. Diffuse transmittance may depend on
several parameters, which include light scattering by the glass particles in the bulk of the
film, surface roughness of the film, and light scattering by gas bubbles which are trapped
in the polymer matrix. In the following, we will analyze the different possible causes for
RESULTS AND DISCUSSION
86
the diffuse part of the transmitted light. The increase in the diffuse transmittance can be
due to different reasons, which include, surface roughness, orientation of the glass flakes,
mis-match of the refractive indices of the flakes and polymer matrix etc.
5.1.2.3 Influence of bulk scattering:
In order to quantify the effect of light scattering of the glass particles in the PVB matrix on
the optical characteristics of the filled PVB layers, optical simulations were performed. For
the optical simulations, an optical model for simulating light scattering and propagation in
polymer filled with particles was developed by Dr. Benjamin Lipovšek from the University
of Ljubljana, Slovenia [210]. The detailed description of the model can be found in
Benjamil et. al., 2015 [210]. The model for particle filled polymers was used by integrating
it in the optical simulator CROWM (Combined Ray Optics/ Wave Optics Model)[210],
[211]. CROWM is based on the combination of three-dimensional ray tracing and transfer
matrix methods to analyze light propagation in thick and thin layers respectively. For
simulations a simple geometry of the flakes is considered – the surfaces are assumed to be
plane-parallel and perfectly smooth.
In a first step, the optical transmittance at 550 nm of a single glass flake of refractive index
n = 1.525 [212] inside a PVB matrix of n = 1.49 [213] is simulated for different tilting
angles of the glass flake with respect to the film surface (Figure 5.14). (Up to tilt angles of
70°, 99 % of light is transmitted through the flake, retaining the specular direction of
propagation. Only above tilt angles of 70°, reflectance is large enough so that the reflected
(scattered) light becomes „visible“ as a loss in the transmitted light.
In a second step, the simulation is performed on PVB films filled with glass flakes at
volume concentrations of 5 to 30%. Three different cases were simulated which include a)
(A) variation of the tilt angle (θ) (surface normals of all glass flakes parallel, i.e., no rotation
around vertical axis of the film) and particle volume concentration (PVC), (B) Fixed tilt,
random rotation (ϕ) around the vertical axis and (C) Random tilt, random rotation around
the vertical axis. The schematic diagrams of simulated cases are shown in Figure 5.15.
RESULTS AND DISCUSSION
87
Figure 5.14: Simulated direct/total transmission of a single glass flake filled in PVB matrix
at a wavelength of 550 nm for different polarizations of the incident light.
Figure 5.15: Simulated tilt (θ) and rotation angles of flakes (ϕ), for conditions a)
variation of the tilt angle of the flakes (0≤ 𝜃 ≤ 180𝑜) and fix rotation, b) Fixed tilt,
rotation around the vertical axis (0≤ 𝜙 ≤ 360𝑜) and c) Random tilt and random
rotation around the vertical axis, where is a vector normal to flake surface.
RESULTS AND DISCUSSION
88
In case A (Figure 5.15a) , the layers show high transmission of the light (> 90%) for all tilt
angles up to 70°. Significant scattering, i.e., haze, takes place only for tilt angles above 60o.
At 80° tilt, total transmission reaches a minimum of 85% for PVC = 25%, while diffuse
transmission (haze) reaches its maximum of 30%. At 90° tilt angle, the incident light
completely „misses“ the flakes, this is why the haze drops to zero in these simulations. In
case B (Figure 5.15b), for glass flakes with fixed tilt and random rotation around the vertical
axis, qualitatively the same behaviour as in case (A) is observed(Figure 5.15b). However,
at 80° tilt angle of the glass flakes, total transmission is only reduced to 88% while haze
reaches a value of 70% for PVC = 25%. In the most realistic case C (Figure 5.15c), the
effects of the tilt angle range (±θ) and particle volume concentration (PVC) are simulated.
The effect of tilting is negligible for angles below 70°. Above 70°, total transmittance
experiences a minute drop from 92% to 91% and haze reaches a maximum of 30% for the
tilt angle range of 90° and a PVC = 25% (Figure 5.16).
Figure 5.16: Simulated transmission (total and diffuse) (@550 nm) of PVB films filled
with glass flakes at different particle volume concentrations havingRandom tilt, random
rotation around the vertical axis.
RESULTS AND DISCUSSION
89
From these simulations, it is to be concluded that light scattering due to the inclusion of
glass flakes into the PVB film does not lead to a significant reduction of total
transmission, which is in perfect accordance with the experimental data. However, the
haze of between 5% and 30% observed in experiment (Figure 5.13) is not reproduced
by the simulations, as the average tilt angle of the glass flakes in the PVB films is less
than 10° for 80% of flake content, even for the sample with the relatively low PVC
(Figure 5.10c). In the following, causes for significant haze other than bulk scattering are
investigated.
5.1.2.4 Influence of the Surface roughness:
From the confocal micrographs (Figure 5.11) it is evident that the layers exhibit
considerable surface roughness (Ra) on the order of 100 nm. This rough surface can be a
potential reason for the high value of diffuse transmission. Therefore, in order to reduce
the surface roughness, an additional UV cured epoxy layer was deposited on both sides
of the glass flakes filled PVB layers. The epoxy was selected because of its easy and
solvent free processing nature. If other polymers are coated on PVB, de-wetting and re-
dissolution of the PVB are observed. Additionally, the refractive index of the epoxy is n
= 1.51, which ideally matches the glass flakes (1.52).
The results of transmittance (total and diffuse) and reflectance (total and diffuse)
measurements of the epoxy coated PVB/glass flakes composite are shown in Figure
5.17(a-f). The epoxy adhesive absorbs in the wavelength range of 300-410 nm, which
leads to a significant drop of transmittance in the UV part of the spectrum, but does not
show measurable absorption in the visible part of the spectrum. Total reflection is slightly
enhanced with respect to PVB/glass composites without epoxy layers to around 10 to
12%, which leads to a slight drop of total transmittance to values between 83% and 89%.
The behaviour of the epoxy coated composite films is summarized in Figure 5.18(a-c)
which shows the dependence of the diffuse transmission at 550 nm on glass loading,
before and after epoxy coating onto both sides of the PVB/glass composite films. For the
flakes with A.R = 200 and 400, about 5% reduction in haze is observed for PVC = 5
vol% and 15 vol%, while about 15% reduction is observed for PVC = 25 vol%. For the
flakes with A.R = 2000, about 15%, 10% and 11% reduction in diffuse transmission is
observed for PVC = 5vol%, 15 vol% and 25 vol%, respectively.
RESULTS AND DISCUSSION
90
These observations suggest that at least part of diffuse transmittance is caused by the
surface roughness. However, the additional epoxy layers do not only reduce the diffuse
part of transmittance, but at the same time also reduce total transmission, due to
absorption of UV light and enhanced reflectance at the surfaces of the epoxy layers.
As diffuse transmission is beneficial for solar cells [214] and the reduction of total
transmittance is detrimental, smoothing layers will not be employed for the following
experiments.
a)
300 400 500 600 700 8000
20
40
60
80
100
Diffuse Transmission
5 vol. %
15 vol. %
25 vol. %
Total Transmssion
5 vol. %
15 vol. %
25 vol. %
Tra
nsm
itta
nce
(%
)
Wavelength (nm)
AR ~ 200
b)
300 400 500 600 700 8000
20
40
60
80
100AR ~ 400
Tra
nsm
itta
nce (
%)
Wavelength (nm)
Total Transmssion
5 vol. %
15 vol. %
25 vol. %
Diffuse Transmission
5 vol. %
15 vol. %
25 vol. %
c)
300 400 500 600 700 8000
20
40
60
80
100
Tra
nsm
itta
nce
(%
)
Wavelength (nm)
AR ~ 2000
Total Transmssion
5 vol. %
15 vol. %
25 vol. %
Diffuse Transmission
5 vol. %
15 vol. %
25 vol. %
d)
300 400 500 600 700 8000
20
40
60
80
100AR ~ 2000
Reflecta
nce (
%)
Wavelength (nm)
Total Reflectance
5 vol. %
15 vol. %
25 vol. %
Diffuse Reflectance
5 vol. %
15 vol. %
25 vol. %
e)
300 400 500 600 700 8000
20
40
60
80
100AR ~ 400
Re
fle
cta
nce
(%
)
Wavelength (nm)
Total Reflectance
5 vol. %
15 vol. %
25 vol. %
Diffuse Reflectance
5 vol. %
15 vol. %
25 vol. %
f)
300 400 500 600 700 8000
20
40
60
80
100
Reflecta
nce (
%)
Wavelength (nm)
Total Reflectance
5 vol. %
15 vol. %
25 vol. %
Diffuse Reflectance
5 vol. %
15 vol. %
25 vol. %
AR ~ 2000
Figure 5.17: Transmission and reflectance spectra of PVB containing glass flakes after
coating epoxy on both sides of PVB filled with flakes. a) Total transmission and diffuse
transmission spectra of PVB filled with 5-15 vol% of 200 aspect ratio glass flakes, b)
Total transmission and diffuse transmission spectra of PVB filled with 5-15 vol% of
400 aspect ratio glass flakes, c) Total transmission and diffuse transmission spectra of
PVB filled with 5-15 vol% of 2000 aspect ratio glass flakes, d) Total reflectance and
diffuse reflectance spectra of PVB filled with 5-15 vol% of 200 aspect ratio glass
flakes, e) Total reflectance and diffuse reflectance spectra of PVB filled with 5-15 vol%
of 400 aspect ratio glass and f) Total reflectance and diffuse reflectance spectra of
PVB filled with 5-15 vol% of 2000 aspect ratio glass flakes.
RESULTS AND DISCUSSION
91
a)
0 5 10 15 20 25 300
5
10
15
20
25
30
35
40
45
50
Diffu
sed T
ransm
issio
n (
%)
@ 5
50 n
m
Vol (%)
A.R ~200 Before
A.R ~200 After
b)
0 5 10 15 20 25 300
5
10
15
20
25
30
35
40
45
50
Diffu
sed T
ransm
issio
n (
%)
@ 5
50 n
m
Vol (%)
AR 400 before
AR 400 After
c)
0 5 10 15 20 25 300
5
10
15
20
25
30
35
40
45
50
Diffu
sed T
ransm
issio
n (
%) @
550 n
m
Vol (%)
AR 2000 Before
AR 2000 After
Figure 5.18: Dependence of diffuse transmission of PVB films @ 550 nm on glass flake
loading before (black squares) and after (red dots) coating epoxy on both sides of the
film, a) layers of flakes with α ~200 b) layers of flakes with α ~400, and c) layers of
flakes with α ~2000.
5.1.2.5 Barrier performance of glass flakes
Pristine PVB films (70 µm) show water vapor transmission rates (WVTR) of 65 g.m-2.day-
1 (@40 °C/85% RH).Upon addition of glass flakes, the WVTR values are reduced
significantly, due to the increasing tortuous path length (Tables. 12 -14). The WVTR
decreases with both, increasing α and growing volume fraction of the glass flakes. The
smallest WVTR value of 0.14 g.m-2.day-1, corresponding to a permeability of 3e-3 g.cm.m-
2.day-1, is achieved for glass flakes with α ~2000 at a volume fraction of 25% (Tab. 12).
This corresponds to a reduction of the permeability with respect to the value of pristine
PVB by a factor of 150, which is termed the barrier improvement factor (BIF).
𝐵𝐼𝐹 = 𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑦𝑡(𝑃𝑉𝐵)
𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦(𝑃𝐵𝑉+𝑔𝑙𝑎𝑠𝑠 𝑓𝑙𝑎𝑘𝑒𝑠) Eq. 55
Table 11 show the development of the BIF with increasing volume fraction of the glass
flakes and the comparison of the experimental BIF to Bhardwaj’s theoretical model for
different aspect ratios (200-2000) are shown in Figure 5.19. BIF of 30, 56.2 and 150 have
been achieved for 200, 400 and 2000 aspect ratio (ɑ) glass flakes respectively for the layers
containing 25 vol% of the flakes. The experimental data is almost in close matching with
Bhardwaj’s BIF except for the higher loadings for large aspect ratio flakes. The reason for
this large difference in BIF is defects within the layer filled with flakes of α ~2000 as shown
in Figure 5.20. These defects act as the diffusion path for the permeating gas and increase
RESULTS AND DISCUSSION
92
permeability. The causes of these defect can be either un-dissolved polymer or improper
stacking of the glass flakes within the layer.
Bharadwaj (S=1, a=200)
Experimental (A.R~200)
Bharadwaj (S=1, a=400)
Experimental (A.R~400)
Bharadwaj (S=1, a=2000)
Experimental (A.R~2000)
0 5 10 15 20 250
50
100
150
200
250B
IF o
f co
mp
osite
s
Concentration (vol %)
Figure 5.19: Experimental barrier improvement factor of the barrier (PVB/glass flakes
composites) compared with BIF of composited according to Bhardwaj’s model.
Experimental BIF of flakes with α ~200 (closed black circle), Bhardwaj’s simulated
(dotted black line), experimental BIF of flakes with α ~400 (blue closed triangle) and
Bhardwaj simulated (dotted blue line) and experimental BIF of flakes with α ~2000 (dark
yellow square) and Bhardwaj simulated (dotted dark yellow line). vs the glass flakes
volume concentration in the PVB layers.
Figure 5.20: SEM cross section image of PVB filled with 25 vol% of glass flakes of α
~2000, highlighted area shows the defects within the layer.
RESULTS AND DISCUSSION
93
Table 11: WVTR (@40oC/85 % RH) of PVB films filled with different volume
concentrations of glass flakes of α ~ 200.
G. flakes vol%
(α ~200)
Film thickness
(µm)
WVTR
g.m-2. day-1
Permeability
g.cm.m-2.day-1
BIF
0 70 65 0.45 1
5 150 10 0.1 4.5
10 260 3.5 0.05 9
15 390 1.6 0.03 15
20 210 1.1 0.02 22.5
25 180 0.8 0.015 30
30 212 0.3 0.013 34.6
Table 12: WVTR (@40oC/85 % RH) of PVB films filled with different concentration of glass
flakes with α ~400 aspect ratio glass flakes with different concentrations.
