large work function shift of organic semiconductors inducing … · 2014. 2. 13. · large work...

Nano Res

1

Large work function shift of organic semiconductorsinducing enhanced interfacial electron transfer inorganic optoelectronics enabled by porphyrinaggregated nanostructures

Maria A. Vasilopoulou1 (), Antonios M. Douvas1, Dimitra G. Georgiadou1, Vassilios Constantoudis1,

Dimitris Davazoglou1, Stella Kennou2, Leonidas C. Palilis3, Dimitra Daphnomili4, Athanassios G. Coutsolelos4,

and Panagiotis Argitis1 Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0428-9

http://www.thenanoresearch.com on February 11 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0428-9

TABLE OF CONTENTS (TOC)

Large Work Function Shift of Organic

Semiconductors Inducing Enhanced Interfacial

Electron Transfer in Organic Optoelectronics Enabled

by Porphyrin Aggregated Nanostructures

M. Vasilopoulou1,*, A. M. Douvas1, D. G.

Georgiadou1, V. Constantoudis1, D. Davazoglou1, S.

Kennou2, L. C. Palilis3, D. Daphnomili4, A. G.

Coutsolelos4 and P. Argitis1

1Institute of Advanced Materials, Physicochemical

Processes, Nanotechnology and Microsystems

(IAMPPNM), National Center for Scientific Research

“Demokritos”, Greece. 2Department of Chemical Engineering, University of

Patras, Greece 3Department of Physics, University of Patras, Greece 4Laboratory of Bioinorganic Chemistry, Chemistry

Department, University of Crete, Greece

Porphyrin nano-aggregated layers are used to induce large work

function shift on organic semiconductors. Significant enhancement in

the efficiencies of organic light emitting diodes and organic

photovoltaics was observed after the insertion at the cathode interface

of a layer consisting of the porphyrin 1 having its molecules arranged

with a face-to-face orientation.

porphyrin 1 porphyrin 2

Large work function shift of organic semiconductors inducing enhanced interfacial electron transfer in organic optoelectronics enabled by porphyrinaggregated nanostructures

Maria A. Vasilopoulou1(), Antonios M. Douvas1, Dimitra G. Georgiadou1, Vassilios Constantoudis1, Dimitris Davazoglou1, Stella Kennou2, Leonidas C. Palilis3, Dimitra Daphnomili4, Athanassios G. Coutsolelos4 and Panagiotis Argitis1

1Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems (IAMPPNM), National Center for

Scientific Research “Demokritos”, 153 10 Aghia Paraskevi Attikis, Athens, Greece. 2Department of Chemical Engineering, University of Patras, 26500 Patras, Greece 3Departmen of Physics, University of Patras, 26500 Patras, Greece 4Laboratory of Bioinorganic Chemistry, Chemistry Department, University of Crete, Voutes Campus, 71003 Heraklion, Crete, Greece

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Porphyrins, OLEDs,

OPVs, aggregates

ABSTRACT

We report on large work function shifts induced by the coverage of several

organic semiconducting (OSC) films commonly used in organic light emitting

diodes (OLEDs) and organic photovoltaics (OPVs) with a porphyrin aggregated

layer. The insertion between the organic film and the aluminum cathode of an

aggregated layer based on the meso-tetrakis(1-methylpyridinium-4-yl) porphyrin

chloride (porphyrin 1), with its molecules adopting a face-to-face orientation

parallel to the organic substrate, results in a significant shift of the OSC work

function towards lower values due to the formation of a large interfacial dipole

and induces large enhancement of either the OLED or OPV device efficiency.

OLEDs based on poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1’,3

-thiadiazole)] (F8BT) and incorporating the porphyrin 1 at the cathode interface

exhibited current efficiency values up to 13.8 cd/A, an almost 3-fold

improvement over the efficiency of 4.5 cd/A of the reference device. Accordingly,

OPVs based on poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61 butyric acid

methyl ester (PC61BM) and porphyrin 1 increased their external quantum

efficiencies to 4.4 % relative to 2.7 % of the reference device without the

porphyrin layer. The incorporation of a layer based on the zinc meso-tetrakis

(1-methylpyridinium -4-yl)porphyrin chloride (porphyrin 2), with its molecules

adopting an edge-to-edge orientation, also introduced improvements, albeit

more modest in all cases, highlighting the impact of molecular orientation.

Nano Research

DOI (automatically inserted by the publisher)

Research Article

1. Introduction

Self-assembly of functional molecular materials into

well-defined nanostructures depending on various

non-covalent interactions has attracted increasing

research interest in both materials study and

devices [1-5]. As the typical representatives of

functional molecular materials with largely

conjugated molecular structure, porphyrins and

phthalocyanines have been extensively studied over

the past decades because of their wide range of

applications [6-15]. Porphyrins, in particular, are

tetrapyrrole derivatives which show up in many

biochemical molecules like chlorophyll, and have

been of interest for many years due to their

light-harvesting and charge transfer functions in

biological photosynthesis [9-10]. The chemical

stability and useful properties of porphyrins,

coupled with the ease with which their structure

can be synthetically manipulated, along with their

organization into nano- and microscale structures

using self-assembled techniques, makes them very

attractive for producing highly functional materials

for optoelectronic applications, such as organic thin

film transistors (OFETs) [16], organic light emitting

diodes (OLEDs) [17-19] and organic photovoltaics

(OPVs) [20-21]. In the latter two, porphyrins are

usually used as additives in a host organic

semiconducting layer destined to emit or absorb

light, respectively.

