electrochromism and over-oxidation in conjugated polymers

Linköping Studies in Science and Technology Licentiate Thesis No. 1236

Electrochromism and over-oxidation in conjugated polymers: Improved color switching

and a novel patterning approach

Payman Tehrani

LiU-TEK-LIC- 2006:17

Dept. of Science and Technology

Linköpings University, LiU Norrköping

SE-601 74 Norrköping

Norrköping 2006

ISBN: 91-85497-34-7 ISSN: 0280-7971

LiU-TEK-LIC- 2006:17

© Copyright 2006

Printed by LiU-Tryck, Linköping 2006

ISBN: 91-85497-34-7

ISSN: 0280-7971

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 this new class of materials that also enable many new areas of electronics applications. In the organic electronics research field conducting organic molecules and polymers are synthesized and used in devices. The reason behind the success of conducting polymers 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 reviews and describes different aspects of the organic electronics, here focusing on electrochromic displays; device improvements, the study of degradation and also patterning technology for rational manufacturing processing. The color contrast in electrochromic displays based on conjugated polymers was increased with approximately a factor of two by adding an extra electrochromic polymer. It was found that electrochemical over-oxidation (ECO) limits the flexibility in choosing desired electrochromic materials. ECO is one of the main degradation mechanisms in electrochromic displays. ECO is an efficient and fast process to permanently reduce the electronic conductivity in polythiophenes. From this, a novel patterning process was developed, in which the films of polythiophenes can be patterned through local and controlled deactivation of the conductivity. The ECO has been combined with different patterning tools to enable the use of existing printing tools for manufacturing. In combination with screen-printing, low-cost and high volume roll-to-roll patterning was demonstrated, while together with photolithography, patterning down to 2 µm can be achieved. Systematic studies have shown that conductivity contrasts beyond 107 can be achieved, which is enough for various simple electronic systems. To generate better understanding of the ECO phenomena 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.

Acknowledgements

I would like to express my sincere gratitude to people who have created the path that has brought me here, especially …

… my supervisor Magnus Berggren, for all the help and support during these years.

… Thomas Kugler, who gave me the opportunity to start working in this research field.

… Nate and Xavier, for all the valuable and interesting scientific discussions and ALL the help with this thesis.

… Joakim, 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 (Magnus, Sophie, Nate, Xavier, Dr David, PeO, Peter, Elias, Joakim, Fredrik, Lars, Oscar, Emilien, Maria, Thomas, Max, Jessica and Linda) for creating an amazing working environment, I especially appreciated all the fun after-work activities.

I want to thank my family and friends for the support and all the good times outside of work.

Last but not least (now anyway) I want to thank my loving wife Yashma and the “kocholo” she has in her stomach, for bringing happiness into my life.

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: 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

Submitted to Thin Solid Films

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 3: 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

Manuscript

Contribution: Most of the electrochemical measurements. Wrote the first draft.

Contents

1. INTRODUCTION 1

2. CONDUCTING POLYMERS 3

2.1. CONJUGATED POLYMERS 3

2.2. CHARGE CARRIERS 7

2.3. CHARGE TRANSPORT 9

2.4. OPTICAL PROPERTIES 10

2.5. CIE L*A*B* COLOR SYSTEM 11

3. ELECTROCHEMISTRY 15

3.1. ELECTROCHEMICAL CELL 15

3.2. ELECTRODES OF CONJUGATED POLYMERS 16

3.3. ELECTROCHEMICAL MEASUREMENTS 18

3.4. ELECTROLYTE 20

3.5. OVER-OXIDATION 20

4. APPLICATIONS OF ORGANIC ELECTRONICS 23

4.1. ELECTROCHEMICAL TRANSISTORS AND LOGIC 24

4.2. ELECTROCHROMIC DISPLAYS 24

4.3. PRODUCTION OF ORGANIC ELECTRONICS 27

5. REFERENCES 30

1. Introduction

The consumer market is stuffed with various forms of electronics, such as computers, liquid crystal displays (LCD) and plasma displays, cellular phones with cameras and MP3-players. The electronics market has been dominated by silicon technology for the last 50 years, now offering inexpensive products for the low-end market. In the past few years a number of electronic products based on organic materials have been demonstrated (see Figure 1a). Some of these products are in fact produced and sold in very large volumes (see Figure 1b). How can this new organic electronics technology compete with the already established inorganic technology that has matured during the last 50 years? Well, it is for sure that organic electronics cannot serve as the transistor switch in fast computers. Today 3.5 GHz processors are being installed in

Figure

(With

Philips

1. a Full color 40” polymer display prototype

permission from Epson). b Polymer display on

shaver from 2002 (With permission from Philips).

