· ! 5! abstract! overthepastthreedecades,tremendousamountofresearchhas...

Degradation Pathways in Organic Small Molecule and Hybrid Solar Cells

GOLNAZ SHERAFATIPOUR NanoSYD-Mads Clausen Institute SDU Sønderborg-University of Southern Denmark

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Degradation Pathways in Organic Small Molecule and Hybrid Solar Cells

Golnaz Sherafatipour

Doctoral Thesis

2018

Nanoscience Centre NanoSYD Mads Clausen Institute Faculty of Engineering

SYDDANSK UNIVERSITET

Supervisor:

Morten Madsen

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Abstract

Over the past three decades, tremendous amount of research has been devoted to the field of renewable energy in order to reduce our dependence on fossil fuels. Among them, organic (OSC) and perovskite (PSC) solar cells have gained enormous attention as they offer unique advantages compared to the traditional silicon solar cells, such as low fabrication cost, mechanical flexibility, light-‐weight modules and semi-‐transparency. These unique properties offer a wide range of applications and integration schemes, which make this field of research even more exciting. OSC and PSC have recently achieved power conversion efficiencies (PCE) of around 15% and 22%, respectively, emphasizing the great potential of the technologies. However, both of these devices suffer from low stability and short lifetime (degradation). Therefore, understanding the degradation mechanisms of these devices, paves the way for viable commercialization of this appealing technology in the market. This work is dedicated to investigate governing degradation mechanisms and pathways taking place inside organic and perovskite solar cell devices. First part of the work focuses on the performance and stability of DBP-‐C70 based organic solar cells in standard and inverted device configurations. We study their device stabilities by aging them under ISOS-‐D-‐3 (darkness, 85℃ and 85% RH-‐humidity) and ISOS-‐T-‐3 (darkness, -‐40℃ and room humidity) conditions. The results show that despite a change in the performance upon aging, there is a pronounced morphological stability at the DBP-‐C70 interface. Possible effects from the electron transport layer (ETL) on the device stability were investigated, demonstrating that this layer contributes significantly to the degradation of the inverted devices. The second part of this work focuses on understanding the degradation mechanisms taking place inside perovskite solar cells under real operational conditions. Results for indoor (ISOS-‐L-‐1, illumination, 60℃ and ambient humidity) and outdoor (ISOS-‐O-‐1, sunlight, ambient) degradation test conditions are conducted, showing reversible and irreversible degradation mechanisms under light-‐darkness cycles, which reveal interesting degradation pathways and emphasize on the importance of including these cycles in experimental protocols for the assessment of long-‐term stability of the PSCs.

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Resume

I de seneste tre årtier har forskningen inden vedvarende energi været stærkt stigende, således at mængden og afhængigheden af fossile brændstoffer kan reduceres i fremtiden. Blandt teknologier indenfor vedvarende energi har organiske (OSC) og perovskite (PSC) solceller tiltrukket meget opmærksomhed, da de tilbyder unikke fordele sammenlignet med traditionelle silicium solceller, såsom lave fabrikationsomkostninger, mekanisk fleksibilitet, samt lette og gennemsigtige moduler. Disse unikke egenskaber tilbyder en lang række applikationer og integrationsmuligheder, som gør dette forskningsfelt yderligere interessant. OSC og PSC har for nyligt opnået ydeevner på hhv. 15% og 22%, hvilket understreger deres store potentiale. Begge disse typer solceller lider dog under relativt lav stabilitet og levetid, hvorfor en forståelse for degraderingsmekanismerne der finder sted i disse celler kan være med til at bane vejen for en kommercialisering af denne lovende teknologi.

Dette arbejde er fokuseret mod at undersøge de grundlæggende degraderingsmekanismer og retninger som finder sted i organiske og perovskite solceller. I første del af arbejdet undersøges ydeevnen og stabilitet af DBP-‐C70 organiske solceller i standard og inverteret konfigurationer. Vi studerer deres stabilitet ved at degradere dem under ISOS-‐D-‐3 (mørke, 85℃ og 85% luftfugtighed) og ISOS-‐T-‐3 (mørke, -‐40℃ og normal luftfugtighed) betingelser. Resultaterne viser at der, på trods af ændringer i ydeevnen under degradering, er en udtalt morfologisk stabilitet ved DBP-‐C70 grænsefladen. Mulige effekter fra elektron transport laget på solcelle stabiliteten blev undersøgt, hvilket demonstrerede at dette lag bidrager væsentligt til degradering af de inverterede solceller. Den anden del af dette arbejde fokuserer på forståelsen af de degraderingsmekanismer der finder sted I perovskite solceller under reelle arbejdsbetingelser. Resultater for indendørs (ISOS-‐L-‐1, belysning, 60℃ og rum luftfugtighed) og udendørs (ISOS-‐O-‐1, sollys) degraderingstest betingelser blev udført, og viste reversible og ikke-‐reversible degraderingsmekanismer under lys-‐mørke overgange, hvilket afslører interessante degraderingsmekanismer og understreger vigtigheden i at inkludere disse overgange i eksperimentelle stabilitetsprotokoller for PSC solceller.

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Acknowledgments

I would like to express my gratitude to everyone who supported me throughout the course of my PhD studies. This work would not have been possible without their help and contribution in one way or another.

First and foremost, I am greatly thankful to my supervisor Dr. Morten Madsen for giving me the opportunity to join NanoSYD and be part of the THINFACE project. His aspiring guidance, patience and enthusiasm have always motivated me, and his friendliness and openness releases inevitable tensions of a PhD work.

Next, I would like to thank my supervisor at TU Dresden, Prof. Koen Vandewal, for sharing his incredible knowledge and skills in the field and all his support and encouragement during my stay at TUD and through this research work.

Thank you Prof. Horst-‐Günter Rubahn and Dr. Katharina Rubahn, for all your support and friendly advice during my PhD studies and THINFACE project.

My colleagues/friends, Mina, Peyman, Andre, Mehrad, Bhushan, Ela, Pawel, Michela, Elodie, Jani, Arkadiusz, Vida, Elahe and Fatemeh thank you for all the nice time and happy memories inside and outside of the office. Mina and Peyman, I was so lucky to have your valuable friendship during the long and dark days when things can challenge you to your very core. You turned all of them into great memories.

Thanks to beautiful souls around the department, Sabina, Charlotte, Lise, Ferran, Zora, Roana and Luciana for their kind and cheering attitude.

I would also like to thank our lab technicians and manager who are the backbone of the lab and the facilities, Mogens, Reiner, Jens and Arkadiusz.

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I would especially like to thank my colleagues/friends at IAPP, TU-‐Dresden Johannes Benduhn for the sEQE measurements, and Dr. Donato Spoltore for his help with the device fabrication. It was a pleasure to work with you. I enjoyed the energetic and friendly research environment of IAPP. You made my stay memorable!

To my love Gerrit, I am deeply thankful for your unconditional support and love. You made it possible to come this far. You are truly one of a kind!

Last but not least, I am extremely grateful to my beloved family for their love and never ending support throughout all these years. In every moment of my life they have always stood by me and cheered me up.

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List Of Papers I. Degradation pathways in standard and inverted DBP-‐C70 based organic solar cells Golnaz Sherafatipour, Johannes Benduhn, Bhushan Ramesh Patil, Mehrad Ahmadpour, Donato Spoltore, Horst-‐Günter Rubahn, Koen Vandewal, Morten Madsen Manuscript II. Dynamics of photoinduced degradation of perovskite photovoltaics: from reversible to irreversible processes Mark V. Khenkin, Anoop K. M., Iris Visoly-‐Fisher, Sofiya Kolusheva, Yulia Galagan, Francesco Di Giacomo, Olivera Vukovic, Bhushan Ramesh Patil, Golnaz Sherafatipour, Vida Turkovic, Horst-‐Günter Rubahn, Morten Madsen, Alexander V. Mazanik, and Eugene A. Katz ACS Appl. Energy Materials III. Reconsidering figures of merit for performance and stability of perovskite photovoltaics Mark V. Khenkin, Anoop K. M., Iris Visoly-‐Fisher, Yulia Galagan, Francesco Di Giacomo, Bhushan Ramesh Patil, Golnaz Sherafatipour, Vida Turkovic, Horst-‐Günter Rubahn, Morten Madsen, Tamara Merckx, Griet Uytterhoeven, João P. A. Bastos , Tom Aernouts, Francesca Brunetti, Monica Lira-‐Cantu and Eugene A. Katz RSC Energy & environmental science IV. Area-‐dependent behavior of bathocuproine (BCP) as cathode interfacial layers in organic photovoltaic cells Bhushan Ramesh Patil, Mehrad Ahmadpour, Golnaz Sherafatipour, Talha Qamar, Antón F. Fernández, Karin Zojer, Horst-‐Günter Rubahn, Morten Madsen Scientific Reports-‐Nature-‐Submitted, 2018

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Contents

List of Figures ......................................................................................................... 15

1 Introduction ....................................................................................................... 19 1.1 A green horizon ........................................................................................................ 19 1.2 Photovoltaic devices .............................................................................................. 20 1.3 organic solar cells .................................................................................................... 21 1.3.1 Challenges toward commercialization of organic solar cells ...... 22

1.4 Perovskite solar cells ............................................................................................. 23 1.4.1 Challenges toward commercialization of perovskite solar cells 23

1.5 Aim of the thesis ...................................................................................................... 24 2 Fundamentals .................................................................................................... 27 2.1 Organic semiconductors ....................................................................................... 27 2.2 Operation principles of organic solar cells (OSCs) ................................... 29 2.2.1 exciton generation ......................................................................................... 29 2.2.2 exciton diffusion and dissociation .......................................................... 30 2.2.3 carrier transport ............................................................................................. 35 2.2.4 charge extraction at electrodes ................................................................ 36 2.2.5 Summery of the operation .......................................................................... 37

2.3 Solar cell architectures ......................................................................................... 38 2.3.1 Planar vs. bulk heterojunction .................................................................. 38 2.3.2 Standard vs. inverted structure ............................................................... 39

2.4 Materials ...................................................................................................................... 40 2.4.1 Donor and Acceptor ...................................................................................... 40 2.4.2 buffer layers ...................................................................................................... 42 2.4.3 Contacts .............................................................................................................. 44

2.5 Fabrication techniques ......................................................................................... 44 2.6 Stability of organic solar cells ............................................................................ 46 2.6.1 Intrinsic degradation .................................................................................... 46 2.6.2 Extrinsic degradation ................................................................................... 46

2.7 Perovskite solar cells ............................................................................................. 47 2.7.1 Fabrication ........................................................................................................ 49 2.7.2 Stability ............................................................................................................... 49

2.8 Characterization ...................................................................................................... 50 2.8.1 J-‐V characteristics .......................................................................................... 50 2.8.2 ISOS degradation tests ................................................................................. 56

2.9 Relation between VOC and CT states ................................................................ 56 3 Experimental Setup ........................................................................................ 61 3.1 Device fabrication ................................................................................................... 61 3.1.1 patterning of the substrates ...................................................................... 61 3.1.2 Pre-‐cleaning the substrates ....................................................................... 62

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3.1.3 Vacuum deposition of organic/recombination layers and top electrode ........................................................................................................................ 63 3.1.4 Final structure ................................................................................................. 66

3.2 Characterization ...................................................................................................... 67 3.2.1 J-‐V measurements .......................................................................................... 67 3.2.2 Sensitive external quantum efficiency (sEQE) measurements .. 68 3.2.3 Degradation protocols ................................................................................. 69 3.2.4 Morphological characterization ............................................................... 70 3.2.5 Photoluminescence quenching measurements ................................. 70

4 Degradation pathways in Standard and Inverted DBP-‐C70 Based Organic Solar Cells ................................................................................. 73 4.1 Device Performance: .............................................................................................. 75 Standard vs. Inverted configuration ....................................................................... 75 4.2 Sensitive EQE measurements ............................................................................ 77 4.3 Morphology investigation at the D-‐A interface .......................................... 78 4.4 Degradation studies ............................................................................................... 80 4.5 ETL charge transport properties and its effect on the stability .......... 85 4.5.1 Electron-‐only devices ................................................................................... 86 4.5.2 Photoluminescence quenching measurements ................................. 88 4.5.3 Improved stability of the inverted devices ......................................... 90

5 Understanding the degradation mechanisms in perovskite solar cells ................................................................................................................... 93 5.1. Outdoor day/night degradation and recovery tests ............................... 94 5.2 Indoor degradation and recovery dynamics ............................................... 99 5.3 Correlation between indoor and outdoor stability measurements . 106

6 Summary and outlook ................................................................................ 109 Appendix A ............................................................................................................ 113 ISOS-‐T-‐3 degradation test for standard structure .......................................... 113

Appendix B ............................................................................................................ 115 TTF as donor material in organic solar cells ..................................................... 115

Bibliography ......................................................................................................... 133

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List of Figures

Figure 1.1. Three generations of solar cells. Mono crystalline and poly crystalline Silicon based solar cells, thin film solar cells and organic solar cells .......................................................................................... 21

Figure 2.1. Illustration of HOMO and LUMO level of an organic material with bonding-‐antibonding interactions. ........................... 28

Figure 2.2. Three differnet types of excitons. a) Wannier-‐Mott excitons, with delocalized and weakily bound presented in inorganic materilas b) Strongly bound Frenklel excitons entirely located on one molecule in organic semiconductotors. ............... 30

Figure 2.3. Left) Charge-‐trasfer excitons, located on the adjacent molecules at the donor-‐acceptor hetetojunction; right a) Ground state at which the molecules are neutral and the HOMO of the molecules is filled; right b) Charge transfer state at which the donor is positively and the acceptor is negatively charges. Electron is transferred from donor to the acceptor molecule and it is still coulombically bounded to the hole in the donor molecule. ........................................................................................................... 31

Figure 2.4. Marcus theory. Electron transport can occur if the initial state overcomes the barrier of ΔG* to reach the crossing point of the potentials. Marcus derived the height of the barrier. ............ 34

Figure 2.5. The principle of charge separation in an organic solar cell operation. Summary of the 4 main steps. ........................................... 37

Figure 2.6. a) Planar heterojunction or bilayer solar cell b)Bulk heterojunction solar cell with a donor/acceptor blended active layer for more efficient charge generation. ....................................... 38

Figure 2.7. Two main geometries of PHJ organic solar cells: a) standard and b) inverted configuration. ............................................. 40

Figure 2.8. Skeletal formula of tetraphenyldibenzoperiflanthene (DBP). ................................................................................................................. 41

Figure 2.9. Schematic drawing (a) soccer-‐ball structure of buckyball C60 and (b) rugby-‐ball structure of common fullerene C70. (c) C70 Powder ............................................................................................................... 42

Figure 2.10. (left) Crystal structure of molybdenum trioxide (Hyung-‐Seok Kim/Nature Materials), (right) MoO3 powder ...................... 43

Figure 2.11. Left) skeletal formula of Bathocuproine, 2,9-‐dimethy-‐4, 7-‐diphenyl -‐1,10-‐phenathroline (BCP), right) BCP powder. ...... 44

Figure 2.12. Schematic drawing of spin coating procedure. Desired solution is applied over a substrate located on a spinner, then

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the spinner is rotated and the material is spread over the surface by centrifugal force to form a film. ........................................ 44

Figure 2.13. Thermal evaporation chamber. Substrate is located upside down above the source. Material is thermally evaporated and is deposited on the surface of the rotating substrate. A Crystal monitors the deposition rate. ................................................... 45

Figure 2.14. Generic perovskite crystalline lattice arrangement of the form ABX3. Note that the lines represent crystal orientation and not the bonding patterns. The two structure are equivalent with left) atom B at the <0,0,0> position and right) atom A at the <0,0,0> position. ............................................................................................. 48

Figure 2. 15. Electric circuit equivalent to an OPV device. .................... 51 Figure 2. 16. Current-‐voltage (I-‐V) curve of an organic solar cell. a)

Without illumination, the solar cell behaves like a diode, b) after illumination, the curve is shifted downward as the cell start to generate power, c) increasing the light intensity results in further shift of the curve downward as the generated power increases. ........................................................................................................... 52

Figure 2.17. Operation of a solar cell under different applied biases; 1) large reverse bias; 2) small reverse bias; 3) positive bias, results in zero initial field, and corresponds to open-‐circuit condition; 4) positive bias and carrier injection .............................. 53

Figure 2.18. Solar cell J-‐V curve metrics. ....................................................... 54 Figure 2.19. a) free energy diagram for the ground state and lowest

excited state. b) Reduced EQE PV and EL spectrum with fits using formulas (10) and (11). Each parameter is indicated in the figure. .................................................................................................................. 59

Figure 3.1. Layout of the patterned ITO coated glass substrates. ...... 62 Figure 3.2. Cryofox deposition cluster system. Organic materials and

metals are deposited in two separated chambers in ultra high vacuum conditions. A robotic arm transfers the sample between the chambers without breaking the vacuum between the steps. Right. The system is connected to a glove box to avoid exposure of the samples to air. (http://www.polyteknik.com/index.php) ............................................................................................................................... 64

Figure 3.3. Organic sources located inside the ultra high vacuum chamber. Each source is placed inside crucible, and a shutter to cover the source while not in use. .......................................................... 65

