Effect of electrode surface on the electrochromic properties of electropolymerized poly(3,4-ethylenedioxythiophene) thin films

Organic Electronics 30 (2016) 67e75

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Effect of electrode surface on the electrochromic properties of electropolymerized poly(3,4-ethylenedioxythiophene) thin films Rekha Singh a, Anil Kumar a, b, c, d, * a

Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India Centre of Excellence in Nanoelectronics, Indian Institute of Technology Bombay, Mumbai 400076, India c National Centre for Photovoltaic Research and Education, Indian Institute of Technology Bombay, Mumbai 400076, India d National Centre of Excellence in Technologies for Internal Security, Indian Institute of Technology Bombay, Mumbai 400076, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 October 2015 Received in revised form 27 November 2015 Accepted 28 November 2015 Available online xxx

Electrochromism or electrochromic contrast in electropolymerized thin films of dioxythiophenes based conjugated polymers is known to be sensitive to the structure of monomer and the polymerizing conditions. However in our studies we found that it is also sensitive to the electrode surface wherein a significantly high electrochromic contrast is observed in the electropolymerized thin films of poly(3,4ethylenedioxythiophene), PEDOT deposited on platinum (71%) as compared to that on indium tin oxide, ITO, coated glass surface (54%). This is attributed to the formation of more conjugated polymer on the metallic surfaces as confirmed by narrow and red shifted absorption peak for PEDOT on platinum compared to broad and blue shifted peak on ITO in the UVevis absorption spectra. This difference in the electrochromic properties of electropolymerized PEDOT thin films on the two surfaces is investigated by studying their electrochemical growth using UVevis absorption, Raman spectroscopy and atomic force microscopy techniques. These results suggest the deposition of more conjugated polymer in the initial stages of growth (
3 mC/cm2) on both the substrates whereas it continues the same way into the intermediate stages (up to ~15 mC/cm2) only on platinum, thereby, resulting in higher electrochromic contrast on platinum. The coloration efficiency of PEDOT thin film was also found to be improved on platinum (465 cm2/C) compared to that on ITO (230 cm2/C). Moreover, we observed that the EC contrast of electropolymerized PEDOT thin films on platinum was found to be insensitive to polymerizing solvent that is generally not the case when polymerized on ITO. The cyclic stability of PEDOT-Pt films are better compared to that of PEDOT-ITO which is attributed to the improved reversibility of these films with respect to potential switching. © 2015 Elsevier B.V. All rights reserved.

Keywords: PEDOT Electrochromic contrast Surface effect Electropolymerization

1. Introduction Electrochromism is referred to a reversible change in color (multichromism) and/or change in transmittance between the coloured and bleached state as a function of applied voltage. Electrochromic materials have attracted considerable attention due to their low power consumption [1,2] and application in various emerging fields such as displays (transmissive/reflective), optical shutters, smart windows for energy control and night vision etc [3,4]. This resulted in large effort being devoted to the development of new materials [5,6] with different colors [7e9], and improved

* Corresponding author. Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India. E-mail address: [email protected] (A. Kumar). http://dx.doi.org/10.1016/j.orgel.2015.11.034 1566-1199/© 2015 Elsevier B.V. All rights reserved.

electrochromic properties as well as development of inexpensive device fabrication techniques [10e14]. Though inorganic materials [15,16] have been known for a long time for their high electrochromic contrast and stability, recent developments have shown the advantages of conjugated polymers over them in terms of ease of fine tuning the properties [7,17], solution processability [18] and their stability towards flexural stress [19e21]. Conjugated polymers based on 3,4-alkylenedioxythiophenes (ADOTs) have been studied extensively due to their low oxidation potentials, high coloration efficiencies, fast switching and increased lifetime. Beaujuge and Reynolds have reviewed recent developments in the field of electrochromism based on poly(3,4-alkylenedioxythiophene) (PADOT) polymers [22]. Understanding the dependence of electrochromic properties of conjugated polymers on various processing conditions is important

