Free-standing polypyrrole films as substrate-free and Pt-free counter electrodes for quasi-solid dye-sensitized solar cells

Organic Electronics 13 (2012) 3032–3039

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Free-standing polypyrrole films as substrate-free and Pt-free counter electrodes for quasi-solid dye-sensitized solar cells P. Veerender, Vibha Saxena, P. Jha, S.P. Koiry, Abhay Gusain, S. Samanta, A.K. Chauhan, D.K. Aswal ⇑, S.K. Gupta Technical Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India

a r t i c l e

i n f o

Article history: Received 9 July 2012 Received in revised form 20 August 2012 Accepted 20 August 2012 Available online 13 September 2012 Keywords: DSSC Free-standing polypyrrole TCO-free counter electrodes Pt-free counter electrodes

a b s t r a c t Substrate-free and Pt-free counter electrodes (CE) were utilized for dye sensitized solar cells for the first time using free-standing polypyrrole (PPy) films. The PPy films were synthesized by liquid–liquid biphasic interfacial polymerization. These films have unique morphologies as they have a dense base (that provides mechanical strength) and porous top (resulting in large surface area for catalytic activity). The electrochemical impedance spectroscopy measurements revealed that free-standing PPy exhibit reasonable catalytic activ ity for I 3 =I redox couple. The dye-sensitized solar cells were fabricated using N3 dye, gel polymer electrolyte and free-standing PPy films as CE. The power conversion efficiency measured under AM 1.5, 100 mW/cm2 illumination was 3.5%. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSSC) have emerged as promising technology for harvesting solar energy since their inception in 1991 by O’Regan and Gratzel [1]. In recent years, DSSCs have been extensively studied owing to their potential for enabling simpler fabrication process at lower cost. Typically, DSSCs comprise a photoanode (sensitized dye adsorbed on mesoporous TiO2), a redox electro lyte (usually I 3 =I redox couple), and a platinized counter electrode (CE). A Pt film coated on transparent conducting oxide (TCO) glass substrates is employed as CE because of its excellent catalytic ability to reduce the triiodide into iodide. However, the commercial viability of Pt as CE has been limited because of (i) its high cost and limited flexibility, (ii) corrosion of the Pt in presence of small traces of water in the electrolyte, (iii) high temperature processing, and (iv) dissolution of Pt films and formation of PtI4 [2]. To address these issues, a great deal of research is oriented towards the development of Pt-free counter elec-

⇑ Corresponding author. E-mail address: [email protected] (D.K. Aswal). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.08.039

trodes. In this regard, conducting polymers hold the promise for enabling the replacement of Pt CEs in DSSC because of their unique properties, including low cost, ease of synthesis, good catalytic activity, and remarkable stability [3]. Many conducting polymers such as, poly(3,4-ethylenedioxythiophene) (PEDOT) [4], poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) [5], polyaniline [6], and polypyrrole [7] were studied as alternative CEs for DSSCs. In most cases, the films of conducting polymers were prepared by spin casting on TCO substrates and annealed at high temperatures in order to improve film adhesion on TCO substrate. However, the conducting polymer films prepared in this manner have limited thickness. Additionally, the poor film adhesion on the substrate requires high temperature processing. In order to avoid the use of high temperature processing, Xia et al. [8] deposited conducting polymer films on TCO substrates by vapour phase polymerization in two steps: first an oxidant material was spin casted on TCO substrate and then the substrate was kept in monomer saturated vapour to polymerize the monomer on the substrate. However, the conducting polymer films formed this way have low surface area owing to the limited thickness of the film.

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However, to obtain high-efficiency DSSCs, the thickness of the counter electrode should be much greater than that of the Pt-FTO electrode for facile triiodide reduction. This issue was addressed by adopting electrochemical deposition of conducting polymer, which resulted in thick and high surface area films [9]. However, the electrochemical deposition of conducting polymers requires TCO substrates, which adds to the cost of the DSSCs. Since the use of TCO substrate and Pt account for 50% of the total cost of DSSCs [10], there is a need for the development of cost effective TCO-free and Pt-free counter electrodes DSSCs. Though there are a few reports available on such TCO-free and Pt-free electrodes that include PEDOT [11], titanium carbide, tungsten oxide, and vanadium nitride [12], to the best of our knowledge, there is no report available on the use of substrate-free and Pt-free counter electrode by employing conducting polymers. The growth of two-dimensional films using interfacial polymerization is a promising technique to prepare template-free nanostructures in a simple and inexpensive way. The soft interface between immiscible liquids (e.g. aqueous-organic) assist in the assembly of nanostructures since highly mobile molecules or monomers can easily reach equilibrium on self-assembly at the interface [13]. In this paper, we have adopted this method to synthesize free-standing films of PPy at aqueous-organic biphase, which yields distinct morphology, that is, a compact bottom and porous top. The prepared freestanding PPy films could be used as a CE for the fabrication of the DSSC because of their good catalytic activity (owing to porous morphology of the top surface) and without any need of substrate (owing to compact morphology of the bottom surface). Further, we have employed a gel electrolyte to fabricate the quasi-solid DSSC. Devices were also fabricated using Pt counter electrode for comparison, and the results show that the free-standing PPy films can be employed efficiently as substrate-free and Pt-free counter electrodes.

