And Ion-Conductive Polymers in Electrochromic Devices Linköping Studies in Science and Technology Applied Physics Thesis No

Department of Phvsics and Measurement Technology

Linköping University Department of Physics and Measurement Technology S-5S1 S3 Linköping, Sweden Electron- and ion-conductive polymers in electrochromic devices Linköping Studies in Science and Technology Applied Physics Thesis No. 401 Catarina Gustafeson IiU-TEK-LIC-1993:43 ISBN 91-7871-181-9 ISSN 0280-7971 Preface

This licentiate thesis is based on work performed in the Conducting Polymer Group, Laboratory of Applied Physics at the Department of Physics, Linköping University, between august 1990 and november 1993. It contains an introduction to the chemistry and physics of electronically conductive polymers, ion-conductive polymers, and electrochromic materials, and the following papers:

Paper 1 Heterocyclic conductive polymers as electrode materials in solid state electrochromic devices J. C. Gustafsson, O. Ingåräs, A. M. Andersson; Synthetic Metals (in press)

Paper 2 Spectroscopic evidence for asymmetric polaron states in pory[3-(4-octylphenyl)- thiophene] J. C. Gusti 'sson, Q. Pei, O. Inganäs; Solid State Commun. 87 (1993) 265

Paper ' In situ ^ rf roscopic investigations of electrochromism and ion transport in a pory(3, ,i Jf /lenedioxythiophene) electrode in a solid state electrochemical celL

J. C. Gu k; fsson, B. Liedberg, O. Inganäs; Submitted to Solid State Ionics Contents Page

1. Electronically conductive polymers 4 1.1. Historical remarks 4 1.2. Chemical and electronic structure 5 1.3. Charge transport mechanism 8 1.4. Electrochemistry 8

2. Ion-conductive polymers 11 2.1. Introduction 11 2.2. Transport mechanisms and polymer architecture 11

3. Electrochromic materials 14 3.1. Introduction 14 3.2. Coloration mechanisms 15 3.3. Electrochromic devices 17

4. Summary of the papers 20

5. Acknowledgement 21

6. References 22

PAPERS 1. Heterocyclic conductive polymers as electrode materials in solid state electrochromic devices 2. Spectroscopic evidence for asymmetric polaron states in poly[3-(4- octylphenyl)-thiophene] 3. In situ spectroscopic investigations of electrochromism tnd ion transport in a poly(3,4-ethylenedioxythiophene) electrode in a solid state electrochemical cell 1. Electronically conductive polymers

1.1 Historical remarks Polymers have traditionally been considered as thermally and electrically insulating, but in the late 1970's Shirakawa and his co-workers [1-3] found out that by letting polyacetylene react with either an electron-acceptor or an electron-donor, an increased electronic conductivity could be achieved. These reactions were called p- and n-doping, respectively, using traditional semi- conductor terminology. During the following years it was found that other conjugated polymers, like poly(p-phenylene) [4], polypyrrole [5-7], polythiophene [8-10] and polyaniline [11] also could be chemically or electrochemically doped to higher conductivity.

polyacetylene

poly(p-phenylene)

polypyrrole

polythiophene

H ff-\ -HN _ polyaniline

Fig.l: Some conjugated polymers

The conjugated polymers were at this early stage believed to provide a way to combine the mechanical properties of ordinary plastics (flexibility, processability, toughness, etc) and the electrical properties of metals. But as research continued, new problems came to light. The conjugated materials mentioned above are not processable, they are not stable in air and they are usually very brittle. Also, there are several questions about the conduction mechanism in these materials. How do they conduct electricity? Does the anisotropic chemical structure have a correspondence in the electrical properties? What is the maximum conductivity and how can it be achieved? The problem with poor processability was overcome by adding alkyl side chains to polythiophene to obtain a material that is fusible and soluble in com- mon organic solvents [12-14]. Unfortunately, interaction between the attached side chains increases the rate of thermal undoping [15], so gain in one property means loss in another. In order to study the anisotropy in electrical properties and models for the transport mechanisms, a material with a high degree of order has to be prepared. That can be done by stretch orienting the polymer with or without a support, using a precursor polymer that can be stretched before conversion [16- 18] or using a template that gives the polymer a more ordered structure [19,20]. Also, by improving the routes of synthesis a type of polyacetylene with fewer sp3 defects has been developed at BASF [21-23], showing a conductivity of about 10 S/cm, which is comparable to that of copper.

