platinum composite counter electrode

Synthetic Metals 197 (2014) 204–209

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Enhanced performance of dye-sensitized solar cells based on an electrodeposited-poly(3,4-ethylenedioxythiophene)/platinum composite counter electrode Gentian Yue a,∗ , Dawei Zhang a , Fuirui Tan a , Jihuai Wu b,∗∗ , Fumin Li a , Chong Chen a , Zhang Lan b a b

Key Laboratory of Photovoltaic Materials of Henan and School of Physics & Electronics, Henan University, Kaifeng 475004, China Institute of Material Physical Chemistry, Huaqiao University, Quanzhou 362021, China

a r t i c l e

i n f o

Article history: Received 25 December 2013 Received in revised form 5 July 2014 Accepted 15 September 2014 Keywords: Poly(3,4ethylenedioxythiophene)/platinum Electrochemical Counter electrode Dye-sensitized solar cell

a b s t r a c t Composite film of poly(3,4-ethylenedioxythiophene)/platinum (PEDOT/Pt) with high efficient is electrodeposited onto rigid fluorine-doped tin oxide substrate by using a facile approach of one step electrochemical polymerization, and its application as counter electrode (CE) in dye-sensitized solar cell (DSSC). The DSSC based on the PEDOT/Pt counter electrode exhibited a high power conversion efficiency of 7.86% under illumination of 100 mW cm−2 , which is higher 13.09% than that of the DSSC based on Pt electrode (6.95%). The extensive electrochemical analyses for the PEDOT/Pt CE made from cyclic voltammetry, electrochemical impedance spectroscopy measurements, and Tafel curves reveal that the PEDOT/Pt hybrid CE possesses high electrocatalytic activity for the reduction of triiodide to iodide and low charge-transfer resistance on the counter electrode–electrolyte interface. Therefore, the PEDOT/Pt CE can be considered as a promising alternative CE for DSSCs. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSSCs) have attracted significant attention in the pursuit of alternatives to silicon solar cells due to their low fabrication costs, environmental friendliness, simple preparation process, and high power conversion efficiency [1,2]. In traditionally, DSSC consists of dye-sensitized photoanode, electrolyte, and counter electrode (CE). Among them, the role of the photoanode is to absorb sun light and generate electrons. The electrolyte is iodine/triiodide (I− /I3 − ) mixture for transferring electrons from CE to excited dyes. The function of CE is to catalyze the I− /I3 − redox couple and completes the electric circuit in the DSSCs. To date, platinum (Pt) as the conventional catalyst for DSSCs has induced an intense level of electrocatalytic activity for the I− /I3 − redox couple [3], and the commercialization of DSSCs is impeded owing to high cost of platinum. This resulted in many efforts to seek suitable Pt substitution in DSSCs with low cost and high electrocatalytic activity. So far, a great deal of alternatives has shown Pt-like functions as CEs, such as the carbon materials (e.g., carbon

∗ Corresponding author. Tel.: +86 371 23881940. ∗∗ Corresponding author. Tel.: +86 595 22693899. E-mail addresses: [email protected] (G. Yue), [email protected] (J. Wu). http://dx.doi.org/10.1016/j.synthmet.2014.09.018 0379-6779/© 2014 Elsevier B.V. All rights reserved.

nanotubes, graphene, activate carbon, and so on) [4–7], conducting polymers (e.g., polyaniline, polypyrrole, polythiophene, and so on) [8–10], metal sulfides (e.g., cabolt sulfide, nickel sulphides, and so on) [11–14], nitrides (e.g., transition metal nitrides) [15,16], and carbides (e.g., tungsten carbide and molybdenum carbide) [17,18]. Among them, conducting polymers have been considered one of the most promising candidates as CE materials in Pt-free DSSCs for their high conductivity, low-cost, large electrochemical surface area, and good electrocatalytic activity for I3 − reduction [19,20]. For instance, poly (3, 4-ethylenedioxythiophene) (PEDOT), as a typical conductive polymer, has been intensively studied due to its unique characteristics, e.g., great conductivity, excellent electrocatalytic activity, good film-forming ability, protracted stability in atmosphere, and low cost availability [21,22]. Furthermore, composite film combining carbon-based materials and conducting polymers have exhibited excellent electrochemical properties and catalytic activity due to their synergistic effects [23,24]. Until now, there are few reports in the literature describing integrated studies of Pt and PEDOT composite film as the CE catalyst in Pt-free DSSC by using a cyclic voltammetry (CV) electropolymerization. In the view of this, a novel composite film with high electrocatalytic activity is designed and prepared based on PEDOT system using electrochemical polymerization method and served as CE catalyst in Pt-free DSSCs, by which the obtained film

