Can Vacuum Deposition Apply to Bismuth-Doped γ-CsPbI Perovskite? Revealing the Role of Bi in the Formation of Black Phase.

The B-site doped CsPbI has been demonstrated to be very promising for photovoltaics owing to its low black phase transition temperature. Though B-site doped black-CsPbI perovskites have been successfully achieved by solution-processing, it is unclear whether these systems are available by other methods such as vacuum deposition. In this work, heterovalent doped CsPbBiI is targeted. To incorporate Bi into the final film via vacuum deposition, the solid solution precursor PbBiI (0.01 ≤ ≤ 0.04) is developed. However, these coevaporated films not only are dominated by another hexagonal perovskite phase but also fail to decrease the black phase transition temperature. The role of Bi in the formation of the black phase is further studied by solution methods with different types of precursors. It is demonstrated that the key factor in the low-temperature black phase transition is small grain size, as well as the colloid size within the precursor solution, rather than simple substitution of Pb with Bi.