There are several products utilizing conjugated polymers on the market, see for instance the review by Schoch and Saunders [24]. Conjugated polymers will certainly be used in many applications in the future. There are promising results indicating practical use of conjugated polymers in the fields of electronic devices [25-27], LED's (light emitting diodes) [28-32], different sensor applications [33-35] and electromagnetic shielding [36-38].

1.2 Chemical and electronic structure Conducting polymers can be divided into two types; those with a degenerate ground-state (the double and single bonds can be interchanged with no cost in

energy), and those where the ground-state degeneracy is broken. Trans-(CH)X has a degenerate ground-state, while cis-polyacetylene, poly(p-phenylene) and

Polyacetylene Polypyrrole

Benzenoid form

Quinoid form

Figure 2. Ground state configuration a: Trarcs-polyacetylene: degenerate ground-state. The two forms are equivalent and have the same energy. b: Polypyrr•:•!«: non-degenerate ground state. The benzenoid form has lower energy. polyheterocyclics (polypyrrole, polythiophene etc.) have a non-degenerate ground-state. For example, in polypyrrole the quinoid form has a higher energy than the benzenoid form.

Polyacetylene is the simplest of all conjugated polymers and also the one most studied. It is often used as a model for all conjugated polymers due to its simplicity. It consists of weakly coupled chains of CH units. Three of the carbon valence electrons are in sp2 hybridized orbitals; two of the o-type bonds con- struct the bond to the neighboring carbon atoms, while the third forms the bond with the hydrogen atom. These three orbitals are as far apart as possible

Fig.3: Solitons in polyacetylene and the associated electronic state in the band gap a) Positively charge soliton b) Neutral soliton c) Negatively charged soliton

in space, resulting in 120° bond angle. This can be satisfied by two possible

arrangements of the carbons,

valence electron belongs to a 2pz orbital and forms a n -bond together with the corresponding electron from a neighboring carbon atom. The carbon-carbon bonds are not equally long, due to Peierls distortion, in which adjacent CH groups move towards each other, forming alternately short (or double) C=C bonds and long (or single) C-C bonds. This dimerization lowers the electronic energy of the system by opening an energy gap at the Fermi level, separating the highest occupied molecular orbital states from the lowest unoccupied molecular orbital state [39].

The soliton in (CH)x is a bond alternation domain wall connecting phases with opposite bond alternation. Calculations [40] have shown that the defect is actually spread over a region of about 14-20 CH units. Associated with the structural defect is a localized electronic state in the middle of the band gap. This electronic state can accommodate 0, 1, or 2 electrons. Figure 3 shows the neutral soliton, which has one electron in the mid-gap state, the positively and the negatively charged solitons with 0 and 2 electrons, respectively.

For polypyrrole the two structures shown in fig.2 are not energetically equivalent. By viewing polypyrrole as a cis-polyacetylene chain, stabilized by the NH-group, one can make use of a model suggested for cis-polyacetylene [41]. The charge injected into polypyrrole upon doping, is stored as polarons or bipolarons depending on the doping level, see fig.4. At low doping levels,

Fig.4: Charged defects of doped polypyrrole. The electronic states in the band gap and the associated optical transitions. a) Polaron in polypyrrole b) Bipolaron in polypyrrole when the defects are not interacting, doping gives rise to singly charged pc- larons with two localized electronic states in the band gap. For a hole polaron created upon p-doping of the polymer, the lower level is half filled and the upper one is empty, allowing four subgap optical transitions, two of them degenerate. At higher doping levels, the probability for recombination of two polarons into one bipolaron is increased. Also, doubly charged bipolarons can be created by direct electron transfer from polarons to dopant molecules. For a hole bipolaron, the two levels are empty and only two subgap transitions are possible.

