Electrochemical Switching in Conducting...
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Electrochemical Switching in Conducting Polymers – Printing Paper Electronics
Payman Tehrani
Norrköping 2008
Electrochemical switching in conducting polymers – printing paper electronics
Payman Tehrani
Linköping Studies in Science and Technology. Dissertations, no.1212
Copyright © 2008 Payman Tehrani, unless otherwise noted.
Printed by LiU‐Tryck, Linköping, Sweden 2008
ISBN: 978‐91‐7393‐801‐3
ISSN: 0345‐7524
Abstract
During the last 30 years a new research and technology field of organic electronic
materials has grown thanks to a groundbreaking discovery made during the late 70’s.
This new field is today a worldwide research effort focusing on exploring a new class
of materials that also enable many new areas of electronics applications. The reason
behind the success of organic electronics is the flexibility to develop materials with
new functionalities via clever chemical design and the possibility to use low‐cost
production techniques to manufacture devices.
This thesis reports different aspects of electrochemical applications of organic
electronics. We have shown that the color contrast in reflective and transmissive
electrochromic displays can be almost doubled by adding an extra electrochromic
polymer. The choice of electrochromic material was found to be limited by its
electrochemical over‐oxidation (ECO) properties, which is one of the main
degradation mechanisms found in displays. The irreversible and non‐conducting
nature of over‐oxidized films encouraged us to use it in a novel patterning process in
which polythiophene films can be patterned through local and controlled
deactivation of the conductivity. ECO can be combined with various patterning tools
such as screen printing for low‐cost roll‐to‐roll manufacturing or photolithography,
which enables patterning of small features. Studies have shown that electronic
conductivity contrasts beyond 107 can be achieved, which is enough for various
simple electronic systems. To generate better understanding of the ECO
phenomenon, the effect of pH on the over‐oxidation characteristics was studied. The
results suggest that a part of the mechanism for over‐oxidation depends on the OH–
concentration of the electrolyte used. Over‐oxidation has also been used in
electrochemical loggers, where the temperature and time dependence of the
propagation of an over‐oxidation front is used to monitor and record the
temperature of a package.
Populärvetenskaplig sammanfattning
Dagligen kommer vi i kontakt med olika plastmaterial. Dessa har vanligtvis mycket
dålig elektrisk ledningsförmåga och används oftast som isolerande material. Det finns
dock en klass av plaster som är halvledande eller ledande. Sedan upptäckten av dessa
material för mer än 30 år sedan har nya material och användningsområden utvecklats
och nu börjar de första produkterna baserad på organisk elektronik komma ut på
marknaden. En stor fördel med de ledande plasterna är att egenskaperna kan
anpassas genom att ändra den kemiska strukturen. Man kan dessutom lösa upp dem
och skapa ledande bläck, som sedan kan användas i vanliga tryckmaskiner. Detta gör
det möjligt att på ett enkelt och billigt sätt tillverka elektronik på liknande sätt som till
exempel tidningar trycks idag.
Den här avhandlingen behandlar en del av det nya området som berör
elektrokemiska komponenter och några av dess tillämpningar. Fokus ligger främst på
billig, tryckt elektronik. Bland annat presenteras ett sätt att fördubbla kontrasten för
tryckta pappersdisplayer, ett nytt sätt att mönstra ledande plaster och elektrokemisk
temperaturloggningsetikett som kan övervaka temperaturen för förpackningar under
transport. Den mekanism som förstör ledningsförmågan vid höga spänningar har varit
ett återkommande inslag i de studier som har genomförts här. Denna mekanism
förstör komponenterna under drift men kan också användas för att ta bort
ledningsförmågan som mönstringsmetod eller för att lagra information, permanent, i
temperaturloggningsetiketten.
Acknowledgements
I would like to express my sincere gratitude to the people who have helped me reach
this point, especially …
… my supervisor Magnus Berggren, for all the help and support during these years
and for creating this very inspiring working environment.
… Thomas Kugler, who gave me the opportunity to start working in this research
field.
… Nate, Xavier and Isak, for ALL the valuable and interesting scientific discussions.
… Sophie for knowing everything and helping with all the administrative duties.
… David, Peter, Joakim and the others that have worked with me in the lab, for being
patient with my very impatient experimental technique.
… the people that I have worked with, especially the co‐authors of the included
papers, who have contributed with their expertise to bring science one tiny step
forward.
A really big thank to all the present and past members of the Organic Electronics
group at ITN (Daniel, David, Edwin, Elias, Elin, Emilien, Fredrik, Georgios, Hiam, Isak,
Jessica, Jiang, Joakim, Klas, Kristin, Lars, Linda, Magnus, Maria, Max, Nate, Olga,
Oscar, PeO, Peter, Sophie, Thomas, Xavier, Xiangjun and Yu) for creating an amazing
working environment (and relaxing coffee breaks).
I want to thank my family and friends for believing in me and giving me all the
support that has brought me here. And of course thanks for all the good times
outside of work.
I also want to thank the two most important persons in my life: My loving wife
Yashma for supporting me in my work. Without you this would have been more
difficult. Last but not least I want to thank my little Kayvan that takes a lot of my time
but gives back so much more.
List of included papers Paper 1: Patterning polythiophene films using electrochemical over‐oxidation
Payman Tehrani, Nathaniel D. Robinson, Thomas Kugler, Tommi Remonen, Lars‐Olov Hennerdal, Jessica Häll, Anna Malmström, Luc Leenders and Magnus Berggren
Smart Materials and Structures 14 (2005) N21–N25
Contribution: All the experimental work. Wrote the first draft and was involved in the final editing of the paper.
Paper 2: The effect of pH on the electrochemical over‐oxidation of PEDOT:PSS films
Payman Tehrani, Anna Kanciurzewska, Xavier Crispin, Mats Fahlman, Nathaniel D. Robinson and Magnus Berggren
Solid State Ionics 177 (2007) 3521–3527
Contribution: Most of the electrochemical measurements. Wrote the first draft and was involved in the final editing of the paper.
Paper 3: Evaluation of active materials designed for use in printable electrochromic
polymer displays
Payman Tehrani, Joakim Isaksson, Wendimagegn Mammo, Mats R. Andersson, Nathaniel D. Robinson and Magnus Berggren
Thin Solid Films 515 (2006) 2485–2492
Contribution: About half of the experimental work (not including the synthesis of the polymers). Wrote the first draft and was involved in the final editing of the paper.
Paper 4: Improving the contrast of all‐printed electrochromic paper‐displays
Payman Tehrani, Lars‐Olov Hennerdal, John R. Reynolds and Magnus Berggren
Manuscript
Contribution: All the experimental work except the synthesis of the polymer and the roll‐to‐roll patterning of PEDOT:PSS. Wrote the first draft and was involved in the final editing of the paper.
Paper 5: Printable organic temperature logger based on over‐oxidation front
propagation in PEDOT:PSS
Payman Tehrani, Isak Engquist, Nathaniel D. Robinson, David Nilsson, Mats Robertsson and Magnus Berggren
Manuscript
Contribution: All the experimental work except the DSC measurement. Wrote the first draft and was involved in the final editing of the paper.
