Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight^1,2. Following a decade of advancement, Cu₂O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials^3,4,5. However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance⁶. Here we demonstrate performance of Cu₂O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu₂O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu₂O 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 Cu₂O 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.

太阳能燃料提供了一种利用阳光提供可持续燃料的有前景的方法^1,2。经过十年的进步，Cu ₂ O 光电阴极能够提供与现有光伏材料的光电极相当的性能^3,4,5。然而，人们对大量电荷载流子复合知之甚少，这仍然限制了性能的进一步进步⁶。在这里，我们通过利用对单晶 Cu ₂ O 薄膜中载流子复合和传输的新概念理解，展示了 Cu ₂ O 光电阴极超越最先进水平的性能。利用环境液相外延，我们提出了一种生长具有三个晶体取向的单晶 Cu ₂ O 样品的新方法。宽带飞秒瞬态反射光谱测量用于量化各向异性光电特性，发现沿[111]方向的载流子迁移率比沿其他方向的载流子迁移率高一个数量级。在这些发现的推动下，我们使用简单且低成本的方法开发了一种具有非常纯的（111）取向和（111）终止面的多晶Cu _{2 O光电阴极，可提供7 mA cm} ^-2电流密度（超过70%）与最先进的电沉积设备相比，在空气质量 1.5 G 照明下，在 0.5 V 电压下与可逆氢电极相比，性能得到了改善，并稳定运行至少 120 小时。