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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.