By letting light pass through a thin film of the polymer and measuring the absorption as a function of wavelength, a good picture of the band structure of the polymer can be obtained [42]. Light with energy matching the energy differences between allowed electron's states of the polymer will be absorbed, causing excitations.

1.3 Charge transport mechanisms During the years several mechanisms for charge transport in conjugated polymers have been suggested. Especially the interchain transport mechanism has been a subject of intense studies. It has been found that different transport mechanisms dominate at different doping levels. Undoped or lightly doped conjugated polymers show a decreasing conductivity as the temperature drops. This has been explained by charge transport through variable-range hopping [43]. In this model, transport occurs by hopping among soliton-like states near the Fermi level. The hopping probability depends on both the energy difference between the initial and the final state and the overlap of the wave functions. Hence, if localization is not very strong, hopping can occur over larger distances. When the temperature is decreased, the thermally activated electron has lower energy. The number of reachable states is therefore reduced and the conductivity decreases. Kivelson introduced the idea of transport by inter-soliton hopping, in which a free neutral soliton located near an impurity can jump to a charged soliton site [44], while Chance et al. developed a model for charge transport based on bipolaron hopping [45]. It has been suggested that small conducting islands are formed in heavily doped conducting polymers. The islands are separated by resistive barriers consisting of weakly doped chains and conjugation defects, through which the electrons have to tunnel [23].

1.4 Electrochemistry Electrochemical methods have shown to be excellent tools for both synthesis and characterization of conjugated polymers. Also, in many applications, like batteries and electrochromic displays, it is the electrochemical aspects of these materials that are utilized. 9 Galvanostatic, potentiostatic and potentiodynamic techniques can be used to electropolymerize monomers and form a conjugated polymer film on a conducting substrate. Usually, the electropolymerisation is carried out in a one- compartment cell with a classical three-electrode configuration; a working electrode, a reference electrode and a counter electrode (fig.5).

Working electrode

Electrolyte with monomers

Figure 5: A classical three-electrode cell for electropolymerisation of conjugated polymers. M=monomer, A"=anion of the supporting electrolyte, C+=cation of the supporting electrolyte

The polymerisation process is initiated by the electrochemical formation of a cation radical M+ achieved by electron transfer from the monomer M.

M -> M+ + (I)

The evolution of the polymerization is depending on the stability of this cation radical and the continuous generation of another cation radical at the anode surface. The polymerization is propagating by diraer formation followed by de- protonation and further oxidation of the dimer D, which has a lower oxidation potential than the monomer [46].

M+ + M -> (M-M)+ -^ D + 2H+ + (ID

At each step of the electropolymerisation reaction, propagation is dependent on the presence of cation radicals in the vicinity of the working electrode surface. 10 Therefore, the electrode potential has to be maintained at the electrochemical oxidation potential of the monomer in order to maintain a tfux of cation radicals. The polymer is produced in its doped state having positive charges distri- buted along the chains in the form of polarons or bipolarons. Anions of the supporting electrolyte are therefore incorporated into the polymer film as charge compensators.