1

I N T R O D U C T I O N

2

many of the computers sold in the market, but the organic electronic components are not performing in the higher MHz region. The manufacturing of silicon chips requires many hundred process steps that involve either vacuum processing, photolithography or other batch-based techniques, which gives that a modern factory for silicon chips requires investments in the range of several billions euros. Organic materials are unique in the sense that they can be dissolved or emulsified in common solvents, which give that processing of devices can be achieved using existing printing technologies. Printing manufacturing is a very old technology that has technologically matured over many 100s of years; today offering an ensemble of different fundamental patterning techniques, that enable printed matter to be made on various substrates at high speeds in a roll-to-roll process. If organic electronic devices could be produced using the same technique as is used for producing newspapers and printed graphics on packages, the cost for individual electronic systems would be far less than 1€ per unit. Imagine a piece of wallpaper produced in a printer shop hosting a display that show motion pictures and do not cost more than an ordinary billboard or a poster.

Another advantage of organic electronics is that the function of the device can be dictated via the design of the organic molecule. The flexibility in the design of functionality of the material and the possibility of printing devices makes organic electronics a promising field that opens new opportunities for including electronic functions on ordinary packages and cheap sensor technologies.

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 a broad overview of the field of organic electronics as well as a description of important applications for the work presented in this thesis.

2. Conducting polymers

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 the electrical conductivity of trans-polyacetylene, a conjugated polymer, can be increased over many orders of magnitudes to finally reach the typical conductivity level of metals by doping it with iodine[1]. In 2000, those gentlemen were awarded the Nobel Prize in chemistry[2] for this discovery. Their finding has resulted in a new class of materials offering novel applications for electronics such as flexible solar cells, low-cost light emitting devices, printed field effect transistors and electrochemical components made on paper. 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 the inherent mobility. These parameters are governed by the chemical structure of the polymer. 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. Conjugated polymers The electronic configuration of a single carbon atom in its ground state is 1s22s22p2, which means that there are two electrons in the 1s-orbital, two electrons are in the 2s-orbital and two electrons in the 2p-orbital (Figure 2). When carbon atoms are bound to other atoms in a molecule, their electronic structure is changed and they acquire different hybridization states. In ethane for example, the four outer electrons in the 2s- and 2p-orbitals are rearranged

3

C O N D U C T I N G P O L Y M E R S

4

Figu

mole

to form fouoriented in

In ethylenhybridizati(Figure 2).the remainethylene orform σ-bonConsequendouble bonatoms closrigid sincebond that rhosts a 2p-which colleis character

re 2. The electronic configuration of the carbon atom in different

cules, as a free radical, as a member in ethane and ethylene.

r equivalent sp3-hybridized orbitals (Figure 2). These orbitals are the space in a tetrahedral fashion.

e one 2p-electron of each carbon atom is not involved in the on, three electrons occupy three equivalent sp2-hybridized orbitals The sp2-hybridized orbitals form a planar-triangular geometry with ing 2p-orbitals pointing perpendicular outwards from this plane. In other sp2-hybridized hydrocarbons, the electrons in the sp2-orbitals ds with neighboring atoms, while the 2p-electrons form π-bonds.

tly, the bond between the two carbon atoms can be considered as a d. The double bond is tighter in the sense it draws the two carbon

er to each other as compared to a single bond. It is also much more rotation along the bond axis is suppressed in contrast to a single otates freely. In larger hydrocarbons, in which each carbon atom electron, each carbon atom generates π-bonds with their neighbors, ctively generates alternating single and double bond structure that istic of conjugated polymers and molecules.


The sp2-orbitals are relatively localized, interacting only with neighboring atoms, but the 2p-orbitals spread in space and can interact with electrons farther away. The interaction between two neighboring 2p-electrons gives rise to two different molecular orbitals, one being bonding to its nature and with a lower energy level (π) and one being antibonding (π*) to its nature located at a higher energy level (see Figure 3). As the number of carbon atoms increases in a molecule, the number of molecular orbitals increases. In large molecules with N carbon atoms, in which every carbon atom is bonded with two neighboring carbon atoms via one single and one double bond, respectively, N 2p-electrons yield N molecular orbitals of various 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 3). However, the energy difference between the HOMO and the LUMO, called the band gap, does not vanish, resulting in a permanent gap between the occupied and the unoccupied states.