G. Yue et al. / Synthetic Metals 197 (2014) 204–209

could possess excellent electrocatalytic activity for the reduction of triiodide to iodide and low charge-transfer resistance at the electrolyte–electrode interface. The DSSC based on the PEDOT/Pt CE shows a high power conversion efficiency of 7.86% under illumination of 100 mW cm−2 , which is much higher than that of DSSC with the conventional Pt electrode or PEDOT CE. The PEDOT/Pt CE preparation with one-step electropolymerization not only maintains excellent electrochemical properties and high conductivity of PEDOT and Pt, but also greatly reduces the cost of production. Thus, the results carried a foreshadowing for the development of a new generation of efficient, low cost DSSC using composite CEs. 2. Experimental 2.1. Materials The Lithium perchlorate (LiClO4 ), polyethylene glycol with average molecular weight 600 (PEG-600), 3, 4-ethylenedioxythiophene (EDOT), H2 PtCl6 ·6H2 O, isopropanol, 4-tert-butyl-pyridine (TBP) and titanium tetrachloride (TiCl4 ) are purchased from Shanghai Chemical Agent Ltd, China. The organometallic dye N-719 [Cis-di(isothiocyanato)-bis-(2,2 -bipyridyl-4,4 -dicarboxylato) ruthenium (II) bis-tetrabutylammonium] is obtained from Solaronix SA (Switzerland). All reagents are of analytical reagent grade. Fluorine-doped tin oxide (FTO) glass substrates (8  cm−2 , Hartford Glass Co., USA) are cut into 1 × 1.5 cm2 carefully and ultrasonically cleaned sequentially in detergent, acetone and distilled water for 10 min, respectively; and then stored in isopropyl alcohol. 2.2. Preparation of PEDOT/Pt CE The electrodeposition of PEDOT and Pt onto FTO glass substrate was carried out with an electrochemical analyzer system (CHI660D, Shanghai Chenhua Device Company, China). All experiments were implemented in a three-electrode cell at room temperature (about 25 ◦ C), including one Pt foil as CE, one Ag/AgCl electrode as reference electrode and FTO glass substrate with an exposed area of 0.8 × 0.8 cm2 as the working electrode. The base polymerization solution consisted of 0.1 M EDOT, 0.1 M LiClO4 and PEG-600 dissolved in anhydrous ethanol. Then the prepared of 0.01 M H2 PtCl6 solution was mixed into above PEDOT base polymerization solution by ultrasonication for 1 h. A constant current density of 10 mA cm−2 was served for electrodeposition (the synthesis route shown in Scheme 1). The FTO glass substrate coated with PEDOT/Pt film was put into anhydrous ethanol for 2 h and vacuum oven at 100 ◦ C for 12 h, respectively. 2.3. Fabrication of DSSC