2. Experimental 2.1. Preparation of free-standing polypyrrole films All chemicals used were of analytical grade and consumed without any further purification. Ferric chloride (FeCl3) was purchased from Thomas baker. Pyrrole (Aldrich, 98%) was double distilled under reduced pressure prior to use. PPy free-standing films were synthesized by interfacial polymerization, as schematically shown in Fig. 1. Typically 20 ml of 0.1 M aqueous FeCl3 and 20 ml of 0.1 M pyrrole (prepared in dichloromethane, DCM) are placed in a beaker. It is found that within a few minutes, a PPy layers start growing at the aqueous/organic interface. This layer grows in thickness with passage of time with a growth direction towards the organic phase. Typical thickness of the PPy film after 18 h was found to be 70 lm. These freestanding PPy films were found to mechanically strong and could be easily lifted onto glass or plastic substrates using a pair of tweezers. These freestanding PPy films were washed with de-ionized water and DCM to

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Fig. 1. (a) Schematic showing the formation of free-standing PPy films at the aqueous (0.1 M FeCl3)/organic (0.1 M pyrrole) interface. A dense PPy film polymerizes at the interface, which with passage of time grows towards the organic phase and forms a porous structure. (b) Photograph of the mechanically lifted free-standing PPy film with dense surface on the front (the porous surface is on the back).

remove un-reacted FeCl3 and pyrrole and later dried overnight. Visual inspection of the film revealed two different textures on either sides of the film: the face formed towards aqueous phase exhibited dense morphology (referred as base or bottom face) while the face towards organic phase appeared porous (referred as top face). This was later confirmed by scanning electron microscopy (SEM). SEM images were recorded using TESCAN (Model:VEGA MV2300T/40) at an accelerating voltage 30 kV. The PPy films were also imaged using atomic force microscopy (AFM, Multiview 4000, Nanonics). Films were further characterized by Ultraviolet–visible (UV–Vis) absorption spectroscopy using a double beam UV/Vis spectrophotometer (Jasco, V 530). The UV–Vis absorption spectra were recorded by dispersing free-standing PPy film in N-methyl2-pyrrolidone (NMP) solvent by ultrasonication. Fouriertransform infrared (FTIR) spectra of the PPy films were recorded using Bruker spectrometer (Vertex 80 V) in reflectance mode at 4 cm1 resolution. Cyclic voltammetry  (CV) measurements for the I 3 =I redox couple was carried out in a three electrode electrochemical cell with pristine free-standing film as working electrode, Pt foil as counter electrode, and an Ag/AgCl reference electrode in 10 mM LiI, 1 mM I2, and 0.1 M tetrabutyl ammonium perchlorate (TBAP) in acetonitrile solution at a scan rate of 50 mV/s. 2.2. DSSC fabrication Fig. 2 shows the schematic device structure of the DSSC using free-standing PPy film as a counter electrode. For the preparation of a photoanode, fluorine-doped tin oxide (FTO) glass substrates were subsequently cleaned in detergent, DI water, and methanol for 30 min each and later dried under nitrogen (N2) stream. Afterwards, the substrates were treated with TiCl4 by dipping in 50 mM aqueous solution of TiCl4 for ½ h at 70 °C followed by annealing at 450 °C for ½ h. This treatment resulted in the formation of a compact TiO2 layer. Further, a nanocrystalline/mesoporous TiO2 layer (Solaronix, HT) was prepared by doctor blading, followed by sequential sintering at 75 °C for

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Fig. 2. Schematic of the DSSC assembly using free-standing PPy film as counter electrode.

