Linköping Studies in Science and Technology

Dissertation No. 996

Surface Energy Patterning and

Optoelectronic Devices

Based on Conjugated Polymers

Xiangjun Wang

Biomolecular and Organic Electronics

Department of Physics, Chemistry and Biology

Linköping University, SE-581 83 Linköping, Sweden

Linköping 2006

i

Cover figures:

Picture on the left: Microscope image of polymer patterns formed directly from

solution by surface energy controlled dewetting.

Picture on the right: Photon to current efficiency curves of solar cells based on

a novel low-bandgap polymer blended with different fullerenes.

ISBN: 91-85497-00-2 ISSN: 0345-7524

Printed in Sweden by UniTryck

Linköping 2006

ii

To Lichun and Gengshi

iii

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Abstract

The work presented in this thesis concerns surface energy modification

and patterning of the surfaces of conjugated polymers. Goniometry and

Wilhelmy Balance techniques were used to evaluate the surface energy or

wettability of a polymer’s surface; infrared reflection-absorption spectroscopy

(IRAS) was used to analyse the residuals on the surface as modified by a bare

elastomeric stamp poly(dimethylsiloxane) (PDMS). The stamp was found to be

capable of modifying a polymer surface. Patterning of a single and/or double

layer of conjugated polymers on the surface can be achieved by surface energy

controlled dewetting. Modification of a conjugated polymer film can also be

carried out when a sample is subjected to electrochemical doping in an aqueous

electrolyte. The dynamic surface energy changes during the process were

monitored in-situ using the Wilhelmy balance method.

This thesis also concerns studies of conjugated polymer-based

optoelectronics, including light-emitting diodes (PLEDs), that generate light by

injecting charge into the active polymer layer, and solar cells (PSCs), that

create electrical power by absorbing and then converting solar photons into

electron/hole pairs. A phosphorescent metal complex was doped into

polythiophene to fabricate PLEDs. The energy transfer from the host polymer

to the guest phosphorescent metal (iridium and platinum) complex was studied

using photoluminescence and electroluminescence measurements performed at

room temperature and at liquid nitrogen temperature. PSCs were prepared

using low-bandgap polyfluorene copolymers as an electron donor blended with

several fullerene derivatives acting as electron acceptors. Energetic match is

the main issue affecting efficient charge transfer at the interface between the

polymers and the fullerene derivatives, ane therefore the performance of the

PSCs. Photoluminescence, luminescence quenching and the lowest unoccupied

molecular orbital (LUMO) together with the highest occupied molecular orbital

v

(HOMO) of the active materials in the devices were studied. A newly

synthesized fullerene, that could match the low-bandgap polymers, was

selected and used as electron acceptor in the PSCs. Photovoltaic properties of

these PSCs were characterised, demonstrating one of the most efficient

polymer:fullerene SCs that generate photocurrent at 1 µm.

vi

Preface

This work has been performed in the Biomolecular and Organic

Electronics group in the Division of Applied Physics, at the Department of

Physics, Chemistry and Biology at Linköping University, Sweden. I enrolled as

a Ph D student under the supervision of Prof. Olle Inganäs in the research

project “From nano-scale interactions to optoelectronic devices of conjugated

polymers” in The National Network Research School in Materials Science,

supported by the Swedish Education Ministry since March 2002 after I had

been a project engineer for more than a year in the same group. The project has

been performed with the cooperation of four Ph D students with backgrounds

in theoretical physics, conjugated polymer synthesis and surface

characterization and device physics applications from Chalmers University of

Technology, Karlstad University and Linköping University.

The work presented in this thesis concerns surface energy modification

and patterning of surfaces of conjugated polymers; and studies of conjugated

polymer-based optoelectronics, including light-emitting diodes (PLEDs), that

generate light by injecting a charge into the active polymer layer, and solar

cells (PSCs), that create electrical power by absorbing and then converting

solar photons into electrons.

Xiangjun Wang, Linköping, Sweden, December 2005

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List of publications:

Papers included in this thesis:

I. X. Wang, M. Östblom, T. Johansson, and O. Inganäs, PEDOT surface energy pattern controls fluorescent polymer deposition by dewetting. Thin Solid Films 449 (2004) 125-132. II. X. Wang, K. Tvingstedt, and O. Inganäs, Single and bilayer submicron arrays of fluorescent polymer on conducting polymer surface with surface energy controlled dewetting. Nanotechnology 16 (2005) 437-433. III. X. Wang, T. Ederth, and O. Inganäs, In-situ Wilhelmy balance surface energy determination of poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene) during electrochemical doping-dedoping. In manuscript. IV. X. Wang, M.R. Andersson, M.E. Thompson, and O. Inganäs, Electrophosphorescence from substituted poly(thiophene) doped with iridium or platinum complex. Thin Solid Films 468 (2004) 226-233. V. X. Wang, E. Perzon, J.L. Delgado, P. de la Cruz, F. Zhang, F. Langa, M.R. Andersson, and O. Inganäs, Infrared photocurrent spectral response from plastic solar cell with low-bandgap polyfluorene and fullerene derivative. Applied Physics letters 85 (2004) 5081-5083. VI. X. Wang, E. Perzon, F. Oswald, F. Langa, S. Admassie, M.R. Andersson, and O. Inganäs, Enhanced photocurrent spectral response in low-bandgap polyfluorene and C70-derivative based solar cells. Advanced Functional Materials 15 (2005) 1665-1670. VII. X. Wang, E. Perzon, W. Mammo, F. Oswald, S. Admassie, N.-K. Persson, F. Langa, M.R. Andersson, and O. Inganäs, Polymer solar cells with low-bandgap polymers blended with C70-derivative give photocurrent at 1 µm. Thin Solid Films, (Accepted for publication).

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My contribution to the above papers:

Paper Experimental work

Writing

I

All experimental work except for the IRAS measurement

First version (90%)

II

Majority (90%)

First version (90%)

III

All experimental work

First version (70%)

IV

All experimental work

First version (90%)

V

All experimental work except for the CV measurement

First version (90%)

VI

All experimental work except for the materials synthesis and CV measurement

First version (80%)

VII

All experimental work except for the materials synthesis and CV measurement

First version (80%)

Related publications, not included in the thesis:

VIII. X. Wang, Patent: Patterning Method, International patent: PCT/GB2003/002917 IX. X. Wang, K. Tvingstedt, and O. Inganäs, Self-organization of polymer from liquid induced by bare PDMS stamping. Proc. of the fifth micro structure workshop, Ystads Saltsjöbad, March 30-31, 2004. 113-116. X. X. Wang, M.R. Andersson, M.E. Thompson, and O. Inganäs, Electrophosphorence from polythiophene blends light emitting diode. Synthetic Metals 137 (2003) 1019. XI. M.D. Johansson, X. Wang, T. Johansson, O. Inganäs, G. Yu, G. Srdanov, and M.R. Andersson, Synthesis of soluble phenyl-structured poly(p-phenylenevinylenes) with a low contents of structural defects, Macromolecules 35 (2002) 4997.

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XII. N.-K. Persson, X. Wang, and O. Inganäs, Optical limitations in low bandgap polymer/fullerene bulk heterojunction devices. Submitted to Advanced Functional Material. XIII. A. Gadisa, X. Wang, S. Admassie, E. Perzon, F. Oswald, F.Langa, M.R. Andersson, and O. Inganäs, Stochiometry dependence of charge transport in polymer/methanofullerene and polymer/BTPF70 based solar cells. Organic Electronics (Accepted for publication). XIV. F. Zhang, E. Perzon, X. Wang, W. Mammo, M.R. Andersson, and O. Inganäs, Polymer solar cell based on a low-bandgap fluorene copolymer and a fullerene derivative with photocurrent extended to 850 nm. Advanced Functional Materials 15 (2005) 745. XV. O. Inganäs, M. Svensson, F. Zhang, A. Gadisa, N.-K. Persson, X. Wang, M.R. Andersson, Low bandgap alternating polyfluorene copolymer in plastic photodiodes and solar cells. Applied Physics A 79 (2004) 37. XVI. F. von Keiseritzky, J. Hellberg, X. Wang, and O. Inganäs, Regiospecifically alkylated oligothiophenes via structurally defined building blocks. Synthesis-Stuttgart(9): (2002) 1195-1200. XVII. L.J. Lindgren, X. Wang, O. Inganäs, and M.R. Andersson, Synthesis and properties of polyfluorenes with phenyl substituents. Synth. Met. 154 (2005) 97. XVIII. E. Perzon, X. Wang, F. Zhang, W. Mammo, J.L. Delgado, P. de la Cruz, O. Inganäs, F. Langa, and M.R. Andersson, Design, synthesis and properties of low band gap polyfluorenes for photovoltaic devices. Synth. Met. 154 (2005) 53. XIX. E. Perzon, X. Wang, S. Admassie, O. Inganäs, and M.R. Andersson, An alternating low band-gap polyfluorene for optoelectronic devices. Submitted to Macromolecules.

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Conference contributions:

1. Oral presentation: Electrophosphorecence from polythiophene doped

light emitting diode. 2002 International Conference of Synthetic Metal,

Shanghai, 30 June-3 July, 2002.

2. Poster presentation: Modification and patterning of PEDOT-PSS

surface. International Symposium on Colloid and Interface Technology.

Fundamentals and Applications, Lund, 6-8 November 2002

3. Oral presentation: Self-assembly polymer array prepared by surface

energy controlled dewetting. The Fifth Micro Structure Workshop, Ystads

Saltsjöbad, March 30-31, 2004.

4. Oral presentation: Broad photocurrent spectral response window of solar

cells with low-bandgap polyfluorene copolymer and C70-fullerene. E-MRS

Spring 2005, Strasbourg, France, May 31- June 3, 2005.

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Acknowledgement

Four-years of Ph D study and research are approaching an end. It is time

to express my sincere gratitude to the people who make my Ph D work fruitful

and my life enjoyable.

I have been very fortunate to get the opportunity to be a project engineer

(for short time) and then subsequently be recruited as a Ph D student working

in Biomolecular and Organic Electronics at the division of Applied Physics, in

the Department of Physics, Chemistry and Biology at Linköping University,

Sweden. The group is filled with very active and positive people; the

atmosphere was extraordinary for conducting multidisciplinary research. The

experience has allowed me to develop my abilities in scientific research and

management. Thank must go first to my supervisor, Prof. Olle Inganäs, for his

creative ideas, continuous encouragement and patient guidance in my hunt for

new insights and understanding behind the phenomena of my studies.

I would also like to gratefully acknowledge Magnus Krogh who

introduced the fantastic soft lithography technique to me and helped me

initially in my work for the European “Highlight Project”. His experience and

knowledge were invaluable for making the project go smoothly.

Thanks also go to Dr Mattias Östblom for his IRAS measurements, giving

clear interpretation of the PDMS stamp modification effect and for general

discussions. Furthermore, Dr Thomas Ederth was more than helpful in sharing

his knowledge on characterizing surface energies using a Wilhelmy Balance.

Prof. Mark E. Thompson at University of California supplied his

phosphorescent dyes for us to use and his kind suggestions and revision of my

manuscript for Paper III, thank you very much.

I am grateful to Erik Perzon, Lars Lindgren, Prof. Mats R Andersson and

their colleagues at Chalmers University of Technology synthesizing novel low-

and high-bandgap polyfluorene copolymers and many years cooperation and

xii

also to people: Juan Luis Delgado, Pilar de la Cruz and Prof Fernando Langa,

in Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La

Mancha, Spain, synthesizing the fullerene derivatives. The solar cells prepared

in Linköping showing some of the best results in this field result from the

efforts. The help obtained made it possible for us to investigate device physics

in such polymer-fullerene bulk heterojunction solar cells.