Related work not included in this thesis:
Printable All‐Organic Electrochromic Active‐Matrix Displays
Peter Andersson, Robert Forchheimer, Payman Tehrani, and Magnus Berggren
Advanced Functional Materials 17 (2007) 3074–3082
Device, kit and method for monitoring a parameter history
US2008013595, EP1862786
Patterning PEDOT:PSS layer by controlled electrochemical reaction
EP1916670
A device for integrating and indicating a parameter over time
Patent pending
Table of contents
1 INTRODUCTION 1
2 CONDUCTING POLYMERS 3
2.1 π-CONJUGATED POLYMERS 3
2.2 EXAMPLES OF CONJUGATED POLYMERS 7
2.3 CHARGE CARRIERS 8
2.4 CHARGE TRANSPORT 9
2.5 OPTICAL PROPERTIES 11
3 ELECTROCHEMISTRY OF CONJUGATED POLYMERS 13
3.1 ELECTROCHEMICAL CELL 13
3.2 ELECTRODES OF CONJUGATED POLYMERS 14
3.3 ELECTROCHEMICAL MEASUREMENTS 17
3.4 ELECTROLYTES 18
3.4.1 ION TRANSPORT 19
3.4.2 IONIC CONDUCTIVITY 19
3.5 OVER-OXIDATION 21
4 APPLICATION OF CONJUGATED POLYMERS 23
4.1 ELECTROCHEMICAL OVER-OXIDATION PATTERNING 24
4.2 ELECTROCHROMIC DISPLAY 26
4.2.1 CIE LAB COORDINATES 29
4.3 ELECTROCHEMICAL TIMER AND LOGGER 32
REFERENCES 35
PAPER 1-5 38
Today, we are surrounded by electronics and they greatly affect our daily life. It all
started with the transistor[1] that was demonstrated by the Bell Laboratories in 1947
(Figure 1). During the last 60 years the technology has been developed into a vast
array of products that we today take for granted. The field of organic electronics was
started by the discovery of conducting polymers[2] in 1977. During the last 30 years
new materials and many different applications have been demonstrated. The
question is how organic electronics will affect our lives in 30 years? The properties of
this class of materials promise novel and exciting applications beyond the potential of
traditional, silicon‐based, electronics. Organic conducting polymers allow …
…simple and fast manufacturing using printing techniques
…use of flexible and lightweight carrier substrates such as paper or plastic films
…novel applications and devices
…unique ways of adapting the device function by tuning the material properties
though chemical engineering, which enables endless opportunities.
Figure 1. The first silicon based transistor demonstrated in Bell Laboratories.
1
1 Introduction
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
Thanks to these benefits, new products have been developed and are now emerging
on the market. The most mature technology is organic light emitting diodes (OLED)
where several companies have released products for the consumer market. The first
product that integrated an OLED display was the Philips shaver (Figure 2a) announced
in 2002. Recently Sony has started selling their first generation of OLED displays as an
11” TV with extraordinary high contrast, superb color accuracy and close to 180°
viewing angle (Figure 2b). These products are only examples of what can be achieved
with this new class of materials and many other technologies are approaching the
market soon.
Figure 2. Two examples of products using organic based displays. (a) A Philips shaver from
2002. (b) The Sony OLED TV XEL‐1, only 3 mm thick was released in the market in 2007.
Copyright © Philips and Sony Corp.
Above, the background and the motivation for using conducting polymers in
electronics were briefly reviewed. In chapter 2 the material and the basic principles
for charge conduction in conjugated polymers are presented. Chapter 3 describes
electrochemistry and its application to conjugated polymers. Chapter 4 gives an
overview of the field of electrochemical organic electronics devices and a more in
depth description of the work presented in this thesis.
2
Traditionally, polymers have been considered as poor electronic conductors and have
extensively been used as insulators in electronics and electric power applications. In
1977, A. Heeger, A. MacDiarmid and H. Shirakawa demonstrated that chemical
doping of trans‐polyacetylene films, a π‐conjugated polymer, can enhance its
electrical conductivity by many orders of magnitude to almost reach the typical
conductivity values of metals[2]. Their finding has resulted in a new class of materials
offering novel applications for electronics such as flexible solar cells, light emitting
devices, printed field effect transistors and electrochemical components made on
paper. In 2000, these gentlemen were awarded the Nobel Prize in chemistry[3] for
their discovery. The wide range of conductivity accessible in polymers originates from
the diverse nature of the carbon atom in different chemical structures. The
conductivity of a material is determined by the number of charge carriers and their
inherent mobility. These parameters are governed by the chemical structure and the
morphology of the polymer film. In this chapter, the relationship between the
chemical structure and the conductivity is discussed, especially the charge carriers
and the charge transport processes in the polymers.
2.1 π-conjugated polymers
Although all organic materials are based on the same building block, the carbon
atom, materials with organic compounds exhibit large variety of properties. Let us
compare diamond and graphite, for instance. Both consist only of carbon atoms, but
they have completely different electrical, optical and mechanical properties:
Diamond is non‐conducting, transparent and considered as one of the hardest
material known to mankind, while graphite is conducting, black and brittle. The
difference can be explained by the different hybridization of the carbon atoms, an
3
2 Conducting polymers
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
atom which has four valence electrons in one 2s‐ and three 2p‐orbitals. In diamond,
the carbon atom is sp3‐hybridized, meaning that the electrons occupy four hybrid
orbitals made of a mixture of one 2s‐ and three 2p‐orbitals. Because of the symmetry
of the four equal hybridized orbitals, the tetrahedral structure that is characteristic
for all sp3‐hybridized materials is created (Figure 3a). The electrons in the sp3‐orbitals
have a small extension in space and form very strong bonds (σ‐bonds) with
neighboring atoms. Therefore materials with sp3‐hybridization are stable and
electrically insulating as there are no free electrons in the system.
Figure 3. The four valence electrons in the carbon form symmetrical σ‐bonds to four other
atoms in case of sp3‐hybridized material (a). In case of sp2‐hybridized materials three
symmetrical are formed with the last valence electron left in the 2p orbital orthogonal to the
plane of the σ‐bonds (b).
In sp2‐hybridized materials, such as graphite, three of the four valence electrons are
in sp2‐orbitals, a mixture of one 2s‐ and two 2p orbitals, while the last electron is in
the remaining 2p‐orbital. The sp2‐electrons form strong bonds (σ‐bonds) with
neighboring atoms and create the backbone of the material, while the 2p‐electron
forms π‐bonds with neighboring 2p‐electrons creating a double bond (together with
the σ‐bond). Because there are three symmetrical sp2‐orbitals, the sp2‐hybridized
materials have a planar‐triangular symmetry (Figure 3b). The 2p‐orbitals are
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CHAPTER 2 – CONDUCTING POLYMERS
orthogonal to the plane of the sp2‐orbitals. In contrast to the sp2‐orbitals they are
delocalized and can reach far beyond the nearest neighboring atom. The sp2‐
hybridization in graphite results in a layered structure that gives softer mechanical
properties as compared to the rigid diamond structure. In the following sections the
specific electrical and optical properties of sp2‐hybridized material are discussed.
