Comparison of PEDOT Films Obtained via Three Different Routes

May 13, 2010 - The performance of different poly(3,4-ethylenedioxythiophene) (PEDOT) films was compared by electrochemical, spectroelectrochemical, an...
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J. Phys. Chem. B 2010, 114, 7445–7451

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Comparison of PEDOT Films Obtained via Three Different Routes through Spectroelectrochemistry and the Differential Cyclic Voltabsorptometry Method (DCVA) S. Duluard,† B. Ouvrard,† A. Celik-Cochet,‡ G. Campet,† U. Posset,‡ G. Schottner,‡ and M.-H. Delville*,† CNRS, UPR-9048, UniVersite´ de Bordeaux, ICMCB, 87 AVenue du Dr. A. Schweitzer, 33608 Pessac cedex, France, and Fraunhofer-Institut fu¨r Silicatforschung (ISC), Neunerplatz 2, D-97082 Wu¨rzburg, Germany ReceiVed: NoVember 24, 2009; ReVised Manuscript ReceiVed: April 21, 2010

The performance of different poly(3,4-ethylenedioxythiophene) (PEDOT) films was compared by electrochemical, spectroelectrochemical, and time-derivative measurements of absorbance versus potential (linear potentialscan voltabsorptometry) for an overall spectroelectrochemical characterization of the electrochromic properties in ionic liquids such as 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMITFSI). The timederivative signals were monitored at different wavelengths, and information obtained therefrom was complementary to that obtained from conventional cyclic voltammetry. PEDOT films prepared via in situ chemical oxidative polymerization appeared to be much more efficient than electropolymerized and PEDOT-poly(styrenesulfonate) (PSS) reference films, in terms of both contrast ratio and coloration efficiency, which was the case even for PEDOT films deposited on less conductive flexible plastic substrates. Introduction Cyclic voltabsorptometry (CVA) is a very useful tool to investigate the electrochemical reactions accompanying the spectral changes of electrochromic materials.1 In comparison with common cyclic voltammetry, it yields more detailed information separating the purely capacitive processes in the electrochemical system from those causing the color changes in the electrochromic layer. Two approaches are usually reported in the literature for voltabsorptometric studies: the absorbance-potential dependencies2 and the differential dA/dt versus potential curves,3 usually called the differential cyclic voltabsorptometry method (DCVA). This technique is also useful to study the electrochromic properties of PEDOT and its derivatives well-known as conducting polymer materials.4 Large surface deposition of PEDOT can be performed by wet-chemical processes with low energy consumption. Due to low processing temperatures the deposition is compatible with flexible plastic substrates. PEDOT and related polymers exhibit coloration efficiencies of several hundreds of cm2/C with switching times of a few seconds.5 Moreover, as most of the poly(thiophene)s, PEDOT is relatively easy to process, can be obtained by oxidative chemical6 or electrochemical7 polymerization of the monomer 2,3-dihydrothieno[3,4-b]-1,4-dioxine, more often called 3,4-ethylenedioxythiophene (EDOT), and is exceptionally stable in its doped state associated with high conductivity and a low optical band gap.8,9 It exhibits a deep blue and a light blue coloration in the reduced and oxidized state, respectively, and as such is commonly used in photovoltaic10 and electrochromic11 devices. In this paper, we aim at comparing the electrochemical properties of PEDOT films obtained by three different processes: (i) deposition from a commercially available PEDOT dispersion in ethanol (DC), (ii) in situ chemical oxidative polymerization * Corresponding author. E-mail: [email protected]. † Universite´ de Bordeaux. ‡ Fraunhofer-Institut fu¨r Silicatforschung (ISC).