I am also indebted to interactions with some of the previous and almost all

the current members of the Biomolecule and Organic Electronics Group, in the

form of performing experiments together and ending with writing a paper. Also

reviewing papers within the internal referee system, being an assistant in the

lab and having fruitful discussions on previous and ongoing projects, had a

huge impact on my achievements and are very appreciated. I learn a lot of from

all of you.

I am grateful for the financial support from the National Graduate School

in Material Science and to all the people with whom I cooperated within the

network project: Erik Perzon, Mats Andersson, Cecilia Björström, Ellen

Moons, Kjell Magnusson, Fengling Zhang and Olle Inganäs. The meetings and

communications were also important.

Discussion with or help from people in Prof. Magnus Berggren’s group of

Organic Electronics in Norrköping Campus are also appreciated.

Thanks to Dr Stefan Welin Klintström for his useful suggestions and

reminders. Thanks also to our secretary Ann-Marie Holm for managing trips

and taking care of documents for group members.

Thanks also go to Dr David Lawrence. With your contribution and effort,

the thesis presents my Ph D work in a much nicer way.

Thanks to technicians Bo Thuner and Wim Bouwens for their support.

I am also grateful to friends in China, Sweden and everywhere over the

world for their support and concern.

xiii

Love, support and understanding from my husband, Dr Lichun Chen, and

my daughter, Gengshi, have been extremely important during the years. I

express my deep gratitude to you both. I wish I could present my love to you

two and be an excellent wife and a mother, in return, for all of our future years.

/Xiangjun Wang

xiv

Table of Contents

1. General introduction 1

2. Conjugated polymers 3

2.1 Chemical configuration and electronic structure 3

2.2 Optical absorption and emission 5

2.3 Electroluminescence 6

2.4 Exciton decay routes and dissociation 7

2.5 From insulator and semiconductor to metal 8

2.6 Solubility of the conjugated polymer 9

3. Surface energy modification 11

3.1 Surface energy 11

3.2 Determination of contact angle by goniometry, Wilhelmy

Balance and capillary rise methods 12

3.3 Surface energy modification 18

3.4 Surface modification by bare PDMS stamp 20

3.5 Surface modification by electrochemical doping 24

4. Patterning of polymers on PDMS modified surface 25

4.1 Patterning of polymers using soft lithography 25

4.2 Patterning of polymers on PDMS patterned surface

by dewetting 28

5. Conjugated polymer-based optoelectronics 38

5.1 Polymer light emitting diodes (PLEDs) 38

5.2 Molecularly-doped PLEDs 40

5.3 Polymer solar cells (PSCs) 44

5.4 PSCs based on low-bandgap polymer 48

6. Future outlook 54

7. Summary of papers 56

Reference 60

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1. General introduction

Conjugated polymers have attracted considerable interest since the first

conjugated-polymer-based light emitting diodes (LEDs) were created in 19901.

Polymers belonging to the conjugated polymer family are capable of acting as

conducting or semiconducting materials and are used in polymer-based

optoelectronics devices, such as light emitting diodes, photovoltaic solar cells,

field effect transistors, lasing devices and actuators1-5. These devices usually

contain several conjugated polymer layers that are prepared from an organic

solution of the polymers. For certain reasons, for instance when fabricating

addressable polymer LEDs for a display, or when optimising optical or electric

behaviour in an optoelectronical device when light propagates through the

device, the film of the polymers is required to be patterned3,5-7. Characterization

of the surface energy of a conjugated polymer gives information about the

surface wettability and the interaction between the surface and a surrounding

liquid. This characterization is important for understanding film formation and

therefore for optimisation of these devices. Surface energy modification can be

used for changing the surface wettability. This may result in a modified surface

with a desirable alternating decreased and increased wettability region.

In this thesis, a brief review of the concept of conjugated polymers and

their properties is given in Chapter 2. Chapter 3 describes the characterization

of the surface energy of conjugated polymers using the Goniometry and

Wilhemy Balance methods. This is one of the major parts of the work

presented in the thesis. In particular, the novel method of surface modification

using a bare elastomeric stamp made from poly(dimethylsiloxane) (PDMS), is

presented for the first time. Soft lithography for the patterning of polymers is

described in Chapter 4. A method, for patterning of polymers on PDMS

modified surfaces with surface energy controlled dewetting, is developed.

1

In chapter 5, the preparation and characterization of conjugated-polymer-

based optoelectronics devices, including LEDs, that generate light through the

injection of charge into the active polymer layer, and solar cells (SCs) that

create electric power by absorbing solar photons and converting them into

current, are presented. Phosphorescent metal complexes were doped into

polythiophene to fabricate LEDs. Energy transfer from the host (the polymer)

to the guest (the phosphorescent metal (iridium and platinum) complex) was

studied using photoluminescence and electroluminescence measurements

performed at room temperature and at liquid nitrogen temperature. Solar cells

were fabricated using low-bandgap polyfluorene copolymers as electron donors

blended with several fullerene derivatives as electron acceptors. Luminescence

quenching measured by photoluminescence and the lowest unoccupied

molecular orbital (LUMO) along with the highest occupied molecular orbital

(HOMO) of the active materials, measured using cyclic voltammograms, were

used to discuss the energetic match for efficient charge transfer at the interface

between the polymers and the fullerene derivatives. Newly synthesized

fullerenes that could match the low-bandgap polymers were selected and used

as electron acceptor for preparing SCs. Photovoltaic properties of these SCs

were characterised, demonstrating that one of the most efficient

polymer:fulleren SCs that generates photon current at 1 µm was obtained.

2

2. Conjugated polymers 2.1 Chemical configuration and electronic structure

A conjugated polymer is a kind of macromolecule that consists of

alternating single and double carbon-carbon bonds in the main chain

(backbone), as illustrated in Fig. 2.1. These bonds are constructed of three sp2

and one pz orbitals that are hybridized from the 2s and 2p electrons of the

carbon atom (see Fig. 2.1). The three sp2-orbitals form a σ-bond that lies in a

plane, with each orbital crossing the others at a common point with an angle of

120º. The electrons are strongly localized. The pz orbitals overlap each other

and form a π-bond, intersecting through the center of a σ-bond at an angle

perpendicular to the plane of the σ-bond. The electron in the π-bond can be

mobile over several carbon atoms and is not as localized as the electron in the

σ-bond. The distance the electrons are allowed to travel within the same chain

is referred to as the conjugation length. N delocalized electrons occupy N/2

molecular orbitals and all molecular orbitals are split into two groups: occupied

(π) and unoccupied (π∗) . Thereby an energy band gap between the highest

occupied molecular orbital (HOMO) and the lowest unoccupied molecular

orbital (LUMO) is formed and stabilization of the electronic structure is

achieved. The band gap is proportional to the reciprocal of the conjugation

length8-10.

Chemical configuration and electronic structure allow the conjugated

polymer to have desirable properties for a wide variety of applications in

optoelectronics and sensors.

The chemical structures of some of the conjugated polymers that are

historically important, or the concern of this thesis, are shown in Fig. 2.2.

3

4

2s

2pz

2pY

2pX

Y

X

XY

X

Y

pz

(1)

X

Y

y

X

Z

sp2

pz

(2)

y

X

Z

sp2

sp2

sp2

(3)

120º

120º120º

Fig. 2.1. The configurations of conjugated polymers. (1) The 2s- and 2p-

orbitals of carbon atoms are hybridised to sp2 and pz –orbitals. (2) σ-bones

(above) and π-bond (below) construction in conjugated polymers. (3) The

chemical structure of a conjugated polymer poly(acetylene).

n

n

(1) Polyacetylene (2) Poly(para-phenylene-vinylene)

S

O O

n

S n

(3) Polythiophene (4) Poly(ethylenedioxythiophene)

PEDOT

S S

NSN

NN

n

S

C8H17

H17C8

n

(6) Poly(dioctylphenylthiophene)(PDOPT) (5) Polyfluorene copolymer

APFO-Green 1

Fig. 2.2. The chemical structures of polymers those are important or

studied in this thesis. (1) The first, synthesized, conducting polymer,

discovered by S. Shirakawa and his coworkers who were awarded the Nobel

Prize in 200011-13. (2) The first polymeric light emitting diode made from

poly(para-phenylene-vinylene) by a spin-coating technique1. Polymers from

(3) - (6) are semiconductors and their derivatives are often used in polymer

opto- or electronics.

2.2 Optical absorption and emission

A conjugated polymer can undergo a transition of its electronic state from

the ground to an excited state (i.e. to an exciton) or from an excited to the

ground state via the absorption or emission of photons. Only singlet-singlet

transition can be induced under optical excitation, according to the spin-

conservation rule. Optical transition between vibronic energy levels in a

conjugated polymer is shown in Fig. 2.3.

5

Fig. 2.3. Optical transitions between vibronic energy levels in a

conjugated polymer.

S0n

S1m S1m

S0n

Configuration coordinate

Ene

rgy

n=1

n=3n=2

n=0n=1

n=3 n=2

n=0

m=1

m=3m=2

m=0

m=1

m=3m=2

m=0

350 400 450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Absorption Luminescence

Wavelength (nm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

S00-S12

S00-S11

S00-S10

S10-S02

S10-S01

S10-S00

2.3 Electroluminescence

When a semiconductor material, for instance a polymer film, is under the

influence of an electric field, electrons may be injected into the conduction

6

band and a hole in the valence band and thereby an exciton is induced.

Recombination of an electron and hole pair can lead to the emission of photons.

This is referred to as electroluminescence. Contrary to optical excitation, both

the singlet and triplet states can be electrically excited. Only the transition of a

singlet-ground state is allowed and generates light. That of a triplet-ground

state is spin forbidden, implying that only one quarter of the available excitons

can be internally converted into light14,15. The remainder of the electric power

is lost. However, the triplet-ground transition might be utilized to generate

light, by doping heavy metal complexes into polymers, enhancing the

electricity-to-light conversion efficiency. Details on how to utilize triplet

exciton will be discussed in section 5.2.

2.4 Exciton decay routes and dissociation

In addition to the radiative decay of an exciton (from the excited state to

the ground state) that gives off photon emission, there are other ways to relax

the excited state (see Fig. 2.4):

a. Internal conversion (IC): relaxation progresses from higher to lower

singlet or from higher to lower triplet state through a non-radiative

decay which gives off energy in the form of torsional and vibrational

quanta.

b. Inter system crossing (ISC): relaxation goes from the lowest singlet to

the lowest triplet or from the lowest triplet to lowest singlet state.

The IC process is common in conjugated polymers because the density of

the vibration levels increases the probability. ISC is important for a molecule

that contains a heavy atom, because of spin-orbital coupling. Polythiophenes

contain heavy sulphur atoms and make the ISC yield as high as 40%16.

In addition, the excitons formed through the absorption of photons can

also be dissociated into a separate electron and hole. For this to happen a higher

affinity molecule must be available and be located close to the polymer. An

electron can then be transferred to the high affinity molecule. Collection of

7

these electrons from the secondary molecule onto an electrode gives a

photoinduced current. This is the basis for photovoltaic or solar cells. Details

about current generation induced by absorption of photons will be described in

section 5.3.

Fig. 2.4. Different transition and excitation decay routes in a

conjugated polymer molecule.

S0

S2

S1ISC A

bs.

