Crystalline TiO₂ protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

The trade-offs between photoelectrode efficiency and stability significantly hinder the practical application of silicon-based photoelectrochemical devices. Here, we report a facile approach to decouple the trade-offs of silicon-based photocathodes by employing crystalline TiO₂ with graded oxygen defects as protection layer. The crystalline protection layer provides high-density structure and enhances stability, and at the same time oxygen defects allow the carrier transport with low resistance as required for high efficiency. The silicon-based photocathode with black TiO₂ shows a limiting current density of ~35.3 mA cm⁻² and durability of over 100 h at 10 mA cm⁻² in 1.0 M NaOH electrolyte, while none of photoelectrochemical behavior is observed in crystalline TiO₂ protection layer. These findings have significant suggestions for further development of silicon-based, III–V compounds and other photoelectrodes and offer the possibility for achieving highly efficient and durable photoelectrochemical devices.