manuscripts [25,26]. Briefly, a thin TiO2 blocking layer was deposited on the FTO substrate by immersing the FTO in the 0.15 M of TiCl4 isopropanol solution at 70 ◦ C for 0.5 h, followed by sintering at 450 ◦ C for 30 min in air. Subsequently, the TiO2 layer with particle size of 10–20 nm was coated onto the blocking layer by using doctor blade method, and then sintered at 450 ◦ C for 30 min in air. A TiCl4 modified layer formed by immersing the above obtained TiO2 electrode in 0.15 M TiCl4 isopropanol solution for another 0.5 h and sintering at 450 ◦ C for another 30 min in air, by which obtained the TiO2 electrode with TiCl4 modified can improve the roughness and increase the adsorption of dye. The dye was loaded by immersing the TiO2 electrode in the 0.3 mM of dye N719 ethanol solution for 24 h. Thus the dye-sensitized TiO2 anode with thickness of 8–10 ␮m was obtained. The DSSC was fabricated by injecting the liquid electrolyte (0.05 M of I2 , 0.1 M of LiI, 0.6 M of tetrabutylammonium iodide and 0.5 M of TBP in acetonitrile) in the aperture between the dye-sensitized TiO2 anode and the PEDOT/Pt CE. The two electrodes were clipped together and wrapped with thermoplastic hot-melt Surlyn. 2.4. Characterization and measurements The surface morphology of the sample was observed by using a JSM-7600F field emission scanning electron microscope (FESEM). Cyclic voltammetry (CV), electrochemical impendence spectroscopy (EIS) and Tafel polarization curves were conducted using a computer-controlled electrochemical analyzer (CHI 660D, CH Instrument). The electrolyte used in the DSSC test was also injected into the symmetric dummy cells for both EIS and Tafel measurements. EIS was carried out using a CHI660D (Shanghai Chenhua Device Company, China) electrochemical measurement system at a constant temperature of 25 ◦ C in ambient atmosphere under dark conditions and leaving an exposed area of 0.64 cm2 . The frequency of applied sinusoidal AC voltage signal was varied from 0.1 Hz to 105 Hz and the corresponding amplitude was kept at 5 mV in all cases. The resultant impedance spectra were analyzed by means of the ZSimdemo version 3.30d. The photovoltaic testing of the DSSC was carried out by measuring photocurrent–photovoltage (J–V) characteristic curves under white light irradiation of a 100 mW cm−2 (AM1.5) from a solar simulator (XQ-500 W, Shanghai Photoelectricity Device Company, China) in ambient atmosphere and leaving an exposed area of 0.25 cm2 . The fill factor (FF) and the power conversion efficiency () of the DSSC was calculated according to the following equations [1]: (%) = FF =

The preparation methods including the synthesized nanocrystalline TiO2 particles, fabrication of multilayer photoanode, and assembling of DSSC is all the same as our previous reported

205

V max ×J max VOC × JSC × FF × 100% = × 100%, Pin Pin

V max ×J max , VOC × JSC

(1) (2)

where JSC is the short-circuit current density (mA cm−2 ); VOC is the open-circuit voltage (V), Pin is the incident light power and Jmax

Scheme 1. The synthesis route of PEDOT/Pt CE.

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Fig. 1. The SEM images of PEDOT (a) and PEDOT/Pt counter electrodes (b). The inset of Fig. 1b represents the cross-sectional image of PEDOT/Pt film.

(mA cm−2 ) and Vmax (V) are the current density and voltage at the point of maximum power output in the J–V curves, respectively. 3. Results and discussions 3.1. Morphology and composition of the CEs Fig. 1 shows the SEM images of the PEDOT and PEDOT/Pt CEs, respectively. The inset of Fig. 1b represents the cross-sectional image of PEDOT/Pt film with a thickness of about 534 nm. From Fig. 1a, the PEDOT film represents a mound-like surface morphology with flower clusters-like nanoparticles aggregations, which is provided with high electrocatalytic activity and stability for I− /I3 − liquid electrolyte. Compared with Fig. 1a, it can be observed that the PEDOT/Pt CEs possesses excellent uniform surface with perfect network structure as shown as in Fig. 1b, thus possibly improving penetration of I− /I3 − liquid electrolyte into the inside of the PEDOT/Pt film, enhancing the contact between counter electrode and I− /I3 − liquid electrolyte, and ultimately creating highly electrocatalytic activity. 3.2. Electrochemical properties of CEs Fig. 2 shows the CV during the electrochemical polymerization process of PEDOT/Pt on FTO substrates in EDOT and H2 PtCl6 mix