There are several electroanalytical methods to characterize conjugated polymers. For example, cyclic voltammetry, in which the applied cell potential is swept and the current flowing through the cell is being recorded. Informa- tion about maximum doping level and electrochemical stability can be obtained using this technique. By varying the sweep-rate in cyclic voltammetry know- ledge about the electronic and ionic transport processes associated with the doping and undoping of the polymer can be obtained too. Generally, with thin poiymer films, the peak current varies linearly with the sweep-rate as if the charges were adsorbed on the surface of the polymer film. When thicker films are used, the peak current varies with the square-root of the sweep-rate, indicating that the charge transfer is governed by the diffusion of electrons and/or ions in the polymeric matrix. More information about the charge transport in the polymer film, may be given by chronoamperometry. In this, the polymer is brought to a potential at which the charge transfer may occur and then, the current decay upon time is analyzed using the Cottrell equation

i(t)=nFAC<\/ — (III) \ t where i(t) is the current as a function of time, n is the number of electrons, F is Faraday's number (96485 As/mol), A is the polymer film area, C is the concen- tration of electroactive sites and D is the apparent diffusion coefficient of the charge carriers.

An alternative method to determine the apparent diffusion coefficient is the pulsed current method [47], that consists in applying a very short anodic pulse (50ms) of desired amplitude to a very thin reduced polymer film. The current pulse causes formation in positive sites in a limited amount in order to keep the polymer in a quasi neutral state and avoid problems related to the switching of the film from the neutral to the doped and conducting state. 11 2. Ion-conducting polymers

2.1 Introduction It was shown in 1966 [48, 49] that complexes between polyethers, like poly(ethylene oxide) (PEO) and polyCpropylene oxide) (PPO), and alkali metal salts could be formed. Further work performed in Sheffield [50,51] and Grenoble [52] lead to that these complexes were suggested for use as solid electrolytes, mainly in solid-state batteries, but also in electrochromic devices and different sensor applications.

Polymer electrolytes are formed by dissolving a salt in an ion coordinating polymer. For the dissolution to occur and produce a homogeneous solution, the interaction between the coordinating atom on the polymer chain and the ions of the salt must be strong enough to overcome the lattice energy of the salt and the loss of conformational entropy as the polymer chain reorients to form a more ordered structure when complexing with the ion. Typically, the polymers forming salt complexes contain oxygen, nitrogen or sulphur; electron donor atoms that can provide the required interaction with the cation of the salt. It is also important that the polymer chain has a suitable distance between coordinating sites and that it can adopt conformations that allow multiple inter- and intramolecular ion bonds. For example, poly(ethylene oxide) can adopt a helica' conformation with an oxygen-lined cavity that provides the optimum distances for oxygen-ion interactions [53].

2.2 Transport rngghanisms and polymer architecture In the most general terms, the conductivity of a homogeneous polymer electrolyte phase at any temperature T, may be written as

where n; is the density of charge carriers of type i, qj is the charge on each and Uj is the mobility. The summation has to be carried out over all charged species, simple ions as well as mobile charged clusters, that are formed especially at intermediate and high salt concentrations [54].

When a salt is dissolved in a polymer matrix, the conductivity at first increases with the amount of added salt. Upon addition of more salt, the conductivity reaches a maximum and then falls [55,56]. This can be explained briefly by considering the salt ions as transient crosslinks [57], reducing the segmental motion of the polymer and, thus, also the mobility uf the ions. At 12 low salt content the mobility of the ions is relatively independent of the con- centration and therefore, the conductivity is controlled by the number of charge carriers, ions. Thus, if more salt is dissolved, there will be more charge carriers and the conductivity will increase. At high salt content, the increase in the number of charge carriers is more than compensated and by the stiffening of the matrix and the associated reduced mobility of the ions. It has also been suggested, that at high salt concentrations, formation of immobile aggregate species v/ill lower the number of real charge carriers and contribute to the fall in conductivity.

