Performance Enhancement of Lead-Free Tin Based Perovskite Solar Cells with Reducing Atmosphere-Assisted Dispersible Additive

Sn-based halide perovskite materials have attracted tremendous attention and have been employed successfully in solar cells. However, their high conductivities resulting from the unstable divalent Sn state in the structure cause poor device performance and poor reproducibility. Herein, we used excess tin iodide (SnI2) in Sn-based halide perovskite solar cells (ASnI(3), A = Cs, methylammonium, and formamidinium tin iodide as the representative light absorbers) combined with a reducing atmosphere to stabilize the Sn2+ state. Excess SnI2 can disperse uniformly into the perovskite films and functions as a compensator as well as a suppressor of Sn2+ vacancies, thereby effectively reducing the p-type conductivity. This process significantly improved the solar cell performances of all the ASnI(3) materials on mesoporous TiO2. Optimized CsSnI3 devices achieved a maximum power conversion efficiency of 4.81%, which is the highest among all inorganic Pb-free perovskite solar cells to date.