Figure 3.4. Final outlook of the DBP-‐C70 based organic solar cells. Each sample consists of 4 cells. Overlap of the top electrode with

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the bottom ITO electrode defines the cell area, which here is 10 mm2 ..................................................................................................................... 66

Figure 3.5. Final illustration of DBP-‐C70 based organic solar cells with a) standard and b) inverted configurations. C and d show energy band diagram for each structure. ........................................................... 67

Figure 3. 6. Sample holder with 4 sample positions ................................ 68 Figure 3.7. Sketch of the sensitive EQE measurement setup120. 69

Figure 4.1. J-‐V curves of DBP-‐C70 organic based PHJ devices having standard or inverted configurations. ........................................................ 76

Figure 4.2. sEQE measurements at 300 K and Marcus fits for standard and inverted structures. Dashed lines are fits to the EQE using Marcus theory. ..................................................................................................... 78

Figure 4.3. AFM images of interface layer (a) DBP on MoO3/ITO, b) C70 on DBP/MoO3/ITO, c)C70 on BCP/ITO, and d) DBP on C70/BCP/ITO .................................................................................................................................... 79

Figure 4.4. J-‐V curves for fresh (solid lines) and aged (dashed lines) devices at a) ISOS-‐D-‐3 b) and ISOS-‐T-‐3 degradation conditions. . 81

Figure 4.5. sEQE measurements and their corresponding Fits for fresh and aged devices at a) ISOS D-‐3 and b) ISOS-‐T-‐3 degradation conditions. ............................................................................................................. 83

Figure 4.6. sEQE measurements and Marcus Fits for fresh and annealed devices at 110 ℃ for 3 hours ......................................................................... 84

Figure 4.7. a) Electron only devices with different ETLs, and with deposited 10 nm BCP and 100 Ag on top b) JV measurements of the fresh EODs c) JV measurements after aging devices for 24 hours in ISOS-‐D3 and d) ISOS-‐T-‐3 degradation conditions. ............ 87

Figure 4.8. Photoluminescence (PL) measurements for five ETLs, fresh and degraded at ISOS-‐D-‐3 and ISOS-‐T-‐3. The stack has the structure: (0.5nm)/BCP (0.5nm) and BCP (2nm)/Ag (1nm)/BCP (2nm)/Ag (1nm)/BCP (2nm)). ........................................................................ 89

Figure 4.9. JV measurements for fresh and aged inverted device with 0.5 nm BCP and BCP/Ag stack. ..................................................................... 91

Figure 5.1. Normalized PCE evolution for indoor contentious simulated sunlight illumination of glass/ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/spiro-‐OMeTAD/Au cells (type I). .................................................................. 96

Figure 5.2. Normalized PCE evolution during two weeks of outdoor exposure to natural sunlight (a) type I (glass/ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/spiro-‐OMeTAD/Au), and (b) type II mini-‐modules

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(glass/ITO/TiO2/CH3NH3PbI3/Spiro-‐OMeTAD/Au). All lines are guides for the eye. .............................................................................................. 96

Figure 5.3. Normalized evolution of daily energy output, Eday, of (a) cell type I, and (b) mini-‐modules type II. All lines are guides for the eye. ........................................................................................................................... 98

Figure 5.4. Evolution of PV parameters of PSCs under continuous 1-‐sun indoor illumination, interrupted at T80 (a) T60 (b), or T50 (c, d). Gray areas show their subsequent recovery in the dark. ............... 100

Figure 5.5. Evolution of PV parameters after turning on the light and continuous simulated 1-‐sun re-‐illumination of the PSCs after T50 and recovery in the dark (gray areas): (a) The cell whose PCE dark recovery reached saturation (as in Figure 5.4.c); (b) The cell whose PCE dark recovery did not reach saturation (as in Figure 5.4.d). ..................................................................................................................... 100

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CHAPTER 1

Introduction

1.1 A green horizon The future generation is facing a broad and complex topic, which

is the foreseeable world’s energy crisis. The main supply of energy

today is Fossil fuels, however, these natural resources are in limited

supply and consumption of them is giving rise to further climate

change. By 2020, the global consumption of energy is expected to

increase by 50 percent, and the main debate is to find alternatives to

meet this demand. The best solution is to replace these finite

resources with renewable ones, known as green energy such as

wind, solar, biomass and geothermal. Green energy utilizes energy

resources that are readily available all over the world.

During the past three decades, there has been a tremendous

amount of research and development in the field of green energy

that has helped to reduce our dependence on fossil fuels. Among all

the renewable resources, solar energy has gained enormous

attention in past decades. Harnessing energy from the sun is the

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most reliable and pragmatic approach to cater the global energy

needs. In fact, the energy from the sun that hits the earth’s surface in

just one hour can power the whole planet in energy for one year.

However, utilizing this great power to its maximum potential is

limited by many factors such as materials, manufacturing, etc.

1.2 Photovoltaic devices In order to harvest the sunlight, we use devices that can convert

sunlight into electricity. These devices are called solar cells, and the

process of converting sunlight into electricity is called photovoltaic;

therefore, solar cells are also known as photovoltaic devices.

Solar cells are categorized into three generations according to the

time sequence1 (see Figure 1.1). First generation solar cells are

crystalline and multi-‐crystalline Silicon (Si), with simple constitution,

but high manufacturing cost. The theoretical efficiency of this type of

solar cells is of about 30%2. Efficiency of a solar cell refers to the

portion of sunlight that can be converted into electrical power via

the photovoltaic effect.

Second generation solar cells or thin film solar cells made out of

amorphous Si with lower manufacturing cost, but poor stability and

inherent problems. The most successful materials of this generation

are Cu(In,Ga)Se2 (CIGS) and CdTe/CdS. The lab efficiency for this

generation is around 19%.

The third generation solar cells introduce new concepts in terms

of device architecture and materials; for example the idea of multi

junction solar cells to improve harvesting of photons and overcome

the 30% limit for efficiency. This 3rd generation includes dye-‐

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sensitized (DSSCs)3,4 and Organic Solar Cells (OSCs), quantum dot

(QD) and Perovskite photovoltaic. DSSCs are fabricated using dyes,

metal oxides and electrolyte, and their efficiencies are in the range of

12% for small lab scale devices. However, the lifetime of the devices

are low compared to inorganic solar cells.

Figure 1.1. Three generations of solar cells. Mono crystalline and poly crystalline Silicon based solar cells, thin film solar cells and organic solar cells

1.3 organic solar cells An organic solar cell is a type of an organic optoelectronic device,

that deals with conductive low weight organic materials such as

small molecules and oligomers, or high weight molecules, i.e.,

polymers, which is vastly used5. These devices use photovoltaic

effect to directly convert sunlight into direct current (DC) electricity.

In 1906, Pochettino observed photoconductivity in an organic

compound named Anthracene6, and it was the beginning of a new era

for application of organic compounds in electronics.

In 1954, high conductivity in perylene-‐iodine complex was

discovered7, after which organic semiconductors attracted great

amount of attention. In recent decades, organic solar cells (OSCs)

have been under intense research due to their interesting

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advantages compared to first and second generation. They exhibit

higher absorption coefficient, which makes it possible to fabricate

very thin films with low amount of materials. Moreover,

manufacturing them is easy and cost-‐effective7–10, and the final

product is flexible, semitransparent and lightweight. They also

provide a huge potential for large area applications and portable

solar panels. This means they can be printed by roll-‐to-‐roll (R2R)

machinery. These advantages connote a great potential for organic

solar cells to make a significant impact on the future of the PV

market. More details and technical information about organic solar

cells is provided in chapter 2.

1.3.1 Challenges toward commercialization of organic solar cells Despite all the advantages for organic solar cells, still achieving a

high efficiency and long-‐term stability11 is remaining the bottleneck

for potential commercialization of this appealing technology. To this

date, a world-‐record of 15% efficiency has been achieved for organic

solar cells12, however, this still needs to be improved to compete

with current photovoltaic technologies on the market.

Low stability and short lifetime of organic solar cells means that

over time, they loose their photovoltaic ability, and their efficiency is

decreased. These changes can be caused by extrinsic or intrinsic

factors such as oxygen and moisture from the air or dynamic and

active nature of organic materials11,13 (more details in chapter 2).

Despite improvements regarding these issues14, loss mechanisms

and degradation patterns yet need to be fully understood. In order to

push the limitations for application of organic solar cell beyond its

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boundaries, a deep understanding of the degradation process that

limits and controls the stability of the devices is crucial. Our

comprehensions can serve as a guide for finding new solutions and

identify new materials for more advanced and stable next-‐generation

solar cells.

1.4 Perovskite solar cells Perovskite solar cells are another type of solar cells that have

attracted prominent attention over the past few years due to their

high efficiency of around 20%15, which is comparable to other types

of inorganic photovoltaic such as Cadmium Telluride. Their high

efficiency have made them rising star of the photovoltaic world since

their breakthrough paper of 201216, and a huge interest to the

academic community (more details in section 2.7).

Over the past two years, improvements in fabrication routines

and engineering of perovskite formulations has led to significant

increase in their power conversion efficiency which reached over

22%, as of April 201717. This dramatic rise is incredibly impressive

and the efficiency is comparable to Cadmium Telluride, which has

been used for near 40 years. Furthermore, their potential for much

lower processing cost makes them significantly interesting for its

future market.

1.4.1 Challenges toward commercialization of perovskite solar cells Despite their high efficiency, perovskite solar cells suffer from a

low stability and short operational lifetime. There are several

reaction pathways involving, oxygen, water and diffusion of metal

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from the electrodes that leads to degradation (more details in section

2.7.2)18–25. Another problem is the use of lead in perovskite

compounds and products for commercial use that is a huge source of

toxic pollution. A lead alternative such as tin-‐based perovskites is

possible, however, the power conversion efficiency of such devices is

much behind the lead-‐based devices26.

Finally another issue is the lower optical density of these

materials, which means a higher thickness of the light-‐harvesting

layer is needed compared to organic solar cells, which again results

in some fabrication limitations in solution processed devices27.

Perovskite solar cells can also be fabricated based on vacuum

deposition techniques to give a better, more uniform film qualities.

However, evaporation of organic and inorganic materials requires

special evaporation chambers that are not available in many

research labs, and not easy to control in large area depositions.

Previously, vacuum deposition techniques offered the highest

efficiency devices, but recently, through the improvements in

solution-‐based deposition techniques, the record-‐breaking devices

has shifted to solution-‐based processing28.

Overall, to enable a reliable low cost-‐per-‐watt energy solution,

perovskite solar cells need to obtain longer device lifetime, and low

manufacturing costs. Although this has not yet been achieved,

perovskite-‐based solar cells still demonstrate enormous potential to

achieve this.

1.5 Aim of the thesis The broad title of this dissertation comprises degradation studies

on organic and perovskite solar cells. The presented work takes the

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opportunity to investigate governing mechanisms taking place inside

these devices. For organic solar cells, we focus on factors affecting

the open-‐circuit value as one of the most important parameters

affecting both performance and stability of the devices. In this work

small molecule solar cells based on DBP and C70 are investigated. For

perovskite solar cells, their stabilities are tested under real

operational conditions and a correction between already existing

stability measurements for indoor and outdoor testing conditions is

suggested. The results points on possible and specific degradation

pathways that are important to understand in detail, in order to

facilitate the viable commercialization of organic and perovskite

solar cells in the future. The work is structured as follows.

Chapter 2 provides background information regarding organic

and perovskite solar cells, principles of operation, materials and

characterization techniques.

Chapter 3 describes the fabrication steps including the

measurement and characterization setups employed in this work.

Chapter 4 presents the results and discussion regarding the

performance and degradation mechanism in the DBP-‐C70 organic

solar cells investigated in this work.

Chapter 5 presents results and discussion related to degradation

mechanism in the perovskite solar cells tested under both indoor

cycling protocols and outdoor testing.

Chapter 6 provides conclusion remarks as well as an outlook for

the future studies.

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CHAPTER 2

Fundamentals This chapter introduces basics of the organic and perovskite solar

cell (OSC) technologies, including the characterization of those.

Understanding these concepts is required to follow the studied

material subjected to this work.

2.1 Organic semiconductors Organic semiconductors are carbon-‐based materials in which

carbon atoms are covalently bonded to each other by alternating

single and double bonds (conjugated π-‐bonds), and a weak Van der

Waal’s force that holds two organic molecules within the solid29,30.

The bonding structure is the property that gives organic

semiconductors its unique advantages such as flexibility, lightweight,

and easy processing.

The band structure of an organic semiconductor can be viewed

similarly as inorganic semiconductor in which the valance band is

filled with electrons and conduction band is free of electrons. In

28

organic semiconductors, the lowest electronic transition (and the

most probable one) is between the π–band, which is the Highest

Occupied Molecular Orbital (HOMO), and the π*–band, which is the

Lowest Unoccupied Molecular Orbital (LUMO), (Figure 2.1). These

types of molecular orbitals are analogous to the valance and

conduction band of an inorganic semiconductor, respectively29.

There are techniques such as cyclic voltammetry and photoemission

yield spectroscopy to measure the HOMO and LUMO level of the

organic materials31.

The energy difference between the HOMO and LUMO in an

organic semiconductor is denoted as the band gap of the material,

which is generally between 1.1-‐3.5 eV. Hence, optical excitation can

happen in the range of visible light and near infra-‐red32.

When an electron is excited from HOMO to LUMO, it causes the

excitation of the whole molecule itself into a higher energy state.

This is different than the actual excitation of a free electron from the

valance band to the conduction band in inorganic semiconductor33 .

Figure 2.1. Illustration of HOMO and LUMO level of an organic material with bonding-‐antibonding interactions.

29

2.2 Operation principles of organic solar cells (OSCs) Operation of an organic solar cell can be simplified into 4 main

steps:

1. Photon absorption resulting in formation of electron-‐hole pairs

(exciton)

2. Exciton diffusion to heterojuction

3. Exciton dissociation at the heterojunction

4. Carrier transport and extraction at the electrodes

Each of these steps will be described here in detail.

2.2.1 exciton generation When light is shined over an organic solar cell, photons are

absorbed at the active organic materials. If the photon energy

exceeds the band gap of the material, it excites an electron to the

LUMO level, while leaving a hole in the HOMO. Due to low dielectric

constant (2-‐4)34 of the organic materials and low screening of

charges, upon transition of an electron from HOMO to LUMO, a

coulombically bounded electron-‐hole pair known as exciton is

formed with a binding energy of 0.1-‐1.4 eV. These types of excitons

are known as Frenkel excitons, which are entirely located on one

molecule, Figure 2.2.b. However, in inorganic semiconductors due to

a higher dielectric constant (12-‐16), the Coulomb attraction is

weakened and screening is increased; therefore, the average radius

between electron and holes is larger than the lattice spacing, leading

to a much lower binding energy of only 5-‐15 meV33. These types of

excitons are known as Wannier-‐Mott excitons and their binding

energies are sufficiently below thermal energy at room temperature

30

(KBT≈25 meV), which results in a higher probability of free charge

carrier generation after absorption of photons and dissociation of

the pairs by absorbing thermal energy, Figure 2.2.a.

Due to the fact that the absorption coefficient of organic materials

is higher than inorganic materials (~105 cm-‐1), a thin layer of a few

hundreds of nanometers of the active layer is sufficient to absorb

enough amount of light and generate carriers.

Figure 2.2. Three differnet types of excitons. a) Wannier-‐Mott excitons, with delocalized and weakily bound presented in inorganic materilas b) Strongly bound Frenklel excitons entirely located on one molecule in organic semiconductotors.

2.2.2 exciton diffusion and dissociation After excitons are formed, next step is to separate them and

generate free charges, which eventually leads to generation of

current (electricity). To achieve an efficient exciton pair separation,

Tang et al.10 provided a solution by employing two different kinds of

organic materials with properly aligned band energies. One material

acts as electron donor and the other is an electron acceptor, resulting

in the basic concept of the so-‐called donor-‐acceptor (D/A) in OSCs.

The interface between the two materials is called the D/A

heterojunction. The acceptor material is a strongly electronegative

31

and when placing the two materials adjacently, the electron can go to

a much lower energy state within the acceptor. This causes the

dissociation of the exciton at the heterojunction. When an exciton is

generated in the electron donor material, it migrates towards the

heterojunction, and charge transfer from donor to acceptor initiates

if the difference between the LUMO energy levels of the donor and

acceptor overcome the exciton binding energy. This makes the

electron to transfer from exciton to LUMO of the acceptor while a

hole remains in HOMO of the donor35, Figure 2.5.3.

2.2.2.1 Charge transfer states After the charge transfer at the heterojunction, the electron-‐hole

pair is still Coulombically bound and are located at the D/A interface

on adjacent but different molecules. This new type of exciton, which

is between the two other types in terms of binding energy and

binding distance, is called charge transfer (CT) exciton and its in a

charge transfer state, Figure 2.3.

Figure 2.3. Left) Charge-‐trasfer excitons, located on the adjacent molecules at the donor-‐acceptor hetetojunction; right a) Ground state at which the molecules are neutral and the HOMO of the molecules is filled; right b) Charge transfer state at which the donor is positively and the acceptor is negatively charges. Electron is transferred from donor to the acceptor molecule and it is still coulombically bounded to the hole in the donor molecule.

