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.