P

IC

Abs. F

IC

IC

T2

T1

2.5 From insulator and semiconductor to metal

Conjugated polymers, in pristine and neutral states, are insulators with

wide band gap or semiconductors with narrow band gap. The energy band gap

of the semiconducting conjugated polymer is normally in the range from 1 to 4

eV, which is similar to that existing in the common inorganic semiconductor.

Conjugated polymers may have metallic properties, i.e. high conductivity, after

the neutral polymers undergo reduction (n-doping) or oxidation (p-doping) by

withdrawing or adding electrons or ions, where a deformation of the main

chain is introduced. This is referred to as doping a polymer. A charged polymer

molecule, if it creates a degenerate state within the band gap, forms a soliton,

such that the conductivity of the polymer is increased. The conductivity of

conjugated polymers can cover the range from insulator (10-10 S/m) to metal

(107 S/m)17. Charged polymer molecules can also generate non-degenerate

electronic states in the band gap, forming polarons or bipolarons, such that the

8

optical properties of the polymer are modified18-20. Fig. 2.5 shows the chemical,

and electronic structures of a polaron and a bipolaron and their charge and spin.

Fig. 2.5. Four new levels are introduced in the bandgap as the

results of polaron and bipolaron formation. Solid and dash-line

arrows represent electron spin and possible transition to/or from the

new states, respectively. The charge and spin of the polarons are

listed to the right.

Chemical structure Electronic structure Charge and spin

CB0 and 0 Neutral state

SS

SS

SS

SS

VB

CB

SS S

SS

SS

S+ +

+e and 1/2 Positive polaron

+2e and 0 Positive bipolaron

-e and ½ Negative polaron

-2e and 0 Negative bipolaron

CB

VB

CB

VB

CB

VB

SS S

SS

SS

S .+

VB

. Ө

SS S

SS

SS

S

ӨӨ

SS S

SS

SS

S

9

2.6 Solubility of the conjugated polymer

Another important property of conjugated polymers is that they can be

made soluble in common organic solvents by adding appropriate functional

side-chain groups. This means that the deposition of polymer films can be done

by spin-coating techniques in air from polymer solutions, which makes

fabrication of conjugated-polymer-based thin-film devices much easier and less

costly when compared with devices made via vacuum deposition techniques.

The interaction at the interface between a polymer in an organic solvent and the

substrate surface affects the formation and morphology of the film. This may

influence the mechanism of exciton and charge carrier transport, dissociation of

excitons and recombination of charges in various electronic devices, and

therefore influence the performance of the resulting devices21. It again shows

that characterization and modification of surfaces are essential in improving the

performance of opto-electronic devices.

10

3. Surface energy modification

3.1 Surface energy

When a liquid drop stands on a solid surface, the shape of the droplet is

determined by a balance between the cohesive force existing between the liquid

molecules and the attractive force formed between the liquid molecules and the

solid surface molecules at the interface. A droplet beads-up when the cohesive

force is stronger than the attractive (adhesion) force. On the other hand, the

liquid drop spreads over a solid surface if the cohesive force is weaker than the

attractive (adhesion) force. The angle formed between the liquid-solid interface

and a tangent to the droplet profile at the liquid-solid-air contact point is

referred to as the contact angle (θ), which is often used to qualitatively

represent surface energy or surface wettability. In Fig. 3.1, a schematic diagram

of a droplet standing on a solid surface at equilibrium is shown, where

σSL, σS, and σL are the surface energies at the interfaces of the solid-liquid,

solid-air and liquid-air, respectively. θ is the contact angle.

Fig. 3.1. Schematic diagram of a liquid

droplet standing on a solid surface.

Liquid droplet

σS

σL

σSL

θ

Solid

At equilibrium, the relationship between the shape of a liquid droplet and

the surface energy or surface tension at the interfaces can be quantitatively

stated by Young’s equation:

11

L

SLS

SLSL

σσσθ

σσθσ−

=⇒

−=⋅

cos

cos

Eqn 3.1

The surface energy or the surface wettability of a solid surface can be

characterized using the contact angle. The relationship between the contact

angle and the surface wettability is illustrated in table 3.1.

Table 3.1. The contact angle of water on a solid surface and its surface

wettability

Higher

Lower

Surface energy

Partially wettable

00 900 ≤< θ

Adhesion force > Cohesive force

Non-wettable

090>θ

Adhesion force < Cohesive force

Completely wettable

00~θ

Adhesion force >> Cohesive force

Surface wetting capability

Contact angle

Forces

In practice, a contact angle when a liquid covers a fresh solid surface (i.e.

the advancing contact angle) differs from that when the liquid recedes (i.e. the

receding contact angle). Values quoted in the literature generally refer to the

advancing contact angle, unless otherwise stated.

3.2 Determination of contact angle with goniometry, Wilhelmy balance and

capillary rise methods

To characterize the surface energy or surface wettability, several methods

have been developed, including (a) measuring the contact angle of a pendant

droplet on a surface using goniometry (drop shape method)22-24 and (b)

calculating the contact angle that is formed between a solid surface and a liquid

when a solid plate is vertically immersed into a liquid, using a Wilhelmy

12

balance (Wilhelmy method)25,26. These are schematically illustrated in Fig. 3.2.

Additionally, capillary rise27-29 can be also used to characterize surface energy.

3.2.1 Determination of the contact angle by a goniometer

Goniometry, measuring the contact angle of a pendant liquid droplet on a

surface, is a convenient and direct method. As shown in Fig. 3.2 (a), a pendant

liquid droplet is dispensed on a surface using a micro syringe attachment. The

contact angle is measured using a goniometer at room temperature and ambient

humidity immediately after equilibrium is established.

Fig. 3.2. Schematic diagrams of contact angle measurement

instruments: (a) Goniometry and (b) Wilhelmy balance. The

substrate in (b) consists of a coated layer and a supporting

substrate layer. In the general case mθθ ≠ .

The contact angles of some of the conjugated polymers, including

polythiophenes, polyfluorene copolymers and a fullerene derivative used to

fabricate the polymer solar cells and light emitting diodes in our lab, were

measured using the drop shape method. The samples were prepared by spin-

coating a polymer film onto a glass substrate from a polymer solution under

ambient conditions. Water was used as the probe liquid for the measurement

and all measurements were made under ambient conditions. The chemical

Hung on a micro balance

θmθ Liquid reservoir

Solid surface

Liquid droplet

Micro syringe

θ

(a) Goniometry (b) Wilhelmy balance

13

structures, film formation, storage conditions and contact angle values are

listed in table 3.2. The polymers from (1) to (11) belong to the polythiophene

family. These polymers can be divided into roughly two groups from a

chemical structure point of view. The first group of polymers, including (1) to

(5) and (10) to (11), has side chains with alkyl, or phenyl alkyl groups attached

to the backbone. These hydrophobic side chains cause the films to be non-

wettable and therefore show higher contact angles (θ > 90˚) as compared with

the other polythiophenes. The second group of polymers consists of (6)-(8),

which have ether groups as side chains with an alkyl terminating group, and

might result in attractive (adhesion) forces (O-HO bond) between the polymer

surface and the water (the testing liquid), resulting in films that are more

wettable (θ < 90˚). Polymer (9) is also a polythiophene and involves both alkyl

and ether groups in its side chains. The contact angle for this polymer is larger

than 90˚, possibly because of the hydrophobic alkyl groups dominating the

surface energy property of the outer layer of the film. Polymers (12) and (17) to

(20) are alternating polyfluorene copolymers and non-wettable, as governed by

the alkyl group. A film from the C60 molecule derivative has a contact angle

less that 90˚, which could be attributed to the ester group involved in the

molecule. The difference in contact angle between polymers (for example

polyfluorene copolymers) and the PCBM fullerene might cause an

inhomogeneous distribution of two phases laterally, and/or a gradient

distribution vertically relative to the surface as spin-coated from a mixture

solution, as revealed by other analysis methods30,31.

The condition for preparing films (spinning speed, spinning duration and

solution concentration), in our case, did not have much influence on the

resulting contact angle as shown in Table 3.2.

3.2.2 Determination of the contact angle using a Wilhelmy Balance

As shown in figure 3.2 (b), a thin plate (generally rectangular) coated with

a thin film layer is immersed into and withdrawn from a liquid at a constant

14

Table 3.2 Films and contact angles

Spin rate

Durat-ion (s)

Drying in

Contact angle (°)

Short name

Chemical structure Vaca Air b Advc Recd

1000 40 X 98 90 (1) PTOPT

1000 40 X 97.5

1000 40 X 105

1000 40 X 109

(2) PDOPT

1000 20 X 107

1000 40 X 101 90 (3) POPT

1000 40 X 99

1000 40 X 98 94 (4) P3HT

1000 20 X 100

(5) P3OT

1000 40 X 106 92

(6) PEOPT

1000 40 X 80 50

S

S

CH3

n

S

CH3

CH3

n

S

CH3

n

S

CH3

n

S

CH3

n

S

OO

O CH3

n

(7) MS2

1000 40 X 58 40

(8) MS3

1000 40 X 78 64

(9) MW60

1000 40 X 93 90

(10) PDOCP-

T

1000 40 X 90 80

(11) PBOPT

1000 40 X 93 91

(12) F8BT

1000 40 X 98 94

(16) APFO2

1000 40 X 95 94

S

CH3

OO

O CH3

n

S

OO

O

CH3

CH3

n

S

O

CH3

S

CH3

n

S S

CH3CH3

n

S

CH3

O

CH3

n

C8H17H17C8

SNN

n

NN

S

S Sn

LBPF

(17) APFO5

1000 40 X 110 98

(18) LBPF4

1000 40 X 95 91

1000 5mg/ml

40 X 95 93 (19) APFO3

1000 10mg/ml

X 97

(20) APFO4

1000 40 X 98

NN

S

S SS

n m o

LBPF3

NN

S

N Nn

LBPF4

NN

S

S Sn

LBPF6

NN

S

S Sn

LBPF5

(21) PCBM

1000 40 78

O

OMe

• All films were deposited from chloroform solution with concentration of

5 mg/ml. Unless otherwise stated.

• Vaca: In a vacuum, 10-6 Torr; Airb: In air; Advc: advancing contact

angle; and Recd: receding contact angle.

speed so that a curved liquid surface is formed on both the back and front sides

of the plate. The force, F, acting on the plate is monitored by a micro-electro

balance and can be described by the following equation:

Ftotal = wetting force + weight of plate (sample) – buoyancy

For a “Sigma 70” Wilhelmy balance, as used in this work, the weight of

the plate is removed by the instrument’s electronics and the buoyancy force is

estimated by extrapolating a graph of force vs immersion depth back to zero

depth. The remaining component of the measured force is the wetting force

which is given by:

Wetting force = 2/)cos(cos mLS P θθγ + Eqn. 3.2

where P is the perimeter of the sample plate, and LSγ is the interfacial

surface tension between the liquid and solid surfaces.θ and are the contact

angles of the liquid formed on the film-coated side and backside of the plate,

respectively. They are normally not equal due to the unequal surface energy of

the two sides. Thus, at any depth, data is collected which can be used to

calculate the contact angle. The contact angle that is obtained from data

generated as the sample plate advances into the liquid, is called the ‘advancing

contact angle’. When the process is reversed, i.e. as the sample plate retreats

from the liquid, a ‘receding contact angle’ is obtained. Here it is assumed that

the surface tension of the liquid and the contact angle of the liquid on the

backside of the substrate, , are known. The dynamic contact angle

measurement can be carried out while the plate is immersed into the liquid. In

paper

mθ

mθ

32, we demonstrated the in-situ surface energy (contact angle)

investigation of conjugated polymers during electrochemical doping in an

aqueous electrolyte using the Wilhelmy method.