of EDOT (ISP), and (iii) electrochemical polymerization of the monomer (EP). We will focus on the role of the film preparation method with regard to the film performance. Experimental Section Standard chemicals (Aldrich) were used as received without any further purification. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and 1-butyl-3-methylimidazolium (BMITFSI) were purchased from Aldrich and Solvionic, respectively. Orgacon EL-P3040 dispersion (PEDOT-poly(styrenesulfonate) copolymer (PEDOT-PSS) in ethanol) was obtained from Agfa. F-doped SnO2 (FTO) coated glass 10 Ω/0, Sn-doped In2O3 (ITO) coated glass 15 Ω/0, (Solems) and Sn-doped In2O3 (ITO) coated PET (Bekaert) 60 Ω/0, were used as transparent conductive substrates. Their sheet resistances, measured using a four probe setup, were 10, 14, and 70 Ω/0, respectively. The conductive substrates were cleaned prior to coating deposition by successive sonication in KOH/HCl solution, Triton X100 detergent solution, ethanol, and acetonitrile. Electropolymerization: A solution of 0.1 M tetra-n-butylammonium perchlorate (TBAP, Aldrich)), 0.005 M EDOT (Aldrich) in acetonitrile (Aldrich) was prepared and used as the electrolyte for the deposition. For the deposition a three-electrode cell configuration was employed. The corresponding substrate was placed as the working electrode. As counter and reference electrodes an FTO glass sheet and a saturated calomel electrode were used, respectively. An oxidative potential of +1.4 V/SCE was applied during a chosen time. In situ chemical oxidative polymerization: The EDOT monomer (Baytron M) was in situ chemically polymerized on transparent conducting oxide (TCO) substrates according to the following procedure: 0.625 mmol of moderator base (imidazol, Aldrich) was dissolved in n-butanol (Aldrich, reagent grade) in an ultrasonic bath until a clear solution was obtained. Subsequently, 2.5 mmol of EDOT and 4.4 mmol of an oxidizing agent (Fe(OTs)3 in n-butanol, Aldrich) were added to the mixture,

10.1021/jp9111712  2010 American Chemical Society Published on Web 05/13/2010

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and the solution was spin-coated by means of a KSM Karl Su¨ss spin coater. After thermal curing at 100 °C for 20 min, the unreacted agents were removed by thorough rinsing in n-butanol and the films dried with compressed air. Deposition of PEDOT dispersion: Orgacon EL-P3040 dispersion from Agfa was spin-coated on the respective substrate with the following sequence: 600 rpm, 15 s (deposition of the solution on the substrate), 1.000 rpm for 60 s (homogenization of the layer) and 2.000 rpm for 3 s (removal of the material accumulated at the edges). Most of the solvent was evaporating during the deposition process. A thermal treatment at 100 °C for 5 min was then performed to evaporate the remaining solvent and improve the connection between the individual PEDOT particles. For multilayer coatings, the thermal treatment was applied for each layer. The electrochemical characterization of the PEDOT films was performed in an oxygen-free electrolyte of the following composition: 0.03 lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) (Aldrich) in 0.97 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMITFSI) (Solvionic) (molar ratio). The PEDOT film was placed as the working electrode, with the counter electrode being a platinum foil and the pseudoreference electrode a freshly prepared Ag/AgCl wire. The electrochemical studies, except for spectroelectrochemical measurements, were performed in a glovebox under inert argon atmosphere using a Voltalab PGZ301 potentiostat. Voltabsorptometry measurements were carried out in an electrochemical cell, placed in an Evolution 100 spectrometer (Thermo Electron Corp.). The working electrodes were either FTO or PET/ITO, the auxiliary electrode was a platinum foil, and a Ag/AgCl wire was used as a pseudoreference electrode. The latter was calibrated against a saturated calomel electrode (SCE). The optical absorption of the PEDOT films was recorded at 620 and 950 nm with a potential scan of 50 mV min-1. The DCVA response was obtained by differentiating the absorbance band with respect to the working electrode potential. Results and Discussion PEDOT can be electrochemically polymerized in both organic12,13 and aqueous14-17 solvents or in mixtures thereof, for example water-acetonitrile.18 The deposition in aqueous solvent is beneficial from a cost, handling, and safety point of view. However, the use of surfactants having the detrimental side effect of decreasing film conductivity19 is mandatory due to the low water solubility of EDOT. Therefore, for the present study the more favorable electrodeposition in organic solutions, such as acetonitrile (up to 0.15 mol/L) was chosen. The electrochemical process offers the possibility of a precise control of the film thickness, from a few angstroms to some micrometers via fine-tuning of the charge passed through the sample. In the in situ chemical oxidative polymerization process,20 the polymerization starts immediately once the monomer and oxidizer solutions are mixed (Figure 1).21 The mixture must be deposited as a thin film on the desired substrate as quickly as possible after preparation and is advantageously stored at low temperature to slow the polymerization kinetics and avoid early aggregation. In addition, a reaction moderator is added to the solution to lower the polymerization rate. After polymerization, the excess of oxidant is removed by rinsing. PEDOT dispersions were first developed by Jonas et al.22 who found a route based on the oxidative polymerization of EDOT in the presence of poly(styrenesulfonic acid) (PSSA) and potassium persulfate (KPS) as oxidative agent. The coating with such PEDOT-PSS dispersions provides a very simple method

