Electrochemical Quantification of Phosphonic Acid Passivated Surface Sites of NiOx for Perovskite Solar Cells

Nickel oxide (NiOx) is among the few p-type metal oxide semiconductors considered a strong candidate for hole transport layers in halide perovskite solar cells (PSCs). However, its reactivity with perovskite ions poses significant challenges to achieving high efficiency and long-term stability. Here, we investigate passivation of detrimental reactive surface sites on NiOx by carbazole phosphonic acids. We leverage electrochemical cyclic voltammetry (CV) of NiOx electrodes as a proxy measure for the redox activity that afflicts PSCs. From the CVs, we derive a metric, N (units cm-2), that relates to the number of redox active sites on NiOx surfaces. We observe a statistically significant negative correlation between PSC efficiency and N-value that indicates PSCs are more efficient on NiOx with lower electrochemical reactivity. The new mechanistic insight into NiOx passivation demonstrates it requires a reducing agent and Bronsted acid combination, providing a broadly applicable approach for evaluating and enhancing the stability and performance of NiOx-based interfaces in photovoltaics.