Understanding of the Band Gap Transition in Cs3Sb2Cl9−xBrx: Anion Site Preference Induced Structural Distortion

Abstract

Energy demand is growing rapidly with economic and population growth, which needs to be fulfilled. Among all kind of energies, solar energy is inexhaustible and easily available for use. In order to convert the solar energy to electrical energy, efficient materials must be developed and some device should be fabricated. The required properties of a material for its suitable applications in photovoltaics are, the material should be a better solar light absorber, it must have higher power conversion efficiency and less toxicity, etc. The discovery of lead halide based hybrid perovskite, with greater power conversion efficiency and good solar light absorber than that of traditional silicon solar cell materials, revolutionize the search for alternative photovoltaic material. The greater power conversion efficiency of these materials is due to their exceptional optoelectronic properties like; suitable band gap, high absorption coefficient, and low exciton binding energy. However, these lead-based perovskites suffer roadblock towards the commercialization of these materials because of the highly toxic nature of lead and instability of these compounds. Search for lead-free perovskite led to the discovery of Bi and Sb-based perovskites, which are very good alternative lead-free halide perovskites. These compounds do not form normal AMX3 (A=Alkali metal; M=Pb; X=Halides) type perovskite with corner shared MX6 Polyhedra, rather have A3M2X9 (A=Alkali metal; M=Bi, Sb; X= Halide) composition with defect halide perovskite structure. In these type of materials direct Band gap materials are suitable candidates for better photovoltaic materials compared to indirect band gap materials. Herein we have synthesized Cs3Sb2Cl9, and Br substituted Cs3Sb2Cl9 using solution method. The detail structural, optical, electronic properties of these Br substituted Cs3Sb2Cl9 compounds as well as a suitable explanation from both experimental and theoretical study for the band gap transition from indirect to direct with Br substitution in Cs3Sb2Cl9 will be presented.