G. flakes vol%
(α ~400)
Film thickness
(µm)
WVTR
g.m-2. day-1
Permeability
g.cm.m-2.day-1
BIF
0 70 65 0.45 1
5 78 9 0.07 6.4
10 140 1.7 0.023 19.5
15 310 0.7 0.021 21.4
20 140 0.9 0.012 37.5
25 240 0.34 0.008 56.2
30 140 0.33 0.004 112
Table 13: WVTR (@40oC/85 % RH) of PVB films filled with different concentrations of
glass flakes with α ~2000.
G. flakes vol%
(α ~2000)
Film thickness
(µm)
WVTR
g.m-2. day-1
Permeability
g.cm.m-2.day-1
BIF
0 70 65 0.45 1
5 78 2.1 0.017 26.4
15 150 0.24 0.0035 128.6
25 160 0.14 0.003 150
RESULTS AND DISCUSSION
94
5.1.2.6 Oxygen permeability
Neat PVB films show OTR values of 110 cm3 m-2 day-1 bar-1. The incorporation of glass
flakes reduces the OTR significantly (Table 14). The PVB film filled with 5 vol%, 15 vol%
and 25 vol% of glass flakes with α = 2000 exhibits OTR values of 4.8 cm3.m-2.day-1.bar-1,
0.45 cm3.m-2.day-1.bar-1, and 0.35 cm3.m-2.day-1.bar-1, respectively. The lowest
permeability of 𝑃 = 7 ∙ 10−3 𝑐𝑚3 ∙ 𝑚𝑚 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 is obtained for 25 vol% ,
which corresponds to a BIF of 140 (Table 14).
Table 14: OTR of PVB films filled with different concentrations of glass flakes with α =
2000.
G. flakes vol%
(α ~2000)
Film
Thickness
(µm)
OTR
cm3.m-2. day-1.bar-1
Permeability
cm3.cm. m-2. day-1.bar-1
BIF
0 90 110 0.98 1
5 80 4.8 0.04 24.8
15 190 0.45 0.008 122
25 210 0.35 0.007 140
0 5 10 15 20 250
20
40
60
80
100
120
140
160
BIF
Vol. Fraction
Oxygen permeability
moisture permeability
Figure 5.21: Barrier improvement factor of the PVB film filled with glass flakes (α ~2000)
against oxygen (red dots) and moisture (Black Square) vs the volume fraction of glass
flakes.
RESULTS AND DISCUSSION
95
The BIF with respect to oxygen permeation shows, within the accuracy limits of the
experiment, the same dependence on the volume fraction of glass flakes as the BIF in the
case of humidity permeation (Figure 5.21). This is in accordance with the tortuous path
model, which assumes that the improvement of the barrier with increasing filler content
does not depend on the nature of the diffusant, as long as it does neither permeate nor
interact with the filler particles. Oxygen permeability can thus also be predicted by the
Bhardwaj’s model.
5.1.2.7 Bendability
Flexibility is one of the key requirements for water and oxygen barriers for flexible OSCs.
Therefore, the barriers based on glass flakes were subjected to bending with a bending
radius of 3 cm (for details see chapter 4). For this purpose, the PVB film containing 15
vol% glass flakes with α = 2000 was selected. The WVTR value of the layer equals 0.24
g.m-2.day-1 before and after 20,000 bending cycles (Figure 5.22). Furthermore, no visible
damage to the film is observable after bending. Obviously, the glass flakes always return
to their initial position after bending, neither loosing adhesion to the PVB matrix nor
damaging it.
0 5000 10000 15000 200000.0
0.2
0.4
WV
TR
(g.m
-2.d
ay
-1)
Bending cycles (No.)
Figure 5.22: WVTR of PVB film with 15 vol% glass flakes (α ~ 2000) vs. number of bending
cycles.
RESULTS AND DISCUSSION
96
5.1.2.8 Encapsulation of organic solar cells
In order to test the barriers based on glass flakes under operational conditions, they are used
for the encapsulation of organic solar cells (OSCs). For this purpose, PVB films filled with
15% v/v glass flakes of α = 2000 are laminated on top of the solar cells (for details see
chapter 4) with a UV curable adhesive. The solar cells thus encapsulated are subsequently
subjected to accelerated lifetime tests, namely damp heat tests and irradiation by a sun
simulator.
5.1.2.9 Photo bleaching of P3HT
J(O2) , P(O2) and OTR of a PVB/glass flakes barrier film of thickness 210 µm to oxygen at
partial pressure p(O2)0.2 bar can be assessed from the rate of photobleaching of a P3HT
film underneath the barrier by using Eq. 52, Eq. 53 and Eq. 54 when the photobleaching
reaction is diffusion controlled.
a)
400 450 500 550 600 650 7000.0
0.2
0.4
0.6
Abs.
Wavelength (nm)
b)
400 450 500 550 600 650 7000.0
0.2
0.4
0.6
0.8
Abs.
Wavelength nm.
c)
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
No
rma
lize
d A
bso
rba
nce
@ 5
25
nm
Time (h)
PVB/Glass flakes
PVB
Figure 5.23: UV–vis spectra of P3HT films on glass encapsulated with a) a PVB layer
and b) a PVB filled with 25 vol% of α ~2000 glass flakes during exposure to the light of
a sun simulator in ambient air at 65 C. c) Normalized absorbance loss at 525 nm of
P3HT films encapsulated with plain PVB, PVB/glass flakes filled barrier.
Using Eq. 52, Eq. 53 and Eq. 54 equations, and inserting the bleaching rate
∆𝐸 ∆𝑡 = 0.03 𝑑𝑎𝑦−1⁄ obtained from Figure 5.23(c) as well as the oxygen partial pressure
difference ∆𝑝(𝑂2) = 0.2 𝑏𝑎𝑟, and assuming the consumption of five moles of molecular
oxygen per mole thiophene rings bleached [6], [79], the OTR value of ~0.5 cm3.m-2.day-
1.bar-1 for PVB/glass flakes layer is obtained, which is in close accordance with the OTR
value of 0.45 𝑐𝑚3 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 measured with the commercial OTR device.
RESULTS AND DISCUSSION
97
5.1.2.10 Lifetime under damp heat
For the damp heat degradation tests, the organic solar cells were laminated with (1) PVB
barrier films and (2) PVB/ glass flakes composite barrier films. Encapsulated solar cells
were placed in a climate chamber with controlled test conditions of 40 oC and 85% relative
humidity (RH). These conditions were chosen to be compatible with those at which the
WVTR were measured. Figure 5.24 provides the degradation data of the two samples
within 168 hours of exposure to damp heat conditions.
0 40 80 120 1600
2
4
6
8
10
12
15 % glass flakes
PVB
Jsc (m
A/c
m2)
Time (h)
0 40 80 120 1600
10
20
30
40
50
60
70
15 % glass flakes
PVB
FF
(%
)
Time (h)
0 40 80 120 1600.0
0.2
0.4
0.6
0.8
15 % glass flakes
PVB
Voc (
V)
Time (h)
0 40 80 120 1600
1
2
3
4
15 % glass flakes
PVB
PC
E (
%)
Time (h)
Figure 5.24: Damp heat degradation test (40 °C/85% RH) of P3HT:PCBM based devices
encapsulated with pristine PVB films and PVB films filled with 15% v/v glass flakes (α
= 2000). Jsc: short circuit current. Voc: open circuit current. FF: fill factor, PCE:.
Power conversion efficiency.
It can be seen from the Figure 5.24 that a dramatic loss of PCE is observed for the device
encapsulated with pristine PVB film. The device loses PCE, fill factor, Jsc and Voc and
thus dies within 24 hours. This effect is known to occur upon ingress of water into the
packaging and a subsequent damage to the interface between the active layer and the
PEDOT:PSS hole extraction layer. In contrast to the sample encapsulated with pristine PVB
RESULTS AND DISCUSSION
98
film, the device encapsulated with the PVB/glass flakes composite film does not show any
degradation during the testing period. According to its WVTR value of 0.24 g.m-2.day-1,
the barrier film has only transmitted 0.0001 g.m-2 of water into the device during the
exposure period, which corresponds to a film of liquid water of around <0.1 µm in
thickness. This is much less than the capacity of the adhesive film used for lamination of
~5 µm and thus not sufficient to damage the active layer/PEDOT:PSS interface [215].
5.1.2.11 Lifetime under irradiation by sun simulator
For the sun degradation test, OSCs encapsulated with PVB / glass flakes composite films
and, for reference purposes with pristine PVB films as well as with commercial Mitsubishi
films were subjected to the irradiation by a sun simulator (1000 W m-² @ 65 °C black body
temperature). Figure 5.25 shows the photovoltaic key parameters (Jsc, FF, Voc and PCE) of
the encapsulated devices subjected to sun test. Devices with Mitsubishi barrier show no
degradation at all after 160 hours of irradiation, which confirms that P3HT:PCBM based
solar cells do not degrade due to thermally induced processes, e.g., morphology changes,
at 65 °C . The slight increase in PCE is due to the increase in Jsc which is caused by the
post annealing effect at the elevated temperature in the sun tester [92], [216], [217]. The
device encapsulated with pristine PVB film, degrades rapidly in both Jsc and FF, and thus
reaches 80% of its initial PCE within less than 20 hours, which infers that the PVB is not a
good barrier against oxygen and hence loses PCE. The device encapsulated with the
PVB/glass flakes composite film has degraded to 80 % of its initial PCE only after 160
hours of the test. The loss in PCE is caused by loss of Jsc, whereas fill factor and Voc remain
stable. This indicates, that degradation is due to the diffusion of oxygen through the barrier
and its subsequent photo-induced reaction with the active layer [6], [218]. This is supported
by the fact that the amount of oxygen which has diffuse through the barrier within 160
hours at 65 °C is sufficient to photo-oxidize about 1% of the thiophene rings in the active
layer, which is the damage at which P3HT:PCBM cells have been reported to loose around
20% of their initial performance [218].
Further support comes from the observation that no degradation in damp heat is observed
(see Figure 5.25). This can be explained by the effect the effect of temperature as in damp
heat conditions the temperature is only 40 °C, i.e., the diffusion rate of oxygen is only a
fifth to a tenth of that in the sun soaker and hence devices degrade faster.
RESULTS AND DISCUSSION
99
0 40 80 120 1600
2
4
6
8
10
12
Mitsubishi
15 vol% (2000 GF)
PVB
Jsc (m
A/c
m2)
Time (h)
0 40 80 120 1600
10
20
30
40
50
60
70
Mitsubishi
15 % glass flakes
PVB
FF
(%
)
Time (h)
0 40 80 120 1600.0
0.2
0.4
0.6
0.8
Voc (
V)
Time (h)
Mitsubishi
15 vol% (2000 GF)
PVB
0 40 80 120 1600
1
2
3
4
Mitsubishi
15 vol% (2000 GF)
PVB
PC
E (
%)
Time (h)
Figure 5.25: Sun degradation test of P3HT:PCBM based solar cells encapsulated in
three different barriers: Mitsubishi barrier film, pristine PVB film, and PVB film filled
with 15% v/v glass flakes (α = 2000). Jsc: short circuit current. Voc: open circuit current.
FF: fill factor, PCE: power conversion efficiency.
5.1.2.12 Conclusion
For the first time, transparent and flexible barriers for organic electronic devices have been
prepared on the basis of glass flakes. WVTR values of as little as 0.14 g.m-2.day-1 have
been achieved, by maximizing the aspect ratio and by aligning the glass flakes with the film
surface, thus maximizing the order parameter of the Bhardwaj equation. It has been shown
that the permeation depends on the volume content of the glass flakes. At volume fractions
of ~15%, the alignment of the flakes is almost perfect. At higher volume fractions of around
25%, the alignment is almost perfect but the layers contain defects. BIF values of around
140 have been achieved against both moisture and oxygen with ~2000 aspect ratio glass
flakes. WVTR is improved by the factor of 150, while the layers maintained the total
transmission of around 90% along with slight increase in the diffuse transmission.
RESULTS AND DISCUSSION
100
The bending results show that the barrier quality remains unchanged even after 20000
bending cycles, which proves the good adhesion of the PVB matrix to the glass flakes.
Application of such barriers as an encapsulation for the OSC also resulted in increase of
the lifetime of the devices. In the case of the sun irradiation test, the lifetime of OSCs was
extended from few hours to beyond 160 h, whereas in the case of damp heat tests, almost
no degradation is observed. Hence, based on these results, barrier layers comprising glass
flakes as fillers in polymeric matrix have a high potential to be used as the encapsulation
for the thin film organic solar cells.
RESULTS AND DISCUSSION
101
5.2 Polysilazane
Perhydropolysilazane (PHPS) is the trade name of a solution of perhydropolysilazane in
di-n-butyl ether. PHPS is an inorganic polymer composed of Si-N, Si-H and N-H bonds.
Proper curing of PHPS leads to formation of silica. Generally SiO2 films deposited from
vacuum assisted techniques show low co-efficient of diffusion of gas molecules. In this
work, PHPS is used as an alternative to get SiO2 networks processed from solution avoiding
expensive techniques. Therefore, processing of PHPS needs optimization before it can be
used as an effective coated barrier for OSCs. Therefore, in this chapter development and
optimization of an in-line encapsulation method for printed electronics based on the
deposition of the barrier material PHPS directly on the device from solution and subsequent
curing by VUV irradiation are discussed. In a first step, we will describe how the properties
of PHPS-based barriers on top of PET films can be optimized with respect to WVTR- and
OTR-values, flexibility, and processing speed. The resulting barrier films are subsequently
tested as encapsulation materials for organic solar cells. Following the design rules thus
developed, PHPS/polymer sandwich layers are then coated directly on P3HT:PCBM based
solar cells by roll-to-roll compatible methods. The protective effect of the directly coated
barrier on the solar cells is finally investigated in damp heat and under irradiation by a sun
simulator.
The successful demonstration of direct coating of PHPS/polymer sandwich layers on top
of organic devices does not only enable higher throughput and lower material consumption
in the roll-to-roll production of printed electronics, it also opens the way for printing
organic electronics onto 3D objects, which cannot be protected otherwise, because
encapsulation by lamination of barrier films is not possible in this case.
