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

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.