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in the development of electrochromic devices. A relation has been established between the chemical structure and EC properties of PADOTs wherein these properties have been observed to improve with increase in ring size or size of the substituent and has been attributed to the change in morphology which leads to an efficient ion exchange [23e25]. Until recently this dependence between the chemical structure and EC properties was thought to be the only criterion affecting these properties. However, recently there are a few reports where it has been shown that EC properties can also be altered without changing the chemical structure of the polymer. In this regard, electrochromic properties of Poly(3,4ethylenedioxythiophene), PEDOT, have been studied and found to be affected by various processing conditions. Generally the electropolymerized PEDOT on ITO has EC contrast of 44e54% [23,24,26]. However, different synthetic methods, which includes electrochemical and chemical, of making PEDOT films can alter the mesoscopic homogeneity, conjugation length, level of doping and conformational changes etc. therefore affecting the optical, electrical and electrochromic properties [27,28]. Significant efforts have been devoted to make the morphology of PEDOT porous which resulted in fast switching kinetics [29e34]. Compositing the PEDOT matrix with conducting nano-objects e.g. carbon nanotube [35], graphene [36], copper [37] and gold [38] nanoparticles also resulted in increased electrochromic response and high coloration efficiency. It has been shown by Poverenov et al. that PEDOT electropolymerized in propylene carbonate has higher EC contrast compared to that in acetonitrile solvent, thereby indicating the effect of solvent on EC properties [39]. In another study, Chiang et al. have demonstrated an increase in EC contrast and cyclic stability of PEDOT by post solvent treatment with dimethyl sulfoxide (DMSO) [40]. In the above mentioned examples, change in the morphology of polymer thin film is the variable which was altered by changing either the chemical structure or the processing conditions (solvent [41,42], electrolyte [43] etc.). Therefore it is a point of great interest to investigate further the dependence of morphology on other conditions which can have bearing on the electrochromic properties. Electrode surface has been shown to induce a change in the morphology and electronic properties of the deposited polymer [44e46]. This is reflected in an observation that a more conducting and crystalline polymer is formed during initial phases of growth and the conductivity of the polymer film diminishes as the thickness increases [47,48]. However this growth pattern of polymer has also been shown to strongly depend on the nature of the electrode surface. For example, in the work by Roncali et al. it has been shown that the poly(3-methylthiophene) grown on platinum surface has higher conductivity than the one grown on ITO [49]. In this regard, it is to be noted that the effect of substrate surface on electrochromic properties is not yet explored and is the theme of this work. In this work, we report on an observed high EC contrast (D% reflectance) of electropolymerized PEDOT thin films deposited on platinum substrate in comparison to that deposited on conventional ITO coated glass substrates. The effect of substrate surface proximity on electrochemical growth of PEDOT has been studied by UVevis absorption, Raman spectroscopy and atomic force microscopy. Different electrochromic properties of PEDOT on platinum and ITO surfaces have been attributed to the difference in the growth mechanism of the polymer leading to a different morphology. 2. Experimental section 3,4-Ethylenedioxythiophene (EDOT, 97%) monomer was purchased from SigmaeAldrich and purified by reduced-pressure

