Binary molecular-semiconductor p–n junctions for photoelectrocatalytic CO2 reduction

In one approach to solar energy conversion, light-harvesting sensitizers absorb and convert photons into electron–hole pairs to drive water splitting or CO2 reduction to produce fuels. Despite recent progress in photoelectrocatalytic cells, experimental realization of a high-performance photocathode for solar-driven CO2 reduction has proven difficult. Here, we use a binary p–n junction strategy to prepare a series of photocathodes that convert sunlight into high-energy electrons for efficient CO2 reduction to formate. The photocathodes integrate a semiconductor p–n junction comprising GaN nanowire arrays on silicon, with molecular p–n junctions self-assembled on the semiconductor surface. Solar irradiation of the photocathodes generates redox-separated states that interact to form an intermediate state with remotely separated electrons and holes at the catalyst and semiconductor, respectively. The photocathodes reduce CO2 to formate at stable photocurrent densities of around −1.1 mA cm−2 during 20 h of irradiation with Faradaic efficiencies of up to 64%. Driven by solar light, photoelectrocatalytic cells can convert CO2 into energy carriers, but strategies to improve their performance are still required. Here the authors combine molecular and semiconductor p–n junctions that have complementary absorption in the visible light range to convert CO2 to formate efficiently.