Current Density / mA cm

-2

8

solution with scan rate of 10 mV s−1 , and the arrows indicate the peaks development with successive scans. The oxidation of EDOT and H2 PtCl6 start at about + 0.7 V with the formation of PEDOT/Pt on FTO surface. As seen as in Fig. 2, it also exhibits broad anodic and cathodic peaks corresponding to the oxidation and the reduction of the preformed PEDOT/Pt hybrid CE; and the peak current densities increase proportionally with the number of scans increasing. With the consecutive scans increasing, the oxidation peaks for the PEDOT/Pt CE become slightly more anodic and reduction peaks become more cathodic as shown as in Fig. 2. This phenomenon is responsible for the formation of the PEDOT/Pt conductive hybrid film on FTO substrate and the film thickness increasing [27]. Fig. 3 shows the CVs for the Pt, PEDOT and PEDOT/Pt CEs at the scan rate of 10 mV s−1 , respectively. To investigate the redox reactions on the CE side, the scan interval ranging from −0.4 to 0.4 V versus. Pt is selected according to the previous reports [28,29]. The anodic peaks are responsible for the reaction 3I− → I3 − + 2e− , and the cathodic peaks are vital for its operation in DSSC and corresponding to I3 − + 2e− → 3I– . The cathodic peak potential (Epp) and current density (Ipc ) are two critical parameters for comparing electrocatalytic activities of different CEs. The more positive Epp means the smaller overpotential for reduction of I3 − to I− , indicating an excellent electrocatalytic activity for triiodide reduction reaction [30]. From Fig. 3, the Epp of the reduction reaction on the PEDOT/Pt CE is more positive than that of the PEDOT and Pt CEs, which increase in the order of Pt (−0.186 V)
scan rate = 10 mV.s-1

6 4 2 0 -2 -0.4

-0.2

0.0

0.2 0.4 0.6 0.8 Potential / V vs. Pt

1.0

1.2

Fig. 2. The cyclic voltammograms during electropolymerization of PEDOT/Pt on FTO substrates in anhydrous ethanol solution with the scan rate of 10 mV s−1 .

Fig. 3. Cyclic voltammograms for the Pt (a), PEDOT (b) and PEDOT/Pt (c) electrodes with the scan rate of 10 mV s−1 .

G. Yue et al. / Synthetic Metals 197 (2014) 204–209

a

207

b

Fig. 4. 40 cycles CVs of the PEDOT/Pt electrode with the scan rate of 10 mV s−1 (a), and the relationship between the cycles and the maximum redox peak currents for the PEDOT/Pt electrode at the rate of 10 mV s−1 (b).

(−0.125 V); the absolute value of the Ipc decrease in the order of Pt (2.94 mA)
PEDOT and PEDOT/Pt CEs and equivalent circuit models for I− /I3 − reaction, in which the high-frequency intercept on the real axis represents the series resistance (Rs). The semicircle at high frequency refers to the charge-transfer resistance (Rct) for the I3 − reduction at the CE electrolyte interface, the corresponding constant phase angle element (C1 ) at the CE electrolyte interface, and the semicircle at low frequency represents the Nernst diffusion impedance (Zw) corresponding to the diffusion resistance of the I− /I3 − redox species, respectively [25,31]. The resultant EIS parameters by fitting with ZSimdemo version 3.30d software are listed in Table 1. From Fig. 5 and Table 1, the Rs for the PEDOT/Pt CE has a similar value of around 3.10  cm2 to the Pt electrode (3.12  cm2 ) and PEDOT electrode (3.13  cm2 ), indicating the PEDOT/Pt hybrid film is firmly bonded to the FTO substrate (discussed at the anterior CV section). The PEDOT/Pt hybrid CE exhibits the lower Rct value of 1.31  cm2 compared to the pristine Pt electrode (1.49  cm2 ) and PEDOT electrode (1.89  cm2 ) CE, revealing a synergistic effect of the Pt and PEDOT on the improvement of electrocatalytic activity and electrical conductivity for the hybrid CE. In other words, the PEDOT/Pt hybrid CE is provided smaller overpotential for an electron transferring from the CE to the electrolyte [32]. Additionally, it should be noted that the Zw for the PEDOT/Pt hybrid CE (1.56  cm2 ) is larger than that of Pt electrode (0.80  cm2 ). This may be attributed to the relatively low electrical conductivity of conductive polymers compared with that of the Pt catalyst. Fig. 6 presents the Tafel curves of the Pt, PEDOT and PEDOT/Pt CEs to aid investigation the interfacial charge-transfer properties of the I− /I3 − redox couple on the CEs. It shows the logarithmic current density (log j) as a function of the potential for the reduction of I− /I3 − . From Fig. 6, it can be found that the anodic or cathodic

2.4

-2 Log j / log mA.cm

1.8 1.2 0.6 0.0 CEs

-0.6

Pt PEDOT PEDOT/Pt

-1.2 -0.9 Fig. 5. EIS of the symmetrical Pt, PEDOT and PEDOT/Pt CEs and equivalent circuit models for I− /I3 − reaction.

