Resolving the discrepancies in the reported optical absorption of low-dimensional non-toxic perovskites, Cs3Bi2Br9 and Cs3BiBr6

Abstract

In the search for lead-free perovskites for solar cells, cesium bismuth halides are emerging as potential optoelectronic materials. Cs⁺, Bi³⁺ and Br⁻ can form multiple compounds with different stoichiometries and crystal structures. Among these, Cs₃Bi₂Br₉ and Cs₃BiBr₆ are stable in air at room temperature. Published optical absorption data for Cs₃Bi₂Br₉ vary significantly from article to article, while there are limited data on the newly discovered Cs₃BiBr₆. Herein we resolve the discrepancies in the literature and fill in the knowledge gaps by depositing phase pure Cs₃Bi₂Br₉ and Cs₃BiBr₆ thin films via co-evaporation of CsBr and BiBr₃ in high vacuum and studying the effects of CsBr:BiBr₃ ratio on the optical properties of well-characterized films. Cs₃Bi₂Br₉ and Cs₃BiBr₆ thin films show absorption peaks centered at 435 nm and 383 nm, respectively. These peaks are associated with excitations localized on [BiBr₆]³⁻ octahedra in the solid phase but the latter also coincides with absorption by [BiBr₆]³⁻ in liquid solutions. We show that the discrepancies in the literature on optical absorption of Cs₃Bi₂Br₉ can have two explanations: Cs₃BiBr₆ phase impurities or the inevitable absorption by dissolved [BiBr₆]³⁻ in colloidal dispersions, which may be easily misinterpreted as originating from the solid product.