Figure 4. As the number of carbon atoms increases, the overlap between the π‐orbitals cause
states to split, eventually giving rise to continuous bands as the polymer extends into a long
chain.
In a molecule with two sp2‐hybridized carbon atoms, the interaction between the 2p‐
electrons results in two molecular orbitals, one being of bonding nature with lower
energy level (π) and one being antibonding (π*) located at a higher energy level
(Figure 4). As the number of carbon atoms increases in a molecule, the number of
molecular orbitals increases. In large molecules with N carbon atoms, the N 2p‐
electrons yield N molecular orbitals all with different energy levels. Because N
electrons can occupy N/2 states, half of the orbitals are filled while the rest are
empty. For electronic and optical applications, the energy difference between the
highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular
level (LUMO) is important, since it predicts the fundamental properties of the
material. As the number of carbon atoms, N, increases, the energy difference
between the levels decreases until they merge in continuous bands (Figure 4).
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ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
Now consider an infinite chain of carbon atoms which are sp2‐hybridized. That would
give an infinite amount of atomic orbitals that are brought together and overlapping.
Because of the energy splitting, the molecular orbitals have to be packed densely
with infinitesimal energy difference between them, creating bands of states in the
energy diagram. This would result in a one dimensional metal without a band gap
(the energy difference between the HOMO and LUMO). But in practice this is not a
stable state and a band gap is formed between the bonding and the antibonding
states. This can be explained by Peierls theorem[4]: He stated that a one‐dimensional
metal is unstable with respect to geometrical modification that leads to lowering of
the symmetry and removal of the degeneracy of the HOMO and LUMO levels, thus
obtaining a semi‐conducting state. The Peierls theorem applied on trans‐
polyacetylene (Figure 6a) yields as a result that the metallic state with delocalized
electrons in the polymer chain (Figure 5a) is unstable. The polymer is stabilized
through a geometrical distortion that results in a dimerization of the unit cell with
alternating single and double bonds (Figure 5b). In this state the energy levels of the
HOMO and LUMO are separated creating a band gap that is characteristic for semi‐
conductors. Therefore all conjugated polymer are semi‐conductors in their undoped
state.
Figure 5. The structure of trans‐polyacetylene (a) results in a degenerated HOMO and LUMO
state, which gives the polymer metallic properties. But according to Peierls theorem this state is
unstable and is instead transformed into the more stable form with alternated double and
single bonds in (b) and a band gap between the HOMO and the LUMO state.
6
CHAPTER 2 – CONDUCTING POLYMERS
2.2 Examples of conjugated polymers
SS
SS
S
OO
OO
OO
OO
OO
N N NH
NH
a
b
c
e
f
d
SS
SS
S
NH
NH
NH
NH
NH
Figure 6. Examples of some conjugated polymers (a) trans‐polyacetylene (b) polythiophene (c)
poly(para‐phenylene vinylene) (d) polypyrrole (e) polyaniline (f) poly(3,4‐
ethylenedioxythiophene)
The simplest conjugated polymer, from a structure point of view, is trans‐
polyacetylene (Figure 6a), being a straight conjugated chain of carbon atoms, each
atom bonded to one hydrogen atom. Although trans‐polyacetylene is easy to model
in theoretical studies, the films are very sensitive for exposure to air and water,
making it a poor candidate in applications. Through the years, different classes of
more stable conjugated polymers have been developed (Figure 6). One of the
characteristics that make polymers interesting is that their fundamental electronic
and optical properties can be tuned by changing their chemical structure. One
polymer that is widely used because of its stability and high conductivity is poly(3,4‐
ethylenedioxythiophene), PEDOT, a polythiophene derivative with a low band gap
(Figure 6f). PEDOT itself is insoluble, but when chemically polymerized with poly(4‐
styrenesulfonate), PSS, as counter ion, a water emulsion can be obtained (Figure 7).
In the PEDOT:PSS couple, the PEDOT part is positively doped making the polymer
highly conducting (at a conductivity sometimes exceeding 10 S/cm)[5].
7
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
S
OO
S
OO
S
OO
S
OO
S
OO
S
OO
S
OO
SO3Na SO3 SO3Na SO3Na SO3Na SO3 SO3Na
+ +
Figure 7. The positively doped PEDOT having PSS as the counter ion.
2.3 Charge carriers
The π‐electrons in a neutral conjugated polymer chain are bound in the π‐orbitals
giving rise to an alternation of single and double bonds. In this state the conjugated
polymer has typical semiconducting properties. To conduct electricity, charges have
to be introduced into the polymer (removing or adding electrons). The charge in the
polymer chain is stabilized by altering the conjugation over several monomers (Figure
8). The charge together with the distortion of the structure of the polymer is denoted
a polaron and can be either negative or positive. When two polarons form a couple, a
so called bipolaron is generated with a charge of +2 (–2).
SS
SS
SSS
SS
SS
SSS
SS
SS
SSS +
+
e-
SS
SS
SSS
SS
SS
SSS
SS
SS
SSS
e-
Figure 8. The doping of the polymer and creation of a positive (left) and a negative (right)
polaron.
The number of free charge carriers can be increased through doping, for instance via
charge injection from electrodes or via chemical doping. Chemical doping involves
8
CHAPTER 2 – CONDUCTING POLYMERS
charging the polymer film through chemical oxidation or reduction by dopant species.
The dopant, with opposite charge, stays in the film and balances the charge of the
polymer. In many electrochemical systems, the polymer film can be doped and
dedoped reversibly via oxidation and reduction.
Figure 9. Positive polaron and bipolaron in a conjugated polymer. Overlap between charge
carriers at high doping levels results in the formation of bands.
Upon doping, the charge carriers alter the structure of the polymer by increasing the
length of the double bonds and shortening the single bonds, thus giving a more
quinoidic character. This results in a decreased energy splitting between the HOMO
and the LUMO levels, moving those states towards each other within the band gap
(Figure 9). As the doping level increases, the number of states within the band gap
increases. At high doping levels, the states within the band gap start to overlap and
create bands of bipolaronic states (Figure 9).
2.4 Charge transport
In the solid state, polymer films are usually disordered with chains forming random
coils. The π‐system of the conjugated polymers makes the polymer straight and rigid
but chemical defects or torsion break the conjugation along the chain. In the
conjugated sections of the polymer (I in Figure 10), the charge is transported through
rearrangement of the bonds which is not as fast as in inorganic crystalline
9
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
semiconductors. The rearrangement is limited by the movement of nuclei that are
about 100 times slower than the electron rearrangement. But still, the limiting step
for charge transport is the hopping of charges in between chains and around defects
present in the bulk (II in Figure 10).