10 min, 135 °C for 10 min, 175 °C for 20 min, 425 °C for 10 min and 450 °C for 10 min [14]. The electrodes prepared this way were immersed in a dye solution consisting of 0.5 mM N3 dye (Solaronix) in ethanol for 24 h. Dye coated TiO2 photoanodes were then rinsed in ethanol and dried under N2 stream. The gel polymer electrolyte (GPE) was prepared by first dissolving 1.338 g lithium iodide (LiI) and 0.254 g Iodine (I2) in 4 ml propylene carbonate and 10 ml acetonitrile and then 3 g of polyethylene oxide (PEO, MW – 1  106, Alfa Aesar) was added to this mixture. Further, the mixture was heated at 70–80 °C under continuous stirring for 7–8 h. Finally, a homogeneous viscous GPE was obtained. Pt counter electrodes were prepared by depositing a drop of 50 mM solution of H2PtCl6 in isopropanol onto FTO substrates followed by annealing at 450 °C in air for 30 min. In case of PPy CE, the electrical contact was taken on top surface of the film by thermally evaporating a thin (50 nm) gold layer in vacuum chamber at 105 bar. The electrical contact was taken by attaching a silver wire (dia.: 50 lm and length: 1 cm) to the gold electrode using a dot of silver paste. The DSSC was fabricated by sandwiching dye coated TiO2 electrode and counter electrode using stretched parafilm for spacer. The GPE was injected between the photoanode and CE using a syringe. The final photovoltaic device area was 0.3 cm2 and characterized using potentiostat/Galvanostat (PGSTAT 30, Autolab, Eco Chemie, Netherlands) under illumination of AM 1.5 G (intensity  100 mW/cm2). For electrochemical impedance spectroscopy (EIS) measurements, the DSSC was kept at open circuit potential and AC amplitude of 10 mV was applied. The measurements were carried out from 500 kHz to 0.05 Hz. The obtained spectra were analyzed using an equivalent circuit model fitted with ZSimpWin 3.22 software. 3. Results and discussion Fig. 3(a) and (b) show the cross-sectional SEM images of the free-standing PPy films polymerized at aqueous/organic interface for 4 and 18 h, respectively. In both cases, it is evident that initially a densely packed polypyrrole film having thickness 1 lm forms, which is followed by a porous and granular film. The porous part grows in size with increasing polymerization time. In order to further investigate the nature of dense and porous surfaces, we have recorded magnified SEM and AFM images for a PPy films obtained after 18 h of interfacial polymerization. Typical obtained results are shown in Fig. 4. It is evident from Fig 4(a) and (b) that the dense face of the PPy films is com-

Fig. 3. Cross-sectional SEM images of the free-standing PPy films polymerized at aqueous/organic interface for: (a) 4 h and (b) 18 h. The film is composed of two parts: (i) dense, which is typically 1 lm thick and does not grow with time, and (ii) porous structure that grows in thickness with time.

posed of plate-like structures with an rms roughness of 15 nm. The formation of plate-like structure, as schematically depicted in Fig. 4(c), is attributed to the fact that the pyrrole molecules are oriented preferentially at the aqueous/organic interface due to H-bonding of pyrrole monomer with water phase [15], and therefore, forming oriented and long polypyrrole chains as the polymerization takes place at 2 and 5 carbon position of the pyrrole ring. The strong p–p interactions among the oriented polypyrrole chains lead to the formation of plate-like structures. On the other hand, the magnified SEM image of the porous surface revealed that PPy has granular (size: 1 lm) structure with deep pores. Due to the deep pores (several microns) the AFM image could not be recorded. In the bulk organic phase (i.e. away from the interface) the pyrrole monomers are randomly oriented, which results in the for-

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Fig. 4. SEM and AFM images of the denser and porous surfaces of the PPy films polymerized for 18 h. (a) and (b) are respectively the SEM and AFM images of the denser surface. (c) Schematic showing the oriented polymerized pyrrole chains forming a dense layer at the aqueous/organic interface due to enhanced p–p interactions. (d) Magnified SEM image of the porous surface. (e) Schematic showing the formation of coil-like structure away from the aqueous/organic interface that leads to the formation of porous structure.