5.2.1 Optimizing the curing method for PHPS
In order to make PHPS compatible to be used as the inline encapsulation for OSCs, the
PHPS curing mechanism needs to be optimized. Hence, PHPS films are irradiated with
deep UV (~172 nm peak wavelength), as this method of curing is reported frequently in the
literature [164], [165], [169], [183]. Therefore, for this purpose, PHPS films coated on PET
substrates are optimized with respect to irradiation distance, composition of the ambient
atmosphere and temperature during VUV irradiation. The conversion mechanism of the
PHPS film coated on polyethylene substrate is monitored by recording FTIR spectra. Figure
5.26 shows IR spectra of PHPS cured at different irradiation distances between sample
RESULTS AND DISCUSSION
102
surface and lamp. PHPS was cured at three distances, namely 100 mm, 30 mm and 5 mm.
All the layers had the same thickness (~ 500 nm) and were exposed to deep UV light for
the same amount of time (~25 minutes). The characteristic peaks in un-cured films appear
at 3400 cm-1 (N-H), 2150 cm-1 (Si-H), and 850 cm-1 (Si-N-Si). In fully cured films, all of
these peaks have disappeared, while additional peaks at 450 cm-1 (Si-O-Si) and 1050 cm-1
(Si-O-Si) appear. The curve representing 100 mm distance in Figure 5.26 shows peaks at
3400 cm-1, 2150 cm-1, 1050 cm-1, 850 cm-1 and 450 cm-1 which means, the uncured PHPS
still exists in the film, because the peak intensity at 850 cm-1 (Si-N-Si) is still higher than
the peak at 1050 cm-1, while in the curve representing 30 mm distance, the peaks
representing NH and SiH bonds at 3400 cm-1 and 2150 cm-1, respectively, are almost gone.
However, the peak at 850 cm-1 is still larger than that at 1050 cm-1, which infers that the
transformation is still incomplete and Si-N-Si still exists in the coating. The spectrum
representing 5 mm distance shows almost no NH and SiH peaks and the peak at 1050 cm-1
is dominant over the 850 cm-1, which suggest that PHPS has been transformed more or less
completely into SiO2 network as the IR spectra does not show any existence of Si-N-Si, Si-
H and NH bonds. Therefore, it can be concluded from the results that the curing with VUV
at smaller distances cures PHPS faster. This is due to the fact that the intensity of the deep
UV increases and forms more and more oxygen radicals near the sample surface that can
react and diffuse through the PHPS film and form SiO2 network. The conversion of PHPS
to SiO is not as significant in 30 mm and 100 mm cured layers compared with 5 mm, hence
the distance of 5 mm is most suitable for curing PHPS.
RESULTS AND DISCUSSION
103
8001600240032004000
5 mm distance
30 mm distance
Wave number (cm-1)
Abro
bance
As deposited
100 mm distance
Figure 5.26:FTIR spectra of a 500 nm thick PHPS film coated on PE substrate cured with
deep UV light for ~25 minutes with irradiation distance of 100 mm (red curve), 30 mm
(blue curve) and 5 mm (purple curve).
5.2.2 Curing by the combination of heat and deep UV at distance of 5 mm:
The combination of curing methods, i.e. of thermal and deep UV curing, at the distance of
5 mm is the most suitable way for curing PHPS as shown in Figure 5.27. This is because
the oxygen radicals generated by deep UV diffuse faster to and within the PHPS layer. Also
the reaction itself is accelerated by increasing temperature. As a result, for thin PHPS films
(< 500 nm) the curing is achieved in ~15 minutes.
RESULTS AND DISCUSSION
104
8001600240032004000
Un-cured PHPS
100 oC + VUV
Wavenumber (cm-1)
Abso
rbance
Figure 5.27: : FTIR spectra of PHPS film (500 nm thick) cured with the combination of
heat (100oC) and irradiation with 172 nm wavelength light at a distance of 5 mm for 15
minutes.
5.2.3 Correlation of WVTR with Infrared peak ratios
The conversion of PHPS films to silica has been reported by exposure to either damp heat
or deep UV (172 nm) irradiation [161], [162], [164], [174], [177]. According to Prager et
al.[164], the faster of the two methods is curing PHPS by deep ultraviolet irradiation, which
can be further accelerated at increased temperature [169]. This makes curing by deep UV
the most favorable candidate for roll-to-roll printing of barrier layers. In order to ensure
reproducible quality of PHPS-based barriers even under varying conditions, a quantitative
endpoint control of the curing process in the roll-to-roll production line is required. Precise
endpoint control also allows minimizing the photon dose used for curing PHPS, which
reduces the photo-damage to the active materials when coating barriers directly on devices.
Therefore, we establish a quantitative correlation between the achieved values of water
vapor transmission rates (WVTR) of the resulting barriers and the peak ratios of infrared
vibrations related to the conversion of PHPS to silica. Using peak ratios instead of
RESULTS AND DISCUSSION
105
intensities of single peaks offers several advantages, e.g., the independence of equipment
characteristics and of absolute film thicknesses. To this end, 500 nm thick PHPS layers are
coated on poly(ethyleneterephthalate) (PET) and polyethylene (PE) substrates for moisture
permeation and Fourier Transform Infrared (FTIR) spectroscopic analysis, respectively,
and irradiated subsequently by deep UV light (wavelength of ~172 nm) at a distance of
around 5 mm at room temperature conditions. The temporal development of the FTIR
spectra during the conversion of PHPS is shown in Figure 5.28 a. In agreement with
literature, the peaks corresponding to the N-H (3400 cm-1), Si-H (2150 cm-1), and Si-N (830
cm-1) stretching vibrations decrease within few minutes of curing. Concomitantly, the peaks
assigned to the Si-O bending and stretching vibrations near 450 cm-1 and 1050 cm-1,
respectively, increase. No significant signal at 3200 cm-1 is observed, indicating a
negligible concentration of OH-groups in the final film. The ratio of the peaks at 830 cm-1
and 1050 cm-1 is plotted in Figure 5.28 b. This peak ratio, I(1050 cm-1)/I(830 cm-1), is the
most appropriate one for the quantification of the reaction progress, whereas the peaks at
450 cm-1 and 3400 cm-1 are less appropriate for quantitative evaluation, due to their low
intensities. The peak ratios suggest that after 16 minutes of irradiation most of the
polysilazane has been transformed to silica. As the curing continues, the conversion process
further proceeds, however, at a much slower rate. Figure 5.28 b demonstrates nicely that
with progressing conversion of polysilazane to SiO2, i.e., with increasing peak ratios, the
barrier quality in terms of WVTR values improves continuously. The closest correlation of
WVTR values and peak ratios is obtained for the peak ratio 1050/830, which indicates full
conversion at a ratio of around 2.2, corresponding to WVTR values of around 0.05 g m-2
day-1 for 500 nm thick films (equivalent to a permeability of 2.5 x10-6 g cm m2 day-1, in
accordance with literature) [183]. FTIR peak ratios thus represent a precise and reliable
tool for determining PHPS barrier qualities, thus avoiding time consuming water vapor
transmission measurements.
RESULTS AND DISCUSSION
106
a)
b)
Figure 5.28. a) FTIR spectra of a 500 nm thick PHPS film, cured by deep UV light for the
times specified in the figure. The corresponding WVTR values are given next to the spectra.
b) FTIR peak ratios (1050 cm-1/(830 cm-1, open circles) of Figure 5.28 (a), correlated with
the corresponding WVTR values (full squares) at different curing times (Published in
reference [184] and reproduced with permission from John Wiley and Sons).
40080012001600200024002800320036004000
40080012001600200024002800320036004000
Abs
orba
nce
2 min
Wave number (cm-1)
0.06 g/m2.day16 min
0.1 g/m2.day10 min
0.8 g/m2.day6 min
2.1 g/m2.day4 min
3.5 g/m2.day
4 g/m2.dayun-cured PHPS
20 min 0.05 g/m2.day
0 5 10 15 200.0
0.5
1.0
1.5
2.0
2.5 1050/830
1/WVTR
Irradiation time (min)
Peak r
atio
0
5
10
15
20
1/W
VT
R (
m2
. day/g
)
RESULTS AND DISCUSSION
107
5.2.4 Correlation of WVTR with IR peak (damp heat)
As mentioned in the previous section about correlation of WVTR and PHPS curing with deep
UV and a nice relationship is obtained which can suggest the WVTR properties of the cured
films. Similarly we tried to develop correleationship of WVTR of the films cured with damp
heat conditions. To develop such a relationship, the PHPS films with thickness of ~800 nm
were exposed to damp heat (65oC, 85%RH) and subsequently monitored with IR and
measuring WVTR (The time during the WVTR measurement (@40oC, 90%RH) is
excluded). FTIR spectra of coating of PHPS and their corresponding WVTR are shown in
Figure 5.29 a,b. The results show that the conversion of PHPS proceeds relatively slower
under these conditions. The characteristic peaks in un-cured film are 3400 cm-1 (N-H), 2170
cm-1 (Si-H), and 830 cm-1 (Si-N-Si). The peaks near 3400 cm-1 (N-H) and 2170 cm-1 (Si-H)
are almost gone after 60 minutes, at the same time, absorption peaks based on a siloxane
bond (-Si-O-Si-) near 450 cm-1 and 1050 cm-1 appeared and increased. These peaks indicate
the formation of SiO2. The spectrum for the exposure time of 60 minutes showed absorption
peaks based on Polysilazane 830 cm-1 (Si-N-Si), indicating that Polysilazane existed and that
the transformation to SiO2 was still incomplete. With increasing exposure time (120 – 180
minutes), the polysilazane-based peaks are reduced, indicating the conversion progressed.
Un-transformed PHPS exhibits poor barrier properties, as can be seen from the WVTR value
of as deposited layer (4 g/m2.day), which corresponds to the permeation value for the PET
substrate. As the exposure time to damp heat increases, the curing continues and polysilazane
transformation proceeds to completion. The trend of conversion of the PHPS is nearly as
same as that of deep UV cured but the barrier behavior is slightly different. However, the
shape of the spectra is slightly different as compared to deep UV with much more sharp peak
at 1050 cm-1 (Figure 5.29a). In the case of damp heat, lowest permeation is obtained with
exposure time of 300 minutes which is 0.09 g/m2.day which corresponds to the peak ratio,
I(1050 cm-1)/I(830 cm-1) of 4.5, which is higher as compared to deep UV curing. With this
higher peak ratio, the corresponding WVTR values for damp heat cured films are clearly
inferior. The possible reason for this can be the interstitial site generated by the diffuse water
while exposure of the damp heat conditions, that served as the permeation paths for the water
molecules. Based on these results, curing of PHPS with deep UV is much better not only in
terms of barrier quality but also in curing time.
RESULTS AND DISCUSSION
108
a)
40080012001600200024002800320036004000
0.09 g/m2.day
0.2 g/m2.day
0.5 g/m2.day
1.6 g/m2.day
2.2 g/m2.day
300 min
200 min
120 min
60 min
Wave number (cm-1)
Abs
orba
nce
un-cured PHPS
30 min
4 g/m2.day
b)
0 50 100 150 200 250 3000
1
2
3
4
5 1050/830
1/WVTR
Curing time (min)
Peak r
atio
0
2
4
6
8
10
12
1/W
VT
R (
m2.d
ay.g
-1)
Figure 5.29: a) FTIR spectra of a 800 nm thick PHPS film, cured by an exposure to damp
heat @ 65oC/85% RH for the times specified in the figure. The corresponding WVTR values
are given next to the spectra. b) FTIR peak ratios (1050 cm-1/(830 cm-1, open circles) of
Figure 5.29a, correlated with the corresponding WVTR values (full squares) at different
curing times.
RESULTS AND DISCUSSION
109
5.2.5 Optimization of the PHPS conversion rate
PHPS (20 wt%) in di-butyl ether when coated by blading at a gap of 50 µm and speed of
10 mm. sec-1, yields a relatively thick films of about 2-3 µm. At this thickness, curing of
PHPS takes relatively long. Therefore, to get optimum thickness, for which the curing time
matches with the roll-to-roll processing, different parameters were varied and the results
are shown in Table 15. An amount of 70 µl of PHPS was coated on PET substrate at
different coating speeds. The blade gap was always maintained at 50 µm. It is observed that
the curing time depends on the final thickness of the film. The thicker the films, the longer
it will take to cure them (the time mentioned in the table is the time when Si-H peak has
vanished completely in the FTIR spectra).
Table 15: Parameters for coating 70 µl PHPS on PET substrate
Coating speed
(mm.s-1)
Dilution
(ratio)
Wet layer
thickness
(µm)
Final
thickness
(nm)
Curing method Curing
time
(min)
1 mm/s 1:6 50 ~ 70 DUV+Temp 1-2
1 mm/s 1:5 50 ~ 100 DUV+Temp 2-3
1 mm/s 1:1 50 ~ 500 DUV+Temp ~10-20
5 mm/s 1:1 50 ~ 700 DUV+Temp ~25-35
10 mm/s 1:1 50 ~ 1200 DUV+Temp >50
15 mm/s 1:1 50 ~ 1500 DUV+Temp >60
20 mm/s 1:1 50 ~ 1600 DUV+Temp >80
30 mm/s 1:1 50 ~ 2500 DUV+Temp >100
As the PHPS layers get thicker, they need more time to be cured completely, due to the low
diffusion rate of the UV-generated oxygen atoms through the upper parts of the PHPS films,
especially after conversion to SiO2 [21]. However, in roll-to-roll coating of barrier films
minimizing the time for curing PHPS is essential. Therefore, we attempt to reduce the
curing time by creating stacks of ultrathin layers. Figure 5.30 shows the FTIR spectra of
two different 400 nm thick PHPS layers which were both cured for a total of 12 min. The
first film (red data) was coated in a single step and subsequently cured by deep UV light
for 12 min. The second film (black data) represents a stack of four subsequently coated thin
PHPS layers on top of each other, each individual layer being 100 nm thick and cured for
three minutes before coating the following one. After 12 min of curing, the film prepared
in a single curing step still contains unreacted PHPS, as obvious from the peak ratio of
RESULTS AND DISCUSSION
110
I(1050 cm-1)/I(830 cm-1) ≈ 1.5. In contrast, the 4-layer stack displays a peak ratio of I(1050
cm-1)/I(830 cm-1) ≈ 2, which indicates that the layer is almost completely transformed.
