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A novel composite polymer electrolyte containing room-temperature ionic liquids and heteropolyacids for dye-sensitized solar cells
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Electrochemistry Communications 9 (2007) 2755–2759 www.elsevier.com/locate/elecom
A novel composite polymer electrolyte containing room-temperature ionic liquids and heteropolyacids for dye-sensitized solar cells Da Chen b
a,b
, Qian Zhang b, Geng Wang b, Hao Zhang b, Jinghong Li
b,*
a Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China Department of Chemistry, Key Lab of Bioorganic Phosphorus Chemistry and Chemical Biology, Tsinghua University, Beijing 100084, China
Received 20 August 2007; received in revised form 18 September 2007; accepted 19 September 2007 Available online 22 October 2007
Abstract A novel composite polymeric gel comprising room-temperature ionic liquids (1-butyl-3-methyl-imidazolium-hexafluorophosphate, BMImPF6) and heteropolyacids (phosphotungstic acid, PWA) in poly(2-hydroxyethyl methacrylate) matrix was successfully prepared and employed as a quasi-solid state electrolyte in dye-sensitized solar cells (DSSCs). These composite polymer electrolytes offered specific benefits over the ionic liquids and heteropolyacids, which effectively enhanced the ionic conductivity of the composite polymer electrolyte. Unsealed devices employing the composite polymer electrolyte with the 3% content of PWA achieved the solar to electrical energy conversion efficiency of 1.68% under irradiation of 50 mW cm2 light intensity, increasing by a factor of more than three compared to a DSSC with the blank BMImPF6-based polymer electrolyte without PWA. It is expected that these composite polymer electrolytes are an attractive alternative to previously reported hole transporting materials for the fabrication of the long-term stable quasi-solid state or solid state DSSCs. 2007 Elsevier B.V. All rights reserved. Keywords: Room-temperature ionic liquid; Heteropolyacid; Composite polymer electrolyte; Ionic conductivity; Dye-sensitized solar cells
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted great attention over the past decade due to their high photoenergy conversion efficiencies and low cost cell fabrication processes [1,2]. For DSSCs, the electrolytes usually consist of an iodide/triiodide redox couple in organic solvents. However, the disadvantages of using liquid electrolytes are less long-term stability, difficulty in robust sealing, evaporation and leakage of electrolyte in case of breaking of the glass substrates [3,4]. Therefore, p-type semiconductor [5], hole-conductor [6], and polymeric materials incorporating triiodide/iodide as a redox couple [7–9] have
*
Corresponding author. Tel./fax: +86 10 6279 5290. E-mail address: [email protected] (J. Li).
1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.09.013
been attempted to substitute the liquid electrolytes for quasi-solid or solid state DSSCs. Recently, growing attention has been paid to quasi-solid DSSCs using polymer gel electrolytes due to their unique hybrid network structure and favorable properties such as thermal stability, non-flammability, negligible vapor pressure and easy solidification, when compared with the liquid electrolytes [10,11]. To pursue high conversion efficiency of DSSCs, it is necessary and pivotal to enhance the ionic conductivities of these polymer gel electrolytes [7,12,13]. However, conventional polymer electrolytes exhibit very low ambient ionic conductivity because of the severe crystallinity of polymers. In this respect, most of the recent studies have been directed to the preparation and characterization of polymer gel electrolytes that have higher ionic conductivity at ambient temperature. It is known that room-temperature ionic liquids (RTILs) have
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D. Chen et al. / Electrochemistry Communications 9 (2007) 2755–2759
wide liquid-phase range, non-flammability and very low vapor pressure at room temperature, wide electrochemical windows, high ionic conductivity, and excellent thermal and chemical stability [14,15]. One of the interesting developments in the field of RTILs is the combination of RTILs and polymers to form gel-like composites for the application in DSSCs [3,16], where the RTILs are both the ionic sources and the plasticizer. In addition, polymer electrolytes doped with proton donors have recently attracted much attention due to their high proton conductivity, chemical and electrochemical stability, and easier processing of polymer matrices. As one of the most attractive inorganic proton donors, heteropolyacids (HPAs) have been demonstrated to be highly conductive and structural stable [17,18]. These striking and significant observations have triggered our interest to explore the use of RTILs and HPAs as ‘‘gelator’’ to enhance the ionic conductivity of polymer electrolytes. To date, however, few work has been reported on composite polymers containing RTILs and HPAs as the polymer electrolytes for the fabrication of DSSCs. Hence, HPAs are combined with RTILs-based polymers to form polymer gel electrolytes for quasi-solid DSSCs in this paper. 