Structure-related bandgap of hybrid lead halide perovskites and close-packed APbX3 family of phases

Abstract

Metal halide perovskites APbX₃ (A⁺ = FA⁺ (formamidinium), MA⁺ (methylammonium) or Cs⁺, X⁻ = I⁻ or Br⁻) are considered as prominent innovative components in nowadays perovskite solar cells. Crystallization of these materials is often complicated by the formation of various phases with the same stoichiometry but structural types deviating from perovskites such as the well-known hexagonal delta FAPbI₃ polytype. Such phases are rarely placed in the focus of device engineering due to their unattractive optoelectronic properties while they are, indeed, highly important because they influence the optoelectronic properties and efficiency of the final devices. However, the total number of such phases has not been yet discovered and the complete configurational space of the polytypes and their band structures have not been studied systematically. In this work, we predict and describe all possible hexagonal polytypes of hybrid lead halides with the APbI₃ composition using the group theory approach, also we analyze theoretically the relationship between the configuration of close-packed layers in polytypes and their band gap using DFT calculations. Two main factors affecting the bandgap were found including the ratio of cubic (c) and hexagonal (h) close-packed layers and the thickness of blocks of cubic layers in the structures. We also show that the dependence of the band gap on the ratio of cubic (c) and hexagonal (h) layers in these structures is non-linear. We believe that the presence of such polytypes in the perovskite matrix might be a reason for the decrease in the charge carrier mobility and therefore it would be an obstacle for efficient charge transport causing negative consequences for the efficiency of solar cell devices.