Tin Oxide as a Protective Heterojunction with Silicon for Efficient Photoelectrochemical Water Oxidation in Strongly Acidic or Alkaline Electrolytes

Photoelectrodes without a p–n junction are often limited in efficiency by charge recombination at semiconductor surfaces and slow charge transfer to electrocatalysts. This study reports that tin oxide (SnO x ) layers applied to n‐Si wafers after forming a thin chemically oxidized SiO x layer can passivate the Si surface while producing ≈620 mV photovoltage under 100 mW cm −2 of simulated sunlight. The SnO x layer makes ohmic contacts to Ni, Ir, or Pt films that act as precatalysts for the oxygen‐evolution reaction (OER) in 1.0 m KOH(aq) or 1.0 m H 2 SO 4 (aq). Ideal regenerative solar‐to‐O 2 (g) efficiencies of 4.1% and 3.7%, respectively, are obtained in 1.0 m KOH(aq) with Ni or in 1.0 m H 2 SO 4 (aq) with Pt/IrO x layers as OER catalysts. Stable photocurrents for >100 h are obtained for electrodes with patterned catalyst layers in both 1.0 m KOH(aq) and 1.0 m H 2 SO 4 (aq).