Function and Electronic Structure of the SnO₂ Buffer Layer between the α-Fe₂O₃ Water Oxidation Photoelectrode and the Transparent Conducting Oxide Current Collector

The tin oxide buffer layer between the transparent conducting oxide current collector and the hematite photoelectrode causes considerable water oxidation enhancement of that electrode. The water oxidation onset potential is lowered by 180 mV. The lifetime of photogenerated charge carriers is increased by a factor of 10. For the investigation of structure and function of the buffer layer, we designed a wedge-shaped multilayer film assembly. Oxygen 1s X-ray photoemission spectra suggest a decrease of oxygen vacancy concentration near the interface of <em>α</em>-Fe<sub>2</sub>O<sub>3</sub> and FTO-SnO<sub>2</sub>, when the SnO<sub>2</sub> buffer layer is introduced. This SnO<sub>2</sub> buffer layer increases the crystallinity of the hematite layer. The oxygen 1s near-edge X-ray absorption fine structure shows that the buffer layer increases the Fe 3d-O 2p hybridization and affects the quasi-Fermi level of electrons in <em>α</em>-Fe<sub>2</sub>O<sub>3</sub>. There is some indication that the <em>α</em>-Fe<sub>2</sub>O<sub>3</sub> layer contains an adverse hole state in the valence band which disappears when the <em>α</em>-Fe<sub>2</sub>O<sub>3</sub> layer is grown on the SnO<sub>2</sub> layer. This layer induces improved orbital overlap with subsequent improved charge transfer between the absorber <em>α</em>-Fe<sub>2</sub>O<sub>3</sub> and the current collector FTO. Our experiments indicate that performance enhancement by this buffer layer is of electronic structure origin.