The values of WVTR of the single layer film and the multilayer stack after 12 min of curing
are 0.15 g m-2 day-1 and < 0.02 g m-2 day-1 (the limit of our measurement setup),
respectively, in accordance with the observed FTIR peak ratios. After 20 min of curing,
also the single layer film reaches an FTIR peak ratio of I(1050 cm-1)/I(830 cm-1) ≈ 2 and a
WVTR of < 0.02 g m-2day-1, demonstrating that in principle also thick PHPS layers can be
fully converted, albeit at much longer reaction times, due to the much lower diffusion
coefficients of the UV generated oxygen atoms at later stages [165].
Figure 5.30. FTIR spectra of a 400 nm thick PHPS film (red curve) and of a stack of four
100 nm thick PHPS layers (black curve), both cured with deep UV light for a total of 12
minutes. (Published in reference [184] and reproduced with permission from John Wiley
and Sons).
40080012001600200024002800320036004000
0.15 g.m-2.day
-1
Wave number (cm-1)
1 thick coat
4 thin coats
<0.02 g.m-2.day
-1
~100 nm ~100 nm
~100 nm
~ 400 nm
~100 nm
RESULTS AND DISCUSSION
111
5.2.6 Hydrophobic nature of the PHPS film:
PHPS as coated on PET substrate in uncured form shows a slightly hydrophobic nature wih
the contact angle of 92.8o but somehow looses it hydrophobic nature when cured with deep
UV but still maintains slightl hydrophocity 88.3o (Figure 5.31). This hydrophic nature of
the PHPS is also favourable for its application as encapsulation of OSCs.
a)
c)
b)
d)
Figure 5.31: Hydrophobic nature of the PHPS, a) droplets of water on un-cured PHPS
surface, b) contact angle (C.A) of 92.8o on un-cured PHPS, c) C.A of 88.3o of PHPS after
curing and d) contact angle of PET substrate 80o.
5.2.7 Flexibility / bendability of PHPS-based barriers
For the encapsulation of flexible devices, barrier materials also need to be flexible without
being damaged. However, due to their chemical nature, fully cured PHPS films are brittle.
Thus, increasing the thickness of the PHPS layers, on one hand, reduces the initial WVTR,
but on the other hand leads to rapid damage of the films in bending tests. Figure 5.33 shows
the comparison of the WVTR values of PHPS layers of different thicknesses with
increasing number of bending cycles. Films with thicknesses of 270 nm gradually lose their
barrier properties, which after 150 bending cycles finally leads to worse values than for
PHPS (Before curing)
C.A = 92.8o
C.A = 80o
PHPS (after curing)
C.A = 88.3o
PET
RESULTS AND DISCUSSION
112
films of only 170 nm. Calcium tests demonstrate that this is due to tensile cracks which
form perpendicularly to the curvature of the substrate (Figure 5.32).
Figure 5.32: Optical calcium test of PHPS barrier (270 nm) on PET film (d = 125 µm)
after 50 bending cycles (bending radius of 3 cm). The PET/PHPS barrier film has been
laminated (using UV curable adhesive) on a calcium film of 200 nm in thickness,
evaporated on a glass slide. Testing conditions: 65oC / 85 %RH. Images taken after a) 1 h,
b) 2 h, c) 5 h, d) 8 h and e) 10 h (Published in reference [184] and reproduced with
permission from John Wiley and Sons).
Figure 5.33. Reciprocal WVTR vs. number of bending cycles for stacks of different
numbers of PHPS layers on PET film ( d = 125 µm) at a bending radius of 3 cm
(Published in reference [184] and reproduced with permission from John Wiley and
Sons)..
0 25 50 75 100 125 150
0
1
2
3
4
5
6
1/W
VT
R (
m2.d
ay/g
)
Bending cycles (No.)
70 nm
170 nm
270 nm
un-coated PET
RESULTS AND DISCUSSION
113
It has been shown before that the barrier properties of PHPS films can be enhanced by
building multilayer stacks of alternating layers of PHPS and organic polymers [21], [182].
In order to make use of this effect for enhancing the bendability of the barrier films, we
deposited multilayer stacks of two thin PHPS layers (ca. 170 nm) alternating with two
polymer films of around 5 – 10 µm thickness on PET substrates (see inset of Figure 5.36).
As obvious from Figure 5.33, cured PHPS films of ~170 nm in thickness should hardly be
affected by bending. For the deposition of the organic interlayers, different polymer
formulations were tested such as a) solutions of the polymers polyvinyl alcohol (PVA),
polyvinylidene fluoride (PVDF), and polyvinyl butyral (PVB) as well as b) solvent-free
epoxy and acrylic adhesives. The adhesives were investigated because of their enhanced
adhesion to the PHPS films with respect to the non-crosslinked polymer films, in order to
explore the possibility of reduced delamination during the bending tests. In addition, the
acrylic adhesive formulation acts as a UV filter (Figure 5.34), which otherwise would have
to be added as an extra layer to the packaging of devices. The resulting initial WVTR values
are 0.14 g m-2day-1, 0.16 g m-2day-1, 0.12 g m-2day-1, 0.10 g m-2day-1and 0.11 g m-2day-1 for
PVA, PVB, PVDF, epoxy adhesive and acrylic adhesive, respectively. The oxygen
transmission rates (OTR) of these multilayer stacks are below the detection limit of our
setup of 0.01 cm3 m-2 day-1 bar-1. It is interesting to note that the WVTR of two 170 nm
thick PHPS films on top of each other, i.e., without an intermediate polymer layer, is about
five times higher, i.e., 0.6 g m-2 d-1 than in the case with the polymer interlayer. This
difference cannot be explained by the additional barrier effect of the polymer layers, as
these showed no measurable reduction of the WVTR when coated on top of PET films,
which is also in accordance with the observation that the WVTR values of the multilayer
stacks are approximately the same for all polymer interlayers. The beneficial effect of the
intermediate polymer layer is either caused by the decoupling of defects in the two PHPS
layers, or, more probably, by their planarization effect, which enables a more defect free
growth of the top PHPS layer [19].
Bending tests were performed on PHPS/interlayer sandwich barriers deposited on PET
substrate with overall sample thickness 140d m. The details of the failure mechanism
is described in Channa et al.[184].
RESULTS AND DISCUSSION
114
Figure 5.34: Ttransmission spectra of PET film and of PHPS/Acrylic/PHPS barrier coated
on PET film
Two types of behavior were observed (Figure 5.36). For all polymer interlayers deposited
from solution and for the UV cured acrylic adhesive, the initial WVTR values remain
almost constant during the first 1000 bending cycles and increase by only around 20% even
after 3000 cycles.
The same behavior is also observed for non-crosslinked polymers (PVDF and PVB)
interlayers, where the barrier films remain stable for two hundred bending cycles (Figure
5.35). In contrast, for the epoxy based adhesive, an increase of the WVTR by 60% after
3000 bending cycles is observed.
400 600 800
0
20
40
60
80
100T
ransm
issio
n (
%)
Wavelength (nm)
PET
PHPS/Acrylic/PHPS on PET
RESULTS AND DISCUSSION
115
Figure 5.35: Normalized reciprocal WVTR of sandwich coatings with alternating PHPS
and organic layers (see inset) vs. the number of bending cycles with a bending radius of 3
cm. Each PHPS layer has a thickness of ~170 nm, while the organic layers are ~ 5-10 um
thick. For the organic interlayers, two non-cross linked polymers, namely PVDF (black
squares) and PVB (red circles) were used. (Published in reference [184] and reproduced
with permission from John Wiley and Sons).
We interpret the observed behavior in terms of internal imperfections within the layers,
which may propagate to form tensile cracks that deteriorate the WVTR. The critical size of
such flaws can be estimated from fracture mechanics considerations which relate tensile
stress , critical flaw size a , and fracture toughness IcK . We use the relation with
Ic 0.30K = MPa m1/2 for the fatigue threshold of silica [219], [220]. With a characteristic
fracture process zone size 0 200a nm as deduced from fracture surface observations on
silica [221], we arrive at a characteristic critical flaw size a = 0.9 μm for a characteristic
tensile stress of 160 MPa. Using the idea that the characteristic size of fabrication induced
flaws is of the order of the layer thickness (i.e., thinner layers contain smaller flaws and are
RESULTS AND DISCUSSION
116
therefore mechanically more stable) this explains why single layers with thicknesses well
below 1 μm become flaw insensitive as seen in Figure 5.33.
Turning to multilayer structures, the main effect of the interlayers, which are both
elastically softer and much thicker than the PHPS layers, consists in mechanically
decoupling the cured PHPS layers. The PHPS layers now essentially deform in parallel and
flaws in different layers do not appreciably interact. Accordingly, the flaw insensitivity of
the single layers shown in Figure 5.33 transfers to the multilayer architecture and the films
maintain their barrier quality during mechanical cycling. The exception are epoxy
interlayers, which because of the brittle nature of the epoxy may themselves be prone to
fracture during cyclic loading and then induce failure of the PHPS barrier layers.
0 500 1000 1500 2000 2500 30000
2
4
6
8
10
1/W
VT
R (
m2day
1/g
)
Bending cycles (No.)
Epoxy adhesive
Acrylic adhesive
PHPSorganic layer
PHPS
organic layer
Figure 5.36: Reciprocal WVTR of sandwich coatings with alternating PHPS and organic
layers (see inset) vs. the number of bending cycles with a bending radius of 3 cm. Each
PHPS layer has a thickness of ~170 nm, while the organic layers are ~ 5-10 µm thick. For
the organic interlayers, two UV-curable adhesives on epoxy (black squares) and acrylic
basis (red circles) were used (Published in reference [184] and reproduced with
permission from John Wiley and Sons).
RESULTS AND DISCUSSION
117
5.2.8 Protection of organic electronic devices by PHPS-based barriers
In order to assess the quality of the barriers described above under real conditions, the
protection of organic solar cells by PHPS/polymer/PHPS sandwich layers on PET against
photo-oxidation and damp heat was evaluated.
First, the protection of P3HT films against photo-oxidation by encapsulation in
PHPS/polymer/PHPS sandwich films was measured and compared with commercially
available barriers. For this purpose, P3HT was coated on glass substrates and subsequently
encapsulated with different barrier foils under nitrogen atmosphere. P3HT was chosen as
the probe material because of its well-characterized behavior towards photo-oxidation[85].
For encapsulation, a PHPS-based barrier film on PET (PET/170 nm PHPS/5 µm acrylic
adhesive/170 nm PHPS/), a glass slide, a Mitsubishi barrier film, or a plain PET film were
rim-encapsulated on top of the glass substrate using a UV-cured epoxy glue in a way that
the adhesive did not cover the P3HT layer (Figure 5.37).
Figure 5.37: Schematic view of encapsulation of P3HT films on glass, rim encapsulated
with PHPS based sandwich barrier on PET film. (Published in reference [184] and
reproduced with permission from John Wiley and Sons).
The samples were illuminated in ambient atmosphere with the light of a sun simulator from
the glass substrate side. The rate of photo-oxidation was quantified by recording UV/Vis
spectra at different time intervals and analyzing the loss of absorbance of the P3HT film at
the wavelength of 525 nm (Figure 5.38).
Adhesive
Glass
AdhesiveP3HT
PET
PHPS
Acrylic
PHPS
RESULTS AND DISCUSSION
118
a)
b)
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8P3HT Encapsulated with
PHPS/Acrylic/PHPS
Abs.
Wavelength (nm)
c)
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
Abs.
Wavelength (nm)
Encapsulated with Mitsubishi
d)
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8Encapsulated with glass
Abs.
Wavelength (nm) e)
PET
Mitsubishi
Glass
PHPS/Acrylic/PHPS
0 300 600 900
0.2
0.4
0.6
0.8
1.0
No
rma
lize
d A
bso
rba
nce
@ 5
25
nm
Time (h)
Figure 5.38: UV/vis spectra of P3HT films on glass encapsulated with a) a plain PET,
b) a PHPS based barrier, c) Mitsubishi ( a commercial barrier) and d) glass during
exposure to the light of a sun simulator in ambient air at 65oC. The increase of absorption
at 400 nm in (b) and (c) is due to the yellowing of the PET substrates. e) Normalized
absorbance loss at 525 nm of P3HT films encapsulated with plain PET, PHPS based
barrier, Mitisubishi and glass on glass. (Published in reference [184] and reproduced
with permission from John Wiley and Sons).
400 450 500 550 600 650 7000.0
0.2
0.4
0.6
0.8A
bs.
Wavelength (nm)
Sun irradiation
T= 504 h
P3HT encapsulated with PET
RESULTS AND DISCUSSION
119
As can be seen from Figure 5.38, that the P3HT film encapsulated with plain PET lost 60%
of its initial optical density (OD) within the first 600 h, as expected from the poor barrier
properties of PET. Samples encapsulated with glass slides or Mitsubishi barrier foils did
not show any degradation over the duration of the experiment of 960 hours. P3HT protected
by the PHPS based barrier lost less than 1% of its initial optical density during the time
span of 960 h. From the bleaching kinetics, the oxygen permeability of the barriers can be
estimated from equations Eq. 52, Eq. 53 and Eq. 54[90], [222]. Assuming the consumption
of five moles of molecular oxygen per mole thiophene rings bleached, this drop in
absorbance corresponds to OTR values of ≈ 8 cm3 m-2 day-1 bar-1 for the plain PET film
and of less than 0.1 cm3 m-2 day-1 bar-1 for the PHPS/polymer/PHPS sandwich film, which
is in accordance with the measurements performed with the commercial OTR device.
Damp heat tests of the PHPS/polymer/PHPS sandwich barrier are conducted on
encapsulated P3HT:PCBM based organic solar cells (OSCs) of inverted architecture,
whose degradation behavior is the most studied one of all organic solar cells and thus well
understood [84]. Standard glass substrates, each having six P3HT:PCBM-based solar cells
with effective areas of 0.1 cm2 each (as discussed in Experimental section Figure 4.1), were
encapsulated using an adhesive and three different barrier foils, namely plain PET,
PHPS/acrylic/PHPS/acrylic films on PET, and a Mitsubishi barrier foil as shown in Figure
5.39.
