Improvement of the DSSC Performance By Using a Phosphonium-Type Ionic Liquid Modified TiO2 Electrode

Abstract

 Dye-sensitized solar cells (DSSCs) have been widely investigated as next generation solar cells because of their simple structures, low-cost and facile manufacturing. However, its low energy conversion efficiency prevents the widespread use of DSSCs. The low efficiency of DSSCs is mainly caused by the dye aggregation and the electron recombination. One effective way to solve the dye aggregation and the electron recombination is the use of coadsorbents like chenodeoxycholic acid (CDCA).1) The coadsorbents form a thin layer on the TiO2 surface, which relaxes dye aggregation and/or suppresses the electron recombination. However, in this case, only slight increase of the photocurrent density often observed because the amount of dyes adsorbed on the TiO2 surface decreases. Therefore, the relaxation of the dye aggregation with keeping the amount of dyes on the TiO2 surface is required in order to improve the DSSC performance.2)  To prevent the dye aggregation, we have used ionic liquid (IL)-modified TiO2 electrode. IL is a salt in the liquid state at room temperature. Recently, we reported the self-assembled monolayers (SAMs) of bulky phosphonium- and ammonium-type ILs with disulfide groups.3-5) The steric and electrostatic repulsion among the bulky IL molecules prevent them from packing closely and form the enough space to entrap external molecules on the surface. We tried to utilize the space formed by the bulky phosphonium-type IL on the TiO2 surface to trap dye molecules. The ILs on the TiO2 surface probably reduce the dyes aggregation with keeping the amount of dyes on the surface. Furthermore, it is also expected that the electron recombination between the electrolyte and the TiO2 surface is suppressed because the surface is covered with the alkyl chains of IL. In this study, a bulky phosphonium-type ionic liquid (IL) with a carboxylic acid group was newly synthesized. Subsequently, TiO2 electrodes modified with N3 6) dye and IL with various ratios were prepared. DSSCs based on the IL-modified TiO2 photoanodes have been fabricated and their photovoltaic performance have been evaluated. The surface ratio of N3 dye and IL were determined by the wavelength dispersive X-ray analysis (WDS). When the TiO2 electrode was dipped into the t BuOH/CH3CN (1/1, v/v) solution containing 0.3 mM N3 dye and 30mM IL, the electrode modified with 1/1 ratio was obtained. The DSSC using N3- and N3, IL-modified TiO2 photoanodes were fabricated and their J-V characteristics were measured. All parameters of the DSSCs (the short-circuit photocurrent (J sc), the open-circuit photovoltage (V oc), fill factor (ff), and the power conversion efficiency (η)) have been improved. These results indicate that the DSSCs based on the IL-modified TiO2 electrodes prevented the charge recombination among dyes and/or between the electrolyte and the TiO2surface.1) M. K. Nazeeruddin,et al.,Nanotechnology,2008, 19, 424005.2) K. Char, et al., ACS Appl. Mater. Interfaces, 2015, 7, 25741.3) H. Masuda, et al., Chem. Commun., 2013, 49, 10184.4) H. Masuda, et al., Chem lett, 2016, 45, 4, 436.5) H. Masuda, et al., Chem. Commun., 2016, 52, 4780.6) M. K. Nazeeruddin, et al., J. Am. Chem. Soc., 1993, 115, 6382. Figure 1