Electrochemical Switching in Conducting...

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


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

4

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).

5


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


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


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


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


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).

10


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


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.

13

3 Electrochemistry of conjugated polymers


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

14

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


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


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


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


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]:


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


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


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


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


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


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


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


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).


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


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


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


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


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

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