Figure 5.39: Schematic view of encapsulation of P3HT:PCBM based solar cell on glass,
rim encapsulated with PHPS based sandwich barrier on PET film.
Glass
Organic solar cell
PHPSAcrylicPHPS
Acrylic
Adhesive Adhesive
PET
RESULTS AND DISCUSSION
120
After lamination, the devices showed efficiencies around 3%. They were placed in a damp
heat chamber at 40oC/85% RH. For measuring the current density vs. voltage
characteristics, the samples were taken out of the damp heat chamber and placed under a
sun simulator in ambient atmosphere. As shown in Figure 5.40, the devices encapsulated
with plain PET died within less than 50 hours of exposure to damp heat due to the ingress
of water and oxygen. On the other hand, the devices encapsulated with PHPS-based barrier
foils and Mitsubishi barrier foils did not show any significant degradation and remained
stable for the period of testing.
a) b)
0 50 100 150 200 250 3000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
PC
E (
%)
Time (h)
PHPS
Mitsubishi
PET
0 50 100 150 200 250 3000
10
20
30
40
50
60
70
FF
(%
)
Time (h)
PHPS
Mitsubishi
PET
c) d)
PHPS
Mitsubishi
PET
0 50 100 150 200 250 3000
2
4
6
8
10
Jsc (m
A/c
m2)
Time (h)
PHPS
Mitsubishi
PET
0 50 100 150 200 250 3000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voc (
V)
Time (h)
Figure 5.40: Efficiency of P3HT:PCBM based organic solar cells encapsulated with plain
PET foils (PET), commercial Mitsubishi barrier foils (Mitsubishi), or PHPS-based barrier
foils (PHPS, for details see text) upon exposure to damp heat (40 °C/85% RH). (Published
in reference [184] and reproduced with permission from John Wiley and Sons).
RESULTS AND DISCUSSION
121
5.2.9 Encapsulation of OSCs by direct deposition of PHPS
After the optimization of PHPS-based barrier layers on PET foils and their successful
testing as encapsulation for OSC devices, PHPS-based barrier layers with a thickness of
170 nm were coated directly on P3HT:PCBM-based solar cells. Two different top electrode
thickness have been used i.e. 50 nm and 100 nm. This is just to avoid the diffusion of PHPS
solvent and possible harm to OSC by deep UV. One, two and three layers on top of each
other were coated by doctor blading on top of the OSCs at room temperature and
subsequently curing each layer by simultaneous exposure to deep UV light (172 nm) and
heat (T = 100 °C) for 3 min. The performance of devices before coating, after coating and
curing is shown in Figure 5.41.
--
50 nm -- --
100 nm --
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
PC
E [
%]
initial
coated
cured
--
50 nm -- --
100 nm --0
10
20
30
40
50
60
70
FF
[%
]
--
50 nm -- --
100 nm --0.0
0.1
0.2
0.3
0.4
0.5
0.6
Vo
c [
V]
--
50 nm -- --
100 nm --0
2
4
6
8
10
12
Jsc [
mA
/cm
²]
--
50 nm -- --
100 nm --0
50
100
150
200
250
Lig
ht
Inje
ctio
n @
1.0
V [
mA
/cm
²]
--
50 nm -- --
100 nm --1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
Le
aka
ge
[A
/cm
²]
Figure 5.41: Device performance of the OSC devices with evaporated silver electrode of
thickness 50 nm and 100 nm, and each device directly coated with one, two and three PHPS
layers each having thickness of 170 nm, showing performance in terms of PCE, FF, Voc,
Jsc,light injection and Leakage of initial (grey), after coating (red) and after curing (green).
RESULTS AND DISCUSSION
122
Both OSC devices with 100 nm and 50 nm thick top silver electrode survived the process,
and no degradation in terms of efficiency is observed hence; neither detrimental
interactions between the PHPS solution and the OSC nor any damage by deep UV light
were observed (Figure 5.41). Therefore, for further experiments, top electrode with 50 nm
thickness is used, because at this thickness silver does not exhibit a significant gas barrier
effect but protects the solar cell from the deep UV light during the curing of PHPS. The
encapsulated devices were subsequently exposed to accelerated lifetime tests. In damp heat
at 40 °C/85% RH, the devices fail rapidly, due to the formation of cracks in the barrier film
and subsequent delamination and ingress of water and oxygen (Figure 5.42).
a)
b)
Figure 5.42: Organic solar cells directly coated with 170 nm PHPS a) before, b) after a
few hours of exposure to damp heat (40oC/ 85%RH) [186].
In order to investigate why the barrier failed catastrophically, OSC structure without top
silver electrode were coated with PHPS coatings (1 layer ~200 nm thickness) with and
without curing and were subsequently positioned in damp heat.
Two different behaviors were observed, when PHPS is cured completely and is place in
damp heat 65oC/ 85 RH, the layer faces catastrophic failures and delamination occurs
(Figure 5.43a,b), as a consequence of the brittleness of the fully cured PHPS films, even
minor stress at the interface to the substrate, due to different thermal expansion coefficients
or due to uptake of water into one of the layers, most probably the PEDOT:PSS layer, and
subsequent volume expansion, leads to delamination of the films. The second behavior is
when PHPS is not cured, spiral cracks are observed, which indicates a possible reaction
between PHPS and PDOT.PSS. This is because PDOT.PSS is of acidic nature and can react
with PHPS, as a result cracks appear which subsequently lead to delamination Figure 5.43
(c,d).
RESULTS AND DISCUSSION
123
Therefore, in order to prevent such catastrophic failures, the use of soft interlayer between
PHPS and OSC is inevitable. The soft interlayer will not only act as a separator but also
compensate the possible volume expansion of PDOT.PSS and will prevent cracking of
PHPS cured layer.
a) b)
c)
d)
Figure 5.43: Cracking in the PHPS layers coated on OSC device without top silver
electrode, after exposure to damp heat 65oC/ 85 RH, a) device coated with cured PHPS
on top, b) micrograph of the device shown highlighted section of (a), shwoing
delaminated areas, c) device coated with un-cured PHPS and d) micrograph of
highlighted section of (c) showing spiral crack..
RESULTS AND DISCUSSION
124
5.2.10 Intermediate layer of ZnO to avoid delamination
In order to decouple the PHPS barrier film from the substrate, intermediate layers have to
be inserted between substrate and barrier layer, which are able to compensate the
mechanical stress. We chose to test two different materials for this purpose, namely ZnO
nanoparticles and a UV-curable acrylic adhesive. Both materials serve the purpose of
decoupling the PHPS barrier and the substrate, ZnO due to its nanoparticulate nature and
the acrylic adhesive due to its elasticity. In addition, both materials provide additional UV
protection during PHPS curing. The layers were deposited on top of the OSC, and cured
subsequently. In the case of ZnO, the 100 nm thick layer was cured by annealing at 120 °C
for 1 min. In the case of the acrylic adhesive, the 5 µm thick layer was cured by exposure
to UV light in inert atmosphere. Subsequently, the PHPS solution was doctor bladed on top
of the intermediate layers and cured by exposure to deep UV light as shown in Figure 5.44.
Figure 5.44: Schematic diagram of an organic solar cell (OSC) on glass, directly coated
with two PHPS layers (170 nm each), alternating with three layers of Rolic (~ 5 µm each)
along with ZnO (100 nm) as a separating layer[186].
OSCs packaged in this way do not show any signs of cracking after exposure to damp heat
as shown in Figure 5.45. Consequently, the barrier remains stable and does not crack, which
makes it ideally suited for encapsulating OSC with direct coating. Hence, directly coated
devices are subjected to lifetime testing.
Glass substrate
Acrylic
Organic solar cell
PHPS
Acryic
PHPS
Acrylic
ZnO
RESULTS AND DISCUSSION
125
Figure 5.45: Organic solar cells directly coated with two PHPS layers (170 nm each),
alternating with three layers of Rolic (~ 5 µm each) along with ZnO (100 nm) as a
separating layer after a few hours of exposure to damp heat (40oC/ 85%RH) [186].
5.2.11 Lifetime tests
Directly coated devices were subjected to lifetime tests under damp heat 40oC/ 85 RH and
sun test and performance was monitored with time intervals. Normalized power conversion
efficiency (PCE) of the test devices are shown in Figure 5.46. During ~700 hours of
exposure to damp heat 65oC/ 85 RH, the initial power conversion efficiency (PCE) of 3%
drops by 20 %. This drop is almost exclusively due to a loss in jsc (see inset of Figure 5.46),
which suggests that the performance loss is mainly caused by ingress of oxygen and
subsequent doping of the active layer [90], rather than by delamination of the PEDOT:PSS
layer after water ingress.
Under constant illumination with 1 sun at 65 °C block body temperature in ambient air the
PHPS-coated devices lose around 29% of the initial performance within 350 hours. The
degradation of the latter is again mainly due to loss of jsc, which indicates that also under
these conditions diffusion of oxygen through the coating and subsequent doping of the
active layer is the culprit. Since the degradation effect caused by oxygen is much more
detrimental in the presence of light [6], more performance loss is observed compared to the
respective devices exposed to damp heat in the dark. The reason for the diffusion of oxygen
RESULTS AND DISCUSSION
126
is the formation of the cracks in the barrier despite of ZnO layer. The oxygen ingresses
through the cracked areas and reacted with the active layer which resulted in loss of Jsc and
subsequently losing PCE.
0 100 200 300 400 500 600 7000.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400 500 600 7000.0
0.2
0.4
0.6
0.8
1.0N
orm
alized J
sc
Time (h)
Damp heat
Sun test
0 100 200 300 400 500 600 7000.0
0.2
0.4
0.6
0.8
1.0
No
rma
lized J
sc
Time (h)
Damp heat
Sun test
No
rma
lize
d P
CE
Time (h)
Damp heat
Sun test
Figure 5.46: Time traces of normalized power conversion efficiency (PCE) of
P3HT:PCBM based OSCs which have been directly coated with a barrier film stack,
consisting of 2 PHPS layers (thickness ~200 nm each), alternating with 3 interlayers of
acrylic adhesive (each ~ 5 µm thick), subjected to accelerated lifetime testing under
damp heat (40°C, 85% RH) and sun soaking at 65 °C. Inset shows the normalized Jsc of
the same devices(Published in reference [184] and reproduced with permission from
John Wiley and Sons).
5.2.12 Investigation on device failure in sun test:
For further investigations, the degradation devices were analyzed by optical microscope.
Visible cracks are observed in the device as shown in Figure 5.47. These defects served as
the channels for the diffusion of oxygen and thus, the devices degraded
RESULTS AND DISCUSSION
127
a)
b)
c)
Figure 5.47: Optical micrographs of cracks in the sun tested device, a) bright field image,
b) dark field image and c) bottom illuminated image.
5.2.13 Conclusion
It is demonstrated that the flexible organic electronic devices can be encapsulated without
efficiency losses by direct coating of PHPS on top of the devices with roll-to-roll
compatible methods and subsequent conversion to SiO2 by deep UV light treatment in
ambient air. The devices encapsulated with such barriers exhibited stable performance for
several hundred hours in accelerated lifetime tests. The quality of the barrier films was
hardly affected by several thousands of bending cycles as long as the thickness of individual
PHPS layers was below 200 nm. Maximum flexibility along with optimum barrier
properties was achieved by fabricating multilayer stacks of alternating ultrathin PHPS
layers and thin interlayers of organic polymers. The polymer interlayers compensate for the
shear stress upon bending and at the same time act as planarization layers, which enhances
the quality of the PHPS barrier films. Mechanical decoupling of the directly coated barriers
from the device surface by flexible interlayers is shown to avoid cracking of the cured
PHPS layers in accelerated lifetime tests caused by the thermal expansion of the device. A
fast, robust, and quantitative endpoint control of the PHPS curing process based on FTIR
peak ratios has been established.
The direct coating of PHPS-based barrier films is ideally suited for temporary protection
of organic electronics, e.g., for storage or transport before integration into the final product,
such as insulating glass windows or other façade elements. The bill of materials, based on
present prices for small scale orders, of a directly coated barrier of the structure shown in
Figure 5.44 is around 40 €/m2, and thus in the same range as the cost of encapsulation with
medium quality barrier films. This makes full in-line encapsulation of electronic devices
RESULTS AND DISCUSSION
128
by roll-to-roll printing/coating methods possible, even of 3D objects, and thus
revolutionizes the backend processing of printed PV.
CONCLUSION
129
CONCLUSION
In this thesis it is shown that coated barriers are a viable alternative to the encapsulation of
flexible opto-electronic devices into barrier films prepared by vacuum assisted methods.
Moreover, direct coating of devices instead of lamination has been demonstrated.
Following Fick’s 1st law of diffusion as a guideline, two approaches towards coatable
barriers have been chosen, namely enhancing tortuosity by filling glass flakes into PVB
films and reducing accessible area by silica coatings obtained by UV curing of
perhydropolysilazanes. Both methods provide good barrier quality at high transparency and
good flexibility. Glass flakes based barriers proved to be intrinsically flexible, whereas the
brittleness of PHPS based barriers had to be accounted for by a PHPS/organic polymer
multilayer approach.
Both, PVB/glass flakes and PHPS/organic polymer based barrier systems could be
successfully applied for the life time enhancement of flexible organic solar cells under
accelerated testing conditions.
Both encapsulation techniques have been optimized with respect to throughput in view of
application in R2R manufacturing. This makes full in-line encapsulation of electronic
devices by R2R printing/coating methods possible, even of 3D objects, and thus
revolutionizes the backend processing of printed PV.
