Charge Transport and Recombination in Perovskite (CH3NH3)PbI3 Sensitized TiO2 Solar Cells

We report on the effect of TiO₂ film thickness on the charge transport, recombination, and device characteristics of perovskite (CH ₃NH₃)PbI₃ sensitized solar cells using iodide-based electrolytes. (CH₃NH₃)PbI₃ is relatively stable in a nonpolar solvent (e.g., ethyl acetate) with a low iodide concentration (e.g., 80 mM). Frequency-resolved modulated photocurrent/ photovoltage spectroscopies show that increasing TiO₂ film thickness from 1.8 to 8.3 μm has little effect on transport but increases recombination by more than 10-fold, reducing the electron diffusion length from 16.9 to 5.5 μm, which can be explained by the higher degree of iodide depletion within the TiO₂ pores for thicker films. The changes of the charge-collection and light-absorption properties of (CH₃NH ₃)PbI₃ sensitized cells with varying TiO₂ film thickness strongly affect the photocurrent density, photovoltage, fill factor, and solar conversion efficiency. Developing alternative, compatible redox electrolytes is important for (CH₃NH₃)PbI₃ or similar perovskites to be used for potential photoelectrochemical applications.