Figure 10. Transport of a polaron in a lightly doped film can be divided in intra‐chain transport
(I) and inter‐chain hopping (II)
In heavily doped polymer films, the polarons interact to form bands inside the band
gap promoting transport of the charge carriers in the bulk. Inhomogeneity in doping
level and morphology creates regions with poor conductivity between the highly
conducting regions. For example PEDOT:PSS films contain highly doped “metallic”
grains surrounded by regions with low concentrations of PEDOT chains diluted in PSS
(Figure 11). Even though the electronic conductivity is high in the grains, the charge
transport is limited by the hopping from grain to grain.
Figure 11. Charge transport in highly doped grains with a low concentration of conjugated
polymer chains in between the grains. Inside the grains, charges are delocalized and migrate
easily (I), while charges needs to hop between the grains passing over the low‐conducting
phases (II).
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CHAPTER 2 – CONDUCTING POLYMERS
2.5 Optical properties
The electronic structure of a material governs its optical properties as a result of the
interaction between electrons and photons. In order to absorb a photon, an electron
has to be excited to a higher available energy level corresponding to an increase of its
energy state equal to the photon energy (Figure 12a). The light emission process is
similar but reversed; a photon is emitted as an electron at a high energy level state is
de‐excited to an available state with lower energy (Figure 12b). Many conjugated
polymers have a band gap that matches the range of visible light (1.7 – 3.2 eV). This
means that conjugated polymers may exhibit absorption and light emission in the
visible region. In an undoped polymer, the band gap equals the lowest energy that
can be absorbed. Upon doping, the formation of polarons and bipolarons changes the
electronic structure and introduces new states within the band gap. These new
electronic states allow absorption at lower photon energies, which drastically
changes the absorption spectrum. As the doping level increases, the original
absorption peak diminishes at the expense of the new polaronic and bipolaronic
peaks. An example of this is given in the spectroelectrochemical measurement of
Figure 13. Here the original peak at 2.3 eV, corresponding to transition between
HOMO and LUMO, decreases as the doping level increases. Instead a new peak
appears at 1.3 eV corresponding to formation of polarons in the film. When the
doping is increased even further, the original peak at 2.3 eV disappears completely
while the main absorption occurs at further lower energies within the infrared region,
which indicates high concentration of bipolarons.
a absorption b emission
Figure 12. In an absorption (a)/emission (b) process an electron increases/decreases its energy
with the same amount that has been absorbed/emitted in a photon.
11
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
If the change in absorption occurs within the visible range, the human eye will
interpret this as a change of color. This means that the color of the film can reversibly
be controlled via the doping and de‐doping which is controlled by an electrical
addressing signal. This effect is known as electrochromism and utilized in displays
(section 4.2)
1.0 1.5 2.0 2.5 3.0 3.5
0.0
0.2
0.4
0.6 0.0 V 0.7 V 0.9 V
Abs
Photon energy (eV)
Figure 13. The graph shows the absorption spectrum of an electrochromic polymer (PProDOT‐
Hx2) at different applied potential which changed the doping level in the polymer film.
12
The doping level of conjugated polymers can be altered through reduction or
oxidation of the polymer film in an electrochemical cell. As discussed in the previous
chapter, a change in doping level results in a modification of the charge carrier
concentration, thus changing the fundamental electronic nature of the material. This
effect is utilized in simple electrochemical devices such as transistors and
electrochromic displays. In this chapter the basic principles of electrochemistry in
organic electronic materials are discussed.
3.1 Electrochemical cell
Chemical processes involving an exchange of electrons are denoted electrochemical
processes. Conventional chemical reactions are controlled by the concentration of
the chemical species and their cross section for spontaneous chemical reactions. In
an electrochemical reaction the potential applied to electrodes adds another degree
of freedom to control the reaction.
Figure 14. A simple electrochemical cell with two electrodes and an electrolyte in between. The
potential is applied so that the left electrode is reduced and the right electrode is oxidized.
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3 Electrochemistry of conjugated polymers
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
An electrochemical setup, also called an electrochemical cell, consists of two
electrodes and an electrolyte that connects them (Figure 14). When a potential is
applied over the cell a current is able to flow through the cell and is a measure of the
reaction rate. Each electrode and the surrounding electrolyte represent a half‐cell
and therefore the reaction at one of the electrodes is called a half reaction. In order
to have a closed circuit in the electrochemical cell (required for the flow of current),
complementary reactions have to occur at the two electrodes. Reduction, where
electrons are consumed (at the left electrode in Figure 14) requires complementary
oxidation, where electrons are generated (at the right electrode in Figure 14). Under
these conditions the current will flow clockwise in the electrochemical cell according
to the scheme given in Figure 14.
3.2 Electrodes of conjugated polymers
An example of a half reaction in a conjugated polymer is the formation of a positive
polaron through oxidation:
−−+− +↔+ eXPXP :
Here, P denotes an undoped site in the polymer and X – an anion. When the polymer
is charged the anion moves inside the polymer film to stabilize the positive charge
(PP
+−−+
−+−++−
+). This is referred to as electrochemical p‐doping. The n‐doping process is similar
but here instead, cations react with the polymer electrode material at the same time
as electrons are delivered to the material at the electrode:
↔++ MPeMP :
Films of PEDOT:PSS includes immobile anions that are taking part in the p‐doping,
therefore the half reaction is slightly different, moving cations in and out of the film
to compensate the negatively charged PSS.
++↔+ eMPSSPEDOTMPSSPEDOT ::
When a potential is applied between the electrodes in an electrochemical cell, the
ions migrate in the electrical field that is created within the cell. At equilibrium, the
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CHAPTER 3 – ELECTROCHEMISTRY OF CONJUGATED POLYMERS
high concentration of ions close to the electrode screens the electrical field from the
bulk of the electrolyte. This results in that the major part of the potential drop
between the electrodes occurs at the electrode/electrolyte interface (Figure 15a).
The layer with most of the potential drop is called the Helmholtz double layer and
consists of negative (positive) charge carriers in the electrode and cations (anions) in
the electrolyte. The double layer is equivalent to a capacitance with a thickness of ∼3‐
5 Å, corresponding to the size of the involved ions. When a current passes through
the cell, i.e. at non‐equilibrium, only part of the potential drop inside the cell occurs
at the electrodes (Figure 15a). This means that a potential drop also occurs within the
bulk of the electrolyte (Figure 15b) resulting in a flow of ions within the electrolyte.
Figure 15. The potential profile within the electrolyte when a potential is applied. (a) shows the
equilibrium case and (b) shows when a current is passing through the cell. In both cases most of
the potential drop is over the Helmholtz layer at the electrode interface.
