Hole-Selective CoO_x/SiO_x/Si Heterojunctions for Photoelectrochemical Water Splitting

Cobalt oxide (CoO_x), an earth-abundant and low-cost oxygen evolving catalyst (OEC), has notable advantages as a top protection layer of photoanodes for solar-driven water oxidation because of its desirable durability. However, cobalt oxides exist as various phases, such as Co_(II)O, Co_2(III)O₃, Co_3(II,III)O₄, and the (photo)electrochemical properties of CoO_x are significantly governed by its phase. Atomic layer deposition (ALD) is a suitable method to form a multifunctional layer for photoelectrochemical (PEC) water splitting because it allows direct growth of a conformal high-quality film on various substrates as well as facile control over its chemical phases by adjusting the deposition conditions. Here, a well-controlled CoO_x/SiO_x/n-Si heterojunction prepared by ALD is demonstrated for solar-driven water splitting. The phase of the ALD CoO_x films can be easily controlled from CoO to Co₃O₄ by varying the deposition temperature. In addition, this systematic study reveals that its energetic as well as electrochemical properties are changed significantly with the phase. Whereas CoO grown at 150 °C produces high photovoltage by building desirable hole-selective heterojunctions with n-Si, Co₃O₄ formed at 300 °C has a better catalytic property for water oxidation. To address this competitive correlation, we developed a double-layered (DL) ALD CoO_x film that has advantages of both CoO and Co₃O₄. The DL ALD CoO_x/SiO_x/Si heterojunction photoanode produces a photocurrent density of 3.5 mA/cm² without a buried junction and maintains a saturating current density of 32.5 mA/cm² without noticeable degradation during 12 h in 1 M KOH under a simulated 1 sun illumination.