2. Experimental 2.1. Materials 2-Hydroxyethyl methacrylate (HEMA) was purchased from Acros and further purified by distillation. 2,2 0 -Azobis-isobutyronitrile (AIBN) and tris(2,2 0 -bipyridyl)-ruthenium(II) chloride were purchased from Aldrich. 1-Butyl3-methyl-imidazolium-hexafluoro phosphate (BMImPF6) was purchased from Solvent Innovation. Phosphotungstic acid (PWA) and other reagents were purchased from Beijing Chemical Reagent Factory and used as received. 2.2. Preparation of composite polymeric gels Composite polymeric gels comprising PWA filler and BMImPF6 in poly(2-hydroxyethyl methacrylate) (PHEMA) matrix were prepared by a similar method to our previous published procedures [19]. Typically, HEMA was mixed with AIBN in 0.5 wt% with respect of HEMA as initiator of the free-radical polymerization. The homogeneous mixture was kept in an oven of 60 C for 0.5 h. Then BMImPF6 were added at the volume ratio of 1:4 to the amount of monomer and kept at 60 C for another 0.5 h. At last, PWA was added to the mixture, and sonicated for a few minutes to make a homogeneous phase. The resulting mixture was cast on polytetrafluoroethylene mould and heated first at 60 C for 5 h, and then increased to 70 C to initiate the free radical polymerization of HEMA for 20 h. The obtained products were further treated at 60 C in a vacuum for 12 h to remove the residual monomer. For comparison, the PHEMA–RTIL composite polymeric gels without PWA was prepared in the same pro-
cedures and labeled as ‘‘PWA00’’; accordingly, the composite polymeric gels with 3 wt% and 6 wt% PWA with respect to the weight of the HEMA monomer were labeled as ‘‘PWA03’’ and ‘‘PWA06’’, respectively. 2.3. Fabrication of quasi-solid state DSSCs Quasi-solid state DSSCs were assembled using a film of the composite polymer electrolyte sandwiched between the TiO2 photoanode and the counter electrode. TiO2 paste for the fabrication of photoanode was obtained by mixing 2 mL of ethanol and 300 mg of TiO2 nanoparticles (Degussa P25) homogeneously. Nanocrystalline TiO2 films were prepared by doctor blading of TiO2 paste onto FTO conducting glass substrate (15 X/square F-doping SnO2), followed by heating in air at 450 C for 2 h. The resultant ca. 4 lm thick TiO2 films were coated with the sensitizer dye tris(2,2 0 -bipyridyl)-ruthenium(II) chloride by immersing the film in a 1.0 · 103 M ethanol dye solution overnight. The transparent platinized counter electrode was prepared by the deposition of a drop of 0.05 M of H2PtCl6 in isopropanol solution onto the FTO conducting glass followed by heating at 400 C in air for 20 min. The composite polymer electrolyte comprised the as-prepared composite polymeric gel, containing 9% and 0.9% (w/w) of NaI and I2, respectively. For deposition, the suitable amount of composite polymeric gel (PWA00, PWA03 or PWA06) was added into an organic solvent mixture containing N,N-dimethyllformamide (DMF), ethylene carbonate (EC) and propylene carbonate (PC) in the ratio of 5:3:2 (w/w) under stirring, followed by the addition of NaI and I2, respectively. Then the resulting mixture was heated in a closed flask at 120 C to dissolve the polymer matrix, followed by cooling down to room temperature to form a homogeneous solution. Afterwards, about 500 lL of this solution was dropped over the sensitized TiO2 electrode in steps, allowed to dry in air, and then pressed against the counter electrode. An external clamp maintained the mechanical integrity of the cell, without any further sealing. 2.4. Measurements Attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectra were carried out using Perkin Elmer spectrometer in the frequency range of 4000– 600 cm1 with a resolution of 4 cm1. The thermal gravimetry (TG) analysis was performed with a STA 409C thermal analyzer (NETZSCH, Germany). Measurements were conducted by heating the as-prepared samples from 50 to 600 C at a heating rate of 10 C/min under flowing argon atmosphere. The SEM images of the samples were characterized by field emission scanning electron microscopy (FE-SEM; JEOL JSM-6700F) operated at 1.0 kV. The ionic conductivity of the composite polymeric gel was evaluated using electrochemical impedance spectra (EIS) method at ambient temperature and humidity. The impedance measurements were carried out on a PARSTAT
D. Chen et al. / Electrochemistry Communications 9 (2007) 2755–2759
2273 Advanced electrochemical system (Princeton Applied Research, USA). The sample was sandwiched between two stainless steel blocking electrodes (1.2 cm2). The impedance spectra were recorded with the help of ZPlot/ZView software under an ac perturbation signal of 5 mV over the frequency range of 1 MHz to 1 Hz. The photovoltaic tests of quasi-solid state DSSCs were carried out by measuring the photocurrent density–voltage (J–V) character curves under irradiation of white light from a 500-W xenon lamp under ambient atmosphere. The incident light intensity and the active cell area were 50 mW cm2 and 1.3 cm2, respectively.