The dynamics of the electrochemical reaction can be limited by the reaction rate
itself along the electrodes, the electrical conductivity of the film or electrode, or the
conductivity of the electrolyte. In non‐crystalline polymers the film is often porous so
that the electrolyte can penetrate into the film making the interface between the
electrode and electrolyte effectively enormous and less well‐defined. This implies
that the Helmholtz double layer that is built at the interface of the electrolyte and the
electrode can be very large and in principle include parts of the bulk electrode. This
suggests that the reaction rate is limited either by the electronic conductivity of the
15
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
film or by the diffusion/drift of ions in the electrolyte. Such limitations have been
observed in two extreme cases (Figure 16) where in the first situation poly(3‐
hexylthiophene) (P3HT) is p‐doped in acetonitrile electrolyte[6] and in the second,
PEDOT is undoped using solidified gelled electrolytes. P3HT, in its undoped state, has
a very poor electronic conductivity. In this case, as a potential is applied, the end of
the film closest to the anode will start to be oxidized (and hence become doped). This
will enhance the conductivity of the film allowing oxidation to occur further along the
film. Eventually the entire film will be oxidized, as the front moves across the film
starting from the anode. In the PEDOT:PSS system PEDOT is doped in its pristine state
and therefore highly conducting. Using a gelled electrolyte with low water content
gives a low ionic conductivity. In a cell including PEDOT:PSS as one of the electrode
materials with a gelled electrolyte including a low water content, the ion diffusion in
the electrolyte entirely dictates the switch characteristics. Therefore the reduction of
the PEDOT:PSS film will start at the end closest to the counter electrode (the source
of ions) and then move towards the cathode as a moving front.
Figure 16. (a) doping process of P3HT in aqueous electrolyte with high ionic conductivity. (b)
undoping of PEDOT:PSS in a gelled electrolyte of low ionic conductivity.
Because PEDOT:PSS is partly doped from its synthesis, it is possible to further oxidize
or reduce the film. Therefore films of PEDOT:PSS can be used as the counter
electrode for either oxidation or reduction. When the working electrode is reduced
the counter electrode has to be oxidized and since a 1:1 charge transfer is required,
one reduced species at the working electrode requires one oxidized species at the
counter electrode. The doping fraction of pristine PEDOT:PSS films is very high[7] (for
the films used in our lab it is estimated to be about 80% of the maximum doping
level). This means that the volume of the counter electrode should be at least four
times larger than that of the working electrode to enable full reduction to occur at
16
CHAPTER 3 – ELECTROCHEMISTRY OF CONJUGATED POLYMERS
the working electrode[8]. This fact is one key criterion in designing electrochemical
devices.
3.3 Electrochemical measurements
Material analysis can be performed using electrochemical measurements. For
example the energy levels (ionization potentials) of a polymer can be estimated using
relatively simple measurement equipment. A simple model of conjugated polymers in
electrochemical measurements is sketched in Figure 17 where the p‐doping of an
undoped polymer is displayed. N‐doping is similar but a negative potential is applied
to the electrode giving that electrons are injected into the polymer at the LUMO level
instead.
Figure 17. A sketch of an electrochemical process in conjugated polymers. (a) The Fermi level of
the metal electrode is in the band gap of the polymer. (b) When a positive potential is applied
to an electrode, the Fermi level is decreased, allowing charge transfer when the Fermi level
matches the HOMO level of the polymer, resulting in p‐doping of the polymer film. (c) The
charge in the polymer film is stabilized by forming a polaron resulting in moving the energy
levels inside the band gap.
In an experiment, where the potential is linearly swept cyclically around the p‐doping
potential of a polymer, the current response would in principle be as in Figure 18. As
the potential is increased the polymer is oxidized (p‐doped) and a positive current is
measured. The width of the current peak is caused by the disorder in the polymer
that causes the chains in the film to have varying oxidation potential. When the
potential is decreased, the p‐doped polymer is reduced to the neutral state and a
17
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
negative current is measured. Note that the reduction potential is lower than the
oxidation potential because of the relaxation described in Figure 17c where the
energy level of the p‐doped species is higher and therefore will be reduced at a lower
potential compared to the oxidation.
Figure 18. Cyclic voltammogram expected when p‐doping (and dedoping) a polymer. The
oxidation and reduction occur at different potentials since relaxation in the polymer occurs as a
consequence of electrochemical reactions.
This technique, where the potential is repeatedly swept up and down in voltage is
called cyclic voltammetry (CV) measurement and is widely used to measure material
characteristics and for evaluation of the electrochemical properties of compounds. In
paper 2, CV measurements have been used to look at the oxidation and over‐
oxidation potential of various polymers. In paper 3, a similar experiment has been
used, but instead of cycling the potential, only one linear sweep is measured, called
linear sweep voltammetry (LSV).
3.4 Electrolytes
The electrolyte is the ionic conduction medium between the electrodes and can be in
a liquid, solid or gelled state. Two examples of common solvents used in liquid
electrolytes are water and acetonitrile. Water is polar and can easily dissolve
common salts, but its stable potential window is quite narrow preventing accurate
electrochemical measurements at higher potentials. In contrast, acetonitrile is more
stable and offers a much wider potential window for the electrochemical
18
CHAPTER 3 – ELECTROCHEMISTRY OF CONJUGATED POLYMERS
19
measurements. Because acetonitrile is less polar, salt with larger ions are preferred,
for example tetrabutylammonium perclorate. Liquid electrolytes are suitable for
electrochemical measurements, but in devices, solid or gelled electrolytes are
preferred due to their non‐volatile nature. Solid electrolytes are usually based on ions
mixed in an ion‐conducting matrix. For example the electrolyte system in paper 5 is
based on polyethylene glycol (PEG) as the ion‐conducting matrix and Lithium
trifluoromethanesulfonate (LiF3CSO3) as the salt. Polyelectrolytes define another class
of electrolytes which also contain ions – either polycations with smaller anions as the
counter ion or polyanions with small cations. An example of the latter is poly(4‐
styrenesufonate) (PSS) that contains immobile sulfonate (‐SO3–) ions and positive
counter ions, such as sodium (Na+) or protons (H+). Gelled electrolytes consist mostly
of liquid (usually water) and a solid network that gives the gel its mechanical
properties. Because of the high liquid content they have normally high ionic
conductivity.
3.4.1 Ion transport
For high ionic mobility, easy and fast transport of ions through the material is
required. The transport mechanism for the ions depends on the type of electrolyte. In
liquids the ions are surrounded by solvent molecules that follow the ions, creating a
large package that has to be transported through the electrolyte. The viscous drag of
this large package within the liquid electrolyte will then be the rate limiting factor for
the ion transport. In solid electrolytes based on a polymer matrix, the flexibility of the
chains in the amorphous phase of the material allows for the ions to be transported.
Like walking through a crowded dance floor, the polymer chains make room for the
ion to travel across the material. But this is not possible in a crystalline phase where
the material is densely packed and does not have enough space to allow fast
transport of ions.
3.4.2 Ionic conductivity
The current in an electrochemical cell can be assigned to two mechanisms: diffusion
caused by a concentration gradient or drift due to an electrical field. The total current
density is thus given by the Nernst‐Planck equation[9]:
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
cDVFczdiffusiondrift −=+= ∇μ − ∇JJJ
where z is the charge of the ions (for example –1 or +2), c is the concentration, F is
the Faradays constant (∼96500 C/mol), μ is the mobility of the ions and D is the
diffusion coefficient which is proportional to the mobility. The applied potential
creates a force that drives the ions in the direction of the field. This drift part of the
current is intuitive, but the applied potential can also build up a concentration
gradient, through accumulation of electrochemical reaction products. The result of
the random movement of the ions is a net flow opposite to the direction of the
concentration gradient, being the diffusion part of the current. Which mechanism
that is dominant depends on the ionic conductivity of the electrolyte and the
geometrical conditions of the setup.