Figur

betw

conti

e 3. As the number of carbon atoms increases, the overlap

een the π-orbitals causing state to split, eventually giving rise to

nuous bands as the polymer is extends into a long chain.

5


6

Figu

HO

acc

into

The fostated modifidegeneconduc5a) giv(Figurdistortand doLUMOconduc

The sipolyacbondedin theoair anddiffere(Figurtheir f

re 4. The structure of trans-polyacetylene in a results in a degenerated

MO and LUMO state, which gives the polymer metallic properties. But

ording to Peierls theorem this state is unstable and is instead transformed

the more stable form with alternated double and single bonds in b.

rmation of the band gap can be explained by Peierls theorem[3]: He that a one-dimensional metal is unstable with respect to geometrical cation that leads to lowering of the symmetry and removal of the racy of the HOMO and LUMO levels, thus obtaining a semi-ting state. The Peierls theorem applied on trans-polyacetylene (Figure es that the metallic state with delocalized electrons in the polymer chain

e 4a) is unstable. The polymer is stabilized through a geometrical ion that results in a dimerization of the unit cell with alternated single uble bonds (Figure 4b). In this state the energy level of the HOMO and is separated creating a band gap that is characteristic for semi-tors.

mplest conjugated polymer, from a structure point of view, is trans-etylene, being a straight conjugated chain of carbon atoms, each atom to one hydrogen atom. Although trans-polyacetylene is easy to model retical studies, films of the material are very sensitive for exposure to water, making it a poor candidate for applications. Through the years, nt classes of more stable conjugated polymers have been developed e 5). One of the characteristics that make polymers interesting is that undamental electronic and optical properties can be tuned by changing


7

S

S

S

S

S

OO

OO

OO

OO

OO

NH

NH

NH

NH

NH

N N NH

NH

a

b

c

e

f

S

S

S

S

S

d

Figure 5. Examples of some conjugated polymers a trans-polyacetylene

b polythiophene c poly(para-phenylene vinylene) d polypyrrole e

polyaniline f poly(3,4-ethylenedioxythiophene)

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 5f). PEDOT itself is insoluble, but when chemically polymerized with poly(4-styrenesulfonate), PSS, as counter ion, a water emulsion can be obtained (Figure 6). In the PEDOT:PSS couple, the PEDOT part is positively doped making the polymer highly conducting (at a conductivity sometimes exceeding 10 S/cm)[4].

S

OO

S

OO

S

OO

S

OO

S

OO

S

OO

S

OO

SO3 Na SO3 SO3 Na SO3 Na SO3 Na SO3 SO3 Na

+ +

Figure 6. The positively doped PEDOT having PSS as the counter ion.

2.2. Charge carriers The π-electrons in a neutral conjugated polymer chain are bound in the p-orbitals giving rise to an alternation of single and double bonds. At this state the conjugated polymer has typical semiconducting properties. To conduct electricity, charges have to be introduced into the polymer (removing or adding electrons). This results in the formation of positive or negative charges together with unpaired electrons in the polymer chain (Figure 7). The charge


8

SS

SS

SS +

Figu

and

together wpolaron ancouple, a so

The numbedoping invreduction bfilm and systems, thand reducti

Figure

Overla

forma

SS

SS

S

SS

SS

S+

e-

SS

SS

SSS

SS

SS

SSS

e-

SS

SS

SS

SS

SS

SS

re 7. The doping of the polymer and creation of a positive (left)

a negative (right) polaron.

ith the distortion of the structure of the polymer is denoted to as a d can be either negative or positive. When two polarons form a called bipolaron is generated with a charge of +2 (–2).

r of free charge carriers can be increased through doping. Chemical olves charging the polymer film through chemical oxidation or y dopant species. The dopant, with opposite charge, stays in the

balances the charge of the polymer. In many electrochemical e polymer film can be doped and dedoped reversibly via oxidation on.

8. Positive polaron and bipolaron in a conjugated polymer.

p between charge carriers at high doping levels results in the

tion of bands.


Inhomogeneity in doping level and morphology creates regions

Upon doping, the charge carriers alter the structures of the polymer and increase the length of the double bonds and shortens the single bond; 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 inside the band gap (Figure 8). As the doping level increases, the number of states inside the band gap increases. At high doping levels, the states inside the band gap start to overlap and create bands of bipolaronic states (Figure 8).