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known to clearly show four typical peaks, referred as fingerprint, in the range of 1100–700 cm1 of their FTIR spectra [20–22]. For the composite system, three fingerprint peaks of the Keggin-type PWA molecules were identified in the spectra at 980, 907 and 807 cm1 (according to pure PWA), respectively, indicating that PWA molecules maintained their Keggin structures in the BMImPF6 plasticized polymer gels, and the fourth peak of the central P–O
3. Results and discussion The ATR-FTIR spectra of the obtained composite polymeric gels are shown in Fig. 1. Keggin-type HPAs are
Absorbance (a.u.)
PWA06
PWA03
PWA00
PWA
1200
1100
1000
900
800
700
Wavenumber (cm-1) Fig. 1. ATR-FTIR spectra of the composite electrolytes between 1200 and 600 cm1. PWA, pure PWA; PWA00, the PHEMA–RTIL composite electrolyte without PWA; PWA03 and PWA06, the composite electrolytes with PWA at the weight percent of 3% and 6%, respectively.
Weight Loss (%)
100 80
PWA PWA00 PWA03 PWA06
60 40 20 0 100
200
300
400
500
600
Temperature (ºC) Fig. 2. TGA curves of the composite electrolytes. PWA, pure PWA; PWA00, the PHEMA–RTIL composite electrolyte without PWA; PWA03 and PWA06, the composite electrolytes with PWA at the weight percent of 3% and 6%, respectively.
Fig. 3. FE-SEM images of the composite electrolytes: PWA00 (A), PWA03 (B) and PWA06 (C).
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Table 1 Ionic conductivities of composite polymer electrolytes and performance of the dye-sensitized solar cells based on these polymer electrolytes
PWA00 PWA03 PWA06
Ionic conductivity (S cm1) 5
5.2 · 10 1.4 · 104 9.7 · 105
Short-circuit current (mA cm2)
Open-circuit potential (V)
Fill factor
g (%)
0.98 2.58 2.14
0.63 0.64 0.61
0.42 0.51 0.47
0.52 1.68 1.23
(1080 cm1) vibration was overlapped by the very strong vibration of the BMImPF6 molecule. The TG curves for the composite polymeric gels are shown in Fig. 2. In the case of pure PWA, it showed the initial weight loss of surface water below 100 C and the following loss of structural water around 200 C, respectively. And the weight loss of pure PWA at the whole temperature range of 50–600 C was less than 10%, which indicated that PWA had the excellent thermal stability even at high temperature (300–600 C). As the PWA was incorporated into the RTIL plasticized polymeric gel, the first weight loss initiated from 100 C, indicating that the water in the gel was strongly adsorbed water. The reason might be that the polymeric gel had been treated at 60 C in vacuum and that the physically adsorbed water had been removed. The weight loss, between 100 and 200 C, was assigned to the release of structural water from the gel. Above 300 C, the PHEMA matrix and RTIL decomposed, showing the excellent thermal stability of the composite polymeric gel. In addition, with the increase of the PWA content in the composite polymeric gel, the final weight loss at 600 C decreased. This decrease was probably ascribed to the excellent thermal stability of PWA in the composite at the high temperature range. Fig. 3 shows FE-SEM images of composite polymeric gels. As shown, the PWA00 film was homogeneous and smooth (Fig. 3A). Compared to the PWA00 film, the PWA03 and PWA06 films were uneven and agglomerated; and nanoparticles could apparently be observed within these films due to the contained PWA filler (Fig. 3B and C). Furthermore, the unevenness, agglomeration and the observed nanoparticles increased with the increase of the containing PWA in the composite polymeric gel. The ionic conductivity of the composite polymeric gel was derived from a complex impedance plot, which was measured by using EIS method at ambient temperature and humidity [19,23,24]. The variations of ionic conductivities for the composite polymeric gels are listed in Table 1. In the absence of PWA, the conductivity of the blank composite polymeric gel (PWA00) was about 5.2 · 105 S cm1 resulting mainly from the intrinsic conductivity of the BMImPF6 ionic liquid itself. The ionic conductivities of the composite polymeric gels increased with the incorporation of the PWA salt. PWA exhibited preferential transport for positively charged ions [25], and it was highly probable that the described conductor was proton conducting because proton was one of the mobile species present in the PWA– BMImPF6-based composite polymer gel system. It can be seen that the conductivity of the PWA03 and PWA06 com-