3.2.3 Determination of the contact angle by capillary rise

The capillary rise, H, due to the capillary interaction of a surface and a

liquid can be used to evaluate the surface energy of a solid surface. H can be

estimated from the following equations for different geometries28,29,33:

15

Cylinder tube: gR

H LV

ρθγ

∆=

cos2 Eqn. 3.3

Plate: g

HgH LV

LV ρθγ

γρθ

∆−

=⇒∆

−=)sin1(2

21sin

2 Eqn. 3.4

where ρ∆ is the difference between the densities of the liquid and vapour,

is the surface tension of the liquid, g is the acceleration due to gravity and

R is the radius of the cylinder tube.

LVγ

In our contact angle determination, we qualitatively demonstrated the

surface energy changes through the capillary rise on the inner and outer

surfaces of a sample consisting of two parallel polymer coated plates when

subjected to electrochemical doping at different doping levels32.

V = 0 V V = 0.3 V V = 0.5 V V = 0.8 V

Fig. 3.3. A front view of a sample that consisted of two P3HT coated

ITO/PET parallel plates facing each other, when subjected to

electrochemical doping at different doping levels.

The capillary rise of an aqueous electrolyte between two parallel plates

coated with P3HT was quite obvious when the samples were subjected to

electrochemical doping.

The meniscus shape in Fig. 3.3 was mainly attributed to the difference

between the capillary fall (H2) on the inner surface, the capillary rise (H1) on

the outer surface of the two parallel plates and to an external term H(d, L)

induced by sample geometry. The total capillary rise difference, , is

given by

totalH∆

),()sin1(2)sin1(2

),( 2121 LdH

ggLdHHHH LVLV

total +∆

−+

∆−

=+−=∆ρ

θγρ

θγ

Eqn. 3.5

16

where 1θ ~76° and 2θ ~110° are the contact angles of the liquid against

the outer and inner sides of the sample, respectively, at initial state. d and L are

the distance between two parallel samples and the width of the samples,

respectively. As shown in Fig. 3.3, decreases with increasing doping

level, revealing a reduced capillary fall on the inner side of the sample,

resulting from a decreased contact angle,

totalH∆

2θ . 1θ did not change, and the

capillary rise H1 was kept unchanged. only underwent a minor change

which was considered negligible, because there was no geometry change. The

trend in wettability changes of P3HT film is consistent with the results obtained

from the Wilhelmy balance and the drop shape measurements

),( LdH

32.

The surface energy or wettability of a solid surface can be characterized

by the contact angle. Three methods including goniometry, Wilhelmy balance

and capillary rise were presented. Goniometry is simple and direct. It only

supplies information at a local point (on the focusing plane of the meridian) for

each measurement. Surface heterogeneity and/or roughness could cause

variations of the contact angle along the three-phase line. Drop size differences

can be a further cause of variation of the contact angle in the measurement34-36.

The contact angle measurement is accomplished with a sensitive microbalance

and hence is free from operator error which would arise if it were determined

by eye. The results also tend to be highly reproducible due to the large scan

area of the samples and the bulk liquid.

On the other hand, the Wilhelmy balance method has the advantage of

giving reliable characterization of the dynamic effect during wetting and

dewetting because of accurate control of the advancing and receding speeds.

The capillary rise on a vertical plate can also give an in-situ contact angle

characterization. It requires precise height monitoring of the sample.

To make the capillary effect more effective, strategies for preparing a

sample should be considered and curvatures formed at the interface between

the three phases (gas, solid and liquid) must be precisely identified and taken

into account in the calculation.

17

Surface wettability is determined by the surface molecules and their

orientation. Additionally, properties of roughness, inhomogeneities and

chemical contamination have been shown to have smaller effects on surface

wettability.

3.3 Surface energy modification

In current materials research and development, a high priority is given to

surface modification techniques to achieve improvements of surface properties

for specific applications requiring high quality, thin films (e.g. for thin film

opto-electronics, for patterning of material on a surface as well as for painting 37-39). Surface energy modification works best when restricted to a very thin

layer at the surface, just a few nm or even smaller. Ideally the modified layer

should be a monolayer of a polymer surface, such that the surface property, for

instance surface wettability, is modified and the bulk properties beneath the

surface are kept unchanged. The common method to modify a surface property

will be discussed in this section. The method using a bare rubber stamp will be

demonstrated in section 3.4.

3.3.1 Mechanical surface modification

Atomic Force Microscope (AFM) was initially developed to image the

surfaces of insulating materials. However, it was discovered that an AFM

probe could cause mechanical modifications to a surface40,41. Mechanical

surface modification can be regarded as the simplest method to modify a

surface: just using a probe of an atomic force microscope scratches a surface

with a controlled force in either contact or semi contact mode. A very thin layer

is removed and the surface morphology is altered. The modified surfaces are

dependent on the shape of the tip, the force strength applied to the tip, the

scanning speed of the tip and the morphology of the surface as well as the

hardness of the surface material. In some cases tip-induced reorganisation of

molecules or chemical reaction of the surface molecules may occur42,43.

18

3.3.2 Plasma treatment of surface

Plasma is an ionised medium consisting of electrons, ions and possibly of

neutral species and photons, which meets some additional criteria44. Plasma

treatment is probably the most versatile surface treatment technique. Plasma

surface treatment can be conducted using several gases: oxygen, nitrogen or

carbon dioxide, depending on the surface’s composition45. Oxygen plasma

treatment resulted from cross-linking of the surface atom with an ionised

oxygen atom can increase surface energy or surface hydrophilicity or surface

wettability, because some of the oxygen sites are readily converted into

hydroxyl bonds and easily attract water molecules. The chemical structure of a

shallow surface layer (a few nm) can be changed significantly, while the bulk

properties remain unchanged. A similar process takes place with N2 plasma

treatment. Schematic illustrations of both treatments of a surface are shown in

Fig. 3.4.

ig. 3.4. A schematic illustration of O2 and N2 plasma treatment of a

The change of surface energy or surface wettibility after plasma treatment

basic

Fsurface.

ally depends on the gas selected, active gas pressure, the voltage applied

CC

O

CC

CC

OH

C

C

OH

O

CC

CC

CC

C Plasma

O2

CC

NO3

CC

CC

NH2 NO3

CC

C

CC

CC

CPlasma

N2

19

to the plasma, the temperature of the plasma gas and the time that the plasma is

in contact with the surface.

3.3.3 Self-assembled monolayer (SAM)

A self-assembled monolayer (SAM) is formed spontaneously by

chemisorption and self-organization of functionalised and long-chain organic

molecules on surfaces of an appropriate substrate. SAMs are usually prepared

by immersing a substrate into a prepared solution containing a ligand that is

reactive with a surface or by exposing the substrate to a vapor or a reactive

species for a period of time. Self-assembled monolayers of thiols on gold

surfaces are widely used to produce model surfaces with well-defined chemical

composition for a variety of applications including electronic devices,

biomaterials and biosensor surfaces4,6,41,46. A SAM layer may also be

transferred to a surface by contact printing using a rubber stamp47. The SAM

layer is sometimes called the ‘ink’. The surface energy or surface wetting

capability can be tailored by selecting the terminating functional group of the

SAM molecule appropriately, by adjusting the length of the moiety and by

adjusting the degree of coverage of the moiety on the surface. A SAM layer

also has the function of blocking any reaction between two interfaces when

situated between them48. It has been demonstrated that some techniques, such

as micro contact printing (µCP) using PDMS49,50, photolithography51 and vapor

deposition can be used to modify a surface and even produce chemically

patterned surfaces.

Further to the above-mentioned methods, thermal annealing is also an

efficient and common way to reorder the orientation of molecules in a film as

well as to modify surface morphology. Photochemistry using UV or ozone can

also be used for surface modification, as well as ion implantation and

electrochemical doping52,53.

3.4 Surface modification by bare PDMS stamp

20

Transferring a poly(dimethylsiloxane) (PDMS) molecule to a surface

when a PDMS stamp is brought into conformal contact with the surface has

been reported by Glasmästar54. In this section a method of surface energy

modification using a bare PDMS stamp is presented. The mechanism for

surface energy modification was investigated by several surface analysis

methods including atomic force microscopy (AFM), infrared absorption

spectroscopy (IRAS) and contact angle measurement.

The possibility of using a bare PDMS stamp to modify the surface energy

of a surface was first observed to generate a thin layer pattern on a surface.

Finally the method is developed and used as an efficient and common tool for

modifying surface energy and for patterning polymers55-57 and

biomolecules58,59. The surface energy modification is performed as illustrated

and described in Fig. 3.5.

Step 1: Prepare a solid surface.

Step 2: A flat or patterned PDMS stamp

makes conformal contact with the prepared

surface for a period of time.

Step 3: Removal of the stamp from the

surface leaves a PDMS modified surface.

Fig. 3.5. A schematic illustration for surface modification by a bare

PDMS stamp.

Surface energy changes with time were monitored when a stamp was

brought into conformal contact with a surface. Fig. 3.6 shows the contact angle

of two surfaces, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

(PEDOT-PSS) and the polyelectrolyte poly(sodium-4-styrenesulfonate)

(NaPSS), as a function of contacting time. Contacting time is defined as the

period from when the sample is brought into contact with the surface until

when the stamp is removed. N-hexadecane was used as the probe liquid. It can

21

be seen that the natural contact angle of PEDOT-PSS (θ1) is ~ 36° and ~ 9° for

NaPSS (θ2); thus the wetting by n-hexadecane on PEDOT-PSS is poorer than

that on NaPSS. The contact angle on PEDOT-PSS or NaPSS changes with

contacting time. The contact angle of the n-hexadecane on PEDOT-PSS

decreases from 22° to 12° logarithmically as the contacting time changes from

25 min. to 100 hrs. For NaPSS, the contact angle increased linearly from 27° to

33° with logarithmic contacting time from 25 min. to 100 hrs. The difference in

contact angle between modified and unmodified films is -25° for the PEDOT-

PSS film. The magnitude of the corresponding angle for the NaPSS was less,

+23° (contacting time: 2 days or more), revealing that the surface energy

modification was undertaken and done mostly during the first four hours.

100 101 102

10

20

30

40

Non modified

Con

tact

ang

le (º

)

Contacting time (Hours)

PEDOT-PSS NaPSSNon modified

Fig. 3.6. The contact angles as a function of contacting time when PEDOT-

PSS and NaPSS surfaces were getting conformal contact by bare stamps.

The surface morphology of the areas that were modified or non-modified

was investigated using atomic force microscopy (AFM)55 revealing that a

larger thickness (a few nanometers) protruded from the modified surface

relative to the non-modified surface. However it was difficult to judge if the

height change was caused by the lifting up of the original film from the surface

22

or due to molecule transfer from the stamp to the surface or possibly due to an

absorbed layer of water, since the measurement was carried out in the air.

The infrared absorption spectroscopy (IRAS) in Fig. 3.7 gives an insight

into the mechanism of the surface energy modification method. The IRAS

investigation, firstly, shows the chemical changes on PEDOT-PSS and NaPSS

surfaces due to the modification55. They are similar with only minor

differences, suggesting that the chemical changes are the same on both

surfaces. The chemical changes are related to the uncured PDMS molecules,

implying small PDMS molecules have been transferred to the surface during

the conformal contacting of the PDMS stamp with the surfaces.

