Modulation of Band Gap in Bi/Sb-Based Halide Defect Perovskites: Substitution Induced Structural Distortion

Abstract

Energy demand is growing rapidly with economic and population growth, which needs to be addressed with exigency. 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 with high power conversion efficiency (PCE) need to be developed. The discovery of lead halide based hybrid perovskite, with greater PCE than that of traditional silicon solar cell materials, revolutionize the search for alternative photovoltaic material. However, the toxicity and lower stability of lead based halide perovskites impede the large applications of these materials. Hence, there has been search for lead free air and moisture stable halide perovskites containing less toxic elements. Sb/Bi based halide perovskites are one of the suitable compounds with better stability and enhance absorption properties. However, the PCE of these compounds are much smaller as compared to lead halide perovskites due to the presence of large number of deep level defects. Cationic charge and structural distortion largely can alter the defect concentration and hence the photovoltaic properties. In this talk, I will discuss our attempt to understand the effect of substitution on the structural distortion and hence the photovoltaic properties. Bi substitution in Cs3Sb2Cl9 does not affect the band type but a structural phase transition from trigonal to orthorhombic is observed beyond 5% of Bi substitution in Cs3Sb2−xBixCl9 and the band type remain unchanged. However, site specific Br substitution in Cs3Sb2Cl9 resulted in a band type transition from indirect to direct type with two mole percentage of Br substitution and the band gap also decreased from 2.9 to 2.4 eV. Further manipulation at halide site was done to achieve low and direct band gap material. The detailed theoretical and experimental study will be highlighted.