A Silicon-Based Heterojunction Integrated with a Molecular Excited State in a Water-Splitting Tandem Cell

Semiconductor-based photocathodeswith high light-absorption capability are of interest in the production of solar fuels, but many of them are limited by low efficiencies due to rapid interfacial back electron transfer. We demonstrate here that a nanowire-structured p-type Si (p-Si) electrode, surface-modified with a perylene- diimidederivative (PDI′), can undergo photoreduction of a surface- bound, waterreduction catalyst toward efficient H2 evolution under a low applied bias. At the electrode interface, the PDI′ layer converts green lightinto high-energy holes at its excited state for extraction of photogenerated electrons at the photoexcitedp-Si. The photogenerated electrons at the reduced PDI′ are subsequently transferred to the molecular H2-evolution catalyst. Involvement of the photoexcitedPDI′ enables effective redox separation between the electrons at the reduced catalyst and the holes at the valence band of p-Si. The heterojunction photocathodewas used in a tandem cell by coupling with a dye-...