8001000120014001600

Abs

orba

nce

Wavenumber (cm-1)

ab

e

cd

f

8001000120014001600

Wavenumber (cm-1)

Abs

orba

nce

PDMS

ODMS

Residual PDMS onPEDOT-PSS

NaPSSResidual PDMS on

Fig. 3.7. IRAS of PEDOT-PSS and NaPSS films before and after

modification by a PDMS stamp. Left: IRAS spectra of a PEDOT-PSS film

on gold before (a) and after (b) PDMS modification and the resulting

difference (b)-(a) spectra (e). The IRAS spectra of a NaPSS film on gold

before (c) and after (d) and the resulting difference (d)-(c) spectra (f).

Right: The resulting spectra of residual PDMS on PEDOT-PSS and

NaPSS are compared with the spectra of oligo-DMS and PDMS. The

spectra of PDMS and ODMS are scaled for easier comparison.

23

3.5 Surface modification by electrochemical doping

As already shown in sections 3.2.2 and 3.2.3, modification of the surface

energy of a conjugated polymer film can also be carried out when subjected to

electrochemical doping in an electrolyte. Polymer surface energy changes can

be attributed to the fact of change in polarisability due to the

semiconductor/metal transition occurring and change of surface chemical

composition due to ingress/egress of anions/cations and associated solvent

molecules during the electrochemical doping. Fig. 3.8 shows the dynamic

surface energy changes of two polythiophene films during electrochemical

doping-dedoping, measured in-situ by the Wilhelmy Balance method.

0 2 4 6 8 10 12 14 160

20

40

60

80

100

120

-0.1V0.7V

Con

tact

ang

le (θ

)

Immersion depth (mm)

a

Neutral state, 0V

0V 0.5V

0 2 4 6 8 10 12 140

10

20

30

40

50

60

70

80

90 -0.8V

0.8V0.8V

-0.8V

0V

C

onta

ct a

ngle

(θ)

Immersion depth (mm)

b

Fig. 3.8. In-situ dynamic contact angle of (a) poly(3-hexylthiophene) and

vapor-phase polymerised poly(3,4-ethylenedioxythiophene) upon

electrochemical doping-dedoping.

24

4. Patterning of polymers on a PDMS modified surface

4.1 Patterning of polymers using soft lithography

Soft lithography has been developed in recent years to generate micro or

nano-patterns for application in biology, biochemistry and electronics7,49,50,62.

Due to its simplicity, low cost, no clean room requirement and the possibility

for patterning on a curved surface and over a large area, soft lithography is an

attractive alternative to photolithography. The big advantage of soft lithography

is that it avoids exposing a polymer to high energy irradiation and therefore

protects a polymer from degradation. The essential tool used in soft

lithographic techniques is a transparent elastomeric (rubber)

poly(dimethylsiloxane) (PDMS) stamp. By using a stamp with the desired

micro or nano-topography patterns, fluids carrying dissolved polymers or

dispersed particles can be directly confined, solidified and patterned onto a

surface by means of methods like micromolding in capillaries (MIMIC)63,

microtransfer molding (µTM)64 and micro contact printing (µCP)49,50. A

polymer layer is softened by heating at certain temperature or solvent vapor

and can be soft embossed (EM) or imprinted7.

A development of soft lithography is patterning of a polymer onto a

chemically patterned or modified surface that is created by transferring a

molecular monolayer pattern using the SAM method or stamping ‘ink’49.

Dewetting of the polymer on some area of the chemically heterogeneous

surface replicates the surface pattern. Intensive experimental studies and

theoretical simulations of this issue have been performed since the end of the

20th century65-76.

The instability of films, as caused by variation of film thickness67,68,76 and

surface energy gradient on a surface, causes dewetting and hence plays an

important role in initiating patterning processes74,77-80. Film instability that

25

induces dewetting or rupture of a thin film can be classified into two regimes:

film instability on a homogeneous substrate and that on a heterogeneous

substrate. A classical model of thin film break up on a homogeneous substrate

is called spinodal dewetting65. The inflection points, given by the second

derivative of changes of Gibbs free energy is equal to zero, are called spinodal

points, which define the thermodynamic limits of metastability. For

heterogeneous substrates, the break-up is described by the capillary instability

mechanism. Surface energy is the driving force for the film rupture process66.

Instability of thin films on a heterogeneous surface results from a spatial

difference in the wettability of the surface or the gradient of surface energy67-

69,81 and not from the surface being completely nonwettable71-73.

A thin-film equation has been used to describe film instability and

rupture, and to predict ideal templating, which must be reviewed in order to get

a desired polymer pattern. The nondimensional thin film equation:

[ ] [ ] .0)(/ 323 =Φ∇⋅∇−∇∇⋅∇+∂∂ HHHTH Eqn. 4.1

has been used to simulate instability of liquid films on heterogeneous substrates

and to predict the conditions for an ideally templating substrate surface. The

nondimensional equation governs the stability and spatio-temporal evolution of

a thin film system on a substrate subjected to excess intermolecular interactions

74,75,81. The terms in Eqn. 4.1 are as follows: is the nondimensional

local film thickness scaled by the mean thickness ;

),,( TYXH

0h

[ ] [ ]HGAh s ∂∆∂×=Φ //2 20π G∆, is the excess intermolecular interaction energy

per unit area, and As is the effective Hamaker constant for van der Waals

interaction; X, Y are the nondimensional coordinates in the plane of the

substrate, scaled by a length scale 22/10

)/2( hAsπγ ; and the nondimensional time

T is scaled by , γ and µ refer to the film surface tension and 250

2 /12 sAhµγπ

26

viscosity, respectively. The terms (from left to right) in the thin film equation

correspond to the accumulation, curvature (surface tension) and intermolecular

force.

The length scale of the spinodal instability on a uniform surface is given

by . ( )[ ] 2/1222 //4 hG ∂∆∂− γπ

On a chemically heterogeneous, striped surface:

. ( )YXH ,,Φ=Φ

At a constant film thickness, variation of Φ in the X direction by a periodic

step function of periodicity Lp, the gradient of force, Φ∇ , at the boundary of

the stripes causes flow from the less wettable (high pressure) regions to the

more wettable (low pressure) regions, even when the spinodal stability

condition is satisfied everywhere. The nonlinear thin film equation

predicts that there is a critical length scale (λ

0/ >∂Φ∂ H

h) that is smaller than the length

scale of the spinodal instability. Ideal replication of substrate surface energy

patterns in thin film morphology occurs only when

(a) the periodicity of the substrate pattern is greater than λh;

(b) the width of the less wettable stripe is within a range bounded by a low

critical length, below which no heterogeneous rupture occurs, and an

upper transition length above which complex morphological features

bearing little resemblance to the substrate pattern are formed;

(c) the contact line eventually rests close to the stripe boundary;

(d) the liquid that forms on the more wettable stripe remains stable.

Conditions (a) and (b) ensure the onset of dewetting at the center of every less

wettable site and conditions (c) and (d) ensure full coverage of every more

wettable site.

In summary, the conditions for ideal replication of a periodic

heterogeneous substrate can be engineered by modulating the pattern

periodicity, the width ratio between less and more wettable strips, the film

thickness and the wettability gradient across the boundary.

27

4.2 Patterning of polymers on PDMS patterned surface by dewetting

As demonstrated in section 3.4, a PDMS stamp is capable of modifying

the surface energy of a surface by simple conformal contacting with the

surface. We developed microcontact printing to create a chemically patterned

surface. The method to pattern a surface is simple and convenient compared

with the SAM method, since only a bare PDMS stamp without any coating is

used. A PDMS stamp leaves a low molecular weight residue, as an ‘ink’ on a

surface, as observed by infrared spectroscopy after removal of the stamp54,55.

Patterning of single or double layer conjugated polymers can be achieved by

dewetting according to the substrate’s or lower layer’s surface energy pattern,

as applicable, from a solution with a low vapor pressure or from a film that is

heated and subsequently melted. Several issues have been addressed in this

thesis work, including solvent effects on the patterning method55,56,82; the

profile of patterns prepared from different methods55,56,82; the thickness

dependence of pattern morphology from melts56,82 and the aspect ratio of the

spherical caps of patterned films formed from melts56. A schematic picture in

Fig. 4.1 summarizes the surface energy controlled dewetting technique. The

details are also briefly discussed in the following sections.

a. How a solvent effects patterning methods:

On a PDMS patterned surface (see the 1st column in Fig. 4.1) there are

two different methods for patterning of the polymers, depending on the solvent

used for dissolving the polymer (see the 2nd to 4th columns in Fig. 4.1). A

polymer dissolved in solvents (toluene, xylene and (di)chlorobenzene) with low

vapour pressure (< 0.03 bar) and low evaporation rate can be patterned directly

from the solution by spin-coating or dip coating (the 2nd column in Fig. 4.1).

However, a polymer dissolved in a solvent (chloroform and hexane) with high

vapor pressure (> 0.2 bar) and fast evaporation rate forms a continuous film on

28

top of the modified surface when spin-coating. The patterning of a polymer can

be obtained when the polymer film is annealed above its glass transition point

(The 3rd and 4th columns in Fig. 4.1).

Surface patterning

Layer 1 Transfer upon annealing

Layer 2 Transfer upon annealing

∆θ2 = 33º ∆θ1 = 24º

Applying PDMS stamp

Substrate

Annealing at T > Tm1

Formation of layer 1Formation of layer 2 on top of layer 1

Annealing at Tm1 > T > Tm2

Substrate coated with the homogeneous polymer 1

Layer 1 coated with the homogeneous polymer 2

Removal of stamp leaves pattern

Layer Transfer from solution directly

Method 1

Method 2

Formed from low vapor pressure solution by spin-coating

Patterning polymer with 2 different methods

Fig. 4.1. The illustration of surface modification as well as two different

patterning methods on a modified surface. Method 1: Formation of a

polymer pattern directly from a solution by spin-coating. Method 2:

Formation of single and double layer polymer pattern from a

homogeneous film upon annealing.

b. Profile of patterned polymer with different methods:

In “method 1”, films patterned directly from a solution by spin-coating

(Fig. 4.2a) typically have a U-shaped profile consisting of two high walls at the

pattern’s edge and a flat interior part between the two walls. The ratio between

the height of the wall and the thickness of the interior part is in the range from

~1 to 7, depending on the deposition conditions (spinning speed, solution

29

concentration and selected solvent)55. The patterned film thickness (flat interior

part) was limited to 200 nm. This profile results from liquid film dewetting,

solvent evaporation and mass transfer. The polymer carried by a solvent first

dewets from a less wettable region and collects onto the more wettable region.

The solute is then deposited (pinned) at the solid-liquid contact line. Mass

transport to the edge is undertaken during solvent evaporation, leading to a

thicker film at the edges. At higher concentration, the height of the U-shapes is

reduced (Fig. 4.2b) due to a short time for mass transfer to the edge83,84. On the

other hand, when homogeneous polymer films form on the PDMS stamp

patterned surface, pattern formation of the polymer can be induced by

annealing at a temperature above Tg. This is “method 2” for replicating a

surface pattern. The polymer film patterns upon annealing at the final stage of

method 2 always have a spherical-cap shape (Fig. 4.2c) due to minimization of

the surface energy.

550

a

D stan e i250

c350

(µ450

m)

150500

51000152000

00

00

Thic

kne s

s (Å

)

Fig. 4.2. The cross sections of a

patterned polymer (a)

polyfluorene from a 10-mg/ml

solution by spin-coating (1500

rpm), (b) polyfluorene from a 20-

mg/ml solution by spin coating

(1000 rpm) and (c) from a

polythiophene 300 nm thick film

upon annealing.