Fig. 5. (a) FTIR and (b) UV–Vis spectra of free-standing PPy films.

mation of coil-like polypyrrole chains, as schematically depicted in Fig 4(e). This coil-like polypyrrole chains eventually lead to the formation of granular and porous structure. The polypyrrole films were characterized by FTIR and UV– Vis spectroscopy. Fig. 5(a) shows the baseline corrected FTIR spectra of PPy film exhibiting all characteristic vibrations of polypyrrole. The peaks observed at 1490 cm1 and 1590 cm1, and 686 cm1 are attributed to symmetric and antisymmetric ring stretching modes and C-H out of plane bending of pyrrole, respectively [16]. The bands observed at 1063 cm1 and 1387 cm1 correspond to C–H and N–H deformation vibrations, respectively. The absorption bands at 950 cm1, 1100 cm1, and 1233 cm1 correspond to stretching vibrations of doped polypyrrole [17]. The doped form of the polypyrrole is also confirmed by UV–Vis spec-

tra (see Fig. 5b), which shows a low energy tail extending into the near IR region. The high intensity of tail is attributed to the electronic transitions between the valence band and bipolaran band. The doped form of PPy is beneficial as this helps in facilitating charge transport in DSSC. In addition, a band is observed in absorption spectra at 390 nm which is attributed to the p–p⁄ transitions [18]. The electrocatalytic activity of free-standing PPy films was examined for I =I 3 redox couple by recording the cyclic voltammograms (CV). The electrochemical reaction at the counter electrode surface is described by following equation:

I3 þ 2e ¼ 3I 

3I2 þ 2e ¼

2I3

ð1Þ ð2Þ

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Fig. 6. Cyclic voltammograms of free-standing PPy films and Pt/FTO in 10 mM LiI, 1 mM I2, and 0.1 M TBAP in acetonitrile solution at 50 mV/s scan rate. The inset shows the magnified peak after background subtraction for free-standing PPy film.

Fig. 6 shows the typical cyclic voltammograms of both Pt and free-standing PPy counter electrodes in 10 mM LiI, 1 mM I2, and 0.1 M TBAP in acetonitrile solution at

50 mV/s scan rate. For Pt, the anodic/cathodic peaks corre sponding to oxidation/reduction of I and I 3 =I 3 =I2 are clearly seen at 0.52 V and 1.02 V, respectively. Of these two redox reactions, the one corresponding to Eq. (1) i.e.  oxidation/reduction of I 3 =I is important from device point of view. However, in the case of PPy, a weak peak at 0.62 V  corresponding to the oxidation potential of I is ob3 =I served (magnified version of the peak is shown in the inset of Fig. 6) [16]. The magnitude of cathodic/anodic current density is quite smaller for PPy as compared to Pt, and this is expected, as the PPy has relatively poor conductivity. In addition, the porous structure of PPy hinders pathways for the transfer of the electrons. Another point of concern in the CV data of Fig. 6 is a slight positive shift (0.1 V) of oxi dation/reduction of I 3 =I peak. The redox potential (EF) of   the I3 =I redox couple is governed by the Nernst equation [19]:

EF ¼ E0 þ ð0:059=nÞ lnðI3 =I Þ

ð3Þ

where n is the number of electron transferred in half reaction. The slightly poor catalytic behavior of PPy film, leads  to an increase in I 3 ions relative to I at the PPy surface. This eventually results in the positive shift in the redox potential.

Fig. 7. Nyquist plots of DSSCs with (a) Pt/FTO, (b) free-standing PPy counter electrodes. Insets on left show the magnified version of charge transfer resistances at counter electrode interfaces and on right shows the porous behaviour of counter electrodes. (c) and (d) are the equivalent circuits employed for fitting the experimentally measured data. (e) and (f) show Bode plots of DSSCs with Pt/FTO, and free-standing PPy counter electrodes.

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Fig. 8. Schematic depiction of (a) Nernst diffusion in Pt/FTO based DSSC and (b) Nernst and pore diffusions in free-standing PPy based DSSC.

