Impact of Anionic Br– Substitution on Open Circuit Voltage in Lead Free Perovskite (CsSnI3-xBrx) Solar Cells

Abstract

Replacement of lead in the hybrid organic–inorganic perovskite solar cells invokes the need for non-toxic materials such as Sn. Although solution processed CsSnI 3 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 CsSnI 3 with Br leading to formation of CsSnI 3- x Br x (0 ≤ x ≤ 3) perovskites. The beneficial effect of Br incorporation for V oc improvement is evident for CsSnI 3 system even without the addition of SnF 2 . There is an evolution of the crystal structure of CsSnI 3 from orthorhombic to cubic for CsSnBr 3 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 10 15 cm –3 and a high resistance to charge recombination in case of Br rich CsSnI 3- x Br x perovskite. By the addition of SnF 2 to CsSnI 3- x Br x perovskite, the current densities are improved significantly.