-0.6

-0.3 0.0 0.3 Potential / V

0.6

0.9

Fig. 6. Tafel curves of the symmetrical Pt, PEDOT and PEDOT/Pt counter electrodes.

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Table 1 EIS parameters of the dummy cells obtained and the photoelectric properties of the DSSCs with various counter electrodes (Voc: open circuit voltage; FF: fill factor; Jsc: short-circuit current density). Electrodes

Rs ( cm2 )

Rct ( cm2 )

Zw ( cm2 )

Voc (mV)

Jsc (mA cm−2 )

FF

 (%)

Pt PEDOT PEDOT/Pt

3.12 3.13 3.10

1.49 1.89 1.31

0.80 2.98 1.56

729 726 743

14.45 13.75 15.55

0.66 0.63 0.68

6.95 6.29 7.86

branches follow the order of PEDOT/Pt >Pt >PEDOT. A steep slope in the Tafel zone for the PEDOT/Pt CEs implies a large exchange current density (J0 ), suggesting higher conductivity and larger active surface area. This might be one of the key factors for its efficient catalytic activities [33]. Furthermore, the PEDOT/Pt hybrid CE also provides an excellent limiting current density (Jlim ), revealing a large diffusion coefficient [34], which is compared to that of a dummy cell based on the Pt CEs. Briefly, the results of EIS and Tafel both indicates that the PEDOT/Pt hybrid CE possesses superior conductivity and excellent electrocatalytic activity compared to that of Pt CE, thus it can be logically expected considerably improved the photovoltaic performance for DSSC. 3.4. Photovoltaic performance of DSSCs with PEDOT/Pt CE Fig. 7 shows the photocurrent density–voltage (J–V) curves for DSSCs based on various CEs under the irradiation of a simulated solar light with an intensity of 100 mW cm−2 , and the derived photovoltaic parameters are summarized in Table 1. From Fig. 7 and Table 1, these DSSCs are provided with comparatively similar Voc (729 mV), which is mainly determined by the energy level difference between the Fermi level of the electron in TiO2 and the redox potential of the electrolyte [1]. The DSSC with the Pt and PEDOT CEs obtained Jsc of 14.45 and 13.75 mA cm−2 , Voc of 729 and 726 mV, FF of 0.66 and 0.63, and corresponding to the  of 6.95 and 6.29%, respectively. In contrast, the DSSC based on the PEDOT/Pt CE obtained Jsc of 15.55 mA cm−2 , Voc of 743 mV, FF of 0.68, and corresponding to the  of 7.86%. This obviously demonstrates that the photovoltaic parameters of the DSSC with the PEDOT/Pt hybrid CE are better than that of the DSSC Pt-based. This is due to the special features of the PEDOT/Pt hybrid CE, such as the unique surface morphology, small overpotential, low Rct and the high cathodic current density provided by the synergistic catalytic effect of the Pt and PEDOT, which is advantageous to increase the I− /I3 − redox reaction rate on the CE side, thus possibly resulting in the enhancement of the Jsc and FF values [35]. The results are agreement with the CV, EIS, and Tafel measurements. Therefore, under the optional

Fig. 8. Equivalent electrical circuit used to analyze electrical resistances of DSSCs (a); EIS spectra of DSSCs with the Pt and PEDOT/Pt counter electrodes (b).

condition, the DSSC with the PEDOT/Pt hybrid CE exhibits a high power conversion efficiency of 7.86%, which is superior to that of the DSSC using Pt CE (6.95%). The EIS spectra of the DSSCs with the Pt and PEDOT/Pt CEs and the equivalent circuit models are presented in Fig. 8 to analyze the cells impedance. The resulting resistance values are summarized in Table 2, in which the Rs represents the series resistance in the DSSCs; Rct1 and Rct2 are the electron transfer resistances at the CE electrolyte interface and the working electrode (WE) electrolyte interface, respectively; CPE1 and CPE2 are constant phase elements of the CE electrolyte interface and the WE electrolyte interface; Zw is diffusion coefficient of ions in the electrolyte. From the Fig. 8, EIS spectra of DSSCs consist of three semicircles; the first semicircle represents the Rct1 at the CE electrolyte interface; the middle semicircle is related to the Rct2 at the WE electrolyte interface; and the third semicircle represents the Zw. It is notable that the DSSCs based on the Pt and PEDOT/Pt CEs possesses the similar Rs of 13.42 and 13.18  cm2 , Rct2 of 10.74 and 11.88  cm2 , and Zw of 8.06 and 7.43  cm2 , which can be attributed to the same FTO substrates (with resistance of 8  cm−2 ), TiO2 photoanodes and electrolyte for the DSSCs. The DSSCs with Pt and PEDOT/Pt CEs exhibit the Rct1 of 2.93 and 1.62  cm2 . It indicates that the DSSC based on the PEDOT/Pt CE reveals much lower Rct1 than that of the DSSC Ptbased due to the unique surface morphology, small overpotential