2.3. 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, the charge transport is not as fast as in inorganic crystalline semiconductors (I in Figure 9) because it involves the rearrangement of bonds in the polymer chains, which involves the movement of nucleus that are about 102 times slower than the electron reaBut still, the limiting step for charge transport is the hopping ofbetween chains and around defects present in the bulk (II in Figure

Figure 9. Tran

polaron in a lig

film can be divid

chain transpor

inter-chain hopp

In heavily doped polymer films, the polarons interact to form bandband gap promoting transport of the charge carriers in

Figur

tion o

charg

betw

sport of a

with poor

rrangement. charges in 9).

htly doped

ed in intra-

t (I) and

ing (II)

s inside the the bulk.

e 10. Charge transport in highly doped grains with a low concentra-

f conjugated polymer chains in between the grains. Inside the grains,

es are delocalized and migrate easily (I), while charges needs to hop

een the grains passing over the low-conducting phases (II).

9


10

2.4. Optical properties ncy υ if it includes a transition with

conductivity between the highly conducting regions. PEDOT:PSS films contain highly doped “metallic” grains surrounded by regions including low concentrations of PEDOT chains diluted in PSS (Figure 10). Even though the electronic conductivity is high in the grains, the charge transport is limited by the hopping from grain to grain.

A polymer can absorb a photon of frequean energy hυ. Because the optical band gap of most conjugated polymers are found in the region of 1.8 to 3 eV, visible light interacts with the π-electrons in conjugated polymers. In light emitting devices or solar cells, this interaction allows absorbing or emitting light in the visible range. The lowest energy that can be absorbed in an undoped polymer film reflects the edge of the optical band gap; this corresponds to a one-electron picture with an electron transition from HOMO to LUMO (Figure 11). As the polymer is doped, (bi)polaronic states appear within the band gap; thus allowing for absorption of photons

Figu

excit

ener

char

re 11. Absorption of light in polymers must correspond to

ation of an electron in the polymer chain. Because of different

gy diagram the absorption spectrum will be different for different

ge carriers.


11

2.5. CIE L a b color system ted or luminescent object is nothing

with much lower energy typically in the near infrared region (see Figure 11). For a highly doped polymer, the formation of (bi)polaronic bands allows for collective electronic excitations, like in metals, resulting in a wide absorption region in the infrared region. Because of the disorder in the polymer film and vibrational contributions, absorption peaks are not confined at one wavelength but appear as broad absorption peaks.

* * *

The photons that escape from an illuminamore 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 and transformed into nerve signals translated into a perception of color by the brain. Because humans have a three-dimensional color space, 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 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[5], corresponding to different biological receptors in the human eye (Figure 12).

Figure 12. The experimentally measured x , y and z , which corres-

pond to the sensitivity of the various receptors in the human eye.


12

as improved in 1976 to be able to further adapt to the way the human eye

Z need to be calculated first in order to estimate the Lcoordinates. Say that the light reflected or emitted from an o

The model w

perceives colors by introducing the CIE L*a*b* color system[5], 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* (see Figure 13). 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).

X, Y and

Figure 13. The

color system.

spectrum given by I (λ). Then X, Y and Z are given by:

( ) ( )( ) ( )( ) ( )∫

∫∫ ⋅⋅= λλλ dxIX

⋅⋅=

⋅⋅=

λλλ

λλλ

dzIZ

dyIY

where x , y and z are the experimentally computed spectra (Fithe X, Y and Z coordinate respectively. From the X, Y and Z the Lcoordinates can be calculated by correcting the coordinates witpoint: XN, YN and ZN in the following expression

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅=

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅=

−⎟⎞

⎜⎛

⋅=3

1

16116* YL ⎟⎠

⎜⎝

31

31

31

31

200

500

*

*

NN

NN

N

ZZ

YYb

YY

XXa

Y

These equations are valid for X/XN, Y/YN and Z/ZN greater than 0.00these equations, the color of an object, and also a polymer f

*, a* and b* bject has a

CIE L*a*b*

gure 12) for *, a* and b*

h the white

8856. With ilm, can be


13

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 light 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[5]. The difference between two colors can be defined as the distance in the color space, ∆E*, using ∆L*, ∆a* and ∆b* as the difference in the individual coordinates:

( ) ( ) ( )2*2*2** baLE ∆+∆+∆=∆

3. Electrochemistry

The doping level of conjugated polymers can be altered through reduction or oxidation of the polymer film in an electrochemical cell. As discussed in 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. These effects are 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 conducted between separate electrodes are denoted as 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 as a degree of freedom to control the reaction.

