Improved dye-sensitized solar cells by composite ionic liquid electrolyte incorporating layered titanium phosphate
Introduction
Mesoscopic dye-sensitized solar cell (DSSC) has attracted considerable attention in the last 19 years as a potential alternative to conventional inorganic photovoltaics (O’Regan and Grätzel, 1991). Although an impressive 11% light-to-electricity conversion efficiency has been achieved for photovoltaic devices with organic solvent-based electrolytes (Grätzel, 2005), the presence of volatile liquid electrolyte in such devices results in some practical limitations for sealing and long-term operation of the devices, especially for dye-sensitized solar cells (DSSCs) on a plastic substrate. Room temperature ionic liquids are attractive candidates for replacement of the volatile organic solvents due to their negligible vapor pressure and high ionic conductivity (Kubo et al., 2002, Shi et al., 2008, Shi et al., 2009, Wang et al., 2003, Wang et al., 2004, Kuang et al., 2006). Notably, 1-propyl-3-methylimidazolium iodide (PMII), a liquid at room temperature with a viscosity of 880 cP at 20 °C, is by far the most commonly reported. However, the energy conversion efficiency of DSSC using ionic liquid solvent could not reach that of DSSC using volatile liquid solvent (Wang et al., 2004, Kuang et al., 2006).
Some important efforts have been made by adding inorganic nanomaterials into various ionic liquid electrolytes (Bruque et al., 1995, Usui et al., 2004, Yanagida, 2006, Wang et al., 2006a) to improve the photovoltaic properties of DSSC. Yanagida (2006) reported an improved DSSC by adding TiO2 nanoparticles into ionic liquid electrolyte consisting of imidazolium cations, and they obtained increased diffusion coefficient of in the ionic liquid and enhanced photo-electric conversion efficiency. Usui et al. (2004) dispersed carbon nanotubes, carbon nanoparticles, and TiO2 nanoparticles, respectively, into 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) ionic liquid electrolyte, leading to an improved photocurrent density and voltage. Recently, Wang et al. (2006a) reported that layered α-zirconium phosphate enhanced the photovoltaic properties of DSSCs using 1-methyl-3-propylimidazolium dihydrophosphate ionic liquid. These striking and significant observations have triggered our interest to explore new layered materials to improve the photovoltaic performances of DSSCs containing PMII ionic liquid. Crystalline α-titanium phosphate (α-TiP) has a two-dimensional layered structure similar to that of layered α-zirconium phosphate (Wang et al., 2006a, Clearfield and Costantino, 1996). However, for layered α-TiP, P atoms in the lower sandwich lie along a perpendicular line drawn from the Ti atom of the upper sandwich, as shown in Fig. 1a. This arrangement renders it with larger inter-laminar cavities and a greater ion exchange capacity than α-zirconium phosphate (Parida et al., 2004). The enlarged spacing would facilitate the diffusion of in the ionic liquid electrolyte, thus enhance the photovoltaic performance of DSSC. In addition, α-TiP has the common Ti ions with TiO2 photoanode, which can avoid the possible effect of foreign zirconium ions on the photovoltaic characteristics of DSSC.
Herein we reported the results on the use of layered α-TiP to improve the photovoltaic properties of DSSC using a binary ionic liquid of 1-propyl-3-methylimidazolium iodide (PMII) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) (volume ratio, 13:7) as the solvent. The use of EmimBF4 ionic liquid in the study was to reduce viscosity of the electrolyte since EmimBF4 exhibits lower viscosity than PMII (Noda et al., 2001). The preliminary mechanism for the enhanced photovoltaic properties of DSSCs was discussed.
Section snippets
Sample preparation
The synthesis of the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4) with a molecular structure presented in Fig. 1b was carried out according to the published procedures (Matsumoto et al., 2004, Matsumoto et al., 2006). The layered α-TiP crystalline was prepared by a method similar to that in the literature (Zhang et al., 2001) and the structure of the synthesized material confirmed by XRD measurement. The precipitate obtained from the hydrolysis of tetrabutyl titanate
The influence of layered α-TiP on the diffusion coefficient of
Fig. 2 presents typical steady-state voltammograms for the thin-layer cells filled with the composite electrolytes containing various contents of layered α-TiP measured at a scan rate of 10 mV/s. Due to the large excess of I− relative to , only the diffusion of limited the current. Therefore the diffusion coefficients of tri-iodide could be calculated from the anodic steady-state currents density (jlim) using the following equation (Hauch and Georg, 2001), and the results for the
Conclusion
In summary, we reported a composite ionic liquid electrolyte including layered α-TiP, which improved photovoltaic properties of the DSSCs. The intercalation of PMI+ cations into layered α-TiP provided fast transport channels for ions in the electrolyte, leading to the increased diffusion coefficient of . Fast diffusion of ions towards the counter electrode decreased the local concentration around TiO2 nanoparticles, which reduced the chance of electron recombination with in
Acknowledgements
The research was supported by National Nature Science Foundation of China (50802051), National High Technology Research and Development Program of China (863 Program, 2006AA03Z218) and China Postdoctoral Science Foundation (20060400055).
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