Overcoming Redox Reactions at Perovskite-Nickel Oxide Interfaces to Boost Voltages in Perovskite Solar Cells

Nickel oxide (NiO_x) hole transport layers (HTLs) are desirable contacts for perovskite photovoltaics because they are low cost, stable, and readily scalable; however, they deliver lower open-circuit voltages (V_OCs) compared to organic HTLs. Here, we characterize and mitigate electron transfer-proton transfer reactions between NiO_x HTLs and perovskite precursors. Using XPS and UPS characterization, we identify that Ni^≥3+ metal cation sites in NiO_x thin films act both as Brønsted proton acceptors and Lewis electron acceptors, deprotonating cationic amines and oxidizing iodide species, forming PbI_2−xBr_x-rich hole extraction barriers at the perovskite-NiO_x interface. Titrating reactive Ni^≥3+ surface states with excess A-site cation salts during perovskite active layer deposition yielded an increase in V_OC values to 1.15 V and power conversion efficiencies of ∼20%. This may be a general finding for metal oxide contacts that act as Brønsted and Lewis acid-base reactants toward perovskite precursors, an observation that has also been made recently for TiO₂ and SnO₂ contacts.