High Fill Factor and Reduced Hysteresis Perovskite Solar Cells Using Small-Molecule-Engineered Nickel Oxide as the Hole Transport Layer

Nickel oxide (NiOx) is an emerging hole transport layer (HTL) material in halide perovskite solar cells (PSCs), combining high hole mobility, transparency, and stability. Current limitations of the device performance are mainly related to the inefficient hole extraction caused by contact problems between NiOx and the perovskite layer. Based on its expected strong interaction with both the NiOx surface and the perovskite layer, we selected 4-dimethylaminopyridine (DMAP) as a molecular passivation agent for the HTL. Photoelectron spectroscopy and photophysical studies demonstrate that DMAP passivation creates a more favorable band alignment at the NiOx/perovskite interface. This leads to decreased carrier recombination near the interface and enhanced hole transfer. In addition, X-ray diffraction reveals reduced strain, improved crystalline quality, and a redistribution of excess PbI2 in perovskite layers grown on DMAP-passivated NiOx. As a consequence, PSCs with the DMAP-modified HTL exhibit a strongly increased fill factor and power conversion efficiency with values close to 80 and 18%, respectively. Moreover, they show negligible hysteresis and enhanced environmental stability compared to devices with untreated HTLs.