Reducing charge-recombination losses in photovoltaic cells by spontaneous reconstruction of n/p homojunction in a monolithic perovskite film using black phosphorus nanosheets

Trap-assisted charge recombination and interfacial charge recombination have limited further improvements in the efficiency and stability of perovskite solar cells. We construct a novel two-dimensional (2D) black phosphorus nanosheet (BP NS)-modified perovskite model that utilizes an upgraded additive post-processing technique to introduce 2D BP NSs, comprising multiple functional groups and carrying a high charge mobility and high work function, into the absorber during Ostwald ripening. 2D BP NSs, while passivating grain boundaries and surface defects, successfully induced the in-situ formation of an intriguing n/p homojunction structure between the underlying bulk and the top in a monolithic perovskite film, i.e., a graded n/p-PVK-BP absorber. This resulted in the synergistic advantages of expanding the built-in electric field to facilitate the oriented transfer of photogenerated carriers, optimizing the energy level alignment, and minimizing the hole injection barrier at the perovskite/hole transporting layer interface, thereby accelerating the selective extraction/collection of interfacial charges and synchronously reducing nonradiative and interfacial charge recombination losses. The 2D BP NSs anchored at the grain boundaries and surface of p-type perovskite form a "fast path" for hole migration, achieving carrier transport balance at the top and bottom interfaces. The optimized n/p-PVK-BP device achieved an efficiency of 23.22 %, with negligible hysteresis. Owing to the synergistic effects of the various advantages of the graded n/p-PVK-BP homojunction absorber, the unencapsulated device exhibited good long-term storage and operational stability, providing potential for commercial conversion of perovskite-based optoelectronics. This work provides a simple homojunction structure design and performance optimization strategy for the simultaneous reduction of trap-assisted charge recombination and interfacial charge recombination losses and provides new insights for the development of more efficient and stable perovskite photovoltaic applications.