posite polymeric gel was enhanced to 1.4 · 104 and 9.7 · 105 S cm1, respectively. Compared with the PWA03 gel, the conductivity of the PWA06 gel decreased to some extent, although PWA06 had the higher loading of HPAs in the composite. The decrease of ionic conductivity was maybe due to the more inhomogeneous and uneven morphology of PWA06 and the relatively strong interaction between BMImPF6 and PWA, which restrained the ion transportation and gave a reduced ionic conductivity. Fig. 4 shows the photocurrent–voltage characteristic curves of quasi-solid DSSCs based on the composite polymer electrolyte. The results for photocurrent density (Jsc), open-circuit voltage (Voc), filled factor (ff), and corresponding photo-energy conversion efficiency (g) are summarized in Table 1. It is found that g can be markedly enhanced by the PWA-containing composite polymer electrolyte. With the 3 wt% content of PWA in composite polymer electrolyte (PWA03), Jsc and Voc were both enhanced, reaching their maximum of 2.58 mA cm2 and 0.64 V, respectively. An efficiency g of 1.68% was achieved, increasing by a factor of more than three compared to a DSSC with the composite polymer electrolyte without PWA (PWA00). As the content of PWA increased to 6 wt% (PWA06), Voc and Jsc both decreased to some extent. From the above results, it was demonstrated that the quantity of PWA incorporated in the composite polymeric gel played a crucial role in the cell performance. With the introduction of the PWA into the composite RTIL-based polymer gel, the ionic conductivity of the gel electrolyte increased, which resulted in the increase of movability of I/I 3 redox couple and in turn 3.0 b
2.5
Current (mA cm-2 )
Composite electrolyte
c
2.0 1.5 1.0
a
0.5 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Potential (V) Fig. 4. I–V characteristics of the dye-sensitized TiO2 nanocrystalline solar cell using the composite polymer electrolyte (a) PWA00, (b) PWA03 and (c) PWA06 irradiated at 50 mW cm2.
D. Chen et al. / Electrochemistry Communications 9 (2007) 2755–2759
led to the increase of the short circuit of the cell. In addition, the stability of the quasi-solid state DSSCs based on these composite polymer electrolytes was also measured. After two months, the overall energy conversion efficiency of the cell kept 70–75% of the fresh one. The excellent thermostability of these novel composite polymer electrolytes is advantageous to the long-term usage of solar cell. 4. Conclusions For the first time we successfully employed PHEMA polymer in combination with RTILs and HPAs as a polymer gel electrolyte for quasi-solid state DSSCs with improved efficiencies. These composite polymer electrolytes offer specific benefits over the ionic liquids and heteropolyacids, which effectively enhance the ionic conductivity of the composite polymer electrolyte and in turn enhance the performance of the DSSCs. Further improvements on device performance should be readily achievable through optimization of the ionic conductivity, the dye-sensitized TiO2 film, cell sealing, etc. It is expected that these composite polymer electrolytes are an attractive alternative to previously reported hole transporting materials for the fabrication of the long-term stable quasi-solid state or solid state DSSCs. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20435010 and No. 20675044) and 863 Project (2006AA05Z123). References [1] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737.
