Acid-Compatible Halide Perovskite Photocathodes Utilizing Atomic Layer Deposited TiO₂ for Solar-Driven Hydrogen Evolution

Although solution-processable halide perovskite semiconductors exhibit optoelectronic performance comparable to the best photoabsorbers for solar fuel production, halide perovskites rapidly decompose in the presence of water or even humid air. We show that a hybrid electron transport layer, a PC₆₁BM + TiO₂ film (18-40 nm thickness) grown over the sensitive absorber by atomic layer deposition, enables photoassisted proton reduction without further encapsulation. These semitransparent photocathodes, when paired with a Pt catalyst, display continuous reduction of H⁺ to H₂ for hours under illumination, even while in direct contact with a strongly acidic aqueous electrolyte (0.5 M H₂SO₄). Under 0.5 Sun illumination, a photocurrent density of >10 mA cm^-2 is observed, and a photovoltage of 0.68 V assists proton reduction, consistent with a structurally related photovoltaic (PV) device. Submersible halide perovskite photoelectrodes point the way to more efficient photoassisted overall water splitting and other solar fuel generation using solution-processed semiconductors with tunable band gaps.