Tailoring of Ligand-Off Nanoparticles Inks for Thin p-Type Oxide Overlayers Formation with Maintaining Intact Halide Perovskite

In n-i-p halide perovskite solar cells (HPSCs), the development of p-type oxides is one of the most noteworthy approaches as hole transport materials (HTMs) for long-term stability and mass production. However, the deposition of oxide HTMs through a solution process over the perovskite layer without damage to the perovskite layer remains a major challenge. Here, the colloidal dispersion of ligand-off NiO nanoparticles (NPs) to form the HTM overlayer on perovskite using appropriate solvents that do not damage the underlying perovskite layer is reported. Monodispersed NiO NPs are synthesized using oleylamine (OLA) ligands via the solvothermal method, and the OLA ligands are then removed to form ligand-off NiO NPs. Based on the Hansen solubility theory, appropriate mixed solvents are found for both the dispersion of NiO NPs without ligands and coating without perovskite damage. The colloidal dispersion form a compact and uniform NiO NPs layer of 30 nm thickness on the perovskite layer, allowing n-SnO₂/Halide/p-NiO HPSCs to be successfully fabricated. The HPSC shows a record power conversion efficiency under one sun illumination for an n-i-p oxide/halide/oxide structure and excellent thermal stability maintaining 98% of the initial efficiency for 580 h under 85 °C and 10% relative humidity condition.