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[2] P. Wang, S.M. Zakeeruddin, P. Comte, I. Exnar, M. Gra¨tzel, J. Am. Chem. Soc. 125 (2003) 1166. [3] W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, Chem. Commun. (2002) 374. [4] H.X. Wang, H. Li, B.F. Xue, Z.X. Wang, Q.B. Meng, L.Q. Chen, J. Am. Chem. Soc. 127 (2005) 6394. [5] G.R.A. Kumara, S. Kaneko, M. Okuya, K. Tennakone, Langmuir 18 (2003) 10493. [6] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weisso¨rtel, J. Salbeck, H. Spreitzer, M. Gra¨tzel, Nature 395 (1998) 583. [7] F. Cao, G. Oskam, P.C. Searson, J. Phys. Chem. 99 (1995) 17071. [8] A.F. Nogueira, J.R. Durrant, M.A. De Paoli, Adv. Mater. 13 (2001) 826. [9] E. Stathatos, P. Lianos, U. Lavrencic-Stangar, B. Orel, Adv. Mater. 14 (2002) 354. [10] A.F. Nogueira, C. Longo, M.A. De Paoli, Coord. Chem. Rev. 248 (2004) 1455. [11] L. Wang, S.B. Fang, Y. Lin, X.W. Zhou, M.Y. Li, Chem. Commun. (2005) 5687. [12] A.F. Nogueira, M.A. De Paoli, I. Montanari, R. Monkhouse, J. Nelson, J.R. Durrant, J. Phys. Chem. B 105 (2001) 7517. [13] E. Stathatos, P. Lianos, S.M. Zakeeruddin, P. Liska, M. Gra¨tzel, Chem. Mater. 15 (2003) 1825. [14] J. Dupont, P.A.Z. Suarez, Phys. Chem. Chem. Phys. 8 (2006) 2441. [15] D. Chen, G. Wang, J.H. Li, J. Phys. Chem. C 111 (2007) 2351. [16] R. Kawano, M. Watanabe, Chem. Commun. (2005) 2107. [17] K. Hatakeyama, H. Sakaguchi, K. Ogawa, H. Inoue, C. Iwakura, T. Esaka, J. Power Sources 124 (2003) 559. [18] N. Lakshmi, S. Chandra, J. Power Sources 108 (2002) 256. [19] Z.Y. Li, Y. Liu, H.T. Liu, P. He, Q. Zhang, J.H. Li, Solid State Ionics 177 (2006) 1281. [20] U.L. Stangar, N. Groselj, B. Orel, P. Colomban, Chem. Mater. 12 (2000) 3745. [21] P. Saiti, S. Freni, S. Hocevar, J. Power Sources 79 (1999) 250. [22] C.R. Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207. [23] A.F. Nogueira, M.A.S. Spinace´, W.A. Gazotti, E.M. Girotto, M.A. De Paoli, Solid State Ionics 140 (2001) 327. [24] X. Zhang, H. Yang, H.M. Xiong, F.Y. Li, Y.Y. Xia, J. Power Sources 160 (2006) 1451. [25] J.D. Kim, I. Honma, Electrochim. Acta 48 (2003) 3633.
Electrochemistry Communications 9 (2007) 2755–2759 www.elsevier.com/locate/elecom
A novel composite polymer electrolyte containing room-temperature ionic liquids and heteropolyacids for dye-sensitized solar cells Da Chen b
a,b
, Qian Zhang b, Geng Wang b, Hao Zhang b, Jinghong Li
b,*
a Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China Department of Chemistry, Key Lab of Bioorganic Phosphorus Chemistry and Chemical Biology, Tsinghua University, Beijing 100084, China
Received 20 August 2007; received in revised form 18 September 2007; accepted 19 September 2007 Available online 22 October 2007
Abstract A novel composite polymeric gel comprising room-temperature ionic liquids (1-butyl-3-methyl-imidazolium-hexafluorophosphate, BMImPF6) and heteropolyacids (phosphotungstic acid, PWA) in poly(2-hydroxyethyl methacrylate) matrix was successfully prepared and employed as a quasi-solid state electrolyte in dye-sensitized solar cells (DSSCs). These composite polymer electrolytes offered specific benefits over the ionic liquids and heteropolyacids, which effectively enhanced the ionic conductivity of the composite polymer electrolyte. Unsealed devices employing the composite polymer electrolyte with the 3% content of PWA achieved the solar to electrical energy conversion efficiency of 1.68% under irradiation of 50 mW cm2 light intensity, increasing by a factor of more than three compared to a DSSC with the blank BMImPF6-based polymer electrolyte without PWA. It is expected that these composite polymer electrolytes are an attractive alternative to previously reported hole transporting materials for the fabrication of the long-term stable quasi-solid state or solid state DSSCs. 2007 Elsevier B.V. All rights reserved. Keywords: Room-temperature ionic liquid; Heteropolyacid; Composite polymer electrolyte; Ionic conductivity; Dye-sensitized solar cells
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted great attention over the past decade due to their high photoenergy conversion efficiencies and low cost cell fabrication processes [1,2]. For DSSCs, the electrolytes usually consist of an iodide/triiodide redox couple in organic solvents. However, the disadvantages of using liquid electrolytes are less long-term stability, difficulty in robust sealing, evaporation and leakage of electrolyte in case of breaking of the glass substrates [3,4]. Therefore, p-type semiconductor [5], hole-conductor [6], and polymeric materials incorporating triiodide/iodide as a redox couple [7–9] have
*
Corresponding author. Tel./fax: +86 10 6279 5290. E-mail address: [email protected] (J. Li).
