Engineering Interfacial Silicon Dioxide for Improved Metal-Insulator-Semiconductor Silicon Photoanode Water Splitting Performance

Silicon photoanodes protected by atomic layer deposited (ALD) TiO₂ show promise as components of water splitting devices that may enable the large-scale production of solar fuels and chemicals. Minimizing the resistance of the oxide corrosion protection layer is essential for fabricating efficient devices with good fill factor. Recent literature reports have shown that the interfacial SiO₂ layer, interposed between the protective ALD-TiO₂ and the Si anode, acts as a tunnel oxide that limits hole conduction from the photoabsorbing substrate to the surface oxygen evolution catalyst. Herein, we report a significant reduction of bilayer resistance, achieved by forming stable, ultrathin (<1.3 nm) SiO₂ layers, allowing fabrication of water splitting photoanodes with hole conductances near the maximum achievable with the given catalyst and Si substrate. Three methods for controlling the SiO₂ interlayer thickness on the Si(100) surface for ALD-TiO₂ protected anodes were employed: (1) TiO₂ deposition directly on an HF-etched Si(100) surface, (2) TiO₂ deposition after SiO₂ atomic layer deposition on an HF-etched Si(100) surface, and (3) oxygen scavenging, post-TiO₂ deposition to decompose the SiO₂ layer using a Ti overlayer. Each of these methods provides a progressively superior means of reliably thinning the interfacial SiO₂ layer, enabling the fabrication of efficient and stable water oxidation silicon anodes.