By using nuclear magnetic resonance (NMR) techniques, Berthier et al. showed, that it is the amorphous part of the polymer that is responsible for the ionic conductivity [58]. Due to their high degree of crystallinity, PEO- based electrolytes generally show low ionic conductivity at room temperature (<7<108 S/cm). Using an all amorphous polymer is one way of enhanc- ing the ionic conductivity and some work has been done on amorphous PEO [59-62]. Poly(propylene oxide), which does not crystallize to form non-conducting regions, has also been studied [55,63-65]. An alternative is to use small amounts of solvent or low molecular weight polyethers as plasti- cisers [66,67]. This has proven to enhance the ionic conductivity, but also leading to problems like softness and liability to creep. By blending a mechanically stable polymer such as polystyrene with PEO on a macro- scopic or microscopic level, materials with reasonable conductivities and improved mechanical properties were produced [56,68]. Also, addition of small amounts of inert fillers like a-alumina has shown to give a signifi- cant improvement in the mechanical stability at elevated temperatures [69]. An alternative way could be crossl; king the electrolyte by y-irradiation. A small reduction in the conductivity has been observed in crosslinked electrolytes, possibly due to restricted backbone segmental motion [70].

Polymer electrolytes are usually studied well above their glass transition temperature. The materials are locally liquid-like and there are many similarities between the mechanisms of conduction in polymer and liquid electrolyte solutions. But there is one fundamental difference: the solvent is mobile in a liquid electrolyte and immobile due to entanglements in the polymer electrolytes. Thus, the polymer electrolytes can be considered as extremely viscous fluids, where the transport depends on local viscosity. 13 The free volume model [71] is often used to explain the transport mechanism of polymer electrolytes. It assumes that molecular transport occurs whenever a local void is available with volume sufficient so a par- ticle can move into it. This is expressed in the Vogel-Tammann-Fulcher (VTF) equation

a(T) = Coe-Ta^-V (V)

l/2 where o is the conductivity, ao=AJT , A is a constant, To is the reference temperature at which the free volume is zero, and TB is a constant temperature. According to the VTF equation, by lowering the glass transition tempera-ture a higher conductivity could be achieved. For example, poly- phosphazene [72,73] and poly(dimethylsiloxane) [74,75] have been used together with PEO in several copolymers showing enhanced ionic conductivity. One problem with siloxane based electrolytes is that the Si-O-C bonds between the polysiloxane and the polyether segments are sensitive to hydro- lysis, causing structural degradation. Lestei et al. tried to avoid this problem by forming Si-C linkages instead [76].

Microscopically, the free volume theory has several disadvantages, the most important being that it does not relate to the specific structures of either host or salt, and that it contains no kinetic components. The dynamic bond percolation theory [77,78] expresses the probability for ion motion if free volume or coordination sites are available, considering a coulombic liquid. In ordinary, static percolation theory [79] the probability of jumping between two sites is assumed constant. In dynamic bond percolation theory, the probability is modified by the segmental motion of the polymer host, so that an ion pathway that is blocked at one time may become open at a later time. This model can be used to explain the frequency dependence of the conductivity as well as its static value and it relates conductivity to host segmental motion. One limitation of this theory is that no ion-ion interactions are included.

In ordinary polymer electrolytes like complexes of PEO and PPO, both anions and cations are free to move [80,81]. Their main application is in solid state batteries and it has been noticed that the ionic conductivity decreases upon continued application of a dc voltage. This is due to the localization of anions at the anode hindering the alkali-metal ions to leave the electrode. Thus, single-ion conductors are required. This could be achieved by immobilizing either the anion or the cation by using a poly- 14 electrolyte, in which the ionic charge is bounded to the polymer backbone. Work on two different polyelectrolytes mixed with PEO showed lower ionic conductivity for these systems than for ordinary PEO electrolytes [82]. The low conductivity was explained by ion pairing, which is always likely to occur in polyelectrolytes. To prevent or at least reduce ion-pairing, several attempts have been made to introduce sterically hindering groups, preventing close approach of the anion [83-85].