1388-2481/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.09.013
been attempted to substitute the liquid electrolytes for quasi-solid or solid state DSSCs. Recently, growing attention has been paid to quasi-solid DSSCs using polymer gel electrolytes due to their unique hybrid network structure and favorable properties such as thermal stability, non-flammability, negligible vapor pressure and easy solidification, when compared with the liquid electrolytes [10,11]. To pursue high conversion efficiency of DSSCs, it is necessary and pivotal to enhance the ionic conductivities of these polymer gel electrolytes [7,12,13]. However, conventional polymer electrolytes exhibit very low ambient ionic conductivity because of the severe crystallinity of polymers. In this respect, most of the recent studies have been directed to the preparation and characterization of polymer gel electrolytes that have higher ionic conductivity at ambient temperature. It is known that room-temperature ionic liquids (RTILs) have
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D. Chen et al. / Electrochemistry Communications 9 (2007) 2755–2759
wide liquid-phase range, non-flammability and very low vapor pressure at room temperature, wide electrochemical windows, high ionic conductivity, and excellent thermal and chemical stability [14,15]. One of the interesting developments in the field of RTILs is the combination of RTILs and polymers to form gel-like composites for the application in DSSCs [3,16], where the RTILs are both the ionic sources and the plasticizer. In addition, polymer electrolytes doped with proton donors have recently attracted much attention due to their high proton conductivity, chemical and electrochemical stability, and easier processing of polymer matrices. As one of the most attractive inorganic proton donors, heteropolyacids (HPAs) have been demonstrated to be highly conductive and structural stable [17,18]. These striking and significant observations have triggered our interest to explore the use of RTILs and HPAs as ‘‘gelator’’ to enhance the ionic conductivity of polymer electrolytes. To date, however, few work has been reported on composite polymers containing RTILs and HPAs as the polymer electrolytes for the fabrication of DSSCs. Hence, HPAs are combined with RTILs-based polymers to form polymer gel electrolytes for quasi-solid DSSCs in this paper. 2. Experimental 2.1. Materials 2-Hydroxyethyl methacrylate (HEMA) was purchased from Acros and further purified by distillation. 2,2 0 -Azobis-isobutyronitrile (AIBN) and tris(2,2 0 -bipyridyl)-ruthenium(II) chloride were purchased from Aldrich. 1-Butyl3-methyl-imidazolium-hexafluoro phosphate (BMImPF6) was purchased from Solvent Innovation. Phosphotungstic acid (PWA) and other reagents were purchased from Beijing Chemical Reagent Factory and used as received. 2.2. Preparation of composite polymeric gels Composite polymeric gels comprising PWA filler and BMImPF6 in poly(2-hydroxyethyl methacrylate) (PHEMA) matrix were prepared by a similar method to our previous published procedures [19]. Typically, HEMA was mixed with AIBN in 0.5 wt% with respect of HEMA as initiator of the free-radical polymerization. The homogeneous mixture was kept in an oven of 60 C for 0.5 h. Then BMImPF6 were added at the volume ratio of 1:4 to the amount of monomer and kept at 60 C for another 0.5 h. At last, PWA was added to the mixture, and sonicated for a few minutes to make a homogeneous phase. The resulting mixture was cast on polytetrafluoroethylene mould and heated first at 60 C for 5 h, and then increased to 70 C to initiate the free radical polymerization of HEMA for 20 h. The obtained products were further treated at 60 C in a vacuum for 12 h to remove the residual monomer. For comparison, the PHEMA–RTIL composite polymeric gels without PWA was prepared in the same pro-