32

The charge separation at the CT state is about 1 nm, which is

roughly the distance between the donor and acceptor molecules. The

energy of the CT state is defined as the energy difference between

the ionization potential of the donor and electron affinity of the

acceptor (IPD-‐EAA) plus EB which is a term to account for the binding

energy of the CT which is typically estimated at approximately 0.1-‐

0.5 eV.

The energy of the CT state depends on different factors such as

the composition of the donor and acceptor molecules, processing

treatments, etc36. For example, Loi et al. noted that increasing the

concentration of the high dielectric constant PCBM in F8DTBT:PCBM

blend results in lowering the CT state energy due to an increased

effective dielectric constant of the blend37–39. In addition, it reduces

the effective Coulombic interaction between electron and hole, and

facilitates CT state dissociation into free carriers40. Moreover, it has

been reported that thermal annealing of the P3HT:PCBM decrease

the energy of its CT state38,41–43.

As mentioned before, CT pairs at the heterojunction are still

Coulombically bound and need to be separated by an internal field

before they recombine. In fact, CT states are intermediate states

between exciton recombination and dissociation; hence, they play a

crucial role in charge generation in organic photovoltaics44–48.

2.2.2.2 Marcus theory From a molecular perspective, charge transfer from donor

molecule to acceptor molecule is describing by a theory known as

Marcus theory49. In 1956 R. A. Marcus won a Nobel Prize for

introducing a method for calculating rates of electron transfer

33

reactions at which electrons can hop from electron donor molecule

to electron acceptor. This theory is now widely used in chemistry

and physics.

In Marcus theory, the potential of reactants and products are

sketched as parabolas, Figure 2.4, In this sketch, ΔG° is free energy

changes between the reactants and products (difference between

potential minima), ΔG* is activation energy and λ is reorganization

energy, which is the energy required to force the reactants to have

the same nuclear configuration as the products without electron

transfer. Electron transfer can only occur if the excited D/A pair

(D*/A) overcomes the barrier of ΔG* to reach the crossing point of

the potential wells. Marcus derived the height of the ΔG* from the

thermodynamic parameters of the system:

ΔG∗ =(𝜆 + 𝛥𝐺°)!

4𝜆 (2.1)

In organic solar cells, the theory has been successfully applied to

describe the absorption and emission shape of charge transfer states

in D-‐A systems.

In Figure 2.4, a parabola with the lowest energy shows the relaxed

ground state for donor and acceptor interface. After absorption of

the light, one of the molecules is excited (D* or A*) and the molecular

conformation changes at the D/A interface. If the excitation energy

can overcome the barrier ΔG*, the Frenkel exciton being localized at

one molecule separates at D/A interface and forms CT exciton (D*/A

D+/A-‐).

34

Figure 2.4. Marcus theory. Electron transport can occur if the initial state overcomes the barrier of ΔG* to reach the crossing point of the potentials. Marcus derived the height of the barrier.

2.2.2.3 Recombination As mentioned earlier, due to the low dielectric constant of the

organic materials, excited electron-‐hole pairs are still Coulombically

bound after dissociation and while they form a CT exciton. The pair

is called a geminate pair and before they fully dissociate into free

charge carriers, they can still recombine back to the ground state in a

process known as geminate recombination. Since charge transfer

process occurs faster than the charge recombination (~45 fm vs. ~1

ns)50,51, efficient exciton dissociation at the heterojunction is

possible.

Even after dissociation, recombination can happen again if the

dissociated free electrons and holes encounter each other once more.

This time, since the two carriers are separated, free carriers, this

process is called a nongeminate recombination.

In addition to these two situations, if after absorption of light,

generated excitons cannot reach the interface, they recombine back

35

to their ground state. The maximum length that an exciton can

diffuse before recombination is called the exciton diffusion length. In

organic materials this distance is a few tens of nanometer52,53. If the

distance where the excitons are generated is longer than their

diffusion length, they recombine before reaching the heterojunction.

Hence, in organic solar cells, the active layer is kept thin to make

sure that it is within the exciton dissociation length. However, a

thinner active layer can result in low absorption efficiency.

Therefore, as a solution to this tradeoff, bulk heterojunction solar

cells were invented to increase the efficient exciton dissociation by

increasing the amount of the interface between the acceptor and

donor materials.

In all cases being explained above, the energy of the photon is lost

in the form of radiation that results in fewer carriers that are

collected at the electrodes.

2.2.3 carrier transport After dissociation, generated electrons and holes have to travel to

electrodes for collection. The two main driving forces for transport

are drift and diffusion currents. Drift current causes the carriers to

move along the potential gradient inside the solar cell and it

determines by the choice of electrodes. Choosing a high work

function anode and low work function cathode creates a built-‐in

electric field, in which applying an external bias modifies the internal

electric field and the drift current. This current leads the carriers

toward the respective electrodes and from there they are collected.

Diffusion current is another mechanism, which transport the

carriers along the concentration gradient inside the solar cell.

36

Concentration of electrons and holes are higher around the

heterojunction due to generation of geminate pairs there; hence,

carriers diffuse away from the interface and cause diffusion current.

After this point, the ability of the material to transport the carriers

is critical. This ability is called mobility and characterizes the speed

of the carrier transport through the material. In general, electrons

have higher mobility54 than holes which means choosing donor and

acceptor materials with high mobility differences can lead to

accumulation of charges at the interface between active layer and

electrodes55. Therefore, balancing hole and electron mobilities is

critical to achieving efficient carrier transport in the organic solar

cells.

2.2.4 charge extraction at electrodes When the carriers reach at the active layer/electrode interface,

they can be extracted. In order to maximize the efficiency in charge

extraction, the potential barrier have to be minimized. Therefore

ideally the work function of the cathode should match the LUMO of

the acceptor and the work function of anode should match the HOMO

of the donor. This way an ohmic contact is formed and carrier

extraction condition is fulfilled. In organic solar cells, indium tin

oxide (ITO) with a work function of ~4.7 eV is commonly used as

anode. On the cathode side, a low work function material such as Al

(4.2 eV) can be used.

Another trick to align the work functions of the electrodes with

HOMO and LUMO of the organic materials is to use the interlayers

known as buffer layers at the active layer/electrode interface.

Molybdenum oxide (MoO3), Zinc oxide (ZnO) and Bathocuproine

37

(BCP) are a few examples of buffer layers, which have shown to

improve the carrier collection56. Apart from changing the electrode

materials and using buffer layers, increasing the roughness or

interface area of the active layer and electrodes can improve the

charge collection efficiency.

2.2.5 Summery of the operation Upon absorption of a photon in an organic solar cell, an exciton is

formed. This exciton travels toward the donor-‐acceptor interface,

and if it can reach the interface before decaying, the exciton can

dissociate, with the electron transferred to the adjacent acceptor

molecule and hole remained at the donor molecule. This state is

called a charge transfer (CT) state in which the charge carriers can

either dissociate or recombine back to the ground state. If they split

into free charge carriers, they can travel toward the electrodes and

be collected, or if they meet at an interface, they can recombine with

Figure 2.5. The principle of charge separation in an organic solar cell operation. Summary of the 4 main steps.

38

another charge carrier and loose their energy. The summery of the

operation is shown in Figure 2.5.

2.3 Solar cell architectures Solar cell architectures can significantly influence the

performance of the devices. Here, two main types with two common

architectures are explained.

2.3.1 Planar vs. bulk heterojunction Planar heterojunction or bilayer cell is the simplest interface

structure in which a layer of donor and a layer of acceptor on top of

each other (joined together) form the active layer. This layer is

sandwiched between charge collection layers (buffer layers) and

electrodes (anode and cathode), Figure 2.6.a. This simple structure is

based on the basic operating principles of the solar cell. However,

due to the low diffusion length of the excitons in organic material

(around 10 nm), there is a thickness limitation for the layers.

Figure 2.6. a) Planar heterojunction or bilayer solar cell, b)Bulk heterojunction solar cell with a donor/acceptor blended active layer for more efficient charge generation.

Furthermore, a thin active layer results in a weak absorption, and

a delicate thickness balance is thus present in these cells. Introducing

39

bulk heterojunctions (BHJ) solar cells in the mid 1990s7 provided the

solution to this tradeoff, Figure 2.6.b. In this structure, mixing or co-‐

evaporation of donor and acceptor materials forms the junction. The

resulting film is a network of donor/acceptor domains, and since the

scales of the domains are within the diffusion length of the excitons,

it provides a path for efficient carrier transport and dissociation.

Therefore, nearly all generated excitons are dissociated and free

charge carriers are transported towards electrodes and are collected.

This advantage makes it possible to form a thicker active layer

compared to bilayer solar cells. Nevertheless, controlling the film

morphology is harder in the films formed by spin coating compared

with vacuum deposition (more information about the techniques are

provided in section 2.5). This means that there are many parameters

that can affect the performance of the BHJ devices. There exist

various methods to improve the morphology of the BHJ solar cells,

such as thermal annealing, solvent annealing or modifying the

functionality of the organic materials57–59. These techniques have

improved the performance of the OSC devices.

2.3.2 Standard vs. inverted structure

There are two different solar cell geometries: standard (or

conventional) and inverted (see Figure 2.7). If the anode is directly

placed on the substrate it is a standard, and if the cathode is on the

substrate it is an inverted configuration. There are advantages and

drawbacks of each geometry. Standard configuration usually results

in higher efficiencies; however, inverted configuration usually has

higher stability60–65, although clear exceptions are present in both

cases.

40

Figure 2.7. Two main geometries of PHJ organic solar cells: a) standard and b) inverted configuration.

2.4 Materials 2.4.1 Donor and Acceptor Oligomers (small-‐molecules) and conjugated polymers are two

main categories of organic materials being commonly used in active

layer of the organic solar cells. Small molecules have low molecular

weights; while polymers are heavy molecules consist of long

molecular chains. Small molecules are usually deposited via ultra-‐

high vacuum thermal deposition whereas polymers are deposited

using solution-‐processed methods, such as spin-‐coating, or printing

techniques such as ink-‐jet printing and doctor blade.

Small molecules have several advantages compared to polymer

materials, which makes them attractive for their application in solar

cells. Some of the main advantages are:

• Ease of synthesis and reproducibility of the process.

• High purity of the material

• Better control of the morphology and structure of the film by

controlling the growth parameters, such as substrate

temperature, deposition pressure and rate.

41

• No need for solvents (some toxic)

• Fabrication of multilayer structures, such as tandem solar

cells is possible by controlling the thickness of the layer in

nanometer scale.

In the following, the chemical structure and key optoelectronic

properties of the materials used in this thesis are presented.

Tetraphenyldibenzoperiflanthene (DBP)

DBP is a p-‐type (electron donor) semiconductor with a

symmetrical molecular structure. This molecule is only composed of

carbon and hydrogen atoms (see Figure 2.8). Among other donor

molecules, DBP is a promising electron donor material, which has

been utilized in many laboratories since 200966–72.

High optical absorption and a deep HOMO level around 5.5 eV are

principle advantages of DBP in application of solar cells66,73. These

main advantages allow fabrication of a thinner active layer, which

facilitates reaching the exciton to the D/A interface. Moreover, its

HOMO level energy of around -‐5.5 eV is energetically compatible

with fullerene acceptors to be used in the active layer. Energy

difference between HOMO of the DBP and LUMO of the fullerene

results in a high open circuit voltage (VOC)4,66,71,74–78.

Figure 2.8. Skeletal formula of tetraphenyldibenzoperiflanthene (DBP).

42

Fullerene C70

Fullerene molecule is a hollow molecule cage consisting of sixty or

more carbon atoms. It can be in the shape of a sphere, ellipsoid, tube

or other shapes. Buckminsterfullerene or buckyball was the first

known example of the spherical shaped fullerenes. C70 is a fullerene

molecule with 70 carbon atoms, and resembles the shape of a rugby

ball. In 1985, Robert Curl, Harold Kroto and Richard Smalley

discovered Fullerenes (with the most common C60 and C70) (see

Figure 2.9), and they were awarded the 1996 Nobel Prize in

chemistry for their discovery79. Since then C70 has been used as

electron acceptor and electron transporting molecule. HOMO and

LUMO of C70 are -‐6.1 eV and -‐4.0 eV respectively80.

Figure 2.9. Schematic drawing (a) soccer-‐ball structure of buckyball C60 and (b) rugby-‐ball structure of common fullerene C70. (c) C70 Powder

2.4.2 buffer layers Molybdenum trioxide MoO3

Molybdenum oxide is known for its catalytic activity and semi-‐

conductive properties. MoO3 is an n-‐type material being used as hole

transport layer in organic solar cells81–84 (Figure 2.10).

Compared to other metal oxides such as WO3 and V2O5,

molybdenum oxide can be evaporated at very low temperatures

(~ 400 ℃) in vacuum from a crucible. Using MoO3 between the ITO

43

and donor layer improves the fill factor of the devices by reducing

the series resistance. This in general improves the power conversion

efficiencies of the solar cells83. Moreover, it has been shown that

using MoO3 instead of PEDOT:PSS in OPV devices improves their

stability85.

Figure 2.10. (left) Crystal structure of molybdenum trioxide (Hyung-‐Seok Kim/Nature Materials), (right) MoO3 powder

Bathocuproine BCP

Bathocuproine, 2,9-‐dimethy-‐4, 7-‐diphenyl -‐1,10-‐phenathroline or

BCP is a well-‐known electron transport layer being used in organic

solar cells86–88. BCP is a crystalline white or yellow powder, and is

insoluble in water (Figure 2.11). This molecule is a wide band gap

material, and importing an 8-‐10 nm thick layer BCP between

acceptor layer and top metal contact such as aluminum improves

electron transport properties by reducing geminate recombination

of the exciton at the acceptor-‐metal interface. Furthermore, it

protects the active layer from damages caused by the metal

deposition87,88.

44

Figure 2.11. Left) skeletal formula of Bathocuproine, 2,9-dimethy-4, 7-diphenyl -1,10-phenathroline (BCP), right) BCP powder.

2.4.3 Contacts

Active layer and buffer layers are sandwiched between indium tin

oxide (ITO) as anode and top Ag or Al as cathode. ITO is a

transparent conducting oxide with electrical conductivity and optical

transparency in the visible regime.

2.5 Fabrication techniques There are two common ways to deposit layers of a solar cell: spin

coating and vacuum deposition. In spin-‐coating processes, a small

amount of material in form of a solution is applied on the center of a

substrate being located on a holder of a spin-‐coater machine. Then

the substrate is rotated at a high speed and the coating material is

Figure 2.12. Schematic drawing of spin coating procedure. Desired solution is applied over a substrate located on a spinner, then the spinner is rotated and the material is spread over the surface by centrifugal force to form a film.

45

spread by centrifugal force (see Figure 2.12). Depending on the

viscosity and concentration of the solution, as well as the speed of

the spinner, a desired thickness of the film is achieved.

Vacuum thermal evaporation is another deposition technique, in

which an organic material is heated up in high vacuum. In this

technique, the substrate is place upside down in a distance above the

Figure 2.13. Thermal evaporation chamber. Substrate is located upside down above the source. Material is thermally evaporated and is deposited on the surface of the rotating substrate. A Crystal monitors the deposition rate.

source, where the evaporated material is directly deposited onto the

substrate, as shown in Figure 2.13. This allows for depositing many

layers of different materials without chemical reactions between the

layers. A crystal monitors the deposition rate to reach the desired

thickness. However, for large-‐area substrates, there may be

problems with film thickness and uniformity. In this work, we used

46

vacuum thermal evaporation technique to fabricate bulk and bilayer

DBP-‐C70 based organic solar cells.

2.6 Stability of organic solar cells One of the main drawbacks of organic solar cells is their low

stability and short lifetime due to device degradation. This means

that over time, they loose their photovoltaic ability to generate

electricity, which results in a decay in their PCE. Depending on the

importance of air exposure, it can be divided into two categories;

intrinsic and extrinsic degradation.

2.6.1 Intrinsic degradation Intrinsic degradation is caused by thermal intrusion of constituent

materials inside the solar cell. This incudes chemical degradation as

well as molecular segregation or rearrangement at the interfaces and

active materials13. This type of degradation can happen either in dark

conditions or under illumination (photo-‐induced)11,89,90.

Over a short period of time at dark conditions, molecular

segregation and rearrangement at material interfaces hinder charge

extraction. For longer periods, degradation causes phase separation

at BHJ solar cells and results in less efficient charge generation11,91–98.

Under illumination, photo-‐induced degradation causes JSC or VOC

losses especially at solution processed active layers due to increasing

the energy disorder of the polymer materials11. In fullerene

acceptors especially C60, photo-‐dimerization causes JSC loss99.

2.6.2 Extrinsic degradation Extrinsic degradation is penetration of air (oxygen and water)

into the active layer and interlayers, and causing chemical reaction.

47

This results in the loss of photocurrent from the active layer and

dead zones are grown in the device100,101. Reaction of oxygen with

the electrodes can increase the work function of metals by forming

surface dipoles and deteriorate the performance of the devices100,102.

Under light condition, photooxidative loss of absorption (bleaching)

of the semiconductor materials can cause degradation97,103,104.

