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.