cedures and labeled as ‘‘PWA00’’; accordingly, the composite polymeric gels with 3 wt% and 6 wt% PWA with respect to the weight of the HEMA monomer were labeled as ‘‘PWA03’’ and ‘‘PWA06’’, respectively. 2.3. Fabrication of quasi-solid state DSSCs Quasi-solid state DSSCs were assembled using a film of the composite polymer electrolyte sandwiched between the TiO2 photoanode and the counter electrode. TiO2 paste for the fabrication of photoanode was obtained by mixing 2 mL of ethanol and 300 mg of TiO2 nanoparticles (Degussa P25) homogeneously. Nanocrystalline TiO2 films were prepared by doctor blading of TiO2 paste onto FTO conducting glass substrate (15 X/square F-doping SnO2), followed by heating in air at 450 C for 2 h. The resultant ca. 4 lm thick TiO2 films were coated with the sensitizer dye tris(2,2 0 -bipyridyl)-ruthenium(II) chloride by immersing the film in a 1.0 · 103 M ethanol dye solution overnight. The transparent platinized counter electrode was prepared by the deposition of a drop of 0.05 M of H2PtCl6 in isopropanol solution onto the FTO conducting glass followed by heating at 400 C in air for 20 min. The composite polymer electrolyte comprised the as-prepared composite polymeric gel, containing 9% and 0.9% (w/w) of NaI and I2, respectively. For deposition, the suitable amount of composite polymeric gel (PWA00, PWA03 or PWA06) was added into an organic solvent mixture containing N,N-dimethyllformamide (DMF), ethylene carbonate (EC) and propylene carbonate (PC) in the ratio of 5:3:2 (w/w) under stirring, followed by the addition of NaI and I2, respectively. Then the resulting mixture was heated in a closed flask at 120 C to dissolve the polymer matrix, followed by cooling down to room temperature to form a homogeneous solution. Afterwards, about 500 lL of this solution was dropped over the sensitized TiO2 electrode in steps, allowed to dry in air, and then pressed against the counter electrode. An external clamp maintained the mechanical integrity of the cell, without any further sealing. 2.4. Measurements Attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectra were carried out using Perkin Elmer spectrometer in the frequency range of 4000– 600 cm1 with a resolution of 4 cm1. The thermal gravimetry (TG) analysis was performed with a STA 409C thermal analyzer (NETZSCH, Germany). Measurements were conducted by heating the as-prepared samples from 50 to 600 C at a heating rate of 10 C/min under flowing argon atmosphere. The SEM images of the samples were characterized by field emission scanning electron microscopy (FE-SEM; JEOL JSM-6700F) operated at 1.0 kV. The ionic conductivity of the composite polymeric gel was evaluated using electrochemical impedance spectra (EIS) method at ambient temperature and humidity. The impedance measurements were carried out on a PARSTAT
D. Chen et al. / Electrochemistry Communications 9 (2007) 2755–2759
2273 Advanced electrochemical system (Princeton Applied Research, USA). The sample was sandwiched between two stainless steel blocking electrodes (1.2 cm2). The impedance spectra were recorded with the help of ZPlot/ZView software under an ac perturbation signal of 5 mV over the frequency range of 1 MHz to 1 Hz. The photovoltaic tests of quasi-solid state DSSCs were carried out by measuring the photocurrent density–voltage (J–V) character curves under irradiation of white light from a 500-W xenon lamp under ambient atmosphere. The incident light intensity and the active cell area were 50 mW cm2 and 1.3 cm2, respectively.
2757
known to clearly show four typical peaks, referred as fingerprint, in the range of 1100–700 cm1 of their FTIR spectra [20–22]. For the composite system, three fingerprint peaks of the Keggin-type PWA molecules were identified in the spectra at 980, 907 and 807 cm1 (according to pure PWA), respectively, indicating that PWA molecules maintained their Keggin structures in the BMImPF6 plasticized polymer gels, and the fourth peak of the central P–O
3. Results and discussion The ATR-FTIR spectra of the obtained composite polymeric gels are shown in Fig. 1. Keggin-type HPAs are
Absorbance (a.u.)
PWA06
PWA03
PWA00
PWA
1200
1100
1000
900
800
700
Wavenumber (cm-1) Fig. 1. ATR-FTIR spectra of the composite electrolytes between 1200 and 600 cm1. PWA, pure PWA; PWA00, the PHEMA–RTIL composite electrolyte without PWA; PWA03 and PWA06, the composite electrolytes with PWA at the weight percent of 3% and 6%, respectively.
Weight Loss (%)
100 80
PWA PWA00 PWA03 PWA06
60 40 20 0 100
200
300
400
500
600
Temperature (ºC) Fig. 2. TGA curves of the composite electrolytes. PWA, pure PWA; PWA00, the PHEMA–RTIL composite electrolyte without PWA; PWA03 and PWA06, the composite electrolytes with PWA at the weight percent of 3% and 6%, respectively.
Fig. 3. FE-SEM images of the composite electrolytes: PWA00 (A), PWA03 (B) and PWA06 (C).
