Nickel OxideThin Films Formed from the pH-AdjustedSolutions for Inverted Perovskite Solar Cells

We report here a facile approach for fabricating a p-type nickel oxide (NiO_<i>x</i>) thin film for inverted perovskite solar cells (PSCs) by a direct conversion of the pH-adjusted aqueous precursor solutions into solid NiO_<i>x</i>. pH control influenced both the p-type characteristics of the NiO_<i>x</i> film and the phase stability of the perovskite layer deposited on the NiO_<i>x</i> film. Formic acid (HCOOH) increased the Ni³⁺/(Ni²⁺ + Ni³⁺) ratio from 4.65% to 5.77%, enhancing charge extraction capability, while ammonium hydroxide reduced it to 4.13%. The HCOOH-derived NiO_<i>x</i> films also improved the perovskite stability by eliminating residual nitrates. As a result, under ISOS-L1 conditions, the <i>T</i>₉₀ lifespan (90% of the initial efficiency) was extended to 900 h, compared to less than 200 h for other films. A power conversion efficiency of 24.19% was achieved by using the HCOOH-derived NiO_<i>x</i> film with carbazole-based phosphonic acid passivation.