Strategic Synthesis of Ultrasmall NiCo2O4 NPs as Hole Transport Layer for Highly Efficient Perovskite Solar Cells

Abstract This study proposes a novel strategy of controllable deamination of Co–NH 3 complexes in a system containing Ni(OH) 2 to synthesize ultrasmall ternary oxide nanoparticles (NPs), NiCo 2 O 4 . Through this approach, ultrasmall (5 nm on average) and well‐dispersed NiCo 2 O 4 NPs without exotic ligands are obtained, which enables the formation of uniform and pin‐hole free films. The tightly covered NiCo 2 O 4 films also facilitate the formation of large perovskite grains and thus reduce film defects. The results show that with the NiCo 2 O 4 NPs as the hole transport layer (HTL), the perovskite solar cells reach a high power conversion efficiency (PCE) of 18.23% and a promising stability (maintained ≈90% PCE after 500 h light soaking). To the best of the author's knowledge, it is the first time that spinel NiCo 2 O 4 NPs have been applied as hole transport layer in perovskite solar cells successfully. This work not only demonstrates the potential applications of ternary oxide NiCo 2 O 4 as HTLs in hybrid perovskite solar cells but also provides an insight into the design and synthesis of ultrasmall and ligand‐free NPs HTLs to enable cost‐effective photovoltaic devices.