3. Electrochromic materials

3.1 Introduction A chromogenic material has optical properties which can be changed by one or several external factors, like application of an electric field, radiation of light with a specific spectral composition, change of temperature, or exposure to a specific chemical substance. Usually the optical changes are divided into two groups: electrically activated and non-electrically activated. Examples of electrically activated materials are electrochromic materials and liquid crystals. There are many applications for chromogenic materials: architectural glazings, automobile sun roofs, information displays, and mirrors. Already on the market are photochromic sunglasses and automobile mirrors, which regu- late glare automatically.

Electrochromism is defined as a reversible switching of a material from a colored to a transparent or differently colored state, caused by an applied field or current. The change can be due to formation of color centers or to an electrochemical reaction producing a colored compound. Electrochromism was first studied by Platt [86], who discussed field-induced spectral shifts in pyridine dyes.

Electrochromism has been observed in a variety of materials, that can be divided into three general classes: 1) transition-metal oxides (primarily hydrous) 2) organic electrochromic materials 3) intercalated electrochromic materials

Most of the work performed has been on transition-metal oxides, especially tungsten oxide, WO3 [87-91], but also on nickel oxide, NiO, [92-95], V2O5 [96-98], indium oxide, IrOx, [99,100] and mixtures of metal oxides [101]. Among the organic compounds, viologens [102-104], pyrazoline [105,106], lanthanide phthalo- 15 cyanines [107-110], ferric ferrocyanide (Prussian Blue) [111], tetrathiafulvalene [112, 113] and conjugated polymers like polypyrrole [114], polyaniline [115-119], polythiophene [114,120] and poly(isothianaphthene) [121,122] exhibit electrochromism. Intercalated graphite has also been investigated [123]. Tables I-III show a summary of materials and their color changes [124] .

Table h Inorganic Electrochromic Materials: Cathodic (Colored by reduction) Tungsten oxide (WO3) Transparent<-»Blue Molybdenum oxide (M0O3) Yellow<->Grey/purple Titanium oxide (TiO2) Transparent<-»Blue Niobium oxide (Nb2Os) Transparent<-»Bronze Vanadium oxide (V2O5) Yellow<-»Blue/Grey/Black

Table II: Inorganic Electrochroniic Materials: Anodic (Colored by oxidation) Nickel oxide (NiO) Transparent<-»Bronze Indium Oxide (IrOx) Transparent*->Black Rhodium ox-'de (Rh2O3) Yellow<-»Green/brown/purple Cobalt oxide (CoOx) Red«->Purple/grey/black

Table ELL Organic Electrochromic Materials Viologens Transparent<-»Blue/violet/red Conductive polymers Polyaniline TransparenU-*Green/blue/purple Polypyrrole Yellow<->Black Polythiophene Red<-»BIue Poly(isothianaphthene) Black<-»Transparent Anthraquinones Red<-^ Blue/green Diphthalocyanines Green<-»Red/violet/blue/purple Tetrathiafulvalenes Yellow<-»Green/purple Pyrazoline Yellow<-»Blue/green Intercalated graphite Black<-»Blue/green/yellow

3.2 Coloration mechanisms Amorphous and polycrystalline films of tungsten oxide (WO3) show electrochromism (see fig.6). They can be converted from transparent into deeply- + + + +> + 2 colored MxW03, where M is H ,Li , Na , K Cs or Mg *, by ion injection from a suitable electrolyte.

The coloration mechanism of transition metal oxides has been up for debate since it first was discovered. Several theories and models have been suggested, in some cases more than one for each material. The reason for this hand- waving is that most of the work done has been on applications more than on the theory behind the phenomena. See however [125], in which an attempt to connect the optical properties to microstructure and electronic bands is made. 16

(eV)

Figure 6: Optical absorption spectra for amorphous WO3. a) As prepared WO3 , b) Lithium intercalated WO3

The organic electrochromic reactions can be divided into three types: 1) A simple redox reaction which gives a colored species. 2) A redox reaction coupled with a chemical reaction converting the material back to the uncolored state. 3) A redox reaction with a chemical reaction producing an insoluble colored species

