Tunable Optical Properties and Charge Separation in CH₃NH₃Sn_xPb_1-xI₃/TiO₂-Based Planar Perovskites Cells

A sharp potential drop across the interface of the Pb-rich halide perovskites/TiO₂ heterostructure is predicted from first-principles calculations, suggesting enhanced separation of photoinduced charge carriers in the perovskite-based photovoltaic solar cells. The potential drop appears to be associated with the charge accumulation at the polar interface. More importantly, on account of both the β phase structure of CH₃NH₃Sn_xPb_1-xI₃ for x < 0.5 and the α phase structure of CH₃NH₃Sn_xPb_1-xI₃ for x ≥ 0.5, the computed optical absorption spectra from time-dependent density functional theory (TD-DFT) are in very good agreement with the measured spectra from previous experiments. Our TD-DFT computation also confirms the experimental structures of the mixed Pb-Sn organometal halide perovskites. These computation results provide a highly sought answer to the question why the lead-based halide perovskites possess much higher power conversion efficiencies than the tin-based counterparts for solar-cell applications. (Chemical Equation Presented).