There are solutions to prevent intrinsic and extrinsic

degradations11,13, such as encapsulation of the devices with low

permittivity materials105, or using pure, dense materials with highly

ordered film morphology to lower the oxygen and water diffusion

into the layers106. Using non-‐fullerene acceptor materials will also

reduce photo-‐induced JSC and VOC losses, which normally results from

photo-‐dimerization of fullerene acceptors107. Moreover, using high

work function metals is desirable to reduce extrinsic degradation102.

2.7 Perovskite solar cells Perovskite solar cells are made out of the perovskite mineral that

was named after Lev Perovskite who was the founder of the Russian

Geographical Society.

The perovskite structure can be simplified as a hybrid molecule

with organic atom A (positively-‐charged cation in the center of the

cube), inorganic atoms B (usually a metal cation) located on the

corners of the cube, and smaller halide atoms X (anion with negative

charge) occupying the faces of the cube108. In the world of solar cells,

perovskites and the perovskite structure are used interchangeably.

Any compound that has the same crystallographic structure as the

perovskite mineral is also called perovskite, which means the generic

48

form ABX3. For example, Perovskite mineral composed of Calcium,

Titanium and Oxygen forms the CaTiO3.

Depending on the atoms/molecules that are used in the structure,

perovskite can show interesting properties that make them very

exciting for physicists, chemists and material scientists. Their

intrinsic properties such as broad absorption spectrum, fast charge

separation and long carrier lifetime and long electron and hole

transport distance (over one micron109,110) have made them

promising materials for solid-‐state solar cells. The perovskite

crystalline lattice arrangement is demonstrated in Figure 2.14.

Figure 2.14. Generic perovskite crystalline lattice arrangement of the form ABX3. Note that the lines represent crystal orientation and not the bonding patterns. The two structure are equivalent with left) atom B at the <0,0,0> position and right) atom A at the <0,0,0> position.

Low costs of both raw materials and fabrication methods, high

efficiency, light weight and flexible perovskite solar cell have made

them commercially attractive. Perovskite conversion efficiency has

reached to about 22.7% in late 2017 (listed in an efficiency chart

provided by NREL). However, one of the main challenges that hold

them back from the market is their low stability and lifetime.

49

2.7.1 Fabrication Perovskite solar cells can be easily incorporated into standard or

thin film architecture. Vacuum deposition can be used to co-‐

evaporate the organic (methylammonium) and inorganic (lead

halide) components to give a uniform and high quality film.

However, co-‐evaporation of these materials requires special

chambers that may not be available in many labs. Moreover, this

may cause cross-‐contamination between organic and non-‐organic

sources in the deposition chambers, which is difficult to clean.

Development of solution deposition processes has provided

simpler methods to fabricate Perovskite solar cells28. A lead halide

and methylammonium halide can be dissolved in a solvent and spin

coated in one-‐step processing onto a substrate. However, other steps

and additives are required in order to achieve a homogenous and

high quality film.

For the interlayers, many of the standard electron and hole

transport layers from the world of organic solar cells such as

PEDOT:PSS, PCBM, ZnO and TiO2 can be used in perovskite solar

cells.

2.7.2 Stability

As mentioned before, one main challenge in the field of perovskite

solar cells is their poor stability and short lifetime. This is mainly

related to influence of the different factors such as oxygen and

moisture18,19, thermal intrinsic stability20, heating under applied

voltage21, UV photodegradation22, and mechanical fragility23. Water

solubility of the organic constituent of the perovskite absorber

material make these devices highly sensitive to moisture in the air

50

and causes rapid degradation24. This sensitivity can be reduced by

reducing the constitute material or by changing the cell’s

architecture. Moreover, encapsulation of the devices prevents the

immediate degradation of these devices under moist conditions;

however, long-‐term studies have yet to be performed24,25. It has been

shown that the UV Photodegradation of the perovskites with TiO2 is

linked to the interaction of the photogenerated holes inside the TiO2

with the oxygen radicals on the surface of the TiO2 111.

These two instability problems can be solved by using

multifunctional coatings to provide strong hydrophobic barrier and

block the UV light of the incident solar spectrum and converting it

into visible light112. In addition to these solutions, Insertion of a

mechanically reinforcing scaffold into the active layer of the

perovskite solar cells has provided a higher fracture resistance by

30-‐fold113. Nevertheless, despite all these efforts, long-‐term stability

of the perovskite solar cells needs to be further enhanced for

successful commercialization of this appealing technology in the

market.

2.8 Characterization

2.8.1 J-‐V characteristics Organic solar cells are typically characterized under 1000W/m2

light (1.5 AM solar spectrum). In the dark, the solar cell acts as a

simple diode. An ideal solar cell can be modeled by an equivalent

circuit in which the current source is in parallel with a diode.

However, since no solar cell is ideal, a series resistance (Rs) and a

shunt resistance (Rsh) component are added to the circuit. This

51

model is useful to understand the electronic behavior of a solar cell

(see Figure 2.15).

Figure 2. 15. Electric circuit equivalent to an OPV device.

Series resistance is resulted from all the resistances at the

interfaces, conductivity of the semiconductors and the electrodes.

Shunt resistance is related to the defects in the film and takes into

account the leakage of the current through these defects. Ideally a

low Rs and a high Rsh is desired. In this circuit, n is the ideality factor

of the diode, ID is the saturation current, which is the current in the

dark at the reverse bias, and Iph corresponds to photocurrent

generated during illumination.

Figure 2.16 shows the current-‐voltage (I-‐V curve) characteristic for a

typical solar cell. Without illumination, the solar cell electrical

characteristics are similar to a diode. After illumination, the IV curve

is shifted down to the 4th quadrant as the cell begins to generate

power. The greater the light intensity, the greater the generated

current and the amount of shift.

52

Figure 2. 16. Current-‐voltage (I-‐V) curve of an organic solar cell. a) Without illumination, the solar cell behaves like a diode, b) after illumination, the curve is shifted downward as the cell start to generate power, c) increasing the light intensity results in further shift of the curve downward as the generated power increases.

The operation of the solar cell under different applied biases can

be summarized into 4 steps (see Figure 2.17):

1. Large reverse bias: when a reverse bias is applied, it

reinforces the built-‐in electric field, and due to presence of a

strong electric field drift current is dominant. This enhances

the exciton dissociation and results in an efficient charge

transport and a large photocurrent.

2. Small reverse bias: when the applied bias is close to

zero, the built-‐in field is the only force exists in the device,

which drives the carriers to the corresponding electrodes for

collection. When the applied bias is increased in positive

direction, it opposes the built in field. Therefore, drift current

decreases and results in decrease of the current.

53

3. Positive bias and zero internal field: at the point where

the applied bias is equal to the built in field, the electric field

inside the device becomes very small and diffusion current

dominates the current. This condition is called open-‐circuit

condition and corresponds to the maximum voltage out of a

solar cell.

4. Positive bias: by further increasing the bias, the

applied bias becomes larger than the built-‐in field that

reverses the potential gradient in the device. Therefore,

carrier injection occurs through the tunneling and results in a

positive current.

Figure 2.17. Operation of a solar cell under different applied biases; 1) large reverse bias; 2) small reverse bias; 3) positive bias, results in zero initial field, and corresponds to open-‐circuit condition; 4) positive bias and carrier injection

It is more common to use current density (J) instead of current (I)

on the y-‐axis. Therefore, the key parameters of the solar cell that

define the performance of the cell can be extracted from its J-‐V curve.

These parameters are open circuit voltage (VOC), short circuit current

(JSC), fill factor (FF), the maximum power conversion efficiency

(Pmax), the voltage and the current at max power point (Vmax and

Imax). Figure 2.18 shows solar cell J-‐V curve metrics. Each of these

parameters is described here:

54

Figure 2.18. Solar cell J-‐V curve metrics.

Open circuit voltage (VOC)

Is defined as the voltage at which the current density output is

zero. The value can be extracted from the crossing point of the J-‐V

curve with V axis at 0 J.

Short circuit current (JSC)

Is defined as the current at which the externally applied voltage is

zero. The value can be extracted from the crossing point of the J-‐V

curve at 0 volt. The value presents the number of the generated

charge carriers that were collected at the electrodes at short circuit

condition. JSC value can be improved by using small bandgap

materials with high absorption coefficient and high carrier mobility.

Moreover, as discussed before, a bulk heterojunction with more

mixed phases (smaller domains) can improve the charge generation

efficiency.

Fill factor (FF)

The shape of the J-‐V curve is defined by the fill factor parameter. It

is defined as the ratio between the maximum obtainable output

55

power (product of Jmpp and Vmpp at maximum power point) and

product of the open-‐circuit voltage and short-‐circuit current.

FF =J!"" V!""J!" V!"

(2.2)

A low electron mobility causes increase in the recombination rate

before they reach the interface therefore a higher external bias is

needed to sweep the carriers to the heterojunction for dissociation.

This results in a strong dependency of the current on the applied

bias and leads to a lower FF.

Power conversion efficiency (PCE)

Efficiency of a solar cell is also known as the solar power

conversion efficiency, that is the ratio of produced electrical power

to the incident irradiation power.

𝑃𝐶𝐸 =𝑉!" 𝐽!"𝐹𝐹

𝑃!" (2.3)

Where Pin is the input power density.

External Quantum Efficiency (EQE)

The percentage of the photons that are converted to charge

carriers and collected at the electrodes is called the external

quantum efficiency (EQE). This parameter is defined as the product

of the efficiency of the four steps; absorption, diffusion, dissociation

and collection,

𝜂!"! = 𝜂! . 𝜂!"## . 𝜂!"## . 𝜂! (2.4)

A narrow absorption band or a thin active layer can lower the

absorption efficiency. Moreover, a high recombination rate and

photo generated loss due to quenching at metal electrodes or high

phase separation in the active layer can lower EQE.

56

2.8.2 ISOS degradation tests

In order to track the behavior of the organic and hybrid devices

over time and to understand the degradation mechanisms, solar cell

devices are tested under various stability test conditions known as

accelerated lifetime measurements. To increase reproducibility of

the conditions across different labs, International Summit on OPV

Stability (ISOS) protocol have been established. There are different

ISOS test conditions, such as dark, illumination, indoor, outdoor,

laboratory weathering, thermal and humidity cycling. These

conditions are simulated in systems known as climate chambers.

After putting the devices in the desired environment, the

performance of the devices is monitored over time and the result is a

plot that shows the efficiency versus time, known as decay curve.

There are three main regimes of degradation of solar cells known as

burn-‐in, long-‐term and failure. Analyzing the behavior of the solar

cells in these three zones can give much information about the

overall stability of the devices114–117.

2.9 Relation between VOC and CT states One of the prominent parameters that limits the efficiency of the

solar cells is open circuit voltage (VOC), that is the maximum possible

voltage out of a solar cell. Numerous studies have shown that VOC can

be predicted from the energy gap between the HOMO of the donor

and LUMO of the acceptor material in a BHJ. Therefore, VOC depends

on the choice of the materials; which means choosing light

harvesting materials with higher energy bandgaps can be beneficial.

However, analyzing VOC across a wide range of materials have shown

57

that although the optical bandgap of the light harvesting materials is

around 1.7 to 2.1 eV, VOC barely exceeds half of the incident photon

energy meaning around 1.0 V118. This difference between the

material bandgap and VOC has experimentally shown to be

approximately 0.6 eV, while it is only 0.3 to 0.45 eV for inorganic

materials such as Si, CIGS, and GaAs119. To date, there have been

many efforts to address the origins of the VOC losses and finding the

solutions to prevent these, however, it remains still an open

question.

There are many factors that can influence the open circuit voltage

in organic solar cells such as energetic disorders or density of states,

charge transfer states, donor-‐acceptor interface, carrier density and

interface morphology. Apart form these, various other factors such

as temperature, light intensity, recombination, etc. can influence the

VOC indirectly120. Among all, we are interested in charge transfer

states, which are relating the properties of the D-‐A interface to the

VOC value. Several studies have demonstrated that there is a strong

relation between VOC and CT state emission (ECT) being correlated

according to:

ECT−qVOC=0.6±0.1 eV (2.5)

Vandewal et al. obtained an analytical expression for VOC by

separating the losses from ECT into radiative and non-‐radiative

recombination:

𝑉!" =𝐸!"𝑞 − Δ𝑉!"# (𝑇)− Δ𝑉!"!#$% (𝑇) (2.6)

The two temperature dependent radiative and non-‐radiative

recombination losses are given by the following equations:

58

Δ𝑉!"# 𝑇 = − 𝑘𝑇𝑞 ln

𝐽!" ℎ!𝑐!

𝑓𝑞2𝜋 𝐸!" − 𝜆 (2.7)

Δ𝑉!"!#$% 𝑇 = −𝑘𝑇𝑞 ln 𝐸𝑄𝐸!" (2.8)

Here J!" represents the short circuit current, and EQE!" is the

external quantum efficiency of electroluminescence. Equation 2.6

shows a linier correlation between the photovoltage with the energy

of the CT states. A detailed deduction can be found in the paper by

Vandewal et al.121.

Since the energy of the CT state is one of the lowest energy states

in the SC, a highly sensitive absorption technique such as sEQE is

used, in which the CT state is determined at the shoulder of the

spectrum at very low energies38. The working principle of the sEQE

measurements is similar to the standard EQE measurements, and

will be explained in Chapter 3.

There are two methods both based on the principles of the Marcus

theory (see section 2.2.2.2) to extract the CT properties.

Based on this theory49,122, a formalism has been raised by

Vanderwal et al.121 to characterize the properties of the CTS, and has

been widely used to investigate the energy of the CT states123–125.

𝐸𝑄𝐸(𝐸) =𝑓!!

𝐸 4𝜋𝜆𝑘𝑇𝑒𝑥𝑝

−(𝐸!" + 𝜆 − 𝐸)!

4𝜆𝑘𝑇 (2.9)

Where k is Boltzmann’s constant and T is the absolute

temperature. ECT is the free energy difference between the CT

complex ground state and the CT excited state, 𝜆 is reorganization

energy, f is a parameter proportional to the density of CT states and

represents the strength of the donor/acceptor material interaction, h

is Planck’s constant, and c is the speed of light in vacuum. Through a

59

Gaussian fit over the shoulder of the sEQE curve at sub-‐bandgap

region, ECT, 𝜆, and f can be extracted (fitting parameter).

Another method is to measure both CT absorption via sEQE, and

CT emission via electroluminescence (EL). The crossing point of

these two spectra equals ECT, and 𝜆 and f can be extracted through a

fit over the sub-‐bandgap region and using Equations below. (see

Figure 2.19)126.

𝐸𝑄𝐸!" 𝐸 .𝐸 =𝑓!"!4𝜋𝜆𝑘𝑇

exp−(𝐸!" + 𝜆 − 𝐸)!

4𝜆𝑘𝑇 (2.10)

𝐸𝐿(𝐸)𝐸 =

𝑓!"4𝜋𝜆𝑘𝑇

exp−(𝐸!" − 𝜆 − 𝐸)!

4𝜆𝑘𝑇 (2.11)

The left hand-‐side of the above equations show a mirror image

relationship, and due to the multiplication and division of the spectra

by E, they are called reduced absorption and emission spectrum,

respectively.

Figure 2.19. a) free energy diagram for the ground state and lowest excited state. b) Reduced EQE PV and EL spectrum with fits using formulas (10) and (11). Each parameter is indicated in the figure.

61

CHAPTER 3

Experimental Setup

Organic solar cells used in this work are fabricated over pre-‐

patterned indium-‐tin-‐oxide (ITO) (≈ 100 nm) coated glass wafers

with a sheet resistance of 9-‐15 Ω/sq (purchased from University

wafer, Inc, USA). Patterning process is carried out in a class ISO7

cleanroom, and then prepared substrates are transferred to a

laboratory equipped with fabrication and measurement systems to

proceed with other steps. All organic active materials, metal oxides

and metal layers are formed using a vacuum deposition system.

In this chapter, each of these device fabrication steps and the

measurement setups are explained in detail.

3.1 Device fabrication

3.1.1 patterning of the substrates Pattering process is done using a positive lithography process,

through which a thin layer of adhesion promoter,

Bis(trimethylsilyl)amine (also known as hexamethyldisilazane, or

62

HMDS) is applied in 120°C, and a photoresist (AZ5214E,

Microchemicals GmbH, Germany) is spin-‐coated and baked at 90°C

for 1 min on a hot plate. Patterns are transferred using a UV Karl

Suss MA 150 Aligner. Then an ITO etching process is performed for 4

min in HCL:HNO3:H2O (1:0.08:1) solution at 40°C (etching rate≈ 0.5-‐

1 nm) to remove excess areas. Then the photoresist is stripped off in

Acetone placed in ultra-‐sonic bath for 10 min to reveal the final

patterns.

3.1.2 Pre-‐cleaning the substrates Patterned wafers are diced into 15 by 15 mm2 substrates, and are

cleaned using detergent, acetone and isopropanol in an ultra-‐sonic

water bath (10 min each), and blow-‐dried with a nitrogen (N2) gun.

Figure 3.1 shows the layout of the pattered ITO samples after being

diced into smaller substrates.