Table 2 EIS parameters of the DSSCs with the Pt and PEDOT/Pt counter electrodes.

Fig. 7. J–V characteristics of the DSSCs fabricated with different CEs under the standard illumination.

Electrodes

Rs ( cm2 )

Rct1 ( cm2 )

Rct2 ( cm2 )

Zw ( cm2 )

Pt PEDOT/Pt

13.42 13.18

2.93 1.62

10.74 11.88

8.06 7.43

G. Yue et al. / Synthetic Metals 197 (2014) 204–209

and great catalytic property for the reduction of I3 − to I− . As we all known, small resistance signifies fast electron transport and high performance in DSSCs. Thus, the results suggest that the PEDOT/Pt CE is provided with an excellent electrochemical performance in DSSCs for the iodide/triiodide electrolyte. 4. Conclusions A highly performance PEDOT/Pt counter electrode with perfect network structure is successfully synthesized by using a polymerization method, and served as CE in Pt-free DSSCs. The DSSC assembled with the PEDOT/Pt CE exhibits the power conversion efficiency of 7.86% under 100 mW cm−2 sunlight illumination (AM1.5 G), which exceeds 13.09% than that of the DSSC with the Pt electrode (6.95%). The electrochemical performance of the PEDOT/Pt CE is characterized by using CV, EIS, Tafel measurements, and demonstrated amazing electrocatalytic activity for the I− /I3 − redox reaction due to its high cathodic current density, small overpotential, and the low Rct of 1.31  cm2 . Thus, the PEDOT/Pt CE prepared by using electropolymerization technique indicates a great potential as low cost and high performance alternative in large-scale DSSCs. Acknowledgements The authors are very grateful to the joint support by the National Natural Science Foundation of China (No. 61306019, No. U1205112 and No. 21103041). This work is also supported by the Natural Science Foundation of Henan Educational Committee (No. 14A430023), the Scientific Research Found of Henan Provincial Department of Science and Technology (No. 132300413210) and the Natural Science Foundation of Henan University (No. 2013YBZRO47). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2014.09.018. References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] A. Yella, H.W. Lee, H.N. Tsao, C.Y. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629–634. [3] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Grätzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316–320. [4] H. Choi, H. Kim, S. Hwang, Y. Han, M. Jeon, Graphene counter electrodes for dyesensitized solar cells prepared by electrophoretic deposition, J. Mater. Chem. 21 (2011) 7548–7551. [5] H. Choi, H. Kim, S. Hwang, W. Choi, M. Jeon, Dye-sensitized solar cells using graphene-based carbon nano composite as counter electrode, Sol. Energy Mater. Sol. Cells 95 (2011) 323–325. [6] F. Gong, H. Wang, Z.S. Wang, Self-assembled monolayer of graphene/Pt as counter electrode for efficient dye-sensitized solar cell, Phys. Chem. Chem. Phys. 13 (2011) 17676–17682. [7] L. Kavan, J.H. Yum, M. Grätzel, Graphene nanoplatelets outperforming platinum as the electrocatalyst in co-bipyridine-mediated dye-sensitized solar cells, Nano Lett. 11 (2011) 5501–5506. [8] G.T. Yue, J.H. Wu, Y.M. Xiao, M.L. Huang, J.M. Lin, Z. Lan, Functionalized graphene/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate as counter electrode catalyst for dye-sensitized solar cells, Energy 54 (2013) 315– 321. [9] H. Sun, Y. Luo, Y. Zhang, D. Li, Z. Yu, K. Li, Q.B. Meng, In situ preparation of a flexible polyaniline/carbon composite counter electrode and its application in dye-sensitized solar cell, J. Phys. Chem. C 114 (2010) 11673–11679.

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