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.