Oxalic-acid-mediated reconstruction of chemical-bath-deposited NiO for High-VOC wide-bandgap perovskite solar cells

Nickel oxide (NiO x ) is a promising hole-transport layer for inverted perovskite solar cells, yet solution-processed NiO x can suffer from morphological inhomogeneity and interfacial defects that limit device performance. Here, we report a chemically regulated chemical bath deposition (CBD) strategy in which an intermediate oxalic acid (OA) treatment is introduced between precursor growth and thermal conversion. During CBD, ammonia-citrate coordination moderates Ni 2+ hydrolysis and promotes the formation of a continuous Ni(OH) 2 precursor film. Subsequent OA treatment reorganizes the loosely stacked nanosheet-like precursor surface, as supported by comparative spectroscopic and morphological analyses, and thermal annealing converts the modified precursor into a smooth and compact NiO film with a Ni 2+ -dominated, NiO-like surface composition. The resulting NiO/SAM stack supports more uniform perovskite film formation and reduced buried-interface non-radiative loss, as evidenced by steady-state PL, TRPL, DLCP, and device-level TPV measurements. Wide-bandgap inverted perovskite solar cells employing the optimized OA-treated CBD-NiO achieve a champion power conversion efficiency of 20.04% with an open-circuit voltage of 1.313 V. The devices also exhibit improved reproducibility and operational stability, retaining 87.0% of their initial PCE after 509 h of continuous MPPT under the specified laboratory conditions. These results demonstrate that intermediate precursor regulation is an effective approach for improving CBD-derived NiO hole-transport layers and their buried interfaces.