All electrochemical reactions are per-formed in some kind of electrochemical cell setup with at least two electrodes connected by a common electrolyte (Figure 14). For simplicity, consider the case of only two electrodes and one common electrolyte. A potential is applied between the electrodes and the

Figure 14. A

chemical ce

electrodes and

between. Th

applied so that

is reduced

electrode is ox

simple electro-

ll with two

an electrolyte in

e potential is

the left electrode

and the right

idized.

15

E L E C T R O C H E M I S T R Y

16

−−+−

−+−++−

current is measured to monitor the reaction in the cell (Figure 14). Each electrode and the electrolyte together 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 requires complementary oxidation, where electrons are generated, at the right electrode. At 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 the conjugated polymers is the formation of a positive polaron through oxidation:

+↔+ eXPXP :

Here, P denotes a 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 in the polymer (P+). The n-doping process is similar but here instead cations react with the electrode material at the same time as electrons are delivered to the material at the electrode. Films of PEDOT:PSS already contain the anion for 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, an electrical field is created within the cell. The ions, in the electrolyte, migrate to compensate for the field inside the electrolyte and are attracted by the electrodes with opposite charge. At equilibrium, the 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 is focused at the electrode/electrolyte interface. 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


17

electrolyte. The double layer is equivalent to a capacitance with a thickness of around ~3-5 Å, corresponding to the size of the ions. When there is a significant current through the cell, i.e. equilibrium is not reached, most, but not all, of the potential drop occurs very close to the electrodes. This gives that a part of the potential drop occurs inside the electrolyte.

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 film or by the diffusion/drift of ions of the electrolyte. This has been observed in the two extreme cases where in the first situation poly(3-hexylthiophene) (P3HT) is p-doped in acetonitrile electrolyte [6] and in the second, PEDOT:PSS is undoped using solidified gelled electrolytes (Figure 15). 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 down the film. Eventually the entire film will be oxidized, as the front moves across the film starting from the anode. PEDOT:PSS is doped in its pristine state and therefore highly conducting. Using a gelled electrolyte with low water content gives that the ionic conductivity is very low. In a cell including PEDOT:PSS as one of the electrode material and this low water content gelled electrolyte, the ion diffusion of the electrolyte entirely predicts 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.


18

Figure 15. a doping process of P3HT in aqueous electrolyte. b undoping

of PEDOT:PSS in a gelled electrolyte.

Because PEDOT:PSS is partly doped during synthesis, it is possible to further oxidize or reduce the film and therefore it 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 gives one oxidized species at the counter electrode. The doping fraction of pristine PEDOT:PSS films is estimated to be about 80% of the maximum doping level, for the films used in our lab. This means that the volume of the counter electrode should be at least five times larger than that of the working electrode to enable full reduction to occur at the working electrode[7]. 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 equipments. A simple model of conjugated polymers in electrochemical measurements is sketched in Figure 16 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.

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 17. 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 negative current is measured. Note that the


19

reduction potential is lower than the oxidation potential because of the relaxation described in Figure 16c where the energy level of the p-doped species have higher energy and therefore reduced at a lower potential than the oxidation.

Figure 16. 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.

Figure 17. 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.


20

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 or 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 measurement (LSV).

3.4. Electrolyte The ionic conduction medium between the electrodes is generally called an electrolyte and can be either liquid or solid. Common solvents used in liquid electrolytes are water or acetonitrile. Water is polar and can easily dissolve common salts, but its stable potential window is quite narrow preventing electrochemical measurements at higher potentials. Acetonitrile is more stable and offers a much wider potential window for the electrochemical measurements. Because acetonitrile is less polar, only salts with larger ions can be dissolved, for example tetrabutylammonium perclorate[8]. Liquid electrolytes are suitable for electrochemical measurements, but in most practical devices, solid electrolytes are a better choice due to the non-volatile nature of solids. Solid or gelled electrolytes are usually based on ions mixed in an ion-conducting matrix. An example of a solid electrolyte is NaPSS (poly(4-styrenesulfonate) with sodium as the counter ion). The negatively charged sulfonate ions are bound to the polymer chain (a poly-anion), which is immobile in the film. The main limitation with solid electrolytes is that their ionic conductivity is relatively low. In liquid electrolytes ions have both higher mobility and more concentrated electrolytes are possible, hence the ionic conductivity can reach very high values. The ionic conductivity of the solid electrolytes is often limited both by low ionic mobility and low concentrations of ions, resulting in a conductivity several orders of magnitude smaller than the conductivity of e.g. highly concentrated aqueous-based electrolytes.

