Facile fabrication of NiOx hole transport layers via thermal oxidation of evaporated Ni films without metallic Ni residues for perovskite solar cells

Nickel oxide (NiO x ) is widely employed as a hole transport layer (HTL) in perovskite solar cells (PSCs). However, recent studies have revealed that metallic Ni residues often remain in NiO x films prepared by conventional methods, which can reduce Pb 2+ to Pb 0 at the perovskite/HTL interface and deteriorate device performance. Here, we report a facile approach to fabricate NiO x thin films by thermal oxidation of evaporated Ni precursor layers. X-ray photoelectron spectroscopy analysis confirms the complete absence of metallic Ni in the resulting NiO x films, in contrast to many previously reported NiO x layers. PSCs employing NiO x HTLs achieve a power conversion efficiency of 18.70%. Eliminating metallic Ni residues through a simple and scalable process provides a clear pathway toward efficient NiO x -based inverted PSCs.