Mixed Ge/Pb perovskite light absorbers with an ascendant efficiency explored from theoretical view

Organic-inorganic methylammonium lead halide perovskites have recently attracted great interest emerging as promising photovoltaic materials with a high 20.8% efficiency, but lead pollution is still a problem that may hinder the development and wide spread of MAPbI₃ perovskites. To reduce the use of lead, we investigated the structures, electronic and optical properties of mixed MAGe_xPb_(1-x)I₃ theoretically by using density functional theory methods at different calculation levels. Results show that the mixed Ge/Pb perovskites exhibit a monotonic decrease evolution in band energy to push the band gap deeper in the near-infrared region and have a red shift optical absorption with an increased proportion of Ge. The results also indicate that lattice distortion and spin-orbit coupling (SOC) strength play important roles in the band gap behavior of MAGe_xPb_(1-x)I₃ by affecting the bandwidths of CBM and VBM. The calculations for short circuit current density, open circuit voltage, and theoretical power conversion efficiency suggest that mixed Ge/Pb perovskite solar cells (PSCs) with efficiency over 22% are superior to MAPbI₃ and MAGeI₃. And notably, MAGe_0.75Pb_0.25I₃ is a promising harmless material for solar cells absorber with the highest theoretical efficiency of 24.24%. These findings are expected to be helpful for further rational design of nontoxic light absorption layer for high-performance PSCs.