Constructing n/n+ homojunction in monolithic perovskite film for boosting charge collection in inverted perovskite photovoltaics

The suboptimal carrier dynamics at perovskite/electron transport layer has largely limited the further performance enhancement of the state-of-the-art inverted p-i-n structured perovskite solar cells. Herein, we discovered that a simple surface modification of perovskite film by a trivalent metal halide salt, InBr3, could convert the doping levels of the top surface to be more n-type, which spontaneously formed an intriguing n/n+ homojunction between the bulk (week n type (n) component) and the surface (more n-type (n+) component) in a monolithic perovskite film. This brought about synergistic advantages of enlarged built-in electric field for facilitated charge separation, as well as optimizing electronic energy level alignment and minimizing electron injection barrier at the perovskite/C60 interface, leading to both accelerated charge extraction and suppressed interfacial charge recombination. Blade-coated inverted PSCs with n/n+ homojunction achieved high power conversion efficiency (PCEs) up to 22.2%, narrowing the efficiency gap with their conventional n-i-p counterparts. The InBr3 surface treatment also improved the operational stability to > 1000 hours under light with 93% initial efficiency retained.