distillation. Tetrabutylammonium perchlorate (TBAP,  99.0%) was purchased from SigmaeAldrich and used as received. Propylene carbonate (PC; Merck,  99.7) and acetonitrile (MeCN; Merck,  99.93) were dried and distilled according to the standard method of purification. Indium tin oxide (ITO) coated glass slides (15e20 U/ ,) were purchased from J. K. Impex, India. Platinum and gold working electrodes were purchased from CH Instruments. Electrochemical studies were carried out using CHI760D potentiostat from CH Instruments. Optical studies were performed using Ocean Optics USB2000þ with LS-1 tungsten halogen lamp. Bifurcated lab grade reflection probe (R200-7-UV-VIS) was used for reflectance measurements. Raman Spectra were recorded using Ramnor HG-2S Spectrometer (Jobin-Yvon) with Argon ion laser (514.5 nm) at a resolution of 0.5 cm1. Scanning electron microscopy (SEM) images of PEDOT films on ITO and platinum substrate were taken with JEOL FEG-SEM instrument (model JSM-7600F) operating at 10 kW. Atomic force microscopy (AFM) was carried out on Veeco Nanoscope IV Multimode AFM in tapping mode using silicon cantilever. 2.1. Cleaning procedure for ITO/Pt/Au working electrodes ITO coated glass electrodes were cleaned by RCA treatment (1: 2: 5 volume ratio of 25% NH3, 30% H2O2 and H2O at 80  C for 15 min) followed by thorough rinsing with deionized water. Pt/Au Electrodes were cleaned by polishing the surface using an alumina slurry of size 1, 0.3 and 0.05 mm, respectively, in the given sequence. A 1200 grit carbimet disk was used to remove visible scratches from the surface. Residual alumina adhered to the electrodes is removed by washing them in deionized water under sonication. 2.2. Preparation of PEDOT films and their electrochromic studies Polymer films were prepared by the electrochemical polymerization of EDOT monomer (10 mM) solution with TBAP (100 mM) as supporting electrolyte in dry acetonitrile/propylene carbonate on ITO (area ¼ 0.5 cm2), platinum and gold (area ¼ 0.3 cm2) surfaces in potentiostatic mode at 1.2 V (vs Ag/Agþ reference electrode). Ag/ Agþ and platinum foil were used as reference and counter electrodes, respectively. Since the optical studies of PEDOT thin films on platinum and gold electrodes cannot be done in transmission mode, for a comparative study of these films with that on ITO, reflection mode is chosen for all the films. To achieve this, the insulating side of ITO was made reflective by depositing a 100 nm thick aluminum using thermal evaporation and the resulting substrate is referred here as rITO. PEDOT electropolymerized on rITO, platinum and gold is referred as PEDOT-rITO, PEDOT-Pt and PEDOTAu in the text. Optical measurements of PEDOT films were carried out on rITO, platinum and gold substrates in reflection (See Fig. SI1 in Supporting Information for more details on reflectance setup) as a function of applied potential (from þ1.0 V to 1.0 V vs Ag/Agþ) in monomer free electrolyte (100 mM TBAP) solution in dry acetonitrile. Cyclic voltammetry and chronoamperometry techniques were used for spectroelectrochemical and optical switching studies, respectively. For optical switching, the potential was switched between þ1.0 V and 1.0 V with a pulse width of 2 s. 3. Results and discussion 3.1. Effect of electrode surface In order to study the effect of electrode surface on the electrochromic properties of PEDOT, EDOT monomer was electrochemically polymerized on platinum, gold and ITO surfaces at a static potential of 1.2 V (v/s Ag/Agþ). This resulted in the formation of PEDOT thin films on the aforementioned electrodes. PEDOT film

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thickness was varied by varying the polymerization charge density (PCD) in order to optimize the electrochromic contrast. UVevis absorption spectra of these PEDOT thin films in their bleached (doped) and colored (dedoped) state are shown in Fig. 1a. The EC contrast (D%R between colored and bleached state) of these PEDOT films at each value of PCD is calculated and the results are plotted in Fig. 1b and c. As expected, the EC contrast as a function of PCD is observed to pass through a maximum (maximum EC contrast) on all the substrate. Surprisingly, a significantly higher maximum EC contrast (at absorption maxima) of 71% is obtained for PEDOT-Pt in comparison to 54% for PEDOT-rITO thin films at polymerization charge density of 15 mC/cm2. Similar high contrast of PEDOT thin films was observed on gold electrode (68%, Fig. SI2 in Supporting Information). Interestingly, when the EC contrast was calculated for the whole visible spectra (380e750 nm), similar trend of higher contrast was observed for PEDOT-Pt and PEDOT-Au as compared to PEDOT-rITO (Table 1). It must be noted that here we are comparing the maximum EC contrast on different substrates which is independent of the thickness of the polymer deposited [50]. Other electrochromic properties such as coloration efficiency and switching speeds are tabulated in Table 1. PEDOT-Pt and PEDOT-Au thin films show higher coloration efficiency (465 and 435 cm2/C) than PEDOT-rITO (230 cm2/C). The higher EC contrast of PEDOT-Pt than PEDOT-rITO is reflected in their UVevis absorption spectra (Fig. 1a) where higher absorbance in colored state combined with more transparency in the bleached state is observed for PEDOT-Pt. UVevis absorption spectra of PEDOT-rITO and PEDOT-Pt thin films also show other considerable differences. The absorption maximum of PEDOT-Pt is red-