In applications, the drawback of reaction 1 is that the colored species tend to drift or diffuse from the electrode structure causing diffuse colors. To avoid this problem, the electrolyte can be stabilized, for example in a gel. For type 2, color drift is eliminated by the conversion back to the uncolored material. This means that, the coloring reaction has to be run continuously, reducing the in- trinsic efficiency. The insoluble colored species of type 3 enhances the stability of the device, so that the coloration can be achieved by a short current pulse and is then maintained until the device is switched back to the transparent state. This makes the type 3 materials the best choice when constructing an electrochromic cell. The desired color stability can also be achieved if the electrochromic molecules are forming a thin insoluble film on the electrode, either by themselves or by attaching them to a polymer backbone [113,126].

Films of conjugated polymers also exhibit electrochromism, and in these materials the phenomena is very well understood. Upon doping, a conjugated polymer undergoes a transition from a semi-conducting to a metal-like state. The new electronic states introduced give rise to optical transitions at energies 17 below the bandgap (see figure 4), hence changing the optical properties [127]. By controlling the molecular structure, a polymer with a desirable band gap and specific optical properties may be designed.

0.5 1.5 2.5 3.5 eV

Figure 7: Optical absorption spectra for poly(3-octylthiophene). a) Neutral state b) Doped state. Doping level -30%

Fig. 7 shows the optical absorption spectrum of poly(3-octyl thiophene), (POT). In the neutral state POT is red, due to the bandgap absorption starting at 2eV and peaking at 2.6eV. Doping results in formation of bipolarons, with two associated optical transitions in the red region of the visible range. Hence, the doped polymer looks blue in transmission. Polypyrrole is black in the doped state and yellow in the neutral state. The principle is the same as for polythiophene; the optical absorption is shifted from the bandgap to lower energies upon doping. PolyGsothianaphthene) [121,122] is made by attaching a benzene ring to the thiophene monomer. This results in shifting the maximum absorption from the blue region to the center of the visible range. Thus, the neutral polymer is black, and becomes transparent upon doping due to the induced subgap ab- sorptions.

3.3 Electrnchrnmic q, The use of information displays are getting more and more important in our daily life and the traditional display technologies (cathode ray tubes [CRT]) can not serve all display needs and requirements. Thus, new display technologies are always interesting. Liquid crystal displays, d-c plasma panels and LED's are three successful alternatives to the CRT technology. Also, there are several concepts of display devices based on electrochemistry or electro-chemical effects: • the electrophoretic display [128], which changes colour as a result of electrophoretic migration of dye particles in the electrolyte • the electrochemiluminescent display [129], that produces light by recombination in the electolyte of two species reduced and oxidized at the electrodes • the fluorescence quenching display [130], that changes colour when the fluorescent ions or molecules are reduced or oxidized at one of the electrodes to nonfluorescent species • the electro(chemi)chromic display, in which the colour change is a result of an electrochemical redox reaction taking place at or near one of the electrodes The most attractive of these systems is the electrochromic display device because it has fewer chemical system constraints, simpler device operation, better memory capability and high contrast under bright conditions.

An electrochromic device consists of several layers of materials with different properties. Most devices can be described according to figure 8.

Conductive layer o . , . , , Counter electrode Conductive layer Electrolyte

Glass" Electrochromic layer

Figure 8: Schematic structure of an electrochromic device

Two general types of electrochromic devices can be distinguished. The trans- missive devices, in which all layers have to be transparent in the bleached state, and the reflective devices, in which the reflectance is modulated by the electrochromic reactions. When choosing materials for an electrochromic device, there are some requirements that have to be regarded. The electrochromic materials should provide the desired color change, the switching should be fast enough and stable upon repeated switching, perhaps up to 107 times. The electrolyte should have high enough ionic conductivity to ensure fast switching and it should have good interfacial behavior and high mechanical stability. The solid phase is bet-ter suited for large-area window applications because of the reduced risk 19 of leakage and is preferred. The counter electrode material could be electrochromic too, but then the two electrochromic reactions of the device would have to be complementary. The two electrodes should be conductors for electrons as well as for ions and all layers would have to be chemically stable upon exposure to UV light, and possible to prepare in the form of thin films.