On the other hand, interface engineering of OLEDs

and OPVs has emerged over the last years as a key

factor in optimizing carrier transport from/to the

electrodes in order to balance the low intrinsic

carrier concentration and electrical conductivity of

organic semiconductors (OSCs) [22-30]. Since most

OSCs exhibit intrinsic p-type conductivity, balanced

electron injection and transport are still limiting

factors for high device efficiency and long operating

lifetime [27-28, 31-33]. To this end, the use of an

electron transport layer has emerged as a method to

circumvent this issue, offering also the possibility to

combine OSCs with air-stable metals with relatively

high work function, as aluminium, and to

demonstrate devices exhibiting high environmental

stability and ease of fabrication, as well as high

efficiency and low operating voltage. Various

examples include metal oxides with a low lying

conduction band, such as TiO2 [34-35], ZnO [36-37],

solution processable Cs carbonate [38-39],

conjugated zwitterionic compounds [40], neutral or

charged conjugated polyelectrolytes [41-44] and,

recently, methyl pyrimidine-based electron

transporting materials [45-46]. In addition, Yoon et

al. have recently demonstrated that a thin layer

consisting of copper hexadecafluoro phthalocyanine

molecules, which self-assemble parallel to the

substrate, increases significantly the performance of

inverted OPVs when inserted between the indium

tin oxide cathode and a ZnO electron transport

layer [47]. This was attributed to more efficient

electron transport due to their face-on π-π stacking

aligned to the current flow. Our group has recently

demonstrated that a porphyrin compound, the free

base meso-tetrakis(1-methylpyridinium-4-yl)

porphyrin chloride, [H2TMPyP]4+Cl4 (termed

hereafter as porphyrin 1), is self-assembled into

aggregated structures when dissolved in an

alcoholic solvent, such as methanol [48]. It was

found that its molecules adopt a face-to-face

orientation and exhibit ferroelectric properties,

arising from the formation of a large dipole moment

aligned to the stacking direction. Moreover, when a

thin layer of the porphyrin 1 was inserted between

the organic photoactive layer and the metal cathode

in an OPV device, significant improvement of the

efficiency was obtained, which was attributed to

enhanced exciton dissociation at the cathode

interface [49]. The efficiency improvement of the

Address correspondence to Maria A. Vasilopoulou, [email protected]

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

same devices was more modest when using a

second porphyrin compound, namely the

Zn-metallated zinc meso-tetrakis

(1-methylpyridinium-4-yl) porphyrin chloride,

[ZnTMPyP]4+Cl4 (termed hereafter as porphyrin 2).

Herein, we move a step forward by exploring in

details the aggregation mode of both porphyrin

compounds when spin coated on top of several

organic semiconductors, namely

poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,

1’,3-thiadiazole)] (F8BT), commonly used in OLED

devices, poly(3-hexylthiophene) (P3HT):

[6,6]-phenyl-C61 butyric acid methyl ester (PC61BM)

blend, which is a common photoactive material in

bulk heterojunction (BHJ) OPVs and, finally, in

pristine PC61BM. By using x-ray diffraction (XRD)

and UV absorption measurements we have verified

the formation of H-aggregates with a face-to-face

orientation when porphyrin 1 is spin coated on top

of the OSC layer, while the formation of

J-aggregates with the edge-to-edge configuration

was evident when spin coating the porhyrin 2 on

top of the same substrates. More importantly, by

using ultraviolet photoelectron spectroscopy (UPS)

measurements we reveal a large decrease in the

organic semiconductors work function when they

are covered with the porphyrin layer, especially

with that consisting of molecules adopting the

face-to-face configuration. OLEDs based on F8BT

show a large increase in current efficiency from 4.5

cd/A for the reference device to 13.8 cd/A for the

devices incorporating the poprhyrin 1. Similarly,

OPVs based on P3HT:PC61BM with the porphyrin 1

at the cathode interface increase their external

quantum efficiencies to 4.4 % relative to 2.7 % of the

reference device. This efficiency enhancement can

be considered as a result of the significant shift of

the WF of the organic semiconductors towards

lower values, arising from the formation of large

interfacial dipoles and reducing the electron

injection/extraction barrier at the cathode interface.

To the best of the authors’ knowledge, this work

represents the first systematic study on the

aggregation mode of porphyrin compounds on top

of organic semiconductors and on its crucial impact

on interfacial charge transport properties.

2. Experimental

2.1 Synthesis of porphyrin compounds. The

porphyrins utilized in this study, tetra-cationic

meso-tetrakis(1-methylpyridinium-4-yl)porphyrin

chloride [H2TMPyP]4+Cl4, and its Zinc (II) derivative

[ZnTMPyP]4+Cl4, were prepared following a

condensation reaction according to literature

procedures (details are included in SI).

2.2 OLEDs fabrication and characterization.

OLEDs were fabricated on ITO coated glass

substrates (2x2 cm) with a sheet resistance 20

Ω/square, which served as the anode electrode.

Substrates were ultrasonically cleaned with a

standard solvent regiment (15 min each in acetone

and isopropanol). A 10 nm sub-stoichiometric Mo

oxide (MoOx) layer was then deposited using a

hot-wire vapour deposition method to modify the

anode interface followed by an approximately 70

nm emissive layer (EML), based on the

green-emitting copolymer

poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,

1’,3-thiadiazole)] (F8BT), spin coated from a

chloroform solution (at a concentration of 6 mg/ml).

After deposition, the EML layer was annealed at 80

ºC for 10 min in air. F8BT was purchased from

American Dye Source and used as received. Then,

in certain devices a thin porphyrin layer was

deposited on top of the EML, spin-coated at 2000

rpm for 15 sec from solutions in methanol with

different concentrations, to form an aggregated

layer of different thickness and serve as an electron

injection/extraction layer. Note that all reference

devices were treated with methanol by spin-coating

it on top of the active layer at 2000 rpm for 15 sec.

The devices were completed with the deposition of

a 150 nm thick aluminium cathode in a deep

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4 Nano Res.

vacuum (10-6 Torr) chamber. Current

density-luminance-voltage characteristics of the

fabricated OLEDs were measured with a Keithley

2400 source-measure unit. Organic film

preparations and device measurements were

performed in ambient conditions.

