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
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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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