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D. Chen et al. / Electrochemistry Communications 9 (2007) 2755–2759
Table 1 Ionic conductivities of composite polymer electrolytes and performance of the dye-sensitized solar cells based on these polymer electrolytes
PWA00 PWA03 PWA06
Ionic conductivity (S cm1) 5
5.2 · 10 1.4 · 104 9.7 · 105
Short-circuit current (mA cm2)
Open-circuit potential (V)
Fill factor
g (%)
0.98 2.58 2.14
0.63 0.64 0.61
0.42 0.51 0.47
0.52 1.68 1.23
(1080 cm1) vibration was overlapped by the very strong vibration of the BMImPF6 molecule. The TG curves for the composite polymeric gels are shown in Fig. 2. In the case of pure PWA, it showed the initial weight loss of surface water below 100 C and the following loss of structural water around 200 C, respectively. And the weight loss of pure PWA at the whole temperature range of 50–600 C was less than 10%, which indicated that PWA had the excellent thermal stability even at high temperature (300–600 C). As the PWA was incorporated into the RTIL plasticized polymeric gel, the first weight loss initiated from 100 C, indicating that the water in the gel was strongly adsorbed water. The reason might be that the polymeric gel had been treated at 60 C in vacuum and that the physically adsorbed water had been removed. The weight loss, between 100 and 200 C, was assigned to the release of structural water from the gel. Above 300 C, the PHEMA matrix and RTIL decomposed, showing the excellent thermal stability of the composite polymeric gel. In addition, with the increase of the PWA content in the composite polymeric gel, the final weight loss at 600 C decreased. This decrease was probably ascribed to the excellent thermal stability of PWA in the composite at the high temperature range. Fig. 3 shows FE-SEM images of composite polymeric gels. As shown, the PWA00 film was homogeneous and smooth (Fig. 3A). Compared to the PWA00 film, the PWA03 and PWA06 films were uneven and agglomerated; and nanoparticles could apparently be observed within these films due to the contained PWA filler (Fig. 3B and C). Furthermore, the unevenness, agglomeration and the observed nanoparticles increased with the increase of the containing PWA in the composite polymeric gel. The ionic conductivity of the composite polymeric gel was derived from a complex impedance plot, which was measured by using EIS method at ambient temperature and humidity [19,23,24]. The variations of ionic conductivities for the composite polymeric gels are listed in Table 1. In the absence of PWA, the conductivity of the blank composite polymeric gel (PWA00) was about 5.2 · 105 S cm1 resulting mainly from the intrinsic conductivity of the BMImPF6 ionic liquid itself. The ionic conductivities of the composite polymeric gels increased with the incorporation of the PWA salt. PWA exhibited preferential transport for positively charged ions [25], and it was highly probable that the described conductor was proton conducting because proton was one of the mobile species present in the PWA– BMImPF6-based composite polymer gel system. It can be seen that the conductivity of the PWA03 and PWA06 com-
posite polymeric gel was enhanced to 1.4 · 104 and 9.7 · 105 S cm1, respectively. Compared with the PWA03 gel, the conductivity of the PWA06 gel decreased to some extent, although PWA06 had the higher loading of HPAs in the composite. The decrease of ionic conductivity was maybe due to the more inhomogeneous and uneven morphology of PWA06 and the relatively strong interaction between BMImPF6 and PWA, which restrained the ion transportation and gave a reduced ionic conductivity. Fig. 4 shows the photocurrent–voltage characteristic curves of quasi-solid DSSCs based on the composite polymer electrolyte. The results for photocurrent density (Jsc), open-circuit voltage (Voc), filled factor (ff), and corresponding photo-energy conversion efficiency (g) are summarized in Table 1. It is found that g can be markedly enhanced by the PWA-containing composite polymer electrolyte. With the 3 wt% content of PWA in composite polymer electrolyte (PWA03), Jsc and Voc were both enhanced, reaching their maximum of 2.58 mA cm2 and 0.64 V, respectively. An efficiency g of 1.68% was achieved, increasing by a factor of more than three compared to a DSSC with the composite polymer electrolyte without PWA (PWA00). As the content of PWA increased to 6 wt% (PWA06), Voc and Jsc both decreased to some extent. From the above results, it was demonstrated that the quantity of PWA incorporated in the composite polymeric gel played a crucial role in the cell performance. With the introduction of the PWA into the composite RTIL-based polymer gel, the ionic conductivity of the gel electrolyte increased, which resulted in the increase of movability of I/I 3 redox couple and in turn 3.0 b
2.5
Current (mA cm-2 )
Composite electrolyte
c
2.0 1.5 1.0
a
0.5 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Potential (V) Fig. 4. I–V characteristics of the dye-sensitized TiO2 nanocrystalline solar cell using the composite polymer electrolyte (a) PWA00, (b) PWA03 and (c) PWA06 irradiated at 50 mW cm2.
D. Chen et al. / Electrochemistry Communications 9 (2007) 2755–2759
led to the increase of the short circuit of the cell. In addition, the stability of the quasi-solid state DSSCs based on these composite polymer electrolytes was also measured. After two months, the overall energy conversion efficiency of the cell kept 70–75% of the fresh one. The excellent thermostability of these novel composite polymer electrolytes is advantageous to the long-term usage of solar cell. 4. Conclusions For the first time we successfully employed PHEMA polymer in combination with RTILs and HPAs as a polymer gel electrolyte for quasi-solid state DSSCs with improved efficiencies. These composite polymer electrolytes offer specific benefits over the ionic liquids and heteropolyacids, which effectively enhance the ionic conductivity of the composite polymer electrolyte and in turn enhance the performance of the DSSCs. Further improvements on device performance should be readily achievable through optimization of the ionic conductivity, the dye-sensitized TiO2 film, cell sealing, etc. It is expected that these composite polymer electrolytes are an attractive alternative to previously reported hole transporting materials for the fabrication of the long-term stable quasi-solid state or solid state DSSCs. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20435010 and No. 20675044) and 863 Project (2006AA05Z123). References [1] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737.
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