Role of Structural Phases and Octahedra Distortions in the Optoelectronic and Excitonic Properties of CsGeX3 (X = Cl, Br, I) Perovskites

Lead-free all-inorganic perovskites have been widely investigated as alternative materials to replace organic lead-based perovskites in solar cells. Although thousands of studies have been reported, several questions remain in debate, e.g., the role of structural phases and octahedra distortions in the optoelectronic and excitonic properties. Here, we report a theoretical investigation of those effects in the CsGeX3 (X = Cl, Br, I) perovskites by the combination of hybrid density functional theory calculations, spin–orbit coupling for the valence states, maximally localized Wannier functions tight-binding framework, and solution of the Bethe–Salpeter equation (BSE). In contrast to CsGeCl3 and CsGeBr3, which has an energetic preference for distorted cubic structures, hexagonal phases yield the lowest energy for CsGeI3, i.e., the X atomic radius plays an important role in the relative stability. Our results show that octahedra distortion lowers the total energy and increases the energy band gap substantially (above 1.0 eV), which can be explained by volume increasing and Jahn–Teller symmetry breaking that affects the character of the valence band maximum and conduction band minimum. From our BSE calculations, the quasi-particle effects are weak in the absorption coefficient; however, its magnitude depends on the structure phase and octahedra distortions. The spectroscopy limited maximum efficiency approach yields almost constant power conversion efficiency, despite substantial variations in the band gaps. The obtained values are consistent with experimental results in contrast to the Shockley–Queisser limit. We also predict the existence of optimal thickness for maximum efficiency, which (for example) is about 11.6 μm for the super cubic CsGeCl3.