Improved film morphology of (CH3NH3)3Bi2I9 via cation displacement approach for lead-free perovskite solar cells.

Methylammonium bismuth iodide (MA₃Bi₂I₉) has been recently investigated as a light absorber in lead-free perovskite solar cells. However, the MA₃Bi₂I₉ film fabricated via conventional one-step spin coating methods usually has poor surface morphology, limiting the device performance. Herein, a cation displacement approach was employed for the fabrication of MA₃Bi₂I₉ film, where (CH₃CH₂CH₂NH₃)₃Bi₂I₉ (PA₃Bi₂I₉) film was first deposited from a solution containing CH₃CH₂CH₂NH₃I and BiI₃ and then transformed into MA₃Bi₂I₉ film in a methylamine atmosphere. With the technique, the MA₃Bi₂I₉ film was realized with smooth, uniform, and compact surface morphology. Using the MA₃Bi₂I₉ film as a light absorber, a mesoporous photovoltaic device was fabricated with a power conversion efficiency of 0.33%, which is about two times higher than the value (0.15%) obtained for the one-step spin coating MA₃Bi₂I₉ device. Moreover, the facile film fabrication strategy utilized in this work paves the way for high reproducibility of lead-free organic–inorganic halide films and devices. [ABSTRACT FROM AUTHOR]