Duluard et al.

Figure 1. In situ chemical oxidative polymerization of EDOT to form PEDOT films.

for the formation of highly homogeneous, thin PEDOT-PSS films on large scale substrates. The various PEDOT-based films investigated in this study are compiled in Table 1. The samples had overall dimensions of 4 × 5 cm2 and were deposited on two types of TCO electrodes: FTO/glass and ITO/PET. PEDOT films obtained by the above listed deposition methods were first studied on FTO/glass substrates (10 Ω/0 sheet resistance). The influence of the PEDOT charge capacity will be detailed for electropolymerized PEDOT (EP1, EP2, EP3) on FTO/glass. The film with optimum properties will then be compared to the films obtained by the two other methods. The comparison between the PEDOT layers coated on glass and plastic substrates will be performed only in the case of the in situ polymerized samples. To find the best compromise between high colorlessness of the bleached state and high contrast ratio, PEDOT layers with several charge capacities and thicknesses were prepared and studied. The PEDOT films were obtained by electropolymerization during different reaction times (5 s for EP1, 10 s for EP2, and 20 s for EP3). The deposition potential was set at 1.4 V vs Ag/AgCl. The resulting electrochemical film capacities measured by cyclic voltammetry between -1.0 and +1.0 V vs Ag/AgCl are given in Table 1. In situ absorbance measurements with applied potential were performed on these EP films in a 0.03LiTFSI/ 0.97 BMITFSI (molar ratio) electrolyte which will further be used to setup full devices with a Prussian Blue counter-electrode (Figure 2). The results do not differ much from those obtained in classical solvents with no major change in the position of the absorbance band maxima with increasing film capacity. The coloration efficiency (CE) defined as the relationship between the injected/ejected charge as a function of electrode area (Q) and the change in optical density (absorbance) (∆A) at different wavelengths is calculated using the formula CE ) ∆A/Q.23 The variation of CE with the wavelength between 350 and 1000 nm is given for each of the electropolymerized films in Figure 2A. At the wavelength of maximum absorbance (around 580 nm) the CE is 190 cm2/C, which is close to published values (180-200 cm2/C5,24,25). As expected, the resulting coloration efficiencies do not depend on the film thickness as they are intrinsic parameters of the material. The contrast ratio that is highly sensitive to absorbance values, reaches a very high level for the thickest sample (cf. Figure 2B). Even if the absorbance of both reduced and oxidized states increases, the absorbance increase of the colored state predominates, this leads to a strong increase in contrast ratio. For further comparison with other deposition methods, the EP2 film with a CR of 4.7, a capacity of 3.6 mC/cm2, and a coloration efficiency of 190 cm2/C will be chosen. The other PEDOT films were prepared via in situ chemical oxidative