Figure 3.1. Layout of the patterned ITO coated glass substrates.

Prior to fabrication process, substrates are treated with air

plasma cleaning at 400 mTorr for 20 min. Then right after the

treatment, they are transferred to a glovebox connected to a high-‐

vacuum deposition system with a base pressure of about 10-‐8 mbar.

63

3.1.3 Vacuum deposition of organic/recombination layers and top electrode The whole fabrication process is done in a Cryofox cluster system

in UHV environment, and via thermal evaporation process (Figure

3.2). Cluster system is connected to a Nitrogen Glovebox to avoid air

exposure during the whole fabrication process. In order to avoid

material contamination in different layers, deposition of organic

materials, and metals are performed in separate chambers having

vacuum pressure of about 5 x 10-‐8 mbar. These chambers are

connected with a transfer chamber with a base pressure of around

10-‐9, where a robotic arm transfers the sample between the

chambers. This allows a sequential deposition in high vacuum

conditions without breaking the vacuum between the steps. Vacuum

is broken only when there is a need to use a different mask, during

which the samples are only exposed to the Nitrogen environment of

the glovebox. The whole process is controlled through a control unit

where sample recipes are loaded for deposition of each layer

separately.

To form the active layer, tetraphenyldibenzoperiflanthene (DBP)

(purchased from Luminescence Technology Corp., Taiwan) and

fullerene (C70) (purchased from Sigma-‐Aldrich, Germany) are used as

electron donor and acceptor, respectively. For exciton blocking layer

Bathocuproine (BCP), and for electron transport layer molybdenum

oxide (MoO3) (both purchased from Sigma-‐Aldrich, Germany) are

deposited.

64

Figure 3.2. Cryofox deposition cluster system. Organic materials and metals are deposited in two separated chambers in ultra high vacuum conditions. A robotic arm transfers the sample between the chambers without breaking the vacuum between the steps. Right. The system is connected to a glove box to avoid exposure of the samples to air127.

Figure 3.3 shows the organic sources for DBP, C70 and BCP inside

the organic chamber. Each crucible is covered with a shutter when it

is not in use, and a quartz crystal microbalances (QCM) are

controlling the deposition rate for each source with tenth of an

angstrom precision. Materials are thermally evaporated and are

deposited over the surface of rotating samples that are placed upside

down in a distance above the sources. This way thickness uniformity

of 10% across a 10cm sample holder is achieved. Moreover, if

needed, there is a possibility to heat up the substrates up to 250℃.

This allows annealing the samples in vacuum during evaporation or

between depositions of each layer.

65

Figure 3.3. Organic sources located inside the ultra high vacuum chamber. Each source is placed inside crucible, and a shutter to cover the source while not in use.

As the final step, 100nm silver (Ag) (purchased from AESpump

ApS, Denmark) is deposited as top electrode to finish the fabrication.

Deposition of MoO3 and Ag is performed in a separate chamber.

Deposition rate is kept at 0.3 Å/s for organic materials and 0.5-‐1 Å/s

for Ag top electrode using quartz crystals. In order to avoid a short

between the bottom ITO contact with the top Ag electrode, a shadow

mask with a wider opening is used to form the active layer, while a

mask with smaller opening is used during the metallization. Overlap

of Ag contact area with its underneath organic layers defines a cell

area of around 10 mm2. These fabrication steps takes around 3-‐4

hours. Figure 3.4 shows the final look of the fabricated sample. The

final sample consists of 4 cells with separated active areas sharing a

common ITO bottom electrode.

66

Figure 3.4. Final outlook of the DBP-‐C70 based organic solar cells. Each sample consists of 4 cells. Overlap of the top electrode with the bottom ITO electrode defines the cell area, which here is 10 mm2

3.1.4 Final structure In this work we fabricate organic solar cells based on the same

active and buffer materials but with two different standard and

inverted configurations, Figure 3.5.a and 3.5.b, respectively. The

order and thickness of each layer is as follow:

• Standard configuration:

ITO/MoO3 (10 nm)/DBP (20 nm)/C70 (30 nm)/BCP (10 nm)/

Ag (100 nm)

• Inverted configuration:

ITO/BCP (0.5 nm)/C70 (30 nm)/DBP (20 nm)/MoO3 (10

nm)/Ag (100 nm).

Note that the thickness of the DBP (donor) and C70 (acceptor) are

kept the same in both configurations. Energy diagram for each

configuration is illustrated in Figure 3.5.c and 3.5.d.

67

Figure 3.5. Final illustration of DBP-‐C70 based organic solar cells with a) standard and b) inverted configurations. C and d show energy band diagram for each structure.

3.2 Characterization

3.2.1 J-‐V measurements Fabricated devices are mounted in a homemade sample holder

(Figure 3.6), and are characterized in ambient condition using a

3000 class AAA solar simulator from Abet Technology Inc., USA with

a calibrated arc lamp. The system is connected to a Keithley 2400

source measure unit (Keithley instrument Inc., USA), and LAbView

software is controlling the measurements. The JV curve is recorded

applying a voltage sweep between +1 to −0.25 V under illumination

intensity of 100 mW/cm2.

68

Figure 3. 6. Sample holder with 4 sample positions

3.2.2 Sensitive external quantum efficiency (sEQE) measurements The working principle of the sEQE measurements is shown in

Figure 3.7, and is similar to the standard EQE measurements.

In this technique, the light from a quartz Halogen lamb (50 W, TS

Electric) is chopped at 140 Hz and is passed through a

monochromator (Cornerstone 260 1/4m, Newport). Then the

monochromatic light is focused onto a solar cell mounted in a holder,

and the produced current at the short circuit condition is sent to a

current amplifier (DHPCA-‐100, FEMTO). This current is then

analyzed by a LOCK-‐IN-‐amplifier (7280 DSP, Signal Recovery, Oak

Ridge, USA) to extract the EQE properties. EQE is defined as the ratio

of the photocurrent to flux of incoming photons, which is obtained

using a calibrated silicon (FDS100-‐CAL, Thorlabs) and/or indium-‐

gallium-‐arsenide (InGaAs) photodiode (FGA21-‐CAL, Thorlabs) 119. In

order to achieve a span spectrum over 5-‐7 decades, the time

constant of the amplifier was set to 1s, and its amplification was

69

increased to resolve low photocurrent. The key points to achieve a

higher sensitivity compared to the standard EQE are to use a halogen

lamp which has a broad and contentious spectrum down to infrared

(IR), avoid optical fibers for illumination, and using high optical

density filter.

Figure 3.7. Sketch of the sensitive EQE measurement setup128.

3.2.3 Degradation protocols Stability measurements are performed in four ISOS aging test

conditions:

1. ISOS-‐D-‐1 (darkness, room temperature and humidity) in a

dark shelf

2. ISOS-‐D-‐3 (darkness, 85℃ and 85% RH-‐humidity) in a

climate chamber with controlled humidity and temperature

3. ISOS-‐T-‐3 (darkness, -‐40℃ and room humidity) in a thermal

chamber with ambient humidity and controlled temperature

4. ISOS-‐L-‐1 (illumination, 60℃ and ambient humidity) in a

setup consisting a solar simulator with monitored ambient

conditions

70

The setup used for ISOS-‐L-‐1 condition is consisting of a wide-‐area

metal halide lamp solar simulator (metal halide display/optic lamp

HMI from Orsam). The lamp is calibrated in ambient air with a Cell

and Meter (91150V, Newport). Using intensity and temperature

sensors, the ambient conditions are monitored and recorded

together with the solar cell parameters. For ISOS-‐D-‐1, D-‐3 and T-‐3, J-‐

V characteristics of the fresh samples are first measured using the

solar simulator under 1 sun illumination, and then devices are placed

in each condition for 24 hours in darkness, after which, J-‐V

characteristics are recorded again for the aged samples.

3.2.4 Morphological characterization Morphological characteristics are recorded using atomic force

microscopy (AFM). For our morphology investigations, a Dimension

3100 Nanoman scanning probe microscope from Veeco is used to

record the AFM images of the samples in air. Between the two modes

of AFM scanning, which are contact mode and tapping mode, the

second one is used for our experiments to avoid adhesions or

frictions between the tip of the cantilever and the surface of the

organic thin-‐films.

3.2.5 Photoluminescence quenching measurements Photoluminescence (PL) intensity measurements are performed

using a florescence microscope (Nikon Eclipse ME600), equipped

with a mercury short arc lamp having a filtered excitation

wavelength centered between 330nm-‐380nm. The system is

connected to a Maya2000Pro Spectrometer from Ocean optics to

71

record the spectra, and PL is collected with a microscope objective

(Nikon E Plan 50X 0.75 EPL) and after 10 sec integration time.

Measurements were repeated for aged samples.

We use the techniques described in this chapter to fabricate and

characterize fresh and degraded DBP-‐C70 based organic solar cells

with standard and inverted configurations. The results are presented

in the next chapter.

73

CHAPTER 4

Degradation pathways in Standard and Inverted DBP-‐C70 Based Organic Solar Cells

Organic solar cells have become an emerging competitive

technology due to their unique advantages such as low fabrication

cost, semi-‐transparency, flexible and lightweight products. However,

as it was discussed previously in this literature, achieving long-‐term

stability in organic solar cells is a remaining bottleneck for the

commercialization of this otherwise highly appealing technology. For

organic solar cells to be commercially viable, at least 10 years of

long-‐term stability is required129,130, which has presented a

significant challenge to this field. Therefore, a deeper understanding

of the degradation mechanism of the devices is needed, which will

pave the way for strategies on device lifetime improvement.

74

Degradation mechanisms in organic solar cells can be divided into

two main categories caused by either intrinsic or extrinsic factors.

The former is caused by diffusion of oxygen and moisture from the

air into device layers, and the latter is due to dynamic and active

nature of organic materials, and includes chemical degradation and

molecular rearrangement in the active materials and interfaces13.

Encapsulation of the organic solar cells minimizes the availability of

air to the different layers, and can significantly slow down the

extrinsic degradation. However, intrinsic degradation happens over

time even for the best-‐encapsulated devices91–96. Degradation

mechanisms in organic solar cells are rather complex and include

degradation of interfaces and interlayers, electrode diffusion, and

morphological changes. This implies the strong impact of the

interface between every two adjacent layers on the stability of the

devices60–65,131. In organic solar cell, the interface between the donor

and acceptor materials plays a critical rule in degradation of the

devices, where chemical and morphological changes over time

directly affect charge photogeneration and recombination pathways.

As introduced in chapter 2, residing at the D-‐A interface, charge

transfer (CT) state represents an intermediate state between charge

generation and recombination and its properties directly affect the

performance of the devices, especially the open-‐circuit voltage

(VOC)38,47,121,132. Moreover, degradation paths are rather intricate and

differ between standard and inverted devices60–63. As an example,

Krebs et al. and Cros et al. reported a higher air stability for inverted

configuration compared to standard configuration, while both

devices have the same type of photovoltaic layer and undergo an

identical processing procedure64,65.

75

In the work presented in this chapter, we study the degradation

pathways in DBP-C70 based organic solar cells, having standard and

inverted device architectures. We utilize sensitive external quantum

efficiency (sEQE) measurements to detect differences in

morphological properties between standard and inverted device

configurations, as well as detecting the potential degradation at the

DBP/C70 interface after degrading the devices at ISOS-‐D-‐3 and ISOS-‐

T-‐3 aging conditions. Our investigations reveal that despite the

variations in the VOC values of the fresh and degraded devices, CT

state properties undergo only very minor changes, suggesting a

pronounced morphological stability at the interface of DBP and C70.

Instead, it is shown that presence of Bathocuproine (BCP) electron

transport layer (ETL) in inverted devices leads to higher VOC losses and

low stability of these devices133. To address this issue, as an alternative

ETL, we introduce BCP/Ag stack with improved exciton blocking

properties and carrier transport efficiency. Our final results show

enhanced device stabilities for inverted devices implementing BCP/Ag

stack as their ETL.

4.1 Device Performance:

Standard vs. Inverted configuration The experiments were performed with DBP-‐C70 based organic

solar cells having standard and inverted structure. More information

about fabrication processes and characterization techniques can be

found in chapter 3.

The performances of the devices were recorded via J-‐V

measurements by applying a voltage sweep between +1 to −0.25 V

76

under 1 sun illumination. Figure 4.1 and Table 4.1 show the J-‐V

characteristics for the two types of devices with identical active

layers and contacts. Solar cell parameters are averages over 7

devices of each type, and J-‐V curves show the devices with

characteristics closest to the average parameters.

Figure 4.1. J-‐V curves of DBP-‐C70 organic based PHJ devices having standard or inverted configurations.

Table 4.1. Photovoltaic performances of DBP-‐C70 organic based PHJ devices having standard or inverted configurations. The values extracted over seven best performing devices of each type.

Devices VOC (V)

Jsc

(mA/cm2) PCE %

FF

St-PHJ 0.85 ± 0.05 6.32 ± 0.36 3.66 ± 0.21 68.0 ± 1.8 In-PHJ 0.72 ± 0.07 5.78 ± 0.40 2.68 ± 0.29 62.0 ± 2.5

Although both devices have identical active layers, standard

structure show an overall better performance with 130 mV higher

77

VOC and higher PCE. However, a recent work has demonstrated that

this trend may depend on device area134. The reported efficiencies are

similar to the ones previously reported for PHJ DBP/C70 cells in the

literature72,135.

Previous studies have shown that morphological differences and

changes in molecular orientations at the D-‐A interface can result in

VOC changes by altering the material energy levels136 and CT states

properties. Based upon Marcus formalism (see section 2.9) and

Gaussian fitting, we investigate the VOC differences through the sEQE

measurements and CT state characteristics. Using this method121,

ECT, lambda and f are determined by fitting the low energy part of the

EQE spectrum with a Gaussian.

4.2 Sensitive EQE measurements Using Eq. 2.9 and fitting to the sEQE spectra, we obtained ECT

values as 1.44 eV for standard vs. 1.37 eV for inverted configuration.

The normalized sEQE spectra and their corresponding fits are shown

in Figure 4.2, and extracted CT parameters (ECT, 𝜆, and f) are listed in

Table 4.2.

In standard configuration, the 70 meV higher ECT is in agreement

with its higher obtained VOC value; however, it does not fully explain

the differences in VOC upon inverting the structure. A slightly higher f

value obtained for standard structure identifies as higher density of

CT states. This is correlated to higher amount of interface between

the donor and acceptor materials, and represent the strength of the

interaction between the donor/acceptor molecules137. Previous

finding show that larger interface area results in more VOC losses due

78

to a higher recombination current138. However, the difference in f

value obtained here is minor, and should not contribute to significant

VOC differences.

Figure 4.2. sEQE measurements at 300 K and Marcus fits for standard and inverted structures. Dashed lines are fits to the EQE using Marcus theory.

Table 4. 2. VOC and CT characteristics extracted from fit parameters.

4.3 Morphology investigation at the D-‐A interface In order to further study the morphological differences at the D-‐A

interface of the devices, AFM imaging was performed. For this

purpose, samples were prepared with following structures:

Devices VOC (V) ECT (eV) f (eV2) λ (eV) St-‐PHJ 0.85 ± 0.007 1.41 0.0015 0.17 In-‐PHJ 0.72 ± 0.07 1.37 0.0008 0.22

79

a) ITO/MoO3 (10nm)/DBP (20nm),

b) ITO/MoO3/DBP (20nm)/C70 (30nm),

c) ITO/BCP (0.5nm)/C70 (30nm),

d) ITO/BCP (0.5nm)/C70 (30nm)/DBP (20nm).

Figure 4.3. AFM images of interface layer (a) DBP on MoO3/ITO, b) C70 on DBP/MoO3/ITO, c)C70 on BCP/ITO, and d) DBP on C70/BCP/ITO

Samples a & b represent layers in standard configuration, and c &

d represent layers in inverted configuration. For each sample

vacuum deposition was performed without breaking the vacuum

between the layers, and AFM images were recorded in air right after

fabrication, Figure 4.3. The root-‐mean-‐square (RMS) and surface

area values were calculated by software techniques applied to image

analysis and are shown in the figure.

Figure 4.3.a and 4.3.c show the surface of the lower layer of the D-‐

A interface in each type of device, respectively. The DBP layer in

standard configuration (Fig. 4.3.a) has a slightly smoother surface

with lower surface area, although very close to the one for inverted

configuration. Non-‐conformal film coverage for DBP on the C70 in

inverted device configuration could lead to the slightly lower f value

80

seen for this type of devices. In general, variations in interfacial

molecular orientations and morphological differences alter the

interaction of the molecules by varying electronic coupling between the

two molecules, and influences the charge generation efficiency, CT

properties and VOC loss paths136,139–142. Therefore, we note that both

modified molecular orientations and interface area can result in slightly

different f values seen here. This can be further investigated using

grazing incident wide-‐angle X-‐ray scattering (GIWAXS), and high-‐

resolution transmission electron microscope (HR-‐TEM)140; however,

difficulties in controlling the molecular orientations and a thin active

layer make this investigations impossible or challenging.

4.4 Degradation studies So far our findings show that morphological changes affect the CT

properties, which can be detected using sEQE measurements.

