Can Vacuum Deposition Apply to Bismuth-Doped γ‑CsPbI3 Perovskite? Revealing the Role of Bi3+ in the Formation of Black Phase

The B-site doped CsPbI3 has been demonstrated to be very promising for photovoltaics owing to its low black phase transition temperature. Though B-site doped black-CsPbI3 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 CsPb1–x Bi x I3 is targeted. To incorporate Bi3+ into the final film via vacuum deposition, the solid solution precursor Pb1–x Bi x I2 (0.01 ≤ x ≤ 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 Bi3+ 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 Pb2+ with Bi3+.