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TABLE 1: Summary of PEDOT Samples Prepared for the Study deposition method and conditions

sample

deposition

EP1 EP2 EP3 ISP1 ISP2 DC1

electrochemical polymerization electrochemical polymerization electrochemical polymerization in situ polymerization in situ polymerization polymer dispersion coating

a

5 s, 1400 mV 10 s, 1400 mV 20 s, 1400 mV 4 successive coatings

substrate

counter anion

capacity of the obtained film (mC/cm2)

FTO/glass FTO/glass FTO/glass FTO/glass ITO/PET FTO/glass

ClO4ClO4ClO4OTs- a OTsPSS-

1.8 3.6 5.8 3.2 3.2 7.5

-

OTs : p-toluenesulfonate.

Figure 2. (A) Variation of the coloration efficiency with λ as the film capacity increases from 1.8 mC/cm2 (EP1, 5 s deposition) to 5.9 mC/cm2 (EP3, 20 s deposition). (B) Contrast ratio for these films EP1 (cyan), EP2 (blue), and EP3 (black).

TABLE 2: Spectroelectrochemical Properties of Different PEDOT Films: Film Capacity, ∆A at λmax, Ableached (Residual Absorbance in the Bleached State at λmax), Coloration and Bleaching Times (for 90% of Maximum Transmittance Change) at -1 and +1 V, Contrast Ratio at λmax sample

deposition method

capacity (mC/cm2)

∆A at λmax

Ableached at λmax

T90% colored (s)

T90% bleached (s)

contrast ratio at λmax (CR)

EP2 DC1 ISP1 ISP2

electropolymerization dispersion coating in situ polymerization in situ polymerization

3.6 7.5 3.2 3.1

0.70 0.97 1.09 1.09

0.23 0.19 0.29 0.27

11.5 22 16 17.9

1.4 2 2 1.4

4.7 6.9 12.3 12.3

polymerization and dispersion coating (ISP1 on FTO/glass, ISP2 on ITO/PET, and DC1, respectively). The spectroelectrochemical properties of these films were studied by means of in situ UV-visible spectroscopy associated with chronoamperometry and are presented in Table 2. The films’ capacities are in the same range of order whatever the deposition method, except for the dispersion coating (DC1) where the double capacity is needed to reach the same level of coloration. The spectral absorbance of the polymer drastically changes depending on the level of PEDOT doping, as illustrated for ISP2 in Figure 3 (identical results were obtained for ISP1 and DC1, not shown). The maximum of absorption of the reduced deeply blue colored polymer is located around 620 nm (1.99 eV) with a band gap (Eg) of 1.63 eV. The absorption band shows a hardly discernible shoulder at 675 nm (Figure 3, curve a). In the neutral (i.e., reduced) state of the polymer, the electrons are delocalized along the conjugated backbones (Figure 4A) through overlap of π-orbitals (Figure 4A2). This high conjugation length is due not only to the presence of the coplanar aromatic heterocycles of the backbone (Figure 4A) but also to the two electrondonating oxygen atoms coupled to the thiophene ring, which contribute to the π-π* transitions. This results in an extended π-system with a filled narrow valence band.26 Therefore, the π-π* interband transition occurs in the visible range, leading to a deep-blue coloration.4

Upon p-doping by oxidative removal of electrons from the π-system, a p-type polaronic semiconductor is obtained, whereby a further oxidation leads to the formation of polarons and bipolarons (Figure 4B) increasing p-type conductivity and introducing sub band gap energy levels.27-29 PEDOT doping decreases the electronic conjugation, by lowering the number of aromatic rings overlapping, and thus increasing the energy of the electronic π-π* transitions. Consequently, an increase

Figure 3. Absorbance of the ISP2 film (on ITO/PET) depending on the applied potential, in LiTFSI 0.03 (molar ratio) in BMITFSI, pseudoref Ag/AgCl: (a) -1.3 V; (b) -1 V; (c) -0.7 V; (d) -0.6 V; (e) -0.5 V; (f) -0.4 V; (g) -0.3 V; (h) -0.2 V; (i) 0 V; (j) 0.2 V; (k) 0.4 V.

