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

Abstract

Ideal bandgap formamidinium tin–lead perovskites (FASn_0.5Pb_0.5I₃) 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 NiO_x hole-transport layer (HTL) as a substrate for growing the FASn_0.5Pb_0.5I₃ film, where TFOA molecules interact with both NiO_x 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 NiO_x HTL systems. Moreover, the stability of unencapsulated devices also showed significant improvements under various test scenarios.