2.3 OPVs fabrication and characterization.

Regioregular poly(3-hexylthiophene) (P3HT) was

purchased from Sigma-Aldrich, while

[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM )

was provided by Solenne. All chemicals were used

as received without further purification. Control

and porphyrin-modified OPV cells were fabricated

on solvent-precleaned ITO coated glass substrates.

A 10 nm thick MoOx layer was also used as an

efficient anode interfacial layer to improve hole

extraction and device reproducibility. Next, an ~100

nm thick photoactive layer consisting of a

P3HT:PC61BM (1:0.8 wt.% ratio) blend was spin-cast

on MoOx from a 10 mg/ml chloroform solution at

600 rpm. After spin coating, the active layer was left

to dry and then annealed at 130 ºC for 15 min in air.

Next, the reference devices were subjected to

methanol treatment similar to that of the OLEDs

while in other devices a porphyrin layer was

deposited on top of the active layer from a 0.7 %

methanol solution via spin coating at 2000 rpm to

serve as the cathode interfacial layer. Finally, a 150

nm thick Al layer was deposited by thermal

evaporation through a shadow mask to define an

active area of 12.56 mm2. Current density-voltage

characteristics of the fabricated solar cells were

measured with a Keithley 2400 source-measure unit.

Cells were illuminated with a Xe lamp and an

AM1.5G filter to simulate solar light illumination

conditions with an intensity of 100 mW/cm2, as was

recorded with a calibrated silicon photodiode.

Device measurements were performed in air,

immediately after cell fabrication.

2.4 Thin films characterization. The thickness of

the active layers and the porphyrin films were

measured with an Ambios XP-2 profilometer and a

M2000 Woolam ellipsometer, respectively. Thin-film

absorption spectra (on a quartz substrate) were

recorded with a Perkin Elmer Lamda 40 UV/Vis

spectrophotometer. Surface morphology and

structure were investigated with an NT-MDT

atomic force microscope (AFM) operated in tapping

mode. Crystallinity of porphyrin layers was probed

using a Bruker X-ray diffractometer (XRD). The

UPS measurements were performed in a

Leybold/Specs MAX 200 spectrometer using the He

I (21.22 eV) radiation line from a discharge lamp,

with a resolution of 0.15 eV. A negative bias of 12.28

V was applied to the sample during UPS

measurements in order to separate sample and

analyzer high BE cut-off and estimate the absolute

work function (WF) value from the spectrum. The

photoemission cut-off is used here to determine the

position of the vacuum level and the work function

of the film, using the pre-determined position of the

Fermi level of the sample and following a

well-established procedure. The samples made for

UPS were prepared ex-situ and then introduced in

the analysis chamber under ultra-high vacuum

UHV (≈10−10 Torr). All organic semiconducting films

were deposited on ITO /MoOx substrates using the

same preparation conditions and films thicknesses

as in the case of device fabrication. The complex

OSC/porphyrin interfaces were also prepared

ex-situ by spin casting the porphyrin layer on top of

the organic semiconducting film from its solution in

methanol with a concentration of 0.7 % w/v.

3. Results and discussion

The chemical structures of porphyrins 1 and 2 as

well as of the OSCs used in this study are shown in

Figure 1. The specific porphyrin molecules were

selected due to their ease of synthesis and their

ability to form aggregates in their solutions in

methanol/water (these aggregates exhibit a rod-like

shape, as was revealed by transmission electron

microscopy (TEM) images shown in Figures S1 and


5 Nano Res.

S2), from which they can directly be processed on

top of the organic semiconducting layers without

disrupting them. The self-assemble aggregation of

porphyrins was probed by using their UV-vis and

FTIR absorption spectra. The UV-vis absorption

spectra of porphyrin 1 are presented in Figure 2 a.

The spectra obtained in solutions with

concentrations varying from 10-6 M to 10-4 M

(0.001-0.1 % w/v) correspond to the molecular

compound exhibiting a sharp Soret band at 425 nm

and four distinct Q bands appear at 515, 558 and 595

and 653 nm [48].

Figure 1 Molecular structures of porphyrin 1, porphyrin 2 and the organic semiconductors used in this study, namely the light emitting F8BT, the donor polymer P3HT and the fullerene acceptor PC61BM.

The spectra obtained in films spin coated on quartz

substrates from a solution with concentration 0.2 %

w/v exhibited the formation of aggregates, which is

associated with the appearance of a new, blue

shifted band at 406 nm [48], co-existing with the

molecular compound. By increasing the solution

concentration at 0.4 % w/v and above, the

aggregation of all molecules is evident from the

complete disappearance of the initial Soret band at

425 nm –which is now blue shifted at 406 nm and

also significantly broadened- followed by a blue

shift also of the Q bands to 482, 517, 567 and 649 nm.

On the basis of Kasha’s exciton theory [49], blue

shifts in the main absorption bands of the metal-free

porphyrin 1 upon aggregation are typically a sign

of the effective π-π interaction between the

porphyrin molecules, indicating the formation of

H-aggregates from this compound in methanol

through a face-to-face orientation of porphyrin

molecules along their normal axis [50-53].

Figure 2 The demonstration of the face-to-face aggregation mode of poprhyrin 1. (a) The UV absorption spectrum of the molecule obtained in methanol solutions with concentrations from 10-6 M up to 10-4 M and the spectrum of the aggregated films formed by increasing the solution concentration above 0.2 % w/v (the films were spin coated at 2000 rpm for 15 sec). (b) The region around 3000 cm-1 of the IR spectra of the molecule and aggregated film (spin coated at 2000 rpm from a 0.7 % w/v solution in methanol) of poprhyrin 1. The concentration of 0.7 % w/v is the optimum for device operation, as will be discussed bellow. (c) Illustration of the self-assembly mode of porphyrin 1 into H-aggregates with a face-to-face alignment. The grey, white and blue balls represent carbon, hydrogen and nitrogen atoms, respectively.