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Figure 4. Molecular structure of neutral PEDOT (A1) and its band gap structure (A2). Bipolaronic species obtained via two-electron oxidation (p-doping) (B1) and band gap structure (B2).

Figure 5. Coloration efficiency and contrast ratio of the different PEDOT films on either FTO/glass for DC1 EP2 and ISP1 or ITO/PET for ISP2.

in optical transparency in the visible domain is then observed as well as additional absorption bands in the near-infrared region (curves g-k) due to polaronic type conduction.4 This NIR large band, however, shows a relatively low intensity as compared to that for the steep absorption edge arising at 1100 nm, corresponding to the beginning of a very broad absorption at wavelengths λmax . 1100 nm. The former absorption band seems to decay while the band above 1100 nm is gradually enhanced with increasing positive potential (E > -0.3 V) (bipolaron subgap state).30-33 Upon further oxidation, the polaronic bands finally merge in the valence and conduction bands and the polymer reaches a “metallic-like” p-type state. Note that PEDOT, as opposed to most other electrochromic materials, is rather special since the bleached film (p-doped form) is more conductive than the colored one (neutral form). In electrochromic devices, PEDOT films will experience consecutive electrochemically induced doping and dedoping. During the oxidation of the polymer the insertion of an anion34 such as ClO4-, OTs-, PF6-, BF4-, or TFSI- is taking place depending on the nature of the electrolytic medium.35-37 In the specific case of the DC1 PEDOT-PSS codeposited film, the anionic PSS polymer is intrinsically retained in the copolymer, acts as a counteranion (doped PEDOT), or is compensated by the diffusion of small cations (undoped PEDOT) to maintain the charge equilibrium.22,38 For electropolymerized PEDOT (see Supporting Information for EP2) two characteristic isosbestic points at 715 and 840 nm

clearly illustrate the presence of an overall constant concentration of absorbing components in the mixture. In contrast to that, for the ISP sample there is no clear wavelength at which the spectra intersect but a range over 50 nm around 1025 nm, even if the first isosbestic point at 724 nm is well identified, as also reported in the literature.39 The samples shown here are representative of many others obtained in both reproducible and repetitive ways. They exhibited a strong blue coloration (wide absorbance around 600 nm). However, the absorption maximum was shifted from 580 nm for EP films to 620 nm for ISP films and 640 nm for the DC film as illustrated in Figure 5, also showing the variation of both coloration efficiencies and contrast ratios with the wavelength. This bathochromic shift points to an increase in the conjugation length from rather short electropolymerized PEDOT chains (EP) via intermediate chain lengths when in situ polymerized (ISP) to a highly conjugated polymer obtained by chemical oxidative bulk polymerization (DC). It should also be mentioned that the neutral form of the DC sample is the only one to exhibit two distinct π to π* interband transitions (640 and 673 nm), such splitting being attributed to vibronic coupling with a high degree of regularity along the polymer backbone.40 In the 700-1100 nm wavelength range the EP and ISP samples exhibit a negative CE due to the higher absorbance of the bleached state in the NIR region and the polaronic conduction. The ISP, EP, and DC films show respective

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Figure 6. Cyclic voltammograms at 500 mV/min and DCVA at λmax in a 0.03/0.97 LiTFSI/BMITFSI mixture (molar ratio), in a three-electrode cell configuration for electropolymerised PEDOT EP2 (A), PEDOT Orgacon DC1 (B), in situ polymerized PEDOT ISP1 (C), and in situ polymerized PEDOT ISP2 (D).

