Phase stability and optoelectronic properties of lead-free CsSn1−Ge I3 mixed halide perovskites: A first-principles study

In this study, the physical properties of inorganic mixed CsSn1−xGexI3 halide perovskites are investigated by performing first-principles calculations. The results reveal that the mixed phase forms solid solutions without a long-range order in the atomic arrangement, which helps increase the carrier lifetime by facilitating the separation of electron–hole pairs. CsSn1−xGexI3 favors to adopt an orthorhombic CsSnI3 structure with highly disordered Sn–Ge arrangements in Sn-rich compositions, whereas it is likely to crystallize into a rhombohedral CsGeI3 structure maintaining some medium-range order in Ge-rich compositions. Regardless of the crystal shape, the bandgap of the mixed phase is less than 2 eV, which indicates that it can efficiently absorb visible light. In addition, electrons and holes exhibit relatively small effective carrier masses (<0.6 m0, where m0 is the electron rest mass), which indicates high mobility. By demonstrating the high phase stability and advantageous photophysical properties of CsSn1−xGexI3, this work paves the way for the development of high-performance photovoltaic devices.