Abstract:

Organic-inorganic hybrid lead halide perovskites have emerged as the most promising materials in the field of optoelectronics due to their unique electronic and optical properties. However, the poor long-term material and device stabilities of these materials have limited their practical application. Compared to organic-inorganic hybrid perovskites, all-inorganic halide perovskites like CsPbX₃ (X = Cl, Br, I) show enhanced thermal stability and the potential to resolve the issue of instability. Nevertheless, the structural and physical properties of all-inorganic CsPbX₃ halide perovskites with multiple structural polymorphs are still under debate. A recent research article on CsPbI₃ reported the wrongly indexed the XRD pattern of γ-CsPbI₃ as α-CsPbI₃. Consequently, the band gap of γ-CsPbI₃ (1.73 eV) was erroneously designated for α-CsPbI₃. Therefore, there is a need for systematic research on the relationship between the structural features and electronic properties of CsPbX₃. Here, we present a comprehensive theoretical study of the structural, thermodynamical and electronic properties of three polymorphic phases, α-, β-, and γ-CsPbX₃. The space group of α-, β-, and γ-CsPbX₃ are Pm${\rm{\bar 3}}$m, P4/mbm, and Pnma, respectively. First-principles calculations indicate that the phase transition from the high-temperature α-phase to the low-temperature β-phase and then to the γ phase is accompanied by an increase in the degree of PbX₆ octahedral distortion. The zero-temperature energetic calculations reveal that the γ-phase is the most stable. This is consistent with the fact that experimentally, the γ-phase is stabilized at a relatively low temperature. Analysis of the electronic properties indicates that all the CsPbX₃ perovskites exhibit a direct-gap nature and the band gap values increase from α to β, and then to the γ phase. From the analysis of the orbital hybridization near the band gap edges, the increase can be explained by the downshift of the valence band edges caused by the gradual weakening of the Pb-X chemical bond. Among all the phases, the strongest Pb-X interaction in the α-phase leads to the most dispersive band-edge states and thus the smallest carrier effective masses, which are beneficial for carrier transport. Additionally, the band gaps decreased by changing the halogen type from Cl to Br and I under the same phase. this is a consequence of the increased X np orbital energies from Cl 3p to Br 4p and then to I 3p that leads to a high valence band edge for CsPbI₃ and results in the smallest band gap. Our results provide deep understanding on the relationship between the physical properties and structural features of all-inorganic lead halide perovskites.

Key words: Inorganic halide perovskite, Optoelectronic property, First-principles calculation, Electronic structure