To further shed light on the self-assembled process

of the porphyrin 1 we also recorded the IR

P3HT


F8BT PC61BM

300 350 400 450 500 550 600 6500,0

0,2

0,4

0,6

0,8

1,0(a)

595 nm 653 nm558 nm515 nm

649 nm567 nm

517 nm482 nm

406 nm 425 nm

10-6M

10-4 M 0.2 % w/v

0.4 % w/v 0.7 % w/v 1.0 % w/v

porphyrin 1

Abs

orba

nce

(a.

u.)

Wavelength (nm)

(c)


6 Nano Res.

absorption spectra of both its molecules (spectrum

recorded in solution) and the nanoaggregates in a

film spin coated from a 0.7 % w/v methanol

solution. In Figure 2 b the area around 3000 cm-1 of

the IR spectra is shown. This spectral region was

focused on, because the frequencies of the C–H

stretching modes, which are affected by the

intermolecular C–H···N hydrogen bonds, appear in

this region. Given that these compounds contain

nitrogen atoms in the pyridine rings at the outer

side of the molecule, they are expected to be

connected by intermolecular C-H···N hydrogen

bonds [51]. It can be observed that, the main peak

in the molecular spectrum appears at a frequency of

2920 cm−1. This main peak is attributed to the C–H

vibrational modes at the neighbours of the nitrogen

atoms in the pyridine rings [51-53]. According to

literature reports, the formation of weak hydrogen

bonds such as C–H···N and C–H···O causes a blue

shift in the frequency of the C–H stretching mode.

The blue shift of this particular absorption band to

3030 cm-1 was observed only in the case of the

aggregated film of porphyrin 1, which can be

therefore attributed to the formation of an extended

network of intermolecular C–H···N hydrogen bonds

between molecules in the aggregates of porphyrin 1,

in accordance with previously reported assignment

for molecules with nearly similar structure [52]. In

Figure 2 c the aggregation mode of the porphyrin 1

is illustrated. The molecules are arranged with a

face-to-face manner with the plane surfaces of the

porphyrin rings facing one another and oriented

parallel to the substrate. The structure shown in

Figure 2 c, thus represents a three dimensional

framework of extensively networked molecules due

to hydrogen bonding induced intermolecular forces

(H-aggregates). The motif described here

represents the aggregation mode exhibited by

porphyrin 1, as derived from our spectroscopic

study presented above.

Figure 3 The demonstration of the aggregation mode of poprhyrin 2. (a) The UV absorption spectrum of the molecule obtained in methanol solutions with concentration between 10-6 M and 10-4 M and the spectrum of the aggregated films (formed via spin coating at 2000 rpm from a solution in methanol with concentrations above 0.2 % w/v). (b)The region around 3000 cm-1 of the IR spectra of the molecules and aggregates of porphyrin 2. (c) Illustration of the self-assembly mode into J-aggregates with an edge-to-edge molecular orientation. The light blue balls represent the central zinc atoms of the porphyrin ring.

Similar study was carried out for the zinc

metallated molecular counterpart, namely

porphyrin 2, whose UV-vis absorption spectra are

presented in Figure 3 a. Again, the spectrum of the

molecular compound was recorded in solutions

with concentrations between 10-6 M and 10-4 M

while the spectrum of the aggregates was taken in

films spin coated from solutions with

concentrations 0.2 % w/v and above. The

introduction of the central zinc atom induces a

small red shift in the Soret band of the molecule,

300 350 400 450 500 550 600 6500,0

0,2

0,4

0,6

0,8

1,0

627 nm597 nm550 nm

585 nm515 nm 555 nm

450 nm

Abs

orba

nce

(a. u

.)

Wavelength (nm)

10-6M

10-4 M 0.2 % w/v 0.4 % w/v 0.7 % w/v 1.0 % w/v

porphyrin 2

425 nm

(a)

(c)


7 Nano Res.

which now appear at 425 nm, while the Q bands

peak at 515, 555 and 585 nm. A significant red shift

of the Soret and the Q bands to 450 nm and to 550,

597 and 627nm, respectively, is observed in the

spectra of the aggregated films of porphyrin 2. This

red shift is indicative of porphyrin self-assembly

aggregation through an edge-to-edge (i.e.

head-to-tail) molecular ordering [54-56]. In the IR

spectra of porphyrin 2 both of molecules and

aggregates (Figure 3 b) no differences in the region

around 3000 cm−1 are observed indicating an

aggregation mechanism different than the

formation of an extended hydrogen bonding

network as in the case of porphyrin 1.

The self-assembled aggregates of compound 2 are

very likely formed via Zn-N coordination bonds

between the central zinc ion of one porphyrin

molecule and a nitrogen atom in the pyridine side

chain of a neighboring molecule as in the case of

other porphyrins with Zn-metallated core [51]. The

edge-to-edge aggregation process of porphyrin 2 (J

aggregates) is illustrated in Figure 3 c. Here,

molecules are arranged with their planar rings

adopting an edge-to-edge configuration and thus

are oriented diagonal to the substrate. Note that the

different aggregation mode of the porphyrin

compounds proposed here was further supported

by optical anisotropy measurements (Figure S3).

One of the main challenges of this study was to

clarify whether the porphyrins exhibit the same

aggregation mode described above when inserted

on top of the hydrophobic organic semiconducting

layers. In order to investigate their orientation we

spin coated each porphyrin from its solution in

methanol with a concentration of 0.7 % w/v on top

of the organic layer and carried out x-ray diffraction

(XRD) measurements. We used the F8BT as the OSC

to study the poprhyrin aggregation on top of it,

since this is a high performing light-emitting

copolymer, widely used in OLED devices. The XRD

diffractograms for the pristine F8BT, the

F8BT/poprhyrin 1 and the F8BT/poprhyrin 2 films

are presented in Figures 4 a, b, and c, respectively.

All samples exhibit the (100) diffraction peak of

F8BT at 2θ=5.4o, which originates from the α–axis

orientation of the F8BT crystallites (polymer

backbone parallel and side chains perpendicular).

