Photocurrent-Determining Processes in Quasi-Solid-State Dye-Sensitized Solar Cells Using Ionic Gel Electrolytes

Dye-sensitized solar cells (DSCs) using ionic liquids, 1-alkyl-3-methylimidazolium iodide (alkyl: C3−C9), were fabricated with and without a low molecular weight gelator. The highest energy conversion efficiency of 5.0% was obtained from a quasi-solid-state DSC using 1-hexyl-3-methylimidazolium iodide (HMImI). Gelation of these ionic liquids demonstrated better high-temperature durability without decreasing the solar cell efficiency. However, the short-circuit currents (JSC) obtained from these DSCs were about 70% of that obtained from DSCs using organic liquid electrolyte (OLE). To explain the difference of the JSC values between the DSCs using ionic liquid electrolyte (ILE) and OLE, four primitive processes in DSCs, that is, charge transport in the electrolytes, light absorption by I3-, electron diffusion in a TiO2 electrode, and charge recombination, were examined. Viscosities of the ILE decreased with increasing I3- concentration and alkyl chain length. In ILE, measured JSC values increased with increasing I3- concentration up to 0.7−1.4 M, depending on the alkyl chain length. Measured JSC values showed the same tendency as that estimated using a calculation with a model in which the redox couple is transported by diffusion in electrolytes. These results suggest that the slower diffusion of I3- to the counter electrode (CE) limits the JSC values and requires a larger amount of I3- in ILE. However, increasing I3- to more than 0.7−1.4 M resulted in the decrease of JSC. At the optimized concentration of I3- in ILE, the influence of the absorption was estimated to be 13% of the decrease on photocurrent. To the estimate electron diffusion length in the TiO2 electrode, the electron diffusion coefficient (De) and electron recombination lifetime (τ) were measured, showing faster De and shorter τ in ILE than in OLE. The faster De was caused by the higher concentration of cation in ILE. However, the shorter τ was caused by the higher concentration of I3- and depended on the concentration. Thus, the electron diffusion lengths (L) in the DSC using ILE were shorter than that using OLE. Their shorter L also reduced the JSC in the ILE and with the increase of I3- concentration. Among the ILE, the increase of alkyl chain length increased τ. This result should explain the highest efficiency observed in HMImI. During the durability test of the DSC at high temperature, a decrease of the efficiency of the cell using ILE was observed in 1000 h. Time course change of I3- concentration measurements revealed that the gelation of the electrolyte depresses a decrease of I3- concentration caused by sublimation of I2. Depression of sublimation of I2 is important to improve the high-temperature durability in nonvolatile ionic liquid electrolyte.