High carrier mobility along the [111] orientation in Cu 2 O photoelectrodes.

Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight 1,2 . Following a decade of advancement, Cu 2 O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials 3-5 . However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance 6 . Here we demonstrate performance of Cu 2 O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu 2 O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu 2 O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu 2 O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm -2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h.