0

Thic

knes

s (Å

)

0500

1500 1000

2000

500200 µm

100 400)

300Distance (

b

c

100 )

1

0

600 900

300

1200 500

Thic

k ne s

s ( n

m)

200 Distance (

300µm

30

c. Influence of film thickness on morphology

The dewetting of a polymer film upon annealing shows that the time

period for patterning varies from a few minutes to several hours. The length of

time required is determined by the parameters of heating rate, temperature,

polymer thickness, molecular weight of the polymer, and surface energy

gradient across the boundary of the less and more wettable regions. Not all

films can generate desirable, perfect patterns. Only those films with initial film

thicknesses in an intermediate range can perfectly replicate surface pattern. A

thinner film is more likely to form holes and then to break into several small

holes and further form smaller defect droplets (called dewetting spots) in less

wettable regions. At low initial thickness, the density of dewetting spots is

larger than that found with larger thickness, and the size of dewetting spots is

smaller than those formed with larger thickness. At intermediate thickness,

perfect patterns with clean and smooth array on the modified substrate surface

can be obtained. The film is hard to move and remains stable when the initial

film thickness is too large.

Fig. 4.3 shows an example of film morphology dependence on thickness

from measurements taken by AFM in tapping mode. The modified surface had

a periodicity of 7.5 µm, consisting of more wettable stripes 3.5 µm wide and

less wettable ones 4.0 µm wide.

(b) (a)

31

(c) (d)

Fig. 4.3. Comparison of PMMA film morphologies with varied initial

film thickness (a) 120 nm, (b) 128 nm, (c) 345 nm, (d) 500 nm.

Similar trends in morphology are observed for all studied periodicities,

from 1-100 µm.

The graph in Fig. 4.4 shows the relationship of pattern morphologies with

pattern periodicity and initial film thickness. It can be divided into three

regions, denoted by I, II, and III. Regions I and III are the undesired regions

where clear and smooth patterns could not be achieved due to either too thick

or too thin films. Region II is the intermediate range of thickness, relative to the

periodicity, that is suitable for creating perfect patterns. This intermediate range

of film thicknesses broadens with an increase of the pattern dimension. To get

desired pattern arrays, for PMMA, for instance, a thicker film (> 200 nm) is

necessary for patterning of a large structure (periodicity larger than 10 µm) and

a thin film (< 50 nm) for a tiny structure (periodicity less than 1 µm). The

thickness of the film influences the morphology of the patterned film more

strongly than other parameters, such as the stamping time and the heating rate

for the melting of the film.

32

1 10 100

0

200

400

600

800

1000

III

II

Film

thic

knes

s, H

, (nm

)

Periodicity, λ, (µm)

I

Fig. 4.4. A plot of the thickness ranges of a film versus the periodicity

of a film pattern. ο and are the higher and lower thickness limits,

respectively, between which perfect patterns may be obtained.

d. Aspect ratio of the spherical cap of a patterned film upon annealing:

During the final stage of annealing, the patterned film has a cross section

of spherical cap shape as shown in Fig. 4.4. To explore the maximum height of

the spherical cap, the patterned polymer structures were measured using AFM.

The patterned polymer structure presented a smooth, clear cylindrical cap. The

polymer structures were physically separated. A plot of the maximum aspect

ratio between the height (hmax) and the width (λw) of the spherical cap is

presented in Fig. 4.5. It shows that the ratio is limited to 0.15 and occurs for

1 10 10.00

0.05

0.10

0.15

00

Fig. 4.5. A plot of the maximum

aspect ratio versus the width of

the obtained spherical cap. The

inset is an illustration of the

cross-section of a patterned film

on a surface.

Width of polymer stripe, λw, (µm)

h m

ax/λ

w

hmax

λw

33

structures with 10 µm periodicity. This reflects the interactions between the

materials of the modified substrate surface and the overlying polymer film.

e. Imaging of patterned polymers:

Fluorescent polymers such as polythiophenes, CDT-green and CDT-red,

can be dissolved in low vapor pressure (low evaporation rate) organic solvents,

and therefore can be patterned on to a PDMS modified PEDOT-PSS surface

directly from solution (Fig. 4.6).

500 µm 100µm 200µm

Fig. 4.6. Photographs of patterned polythiophene (a), CDT-green (b)

and CDT-red (c) polymer with different geometry spin-coated from

dichlorobenzene solution (10 mg/ml of and 1000 rpm), xylene (10

mg/ml and 100 rpm), respectively. The colored areas are patterned

polymers under light excitations. The dark areas are exposed

substrate surface (PEDOT-PSS).

When a polymer has a desirable glass transition point, such as

polythiophene (PDOPT Tg = 90°C,) and poly(methyl methacrylate) (PMMA Tg

=106°C) and can be dissolved in a high vapor pressure (high evaporation rate)

solvent, patterning single or bilayer polymer can be achieved from an initially

flat film upon annealing (see Fig. 4.7, Fig. 4.8 and Ref.56 for details).

34

(b) (a)

(c)

Fig. 4.7. AFM height image of structures formed upon annealing

from continuous films. (a) Patterned PDOPT film with periodicity of

1.66 µm and (b) patterned PMMA film with a periodicity of 450 nm

and (c) submicron diameter ordered spherical dots of PMMA. The

initial film thicknesses are 120 nm, 20 nm and 80 nm for (a), (b)

and (c), respectively.

35

(a) (b)

(c)

30 µm

(d)

Fig. 4.8. (a) AFM height image of a PMMA array on a PEDOT-PSS

surface. (b) AFM height image of a bilayer array, consisting of PMMA

on the bottom layer and PDOPT on the top layer. (c) AFM phase

image of the same bilayer PMMA/PDOPT array. (d) Fluorescence

microscope image of the bilayer PMMA/PDOPT array. PDOPT emits

red light under UV-light excitation. The square arrays have a side

length of 5 µm with a separation of ~ 5µm.

In summary, patterning of a polymer from a solution or its melt by surface

energy controlled dewetting on a bare PDMS modified surface has been

demonstrated. Physical properties of the solvent are key factors that affect the

methods for the patterning process. The profile of a patterned polymer directly

from a polymer solution can be controlled by the deposition conditions. If a

continuous polymer film is formed, patterning of the polymer can be achieved

by dewetting upon annealing. The morphology of patterned films is strongly

36

influenced by the initial film thickness. A desirable film pattern can be formed

at an intermediate range of the film thickness. The range enlarges with an

increase of pattern periodicity. The cross section of a pattern after the final

annealing stage has a spherical shape and is affected mainly by the interactions

between the polymer melts and the solid surface.

The surface energy controlled patterning technique supplies a convenient

and quick way for creating a modified surface and generating smooth,

completely physically separated polymer structures. The method that can

generate a conjugated polymer pattern on a conducting polymer surface is

unique. It is very promising in generating double and multilayer polymer arrays

[see Fig. 4.8 and Ref.56] and consequently fabricating conjugated polymer-

based opto-electronics. Properties such as photonic bandgap structure on a

surface, polarized or white light emitting diodes prepared with the method

might be achieved85.

37

5. Conjugated polymer-based optoelectronics

The conjugated polymer-based optoelectronics dealt with in this thesis

include polymer light emitting diodes (PLEDs) prepared from polymers doped

with phosphorescent metal compounds and polymer/fullerene bulk

heterojunction solar cells (PSCs). The conjugated polymers are the main

component of an active layer (~ 100 nm thick), sandwiched between two

electrodes. PLEDs generate light, namely electroluminescence, by injection of

charges into the polymer layer, where recombination of the charges takes place

(see Fig. 5.1). PSCs create an electric current via the process of absorbing light

into a polymer, and through the following steps: exciton dissociation, charge

transport and collection (see Fig. 5.4). PLEDs and PSCs are devices which

convert energy in opposite directions: PLEDs take electricity and convert it to

light, while PSCs do the reverse.

5.1 Polymer light emitting diodes (PLEDs)

The first PLED was reported in 19901. It attracted considerable interest

due to the advantages of low operating voltage, easy fabrication and wide-view

angle. The advantage of thin and flexible PLEDs is that they are made

completely from polymers. The main applications of PLEDs are as indicators,

displays and lightning sources where enhancement of the light output power

efficiency is of importance.

38

Fig. 5.1. Schematic diagram of working mechanism of a single-layer PLED. Procedure ①: Injection of charges to the active polymer layer

from two electrodes; Procedure ②: charge transport in the polymer

layer; Procedure ③ recombination of charge gives off light.

Fig. 5.1 shows the schematic working principle of a single-layer PLED.

Three steps are involved: injection of charges into the active polymer layer

from two electrodes; charge transport in the polymer layer and formation of an

exciton or excited state; and then recombination of the charges to give light.

The external quantum efficiency (EQE) is one of parameters used to indicate

the light output efficiency of PLEDs, which is defined as the ratio between the

number of injected electrons to the number of emitted photons.

In single-layer devices, it is hard to reach balanced charge injection and

therefore get high EQE, because the conjugated polymers are mostly hole

transporting86,87. Balanced charge injection can be achieved by preparing

multiple layer devices with proper geometry, where electrons and holes are

injected into the electron and hole-transporting layers, respectively, and

transported efficiently to the recombination sites, resulting in light88-90.

Polymer layer

Light

1

1

3

- -

Low work function electrode

High work function electrode

+++

2

2

+ -

39

Optimization of charge injection has also been done by modifying an electrode

to decrease the injection barrier, by using a metal with a low work function as

electrodes or inserting a thin alkali fluoride or alkali chloride, such as lithium

fluoride or sodium chloride between the active layer and the metal electrode91-

93.

5.2 Molecularly-doped PLEDs

One crucial aspect that limits the EQE of PLEDs is the formation of

excited states. From quantum mechanics, injection of charges into the active

polymer layer of PLEDs leads to both singlet and triplet excited states, with a

ratio of 1:315. In reality, however, the ratio of singlets to triplets may be related

to a material’s and device’s properties14,94. It is generally agreed, however, that

both the singlet and triplet excited states are formed near 1:394,95. Only

transition from singlet exction to the ground state is allowed, giving rise to

photons. The transition from the triplet to the ground state is forbidden,

following spin conservation of quantum mechanics’ rules. If nothing is done to

harvest the triplet excited states, they are lost in non-radiative relaxation,

guaranteeing a reduced light output efficiency.

Doping a heavy metal complex guest into a polymer host in a PLED may

overcome the above limitation96. In the ideal case (see Fig. 5.2), the energy

levels of the singlet and triplet states of the polymer host should be higher than

that of the metal complex, making the energy transfer from the polymer host to

the metal complex guest favorable. The heavy metal-induced spin-orbit

coupling breaks spin conservation rules and allows the transition from both

singlet and triplet excited states to the ground state to occur through radiative

emission, resulting in electrophosphorescence.

Intensive research on electrophosphorescence started by doping metal

complexes into small organic molecules to prepare LEDs97 in 1998. This field

then turned to polymer-based LEDs98,99 where polyvinylcarbazol99, its

derivatives100 and polyfluorene derivatives99,101 were used as a polymer host.

40

Increased EQE values up to 20% were demonstrated96. Furthermore, these

early works improved understanding of energy transfer mechanisms and

exciton formation at the guest and led to efficient white light LEDs100,102.

In the following electroluminescence, including electrofluorescence and

electrophosphorescence, energy transfer types, and the formation of excited

state at metal complex sites are described.

