Pressure-induced effects in the inorganic halide perovskite CsGeI₃

Perovskite photovoltaic materials are gaining significant attention due to their excellent photovoltaic properties. In this study, density functional theory calculations were performed to investigate the structure and electronic and optical properties of CsGeI₃ under hydrostatic strain. The results show that the band gap of CsGeI₃ can be tuned from 0.73 eV to 2.30 eV under different strain conditions. The results indicate that the change in the band gap under strain is likely to be determined by the Ge-I-Ge bond angle. Interestingly, the length of the short Ge-I bond remains unchanged, whereas that of the long Ge-I bond exhibits an evident increment with strain ranging from -4% to 4%. A suitable band gap (1.36 eV) of CsGeI₃ can be obtained under a strain of -1%. Both the calculated elastic constants and the phonon spectrum imply that this structure is stable under the abovementioned condition. Bandgap narrowing induces a red shift of the light absorption spectrum of CsGeI₃ by extending the onset light absorption edge. These results are important for understanding the effects of strain on the halide perovskites and guiding the experiments to improve the photovoltaic performance of the perovskite solar cells.