coloration efficiencies of CEmax ) 345 cm2/C, CEmax ) 205 cm2/C, and CEmax ) 125 cm2/C), which clearly indicates a better electrochromic performance in the visible region when the in situ polymerization procedure is applied, independent of the substrate used (FTO/glass or ITO/PET). In the following part, we describe the time derivative voltabsorptometric measurements that were performed to further characterize the electrochromic PEDOT films. This technique has already been applied to study Prussian Blue41 and some conducting polymers.42,43 As mentioned before, the electrochromic activity of PEDOT is related to the injection/ejection of charge carriers into/from the material. On this basis, the absorbance (A) is related to current density according to

A ) εbc

(1)

dA/dt ) εb(dc/dt) ) iε/nF

(2)

where ε is the molar absorptivity of the film, b is the film thickness, c is the concentration of optically active sites on the electrodes, i is the current density, n is the number of electrons participating in the redox reaction, and F is the Faraday constant. If the reaction responsible for the color change in the film also generates a redox wave during a cyclic voltammetry scan and if all the current is consumed in the electrochromic reaction (no parasite reactions), the dA/dt versus E profile should follow the same trends as the cyclic voltammetry pattern obtained under similar experimental conditions.

The cyclic voltammograms for each type of PEDOT film are given in Figure 6: as well as the corresponding voltabsorptometric curves (dA/dt ) f(V)). By comparison of cyclic voltammetry and voltabsorptometry, information on the amount of charge that is effectively used for the modulation of the absorbance is obtained. The main advantage of DCVA is the possibility of discriminating between different electrochemical processes (e.g., faradaic and capacitive behavior) by choosing a proper spectral domain as well as of detecting side reactions occurring in the system and not directly linked to the coloration (e.g., electrolyte degradation, water decomposition, TCO degradation, ...). The cyclic voltammetry and voltabsorptometry curves of electropolymerized (EP) and in situ polymerized (ISP1/2) films were rather similar. A comparison of the curves showed, for these films, several electrochemical waves, as well as concomitant positions of the anodic and cathodic voltabsorptometric peaks, which points to short chain polymers present in the layers. As the applied potential increased in the positive direction, the π-π* transition band at 620 nm disappeared. This band is characteristic of the reduced neutral state and matches well with the CV of the polymer films. DC1 films exhibited different shapes of both the cyclic voltammetry and voltabsorptometry curves with (i) a single reduction wave significant of a long chain polymer and (ii) a ∆A covering the whole studied potential range. Indeed, in the case of EP and ISP, the voltabsorptometry dA/dt curves show that the bleaching was obtained as soon as a potential of +270 mV (500 mV for ISP) was applied. Applying a potential between +270 mV (+500 mV for ISP) and +1 V is then theoretically useless since there is no additional

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Figure 7. Cyclic voltammogram at 500 mV/min, and volta-absorptometry at 950 nm in a 0.03/0.97 LiTFSI/BMITFSI mixture (molar ratio) of ISP2 (in situ polymerized PEDOT on ITO/PET).

Figure 8. Variation of the absorbance with the coloration/bleaching cycles (-1.0 V/+1.0 V) in percent of maximum absorbance attained at the end of this conditioning step.