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shifted compared to PEDOT-rITO. In addition the absorption spectra of PEDOT-Pt thin films are narrower (the full width at half maxima is 168 nm) than those of PEDOT-rITO thin films (the full width at half maxima is 225 nm). These differences in the UVevis absorption spectra of PEDOT thin films on two different substrates is an indication of morphological transformation of polymer chains which is due either to their extension in conjugation or a strong aggregation on platinum substrate. Similar red-shift and narrowing of absorption spectra has been reported by Roncali et al. for poly(3methylthiophene) on ITO substrates where they attributed this shift also to an increase in mean conjugation length of chains and narrow distribution of various conjugation lengths [49]. Spectroelectrochemistry of electropolymerized PEDOT thin films on rITO and platinum was performed by recording the UVevis spectra as a function of applied potential (1.0 V to þ1.0 V). A typical spectroelectrochemical behavior is observed for PEDOTrITO thin films (Fig. 2a), wherein the absorbance of these films at lmax ~615 nm (due to п to п* transition) decrease with concomitant increase in absorption in the near infrared region (>800 nm, due to the formation of polarons) as the potential is increased. Absorption spectrum of PEDOT-rITO reaches a steady state at þ1.0 V with having a residual weak absorption tail in the visible region. However, in case of PEDOT-Pt thin films (Fig. 2b), this absorption tail is fairly weak in the visible region thereby implying a lesser absorption in the bleached state as compared to PEDOT-rITO thin films in the same state. A similar behavior was observed in substituted EDOT and ProDOT based polymers and has been attributed to their open or porous morphology [23]. These differences in the absorption spectra and EC contrast of PEDOT formed on the two substrates

Fig. 1. (a) Reflectance specta of electropolymerized thin films of PEDOT (bleached and colored) on rITO (dotted) and platinum (solid) prepared at 15 mC/cm2 polymerization charge density. Electrochromic contrast of PEDOT on (b) rITO and (c) platinum electrode surfaces for different polymerization charge densities.

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Table 1 Electrochromic contrast, coloration efficiency and switching time of PEDOT prepared on different electrode. Switching was done between þ1.0 and 1.0 V vs Ag/Agþ in TBAP/ MeCN. Polymer-substrate

PEDOT-rITO PEDOT-Pt PEDOT-Au a

lmax (nm)

615 650 645

%Rb

75 80 79

%Rc

21 9 11

EC contrast at lmax (%)

54 71 68

Average EC contrast 380e750 nm (%)

39 51 45

a

CE (cm2/C)

230 465 435

a

Switching time (s)

tbleach

tcolor

0.2 0.1 0.1

0.6 0.2 0.2

Calculated for 90% color change/switch.

Fig. 2. Spectroelectrochemistry of PEDOT thin films electropolymerized on (a) rITO and (b) Pt. The applied potential range is 1.0 V to þ1.0 V vs Ag/Agþ in monomer free TBAP/ MeCN.

led us to investigate the growth of PEDOT in more detail. We have used UVevis absorption spectroscopy to investigate the growth of PEDOT at different thicknesses and the results were further corroborated with Raman spectroscopy and atomic force microscopy studies in sub-section 3.2 and 3.3, respectively. Electrochemical studies of PEDOT thin films deposited on rITO and platinum were performed by recording cyclic voltammogram (Fig. SI3 in Supporting Information) in monomer free 100 mM TBAP-MeCN electrolyte solution. The onsets of oxidation and E1/2 for the two polymer films are comparable (Table SI1 in supplementary information). Interestingly, the band gap is also not much affected for the two films and found to be same. This is due to the fact that the broadening of UVevis absorption spectra happens towards lower wavelength side leaving the onset at higher wavelength unaffected. Optical switching studies of the PEDOT thin films were carried out in acetonitrile by switching the potential between þ1 V and 1 V. The percentage reflectance, at 615 nm for PEDOT-rITO and 650 nm for PEDOT-Pt, was monitored as a function of time. The EC contrast of PEDOT-rITO and PEDOT-Pt decreases upto 26% and 16%, respectively in acetonitrile after 200 double potential cycles (Fig. 3). The better cyclic stability of PEDOT-Pt films compared to PEDOT-rITO thin films can be attributed to the more conjugated polymeric chains in PEDOT-Pt thin films which show better reversibility towards potential switching. Apart from the substrate surface, solvent could also alter the electrochromic properties of PEDOT. Poverenov et al. have shown that PEDOT electropolymerized on ITO in propylene carbonate (PC) solvent shows higher EC contrast compared to that grown in acetonitrile [39]. In order to check whether solvent has same effect on electrochromic properties when EDOT is electopolymerized on platinum surface, electropolymerization of EDOT was carried out on platinum in PC/TBAP. EC contrast as a function of PCD was studied as shown in Fig. 4. It was interesting to note that PEDOT electrodeposition was slow when propylene carbonate is used as

