Impact of anionic Br^- substitution on open circuit voltage in lead free perovskite (CsSnI_3-xBr_x) solar cells

Replacement of lead in the hybrid organic-inorganic perovskite solar cells invokes the need for non-toxic materials such as Sn. Although solution processed CsSnI₃ has been demonstrated as a lead-free halide perovskite which can function as a light absorber with high photocurrent densities, the power conversion efficiencies were bottlenecked by low open circuit voltages. In this work, the open circuit voltages are modulated by chemical doping of CsSnI₃ with Br leading to formation of CsSnI_3-xBr_x (0 ≤ x ≤ 3) perovskites. The beneficial effect of Br incorporation for Voc improvement is evident for CsSnI₃ system even without the addition of SnF₂. There is an evolution of the crystal structure of CsSnI₃ from orthorhombic to cubic for CsSnBr₃ accompanied by changes in its optical properties with a blue shift of the absorption and IPCE onset, as the Br^- doping is increased. The V_oc enhancement is attributed to the decrease in Sn vacancies which is reflected by the lower charge carrier densities of 10¹⁵ cm^-3 and a high resistance to charge recombination in case of Br rich CsSnI_3-xBr_x perovskite. By the addition of SnF₂ to CsSnI_3-xBr_x perovskite, the current densities are improved significantly.