In order to further gain an insight into the catalytic behavior of free-standing PPy films, electrochemical impedance spectroscopy measurements were carried out. Typical Nyquist plots recorded in the frequency range 500 kHz–0.05 Hz for Pt/FTO CE and PPy CE based DSSCs are shown in Fig. 7(a) and (b), respectively. In both the cases, two arcs, one small and another large, are clearly seen. The inference drawn from these figures are as follows: (i). The smaller arc correspond to high frequency range (500 kHz–500 Hz) and represents charge transfer resistance at CE/electrolyte interface (R1). The magnified version of the smaller arc is shown in the left insets of the Fig. 7(a) and (b), and their further magnified versions of high frequency end are shown in the right insets. It is evident that PPy CE exhibits a smaller arc at very high frequency, which is attributed to the porous morphology of the PPy. A similar observation was reported for porous graphite nanofibers earlier [20]. Absence of this arc for Pt CE is evident as it does not have a porous surface. (ii). The larger arc (corresponding to mid frequency range 500–0.1 Hz) represent the charge transfer resistances at TiO2/dye/electrolyte interface (R2). Based on the above results we infer that Pt/FTO CE has two-dimensional planar structure (i.e. without any porosity), and PPy CE has three-dimensional porous structure. Thus the schematic of DSSC fabricated using Pt and PPy CEs are shown in Fig. 8. In the case of Pt based DSSC, the  diffusion of ions ðI 3 =I Þ between photoanode and Pt CE can be described by Nernst diffusion (Nd). Therefore, the Nyquist plots can be fitted using an equivalent circuit comprising of a contact resistance (Rs) in series with two RQ elements (R: resistance and Q: constant phase element), see Fig. 7(c). Whereas for PPy based DSSC, use of only Nernst diffusion is not sufficient and pore diffusion (i.e. short-distance diffusion within the free-standing PPy film, Np) need to be considered. Therefore, for PPy CE based DSSCs an equivalent circuits, see Fig. 7(d), comprising of 1Rs and 3 RQ elements are used. Using these equivalent circuits, a very good fitting of experimental data is observed with chi-squared value (v2, sum of the squares of the residual) <103. The various estimated parameters for the equivalents circuits are summarized in Table 1. It is evident that the values of Rs, R1 and R2 are higher for the

Table 1 Fitted impedance parameters of Pt/FTO and PPy CE based DSSCs using equivalent circuits, as shown in Fig. 7.

Pt/FTO PPy

Rs (O cm2)

Ro (O cm2)

R1 (O cm2)

R2 (O cm2)

se (ms)

9.55 13.3

– 2.7

4.14 38.4

76.7 302.4

17.3 11.9

PPy CE as compared to the Pt CE, which may be attributed to low conductivity and porosity of PPy films. However, the value of R1 (38.4 X cm2) obtained in our case is lower than that obtained (i.e. 46 X cm2) using electropolymerised PPy films [21]. In order to estimate the electron lifetime (se) in TiO2 films, the Bode plots (phase vs frequency) were recorded for both the DSSCs fabricated using Pt CE and PPy CE, and the results are shown in Fig. 7(e) and (f), respectively. The value of se was estimated from the mid frequency peak (xmid) of Bode plots using the following equation [22]:

se ¼ 1=xmid ¼ 1=2pfmid

ð4Þ

The obtained se values are shown in Table 1. It is seen that PPy (11.9 ms) and Pt (17.3 ms) CEs have comparable recombination lifetimes. Above results clearly show that freestanding PPy films have reasonable electrocatalytic activity as well as recombination lifetimes. These features along with higher surface area of free-standing PPy films make them suitable candidate as counter electrode in DSSCs. The photovoltaic performance of the DSSCs using PPy and Pt CE were evaluated under AM 1.5 (100 mW/cm2) light illumination. The recorded current–voltage characteristics are shown in Fig. 9. The obtained photovoltaic parameters for both the DSSC’s are summarized in Table 2. The main inferences drawn from this table are as follows. (i). The open-circuit voltage (Voc) of the DSSC fabricated using PPy CE is slightly lower as compared to that of Pt CE. In general, the Voc of DSSC depends on the difference between quasi-Fermi level of the electrons in the TiO2 and the redox potential of the electrolyte. In the present case, DSSC’s were fabricated using identical photoanode (N3 dye coated TiO2) and electrolyte (polymer gel) but with different counter electrodes i.e. Pt/FTO or free-standing PPy film.

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Fig. 9. Current–voltage characteristics of DSSCs with Pt/FTO and freestanding PPy counter electrodes under one sun illumination.

includes contributions from Rs, Ro, R1 and R2, as shown in Table 1. (iv). The power conversion efficiency (PCE) of PPy and Pt based DSSCs are respectively 3.5% and 5.3%. Apart from the low Voc and FF, the adsorbed oxygen on porous PPy (known to be a serious impurity for the organic materials) can reduce the efficiency. Nevertheless, considering the first set of experiments on employing freestanding conducting polypyrrole as counter electrode for the fabrication of DSSCs, the measured efficiency is respectable. The efficiency can be further enhanced by taking appropriate steps, such as, improving the conductivity of the PPy films, optimizing the thickness, reducing the adsorbed oxygen content etc. 4. Conclusions

Table 2 Photovoltaic parameters of Pt/FTO and free-standing PPy CE based DSSCs.