In this section, we benefit from this finding to detect degradation

mechanisms caused by morphological changes or molecular

rearrangement at the D-‐A interface. In order to track dark

degradation, standard and inverted devices were aged under ISOS-‐D-‐

3 (85 ℃, 85% RH-‐darkness) and ISOS-‐T-‐3 (-‐40℃ and room humidity-‐

darkness) degradation protocols for 24 hours without encapsulation.

Figure 4.4 shows J-‐V characteristics measured for both fresh and

aged devices under 1 Sun illumination using a solar simulator.

Average values over 7 devices of each set are presented in Table 4.3.

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Figure 4.4. J-‐V curves for fresh (solid lines) and aged (dashed lines) devices at a) ISOS-‐D-‐3 b) and ISOS-‐T-‐3 degradation conditions.

Table 4.3. Photovoltaic performance parameters of fresh and aged devices under ISOS-‐D-‐3 and ISOS-‐T-‐3 degradation conditions. Device parameters for standard configuration are marked in blue, and for inverted devices in red.

Devices VOC (V) Jsc (mA/cm2) PCE % FF

St-Fresh 0.85 ± 0.05 5.8 ± 0.26 3.50 ± 0.17 71.2 ± 1.1

In-Fresh 0.72 ± 0.07 5.88 ± 0.57 2.68 ± 0.29 62.0 ± 2.5

St-ISOS-D-3 0.36 ± 0.28 4.90 ± 1.44 0.68 ± 0.73 23.3 ± 16.7

In-ISOS-D-3 0.03 ± 0.05 5.60± 0.54 0.01 ± 0.02 7.3 ± 14.5

St-ISOS-T-3 0.87 ± 0.05 5.67 ± 0.34 2.94 ± 0.52 59.3 ± 6.7

In-ISOS-T-3 0.45 ± 0.07 5.86 ± 0.93 1.02 ± 0.55 37.9 ± 13.8

Results for ISOS-‐D-‐3 condition show a significant drop in the PCE

values of both devices. However, it is most significant for inverted

cells. This drop is mostly affected by reductions in VOC and FF values,

which may be resulted from an increase in the density of deeper

traps due to oxidation of the active layer143–145. These traps can act as

recombination sites and disturb the internal electric field

distribution144. Moreover, at high temperature condition, diffusion of

metal from the top contact into the buffer layer results in a

decreased FF90,146. Results for ISOS-‐T-‐3 condition show that the PCE

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value of the standard devices is less affected by this aging condition

and the VOC value remains intact. This could be due to the used low

temperature resulting in a slowed down chemical reactions taking

place in the organic materials. This effect is demonstrated in

Appendix A, Figure A.1, where degradation of encapsulated standard

devices in ISOS-‐T-‐3 condition is compared with the ones in ISOS-‐D-‐1

(darkness, room temperature) condition. We observe that for ISOS-‐

T-‐3 condition, after 3500 hours, the VOC value of the devices remains

above 80% of its initial value, while for the devices kept at room

temperature (ISOS-‐D-‐1), T80 point is reached much earlier at around

500 hours121,147. However, results for inverted devices in ISOS-‐T-‐3

condition (Figure 4.4), show a notable change in the device stability

compared to standard devices. Since both devices have the same

material system, these results indicate a degradation process not

related to standard photo-‐oxidation of the active layer. Under ISOS-‐

D-‐3 conditions, when high temperature and humidity are present,

degradation is accelerated and is influence by both chemical

reactions in the active layer and instabilities of the interlayers

resulting in pronounced degradation.

In order to detect possible degradations due to morphological

changes at the D-‐A interface, sEQE measurements were performed

after aging the devices. Figure 4.5 shows normalized sEQE spectra

and their corresponding fits for fresh and degraded devices at each

degradation condition. Table 4.4 represents the extracted CT

parameters.

83

Figure 4.5. sEQE measurements and their corresponding Fits for fresh and aged devices at a) ISOS D-‐3 and b) ISOS-‐T-‐3 degradation conditions.

Table 4.4. Extracted Voc and CT parameters through fits on sEQE spectra for fresh and aged devices.

Devices VOC (V) f (eV2) λ (eV) ECT (eV)

St-PHJ-Fresh 0.85 ± 0.05 0.0017 0.17 1.44

In-PHJ-Fresh 0.72 ± 0.07 0.0008 0.22 1.37

St-PHJ-ISOS-D-3 0.36 ± 0.28 0.0017 0.19 1.44

In-PHJ-ISOS-D-3 0.03 ± 0.05 0.0011 0.23 1.38

St-PHJ-ISOS-T-3 0.87 ± 0.05 0.0014 0.18 1.44

In-PHJ-ISOS-T-3 0.45 ± 0.07 0.0006 0.20 1.40

The sEQE results indicate that regardless of a change in VOC value

of the devices after degradation, the CT properties remain almost the

same. Unchanged f value after degradation suggests a morphological

stability at the DBP/C70 interface that means morphological changes

at the D/A interface are not responsible for a drop in voltage upon

aging of the devices. We further supported this by investigations on

morphological stability of the DBP-‐C70 interface through annealing

standard PHJ and BHJ devices. We fabricated these devices and

84

annealed them at 110 ℃ for 3 hours in dark inside the glovebox.

Extracted J-‐V parameters and CT properties for fresh and annealed

devices are presented in Figure 4.6 and Table 4.5.

Figure 4.6. sEQE measurements and Marcus Fits for fresh and annealed devices at 110 ℃ for 3 hours

Table 4.5. Voc and CT parameters extracted through fits on sEQE spectra for fresh and aged devices.

The results show that although, as expected, a significant higher f

value (higher amount of interface) is observed for BHJ cells, no

changes in CT properties and f value of the devices is seen upon

annealing.

85

For the inverted configuration (Figure 4.5), during degradation,

the C70 absorption (shoulder at 1.8 eV) decreases slightly relative to

DBP absorption. We ascribe the reason for this instability of C70 to

changes in the underlying BCP layer. It has been reported that

insertion of a BCP as ETL in inverted devices, depending on the

device area134, can hamper the device performance with low FF and

PCE133,148, and lead to poor stabilities148. Moreover, BCP tendency for

crystallization can induce defects at the ETL-‐C70 interface that can

give rise to increased VOC losses149.

In standard configuration, due to diffusion of the top Ag electrode

into the BCP layer, a Ag-‐BCP complex is formed87. This complex

introduces a new LUMO level aligned with the LUMO of the C70 that

facilitates electron transfer150. Nevertheless, the inverted structure

suffers from an inefficient electron extraction due to lacking this

metal-‐BCP complex151. Small device area of 10 mm2 being studied

here show reasonable device efficiencies for inverted architectures,

but with limited device stability.

We further investigated the instabilities by assessing the ETL

properties of different thicknesses of BCP in inverted configuration.

Moreover, two BCP ETL stacks based on C70 and Ag were tested in

these devices.

4.5 ETL charge transport properties and its effect on the stability We investigated the possible effect of the ETL contact on stability

of the inverted devices by assessing the ETL properties of three

different thicknesses of BCP (0.5, 5 and 10nm) and two BCP ETL

86

stacks based on C70 and Ag. It has been shown that deposition of an

ultrathin layer of C70 between ITO and BCP can improve the ETL

properties and increase the device yield149. The BCP-‐Ag stack can

provide a BCP-‐Ag semi-‐complex without a need for directly doping

the BCP layer.

Electron transport and exciton blocking properties of each

implemented ETL were examined via Electron-‐Only Devices (EODs)

and photoluminescence (PL) spectroscopy, respectively. These

measurements were performed for both fresh and degraded devices.

4.5.1 Electron-‐only devices Electron transport properties of the implemented ETL films were

assessed by performing space-‐charge limited current (SCLC)

measurements on EODs, Figure 4.7.a. EODs were fabricated with the

same methods described in chapter 3 by sandwiching the C70 layer

between the respective bottom ITO/ETL and top BCP/Ag contact. In

these devices, electrons are injected from the Ag electrode into the

devices and extracted from the ITO side. Five following devices were

fabricated:

• ITO/No BCP (0nm)/C70/BCP/Ag

• ITO/BCP (0.5nm)/C70/BCP/Ag

• ITO/BCP (5nm)/C70/BCP/Ag

• ITO/C70 (0.5nm)/ BCP (0.5nm)/C70/BCP/Ag

• ITO/BCP (2nm)/Ag (1nm)/BCP (2nm)/Ag (1nm)/BCP (2nm)/

C70/BCP/Ag

J-‐V characteristics of the EODs were recorded by applying a

sweeping voltage between +1 and −1 V at reverse bias using a

87

Keithley 2400 source measure unit. Measurements were performed

directly after fabrication and repeated after degradation period.

Figure 4.7. a) Electron only devices with different ETLs, and with deposited 10 nm BCP and 100 Ag on top b) JV measurements of the fresh EODs c) JV measurements after aging devices for 24 hours in ISOS-‐D3 and d) ISOS-‐T-‐3 degradation conditions.

Figure 4.7.b and 4.7.c show EOD characteristics for fresh and

degraded devices in ISOS-‐D-‐3 and ISOS-‐T-‐3 conditions for 24 hour.

J-‐V measurements show a lower series resistance for EODs with

BCP/Ag stack, which indicates better electron transport properties

with ease of electron transportation through the implemented ETL

layer. Among the investigated BCP thicknesses, interestingly, devices

without BCP layer at the bottom (0nm BCP) show no or very minor

deterioration. Noting that these devices have a top BCP layer, these

results indicate that the bottom BCP layer is indeed responsible for

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the degradation in inverted cells, while the top BCP layer has a high

stability and no major rule in degradation of the devices. Therefore, a

pure BCP layer hampers electron extraction in the inverted devices,

which is expected due to lack of a metal-‐BCP complex.

4.5.2 Photoluminescence quenching measurements The exciton blocking properties of the ETLs were characterized

using PL measurements (for more information see 3.2.5). Samples

are consisted of the ETL and 100nm C70 deposited on top:

• ITO/No BCP (0nm)/C70

• ITO/BCP (0.5nm)/C70

• ITO/BCP (5nm)/C70

• ITO/C70 (0.5nm)/ BCP (0.5nm)/C70

• ITO/BCP (2nm)/Ag (1nm)/BCP (2nm)/Ag (1nm)/BCP (2nm)/

C70

Figure 4.8.a demonstrates that upon increasing the BCP thickness,

PL intensity of the C70 layer increases, which means an increased

exciton blocking capability for the ETL and thus minimum quenching

at the cathode interface. Among all samples, BCP/Ag stack shows

stronger PL emission, and hence minimized exciton quenching at the

cathode interface.

After aging the samples in ISOS-‐D-‐3 and ISOS-‐T-‐3 conditions for

24 hours, stabilities of the ETLs were tested by repeating the PL

measurements. Figure 4.8.b and 4.8.c represent the results for each

aging condition, and show overall decreased PL intensity in both

conditions compared with the fresh samples. However, in each set,

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BCP/Ag stack shows higher PL intensity meaning better exciton

blocking properties.

Figure 4.8. Photoluminescence (PL) measurements for five ETLs, fresh and degraded at ISOS-‐D-‐3 and ISOS-‐T-‐3. The stack has the structure: (0.5nm)/BCP (0.5nm) and BCP (2nm)/Ag (1nm)/BCP (2nm)/Ag (1nm)/BCP (2nm)).

90

From these results we conclude that the BCP/Ag stack ETL

provides both improved electron transport and exciton blocking

properties as well as less interface degradation. Therefore, we

implemented this ETL candidate as an alternative to pure BCP in full

inverted devices, and evaluated the performance and stability of the

devices.

4.5.3 Improved stability of the inverted devices

The stability of the inverted devices with the following structure

were tested: ITO/BCP (2nm)/Ag (1nm)/BCP (2nm)/Ag

(1nm)/BCP (2nm)/C70 (30nm)/DBP (20 nm)/MoO3 (10 nm)/Ag

(100 nm)

Devices were aged at ISOS-‐D-‐3 condition for 24 hours and J-‐V

characteristics were recorded under 1 sun illumination for fresh and

aged devices. Results are illustrated in Figure 4.9 together with the

previously obtained results for 0.5nm BCP as ETL. Table 4.6 shows

the extracted J-‐V parameters for both types of devices.

The obtained results show both improved VOC and stability for the

inverted devices with BCP/Ag stack ETL, demonstrating that for

inverted cells, the initial larger VOC drop along with the accelerated

degradation compared to standard devices is caused by the BCP ETL.

Since J-‐V characterizations were performed through a bottom

illumination, slightly lower JSC is achieved due to a reflection from the

thin layer of Ag at the lower layers. Interestingly, performance of the

degraded inverted devices based on BCP-‐Ag stack in ISOS-‐D-‐3

condition is similar to the performance of the standard configuration

cells after aging at the same condition. This indicates that now the

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interlayers possess a similar stability as those implemented in the

devices with standard configuration.

Figure 4.9. JV measurements for fresh and aged inverted device with 0.5 nm BCP and BCP/Ag stack.

Table 4.6. Photovoltaic characteristics of fresh and aged inverted devices with 0.5 nm BCP or BCP/Ag stack as their HTL.

In summary the results show that despite the differences in VOC of

the standard and inverted devices upon degradation in ISOS-‐D-‐3 and

ISOS-‐T-‐3 conditions, no morphological or CT state changes are

observed. These results suggest that instabilities are mainly

originated from electrode or interlayer degradations, and in inverted

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devices the BCP ETL has a significant effect on the performance and

degradation of these devices. Evaluating the performance of the

implemented ETLs via EODs and PL measurements backed up these

conclusions. We showed that implementing an alternative BCP/Ag

stack as ETL for inverted devices can enhance both performance and

stability of these devices.

93

CHAPTER 5

Understanding the Degradation Mechanisms in Perovskite Solar Cells

The efficiency of hybrid organic-‐inorganic metal halide perovskite

solar cells (PSCs) have reached PCE of around 22%15 and 12%152 for

laboratory scale and large area cells, respectively. However,

combining high efficiency with high stability in a device still remains

challenging. There are numerous reports addressing the instability

of the active and transport layers in PSCs under exposure to heat,

light, electric field and air153–160. In real operational outdoor

conditions, these factors can simultaneously take part in degradation

processes making it complicated to understand the precise origin of

the degradation mechanisms involved161,162.

Recently, reversibility of certain PSC degradation processes under

illumination-‐darkness cycling has drawn a lot of attention163–166,

94

since it provides a more complicated stability issue than typically

seen in photovoltaic cells. The loss in the device performance being

caused by the illumination can be recovered fully or partially by

resting at dark conditions. This means that in real operational

conditions with diurnal cycling, a dark period following natural

sunlight might help the cell to recover.

On the other hand, some groups reported recently an opposite

behavior in which recovery occurs under illumination in darkness-‐

light cycles, which is known as “fatigue” behavior. This behavior is

related to the well-‐known light soaking effect, and is mostly

attributed to trap filling upon illumination157,163–168. To observe the

aforementioned two types of behaviors for PSCs during day/night

cycle, we have tested long-‐term stability of the PSCs in outdoor and

indoor conditions, and compared the results with already existing

figures of merit in stability measurements. Then the degradation and

recovery dynamics observed under outdoor day-‐night cycling were

related to the ones from constant illumination condition. This

strategy helps us to understand the degradation mechanism that

may be responsible for both types of diurnal behaviors. This chapter

summarizes the results presented in paper II and III, and the work is

done in collaboration with Ilse Katz institute for Nanoscale Science

and GTechnolog (outdoor testing) and the Holst center (perovskite

cell fabrication). Indoor testing was done at MCI, SDU.

5.1. Outdoor day/night degradation and recovery tests Outdoor degradation experiments were performed with two types of

PSCs:

95

glass/ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/spiro-‐

OMeTAD/Au cells (type I) and glass/ITO/TiO2/CH3NH3PbI3/Spiro-‐OMeTAD/Au mini-‐modules (type II).

Devices were degraded indoor by continuous illumination of

simulated sunlight (ISOS-‐L-‐1 protocol) at 60°C, and outdoor natural

sunlight in the Negev desert (the spectrum was measured very close

to the AM1.5G, ISOS-‐O-‐1 protocol) with performance testing under

simulated sunlight three times a day. The initial PCE of the devices

were ~15% and ~10% for the devices of type I and type II,

respectively. Devices were degraded until their PCE dropped to 80%

of its initial value (T80). This point plays a crucial rule as it defines

the overall lifetime of the solar cell and total energy generated by the

cell during its lifetime169. For indoor measurements, the type I

devices were placed under continuous illumination. At this

condition, T80 was reached in about 1 hour (𝑇!"!"#$~1 ℎ), Figure 5.1,

while in outdoor test conditions for both types of devices the

degradation happened at much slower speeds, also due to the effect

of the day/night cycles, which appeared as fluctuations in PCE values

during day and night periods, Figure 5.2.a and 5.2.b. For type I

devices, in the first 11 days of two weeks exposure, the cell

performance degraded during the days and recovered during the

nights, which resulted in higher “morning” PCE values compared to

“evening” PCE values. Therefore, the effect of the night recovery

resulted in much slower long-‐time degradation dynamics compared

with continuous illumination. In this pattern since the cell PCE would

cross the 80% mark multiple times, T80 as a point for lifetime

determination is a misleading parameter. This result demonstrates

96

the significance of including a light/dark cycling in stability

measurements for defining the cell lifetime.