change in absorbance: dA/dt ) 0. The remaining current density in this 0.27-1.0 V range (either in reduction or in oxidation) corresponding to a capacity of about 33% of the total capacity does not affect the optical contrast in the visible region. Part of this capacity is used to induce changes in the infrared region, as exemplified in Figure 7 for ISP2 with a DCVA curve in the NIR region (950 nm) that shows different tendencies when compared to the CV curve of the polymer film (see also ref 39). This observation is correlated to the concentration of the p-type polaronic carriers during the oxidation of PEDOT, these species being more or less delocalized over the polymer backbone. The oxidized states absorbing at 950 nm do not exist simultaneously with the reduced states absorbing at 620 nm, which was expected from comparison with Figure 3. Moreover, voltabsorptometry is a useful tool to determine which reduction and oxidation potentials are most appropriate to minimize energy consumption while maintaining the optimum optical contrast in the visible. The potentials determined are -1.0/+0.27 V vs Ag/AgCl for the electropolymerised film and -1.0/+0.5 V vs Ag/AgCl for the in situ polymerized ones. On the contrary, for DC1 films (dispersion coating), the sample needs to be cycled over the whole -1.0 to +1.0 V range to switch from its totally bleached state to its totally colored one (the range of potential with dA/dt ) 0 is very narrow). Consequently, EP or ISP films could be considered better anodes than PEDOT/PSS films when transparent electrodes are required. The values of dA/dt also provide some information on the kinetics of coloration and bleaching. Fast coloration is evident at high dA/dt (ISP film > EP sample and DC sample). To complete this study, the switching times in real operating conditions (application of potentials) are presented. The dynamic changes of absorbance upon oxidation and reduction were measured for coloring and bleaching potentials of -1.0 and +1.0 V vs Ag/AgCl (Table 2). For all the PEDOT layers under study, the coloration took much longer than the bleaching (tens of seconds versus 1-2 s). The oxidation of EP and ISP polymer films corresponds to the formation of cationic charges along the polymer chains. The overall electroneutrality is attained by incorporating the same amount of anionic compensating charges. The incorporation of the TFSI anion upon oxidation of the polymer seems to be easily obtained; however, its extraction from the polymer structure is rather slow. For the PEDOT-PSS dispersion film (DC1), coloration and bleaching times are longer than for EP and ISP films, for which two main reasons can be proposed. The first concerns the high resistivity of the insulating polymer PSS hindering the charge transport. The second is linked to the

specific coordination of the lithium cation in ionic liquids. As a matter of fact, we have shown that in pure ionic liquid (1 - x)(BMITFSI)/xLiTFSI with x < 0.2, the lithium solvation essentially occurs by two TFSI- forming [Li(TFSI)2]- anionic clusters,44 which must overcome dissociation energy to intercalate into the PEDOT-PSS film as a Li+ species and compensate for the negative charges of the -SO3- groups on the PSS polyanions. The polymer also becomes more and more isolating, thus limiting the switching rate of the layer. The absorbance in the bleached and colored states was measured for the first cycles of coloration and bleaching of all three types of films and compared to a film sample in situ polymerized on the ITO on glass substrate. The coloring (-1.0 V) and bleaching (+1.0 V) potentials were applied for about 4 min. Astab, the stabilized absorbance for each cycle, is plotted as the percentage of Amax in Figure 8. When ISP films are compared, there is a difference between those performed on glass substrate-based TCOs with low resistivity (FTO and ITO) and that based on PET (60 Ω/0). The same observation was also made in cyclic voltammetry sequences for which ISP films on ITO/PET require at least 20 cycles between -1.0 and +1.0 V at 100 mV/s to reach their highest ∆A values (results not shown). The chemical nature of the TCO (FTO or ITO) does not play a strong role (identical formatting behavior), whereas the deposition of the conductive layer seems to be crucial, since the coating of PET cannot be performed at temperatures as high as that used for deposition on glass. At these low deposition temperatures (130-150 °C), ITO is still conductive, but amorphous. The ISP films were all prepared in the same way, via EDOT polymerization in the presence of an oxidative agent (i.e., reducible agent); this agent is mostly removed by rinsing of the films. However, the remaining traces of this product may explain this long formatting time (beyond 5 cycles). Furthermore, this oxidative agent provides charge compensating tosylate anions that can remain entrapped in the polymer during polymerization. Upon cycling, these counteranions are exchanged against TFSI- anions from the electrolyte;21 this, however, does not seem to be the limiting factor. As far as the bleaching state is concerned, the minimum value of the absorbance was obtained as soon as the first oxidative sequence was performed for all the samples. This clearly indicates a faster kinetics of the doping ion diffusion in the oxidation process than in the reduction one. This faster bleaching kinetics as compared to coloration shown by all the films is probably a consequence of a lower resistance to diffusion of the doping ions through the bulk of the film during oxidation. Other factors