For the F8BT/porphyrin 1 complex film the

foremost diffraction peak at 2θ=32.8o corresponds to

a lattice spacing d=2.7 Ǻ which indicates that the

porphyrin molecules adopt a face-to-face

orientation parallel to the organic substrate [57].

Figure 4 The aggregation mode of porphyrins on top of the F8BT layer. XRD diffractograms of (a) pristine F8BT layer, (b) F8BT/porphyrin 1 and (c) F8BT/porphyrin 2 bilayers. The porphyrins were spin coated on top of the organic layer at 2000 rpm from their 0.7 % w/v solutions in methanol.

For the porphyrin 2 deposited on F8BT the situation

is significantly different. The dominant diffraction

peak (except for that attributed to F8BT) emerges at

2θ=4.4o which unambiguously is assigned to a

completely different lattice spacing d=19.9 Ǻ. This

means that crystallites are predominantly oriented

with the porphyrin molecular plane diagonal to the

organic substrate surface, or in other words, in an

5 10 15 20 25 30 35

5 10 15 20 25 30 35

5 10 15 20 25 30 35

F8BT/poprhyrin 22=4.4o

(F8BT)

Inte

nsi

ty (

a.u

.) (c)

(b)

2=32.8o(F8BT)

2xtheta (deg)

F8BT/poprhyrin 1

In

tens

ity (

a.u.

)

Inte

nsity

(a

. u.) F8BT

2=5.4o

(a)


8 Nano Res.

edge-to-edge orientation. Also, visible in the XRD

measurement is a relatively weak feature at

2θ=28.36o (d=3.1 Ǻ), indicating that some crystallites

adopt the face-to-face orientation, though the

percentage of such crystallites within the film is

assumed to be minimal.

We also verified the preservation of the aggregation

type of each porphyrin when coated on top of the

F8BT layer by measuring the UV-vis absorption

spectra of an F8BT thin film, pristine and with each

porphyrin compound on top of it. The results are

shown in Figure 5 a.

Figure 5 (a) UV absorption spectra of F8BT and F8BT/porphyrin layers. (b) Molecular arrangement of each porphyrin in their aggregates formed on top of F8BT; the lattice distances in each case are also shown.

F8BT exhibits a characteristic absorption shoulder

centred at 380 nm. In the case of the

F8BT/poprhyrin 1 bilayer, a second absorption peak

appear at 406 nm which may be assigned to the

Soret band of the porphyrin 1 H- aggregates (see

Fig. 2 b). The appearance of several weak peaks (i. e.

at 517 and 567 nm) in the wavelength region -where

F8BT is completely transparent- may attributed to

the porphyrin Q bands and is further supportive for

the H-aggregation mode of the porphyrin 1 on top

of the F8BT. Similarly, the appearance of the

absorption peaks at 450 nm and 597 nm in the

spectrum of the F8BT/porphyrin 2 bilayer is a clear

indication for its self-assembly into J-aggregates

when is spin-coated on top of F8BT. The schematic

representation of the molecular arrangement and

characteristic planar distances of both porphyrin

compounds on top of F8BT, are shown in Figure 5 b.

In order to probe the effect of molecular orientation

of porphyrins on the electronic properties of an

organic semiconductor, we carried out ultraviolet

photoemission spectroscopy (UPS) measurements

and the results are shown in Figure 6 a, which

depicts the high-bind energy cut-off of the UPS

spectra of F8BT without and with the porphyrin

layers on top of it. Note that the F8BT layer was

treated with methanol (spin coated at 2000 rpm for

15 sec) in order to take into account the recently

introduced reduction of work function attributed to

solvent effect [58]. The work-function of F8BT film

was estimated about 4.4 eV, which is slightly

reduced compared with the previously reported

values of 4.5-4.6 eV [59], due to the solvent effect

[58]. However, a significant further decrease in the

work function of F8BT film is observed after

inserting the porphyrin layers and especially the

porphyrin 1. According to the measured cut-off

value of the high binding energy regions the

estimated work functions are 3.7 eV and even lower

3.0 eV after the F8BT coverage with porphyrin 2

and 1, respectively. Despite the fact that the work

function shift is quite large in the case of

F8BT/poprhyrin 2 interface (ΔWF=0.7 eV) it becomes

extremely large (ΔWF=1.4 eV) when the F8BT

surface is covered with the porphyrin 1. Such a

significant work function shift can be associated

with the formation of a large molecular dipole

300 400 500 600 700 800

(a)

597 nm

567 nm517 nm

Wavelength (nm)

Ab

sorb

an

ce (

au

)

F8BT F8BT/porphyrin 1 F8BT/porphyrin 2

380 nm(F8BT)

406 nm

450 nm

F8BT

d=2.7 Ǻ

F8BT

d=19.9 Ǻ


(b)


9 Nano Res.

vector oriented perpendicular to the plane of the

porphyrin rings. The magnitude of such vector is

expected to be higher when molecules are parallel

to the organic substrate.

Figure 6 (a) The high binding energy cut-off region of the UPS

spectra of F8BT films either treated with methanol or covered

with the porphyrin layers (formed from solutions in methanol

with a concentration of 0.7 % w/v in order to obtain the best

coverage) for the estimation of the work function shift of F8BT

in each case. (b) Lowering of the electron injection barrier

through the formation of large interfacial dipole after the

coverage of the F8BT film with the porphyrin layer consisting

of aggregated nanostructures.

This molecular dipole moment induces the

formation of an interfacial dipole with the negative

charge end pointing toward the Al electrode and

the positive charge end pointing toward the organic

layer (Figure 6 b), which causes the large work

function shift. The direction of this dipole is aligned

with the built-in potential originated from the

asymmetric contact at the electrodes; therefore, the

actual built-in potential across the device is

reinforced as a result of the superposition. Such a

large interfacial dipole may have a significant

impact in the electron injection/ extraction barrier.