Figure 5.1. Normalized PCE evolution for indoor contentious simulated sunlight illumination of glass/ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/spiro-‐OMeTAD/Au cells (type I).

Figure 5.2. Normalized PCE evolution during two weeks of outdoor exposure to natural sunlight (a) type I (glass/ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/spiro-‐OMeTAD/Au), and (b) type II mini-‐modules (glass/ITO/TiO2/CH3NH3PbI3/Spiro-‐OMeTAD/Au). All lines are guides for the eye.

Nighttime recovery of the PSCs does not result in 100%

restoration of the initial PCE due to two superimposed factors:

presence of a permanent damage, i.e., irreversible degradation

97

mechanism and/or a recovery requiring longer time than one night.

Moreover, a number of different degradation mechanisms can occur

simultaneously and some can dominate at different degradation

stages and define the dynamics of the PCE changing during the day.

In the curve shown in Figure 5.2.a after a certain aging time, the

effect of these factors lead to much closer “morning” and “evening“

PCE values and even invert the pattern during days 12-‐14, at which

the cell demonstrates “fatigue-‐like” behavior, similar to type II

devices (Figure 5.2.b).

Cell lifetime can for example be estimated using the maximum

PCE values measured every day, so that 𝑇!"!"#~4 d (i.e., morning PCE

values up to day 10 in Figure 5.2.a). 𝑇!"!"# counts for the irreversible

losses and/or incomplete recovery during one night; however, since

the dynamics of the diurnal degradation are not taken into account,

the total energy generation by the cell during its lifetime can be

overestimated from this approach170,171.

𝐸!!"!"# = 𝑃𝐶𝐸 𝑡 ∗ 𝑃!" 𝑡 ∗ 𝑑𝑡

!!"!"#

! (5.1)

Where Pin is the power of incoming sunlight (for simplicity, Pin=1

sun=100 mW/cm2).

The evolution of the daily generated energy output by cell type I is

depicted in Figure 5.3.a.

𝐸!"# = 𝑃𝐶𝐸 𝑡 ∗ 𝑃!" 𝑡 ∗ 𝑑𝑡!!"#

! (5.2)

Where 𝑡!"# is the illumination time during one day. Now, we can

estimate 𝑇!"! ≈ 9 d at which Eday drops to 80% of its value. Therefore,

from this definition of T80, both the reversible and irreversible

98

degradation of the cell performance are taken into account, which

suggests a reliable figure of merit for PCE stability.

Figure 5.3. Normalized evolution of daily energy output, Eday, of (a) cell type I, and (b) mini-‐modules type II. All lines are guides for the eye.

Now, using 𝑇!"! ~9 d, we calculate the total energy generated by the

cell during its lifetime as:

𝐸!!"! = 𝑃𝐶𝐸 𝑡 ∗ 𝑃!" 𝑡 ∗ 𝑑𝑡!!"!

! (5.3)

This value connects the cell performance and its stability whose

overall improvement is the ultimate goal of the photovoltaic

technology. This relation does not depend on the certain PCE

changes day/night (illumination/darkness) cycle due to the reason

that in both device types, we observe a behavior resulted from

superposition of reversible and irreversible mechanism, which is

very different from that of continuous illumination experiment.

Overall these results suggest that a light-‐dark cycling is to be used

for precise stability measurements of PSC, and that a new set of

figures of merit are required to describe the performance, stability

and the interplay between them.

99

5.2 Indoor degradation and recovery dynamics To understand the dynamics of degradation and recovery process

under light/dark cycles further, we performed indoor degradation

under continuous light illumination and recovery at dark to different

extends. This allows us to suggest degradation mechanism that may

be responsible for both types of behaviors and relate them to the

results from continuous illumination tests.

For this purpose, we used:

glass/ITO/SnO2/Cs0.05((CH3NH3)0.15(CH(NH2)2)0.85)0.95PbI2.55Br0.45/

spiro-‐OMeTAD/Au cell (type I) (more information regarding the

materials and methods can be found in paper II172).

Indoor illumination was performed in ISOS-‐L-‐1 protocol in

ambient air and using a degradation setup equipped with a wide

area solar simulator (metal halide display/optic lamp HMI from

Osram). The ambient temperature (60 °C ) and intensity were

monitored by sensors, and recorded along with the solar cell

parameters. During the exposure test, solar cell performance was

recorded in situ every 2 minutes via J-‐V measurements in forward

direction, and the parameters were tracked. During the dark

recovery, the cells were kept in the dark at room temperature and

were exposed to light only when J-‐V measurements were conducted

every hour for the first 8 h of degradation.

Samples were divided into three batches and aged until their

efficiency value reached 80% (T80), 60% (T60) and 50% (T50) of the

initial value. Then cells were located in dark condition for recovery.

Figure 5.4 shows the evolution of cell performance parameters.

100

Figure 5.4. Evolution of PV parameters of PSCs under continuous 1-‐sun indoor illumination, interrupted at T80 (a) T60 (b), or T50 (c, d). Gray areas show their subsequent recovery in the dark.

Figure 5.5. Evolution of PV parameters after turning on the light and continuous simulated 1-‐sun re-‐illumination of the PSCs after T50 and recovery in the dark (gray areas): (a) The cell whose PCE dark recovery reached saturation (as in Figure 5.4.c); (b) The cell whose PCE dark recovery did not reach saturation (as in Figure 5.4.d).

For cells, which their PCE reached to 80% of its initial value, full

recovery was observed after a few hours of resting in dark (Figure

101

5.4.a). This result is in accordance with the previously reported

observation for indoor light-‐darkness cycling of state-‐of-‐the-‐art PSCs

(PCE~20%)164. At this stage, degradation is determined by the decay

of mainly open circuit voltage (VOC) and fill factor (FF) and minor

contribution of short circuit current (JSC). This result suggests that

this reversible mechanism with fast recovery should not cause a

permanent damage and a long-‐term deterioration of the PSC

performance under day/night cycle. For reversible degradation in

PSCs, three different mechanisms were previously suggested: (1)

migration of ion vacancies, upon which both halide and cation

vacancies are moving in the perovskite toward the interfaces with

charge transport layers that can affect the charge extraction from the

active layer164; (2) in inverted planer PSCs, light induced reversible

JSC degradation is attributed to the formation of metastable deep

traps in the perovskite layer163; (3) a fully reversible halide

segregation into Br-‐rich and I-‐rich domains, which can be observed

as two photoluminescence peaks that merge into one when allowed

to relax in dark166,173,174. These domains give rise to trap assisted

recombinations for photo-‐excited charge carriers175,176. However, in

our case, PL measurements reveal that phase segregation is not

likely the reason of our results, probably as the Br content is very

low (~15%) in our perovskite layer (more information is provided in

paper II172 and its supplementary information177). Moreover, we

observed that under illumination, both PL intensity and JSC decreased

and recovered in the dark. While the degradation caused by

interfacial effects would result in decreased JSC and increased PL

yield due to deterioration of charge transfer from the active layer.

Therefore, similarity in reversible degradation dynamics for these

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parameters may be attributed to light induced formation of bulk

nonradiative recombination centers in the photoactive layer, which

may be related to the metastable deep traps (mechanism number 2)

or to lattice point defects178. Hence, formation of bulk traps is one of

the possible mechanisms that contribute to reversible degradation.

Reversible VOC and FF changes may have been caused by trap

formation/annihilation and/or light-‐induced migration of ionic

species in the perovskite layer, which lead to changes in the charge

extraction and electric field across the cell164,165.

To study the dynamics of later degradation stages (t>T80), the

second and third batches were illuminated further until the PCE of

the cell reached 60% and 50% of their initial value, and then the

devices where placed in dark condition for recovery (Figure 5.4.b

and Figure 5.4.c,d respectively). Dynamics of further aging the cells

to t~T50 were found to be dramatically different from early stage

degradation. Within the first 10 minutes of the dark storage, a rapid

drop in JSC and FF was observed for all cells resulted in further decay

of PCE from 50% to around 10% of its initial value (Figure 5.4.c,d).

Corresponding J-‐V curve showed an “s-‐shape” distortion (see Figure

S4 in supplementary information177 of paper II172), which has been

attributed to surface recombination or charge accumulation at the

interface between perovskite and transport layer179.

The PCE drop in the dark then followed by a slow recovery

process, which ended up in two cases: either reached saturation over

5 days of measurements or continued its evolution (Figure 5.4.c and

5.4.d respectively, after turning off the light). It is not clear if the

observed difference is qualitative or is due to a different recovery

time scale. Nevertheless, in both cases, due to occurrence of an

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irreversible mechanism, a full recovery of the PCE was not observed,

and only reached a maximum saturation level of 70-‐80% of its initial

value for the behavior shown in Figure 5.4.c.

In order to investigate the dynamics of PCE drop after turning off

the light, the cells from both cases were re-‐illuminated (Figure 5.5).

For the cells whose PCE reached saturation in dark (as in Figure

5.4.c), no further light soaking improvement were observed (Figure

5.5.a). However, the cells for which the PCE did not saturate (as in

Figure 5.4.d), a rapid PCE improvement (mostly resulted from

increases in JSC and FF) was observed, which was then degraded

upon further illumination (Figure 5.5.b). Qualitatively, this type of

behavior is similar to type II behavior observed in outdoor

measurements in which the cells showed recovery under subsequent

illumination (Figure 5.2.b).

An intermediate case between degradation times of T80 and T50 is

when the cell is degraded to 60% of its initial PCE (T60) (Figure

5.4.b). After this point, a dark storage results in the following

dynamics: VOC was recovered rapidly (as in the T80 case) and drops in

JSC and FF were slowly recovered (as in the T50 case, but much

slower). The result was reflected in a slow PCE recovery that took

significantly longer than one night in time scale. Therefore, to our

view Figure 5.4.b and 5.4.d illustrate apparently irreversible

dynamics due to the fact that the recovery rate is such slow that the

full recovery is not practical under real operational day-‐night

stressing. At this stage J-‐V curve shows an s-‐shape when measured

without light soaking (see Figure S4 in supplementary information177

of paper II172).

Until now these dynamics have not been described in the

104

literature. Therefore, to explain this behavior we discuss a possible

scenario172: at later aging stages, shallow interfacial traps of a

different origin than the deep bulk traps are generated under

illumination. While the cell is under illumination, these traps are

being occupied by photogenerated carriers and are neutralized

which mitigate their effect on PSC performance. When the light is

turned off, detrapping of the charge carriers leads to charging of

these states that forms an interfacial charge extraction barrier.

Therefore, it may result in the J-‐V curve’s s-‐shape distortion and

correspondingly JSC and FF decrease as observed in our

experiment180,181.

These results reveal that, although the traps are formed during

illumination, their effect is appearance only after the light is turned

off, which is not the case for the experiments commonly performed

under continuous illumination. This emphasizes the importance of

considering a dark-‐light cycling as a viable strategy for stability

testing of the PSCs, and thus lifetime determination164.

As shown in Figure 5.5.b, by re-‐illuminating the cell whose PCE

did not reach saturation (as in Figure 5.4.d), both JSC and FF

increased significantly within the first 10 minutes due to trap filling

upon light soaking before further degradation takes place. According

to the literature, the effect of light soaking can take from minutes to

hours to reach saturation167,168. Notably, our findings are

considerably different than commonly observed light soaking effect

in the following way: (1) in our case, the cell developed following

~50% PCE degradation in contrast to a feature of fresh devices; (2)

the PCE improvement under illumination was mainly related to

increase in JSC and FF in our case rather than commonly observed

105

improvements in the VOC from light soaking182–186; and (3) in our case

the performance improvement was followed by degradation under

further illumination (Figure 5.5.b). As it has been previously

suggested, deep traps lead to VOC decrease, while shallow ones, being

located near the charge extraction interface, may contribute to JSC

variations by inducing interface charging and poor charge extraction.

Hence, the hypothesis of formation of shallow interfacial traps

suitably describes our observations. These shallow traps can be

originated from interstitial ions or ion vacancies at the

perovskite/transport layers interface178,187.

For the type of behavior shown in Figure 5.4.c, the PCE partially

recovered and saturated after a long time in the dark. Due to the fact

that upon re-‐illumination of the recovered cell no light soaking effect

was observed (Figure 5.5.a), this type of slow recovery can be related

to trap states disappearing, (e.g., due to back-‐migration of ionic

species). To look further into this matter, more investigations using

secondary ion mass spectroscopy (SIMS) are under way.

Figure 5.4.b and 5.4.c show irreversible long-‐term degradation in

which the PCE value reaches saturation level without full recovery.

This behavior was also noted in long-‐term outdoor degradation of

the cells172. Irreversible degradation mechanism may be due to

decay of the perovskite photoactive layer111,188–193, as well as

transport layers and contacts22,194,195, especially catalyzed by

interlayer ion diffusion161,195–197. We should also take into account

the effect of oxygen and water penetration through encapsulation as

our indoor and outdoor experiments were conducted in air.

There are factors that have been reported to contribute to the

irreversible degradation mechanism that can be considered, such as

106

chemical decomposition of the perovskite layer upon exposure to

light, effect of humidity, and temperature188–191, as well as

irreversible trap generation192 and ion migration to the perovskite

from contact/transport layers193,198.

The chemical decomposition is typically accompanied by PbI2

formation resulting in a color change to yellow in the samples. In our

case, we did not visually observe any color change in the cells during

the degradation. Moreover, Raman scattering measurements did not

reveal PbI2 peaks even after cells were degraded to 50% of the initial

PCE (more information can be found in section 7 in the supporting

information177 of paper II172). Although the sensitivity of the Raman

technique to small PbI2 quantities can be limited, we suggest that the

irreversible loss of the cell performance was not resulted from

significant decomposition of the perovskite layer itself.

5.3 Correlation between indoor and outdoor stability measurements Reversible and irreversible degradation mechanisms were

observed for both indoor and outdoor measurements.

In case of outdoor measurements, in the first stage of the aging

process when the cell is degraded to not more than 80% of its initial

PCE, the decrease in the VOC and FF was partially compensated by

nighttime recovery172 (type I dynamics). This effect slows down the

rate of the long-‐term degradation compared to a constant

illumination condition199, This mechanism is similar to the one

depicted in Figure 5.4.b (t<T50) in which the PCE decay is mainly

caused by the decrease in VOC and FF with minor contribution of JSC

107

deterioration, and then the performance is significantly recovered in

the dark. Full recovery was not reached in outdoor conditions due to

“apparently irreversible” processes occurring at t>T80 and a recovery

time longer than one night. However, we observed that increasing

the dark recovery time did not significantly improve the degree of

recovery. At later aging stages of outdoor degradation, the diurnal

behavior of the cells is switched from type I to type II in which PCE

decreased over night and increased under subsequent illumination.

This behaviors is roughly corresponding to the one depicted in

Figure 5.4.d in which after PCE decreased to less than 50% of its

initial value, a sharp PCE decrease is observed immediately after

turning off the light. At this stage both indoor and outdoor

experiments show a decrease in the PCE during the darkness and its

increase under subsequent illumination, which suggests a similar

degradation mechanism(s) for both cases.

Overall, summarizing the results that were shown for indoor and

outdoor degradation tests, we suggest the following scenario for cell

degradation: at the very early aging stages (t≤T80 in our case),

degradation mechanism is reversible. Then at t>T60, both reversible

and irreversible degradation mechanism take place. Finally, at t~T50,

a third type of unique dynamic kick in that changes the diurnal

dynamics of type I to that of type II, which result in further

degradation after turning of the light and subsequent improvements

in PCE under light soaking.

In summery, these results reveal that understanding the

degradation processes under real operational conditions is only

achievable by including the light-‐darkness cycles in experimental

protocols for the assessment of PSCs long-‐term stability.

109

CHAPTER 6

Summary and Outlook Organic and hybrid solar cells have become appealing

technologies due to their unique advantages. Efficiencies of organic

and perovskite solar cells have recently reached world records of

15% and 22%, respectively, however, in order for them to be

commercially viable they need to have at least 10 years of stability,

which has introduced challenges to the field.

In this work, we studied the stability of both OSC and PSC devices.

For organic solar cells, we fabricated and evaluated performance and

stability of DBP-‐C70 based organic solar cells with standard and

inverted device configurations. We focused on the morphological

changes at the DBP-‐C70 interface, which can lead to intrinsic device

degradations. We performed sEQE measurements to identify

molecular and morphological changes at D-‐A interface after aging the

devices at two different ISOS degradation test conditions (ISOS-‐D-‐3

and ISOS-‐T-‐3). The results revealed a pronounced stability at the D-‐A

interface for both devices. Instead, the results suggest that the drop

110

in performance and stability of inverted devices is resulted from

using a BCP bottom ETL in this configuration. This shows the

potential strong impact of all layers and interlayers on the stability of

the devices. To address this, we evaluated exciton blocking and

electron transport properties of different BCP film thicknesses as

ETL, and two alternatives based on C70/BCP and BCP/Ag stacks. The

ETL layers were evaluated using standard JV, SCLC and PL

measurements together with stability tests. The results showed

improved device performance and stability for inverted devices

implementing the BCP/Ag stack as ETL, which implies the significant

contribution of a pure BCP ETL to the hampered stability of the

inverted devices.

