Reductive ionic liquid-mediated crystallization for enhanced photovoltaic performance of Sn-based perovskite solar cells

Tin (Sn)-based perovskite solar cells (PSCs) have recently made inspiring progress, and certified power conversion efficiency (PCE) has reached impressive value of 14.8%. However, it is still challenging to realize efficient and stable 3D Sn-based PSCs due to the fast crystallization and easy Sn2+ oxidation of Sn-based perovskite. Herein, we reported the utilization of a reductive ionic liquid, methylamine formate (MAFa), to drive the controlled crystallization process and suppress Sn2+ oxidation of FASnI3 perovskite film. The coordination of C=O and Sn2+ and the hydrogen bonding of N-H⋯I between the MAFa and FASnI3 precursors are shown to be responsible for retarding the crystallization of FASnI3 during film-forming process, which promotes the oriented growth and reduced defect traps of the film. Moreover, the strong reducibility of −CHO groups in Fa− suppresses the oxidation of Sn2+ in the film. As a result, MAFa-modified 3D PSCs device could reach champion PCE of up to 8.50%, which is enhanced by 26.11% compared to the control device with PCE of 6.74%. Most importantly, the MAFa-modified device shows much improved stability compared to the control device under same conditions without encapsulation. This work adds key building blocks for further boosting the PCE and stability of Sn-based PSCs.