High-Quality (CH₃NH₃)₃Bi₂I₉ Film-Based Solar Cells: Pushing Efficiency up to 1.64

Bismuth-based solar cells have exhibited some advantages over lead perovskite solar cells for nontoxicity and superior stability, which are currently two main concerns in the photovoltaic community. As for the perovskite-related compound (CH₃NH₃)₃Bi₂I₉ applied for solar cells, the conversion efficiency is severely restricted by the unsatisfactory photoactive film quality. Herein we report a novel two-step approach- high-vacuum BiI₃ deposition and low-vacuum homogeneous transformation of BiI₃ to (CH₃NH₃)₃Bi₂I₉-for highly compact, pinhole-free, large-grained films, which are characterized with absorption coefficient, trap density of states, and charge diffusion length comparable to those of some lead perovskite analogues. Accordingly, the solar cells have realized a record power conversion of efficiency of 1.64% and also a high external quantum efficiency approaching 60%. Our work demonstrates the potential of (CH₃NH₃)₃Bi₂I₉ for highly efficient and long-term stable solar cells.