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LIST OF TABLES
a
LIST OF TABLES
TABLE 1: SOME CHARACTERISTICS OF THE MODELS DEVELOPED TO STUDY REGULARLY DISTRIBUTED AND PERPENDICULARLY
ORIENTED FILLERS IN POLYMER NANOCOMPOSITES. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝜎 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑙𝑖𝑡𝑠. ---------------------------------------------- 18
TABLE 2: SOME CHARACTERISTICS OF THE MODELS DEVELOPED TO STUDY RANDOMLY DISTRIBUTED AND PERPENDICULARLY
ORIENTED FILLERS IN POLYMER NANOCOMPOSITES. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝛾 = 𝑠𝑡𝑎𝑐𝑘𝑖𝑛𝑔 𝑝𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟, 𝜇 = 𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐𝑎𝑙 𝑓𝑎𝑐𝑡𝑜𝑟 . ------------- 20
TABLE 3: DEGRADATION OF THE OSC PARAMETERS AND THEIR POSSIBLE EFFECT ON THE DEVICE PERFORMANCE AS DESCRIBED BY
GROSSIORD ET AL,. [86] -------------------------------------------------------------------------------------------------------- 30
TABLE 4: IRRADIATION CONDITION AND WVTR OF EACH SAMPLE ------------------------------------------------------------------- 50
TABLE 5: WVTR OF PHPS COATED ON POLYIMIDE SUBSTRATE ON BOTH SIDES VIA SPIN COATING AND CURED VIA VUV
IRRADIATION AT DIFFERENT TEMPERATURES FOR 20 MINUTES (DATA EXTRACTED FROM [162]). ------------------------- 51
TABLE 6: BIFUNCTIONAL SILANES R’ (CH2)NSI(OR)3, FEW FUNCTIONAL ORGANIC GROUPS R’ FOR PRODUCING AN ORGANIC
NETWORK AND FUNCTIONALIZATION OF THE MATRIX. DATA EXTRACTED FROM [192]. ------------------------------------ 54
TABLE 7: MATERIALS USED IN THE EXPERIMENTS ------------------------------------------------------------------------------------- 61
TABLE 8: COATING PARAMETERS FOR COATING PHPS LAYERS FROM AN AMOUNT OF 70 µL ON PET SUBSTRATE, SUBSEQUENTLY
CURED WITH DEEP UV IRRADIATION IN COMBINATION WITH TEMPERATURE. ----------------------------------------------- 63
TABLE 9: MOISTURE PERMEATION OF THE PVA LAYER HAVING DIFFERENT THICKNESS VALUES. ----------------------------------- 75
TABLE 10: CALCULATED MOISTURE PERMEATION VALUES OF PVOH AND ITS COMPOSITES AT CONDITIONS 40OC / 65%RH; FILM
THICKNESS OF 100 µM IN ALL CASES EXCEPT PET (125 µM). ---------------------------------------------------------------- 76
TABLE 11: WVTR (@40OC/85 % RH) OF PVB FILMS FILLED WITH DIFFERENT VOLUME CONCENTRATIONS OF GLASS FLAKES OF Α
~ 200. --------------------------------------------------------------------------------------------------------------------------- 93
TABLE 12: WVTR (@40OC/85 % RH) OF PVB FILMS FILLED WITH DIFFERENT CONCENTRATION OF GLASS FLAKES WITH Α ~400
ASPECT RATIO GLASS FLAKES WITH DIFFERENT CONCENTRATIONS. ----------------------------------------------------------- 93
TABLE 13: WVTR (@40OC/85 % RH) OF PVB FILMS FILLED WITH DIFFERENT CONCENTRATIONS OF GLASS FLAKES WITH Α ~2000.
------------------------------------------------------------------------------------------------------------------------------------ 93
TABLE 14: OTR OF PVB FILMS FILLED WITH DIFFERENT CONCENTRATIONS OF GLASS FLAKES WITH Α = 2000. ------------------ 94
TABLE 15: PARAMETERS FOR COATING 70 µL PHPS ON PET SUBSTRATE---------------------------------------------------------- 109
LIST OF FIGURES
b
LIST OF FIGURES
FIGURE 1.1. WATER VAPOR TRANSMISSION RATES (WVTR) AND OXYGEN TRANSMISSION RATES (OTR) OF BULK POLYMERS, FOOD
PACKAGING, AS WELL AS OF SOLUTION AND VACUUM PROCESSED HIGH QUALITY BARRIERS. (REPRODUCED FROM [22] WITH
THE PERMISSION FROM ELSEVIER, WITH MODIFICATIONS) .................................................................................... 7
FIGURE 1.2. ILLUSTRATION OF THE AMOUNTS OF WATER TRANSMITTED THROUGH BARRIER FILMS OF THE SIZE OF A FOOTBALL FIELD
(5000 M2) OVER A PERIOD OF 1 MONTH AT THE WVTR VALUES GIVEN (IN G.M-2.DAY-1). DATA EXTRACTED FROM [23]. 7
FIGURE 2.1: RELATION OF WATER VAPOR PRESSURE VS TEMPERATURE (DATA TAKEN FROM DORTMUND DATA BANK, LICENSED BY
CC BY 3.0) ............................................................................................................................................. 11
FIGURE 2.2: MOISTURE PERMEATION OF BIAXIALLY ORIENTED POLYPROPYLENE AND BIAXIALLY ORIENTED PVA, A) MOISTURE
PERMEATION DEPENDENCE ON TEMPERATURE AT 50% RH, B) MOISTURE PERMEATION DEPENDENCE ON RELATIVE
HUMIDITY (RH%) AT 23OC, C) OTR VALUES OF BIAXIALLY ORIENTED PVA AT DIFFERENT TEMPERATURES AT 50% RH AND
D) OTR VALUES OF BIAXIALLY ORIENTED PVA AT DIFFERENT RELATIVE HUMIDITY (RH%) AT 23˚C (COPIED FROM [31]
LICENSED BB CC BY 4.0) . .......................................................................................................................... 13
FIGURE 2.3: SCHEMATIC DIAGRAM A FILM (I) WITHOUT FILLERS OFFERING NO HINDRANCE, (II) FILM FILLED WITH REGULARLY
ARRANGED PLATELETS PERPENDICULAR TO THE DIRECTION OF DIFFUSION, CREATING A TORTUOUS PATH. (REPRODUCED
FROM [45] WITH PERMISSION FROM ELSEVIER). .............................................................................................. 17
FIGURE 2.4: THE ORDER PARAMETER S FOR THREE DIFFERENT CASES; WHEN ALL FILLER PARTICLES ARE PARALLEL TO THE DIFFUSION
DIRECTION (S=-1/2), WHEN THEY ARE PERPENDICULARLY ORIENTED (S=1) AND WHEN THEY ARE RANDOMLY ORIENTED
(S=0). (REPRODUCED WITH MODIFICATIONS FROM [42] WITH PERMISSION FROM AMERICAN CHEMICAL SOCIETY (ACS))
............................................................................................................................................................. 21
FIGURE 2.5: A) SCHEMATIC STRUCTURE OF A TYPICAL ORGANIC SOLAR CELL SHOWING A GLASS OR PET: POLYETHYLENE
TEREPHTHALATE (SUBSTRATE), INDIUM TIN OXIDE: ITO (BOTTOM ELECTRODE), ZNO: ZINC OXIDE (ELECTRON EXTRACTION
LAYER), BLEND OF P3HT:PCBM (ACTIVE LAYER), PEDOT:PSS: POLY(3,4-ETHYLENEDIOXYTHIOPHENE) POLYSTYRENE
SULFONATE (HOLE EXTRACTION LAYER) AND SILVER:AG (TOP ELECTRODE) B) ENERGY BAND DIAGRAM OF A NORMAL CELL
STRUCTURE AND C) ENERGY BAND DIAGRAM OF AN INVERTED CELL STRUCTURE. DATA EXTRACTED FROM [69]. ........... 25
FIGURE 2.6: EQUIVALENT CIRCUIT OF AN ORGANIC SOLAR CELL (ONE DIODE MODEL) (REPRODUCED FROM [74] WITH PERMISSION
FROM ELSEVIER) ....................................................................................................................................... 27
FIGURE 2.7: A SCHEMATIC DIAGRAM OF FEW PROCESSES RESPONSIBLE FOR DEGRADATION IN OSC WITH P3HT:PCBM AS
PHOTOACTIVE LAYER, (REPRODUCED FROM [80] WITH PERMISSION FROM ELSEVIER). ............................................ 28
FIGURE 3.1: OTR AND WVTR OF DIFFERENT BULK POLYMERS NORMALIZED TO 100 µM THICKNESS. [56] PE-LD =POLYETHYLENE
LOW DENSITY, PE-HD= POLYETHYLENE HIGH DENSITY, PP=POLYPROPYLENE, PS=POLYSTYRENE, BOPP=BIAXIALLY ORIENTED
POLYPROPYLENE, PLA=POLYLACTIC ACID, PVC=POLYVINYL CHLORIDE, PA6=POLYAMIDE 6, LCP=LIQUID CRYSTALLINE
POLYMER, EVOH=ETHYL VINYL ALCOHOL, PAN= POLYACRYLONITRILE, PEN= POLYETHYLENE NAPTHALENE, PET=
POLYETHYLENE TEREPHTHALATE, PVDC= POLYVINYLIDENE CHLORIDE, PC= POLYCARBONATE, PVC-P=POLYVINYL
CHLORIDE-PLASTICIZED, PVC-U= POLYVINYL CHLORIDE-UNPLASTICIZED, PVB= POLYVINHYLA BUTYRAL. (REPRODUCED
FROM [100] LICENSED BY CC BY 3.0). ......................................................................................................... 33
LIST OF FIGURES
c
FIGURE 3.2 SCHEMATIC REPRESENTATION OF CLAY MORPHOLOGY WHEN MIXED WITH POLYMERS. (REPRODUCED FROM [116]
WITH PERMISSION FROM ELSEVIER) .............................................................................................................. 36
FIGURE 3.3: LIFETIME OF ORGANIC SOLAR CELLS TESTED UNDER IRRADIATION WITH A SOLAR SIMULATOR (AM 1.5G, 30 OC,
AMBIENT RH 30-40%): NORMALIZED POWER CONVERSION EFFICIENCY (PCE) OF OSCS ENCAPSULATED WITH PET FILM,
PVA COATED PET FILM, AND PET COATED WITH PVA-MMT 5 WT% NANOCOMPOSITE (COPIED FROM [119] WITH
PERMISSION FROM ELSEVIER). ..................................................................................................................... 37
FIGURE 3.4: WVTR (G/M2.H) OF COMPOSITES OF POLYANILINE / GRAPHENE AND POLYANILINE / CLAY AS FUNCTION OF GRAPHENE
LOADING. (COPIED FROM [45] WITH PERMISSION FROM ELSEVIER)..................................................................... 39
FIGURE 3.5: RELATIVE OXYGEN PERMEABILITY FOR PVA, MIXTURE OF PVA/GO COATING AND HYBRID PVA/GO FOR 0.07 VOL%
LAYER IN COMPARISON TO PREDICTIVE PERMEATION CURVES PROPOSED BY THREE MODELS ( I.E. NIELSEN, MODIFIED
NIELSEN AND CUSSLER) FOR DIFFERENT ASPECT RATIOS (𝛼) (COPIED FROM [132] WITH PERMISSION FROM ELSEVIER, 40
FIGURE 3.6: TRANSPARENCY VS REDUCTION IN PERMEATION FOR DIFFERENT FILLER TYPES AND LOADINGS IN POLYMER MATRICES.
............................................................................................................................................................. 41
FIGURE 3.7: TRANSMISSION AND APPEARANCE OF CURED PHPS FILMS, A) SHOWING THE TRANSPARENT APPEARANCE, B)
BENDABLE TRANSPARENT CURED PHPS COATING AND C) TRANSMISSION SPECTRA OF PET FILM AND DIFFERENT TYPES OF
SIO2 COATINGS.(COPIED FROM [169] LICENSED BY CC BY 4.0) ........................................................................ 44
FIGURE 3.8: FTIR SPECTRA OF UNCURED PHPS AND PHPS CURED WITH DIFFERENT METHODS. A) IR SPECTRA OF UNCURED PHPS
(SOLID LINE) AND IR SPECTRA OF PHPS CURING AT 180 °C UNDER MOISTURIZED ATMOSPHERE FOR 300 MIN (DASHED
LINE), B) IR SPECTRA OF UNCURED PHPS (SOLID LINE), PHPS CURED AFTER EXPOSURE TO AMMONIA VAPOR FOR 60
MINUTES (DASHED LINE), C) IR SPECTRA OF UNCURED PHPS (SOLID LINE) AND PHPS CURED BY SUBMERGING INTO 20%
AQUEOUS HYDROGEN PEROXIDE SOLUTION FOR 10 MINUTES. (COPIED FROM [165] WITH PERMISSION FROM ELSEVIER)
............................................................................................................................................................. 47
FIGURE 3.9: TEMPERATURE DEPENDENCE OF WATER VAPOR TRANSMISSION RATES OF THE POLYSILAZANE DERIVED SIO2 COATINGS
(2 COATES) (COPIED FROM [169] LICENSED BY CC BY 4.0). ............................................................................ 52
FIGURE 3.10:PERFORMANCE OF ORGANIC SOLAR CELLS IN TERMS OF NORMALIZED POWER CONVERSION EFFICIENCY (PCE) AND
NORMALIZED SHORT CIRCUIT CURRENT (JSC) DURING THE EXPOSURE TO AM 1.5, 10 0 0 W M –2 LIGHT SOAKING,
ENCAPSULATED WITH (A) PET HAVING THICKNESS OF 50 Μ M, (B) A COMMERCIAL BARRIER, (C) PHPS BASED BARRIER
HAVING ONE PHPS (250 NM) COAT ON BOTH SIDES OF PET, (D) PHPS BASED BARRIER, HAVING 5 LAYER STRUCTURE
(PET/PHPS250 NM/PHPS250 NM/PVA1 Μ M /PHPS250 NM/PHPS250 NM) ON ONE SIDE OF PET AND (E) UN-ENCAPSULATED
OSC DEVICE DEGRADED UNDER IRRADIATION IN GLOVEBOX. (COPIED FROM [182] WITH PERMISSION FROM ELSEVIER). 53
FIGURE 3.11: SCHEMATIC DIAGRAM FOR ROLL-TO-ROLL PRODUCTION OF ORMOCER/INORGANIC OXIDE HYBRID BARRIER FILMS.