Indeed, by taking into account the relative energy

positions of the lowest occupied molecular orbital

(LUMO) of F8BT (about 3.2 eV [59]) and the WF of

the Al contact, a significant energy barrier for

electron injection (Φe) at the F8BT/Al interface is

derived (Figure 6 b), resulting in poor electron

injection in the reference device. The large

interfacial dipole formed after the coverage of F8BT

with the porphyrin layers –especially of that

consisting of porphyrin 1- results in the significant

reduction of the electron injection barrier (Figure 6

b), which may have a direct effect in the device

performance.

However, because different morphology of the

porphyrin layers could hamper the direct

comparison of the two orientations (i. e. an increase

in the surface area is directly correlated to the

device current), prior to device fabrication we

explored if the different orientation leads to a

significant difference in topography. In Figure 7 a

atomic force microscopy (AFM) 2D 6x6 μm images

of methanol treated F8BT and F8BT/poprhyrin in

either orientation films, are presented, with the

porphyrins spin coated from a 0.7 % w/v solution

which gave the better coverage of F8BT (see also

experimental results and analysis presented in SI,

Figures S4 and S5). From this topographies can be

deduced that both poprhyrin materials form a

nanoaggregated layer that uniformly coats the

entire area of the underlayer. The size and distances

between these aggregates strongly depend of the

solution concentration.

The main result derived by comparing these images

is that, while methanol treatment has negligible

effect on F8BT surface morphology, the deposition

of the porphyrin layers slightly increases surface

roughness of the underlying F8BT film; the RMS

19 18 17 16 15 14 13 12 11

Inte

nsity

(a

. u.

)

F8BT/porphyrin 1 W

F=3.0 eV

F8BT/porphyrin 2 W

F=3.7 eV

Binding energy (eV)

F8BT W

F=4.4 eV

(a)


(b)


10 Nano Res.

value from 0.45 nm elevates to 0.78 nm and 0.83 nm

when compounds 1 and 2 are deposited on top of

the F8BT layer, respectively.

Figure 7 (a) 2D AFM 6x6 μm2 topographies of F8BT,

F8BT/poprhyrin 1 and F8BT/poprhyrin 2 (left, middle and

right), and (b) the corresponding circularly averaged Fourier

transforms. The concentrations of both porphyrins solutions in

methanol were 0.7 % w/v.

The Fourier transforms of both porphyrin layers

(Figure 7 b) exhibit a two broad peak structure

demonstrating a two-scale organization of

nanoaggregates. The high frequency peak reveals

an ordered arrangement of aggregates with period

~200 nm, while the peak at the low frequencies may

show a slight tendency for clustering of aggregates.

Taking into account the similarities in the films

morphology exhibited by both porphyrins

processed under identical conditions we conclude

that the influence of surface topography on the

resulting device performance comparison is

negligible.

Next, porphyrin aggregated layers were inserted in

OLED devices with the structure ITO/MoOx (10

nm)/F8BT (70 nm)/porphyrin (5 nm)/Al to facilitate

electron injection/transport. The device

architectures for the different porphyrins

aggregation modes are presented in Figure 8 a. All

devices have its anode interface optimized through

the insertion of an under-stoichiometric Mo oxide

layer [60].

Figure 8 OLED devices. (a) The device architecture. (b)

Current density-voltage (open symbols) and luminance-voltage

(in log scale) (solid symbols) characteristic curves. (c) Current

efficiencies of the same OLEDs.

We first conducted an experimental study to

explore the dependence of OLED device efficiency

on the concentration of the porphyrin solution and

on the porphyrin film thickness. Results are

presented in Figure S6 for the devices based on

porphyrin 1. It was found that, after increasing the

concentration of porphyrin 1 above 0.4 % w/v (we

used solutions with concentrations higher than 0.2

% w/v in order to obtain the formation of fully

aggregated films) the device efficiency remains

nearly similar with slightly better performance of

that incorporating the porphyrin compound with

an approximate thickness of 5 nm, spin coated from

the 0.7 % w/v solution. The latter can be probably

10-4 10-3 10-2

102

103

Fo

uri

er T

ran

sfo

rm F

(k)

Spatial frequency (nm-1)


(a)

(b)

ITO/GLASSMoOx

F8BT

ITO/GLASSMoOx

F8BT

Al

Porphyrin aggregates

(a)


11 Nano Res.

attributed to better coverage of the underlying OSC

with porphyrin as derived from the AFM study

discussed above. For solutions with concentrations

above 0.7 % w/v a small reduction of the device

performance was observed and may be correlated

with the increased size of the nanoaggregates in the

porphyrin layer. Similar results were obtained from

the study of the OLED devices performance versus

(not shown). The current density-voltage-luminance

(J-V-L) characteristics of the F8BT based OLEDs

without and with the 5 nm porphyrin layers, are

shown in Figure 8 b. In Figure 8 c the current

efficiencies of the same devices are presented, while

the results are also summarized in Table 1.

Table 1: Device characteristics of OLEDs with the structure ITO/MoOx/F8BT/porphyrin/Al. Data based on 96 cells of each type.

Note that the reference device without the

porphyrin interlayers was methanol treated prior to

Al deposition. This device exhibits a peak

luminance of about 18000 cd/m2 and current

densitiy of 4000 A/m2, at about 5.0 V, and a

maximum current efficiency of approximately 4.5

cd/A. The device with the porphyrin 2 reaches

luminance values up to 45000 cd/m2 and current

densities of 4500 A/m2, while it also exhibits a peak

current efficiency of 10.0 cd/A. However, the

performance of the device with porphyrin 1 is

outstanding; high luminance, such as 76000 cd/m2

and current density of 5500 A/m2 with a peak

efficiency of 13.8 cd/A can be included among the

best values reported for thin (<100 nm) F8BT-based

OLEDs [61], while the efficiency remained at a high

level of 10 cd/A even at high voltages. This efficiency

enhancement can be explained by the significant

reduction of electron injection barrier at the

F8BT/porphyrin 1/Al interface due to the formation

of a large dipole, as was further supported from

open-circuit voltage (Voc) measurements, presented

in Figure S7.