3.5. Over-oxidation At low potentials electrochemical switching of conjugated polymers is reversible, giving that a large number of reductions and oxidations can be


21

done repeatedly. However, degradation of the electroactivity has been observed at anodic potentials in polythiophenes[9]. 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 therefore an electrochemically inactive film. In a study made by Barsch et. al. on the over-oxidation phenomenon of polythiophene films[10], 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 involves several steps that step-wise degrades the conjugation in the polymer (Figure 18). This mechanism suggests that oxygen binds to the sulfur atom in the thiophene ring and then SO2 is detached from the chain. Thus after losing the conjugation in the polymer chain, the backbone of the chain is ripped apart.

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 [11]. The fourier transform infrared spectroscopy (FTIR) measurements included in paper 3 supports this hypothesis.


22

Figu

Bars

Other formdegradationcombinatioconjugatedThis photogroup formor carboxywhere oxidUnder theswith time a

re 18. Over-oxidation reaction of polythiophene proposed by

ch et. al.[10]. © 1995 with permission from Elsevier.

s of degradation, besides electrochemical over-oxidation, lead to a of the conductivity of PEDOT films. Ultraviolet light in n with oxygen is known to induce photo-oxidation of the PEDOT material, resulting in a reduction in electrical conductivity. -oxidation leads to shorter conjugation lengths, due to sulfone ation and chain scission accompanied by the addition of carbonyl lic groups [12, 13]. Similar chemistry occurs upon heating in air, ative degradation of conjugated polymers has also been reported[14]. e conditions, the conductivity of PEDOT decreases exponentially ccording to an Arrenius law[15] type of behavior.

4. Applications of organic electronics

For more than 25 years, the field of conducting polymer has been developing. Major progress has been made in increasing the stability and functionality of materials for various applications. One very simple application where conducting polymers are used is as antistatic coatings, for example on the backside of a photo papers and foils[4]. A more sophisticated application is a new type of solar cells with organic materials that are being developed in several research groups. Even though the efficiency of the organic solar cells is not comparable with conventional inorganic solar cells, flexible substrates and large-scale manufacturing at low cost make organic solar cells interesting. Organic light emitting diodes (OLEDs)[16] offers displays with improved viewing angle, color contrast, brightness and the possibility of a much wider range of colors compared to conventional LCD technology. OLEDs can also be made in a partly printed production process with reduced production costs. Organic thin film field effect transistors (OFETs) can be used together with the OLEDs in active matrix displays to make high-resolution panels. OFETs are also employed in RFID tags for low-cost applications. Light-emitting electrochemical cells (LEC)[17] are slower than OLEDs, but can operate at a lower driving voltage and are less sensitive to film thickness and roughness making them easier to manufacture, especially using printing technologies[18]. Other electrochemical devices are electrochromic displays, electrochemical transistors and diodes. In electrochemical systems PEDOT is often used. When emulsions are made with the PSS as counter ion, an easy-to-process material is achieved. Films from the emulsion have good conductivity, which makes PEDOT:PSS films usable without a supporting conductor.

23

A P P L I C A T I O N S O F O R G A N I C E L E C T R O N I C S

24

4.1. Electrochemical transistors and logic The conductivity of PEDOT:PSS can be modulated by changing the doping level, as described above. In transistors this is utilized to electrochemically control the conductivity of the channel[19]. A simple design with three connectors (gate, drain and source) is used in a lateral (Figure 19) or vertical design. The channel starts at a high conducting state and becomes low conducting as a positive potential is applied on the gate. The transistor have been implemented in logic circuits like inverter, NAND, NOR and oscillator circuit[20]. These circuits can be combined to form more complex structures and manufactured in a roll-to-roll printing process. Low-cost logic circuits on flexible substrates manufactured at high speed/volumes enable simple electronics for smart packages, cheap one-time-use sensors and other applications where the cost is important and slow response acceptable.

Figu

desi

4.2. EleThe band gvisible wadecreased (ECD) theelectrochemelectronic PEDOT[21]

Acreo AB in collabo(Figure 20quite fast aThe contra

re 19. A sketch of an electrochemical transistor in a lateral

gn. © David Nilsson.

ctrochromic displays ap of many polymers is in the range of energy that corresponds to

velengths. As previously described, upon doping, the band gap is and the polymer becomes transparent. In an electrochromic display doping level of the polymer film can be controlled in an ical system, thus switching the color of the film using an

signal. One polymer that has received a lot of attention in ECDs is . The ease of processing PEDOT:PSS has encouraged scientist at in Norrköping to develop an all-organic display on paper substrate

ration with the organic electronic group at Linköping University ). Even though the blue to transparent switching of the PEDOT is nd can be used for simple displays the color switch contrast is low. st of monochromatic displays is measured in the difference in the


25

lightness of the display, ∆L*. A transmission display based on PEDOT:PSS has a contrast of ∆L*=15, while a reflective display of PEDOT:PSS has around ∆L*=19, which is far from the maximum contrast for a black to white switching display with ∆L*=100.

