Effect of SnF2 concentration on the optoelectronic and PV cell properties of CsSnBr3

Lead-based halide perovskites have shown a dramatic improvement in photo-conversion efficiencies in the past decade. However, the toxicity of lead (Pb2+) may present barriers or even prevent it from commercialization. On the other hand, Sn2+ is a potential candidate as a replacement of Pb2+, while maintaining the perovskite crystal structure. However, Sn2+ suffers from easy oxidation to Sn4+, resulting in Sn vacancy doping which limits the device efficiencies. It has been generally reported in earlier studies that addition of tin fluoride (SnF2) restricts the oxidation of Sn2+ to Sn4+, resulting in improvement in solar cell efficiencies. We report our findings on the role of SnF2 concentration on the photovoltaic properties and energy level positions of cesium tin bromide (CsSnBr3). The device properties were found to be affected strongly by SnF2 concentration, which could not be explained simply by filling of Sn vacancies by excess tin. Observed changes in CsSnBr3 energy levels also could not explain the improvement. Time-resolved surface photovoltage results are consistent with reduction in trap state density (assumed to be Sn4+) upon SnF2 addition, probably due to the reducing character of SnF2.