1. Fluorescence and electrofluorescence (EF)

The radiative transition from a singlet excited state (S1,S2 …) to the

ground state (G) is called fluorescence. If the singlet state is electrically

excited, the radiative decay from the singlet excited state to the ground state is

referred to as electrofluorescence.

2. Phosphorescence and electrophosphorescence (EP)

The radiative transition from a triplet excited state (T1, T2 and T3…) to the

ground state, when the spin conservation rule is broken, is called

phosphorescence. Electrically induced phosphorescence is referred to as

electrophosphorescence.

Fig. 5.2. Schematic diagram of electrofluorescence (EF) and

electrophosphorescence (EP) of radiative transition from the first

singlet (S1) and first triplet excited states (T1).

Sn

S2

Electrical excitation

T1

EF EP

T2

T3

S1

G

41

3. Förster energy transfer

Förster energy transfer refers to the non-radiative energy transfer from the

singlet (S1) of a polymer host to the singlet (S1*) of a metal complex guest,

S1→ S1*. It is a dipole-dipole interaction and requires spectral overlap between

the emission of the polymer host and the absorption of the metal complex

guest. The energy transfer efficiency is inversely proportional to the sixth

power of the distance between the host polymer and the complex guest side.

Typically the exciton diffusion length is less than 10 nm.

4. Dexter energy transfer

Dexter energy transfer describes the energy transfer form the triplet (T1) of

the polymer host to the triplet (T1*) of a metal complex site, T1→ T1*. It is an

electron-electron exchange interaction between a polymer and metal complex,

within which the total spin of the system is conserved. This process occurs

when there is a spectral overlap between the phosphorescence emission

spectrum of the host and the singlet ground state to first excited state

absorption spectrum of the guest. An even shorter separation distance between

the polymer host and the metal complex guest favors Dexter energy transfer

over Förster energy transfer.

Fig. 5.3 Illustration of Förster and Dexter Energy transfer from a polymer

host to a metal complex.

Polymer host

S1

S1*Tn

Tn*

Electrical excitation EF

EF EP

Förster energy transfer S1→ S1*

Dexter energy transfer Tn→ Tn*

Energy transfer

S0

Metal complex guest

42

5. Electroplex formation

A metal complex guest may also trap charges injected from an electrode.

These charges can combine with opposite charges at a neighboring molecule

and form an excited stated, namely an electroplex99,103. Radiative decay of the

excited state can also lead to electrophosphorescence.

6. Triplet-triplet annihilation

Triplet-triplet annihilation is a potential process existing in both the

polymer host and the metal complex, which results in singlet excited state

formation in the polymer or metal complex. The process in the metal complex

is affected by the concentration of the metal complex99,102.

Poly(thiophenes) form another conjugated polymer family, and have been

successfully used for fabricating PLEDs that emit light covering the range from

violet and visible to infrared. It is noted that the inter-system-crossing yield is

higher in general as compared to other polymers due to the heavy sulfur atom16.

It is very necessary to harvest the triplet excited state of polythiophe based

LEDs in order to enhance performance of this kind of PLED. Our study

selected poly(3-methyl-4-octylthiophene) (PMOT) as a polymer host and an

iridium or platinum complex as guest to fabricate the molecularly-doped LEDs.

Energy transfer from the polymer host to the two kinds of guest and the

formation of an exciton at the guest side were studied using photoluminescence

and electroluminescence. Our studies showed that energy transfer dominated as

the route for exciton formation in the iridium complex containing LEDs.

Exciton formation by charge-trapping was involved in the platinum containing

LEDs.

PMOT used as polymer host in our work did not have a sufficiently large

bandgap, which limited the performance of the molecularly doped LEDs. The

study, however, demonstrated a new direction in which polythiophene can be a

candidate as a host to realize electrophosphorescence in PLEDs. Large bandgap

polythiophenes are required to optimize the performance.

43

5.3 Polymer solar cells (PSCs)

Polymer solar cells (PSCs) form another conjugated polymer-based group

of optoelectronics, which profit well from the development of PLEDs because

of the use of similar materials and fabrication technologies. The same reasons

as for PLEDs, ease of preparation, low cost, low weight and flexibility of the

devices, make the polymer-based solar cells very attractive, although the power

conversion efficiency is not as high as that of silicon-based solar cells (10-20

%).

There are number of different types of conjugated polymer-based solar

cells: polymer/polymer blends104,105, polymer/polymer double layer106,

polymer/fullerene double layer107,108, polymers blended with soluble fullerene

derivatives (also called polymer/fullerene bulk heterojunction)109,110 and

polymer/inorganic hydride111,112 solar cells. Amongst PSCs, polymer/fullerene

bulk heterojunction solar cells are promising because they have considerably

high power conversion efficiency and easiest fabrication. The work in this

thesis deals with polymer/fullerene bulk heterojunction solar cells, unless

otherwise stated.

5.3.1 Basic working principle

The process of converting solar light into electric current in PSCs is

accomplished by the following steps and illustrated in Fig. 5.4.

(1) Light (solar photons) passes through a transparent electrode and is

absorbed by the active layer (mainly by the polymer), leading to the

formation of an excited state (an exciton), i.e. a bound electron-hole

pair, the electron is in the LUMO and the hole in the HOMO of the

polymer.

(2) The exciton diffuses to the region where exciton dissociation occurs,

for instance, to the interface of the electron donating polymer that has

44

high ionisation potential and the electron accepting material that has a

high electron affinity. The exciton then dissociates into free charges.

The electron and hole are transferred to the electron donating and

accepting materials, respectively. The driving force for charge

separation and transfer, in this case, is the difference in electron

affinity or in LUMO levels between two materials at the interface,

with the relationship of LUMOacceptor > LUMOdonor.

(3) Charges are transported under an electric field. These charges might

recombine when an electron and a hole are close enough during the

transport.

(4) Charges are collected by the two electrodes.

LUMOacceptor > LUMOdonor !!!

Polymer:fullerene Polymer:fullerene

Cathode

AnodeHOMO

LUMO4

2

3

Cathode

Anode

LUMO

1 4

3

Fig. 5.4. Schematic diagram of the working principle for

polymer/polymer or polymer/fullerene bulk heterojunction solar

cells. Procedure (1): Absorption of light by the polymer and

formation of exciton; Procedure (2): Exciton diffusion and charge

dissociation at the interface between the electron donating polymer

and the electron accepting material; (3): Charge transport and/or

recombination; (4): Collection of charges by the two electrodes.

5.3.2 Characterization of solar cells

45

HOMO

The following parameters are important in characterizing the performance

of SCs:

(1) Current density-voltage (J-V) characteristics:

● When a SC is in darkness, current is increased exponentially with

the application of a forward bias, and almost blocked when a

backward bias is applied. This is the rectification property of a diode

(see Fig. 5.5), often characterized by the rectification ratio, RR, given

by the ratio of current densities between two applied voltages with the

same magnitude but opposite sign.

● When a SC is illuminated, with zero bias (short circuit condition),

current flowing in the circuit is pure photo-induced current, namely

short circuit current. Short circuit current density, Jsc, is used to

characterize the performance of a SC. With increasing applied forward

bias, the electric field-induced current (injection current) competes

with the photo-induced current and at a certain applied voltage, the

two currents cancel each other. This voltage is known as the open-

circuit voltage, Voc. The bandgap between the LUMO of the electron-

accepting molecule and the HOMO of the electron-donating polymer

has a major effect on Voc113,114 while the work function of electrodes

has only a weak influence on Voc115. Due to the complex physical

processes, many factors affect Jsc, such as mobility of charge carriers,

the molecular weight of the active materials116, morphology of the

active layer117-119 and the electrodes used120.

● For a J-V curve of a diode that is illuminated by light, the ratio of

the maximum of the product of J and V to the products of Jsc and Voc,

is referred to as the fill factor, FF, and reflects the charge transport

properties in the diode.

ocsc VJ

VJFF⋅

⋅= max)(

46

J

V

Under Dark

Under illumination

JSC

VOC

FF

0

Fig. 5.5. Typical J-V characteristic of a solar cell under the dark and

the illumination.

(2) Power conversion efficiency (ηPCE) and incident photons to current

conversion efficiency (IPCE):

● Power conversion efficiency, ηPCE, is defined as the ratio of input

light power to the output electric power of the device. It can be

determined from the J-V characteristic, and written as

0LVJFF

PP ocsc

in

outPCE

⋅==η

● The incident photons to current conversion efficiency, IPCE is

defined as the ratio between the total number of collected electrons in

an electric circuit and the total number of photons incident to a SC.

The IPCE is also known as the external quantum efficiency (EQE). It

can be determined by measuring the photo-induced current of a SC

under illumination from an incident monochromic light under short

circuit conditions and is given by:

0

1240)()(L

JEQEIPCE sc

⋅==

λλλ

47

where Jsc is the photo-induced photocurrent density in units of µA/cm2

under short circuit conditions, λ and L0 are wavelength and power density of

the incident light, in units of nm and Watt/m2, respectively.

5.4 PSCs based on low-bandgap polymer

Polymers with a high absorption coefficient and broad spectra are

essential for harvesting solar photons. The latter properties increase the number

of converted electrons and the power conversion efficiency. However, most

conjugated polymers have a bandgap above 2 eV, and absorb only visible

wavelength light, which only accounts for about 40% of solar irradiation

energy (see Fig. 5.6). To use longer wavelength solar photons from solar

irradiation, low-bandgap polymers that extend the absorption spectra into the

red and infrared regions are necessary.

Fig. 5.6. Solar spectrum irradiation (solid line) and solar energy

irradiation distribution (dash line) as a function of wavelength. The AM

1.5 Spectrum from American Society for Testing and Material (ASTM)

Terrestrial Reference Spectra for Photovoltaic Performance

Evaluation121. The total power density of the irradiance calculated from

the spectrum is 90 mW/cm2. Details can be found in121.

500 1000 1500 2000 2500 3000 3500 40000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0

200

400

600

800

1000

Sol

ar e

nerg

y irr

adia

nce

(W/m

40% 80%

Sol

ar S

pect

ral i

rrad

ianc

e (W

/mm

)

Wavelength (nm)

99%

2 )

2 .n

48

A consequence of lowering the bandgap of a polymer is an energy lev

ift in one or both of LUMO and/or HOMO. This might induce energ

ismatch of LUMO levels between electron donating nd

el

sh etic

m a accepting materials

in a S

e)

de

his is

the c

seri of fullerene derivatives (see Fig. 5.7) was

selec

C hence weakening the driving force for exciton dissociation, resulting in

a reduction of photo-induced current generation, and making IPCE and ηPCE

small. This has been shown for some low-bandgap polymers blended with the

famous phenyl-C61-butyric acid methylester, PCBM, to fabricate solar cells122-

124, although PCBM has been demonstrated to have very good electron

accepting properties when blended with some visible-light absorbing polymers,

for example, polythiophene derivatives21,109, poly(p-phenylene vinylen

rivatives110,119, polyfluorene copolymers125,126. Electron-accepting materials

whose LUMO levels match that of a low-bandgap polymer are required.

On the other hand, a loss of optical absorption from a low bandgap

polymer subjected to visible light maybe another consequence when the

absorption is extended to both visible and infra-red wavelength regions. T

ase for some alternating polyfluorene copolymers125-127. Properly selecting

electron accepting molecules may also compensate for the loss of absorption by

electron donating polymers128,129.

In this thesis work, we used a series of newly synthesized, novel, low-

bandgap polyfluorene copolymers (see Fig. 5.7) as electron donating polymers

to fabricate solar cells. A es

ted and used as electron accepting materials in SCs, in order to match the

energetic LUMO levels between the polymers and the fullerenes.