(RE DRAWN FROM [167]). ......................................................................................................................... 55
FIGURE 3.12:(A) CHEMICAL STRUCTURE FOR CYTOP TM (B) SPIN-COATED CYTOP FILM ON GLASS SUBSTRATE UNDER ATOMIC
FORCE MICROSCOPE (COPIED FROM [202] WITH PERMISSION FROM AIP PUBLISHING). ......................................... 57
FIGURE 3.13: CHARACTERIZATION OF HYDROPHOBICITY IN TERMS OF WATER DROPLET CONTACT ANGLES AND THICKNESS VALUES
OF FILMS WITH RESPECT TO WEIGHT PERCENTAGES (WT%) OF CYTOP IN SOLUTION. IMAGES BELOW DROPLETS ARE
MEASURED BY ATOMIC FORCE MICROSCOPY (AFM) (COPIED FROM [203] WITH PERMISSION FROM ELSEVIER). .......... 58
LIST OF FIGURES
d
FIGURE 3.14: CALCIUM DEGRADATION MECHANISM, TYPE A CA FILMS AS A FUNCTION OF TIME FOR VARYING CYTOP FILM
THICKNESSES AND TTYPE-B CA FILMS AS A FUNCTION OF TIME FOR VARYING CYTOP FILM THICKNESSES HAVING SINX
INTERLAYER (COPIED FROM [205] WITH PERMISSION FROM AIP PUBLISHING). ..................................................... 59
FIGURE 4.1: DETAILS OF THE OSC DEVICE, A) LAYOUT OF THE COMPLETE OSC DEVICE, B) SCHEMATIC DIAGRAM OF THE WORKING
CELL........................................................................................................................................................ 64
FIGURE 4.2: SCHEMATIC DIAGRAMS OF THE ENCAPSULATED SOLAR CELLS, A) SOLAR CELLS ENCAPSULATED WITH TRADITIONAL
LAMINATION OF THE BARRIER FILMS USING EPOXY AS AN ADHESIVE, B) DIRECTLY COATED SOLAR CELL. ........................ 65
FIGURE 4.3: A) SCHEMATIC DIAGRAM SHOWING CUP TEST USING WATER B) CUP TEST USING DESICCANTS, C) ALUMINUM CUP
ACCORDING TO ASTM STANDARD E96, B) SYSTECH 7002 METHOD................................................................. 66
FIGURE 4.4: A) PHOTOGRAPH OF THE WVTR DEVICE (SYSTECH 7002), B) SCHEMATIC VIEW OF THE PERMEATION CELL OF
SYSTECH 7002 DEVICE SHOWING THE FLOW OF THE DRY AND WET NITROGEN THROUGH THE CELL CHAMBER. ........... 67
FIGURE 4.5: SCHEMATIC VIEW OF THE OXYGEN PERMEATION CELL ................................................................................ 67
FIGURE 5.1: FT-IR TRANSMISSION SPECTRA OF PVA FILMS AND ITS COMPOSITES WITH CLAY CONCENTRATION OF 6 VOL. % IN THE
RANGE OF 1500–500 CM-1. ....................................................................................................................... 72
FIGURE 5.2: OPTICAL MICROGRAPHS OF THE PVA-CLAY NANOCOMPOSITE, WHERE PVA CONTAINS A) 2 VOL%, B) 4 VOL%, AND
C) 6 VOL% MMT-NA+ NANOCLAY, RESPECTIVELY. .......................................................................................... 73
FIGURE 5.3: UV-VIS SPECTRA OF PVA AND ITS COMPOSITES WITH DIFFERENT CLAY CONCENTRATIONS COATED ON GLASS
SUBSTRATES, A) TOTAL TRANSMITTANCE SPECTRA OF PVA AND ITS COMPOSITES WITH CLAY, B) TOTAL TRANSMITTANCE AT
600 NM AS A FUNCTION OF CLAY CONTENT, C) DIFFUSE TRANSMITTANCE SPECTRA OF PVA AND ITS COMPOSITES WITH CLAY,
D) DIFFUSE TRANSMITTANCE AT 600 NM AS A FUNCTION OF CLAY CONTENT. ......................................................... 74
FIGURE 5.5: BLOCKING EFFECT OF PVOH LAYERS HAVING DIFFERENT THICKNESSES. ........................................................ 76
FIGURE 5.6: WATER WEIGHT LOSS VS TIME FROM CUPS SEALED WITH FILMS OF PVA AND ITS COMPOSITES WITH MMT CLAY AT
40OC / 65%RH........................................................................................................................................ 77
FIGURE 5.7: EVOLUTION OF RELATIVE PERMEABILITY OF PVA FILMS WITH INCREASING CONTENT OF NANOCLAY PARTICLES.
(TRIANGLES – EXPERIMENTAL DATA. DDOTTED LINE - CALCULATED DATA) ACCORDING TO THE BHARADWAJ MODEL FOR AN
ASPECT RATIO OF Α = 500 AND ORDER PARAMETERS OF S = 0. ........................................................................... 78
FIGURE 5.8: RECIPROCAL WVTR OF PVA AND ITS MMT-NA+ CLAY COMPOSITE WITH LOADING CONCENTRATIONS OF 0-10
VOLUME %.VS NUMBER OF BENDING CYCLES WITH BENDING RADIUS OF 5 CM. EACH LAYER HAS A THICKNESS OF 100 µM.
............................................................................................................................................................. 79
FIGURE 5.9: OPTICAL MICROGRAPHS OF GLASS FLAKES. (A) GLASS FLAKES WITH THICKNESS ~ 1 µM, (B) GLASS FLAKES WITH
THICKNESS ~0.5 µM, (C) GLASS FLAKES WITH THICKNESS OF ~0.1 µM .................................................................. 81
FIGURE 5.10: MICROGRAPHS OF PVB FILMS CONTAINING 15 VOL% GLASS FLAKES OF AR = 400. A) OPTICAL MICROGRAPH (TOP
VIEW) (B) SEM CROSS SECTION OF A SEMI-POLISHED PVB/GLASS FLAKES COMPOSITE FILMC) EDX IMAGE OF THE
HIGHLIGHTED SECTION (FULLY POLISHED) OF THE PVB/GLASS FLAKES LAYER WITH SI MAPPING. ................................ 82
FIGURE 5.11: CONFOCAL MICROGRAPH SHOWING THE SURFACE OF THE PVB FILLED WITH 25 VOL% OF THE GLASS FLAKES, A)
GLASS FLAKES WITH A.R~200, B) GLASS FLAKES WITH A.R~400, AND C) GLASS FLAKES WITH A.R~2000. ................. 83
LIST OF FIGURES
e
FIGURE 5.12: SPECTRA OF A PRISTINE PVB FILM HAVING A THICKNESS OF ~ 70 µM, A) TOTAL TRANSMISSION (OPEN TRIANGLES IN
BLACK), DIFFUSE TRANSMISSION (FULL TRIANGLES IN RED), B) TOTAL REFLECTANCE (FULL SQUARE IN BLACK) AND DIFFUSE
REFLECTANCE (BLACK SQUARES). .................................................................................................................. 84
FIGURE 5.13: TRANSMITTANCE AND REFLECTANCE SPECTRA OF PVB CONTAINING GLASS FLAKES. A) TOTAL TRANSMITTANCE AND
DIFFUSE TRANSMITTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES (A.R = 200), B) TOTAL
TRANSMITTANCE AND DIFFUSE TRANSMITTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES (A.R = 400),
C) TOTAL TRANSMITTANCE AND DIFFUSE TRANSMITTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES (A.R
= 2000), D) TOTAL REFLECTANCE AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES
(A.R = 200), E) TOTAL REFLECTANCE AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS
FLAKES OF A.R = 400, AND F) TOTAL REFLECTANCE AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL%
OF GLASS FLAKES OF A.R = 2000. ................................................................................................................ 85
FIGURE 5.14: SIMULATED DIRECT/TOTAL TRANSMISSION OF A SINGLE GLASS FLAKE FILLED IN PVB MATRIX AT A WAVELENGTH OF
550 NM FOR DIFFERENT POLARIZATIONS OF THE INCIDENT LIGHT. ....................................................................... 87
FIGURE 5.15: SIMULATED TILT (Θ) AND ROTATION ANGLES OF FLAKES (Φ), FOR CONDITIONS A) VARIATION OF THE TILT ANGLE OF
THE FLAKES (0≤ 𝜃 ≤ 180𝑜) AND FIX ROTATION, B) FIXED TILT, ROTATION AROUND THE VERTICAL AXIS (0≤ 𝜙 ≤
360𝑜) AND C) RANDOM TILT AND RANDOM ROTATION AROUND THE VERTICAL AXIS, WHERE 𝑅 IS A VECTOR NORMAL TO
FLAKE SURFACE. ........................................................................................................................................ 87
FIGURE 5.16: SIMULATED TRANSMISSION (TOTAL AND DIFFUSE) (@550 NM) OF PVB FILMS FILLED WITH GLASS FLAKES AT
DIFFERENT PARTICLE VOLUME CONCENTRATIONS HAVINGRANDOM TILT, RANDOM ROTATION AROUND THE VERTICAL AXIS.
............................................................................................................................................................. 88
FIGURE 5.17: TRANSMISSION AND REFLECTANCE SPECTRA OF PVB CONTAINING GLASS FLAKES AFTER COATING EPOXY ON BOTH
SIDES OF PVB FILLED WITH FLAKES. A) TOTAL TRANSMISSION AND DIFFUSE TRANSMISSION SPECTRA OF PVB FILLED WITH
5-15 VOL% OF 200 ASPECT RATIO GLASS FLAKES, B) TOTAL TRANSMISSION AND DIFFUSE TRANSMISSION SPECTRA OF PVB
FILLED WITH 5-15 VOL% OF 400 ASPECT RATIO GLASS FLAKES, C) TOTAL TRANSMISSION AND DIFFUSE TRANSMISSION
SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 2000 ASPECT RATIO GLASS FLAKES, D) TOTAL REFLECTANCE AND DIFFUSE
REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 200 ASPECT RATIO GLASS FLAKES, E) TOTAL REFLECTANCE AND
DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 400 ASPECT RATIO GLASS AND F) TOTAL REFLECTANCE
AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 2000 ASPECT RATIO GLASS FLAKES. ............ 90
FIGURE 5.18: DEPENDENCE OF DIFFUSE TRANSMISSION OF PVB FILMS @ 550 NM ON GLASS FLAKE LOADING BEFORE (BLACK
SQUARES) AND AFTER (RED DOTS) COATING EPOXY ON BOTH SIDES OF THE FILM, A) LAYERS OF FLAKES WITH Α ~200 B)
LAYERS OF FLAKES WITH Α ~400, AND C) LAYERS OF FLAKES WITH Α ~2000. ........................................................ 91
FIGURE 5.19: EXPERIMENTAL BARRIER IMPROVEMENT FACTOR OF THE BARRIER (PVB/GLASS FLAKES COMPOSITES) COMPARED
WITH BIF OF COMPOSITED ACCORDING TO BHARDWAJ’S MODEL. EXPERIMENTAL BIF OF FLAKES WITH Α ~200 (CLOSED
BLACK CIRCLE), BHARDWAJ’S SIMULATED (DOTTED BLACK LINE), EXPERIMENTAL BIF OF FLAKES WITH Α ~400 (BLUE CLOSED
TRIANGLE) AND BHARDWAJ SIMULATED (DOTTED BLUE LINE) AND EXPERIMENTAL BIF OF FLAKES WITH Α ~2000 (DARK
YELLOW SQUARE) AND BHARDWAJ SIMULATED (DOTTED DARK YELLOW LINE). VS THE GLASS FLAKES VOLUME
CONCENTRATION IN THE PVB LAYERS. ........................................................................................................... 92
LIST OF FIGURES
f
FIGURE 5.20: SEM CROSS SECTION IMAGE OF PVB FILLED WITH 25 VOL% OF GLASS FLAKES OF Α ~2000, HIGHLIGHTED AREA
SHOWS THE DEFECTS WITHIN THE LAYER. ........................................................................................................ 92
FIGURE 5.21: BARRIER IMPROVEMENT FACTOR OF THE PVB FILM FILLED WITH GLASS FLAKES (Α ~2000) AGAINST OXYGEN (RED
DOTS) AND MOISTURE (BLACK SQUARE) VS THE VOLUME FRACTION OF GLASS FLAKES. ............................................ 94
FIGURE 5.22: WVTR OF PVB FILM WITH 15 VOL% GLASS FLAKES (Α ~ 2000) VS. NUMBER OF BENDING CYCLES. ................. 95
FIGURE 5.23: UV–VIS SPECTRA OF P3HT FILMS ON GLASS ENCAPSULATED WITH A) A PVB LAYER AND B) A PVB FILLED WITH 25
VOL% OF Α ~2000 GLASS FLAKES DURING EXPOSURE TO THE LIGHT OF A SUN SIMULATOR IN AMBIENT AIR AT 65 C. C)
NORMALIZED ABSORBANCE LOSS AT 525 NM OF P3HT FILMS ENCAPSULATED WITH PLAIN PVB, PVB/GLASS FLAKES FILLED
BARRIER. ................................................................................................................................................. 96
FIGURE 5.24: DAMP HEAT DEGRADATION TEST (40 °C/85% RH) OF P3HT:PCBM BASED DEVICES ENCAPSULATED WITH PRISTINE
PVB FILMS AND PVB FILMS FILLED WITH 15% V/V GLASS FLAKES (Α = 2000). JSC: SHORT CIRCUIT CURRENT. VOC: OPEN
CIRCUIT CURRENT. FF: FILL FACTOR, PCE:. POWER CONVERSION EFFICIENCY. ........................................................ 97
FIGURE 5.25: SUN DEGRADATION TEST OF P3HT:PCBM BASED SOLAR CELLS ENCAPSULATED IN THREE DIFFERENT BARRIERS:
MITSUBISHI BARRIER FILM, PRISTINE PVB FILM, AND PVB FILM FILLED WITH 15% V/V GLASS FLAKES (Α = 2000). JSC: SHORT
CIRCUIT CURRENT. VOC: OPEN CIRCUIT CURRENT. FF: FILL FACTOR, PCE: POWER CONVERSION EFFICIENCY. .................. 99
FIGURE 5.26:FTIR SPECTRA OF A 500 NM THICK PHPS FILM COATED ON PE SUBSTRATE CURED WITH DEEP UV LIGHT FOR ~25
MINUTES WITH IRRADIATION DISTANCE OF 100 MM (RED CURVE), 30 MM (BLUE CURVE) AND 5 MM (PURPLE CURVE).
