Band gap tuning and defect tolerance of inorganic RbGeX₃ (Cl, Br, I) halide perovskites: A first-principles study

To identify optimum candidate materials that can be used as light absorbers in solar cells, we performed calculations to simulate the structural, electronic, mechanical, optical, and thermoelectric properties of cubic inorganic RbGeX₃(Cl, Br, I) perovskites in their pristine and defect states with density functional theory as implemented in Vienna Ab initio Simulation Package (VASP). Our results reveal the unique characteristics of the band gaps, defect tolerance, and their effects on the optoelectronic properties of the perovskites with the introduction of the defect states. The cation site defects V_Ge can form shallow energy levels to provide p-type conductivity, while the anion site defects V_{Cl, Br,I}, can act as recombination centres to influence the performance of the devices. The effects of the defects on the mechanical and thermoelectric properties of the perovskites are also discussed. These results provide in-depth information into defect behaviors in the Rb-based perovskites and shed light on related optoelectronic applications.