Ultrathin HfO₂ Protection Layer and Pt Cocatalyst Synergy in TiO₂–Passivated Inverted Pyramid Si Photocathodes for Efficient Solar Hydrogen Production

Silicon-based photocathodes often face trade-offs between light absorption, stability, and charge separation. Here, we develop an inverted pyramid black silicon (SiIP) photocathode via copper-assisted chemical etching (Cu-ACE), followed by thermal oxidation and tetramethylammonium hydroxide (TMAH) etching to minimize surface defects. A pn junction was engineered through phosphorus ion implantation, while a TiO2/HfO2 bilayer was deposited to enhance light absorption and interfacial stability. The high-κ HfO2 layer strengthened the built-in electric field, promoting charge separation, while the TiO2/HfO2 synergy enabled uniform Pt catalyst dispersion. The photocathode exhibited an average reflectance of 9.06% in the 300–1100 nm wavelength range. Furthermore, the introduction of HfO2 significantly enhanced the photoelectrochemical performance of the n+p-SiIP1-O2/TiO2/HfO2/Pt electrode, yielding a 4-fold increase in current density to −20.2 mA cm–2 at 0 V vs RHE, a positively shifted onset potential of 0.52 V vs RHE, and an applied bias photon-to-current efficiency (ABPE) of 2.73%. Moreover, the photocathode maintained 65% of its initial current density during 24-h continuous J–t testing, with nearly complete performance recovery observed in poststability LSV measurements. This work provides a feasible strategy to harmonize optical and electronic design in silicon photocathodes, offering insights for efficient and stable solar hydrogen production.