To demonstrate if electron injection/transport is the

main factor that is affected from the incorporation

of porphyrins at the F8BT/metal cathode contact we

fabricated electron only devices with the structure

Al (150 nm)/F8BT (70 nm)/poprhyrin (5 nm)/Al (150

nm), while in certain devices the porphyrin layer

was not incorporated in order to serve as reference

(the F8BT layer, however, was methanol treated). A

comparison of the current density–voltage (J-V)

characteristics of such devices is shown in Figure 9

a in semi-log scale.

Figure 9 Demonstration of enhanced electron injection. J-V

characteristic curves in semi-log scale of (a) electron only and

(b) hole only devices.

Cathode interface

Jmax (A m-2)

Lmax (cd m-2)

Max current efficiency (cd A-1) [at voltage]

Max power efficiency (lm W-1) [at EQE]

F8BT 4000 18000 4.5±0.2 (5.0 V)

2.8±0.1 (1.5%)

F8BT/ porphyrin 1

5500 76000 13.8±0.2 (4.5 V)

9.6±0.1 (4.6 %)

F8BT/ porphyrin 2

4500 45000 10.0±0.2 (4.5 V)

7.0±0.1 (3.3 %)

10-3

10-2

10-1

100

101

102

103

104

105

Cur

rent

den

sity

(A

/m2 )

F8BT F8BT/porphyrin 1 F8BT/porphyrin 2 SCLC

(a) electron only

0 1 2 3 4 5 6 710-3

10-2

10-1

100

101

102

103

104

105

Cur

rent

den

sity

(A

/m2 )

Voltage (V)

F8BT F8BT/poprhyrin 1 F8BT/poprhyrin 2

(b) hole only


12 Nano Res.

The device without the porphyrin layer presents a

J-V characteristic, resembling that of a diode, with

the electrons current to be limited because of the

low bulk mobility of F8BT and the large barrier

with the Al metal contact [62]. After the porphyrin

layer insertion though the electrons current is

significantly increased, especially in the case of the

porphyrin 1. From the J-V characteristics, it is

evident that the electron current turn-on voltage for

the F8BT/porphyrin 1/Al device is in the sub volt

range and the current density is up to two orders of

magnitude higher than in the F8BT/Al device.

Moreover, the F8BT/porphyrin 1/Al device shows a

space-charge limited current (SCLC) behaviour, as

verified by the overlap of the experimental data

with the calculated J-V characteristics (using the

Mott Gurney equation), thus revealing the existence

of a truly ohmic F8BT/porphyrin 1/Al

electron-injecting/transport contact. The increase in

electron current in the case of F8BT/porphyrin 2/Al

device is significant, but remains injection limited,

verifying the critical role of porphyrins molecular

orientation in the alteration of interfacial electron

injection/transport. Note that, an injection limited

hole current was observed when we fabricated and

measured hole only devices with the structure Au

(80 nm)/F8BT (70 nm)/Au (80 nm) with or without

the porphyrin layers at the cathode interface, as

shown in J-V characteristic curves presented in

Figure 9 b, indicating that the porphyrin layers have

a clear effect only on the electron injection/transport

characteristics of the devices.

In order to demonstrate the universal impact of our

approach, we also studied the influence of the

porphyrins deposition on top of organic

semiconductors commonly used in organic

photovoltaic cells, as P3HT:PC61BM and PC61BM

alone. Figure 10 a presents the high binding energy

cut-off of the UPS spectra of the pristine

P3HT:PC61BM (1:0.8 weight ratios) film subjected to

methanol treatment and of the films with the

porphyrin layers on top. The work-function of the

methanol treated film was found 4.5 eV, while it

was significantly reduced to 3.7 eV and even further

to 3.2 eV after the coverage of the organic

semiconducting film with the porphyrin 2 and 1,

respectively. In addition, we also explored possible

electronic alterations in the PC61BM/porphyrin

interfaces, where electron transfer from the

fullerene acceptor to porphyrin is likely to occur. In

Figure 10 b the high binding energy cut-off of the

UPS spectra of pristine (subjected to methanol

treatment) and porphyrin covered PC61BM films,

are shown. It is observed that the deposition of the

porphyrins on top of the PC61BM films results in a

significant WF shift, from 4.9 eV for the methanol

treated film to 3.7 eV and further to 3.3 eV for the

porphyrin 2 and 1, respectively, covered films.

.

Figure 10 The high binding energy cut-off region of the UPS

spectra of (a) P3HT:PC61BM (1:0.8) and (b) PC61BM films

without and with the porphyrin layers for the estimation of their

work function. Porphyrin layers were spin coated from their

solutions in MeOH with concentration 0.7 % w/v in order to

obtain the optimum coverage.

19 18 17 16 15 14 13 12 11

Binding energy (eV)

PC61

BM/porphyrin 1

WF=3.3 eV

(b)

Inte

nsity

(a

. u.)

PC61

BM/porphyrin 2

WF=3.7 eV

PC61

BM

WF=4.9 eV

(a)

(WF=4.5 eV)

(WF=3.7 eV)

Inte

nsity

(a.

u.)

P3HT:PC61

BM/porphyrin 2

P3HT:PC61

BM (1:0.8)

P3HT:PC61

BM/porphyrin 1

(WF=3.2 eV)


13 Nano Res.

This large WF shift of OSCs resulted in significant

improvement of OPV devices based on

P3HT:PC61BM as the photoactive layer. The devices

were fabricated according to the architecture:

Glass/indium tin oxide (ITO)/MoOx (10

nm)/photoactive layer (100 nm)/porphyrin layer (5

nm)/Al. Reference devices without porphyrin layers

with the OSC films subjected to simple methanol

treatment were also fabricated for comparison.