solvent compared to acetonitrile (Fig. 4c) and reflected in slow increase in EC contrast with PCD (Fig. 4a). This is due to higher viscosity of propylene carbonate (2.51 mPa s at 25  C) compared to MeCN (0.34 mPa s at 25  C), resulting in slower mass transport and lower diffusion coefficients [51,52]. However, the maximum EC contrast was found to be comparable in both the cases suggesting the dominance of electrode surface over solvent in the final EC contrast when platinum was used as the surface. This is in contrast to what has been observed in case of PEDOT electropolymerized on ITO surface. The EC contrast and absorption spectrum (Fig. 4b) for both the PEDOT films (PEDOT-PC and PEDOT-MeCN) is same on platinum but it is significantly different when polymerization is done on ITO. This indicates the dominant effect of substrate on electrochromic properties of PEDOT than solvent (Table SI2 in Supporting Information). 3.2. Effect of electrode surface on electrochemical growth In general, the substrate surface has more pronounced effect on the initial few layers of electropolymerized polymer and is expected to diminish as the number of layers increase [47e49,53]. Therefore, different substrate, owing to their different mechanism of interaction with monomer and polymer, can have different effect on morphology of initial few layers during the growth. Roncali et al., have studied the growth of poly(3-methylthiophene) (PMeT) thin films with the increasing film thickness on ITO and platinum substrates using optical measurements on ITO and conductivity measurements on both the ITO and platinum [49]. They reported higher conductivity of thin films compared with the thick films and for the thin films it was reported to be higher on platinum than on ITO. We did the similar study for electrochemical growth of PEDOTPt and PEDOT-rITO films by recording their absorbance at peak maxima as a function of polymerization charge density. The results are shown in Fig. 5a. It can be seen from the Fig. 5a that initially the absorbance increases linearly with PCD (which is related to

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Fig. 3. Optical switching of PEDOT thin films during switching between ±1 V in TBAP/MeCN on rITO and Pt surfaces.

Fig. 4. (a) Electrochromic contrast of PEDOT deposited on Pt in acetonitrile (solid square) and propylene carbonate (hollow square) for different polymerization time. (b) Visible absorption spectra of PEDOT deposited on platinum in acetonitrile (solid) and propylene carbonate (dashed). (c) Rate of PEDOT electrodeposition on platinum in acetonitrile (solid square) and propylene carbonate (hollow square).

thickness) followed by a decrease in the slope and reaching a saturation plateau at higher values of polymerization charge density. The higher slope in the initial growth indicates the formation of polymer of higher absorptivity at the proximity of electrode surface. This higher absorptivity is generally associated with increase in conjugation [54,55]. Chaing et al. have also reported the increase in optical density of PEDOT thin films on changing

conformation from coil to extended linear chains. For PEDOT-Pt the slope is observed to be higher than for PEDOT-rITO which indicates the formation of polymer with higher conjugation length. This surface effect diminishes in later stages of growth leading to the formation of less absorptive polymer resulting in the observed deviation from linearity. Along with the change in absorption behavior, a significant

