Defect-less formamidinium Sn-Pb perovskite grown on a fluorinated substrate with top-down crystallization control for efficient and stable photovoltaics

Ideal bandgap formamidinium tin-lead perovskites (FASn0.5Pb0.5I3) are appealing for fabricating thermally stable single-junction and tandem perovskite photovoltaics. However, attempts to obtain high device efficiency are limited to the preparation of defect-less perovskite films. Herein, we propose a trifluoroacetamide (TFOA)-modified NiOx hole-transport layer (HTL) as a substrate for growing the FASn0.5Pb0.5I3 film, where TFOA molecules interact with both NiOx and perovskite layers, resulting in a less stressed, more intimate, and energetically favorable bottom contact. Besides, the thermo-driven iodide migration in the perovskite film is also suppressed. Furthermore, dual-function formylhydrazide vapor (FHV) treatment was applied during the annealing process, which not only offers a reducing atmosphere to prevent Sn2+ oxidation but also helps to achieve top-down crystallization control in the presence Lewis basic formamide groups, leading to the formation of a homogeneous perovskite film with closely packed large grains and reduced defects. Ultimately, an optimal efficiency of 21.12% with a nearly 80% fill factor (FF) was achieved for the TFOA-FHV co-treated device, which is currently the highest one for FA-based tin-lead perovskite solar cells (PSCs) in NiOx HTL systems. Moreover, the stability of unencapsulated devices also showed significant improvements under various test scenarios. A highly efficient and stable ideal-bandgap perovskite solar cell based on a defect-less formamidinium (FA) Sn-Pb perovskite light-absorbing layer is grown on a fluorinated substrate via top-down crystallization.