Current density-voltage (J-V) characteristics of

porphyrin-modified solar cells based on

P3HT:PC61BM showed an improved performance

compared with the reference devices under AM 1.5

G illumination (Figure 11 a).

Figure 11 OPV devices performance based on

P3HT:PC61BM photoactive layer (a) Photocurrent

measurements (J-V characteristics) of the reference device

(methanol treated) and of devices with the porphyrin

layers. (b) Dark current measurements (J-V characteristics)

of the same devices.

The incorporation of both porphyrins, especially of

the porphyrin 1, leads to significant improvement in

the short-circuit current (Jsc), the open circuit voltage

(Voc) and the devices fill factor (FF) compared the

control device (Table 2). More specifically, the

short-circuit current, Jsc, increases from 9.0 mA/cm2

to 10.2 mA/cm2 and even further to 11.0 mA/cm2 for

the porphyrin 2 and 1 incorporating devices while

the open-circuit voltage, Voc, increases from 0.60 V

for the control device to 0.63 V for the one with the

porphyrin 2 layer and reaches the high value of 0.68

V for the device with the porphyrin 1. Accordingly,

the fill factor, FF, is also improved from 0.50 for the

control to 0.56 and 0.59 for the modified devices. As

a result, a high PCE of 4.4 % for the device with the

porphyrin 1, relative to 2.7 % for the reference and

3.6 % for the device with the porphyrin 2, was

estimated.

Table 2. Device characteristics of organic solar cells with the structure ITO/MoOx/active layer/porphyrin/Al. Data based on 48 cells of each type.

Analysis of the dark J-V characteristics of the

devices (Figure 11 b) revealed a larger turn-on

voltage (in the range of 0.5-0.6 V) for the porphyrin

modified devices relative to the control device

(0.4-0.5 V). This implies that the built-in potential

(Vbi) across the device is significantly increased

upon porphyrin interlayer utilization in agreement

with the large work function shift which they

induced after their insertion on top of the organic

semiconductor. In addition, significant increase of

the diodes forward current after the incorporation of

porphyrin layers is observed, which indicates that

Active layer Jsc (mA/cm2

)

Voc (V)

FF PCE (%)

P3HT:PC61BM 9.0 0.60 0.50 2.7±0.1P3HT:PC61BM/porphyrin 1 11.0 0.68 0.59 4.4±0.1P3HT:PC61BM/porphyrin 2 10.2 0.63 0.56 3.6±0.1

-0,2 0,0 0,2 0,4 0,6 0,8

-12

-8

-4

0

4

8

(a) P3HT:PC

61BM

P3HT:PC61

BM/porphyrin 1

P3HT:PC61

BM/porphyrin 2

Cu

rre

nt (

mA

/cm

2)

-0,8 -0,4 0,0 0,4 0,810-6

10-5

10-4

10-3

10-2

10-1

100

101

102

Voltage (V)

J dark (

mA

/cm

2 )

(b)


14 Nano Res.

the introduction of porphyrins also significantly

improves electron injection/transport.

Note that, regarding the mechanism affecting the

OPV device efficiency enhancement after the

porphyrins incorporation, we also propose the

enhanced ecxiton dissociation due to the

strengthening of the device built-in field coming

from the large shift of the work function. This

argument and additional experimental results of

devices based on different OSCs are published

elsewere [49]. However, the results presented here

clearly indicate that the proposed approach in

lowering the work function of organic

semiconductors is universal and, thus, it provides a

very simple and versatile method to optimize

solution-processed organic light emitting diodes,

organic solar cells or other optoelectronic devices by

using layers consisting of porphyrin aggregated

nanostructures with appropriate, well defined

molecular arrangement.

4. Conclusions

This work presents the application of porphyrin

aggregated layers on top of organic semiconductors

to lower their work function and enhance interfacial

electron transport. First, we have shown that the

free base [H2TMPyP]4+Cl4, porphyrin 1,

self-assembles into H-aggregates with a face-to-face

molecular orientation in methanol solutions with

appropriate concentration, while its zinc metallated

counterpart, the [ZnTMPyP]4+Cl4, porphyrin 2,

self-assembles into J-aggregates with an

edge-to-edge molecular orientation. We have also

verified with XRD and UV absorption

measurements that the same self-assembling mode

is achieved when porphyrins are spin coated on top

of an organic semiconducting layer. OLEDs based

on F8BT show a large increase in current efficiency

from 4.5 cd/A for the reference device to 10.0 cd/A

and further to 13.8 cd/A for the devices

incorporating the poprhyrin 2 and 1, respectively.

Similarly, OPVs based on P3HT:PC61BM increased

their external quantum efficiencies to 3.6 % and 4.4

%, relative to 2.7 % of the reference device, when

incorporating the porphyrin 2 and 1 layers,

respectively. We show that the large enhancement

in devices performance is mainly attributed to the

large work function shift (ΔWF) of the organic

semiconductors when they are covered with the

poprhyrin layers, as it was found by using UPS

measurements and was explained by the formation

of a large dipole moment vector perpendicular to

the substrate, arising from the perfect alignment of

the porphyrin 1 molecules parallel to the substrate.

This large ΔWF results in significant reduction of the

electron injection barrier and, consequently, in

improved electron injection. We demonstrate with

these results that the use of porphyrins, and in

general of planar aromatic molecules, as interfacial

device components to control their efficiencies by

taking advantage of the organization of their matter

(which has so far very little explored in the

literature) may be of great interest in the field of

OLEDs and OPVs and in general of organic

electronics.

Acknowledgements

The European Commission funded this research by

FP7-REGPOT-2008-1, Project BIOSOLENUTI No

229927, Special Research Account of UoC,

Heraklitos grant from Ministry of Education, and

GSRT.

Electronic Supplementary Material: Supplementary

material containing additional data (discussion and

Figures S1-S7) is available in the online version of

this article at

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