There are several prototypes for solid-state electrochromic devices reported in the literature [131-139]. Some include conductive polymers like polyaniline, but most of the work is on tungsten oxide and other transition metal oxides. 20 4. Summary of the papers

4.1 Paper 1 A solid state electrochemical cell consisting of a conjugated polymer electrode, a solid polymer electrolyte and a transition metal oxide electrode was con- structed. By changing the applied cell voltage, the optical absorption of the cell could be changed. The polymers used in this work have the advantage of easy fabrication; spin-coating or solution-casting for poly(3-octylthiophene) and template polymerisation for polypyrrole. Due to the difference in coulombic capacity and coloration efficiency between the conjugated polymers and the transition metal oxide, the optical changes of the cell are mainly due to optical changes of the polymer. Hence, a solid state electrochemical cell can be used for studying the doping process of conjugated polymers, and the stability of the doping induced states. A drawback is that no absolute electrode potentials can be determined, due to the lack of reference electrode in the system.

4.2 Paper 2 The solid state electrochemical cell, described in paper 1, is used for optical characterisation of the conjugated polymer polyf3-[4-octylphenyl)-thiophene]. In this polymer, the benzene ring acts as a rigid separator between the conjugated backbone and the flexible alkyl side chain. At low doping levels this new polymer of the thiophene family exhibits asymmetric polarons, while poly(3- alkylthiophenes) only show bipolarons, even at low doping levels. Coupling between the main chain n system and the aromatic ring in the side chain is suggested to be the cause of the stabilisation of the polarons and the broken sym- metry in this material.

4.3 Paper 3 Attempts are made to use in situ Raman spectroscopy for identification of the ions moving into and out of a poly(3,4-ethylenedioxythiophene), (PEDOT) polymer electrode in a solid state electrochemical cell. Since poly(styrene sul- fonate) (PSS") is used as charge compensating counter ion, the main idea is that Li+ ions should move into the PEDOT electrode upon reduction of the polymer, and associate with the SO3" groups of the PSS anions. Thus, shifts in the SO3* vibrations are expected. Unfortunately, the Raman signal from the conjugated material is strong enough to dominate the cell spectra, preventing the identification of the moving ions. But this also means that the solid state electrochemical cell can be used for in situ Raman studies on conjugated polymers during electrochemical doping. 21 An estimation of the diffusion coefficient for the ionic charge carriers in the PEDOT electrode gave a remarkably high value, approximately three orders of magnitude higher than value reported for other conjugated polymers. So far no good explanation of this unexpected improvement on the ionic conduction mechanism in this new polymer has been found. Results of optical absorption spectroscopy during doping/undoping of PEDOT acting as an electrode in a solid state electrochromic cell are presented. The results indicate that PEDOT is well suited for applications in electrochromic displays. The change in optical absorption in the visible region is appropriate for a smart window and the required applied voltage is small (±1.5V). The switching time at room temperature, from fully coloured to fully bleached is about 4 seconds and the stability upon repeated switching is very good. Also, the coloration efficiency of the PEDOT material is about 10 times higher than reported for transition metal oxides, showing on the possibility of achieving electrochromic devices with lower power consumtion.

5. Acknowledgements This thesis would never have been without the following people:

Doc. Olle Inganäs, my supervisor and guide into the fields of conjugated polymers, electrochemistry and polymer electrolytes

My co-workers in the Conjugated Polymer Group at the Laboratory of Applied Physics, discussing chemistry, physics and every day life and keeping my spirits high during long and frustrating laboratory hours. 22 6. References

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