PEDOT Films such as surface roughness, porosity, and steric factor and film breathing between the oxidized and reduced states also modify diffusion-controlled kinetics.45 Conclusion In this paper, we have compared three deposition methods for thin PEDOT polymer films and shown the influence on the electrochemical and electrochromic responses of the films. We have illustrated that, depending on the synthetic process for the polymer layer formation, the band gap and sub-band gap levels of conducting polymers can be controlled through several factors like mesoscopic ordering, conjugation length, bond length, doping level, conformational changes, and so forth, affecting optical density and coloration efficiency of the materials. In situ polymerized poly(3,4-ethylenedioxythiophene) films showed superior redox activity and twice as much coloration efficiency in the photonic region in comparison to the classically electropolymerized or spin coated deposits. The nature of the doping ion does not seem to be as crucial as the TCO resistivity when the formatting period is considered. An important feature shown by the derivative curves of absorbance versus potential at 620 and 950 nm, respectively, is that the generation of either colored or bleached state can be easily monitored with cyclic voltabsorptometry in electrolytes such as ionic liquids. When combined with conventional cyclic voltammetry, DCVA may serve as a good diagnostic tool and provide structural information with regard, for example, to the polymer chain length. Acknowledgment. Financial support from the European Commission Specific Targeted Research Project NANOEFFECTS (www.nanoeffects.eu) is gratefully acknowledged. Supporting Information Available: UV-visible spectrum of EP2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. In Electroanalytical Chemistyry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 1. (2) (a) Kuzmany, H.; Sariciftci, N. S. Synth. Met. 1987, 18, 353–358. (b) Kalaji, M.; Peter, L. M. J. Chem. Soc. Faraday Trans. 1991, 87, 853– 860. (c) Aoki, K.; Teragishi, Y. J. Electroanal. Chem. 1998, 441, 25–31. (e) Aoki, K.; Edo, T.; Cao, J. Electrochim. Acta 1998, 43, 285–289. (3) (a) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1989, 136, 427–433. (b) Lapkowski, M. Bull. Electrochem. 1989, 5, 792–799. (c) Gazotti, W. A.; Jannini, M. J. D. M.; De Torresi, S. I. C.; De Paoli, M. A. J. Electroanal. Chem. 1997, 440, 193–199. (d) Visy, C.; Krivan, E.; Peintler, G. J. Electroanal. Chem. 1999, 462, 1–11. (4) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism and electrochromic deVices; Cambridge University Press: Cambridge, U.K., 2007. (5) (a) Gaupp, C. L.; Welsh, D. M.; Rauh, R. D.; Reynolds, J. R. Chem. Mater. 2002, 14, 3964–3970. (b) Lin, T. H.; Ho, K. C. Sol. Energy Mater. Sol. Cells 2006, 90, 506–520. (6) (a) Jyongsik, J. AdV. Polym. Sc. 2006, 199, 189–259. (b) Jonas, F.; Kraft, W.; Muys, B. Macromol. Symp. 1995, 100, 169–173. (c) Sahin, E.; Sahmetlioglu, E.; Akhmedov, I. M.; Tanyeli, C.; Toppare, L. Org. Electron. 2006, 7, 351–362. (7) (a) Piron, F.; Leriche, P.; Maboupn, G.; Grosu, I.; Roncali, J. Electrochem. Commun. 2008, 10, 1427–1430. (b) Wagner, K.; Pringle, J. M.; Hall, S. B.; Forsyth, M.; MacFarlane, D. R.; Officer, D. L. Synth. Met. 2005, 153, 257–260. (8) Roncali, J. Chem. ReV. 1992, 92, 711–738. (9) Barbella, G.; Melucci, M.; Sotgiu, G. AdV. Mater. 2005, 17, 1581– 1593.

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