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

This study proposes a novel strategy of controllable deamination of Co-NH3 complexes in a system containing Ni(OH) 2 to synthesize ultrasmall ternary oxide nanoparticles (NPs), NiCo2O4. Through this approach, ultrasmall (5 nm on average) and well-dispersed NiCo2O4 NPs without exotic ligands are obtained, which enables the formation of uniform and pin-hole free films. The tightly covered NiCo2O4 films also facilitate the formation of large perovskite grains and thus reduce film defects. The results show that with the NiCo2O4 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 approximate to 90% PCE after 500 h light soaking). To the best of the author's knowledge, it is the first time that spinel NiCo2O4 NPs have been applied as hole transport layer in perovskite solar cells successfully. This work not only demonstrates the potential applications of ternary oxide NiCo2O4 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.