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Fig. 5. (a) Absorbance, (b) absorption maxima (lmax)and difference in absorbance maxima of PEDOT-Pt and PEDOT-rITO (Dlmax, hollow triangle) vs polymerization charge density curve for PEDOT thin films prepared on rITO (blue triangle) and platinum (black square). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

change in absorption maxima (lmax) is also observed as growth of polymer proceeds (See Fig. SI4 and Fig. SI5 in Supporting Information). As can be seen from Fig. 5b, PEDOT-Pt thin films show a higher lmax which remains almost unchanged from initial to intermediate stages of growth (till 15 mC/cm2). This nearly constant value of lmax clearly suggests a nearly homogeneous morphology of the polymer film deposited up to this thickness. This is in sharp contrast to what is observed for PEDOT-rITO, where the lmax changes rapidly from an initial high value to a rapid blue shift (Dlmax ¼ 35 at 15 mC/cm2) in the intermediate stage of growth, which indicates a rapid change in the morphology of deposited polymer during intermediate phase of growth. These results suggest that there is pronounced substrate effect of platinum surface during the initial and intermediate stages of growth and this effect decreases in the later stage of growth. In order to further study the effect of electrode surface on polymer growth, Raman studies were carried out. Raman spectroscopy is a very sensitive tool for determining structural or conformational changes in the molecule. Ca ¼ Cb symmetiric stretching band (1400e1450 cm1) due to five-membered thiophene ring is the most intense band in the raman spectrum of PEDOT and is the most affected by change in confirmation. In addition to that, his band is very much sensitive to doping level of PEDOT also. Generally a shift towards lower wavenumber has been observed when benzenoid to quinoid conformational change

happens as observed in case of PEDOT and PEDOT-PSS [40,56]. Neutral PEDOT has dominant Raman peak at 1415 cm1 for Ca ¼ Cb (eOe) symmetric stretching which on oxidative doping becomes broad and maxima is shifted towards higher wavenumber (~1450 cm1). This observed shift in the band towards higher wavenumber is due to increase in the contribution of benzenoid structure [57]. Ex-situ Raman spectra were recorded (514.5 nm excitation line) for doped PEDOT thin films prepared at different PCDs on rITO (Fig. 6a and b). These PEDOT thin films were prepared by oxidative electropolymerization, subjected to dedoping by applying 1 V for 60 s, and then doped again at 0.6 V for 60 s till the steady state current is reached prior to measurements. As can be seen from Raman spectra of PEDOT thin films at different thickness, representing the films at different growth stages on rITO, a broad peak becomes narrower with its centroid being shifted from 1438 cm1 to 1429 cm1 with increase in film thickness. Broadening and shift of Raman band towards higher wavenumber in the case of thin PEDOT films is an indication of their higher doping level compared to the thicker films. Although all the films were doped at a constant potential for the same duration, the inhomogeneity in the films leads to different doping levels. This can be understood in the Prigodin-Epstein model in which it is shown that, conducting polymers cannot be 100% doped or dedoped due to inhomogeneous network of small conducting or crystalline domains embedded into an insulating or disordered polymer matrix [58]. On dedoping some residual charge remain trapped within the nanometer-size crystalline domains surrounded by nonconducting phase. Higher doping level of thin PEDOT films therefore is an indication of more ordered or conjugated domains in the thin films compared to the thicker films of PEDOT. A similar difference was observed in the Raman spectra of doped PEDOT-rITO and PEDOT-Pt thin films where PEDOT-Pt film shows broader peak indicating higher doping level thus supporting our hypothesis of having more conjugated domains (Fig. 6c). 3.3. Morphology studies using FEG-SEM and AFM FEG-SEM and AFM techniques were used for studying the morphology of electropolymerized PEDOT on rITO and platinum substrates. FEG-SEM images of PEDOT-rITO and PEDOT-Pt thin films (15 mC/cm2) at a magnification of 10,000X (scale bar is 1 mm) are shown in Fig. 7. A distinct difference in the bulk morphology of the two films was observed. PEDOT-Pt thin film exhibits a smooth morphology (Fig. 7b) whereas the PEDOT-rITO thin film has prominent features visible at this magnification (Fig. 7a). These images are similar to the one reported by Poverenov et al. for the electropolymerized thin films of PEDOT prepared in propylene carbonate and acetonitrile, respectively, on ITO [39]. No distinct features are observed in SEM images of PEDOT-Pt thin films indicating the small size of grains in these films. Atomic force microscopy is used to investigate the morphology of PEDOT thin films in more detail at different growth stages. As can be seen from the AFM images in Fig. 8, the morphology of the deposited PEDOT changes with thickness. Also this change in morphology with thickness is different for PEDOT on platinum and rITO. However, in the initial stages of growth the morphology of these films is quite similar on both the substrates with compact arrangement of small grains of PEDOT spread homogeneously throughout the whole surface (Fig. 8a and e). In the intermediate stages of growth, PEDOT formed on platinum has more compact morphology with small fibrous features (Fig. 8f and g) while on rITO the deposited PEDOT has bigger grain (Fig. 8b and c) and therefore more roughness of the films. The compact morphology at initial stages in both platinum and rITO and in intermediate stages in platinum can be attributed to the extended conjugation in PEDOT

