First-principles calculations of optical properties and defect structures of Ge-alloyed Sn perovskites

Solar cells using methylammonium lead perovskite as a solar cell light absorbing material have achieved an astounding 25.5% conversion efficiency improvement, equivalent to silicon solar cells as of 2023. There are concerns about the toxic effects of lead in perovskite solar cell materials on human health and the environment, and there is an urgent need to completely replace lead with a more inert metal. In this study, optical properties and defect structures of double perovskite solar-cell materials in which lead is replaced by tin and germanium are analyzed by first-principles calculations to evaluate and design novel lead-free perovskite materials. First-principles calculations were performed using the Vienna ab initio package (VASP) with PBE and HSE06 for the functional. In the level diagram of defects of Cs₂SnGeI₆, many defect levels appeared in the band gap, but the formation energies of many of them were found to be high and difficult to generate. The V_Sn(-/0) and V_Ge(-/0) defects appearing near the VBM and CBM are considered to be defects that trap photogenerated carriers and reduce the conversion efficiency (V_Sn(-/0) refers to defect levels that ionize at the Sn vacancy site). The formation energies of these defects are strongly dependent on the chemical potentials of the constituent elements (Sn, Ge, and I), and defect formation can be controlled by changing the crystal growth conditions (chemical potentials). Optical properties of Cs₂SnGeI₆ will be discussed in the poster.