For perovskite solar cells, we tested long-‐term stability of the

devices under indoor and outdoor conditions during day/night

cycles, revealing both reversible and irreversible degradation

dynamics. We compared the results with already existing figures of

merit for stability measurements, and related the ones observed for

outdoor degradation tests to the results obtained from indoor

constant illumination condition. We concluded that under

illumination/dark cycles, at early stages of the aging process (t≤T80),

degradation mechanism is still reversible; however, as the aging

process continues (t>T60), irreversible degradation mechanism

appears. At a stage when t~T50, a third type of mechanism kicks in,

which changes the degradation pattern and degradation in dark is

seen, instead of typically observed degradation under light. These

results revealed the importance of including the light-‐darkness

cycles in the experimental stability protocols to achieve a deeper

111

understanding of the degradation processes under real operational

conditions.

This work thus touches upon some essential points in the stability

of both organic and hybrid solar cells, however, there are still lots of

investigations to be made and unknown to be explored. For the OPV

devices investigated here, there is still a need for a better ETL

candidate with improved properties to be used in inverted device

configuration. Moreover, as a useful method, Fourier-‐transform

infrared spectroscopy (FTIR) can be used to study the degradation

mechanisms related to pure chemical changes in the investigated

materials.

Further improvements in performance and stability of the organic

and hybrid solar cell devices can be achieved through systematic

investigations and proper characterization methods. Degradation

studies under real operational conditions can reveal more

degradation paths taking place inside the devices and link our

findings to the broader context in the literature. It is expected that

these results will be a path to further study the degradation

mechanisms, and will be built upon so that true implementation of

organic and hybrid solar cells in our everyday life will be realized

soon from all the joint efforts made on these topics.

113

Appendix A

ISOS-‐T-‐3 degradation test for standard structure DBP-‐C70 based organic solar cells with standard strictures were

encapsulated using a Dow Corning 732 silicon vacuum sealant and a

glass on top in order to decrease the effect of the air and extrinsic

degradation. Then half of the devices were placed inside a thermal

chamber under ISOS-‐T-‐3 test condition ((-‐40℃ and room humidity-‐

darkness), and the other half were stored in a shelf in darkness at

room temperature (ISOS-‐D-‐1). JV characteristics of the devices were

recorded regularly using a solar simulator until they reached their

T20 (where the efficiency reaches 20% of its initial value). Figure A.1

presents the decay curves of the devices.

As it can be seen, for the devices which were kept under ISOS-‐T-‐3

at low temperature, the VOC value remain above 80% of its initial

value after 3500 hours (almost 145 days). However, for devices kept

at room temperature, T80 point is reached much earlier and around

500 hours (20 days).

114

Figure A.1. Lifetime curves of the DBP-‐C70 based organic solar cells with Standard planar structure. Top” Under ISOS-‐D-‐1 (darkness, room temperature and room humidity) and bottom: under ISOS-‐T-‐3 (-‐40℃ and room humidity-‐darkness) aging condition

115

Appendix B

TTF as donor material in organic solar cells Tetrathiafulvalene (TTF)200–202 is recognized as a strong π-‐

electron organic donor and has been of great interest since the early

1970s due to the discovery of the first organic metal203,204. TTF is an

small organic molecule with good mobility and a wide spectrum of

applications205–207(Figure B.1). This molecule can reversibly undergo

a two-‐step oxidation process to form the radical cation and the

dication forms of the TTF and is in general thermodynamically stable

to many synthetic transformations. Moreover, it is relatively stable in

air, and its oxidation potentials can be finely tuned by the attachment

of electron-‐donating or electron-‐withdrawing substituents208.

Overall, interesting charge-‐transport properties, photoinduced

electron transfer leading to highly stabilized ion radical pairs,

together with possibility of tailoring the properties, makes TTF

derivatives appealing candidate for organic photovoltaic

applications.

116

Figure B.1. Various applications of TTF in super molecular chemistry and material chemistry202,209.

TTF is widely researched in charge transfer salts where it conducts

electrons200, and it is possibly the best organic electron-‐donor

material and used extensively in supramolecular chemistry210.

However, surprisingly their application in organic and dye

synthesized solar cells has been very limited211,212.

In this work we tested three generations of novel TTF derivatives

as electron donor with acceptor PC60BM in organic solar cells. We are

aiming to improve the performance and stability of the organic solar

cells by implementing and optimizing the properties of this molecule

in the active layer of the devices.

The novel TTF molecules have been synthesizes by our

collaborator Dr. Steffen Bähring at SDU FKF Odense, Denmark, and

solar cell device fabrication and optimization has been performed at

SDU NanoSYD.

117

B.1 First-‐generation TTF molecules We started the process by testing the received first generation of

the synthesized novel TTF derivatives named as TTF1 and TTF2 with

different electron-‐donor and absorption properties (Figure B.2).

Cyclic voltammetry (CV) and HUMO-‐LUMO levels of each molecule

are shown in Figure B.3. The molecules were tested in TTF:PC60BM

based solar cell devices with the structure of ITO/ZnO/TTF:PC60BM

/MoO3/Ag.

Figure B.2. Skeletal formula of TTF1 and TTF2 molecules.

Figure B.3. (left) Cyclic voltammetry and (right) HUMO LUMO levels for TTF1 and TTF2 molecules

118

To fabricate the devices, pre-‐patterned ITO coated substrates

were cleaned using acetone and isopropanol in ultrasonic bath and

blow-‐dried with nitrogen gun. In order to avoid exposure to air, the

whole fabrication process was done in a nitrogen glove-‐box. A

commercial ZnO semi-‐conductive ink (Genes’ Ink, France) was spin-‐

coated on top of ITO with 1000 rpm for 60 sec and annealed on a hot

plate at 130°C for 15 min. First, we started with TTF1 molecule from

which TTF1:PC60BM solutions were prepared by dissolving 20 mg of

TTF1 and 20 mg of PC60BM in Chlorobenzene (CB) solvent to result

in 40 mg/ml concentration with the ratio (1:1). To spin-‐coat the

active layer, three different rpms were used: 500, 1000 and 2000. In

parallel with device fabrication, the solutions were separately spin-‐

coated over cleaned glass substrates in order to investigate the

quality and thickness of the films. Finally, 10 nm MoO3 and 100 nm

Ag were vacuum deposited on top of the deposited films, and a mask

was used to define a cell area of 10 mm2 for each cell (design of the

devices are similar to the ones described in chapter 3).

J-‐V characteristics of the devices were recorded under 1 sun

illumination in a solar simulator (3000 class AAA solar simulator

from Abet Technology Inc., USA). Among three types of devices, only

the ones with deposited active layer at 2000 rpm showed

photovoltaic effect with 0.001% PCE and 0.08 mA/cm2 JSC. The

deposited films over the cleaned glasses were very thin and showed

absorption only in the UV range (Figure B.4).

119

Figure B.4. Absorption spectra of the three types of TTF1:PC60BM films deposited at three different rpms (500, 1000 and 2000).

B.1.1 increasing the concentration to increase

absorption In the next step, a solution with a 3 times higher concentration

(120 mg/ml) was made, and the whole experiment was repeated.

However, similar results were obtained without any improvement,

confirming that the devices were hampered by the poor visible

absorption from the material. Moreover, phase segregation of

PC60BM and TTF1 molecules were seen, which resulted in low

quality and uneven films.

B.1.2 Changing the solvent and using dynamic dispense To improve the solubility of the molecules, we used Chloroform

(CF) solvent instead and annealed the solution before and during

spin coating. In this experiment, 2 solutions with 40 mg/ml and 120

mg/ml concentrations were prepared over night by stirring on a hot

plate at 80 °C. This time 1000, 2000 and 3000 rpms were used to

120

deposit the films. Moreover, since CF is a high vapor pressure

solvent, each solution was applied while the substrate was spinning

(dynamic dispense) to avoid evaporation of the solvent and resulting

in a better film uniformity.

For each sample, the quality of the deposited films and their

absorption properties were evaluated (Figure B.5), and devices were

fabricated based on the highest absorber film. However, none of the

developed devices worked. It was noticed that right after deposition

of the films, crystalline zones were growing very fast on each

substrate, which may be the main result for the failure of this

experiment (Figure B.6).

Figure B.5. Absorption spectrum for deposited films from TTF1:PC60BM solutions with 40mg/ml and 120mg/ml concentration, deposited at 1000, 2000 and 3000 rpm.

Figure B.6. Crystallized zones formed right after spin coating.

121

Crystallization can occur either due to the reason that the two

materials reject each other (originating from their structure or

dipole dipole interaction) or due to a very strong attraction between

the TTF molecules. Unlike polymers, this often happens with small

molecules, as they are not connected in long chains, which makes

them more mobile. To solve this problem, we added a polymer

(polystyrene=PS) into the solution, which would act as a sort of

'spacer' that prevents them from aggregating.

B.1.2 Adding polystyrene (PS)

The experiments with both TTF1 and TTF2 molecules were

repeated, and TTF1:PC60BM and TTF2:PC60BM solutions with 40

mg/ml and 80 mg/ml concentrations containing either 10% or 5%

PS were made. SPS with 8 mg/ml concentration (10% PS in a 80

mg/ml concentration) was prepared by dissolving a 33mg

polystyrene bead in about 4.12 ml chloroform, and left for stirring

for a few days on a hot plate at 50 °C. Next, 8 different types of

TTF:PC60BM solutions were prepared by adding different ratios of

SPS (Table B.1).

Films were spin-‐coated over cleaned glass substrates with 1000,

2000 and 3000 rpms. Photographs and optical microscope images of

each substrate is shown in Figure B.7 and B.8. Absorption properties

of each film was recorded and is illustrated in Figure B.9

122

Table B.1. 8 different solutions with TTF1 and TTF2, with two different concentrations (80 and 40mg/ml) containing 5% and 10% PS.

TTF1:PC60BM

40mg/ml

5% PS 10% PS

20mg TTF1+20mg PCBM

0.25ml SPS+0.75ml CF

20mg TTF1+20mg PCBM

0.5ml SPS+0.5 ml CF

80mg/ml

5% PS 10% PS

40mg TTF1+40mg PCBM

0.5ml SPS+0.5ml CF

40mg TTF1+40mg PCBM

1ml SPS

TTF2:PC60BM

40mg/ml

5% PS 10% PS

20mg TTF2+20mg PCBM

0.25ml SPS+0.75ml CF

20mg TTF2+20mg PCBM

0.5ml SPS+0.5 ml CF

80mg/ml

5% PS 10% PS

40mg TTF2+40mg PCBM

0.5ml SPS+0.5ml CF

40mg TTF2+40mg PCBM

1ml SPS

Figure B.7. Deposited films from TTF1:PC60BM solutions with 40mg/ml and 80mg/ml concentration containing 5% and 10% PS.

123

Figure B.8. Deposited films from TTF2:PC60BM solutions with 40mg/ml and 80mg/ml concentration containing 5% and 10% PS.

Figure B.9. Absorption spectra of films deposited from 4 different solutions based on TTF2:PC60BM, each deposited with 3 different rpms.

124

Overall, TTF2:PC60BM films showed better film quality, and

stronger absorption properties in visible wavelengths. Therefore, the

TTF2 molecules were evaluate as the active layer material in the

solar cell devices.

B.1.3 Assessing TTF2 molecules in organic solar cells

As the next step, 4 different TTF2:PC60BM solutions were coated

over ITO/ZnO substrates to investigate the quality of the deposited

films. Figure B.10 shows optical and AFM images of the spin-‐coated

solution over ITO/ZnO substrates, and Figure B.11 illustrates

absorption properties for each sample.

Figure B.10. Optical microscope and AFM images of the films deposited from TTF2:PC60BM solutions with two different concentrations and two different PS concentrations

125

Figure B.11. Absorption properties of TTF2:PC60BM with 40mg/ml and 80mg/ml containing 5% and 10% PS.

As the results show, the quality and uniformity of the deposited

films has significantly improved. Based on these results, solar cell

devices based on TTF2:PC60BM active layer were fabricated. In this

process, the 4 types of solutions were deposited over ITO/ZnO

substrates with 1000 rpm, followed by vacuum deposition of MoO3

and Ag on top. Figure B.12 shows J-‐V curves and extracted

parameters for each type of device.

As it can be seen, although the efficiency is very low, this time the

cells show reproducible performance results. Low PCE is resulted

from a low current (JSC), which is expected as very little visible light

is absorbed. Therefore, the challenge were to incorporate modified

TTF molecules that absorb in the visible area.

126

Figure B.12. J-‐V curves and extracted parameters of 4 different types of devices based on TTF2:PC60BM active layer.

B.2 Second-‐generation TTF molecules For the second-‐generation TTF molecules, we introduced an

electron-‐accepting moiety (benzoyl groups) to lower the LUMO

energy level, and thereby increase the absorption band between 400

and 600 nm of varying intensity. We assessed two TTF editions in

our solar cells (Figure B.13).

127

Figure B.13. Skeletal formula of TTF2I and TTF2II molecules.

Devices were fabricated with the new TTF2 compounds in their

active layers. TTF2:PC60BM solutions with 40 mg/ml and 80 mg/ml

concentrations were prepared, and devices were fabricated as

described before. Final results showed only very low PCE around

0.004% for 80 mg/ml solutions, and not much improvement.

B.3 Third-‐generation TTF molecule This time our collaborator synthesized third-‐generation TTF

molecules with better absorption properties, which can assist with

improved cell characteristics. These Molecules are editions of the

previously incorporated TTF molecules in which an organic dye is

used as a platform (quinoidal porphyrin), which has higher visible

absorption with the TTF still being able to modulate the electronic

properties (ex-‐TTF porphyrin) (Figure B.14).

128

Figure B.14. Absorption spectra of exTTF-‐Porphyrin molecule

We received two derivatives of this ex-‐TTF porphyrin and for

convenience, we named them TTF56 and TTF80 (Figure B.15). CV

measurements of the two molecules are shown in Figure B.16. Fine-‐

tuning the electronic properties of the molecules to introduce more

electron withdrawing moieties such as COOMeTTF56 results in

0.227 V higher oxidation potential compared to SEt TTF80.

Figure B.15 Skeletal formula of TTF80 and and TTF56 molecules.

129

Figure B.16. CV recorded for TTF80 and TTF56.

With the same procedure as explained before, we made 4

solutions with these two molecules and PC60BM (1:1) having either

40 mg/ml or 80 mg/ml concentration, and containing 5% PS. This

time we slightly changed the recipe. We spin-‐coated ZnO over pre-‐

patterned ITO substrates at 2000 rpm for 60s and baked it at 130°C

for 15 min. Each substrate left in the vacuum for 30 min for complete

drying. Then, the TTF:PC60BM solution was dynamically spin-‐coated

at 1000 rpm for 60s followed by 1500 rpm for 5s resulted in 200-‐

260 nm thick film. Again, a vacuum drying procedure was carried out

for 30 min to give enough time for evaporation of the solvent. After

that, samples were transferred into a vacuum deposition system

where 10 nm MoO3 and 100 nm Ag were deposited as HTL and

electrode on top.

Final device characterization shows reproducible results with

very low PCE of 0.04% and 0.01% for TTF56 and TTF80,

respectively. The measured VOC is comparable to reference donor

130

materials (P3HT); however, the JSC and FF are very low (Figure B.17).

This may be due to a poor morphology, which may be improved

using a different donor and acceptor ratio in the active layer.

Figure B.17. J-‐V characteristics and extracted device parameters for solar cell devices based on TTF56:PC60BM and TTF80:PC60BM with (1:1) ratio.

B.3.1 changing the donor acceptor ratio In another effort, ratio of the PC60BM was increased three times

more and solutions were made with TTF (80 or 56):PC60BM (1:3)

with 40 mg/ml concentration containing 2.5% PS (2 solutions). Solar

cell devices were fabricated with these solutions and their

performances were characterized. Results show an improved PCE to

0.26% and 1.11 mA/cm2 JSC for devices based on TTF56:PC60BM

(Figure B.18).

131

Figure B.18. J-‐V characteristics and extracted device parameters for solar cell devices based on TTF56:PC60BM and TTF80:PC60BM with (1:3) ratio.

B.3.2 Degradation measurements As a final attempt, we tested stability of the TTF56:PC60BM based

solar cells by aging them in ISOS-‐D-‐3 condition for 24 h. Results

show a degradation path in which 50% drop in PCE is mostly caused

by a drop in VOC and JSC and less related to a change in the FF (Figure

B.19). Further investigations are needed to identify the origin of the

degradation mechanisms taking place in these devices.

132

Figure B.19. Stability characterization of TTF56:PC60BP based solar cells in ISOS-‐D-‐3 condition Overall, the results so far indicate the potential application of the

TTF molecules as donor material in organic solar cells, which

however requires much optimization of the material and device

properties. Ex-‐TTF derivatives show interesting electronic

properties to be used in organic photovoltaics. Although the results

for BHJ type OSC are poor, they show a potential to improve the

overall performance by a systematic approach to modify the

electronic properties of the molecules and device fabrication

techniques.

133

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