Different structural evolutions of inorganic perovskite CsGeI3

Distinct from CsSnI3 perovskite systems, the most stable non-centrosymmetric trigonal structure (space group of R3m) of the CsGeI3 perovskite system was found in our calculations. We systematically studied the differences between two all-inorganic perovskites (CsSnI3 and CsGeI3) and proposed an unexpected distorted stable structure of the mixed Sn-Ge perovskite system (CsSn0.5Ge0.5I3). All calculations of structural configuration and electronic properties were performed in the projector augmented-wave density-functional theory framework via the generalized gradient approximation (GGA). We found that the energy band evolutions of all-inorganic perovskites CsSnxGe1-xI3 are completely different from those of the Sn-Pb perovskite compounds. After considering the spin-orbit coupling (SOC) effect, the bandgap evolution of the stable structure shows a linear change. The results unravel the previous controversial understanding of the atomic and electronic properties of Ge-based perovskite solar cells.