49

APFO-Green1

S S

NSN

NN

n

S S

NSN

NN

O

O

APFO-Green4

n

S S

NSN

NN

n

APFO-Green3

NN

NO2

F3C

F3C

Fig. 5.7. Chemical structures of a serie f polyfluorene copolymers:

APFO-Green1, APFO-Green3 and APFO-Green4; and different fullerene

derivatives: phenyl-C61-butyric acid methylester, PCBM, 3'-(3,5-Bis-

and obtained with SCs where the low-bandgap polymers

lended with the C70 derivative were used as the active layer. In particular an

EQE of ~25% at λ = 400 nm, 9.4% at λ = 800 nm and with the onset of current

BTPF60

NN NO 2

F3C

F3C

BTPF70

O O

PCBM

s o

trifluoromethylphenyl)-1'-(4-nitrophenyl)pyrazolino[60]fullerene, BTPF60

and 3'-(3,5-Bis-trifluoromethylphenyl)-1'-(4-nitrophenyl)pyrazolino[70]-

fullerene, BTPF70.

The photocurrent responses, EQE, (see Fig. 5.8) in both the near infrared

visible regions were

b

50

gene

Fig. 5.8. (a) Absorption spectra of the low-bandgap polyfluorene

(solid square), APFO-

ration at l = 1µm was demonstrated in one of the SCs in the series,

exhibiting one of the best results in the field. The Jsc, Voc and ηPCE of 3.4

mA/cm2, 0.54 V and 0.7% were obtained for this SC.

copolymer APFO-Green1 (solid square), APFO-Green3 (solid square),

APFO-Green4 (open triangle) and the fullurene derivative, BTPF70 (solid

line) and (b) EQEs of APFO-Green1:BTPF70

Green3:BTPF70 (open circle), APFO-Green4:BTPF70 (open triangle)

based devices. The blend ratio between these low-bandgap polymers and

BTPF70 is 1:4.

300 400 500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

1.0

BTPF70 APFO-G1

APFO-G3

APFO-G4

Abs

orpt

ion

(Nor

mal

ized

)

Wavelength (nm)

a

300 400 500 600 700 800 900 10000

5

10

15

20

25

30

APFO-G3

APFO-G4

APFO-G1

EQ

E (%

)

Wavelength (nm)

51

These outstanding properties were attributed to the following:

a. the low-bandgap polyfluorene copolymers whose absorption

spectra cover both visible and infrared regions;

0.8

1.0

400 500 600 700 800 900 1000 11000.0

0.2

0.4

0.6

Abs

orpt

ion

(Nor

Wavelength (nm)

ig. 5.9TPF60 (open triangle) and PCBM (solid circle) and (b) EQE of

ese fullerene blends with APFO-Green1 based devices, where

e fullerenes were BTPF70 (open square), BTPF60 (open triangle)

300 400 500 600 700 800 900 10000

5

10

15

20

25

30

mal

ized

)

EQ

E (%

)

Wavelength (nm)

b

F . Comparison of (a) absorptions of BTPF70 (open square),

B

th

th

and PCBM (solid circle) and the blend ratio between APFO-Green1

to individual fullerene was 1:4, respectively.

52

b. the energetic match in LUMO levels between the polymers

the C70 derivative, which supplies the driving force for ch

transfer;

and

arge

c. the existence of intermolecular charge transfer130-132 between the

d.

sponse (see Fig. 5.9) over a certain part of the visible

ption

quenching an

polymers and

C70 molecules that contribute to the photocurrent;

the contribution from the absorption of the C70 derivative to the

current re

region, which compensates for the loss of absor of polymers

in the region.

These were suggested by studies of optical absorption, photoluminescence

d electrochemical determination of LUMO levels of the series of

different fullerene derivatives133-135.

53

6. Future outlook

.1 Increasing aspect ratio of the polymer pattern using thermal reflow

As demonstrated in our paper56, we are able to pattern single and double-

ubstrate by surface energy controlled dewetting,

specially conjugated polymers on a conducting surface, which is very useful

ganic opt

limited, w

e simple

and very promising for fabricating biochips for the detection of

olecul

using surface energy controlled

dewetting have been shown to be able

hich emit polarized light when optically or electrically

excited.

ace energy changes of PEDOT in the forms of VPP-PEDOT and

PEDOT:PSS) during electrochemical redox have been explored32.

reason for that m

backbone of the polymer and swelling of polymer, but might also be due to the

6

layer polymer arrays onto a s

e

for or oelectronics applications. However the aspect ratio of patterns is

hich makes it hard to meet the requirements for optoelectronics’

applications. A pre-patterned double-layer array that consists of two polymer

layers with different glass transition points and proper thickness, might be used

to prepare a high aspect ratio array when reflowing136 upon annealing.

6.2 Integrating of surface energy controlled patterning techniques into

biochips or optoelectronics fabrication

Using a bare stamp to modify the surface en rgy of a surface is

biom es137.

Submicro-scaled polymer stripes prepared

to act as a template and align added

polymer molecules orientation, which will be helpful for fabricating conjugated

polymer layers w

6.3 Surface chemical analysis on PEDOT before and after electrochemical

doping

Surf

Orgacon (

The ay not be only due to the polarization development on the

54

surfa

roup in Louisiana Tech University has shown that a

uantum efficiency of more than 1 was obtained in a polymer–nanocrystal

tu

gene refore enhancing the charge carrier

gene

ce composition and molecular orientation. Therefore it is useful to analyse

the surface chemicals by some surface analysis techniques, for instance

photoelectron spectroscopy, IRAS or Raman, in order to fully understand the

mechanism of surface energy changes during electrochemical redox in an

aqueous electrolyte.

6.4 Carrier multiplication via multiple exction generation in polymer-

nanocrystal quantum dot solar cells

One research g

q

quan m dot photodetectors138. This was attributed to the multiple exction

ration in the quantum dot and the

ration. This concept is quite interesting and should be used in application

of polymer- nanocrystal solar cells because of the similarity of photovoltaic for

the both type devices. It is important to understand the mechanism of

multiplication of exciton (one photon to more than one excitons), and to know

how to efficiently carried out charge separation, transport and …, in order to

construct highly efficient polymer-nanocrystal quantum dot solar cells.

55

7. Summary of papers .1 Surface energy modification and patterning of conjugated polymers

Paper I. PEDOT surface energy pattern controls fluorescent polymer

ethod of patterning surface energy on surfaces

ing a bare elastomeric stamp and furthermore patterning a conjugated

ectly from an organic solution with low vapor pressure and

tu Wilhelmy balance surface energy determination of

poly

doping. Electrochemical doping of conjugated

ymer displays another way of modifying a surface. The dynamic surface

7

deposition by dewetting. A m

us

polymer directly from an organic solution on a conducting polymer surface by

surface energy controlled dewetting was invented. The feature sizes of the

patterned polymers are in the 10 - 100 µm range. Surface energy modification

is due to the transfer of “inks” on the stamp as revealed by infrared reflection

absorption spectroscopy. The dynamics of the modification process were

characterized by contact angle measurement.

Paper II. Single and bilayer submicron arrays of fluorescent polymer on a

conducting polymer surface with surface energy controlled dewetting. For

the work in the previous paper we were able to pattern a polymer by spin-

coating dir

evaporation rate. When an organic solution with a high vapor pressure and

evaporation rate, on the other hand, a flat film is formed. In this paper we

demonstrated a method of patterning single or double-layer polymers on a

PDMS-modified surface upon annealing of the polymer film above its glass

transition point.

The influence of pattern geometry and film thickness on the patterned film

morphology were studied.

Paper III. In-si

(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene) during

electrochemical doping-de

pol

56

energy changes with doping levels were investigated in-situ with a Wilhelmy

balance. Capillary rise has also exhibited the possibility for monitoring in-situ

of surface energy changes.

7.2 Opto-electronics devices based on conjugated polymers

Paper IV. Electrophosphorescence from substituted Poly(thiophene)

doped with iridium or platinum complex. Converting nonradiative triplet

xcitons to radiative excitons in PLEDs is the key issue to overcoming the low

fficiency can be

bulk heterojunction

s is one strategy for harvesting more solar photons with red and infrared

e

external quantum efficiency of PLEDs. An increased e

achieved by doping phosphorescent heavy metal complex into a polymer

matrix, because spin-orbital coupling, induced by the heavy metal, breaks the

transition rule and allows the triplet exciton to ground state transition occur

radiatively, hence enhancing the efficiency. Two metal complexes (Ir and Pt)

were doped into a polythiophene based PLED. The energy transfer was

investigated with electroluminescence and photoluminescence methods,

revealing that complete energy transfer from the polymer matrix to the metal

complex in the Ir-containing PLEDs was the main source of the

electrolumiscence. In the Pt-containing PLEDs on the other hand, charge-

trapping effects also contribute to the electroluminscence.

Paper V. Infrared photocurrent spectral response from a plastic solar cell

with low-bandgap polyfluorene and fullerene derivative. Using a low band-

gap, conjugated polymer blend with a fullerene to fabricate

SC

wavelength from solar energy radiation and then to convert the photons into

current. Lowering the band-gap results in an energy level shift in one or both of

LUMO and/or HOMO levels. This may lead to energetic mismatch in LUMO

levels between the polymer and the fullerene and weaken the driving force for

charge transfer and thereby make the SCs inefficient. A novel low-band gap

polyflourene copolymer (APFO-Green1) was synthesized and blended with

57

two different C60-derivates to fabricate SCs. The resulting diodes demonstrated

that efficient red and infrared photo-induced current responses can be obtained

in diodes where the fullerene LUMO matches that of the polymer. An external

quantum efficiency of 8.4% at 840 nm and 7% at 900 nm and the onset of

photocurrent generation at 1µm were achieved, which demonstrated one of the

best results in the field.

Paper VI. Enhanced photocurrent spectral response in low-bandgap

polyfluorene and C70-derivative based solar cells. Another consequence of

lowering the band-gap of a polymer is the loss of absorption in the visible

avelength, as the absorption of the polymer has been extended to both the

ptor moieties has

een synthesized. The bandgap values of the polymers of 1.2-1.5 eV, were

w

visible and infrared wavelength regions. Determination of energy bandgap of

the polymer and a C70-derivative by optical absorption and electrochemistry

implied that the absorption of the C70-derivative could compensate for the loss

of absorption by the polymer. The LUMO of the C70-derivative matches the

low bandgap polymer (APFO-Green 1). SCs prepared using the APFO-green 1

as electron donor and the C70-derivative as electron acceptor resulted in an

enhanced photocurrent spectral response as compared to that presented in Paper

V. The external quantum efficiency of 9.4% at 800 nm and 7% at 900 nm, with

the onset of photocurrent generation at 1µm were obtained. The power

conversion efficiency of 0.7%, is the best result in the field.

Paper VII. Polymer solar cells with low-bandgap polymers blended with

C70-derivative give photocurrent at 1 µm. A new series of low-bandgap

polyfluorene copolymers with different acceptor-donor–acce

b

determined using optical absorption and electrochemistry. The LUMO and

HOMO varied with the strength of donor-acceptor-donor moieties. These

polymers, blended with the C70-derivative used in Paper VI, were used to

prepare SCs, which showed promising photovoltaic properties. The spectral

58

response of the photocurrent covers visible and near infrared wavelength

regions with the onset of photocurrent generation extended to 1µm. Meanwhile

the correlations of open-circuit voltage of the diodes on the LUMO and HOMO

values of the fullurene and the polymers were discussed.

59

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