Reducing surficial and interfacial defects by thiocyanate ionic liquid additive and ammonium formate passivator for efficient and stable perovskite solar cells

Organic-inorganic metal halide perovskites have attained extensive attention owing to their outstanding photovoltaic performances, but the existence of numerous defects in crystalline perovskites is still a serious constraint for the further development of perovskite solar cells (PSCs). In particular, the rapid crystallization guided by anti-solvents leads to plenty of surficial and interfacial defects in perovskite films. Herein, we report the adoption of a pseudo-halide anion based ionic liquid additive, 1-butyl-3-methylimidazolium thiocyanate (BMIMSCN) for growing ternary cation (CsFAMA, where FA = formamidinium and MA = methylammonium) perovskites with large-scale crystal grains and strong preferential orientation via the enhanced Ostwald ripening. Meanwhile, a novel halide-free passivator, benzylammonium formate (BAFa), was employed as a buffering layer on the perovskite films to suppress surface-dominated charge recombination. As a result, the cooperative effects of BMIMSCN additive and BAFa passivator lead to significant enhancements on fluorescence lifetime (from 79.41 to 201.01 ns), open-circuit voltage (from 1.13 to 1.19 V), and photoelectric conversion efficiency (from 18.90% to 22.33%). Moreover, the BMIMSCN/BAFa-CsFAMA PSCs demonstrated greatly improved stability against moisture and heat. This work suggests a promising strategy to improve the quality of perovskite materials via reducing the surficial and interfacial defects by the synergistic effects of lattice doping and interface engineering.