The ionic conductivity (κ) of an electrolyte can be calculated from the potential drop
(V) along the distance (l) and the current density which is the current (I) that passes
through the cross section area (A):
AVI l⋅=κ
Figure 19. A simplified geometry for measuring the ionic conductivity. I is the current through
the cell, V is the potential difference at a distance l and A is the area of the cross section of the
ion path.
One method for measuring the ionic conductivity is to utilize a four‐point‐probe
measurement[10], which in comparison to the more common two‐electrode
20
CHAPTER 3 – ELECTROCHEMISTRY OF CONJUGATED POLYMERS
measurement is easier to analyze. This has been employed in paper 5 to measure the
ionic conductivity of PEG based electrolyte at different temperatures.
3.5 Over-oxidation
At low potentials electrochemical switching of conjugated polymers is reversible,
meaning that a large number of reductions and oxidations can be done repeatedly.
However, degradation of the electroactivity has been observed at high anodic
potentials in polythiophenes[11‐13]. The phenomenon is called over‐oxidation and
reduces the potential window available for the reversible reactions. Over‐oxidation is
an irreversible reaction that results in a non‐conducting and electrochemically
inactive film as it breaks the conjugation in the polymer chains. In a study by Barsch
et. al. on the over‐oxidation phenomenon in polythiophene films[14], electrolytes
based on acetonitrile with different concentrations of water were used to
electrochemically switch polythiophene films. They observed that both the degree of
over‐oxidation and the potential at which it occurs is highly dependent on the
amount of water in the electrolyte. They proposed a mechanism that step‐wise
degrades the conjugation in the polymer (Figure 20). This mechanism suggests that
first oxygen binds to the sulfur atom in the thiophene ring and then SO2 is detached
from the chain. After losing the conjugation in the polymer chain, the backbone of
the chain is ripped apart in the last step.
S S
O
SO O
O OO
OH
O
O
OH
Figure 20. Proposed mechanism for over‐oxidation of polythiophene[14].
21
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
Even though a thorough study of the over‐oxidation of PEDOT has not been published
yet, it is reasonable to suspect that the over‐oxidation of PEDOT follows a similar
mechanism. Coulometry measurements on PEDOT suggest SO2 formation on the
thiophene rings[15]. Also the Fourier transform infrared spectroscopy measurements
included in paper 3 support this hypothesis.
The PEDOT films can be over‐oxidized in other ways, besides electrochemical, which
lead to a degradation of the conductivity. Ultraviolet light in combination with oxygen
is known to induce photo‐oxidation of the conjugated PEDOT material, resulting in a
reduction in electrical conductivity. This photo‐oxidation leads to shorter conjugation
lengths, due to sulfone group formation and chain scission accompanied by the
addition of carbonyl or carboxylic groups[7, 16]. Similar chemistry occurs upon heating
in air, where oxidative degradation of conjugated polymers has also been
reported[17]. Under these conditions, the conductivity of PEDOT decreases
exponentially with time according to an Arrhenius law[18] type of behavior.
Over‐oxidation is one of the main sources of failure in many devices and is therefore
avoided as much as possible. The mechanism is irreversible and results in a non‐
conducting film phase which can be locally defined. Controlled electrochemical over‐
oxidation can be used as a method for deactivation of the conductivity in a
subtractive patterning process (see further in section 4.1 and paper 1). Because of its
irreversibility, the over‐oxidation front propagation can be utilized in electrochemical
timer and logger (see further in section 4.3 and paper 5).
22
For more than 30 years, the field of conducting polymers has been under
development. Major progress has been made in increasing the stability and
functionality of materials for various applications. One of the most simple
applications where conducting polymers are used is antistatic coatings, used for
example on photographic foils[5]. More sophisticated applications include new solar
cells[19], light emitting devices[20], field effect transistors[21] and different
electrochemical devices[22‐26]. In some applications, organic materials offer better
performance or new functionalities as compared to their inorganic counterparts,
exemplified by superior performance of OLED displays compared to LCD displays
when it comes to energy consumption, weight, color accuracy and viewing angle. In
other cases simplified manufacturing gives huge benefits in comparison with existing
inorganic technologies. For example, using flexible substrates and low‐cost
manufacturability makes the organic solar cell interesting despite their low efficiency.
Another technology where low manufacturing costs enable new markets is RFID
(radio frequency identification) added onto packages and low cost surfaces. Such
device systems can be realized with organic circuitry made out of organic field effect
transistors (OFET). Several companies are now targeting such RFID products for track
and trace applications for logistics.
One class of devices of special interest for this thesis is the electrochemical devices,
including electrochromic displays[5, 26, 27], electrochemical transistors[24, 28] and
actuators[29, 30]. Electrochemical devices are robust and operating at low voltage and
well suited for manufacture using printing techniques. In these devices desired nano‐
features/dimension are spontaneously formed and defined by the Helmholtz layer
that reduces the significance of controlling the film thickness that otherwise is crucial
23
4 Application of conjugated polymers
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
for the devices driven by charge injection or field effects. This makes it possible to
manufacture fully printed devices, which will give very low manufacturing costs on
for instance paper and other non‐ideal surfaces for electronics. A common drawback
of these devices is limited ionic conductivity, which results in rather slow devices.
4.1 Electrochemical over-oxidation patterning
The benefits of organic electronics include new functionality, materials with low
environmental impact and the possibility to use printing tools in the manufacturing
process. Printing is an ancient technology that has been developed over the past
centuries to be a fast and high volume production technique for reproducing text and
images. When now using printing for manufacturing organic electrics, a well
developed technique with little need for major modifications is available for high
volume manufacturing of electronics. Today, many different techniques have been
demonstrated for printing and patterning conducting polymers, such as inkjet[21, 31, 32],
screen[33], offset[34], gravure[34] and flexographic[34] printing the most common being
the inkjets and screen‐printing technology. These additive techniques can be used to
print polymers and to create patterns down to resolutions of around 100 µm, which
is enough for large area displays[32, 33] and solar cells[35]. Coated polymer films often
possess better and more homogenous solid‐state properties compared with those
printed through additive processes. The coated film can then be patterned through
subtractive patterning techniques such as chemical deactivation at the polymer
surface in which a chemical agent converts the conjugated material to a non‐active
material[36, 37]. These techniques are typically very slow and often require hazardous
reagents, but most importantly the pattern resolution is limited by diffusion of the
agent into and along the polymer film. Plasma‐etching techniques can be used in
combination with photolithography; however, these techniques require a vacuum
and are typically not suitable for low‐cost roll‐to‐roll processing. Another technique
for subtractive patterning of conductive films of PEDOT:PSS is to use electrochemical
over‐oxidation (ECO). As described in section 3.5, over‐oxidation is an electronically
controlled, irreversible process that results in a non‐conducting film. ECO can be
utilized in combination with various patterning tools and offers an electrically
24
CHAPTER 4 – APPL ICATION OF CONJUGATED POLYMERS
controlled deactivation method. It can be used together with photolithography to
achieve high resolution µ‐patterns or be used in a screen printing step in order to
produce patterns at high volumes in a roll‐to‐roll process. The conductivity ratio
between the conducting and insulating parts of a patterned film can reach 107, which
is enough for various simple electronics with relatively low packing density.