........................................................................................................................................................... 103
FIGURE 5.27: : FTIR SPECTRA OF PHPS FILM (500 NM THICK) CURED WITH THE COMBINATION OF HEAT (100OC) AND
IRRADIATION WITH 172 NM WAVELENGTH LIGHT AT A DISTANCE OF 5 MM FOR 15 MINUTES. ................................. 104
FIGURE 5.28. A) FTIR SPECTRA OF A 500 NM THICK PHPS FILM, CURED BY DEEP UV LIGHT FOR THE TIMES SPECIFIED IN THE
FIGURE. THE CORRESPONDING WVTR VALUES ARE GIVEN NEXT TO THE SPECTRA. B) FTIR PEAK RATIOS (1050 CM-1/(830
CM-1, OPEN CIRCLES) OF FIGURE 5.28 (A), CORRELATED WITH THE CORRESPONDING WVTR VALUES (FULL SQUARES) AT
DIFFERENT CURING TIMES (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN WILEY AND
SONS). .................................................................................................................................................. 106
FIGURE 5.29: A) FTIR SPECTRA OF A 800 NM THICK PHPS FILM, CURED BY AN EXPOSURE TO DAMP HEAT @ 65OC/85% RH FOR
THE TIMES SPECIFIED IN THE FIGURE. THE CORRESPONDING WVTR VALUES ARE GIVEN NEXT TO THE SPECTRA. B) FTIR PEAK
RATIOS (1050 CM-1/(830 CM-1, OPEN CIRCLES) OF FIGURE 5.29A, CORRELATED WITH THE CORRESPONDING WVTR
VALUES (FULL SQUARES) AT DIFFERENT CURING TIMES. ................................................................................... 108
FIGURE 5.30. FTIR SPECTRA OF A 400 NM THICK PHPS FILM (RED CURVE) AND OF A STACK OF FOUR 100 NM THICK PHPS LAYERS
(BLACK CURVE), BOTH CURED WITH DEEP UV LIGHT FOR A TOTAL OF 12 MINUTES. (PUBLISHED IN REFERENCE [184] AND
REPRODUCED WITH PERMISSION FROM JOHN WILEY AND SONS). ..................................................................... 110
FIGURE 5.31: HYDROPHOBIC NATURE OF THE PHPS, A) DROPLETS OF WATER ON UN-CURED PHPS SURFACE, B) CONTACT ANGLE
(C.A) OF 92.8O ON UN-CURED PHPS, C) C.A OF 88.3O OF PHPS AFTER CURING AND D) CONTACT ANGLE OF PET
SUBSTRATE 80O. ..................................................................................................................................... 111
FIGURE 5.32: OPTICAL CALCIUM TEST OF PHPS BARRIER (270 NM) ON PET FILM (D = 125 µM) AFTER 50 BENDING CYCLES
(BENDING RADIUS OF 3 CM). THE PET/PHPS BARRIER FILM HAS BEEN LAMINATED (USING UV CURABLE ADHESIVE) ON A
CALCIUM FILM OF 200 NM IN THICKNESS, EVAPORATED ON A GLASS SLIDE. TESTING CONDITIONS: 65OC / 85 %RH. IMAGES
LIST OF FIGURES
g
TAKEN AFTER A) 1 H, B) 2 H, C) 5 H, D) 8 H AND E) 10 H (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH
PERMISSION FROM JOHN WILEY AND SONS). ................................................................................................ 112
FIGURE 5.33. RECIPROCAL WVTR VS. NUMBER OF BENDING CYCLES FOR STACKS OF DIFFERENT NUMBERS OF PHPS LAYERS ON
PET FILM ( D = 125 µM) AT A BENDING RADIUS OF 3 CM (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH
PERMISSION FROM JOHN WILEY AND SONS).. ............................................................................................... 112
FIGURE 5.34: TTRANSMISSION SPECTRA OF PET FILM AND OF PHPS/ACRYLIC/PHPS BARRIER COATED ON PET FILM ......... 114
FIGURE 5.35: NORMALIZED RECIPROCAL WVTR OF SANDWICH COATINGS WITH ALTERNATING PHPS AND ORGANIC LAYERS (SEE
INSET) VS. THE NUMBER OF BENDING CYCLES WITH A BENDING RADIUS OF 3 CM. EACH PHPS LAYER HAS A THICKNESS OF
~170 NM, WHILE THE ORGANIC LAYERS ARE ~ 5-10 UM THICK. FOR THE ORGANIC INTERLAYERS, TWO NON-CROSS LINKED
POLYMERS, NAMELY PVDF (BLACK SQUARES) AND PVB (RED CIRCLES) WERE USED. (PUBLISHED IN REFERENCE [184] AND
REPRODUCED WITH PERMISSION FROM JOHN WILEY AND SONS). ..................................................................... 115
FIGURE 5.36: RECIPROCAL WVTR OF SANDWICH COATINGS WITH ALTERNATING PHPS AND ORGANIC LAYERS (SEE INSET) VS. THE
NUMBER OF BENDING CYCLES WITH A BENDING RADIUS OF 3 CM. EACH PHPS LAYER HAS A THICKNESS OF ~170 NM, WHILE
THE ORGANIC LAYERS ARE ~ 5-10 µM THICK. FOR THE ORGANIC INTERLAYERS, TWO UV-CURABLE ADHESIVES ON EPOXY
(BLACK SQUARES) AND ACRYLIC BASIS (RED CIRCLES) WERE USED (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH
PERMISSION FROM JOHN WILEY AND SONS). ................................................................................................ 116
FIGURE 5.37: SCHEMATIC VIEW OF ENCAPSULATION OF P3HT FILMS ON GLASS, RIM ENCAPSULATED WITH PHPS BASED
SANDWICH BARRIER ON PET FILM. (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN
WILEY AND SONS). .................................................................................................................................. 117
FIGURE 5.38: UV/VIS SPECTRA OF P3HT FILMS ON GLASS ENCAPSULATED WITH A) A PLAIN PET, B) A PHPS BASED BARRIER, C)
MITSUBISHI ( A COMMERCIAL BARRIER) AND D) GLASS DURING EXPOSURE TO THE LIGHT OF A SUN SIMULATOR IN AMBIENT
AIR AT 65OC. THE INCREASE OF ABSORPTION AT 400 NM IN (B) AND (C) IS DUE TO THE YELLOWING OF THE PET SUBSTRATES.
E) NORMALIZED ABSORBANCE LOSS AT 525 NM OF P3HT FILMS ENCAPSULATED WITH PLAIN PET, PHPS BASED BARRIER,
MITISUBISHI AND GLASS ON GLASS. (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN
WILEY AND SONS). .................................................................................................................................. 118
FIGURE 5.39: SCHEMATIC VIEW OF ENCAPSULATION OF P3HT:PCBM BASED SOLAR CELL ON GLASS, RIM ENCAPSULATED WITH
PHPS BASED SANDWICH BARRIER ON PET FILM. ........................................................................................... 119
FIGURE 5.40: EFFICIENCY OF P3HT:PCBM BASED ORGANIC SOLAR CELLS ENCAPSULATED WITH PLAIN PET FOILS (PET),
COMMERCIAL MITSUBISHI BARRIER FOILS (MITSUBISHI), OR PHPS-BASED BARRIER FOILS (PHPS, FOR DETAILS SEE TEXT)
UPON EXPOSURE TO DAMP HEAT (40 °C/85% RH). (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION
FROM JOHN WILEY AND SONS). ................................................................................................................. 120
FIGURE 5.41: DEVICE PERFORMANCE OF THE OSC DEVICES WITH EVAPORATED SILVER ELECTRODE OF THICKNESS 50 NM AND 100
NM, AND EACH DEVICE DIRECTLY COATED WITH ONE, TWO AND THREE PHPS LAYERS EACH HAVING THICKNESS OF 170 NM,
SHOWING PERFORMANCE IN TERMS OF PCE, FF, VOC, JSC,LIGHT INJECTION AND LEAKAGE OF INITIAL (GREY), AFTER COATING
(RED) AND AFTER CURING (GREEN). ............................................................................................................ 121
FIGURE 5.42: ORGANIC SOLAR CELLS DIRECTLY COATED WITH 170 NM PHPS A) BEFORE, B) AFTER A FEW HOURS OF EXPOSURE TO
DAMP HEAT (40OC/ 85%RH) [186]. ......................................................................................................... 122
LIST OF FIGURES
h
FIGURE 5.43: CRACKING IN THE PHPS LAYERS COATED ON OSC DEVICE WITHOUT TOP SILVER ELECTRODE, AFTER EXPOSURE TO
DAMP HEAT 65OC/ 85 RH, A) DEVICE COATED WITH CURED PHPS ON TOP, B) MICROGRAPH OF THE DEVICE SHOWN
HIGHLIGHTED SECTION OF (A), SHWOING DELAMINATED AREAS, C) DEVICE COATED WITH UN-CURED PHPS AND D)
MICROGRAPH OF HIGHLIGHTED SECTION OF (C) SHOWING SPIRAL CRACK............................................................. 123
FIGURE 5.44: SCHEMATIC DIAGRAM OF AN ORGANIC SOLAR CELL (OSC) ON GLASS, DIRECTLY COATED WITH TWO PHPS LAYERS
(170 NM EACH), ALTERNATING WITH THREE LAYERS OF ROLIC (~ 5 µM EACH) ALONG WITH ZNO (100 NM) AS A
SEPARATING LAYER[186]. ......................................................................................................................... 124
FIGURE 5.45: ORGANIC SOLAR CELLS DIRECTLY COATED WITH TWO PHPS LAYERS (170 NM EACH), ALTERNATING WITH THREE
LAYERS OF ROLIC (~ 5 µM EACH) ALONG WITH ZNO (100 NM) AS A SEPARATING LAYER AFTER A FEW HOURS OF EXPOSURE
TO DAMP HEAT (40OC/ 85%RH) [186]. ..................................................................................................... 125
FIGURE 5.46: TIME TRACES OF NORMALIZED POWER CONVERSION EFFICIENCY (PCE) OF P3HT:PCBM BASED OSCS WHICH HAVE
BEEN DIRECTLY COATED WITH A BARRIER FILM STACK, CONSISTING OF 2 PHPS LAYERS (THICKNESS ~200 NM EACH),
ALTERNATING WITH 3 INTERLAYERS OF ACRYLIC ADHESIVE (EACH ~ 5 µM THICK), SUBJECTED TO ACCELERATED LIFETIME
TESTING UNDER DAMP HEAT (40°C, 85% RH) AND SUN SOAKING AT 65 °C. INSET SHOWS THE NORMALIZED JSC OF THE
SAME DEVICES(PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN WILEY AND SONS).
........................................................................................................................................................... 126
FIGURE 5.47: OPTICAL MICROGRAPHS OF CRACKS IN THE SUN TESTED DEVICE, A) BRIGHT FIELD IMAGE, B) DARK FIELD IMAGE AND
C) BOTTOM ILLUMINATED IMAGE. ............................................................................................................... 127
LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS
i
LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS
∆𝑚 Change of flux in gram
∆𝑡 Change of time
∆𝑝 Change of pressure
𝜏 Tortuous path
𝜋 Pi ~ 3.152
Φ Volume fraction
Å Angstrom
ASTM American standard for testing of materials
ALD Atomic layer deposition
A.R Aspect ratio
AFM Atomic force microscope
ATR Attenuated total reflection
AIP Aluminum iso-propoxide
atm Atmospheric
𝛼 Aspect ratio
BA Benzyl alcohol
BIF Barrier improvement factor
BOPP Biaxially oriented polypropylene
C.A Contact angle
cm3 Cubic centimeter
LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS
j
𝐷 Co-efficient of diffusion
DUV Deep ultra violet
EVOH Ethylene vinyl alcohol
EVA Ethylene vinyl acetate
FF Fill factor
FTIR Fourier-transform infrared spectroscopy
GO Graphene oxide
gALD Gas atomic layer deposition
g gram
HgLP Low pressure mercury lamps
HDPE High density polyethylene
HEC Hectorite
HTL Hole transport layer
ITO Indium tin oxide
IMI ITO-metal(ag)-ITO
I Intensity
Io Initial intensity
Jsc Short circuit current
kJ/mol Kilo joul per mole
L Length
LDPE Low density poly ethylene
LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS
k
LCP Liquid crystal polymer
MMT Montmorillonite
mm/s Millimeter per second
m2 Square meter
N2 Nitrogen
nm nanometer
OFETs Organic field-effect transistors
OLED Organic light emitting diodes
O2 Oxygen
OPV Organic photovoltaic
OSC Organic solar cells
OEDs Organic electronic devices
OTR Oxygen transmission rate
O3 Oxone
ORMOCERS Organically modified ceramics
PV Photovoltaic
PET Polyethylene terephthalate
PEN Polyethylene naphthalene
PAN Polyacrylonitrile
PECVD Plasma enhanced chemical vapor deposition
PEPVD Plasma enhanced physical vapor deposition
LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS
l
PS Polystyrene
PTFE Polytetrafluorethylene
PCdC Polycarbonate di chloride
PC Poly carbonate
PA Poly amide
PLA Poly lactic acid
PA 6 Polyimide 6
PVA Polyvinyl alcohol
PCE Power conversion efficiency
PBS Polybutylene succinate
PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
PC60BM Phenyl-C61-butyric acid methyl ester
P3HT Poly(3-hexylthiophene)
PVOH Poly vinyl alcohol
PVB Polyvinyl butylene
PMMA Poly(methyl methacrylate)
PHPS Perhydropolysilazane
RH Relative humidity
R Resistance
𝑟 Radius
sALD Solution atomic layer deposition
LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS
m
SEM Scanning electron microscope
S Coefficient of solubility
TFT Thin-film-transistor
TEOS Tetraethyl orthosilicate
TEAOH Tetraethylammonium hydroxide
TGA Thermo gravimetric analysis
UVA Ultraviolet (category A)
UK United kingdom
UV Ultra violet
µm Micro meter
µl Micro liters
Voc Open circuit current
Vol% Volume percent
VUV Vacuum ultraviolet
Wt% Weight percent
W Width
WVTR Water vapor transmission rate
Wm-2 Watt per sq. meter
x Thickness
ZnO Zinc oxide