Figure 20. 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.

A high-contrast monochromatic display can be made by improving the contrast of a PEDOT:PSS display, exploiting the stability and ease of manufacture of PEDOT:PSS films. The absorption spectrum of PEDOT:PSS shown in a display in Figure 21 shows little absorption at wavelengths below 550 nm. One route to improve the contrast is to add another polymer on top of the PEDOT:PSS layer which gives additional absorption at lower wavelengths and makes the display cell to exhibit a color state closer black in the reduced state. The oxidized states of the additional polymer should be as transparent as possible so that the color state of the display is as white (transparent) as possible. This was examined in a study presented in paper 2.


26

0.35

0.40 Reduced Oxidized

Figu

The

the

The simreflectiomode dreflectioaffects developdifferentransmimode d

ECDs cupdatedbe updaother ndisplay is combaddressturned turned oThe aut

0.05

0.10

0.15

0.20

0.25

0.30

Abso

rptio

n

300 400 500 600 700 800 9000.00

Wavelength (nm)

re 21. The absorption spectrum of PEDOT:PSS film in an ECD.

ripples at the lower wavelengths are because of interference in

plastic film.

ple PEDOT:PSS display can be used in both a transmission and n mode. The difference lies in the path of the light. A transmission isplay is backlit while the light has to be reflected or scattered in a n mode display. The difference in the illumination of the displays how the eye measures the color of the pixels. At this stage of ment of the ECDs, the focus is on the material and therefore these ces are not considered. As the measurements are easier to perform in ssion mode, much of the work in paper 2 is based on a transmission isplay.

an also be used in matrix displays if each pixel can be individually . Just a matrix of display cells is insufficient because one pixel cannot ted without affecting the state or bleeding voltage signals to of the eighboring pixels. Andersson et. al. invented the electrochemical smart pixel in 2002 (Figure 22a) in which an electrochemical transistor ined with an electrochromic display in order to enable for individual ing of discrete pixels throughout a whole matrix. The transistor is on (high conducting channel) when the pixel is being updated, but ff (low conducting channel) when the pixel should store its state[22]. hors demonstrated a simple 4x10 active addressable matrix display


Figur

cell.

addre

Ande

InterS

shown in Factive addre

4.3. ProThe benefitlow envirocan be procbeen democommon artechniques resolutions solar cellshomogenouadditive propatterning twhich a chmaterial[28].hazardous r

27

e 22. a The smart pixel consisting of a transistor and a display

b a 4x10 displays that shows “ITN”. c New design on the active

ssable matrix displays with higher fill factor (Photo Peter

rsson). Figure a and b are © 2002 with permission from Wiley

cience.

igure 22b. Further work[23] has resulted in a fully printable, organic ssable matrix display with improved fill factor (Figure 22c).

duction of organic electronics s of organic electronics include new functionality, materials with nmental impact and the fact that the organic electronic materials essed using printing tools. Today, many different techniques have

nstrated for printing and patterning conducting polymers, the most e the inkjets[24, 25] and screen-printing technology[26]. These additive can be used to print polymers and to create patterns down to of around 100 µm, which is enough for large area displays[24] and [27]. Coated polymer films often possess better and more s solid-state properties compared with those printed through cesses. The coated film can then be patterned through subtractive echniques such as chemical deactivation at the polymer surface in emical agent converts the conjugated material to a non-active

These techniques are typically very slow and often require eagents, but most importantly the pattern resolution is limited by


28

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 that offers an electrically controlled deactivation method that can be used together with photolitho-graphy 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 conductive and insulating parts of a patterned film can reach 107, which is enough for various simple electronics with relatively low packing density.

Figure 23. Nilpeter printing press used to print organic electronics.

© Niclas Kindahl, Fotofabriken.


29

In the recent years Acreo AB in Norrköping has, in collaboration with Linköping University, developed a process for Nilpeter printing press (Figure 23) 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 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 50 µm at around 5 meters per minute. In the next 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 it can produce devices at 5 meters per minute producing around 50 million devices (10x10 cm) each year.