Figure 21. The Nilpeter printing press, within the Printed Electronics Arena Manufacturing, is
used to print organic electronics. © Niclas Kindahl, Fotofabriken.
In recent years Acreo AB in Norrköping has, in collaboration with Linköping
University, developed a process for a Nilpeter printing press (Figure 21) to produce
electrochemical devices in a roll‐to‐roll process. The starting material is Orgacon™,
which is a pre‐coated PEDOT:PSS thin film, here deposited on paper or plastic
substrates, supplied by Agfa‐Gevaert in Belgium. Manufacturing of electrochemical
devices then basically requires only three printing steps. First, subtractive patterning
of the Orgacon™ through ECO patterning in a screen printing step results in a pattern
with a resolution down to 100 µm at around 5 – 50 meters per minute. In the next
25
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
step, the electrolyte is applied in another screen‐printing step. Finally, the devices are
encapsulated, by a plastic film, for mechanical protection. This Nilpeter printing press
is small and simple but at 5 meters per minute it can produce more than 50 million
devices (1 dm²) each year.
4.2 Electrochromic display
The color of conjugated polymers can be changed reversibly by controlling their
doping level while they are included as electrode in an electrochemical cell. This
effect is utilized in the electrochromic displays (ECD) where an electrical signal is used
to change the color state of the display. ECDs can be used in different types of
applications such as in smart windows to control the transmission of sunlight[38], as
simple indicators in smart package applications or as the individual display pixel in
matrix addressed e‐paper[22, 39].
Usually, the electrochromic (EC) polymer is cast or coated on a conducting electrode,
such as onto indium tin oxide (ITO) or gold, to achieve transparent or reflective
device configurations, respectively. But if the electrochromic polymer has high
intrinsic conductivity in both the doped and de‐doped states, there is no need for
such conducting substrate electrode; the electrochromic polymer can be its own
electrode. This gives a simplified device architecture that also is simpler to
manufacture. For this reason polymers like PEDOT, with high intrinsic conductivity,
have received major attention in ECDs[26]. The ease of processing PEDOT:PSS has
encouraged scientists for instance at Acreo AB in Norrköping to develop an all‐
organic display on paper substrates in collaboration with the Organic Electronics
group at Linköping University[40]. These displays are flexible, lightweight and can
easily be manufactured into any shape or design. In Figure 22 seven‐segment displays
are shown that have been demonstrated by LiU and Acreo. The PEDOT:PSS display
has clear color switching between blue color in the reduced state and nearly
transparent in the oxidized state.
26
CHAPTER 4 – APPL ICATION OF CONJUGATED POLYMERS
Figure 22. Two seven‐segment digits made with ECD in PEDOT:PSS. The display is made with
printable techniques by Acreo AB and Organic Electronics at Linköpings University. © Niclas
Kindahl, Fotofabriken.
Even though the color switch contrast of PEDOT:PSS displays are satisfied for
indicators and simple displays, higher contrast is desirable for improved performance
in more advanced displays. Several routes have been presented for improving the
optical contrast of PEDOT:PSS displays. Admassie et. al. introduced a grating pattern
(µm‐scale) in the PEDOT:PSS film that through diffraction highly increased the
absorption of the PEDOT:PSS film compared to an unpatterned film[41]. With the right
material choice, this method can be used for improvement of the optical contrast.
The most common method, though, is to add an extra EC polymer that complements
the color of PEDOT:PSS. For significant improvement of the optical contrast, the dark
state should complement the blue color of PEDOT:PSS and together cover as much as
possible of the visible spectrum. At the same time, the uncolored state should be
highly transparent at all optical wavelengths in order to provide nearly a translucent
state.
The extra EC polymer material can be added to the ECD structure in different ways
depending on its optical and electrochemical properties (Figure 23). When the EC
polymer has opposite coloring polarity as compared to PEDOT, i.e. colored while
oxidized and transparent when it is reduced, the extra EC film is best utilized as the
counter electrode in vertical structures (Figure 23a). Contrary, if the EC polymer has
the same optical “polarity” as PEDOT it is better included as a film on top of the active
27
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
PEDOT:PSS electrode, which can be either in a vertical (Figure 23b) or lateral
structure (Figure 23c). Usually the rate limiting step in the displays is the ionic
conductivity of the electrolyte, which means that a vertical display will switch faster
and more homogeneously. However, a lateral structure offers a more simplified
manufacturing process.
PEDOT PEDOT PEDOT
Electrolyte Electrolyte
Substrate Substrate Substrate
Substrate SubstratePEDOT PEDOT
a b c
EC‐polymer
EC‐polymer
EC‐polymer
Electrolyte
Figure 23. Different ways to improve the contrast in an ECD.
In paper 3 and paper 4, the improvement of optical contrast is discussed. The work in
paper 3 is focused on the design rules for choosing materials in contrast
improvement of PEDOT:PSS displays. Here transmissive measurements were
conducted with a standard absorption spectrometer using transparent plastic
substrates (PET‐foils).
But the observed switch contrast values of the transmissive displays presented in
paper 3 cannot be directly translated to reflective display. Paper displays are
interesting from a device point‐of‐view, therefore the work presented in paper 4 was
angled towards devices and important parameters used in applications. ECDs with
paper substrates manufactured in the printing press were modified by adding an
extra layer of an EC‐polymer. Both contrast and switching speed measurements was
based on reflective setups.
There are several practical issues that put constrains on the design of printable EC
devices. The main problem is to choose an electrolyte that has high enough ionic
conductivity, to achieve reasonably fast switching, is printable and also gives proper
mechanical stability. The electrolyte should also maintain its high ionic conductivity
even after long storage times in dry conditions. This turns out to be a great challenge
as high ionic conductivity usually requires high water content, which eventually will
evaporate in dry storage conditions. One route could be to use encapsulation that
28
CHAPTER 4 – APPL ICATION OF CONJUGATED POLYMERS
29
does not allow the drying out of the electrolyte, but water molecules are small and
can go through most materials. Only metal films could give the desired protection but
those are not transparent and cannot be used for displays. Different plastic films are
not dense enough but could be noticeably improved by using additives such as clay
nano‐particles. Scientists are working hard to improve the barrier function of plastic
films but the materials are expensive and still not good enough as a barrier for water
molecules. Another route could be to develop electrolyte materials with high ionic
conductivity also at low humidity levels and only use the encapsulation as a
mechanical protection.
Another issue that has to be considered as well is the design of the counter electrode.