Pt/FTO PPy

Jsc (mA/cm2)

Voc (V)

FF

g (%)

11.01 11.31

0.66 0.49

0.73 0.63

5.3 3.5

Therefore, the difference in Voc can be attributed to  the change in the redox potential of I 3 =I . As discussed earlier, due to the slightly poor catalytic activity of PPy, the redox potential is increased by 0.1 V, which is reflected in the reduced Voc. Further, Voc is related to I 3 concentration using the following equation [23]:

 V oc ¼ ½KT=e ln Iinj =ncb ket ½I3 

ð5Þ

where k is the Boltzmann constant, T is absolute temperature, e is an electronic charge, Iinj is the flux of charge resulting from sensitized injection, ncb is the concentration of electrons on the surface of TiO2, ket is the reaction rate constant of I 3 dark reac tion on TiO2, ½I  is the concentration of I 3 3 in electrolyte. The high concentration of I 3 ions at PPy surface clearly implies the low Voc. (ii). On a positive note, the short circuit current density, Jsc (11.31 mA/cm2), of PPy based DSSC is slightly higher than that of Pt based DSSC (11.01 mA/cm2). Jsc in DSSC depends on electron-injection, charge transfer, and charge-recombination processes. Slightly higher Jsc in this particular case is attributed to the large surface area of the freestanding PPy CE owing to its porous structure. (iii). In addition to Jsc and Voc, the fill factor (FF) is another important parameter that determines the performance of the DSSCs. FF is defined as the ratio of the actual maximum obtainable power (Pmax) to the theoretical power (Pt = Jsc  Voc). The FF (0.63) of PPy based DSSC is lower than that of Pt based DSSC (0.73). The reduced FF of PPy based DSSC is attributed to their enhanced series resistance, which

Free-standing PPy films synthesized by interfacial polymerization were successfully employed as counter electrodes for quasi-solid dye-sensitized solar cells. The cyclic voltammetry and electrochemical impedance spectroscopy measurements revealed that free-standing PPy counter  electrodes show reasonable catalytic activity for I 3 =I redox couple. The DSSCs fabricated using free-standing PPy counter electrodes exhibited power conversion efficiency of 3.5% under AM 1.5, 100 mW/cm2 illumination. Our results demonstrate that free-standing PPy films are potential candidate as substrate-free and Pt-free counter electrodes for DSSC fabrication. Acknowledgements This work is partly supported by ‘‘DAE-SRC Outstanding Research Investigator Award’’ (2008/21/05-BRNS/2743) and ‘‘Prospective Research Funds’’ (2008/38/02-BRNS/ 2858) granted to DKA. References [1] B. O’Regan, M. Gratzel, A low-cost high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] E. Olsen, G. Hagen, S. Eric Lindquist, Dissolution of platinum in methoxy propionitrile containing LiI/I2, Sol. Energy Mater. Sol. Cells 63 (2000) 267–273. [3] T.N. Murakami, M. Grätzel, Counter electrodes for DSC: application of functional materials as catalysts, Inorg. Chim. Acta 361 (2008) 572–580. [4] S. Ahmad, J.-H. Yum, Z. Xianxi, M. Gratzel, H.-J. Butt, M.K. Nazeeruddin, Dye-sensitized solar cells based on poly(3,4ethylenedioxythiophene) counter electrode derived from ionic liquids, J. Mater. Chem. 20 (2010) 1654–1658. [5] K.-M. Lee, C.-Y. Hsu, P.-Y. Chen, M. Ikegami, T. Miyasaka, K.-C. Ho, Highly porous PProDOT-Et2 film as counter electrode for plastic dyesensitized solar cells, Phys. Chem. Chem. Phys. 11 (2009) 3375–3379. [6] S. Cho, S.H. Hwang, C. Kim, J. Jang, Polyaniline porous counterelectrodes for high performance dye-sensitized solar cells, J. Mater. Chem. 22 (2012) 12164–12171. [7] J. Wu, Q. Li, L. Fan, Z. Lan, P. Li, J. Lin, S. Hao, High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells, J. Power Sources 181 (2008) 172–176. [8] J. Xia, L. Chen, S. Yanagida, Application of polypyrrole as a counter electrode for a dye-sensitized solar cell, J. Mater. Chem. 21 (2011) 4644–4649. [9] Z. Li, B. Ye, X. Hu, X. Ma, X. Zhang, Y. Deng, Facile electropolymerizedPANI as counter electrode for low cost dye-sensitized solar cell, Electrochem. Commun. 11 (2009) 1768–1771.

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