5. References

1. Chiang, C.K., J. C. R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau and A.G. MacDiarmid, Physical review letters, 1977 39 (17) p. 1098-1101.

2. Nobel Prize. [cited 30 January 2006]; Available from: http://nobelprize.org/chemistry/laureates/2000.

3. Kittel, C., Introduction to solid state physics. 7th ed. ed. 1996: John Wiley & Sons Inc.

4. Groenendaal, B.L., F. Jonas, D. Freitag, H. Pielartzik and J.R. Reynolds, Advanced Materials, 2000 12 (7) p. 481-494.

5. Berns, R.S., Billmeyer and Saltzmann Principles of Color Technology. 3rd ed. 2000: John Wiley & Sons, Inc.

6. Johansson, T., N.K. Persson and O. Inganas, Journal of the Electrochemical Society, 2004 151 (4) p. 119-24.

7. Larsson, O., Empirical parameterization of organic electrochemical transistors, in ITN. 2004, Linköpings University: Norrköping.

8. Johansson, T., W. Mammo, M. Svensson, M.R. Andersson and O. Inganas, Journal of Materials Chemistry, 2003 13 (6) p. 1316-1323.

9. Ahonen, H.J., J. Lukkari and J. Kankare, Macromolecules, 2000 33 (18) p. 6787-6793.

10. Barsch, U. and F. Beck, Electrochimica Acta, 1996 41 (11-12) p. 1761-1771.

11. Zotti, G., S. Zecchin, G. Schiavon and L.B. Groenendaal, Chemistry of Materials, 2000 12 (10) p. 2996-3005.

12. Crispin, X., S. Marciniak, W. Osikowicz, G. Zotti, A.W.D.v.d. Gon, F. Louwet, M. Fahlman, L. Groenendaal, F.D. Schryver and W.R. Salaneck, Journal of Polymer Science Part B: Polymer Physics, 2003 41 (21) p. 2561-2583.

13. Marciniak, S., X. Crispin, K. Uvdal, M. Trzcinski, J. Birgerson, L. Groenendaal, F. Louwet and W.R. Salaneck, Synthetic Metals, 2004 141 (1-2) p. 67-73.

14. Mohammad, F., P.D. Calvert and N.C. Billingham, Synthetic Metals, 1994 66 (1) p. 33-41.

15. Rannou, P. and M. Nechtschein, Synthetic Metals, 1999 101 (1-3) p. 474.

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http://nobelprize.org/chemistry/laureates/2000

16. Burroughes, J.H., D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature, 1990 347 (6293) p. 539-541.

17. Pei, Q., G. Yu, C. Zhang, Y. Yang and A.J. Heeger, Science, 1995 269 (5227) p. 1086-1088.

18. Add-Vision. [cited 30 January 2006]; Available from: www.add-vision.com.

19. Nilsson, D., M. Chen, T. Kugler, T. Remonen, M. Armgarth and M. Berggren, Advanced Materials, 2002 14 (1) p. 51-54.

20. Nilsson, D., N. Robinson, M. Berggren and R. Forchheimer, Advanced Materials, 2005 17 (3) p. 353-358.

21. Pei, Q., G. Zuccarello, M. Ahlskog and O. Inganas, Polymer, 1994 35 (7) p. 1347-1351.

22. Andersson, P., D. Nilsson, P.-O. Svensson, M. Chen, A. Malmström, T. Remonen, T. Kugler and M. Berggren, Advanced Materials, 2002 14 (20) p. 1460-1464.

23. Andersson, P., Electrochromic Polymer Devices: Active-Matrix Displays and Switchable Polarizers, in ITN. 2006, Linköpings University: Norrköping.

24. Bharathan, J. and Y. Yang, Applied Physics Letters, 1998 72 (21) p. 2660-2662.

25. Sirringhaus, H., T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu and E.P. Woo, Science, 2000 290 (5499) p. 2123-2126.

26. D. A. Pardo, G.E. Jabbour and N. Peyghambarian, Advanced Materials, 2000 12 (17) p. 1249-1252.

27. Krebs, F.C., J. Alstrup, H. Spanggaard, K. Larsen and E. Kold, Solar Energy Materials and Solar Cells, 2004 83 (2-3) p. 293-300.

28. Agfa-Gevaert, Patterning Orgacon™ film by means of UV lithography, Guidelines. 2001.

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electrochromism and over-oxidation in conjugated polymers

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