As mentioned earlier any electrochemical half reaction must be balanced in the
counter electrode. When using PEDOT in the counter electrode, care must be taken
to ensure that enough redox sites are available for active electrode to be fully
switched. In a lateral design with the same film thickness, the area of the counter
electrode should be at least four times larger than the active pixel (see section 3.2).
4.2.1 CIE Lab coordinates
The light that escapes from an illuminated or luminescent object is nothing more that
a spectrum of photons of various wavelengths and intensities. When that light hits
our eyes, it is absorbed by the rods and cones on the retina, transformed into nerve
signals and translated into a perception of color in the brain. Because humans have a
three‐dimensional color space (given by the three different cones that are sensitive
to different colors), many of the existing color systems have three coordinates. In
light emitting devices, three pixels with red, green and blue (RGB) color are used to
span the color space. The RGB color system is practical for creating a display but is
actually not a good description of the full color space accessible with human eyes. In
1931, the International Commission on Illumination (Commission Internationale de
l'Eclairage, CIE) developed a model defining colors by three coordinates by X, Y and
Z[42], corresponding to different biological receptors in the human eye (Figure 24).
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
Figure 24. The experimentally measured x , y and z , which correspond to the sensitivity of
the various receptors in the human eye.
The model was improved in 1976 to be able to further adapt to the way the human
eye perceives colors by introducing the CIE L*a*b* color system[42], where the non‐
linear behavior of the eye is considered. In the CIE L*a*b* system the color is
represented by L*, a* and b* coordinates. L* gives the luminance (lightness), while the
chroma is given by the values of the a* and b* (Figure 25). L* has values from 0 (dark)
to 100 (light), a* goes from –128 (green) to 127 (red) and b* goes from –128 (blue) to
127 (yellow).
Figure 25. The CIE L*a*b* color system.
30
CHAPTER 4 – APPL ICATION OF CONJUGATED POLYMERS
X, Y and Z need to be calculated first in order to estimate the L*, a* and b*
coordinates. Say that the light reflected or emitted from an object has a spectrum
given by I (λ). Then X, Y and Z are given by:
( ) ( )
( ) ( )
( ) ( ) λλλ
λλλ
λλλ
dIZ
dIY
dIX
⋅⋅=
⋅⋅=
⋅⋅=
∫∫∫
z
y
x
where x , y and z are the experimentally computed spectra (Figure 24) for the X, Y
and Z coordinate respectively. From the X, Y and Z the L*, a* and b* coordinates can
be calculated by correcting the coordinates with the white point: XN, YN and ZN in the
following expression:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛−⎟⎟
⎠
⎞⎜⎜⎝
⎛⋅=
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛−⎟⎟
⎠
⎞⎜⎜⎝
⎛⋅=
−⎟⎟⎠
⎞⎜⎜⎝
⎛⋅=
31
31
31
31
3
200*
500*
16116*
NN
NN
N
ZZ
YY
b
YY
XX
a
YY
L
1
These equations are valid for X/XN, Y/YN and Z/ZN greater than 0.008856. With these
equations, the color of an object, and also a polymer film, can be represented by
three independent coordinates that indicate how our eyes measure the color of an
object. The coordinate of the white point is chosen to follow different standards
depending on the lighting condition. For measuring on paper in bright conditions, the
D65 10° standard observer is used, measured to be XN = 94.81, YN = 100.00 and ZN =
107.30 by CIE[42]. The difference between two colors can be defined as the distance in
the color space, ΔE*, using ΔL*, Δa* and Δb* as the difference in the individual
coordinates:
( ) ( ) ( )222 **** baLE Δ+Δ+Δ=Δ
31
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
4.3 Electrochemical timer and logger
An electrochemical timer can be used to keep track of time. It is a simple component
that is easy to manufacture. An electrochemical timer can be realized with two
electrodes of PEDOT:PSS in a lateral configuration, where one electrode acts as a
timer‐stripe and the other as a counter electrode. A gelled or solid electrolyte with
low ionic conductivity connects the two electrodes (Figure 26a). As described in
section 3.2, the applied potential drives a front of electrochemical reaction through
the timer‐stripe (Figure 26b). There are two modes for driving the timer: either
applying a negative potential to start a reduction front or a high positive potential for
an over‐oxidation front. Both the reduction and over‐oxidation highly reduces the
conductivity, which can be used to detect the position of the front by monitoring the
potential in the timer‐stripe (Figure 26d). The reduction front is also clearly visible
due to the color difference between the reduced and the pristine PEDOT:PSS film
while the irreversible nature of the over‐oxidation reaction allows for long term
storage of the position of the front.
Figure 26. A sketch of an electrochemical timer in progress (a‐c). Applying 5 V starts an over‐
oxidation front that propagates through the timer stripe (right electrode). The sensing fingers
measure the potential and can be used to monitor the position of the front (d).
Timers can also be made as sensors for different parameters, such as temperature or
humidity. For example a temperature dependent timer, where the propagation of the
front depends on temperature, can be realized by using an electrolyte with a phase
transition at a desired temperature. The ionic conductivity of the electrolyte changes
32
CHAPTER 4 – APPL ICATION OF CONJUGATED POLYMERS
dramatically upon a phase transition, such as melting or crystallization. Of course the
ionic conductivity in all electrolytes is temperature dependent but the specific
temperature dependent timer described above is much more affected by the
temperature than ordinary timers with electrolytes without a phase transition. In
paper 5 such a timer has been realized and is described further.
By branching one temperature independent timer† and connecting an array of
temperature dependent timers to it (Figure 27), a system is created for logging the
temperature history. The temperature independent timer keeps track of the time and
one by one shuts down the branched timers that are tracking the temperature.
Therefore the information in each temperature dependent timer is based on the
temperature history from start to the moment the front passed and cut off the
potential to that timer.
Figure 27. Sketch of a logger with a temperature independent timer (on the top) that is
branched and connected to an array of temperature dependent timers. The potentials are such
that over‐oxidation fronts are propagating in all timers.
For the logger device, an over‐oxidation front has to be used to retain the
information until the end of the run. The easiest way to read out the information, i.e.
find out the positions of the over‐oxidation fronts in each timer, is by optical read
† A timer with less temperature dependence than a timer with a phase changing electrolyte.
33
ELECTROCHEMICAL SWITCHING IN CONDUCTING POLYMERS – PRINTING PAPER ELECTRONICS
out. But because the optical contrast between the pristine and over‐oxidized
PEDOT:PSS is low, the active (not over‐oxidized) part of the electrode has to be
reduced, chemically or electrochemically. This will create the necessary optical
contrast for knowing the position of the over‐oxidized fronts. The temperature logger
is presented in more detail in paper 5.
34
References 1. Bardeen, J. and W.H. Brattain, The Transistor, A Semi‐Conductor Triode.
Physical Review, 1948. 74(2) p. 230.
2. Chiang, C.K., et al., Electrical Conductivity in Doped Polyacetylene. Physical review letters, 1977. 39(17) p. 1098‐1101.
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