Fabrication of TiO_x–Si photoanode and its energetic photoelectrochemical performance

Pristine Si is oxidized to insulative SiO₂ when it comes in contact with air and water. Covering it with a protection layer inhibits passivation of Si and significantly improves its photoelectrochemical performance. In this study, TiO_x with gradient change of oxygen stoichiometry ratio (TiO_x) was designed as a protection layer and fabricated via a chemical vapour deposition process in Ar flow under 400 °C for 1 min. The anaerobic atmosphere and short heating duration synergistically produced the ratio of O and Ti lower than two in the prepared film. XPS analysis suggested the existance of TiO₂ only at the surface of TiO_x film and Ti³⁺ and Ti²⁺ appeared successively with the increase of distance to the surface. The first advantage of lower-valence-state Ti and oxygen deficiency was to inhibit the oxidation of Si and to reduce electric resistance of the interface and the protection layer. The second advantage was to create a defect energy level under the conduction band of TiO₂ which provided the possibility for holes in the valence band of Si to be transferred to this defect level. This tunnel like transfer enhanced the photogenerated charge separation and redox ability of TiO_x–Si which brought a 3.25 folds enhancements in photocurrent density compared to that of stoichiometric TiO₂–Si at 0 V (SCE) under simulated sunlight. This study highly motivates further research on transparent and conductive protection layer of Si photoelectrode.