R. Singh, A. Kumar / Organic Electronics 30 (2016) 67e75

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Fig. 6. (a) Raman spectra of bare ITO (1), electropolymerized PEDOT on ITO at 1 mC/cm2 (2), 2 mC/cm2 (3), 3 mC/cm2 (4), 6 mC/cm2 (5), 16 mC/cm2 (6) and 31 mC/cm2 (7) polymerization charge densities. (b) Expanded Raman spectra in 1380e1480 cm1 region. (c) Raman spectra of electropolymerized PEDOT on rITO (hollow square) and platinum (solid square) at 15 mC/cm2 polymerization charge density.

Fig. 7. FEG-SEM images of PEDOT thin films on (a) rITO and (b) platinum electropolymerizedat15 mC/cm2 polymerization charge density. Magnification ~10,000, scale bar is 1 mm.

chains induced by platinum substrate as explained before in Section 3.1. In later stages both PEDOT-rITO (Fig. 8d) and PEDOT-Pt (Fig. 8h) thin films show bumpy and rough morphology lacking the fibrous or grainy features. The study of both FEG-SEM and AFM suggest that platinum surface induces morphological changes that are responsible for the enhanced electrochromic properties of electropolymerized PEDOT thin films. 4. Conclusions In summary, we have shown that the electrode surface plays a

critical role in the electrochromic properties of PEDOT thin films wherein improved electrochormic properties were observed when ITO was replaced with more conducting platinum or gold. We have studied the effect of surface proximity and the effect of different electrode surfaces on the growth of PEDOT thin films. The growth pattern of PEDOT is observed to be different on ITO and platinum, thereby implying different morphologies of the polymer formed which results in improved electrochromic properties. This is attributed to either the catalytic effect or high conductivity of metallic electrodes as compared to ITO. It was also interesting to note that substrate surface play a dominant role as compared to

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Fig. 8. AFM height images of PEDOT films electropolymerized on ITO (aed) and on platinum (eeh) at (a, e) 3 mC/cm2 (b, f) 8 mC/cm2 (c, g) 15 mC/cm2 and (d, h) 35 mC/cm2 polymerization charge density. Scan size is 1 mm  1 mm.

solvent or electrolyte when platinum was used as working electrode. This is in clear departure from the present understanding wherein solvent and electrolyte played a major role in the final electrochromic properties when ITO was used as working electrode. Acknowledgments We acknowledge the Department of Electronics and Information Technology (DeitY) and Ministry of New and Renewable Energy (MNRE), India, for the financial support. We would also like to thank SAIF, IIT Bombay and IRCC for availing the FEG-SEM and AFM facilities. Rekha Singh would like to acknowledge the financial assistance in the form of a fellowship from the University Grant Commision (UGC), India. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.orgel.2015.11.034.

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

Abbreviations

[22] [23]

EDOT 3,4-ethylenedioxythiophene PEDOT poly(3,4-ethylenedioxythiophene) ADOTs 3,4-alkylenedioxythiophenes ITO indium tin oxide MeCN Acetonitrile TBAP tetrabutlyammonium perchlorate EC contrast electrochromic contrast CE coloration efficiency PCD polymerization charge density AFM atomic force microscopy FEG-SEM field emission gun-scanning electron microscopy

[24] [25]

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