Directing The Path with Directional Properties: Unravelling Anisotropic Carrier Mobility, Lifetime and Surface Defects in Oxides for Solar Fuels Generation

The concept of solar fuels holds great promise for the sustainable production of fuels through the utilization of solar energy. Over the last ten years, oxide photocathodes, such as Cu₂O, have showcased performance comparable to that of photoelectrodes relying on well-established photovoltaic materials. Our work has demonstrated several record tandem solar water splitting devices based on the state-of-the-art Cu₂O photocathodes featuring a radial junction design and effective hole transport strategies. Nevertheless, a significant challenge found was the occurrence of considerable charge carrier recombination within the bulk of the photoabsorber, a common issue commonly observed in oxide semiconductors. Here, we demonstrate Cu₂O photocathodes with performance beyond the state-of-the-art by exploiting a novel conceptual understanding of carrier recombination and transport in single-crystal Cu₂O thin films with adjustable crystal orientations. The combination of the unique thin film materials platform and a series of optoelectronic characterizations, including a customized broadband femtosecond transient reflection spectroscopy, precisely quantified unprecedented anisotropic carrier mobilities, lifetimes and diffusion lengths. Notably, it was discovered that these properties exhibit significant disparities along different crystal orientations, with one orientation manifesting significantly superior properties compared to the others. To exploit the findings, polycrystalline Cu₂O photocathodes with extremely pure selected crystalline orientations and terminating facets were fabricated, delivering current density 70% more current density compared to the state-of-the-art electrodeposited devices at 0.5 V versus a reversible hydrogen electrode under simulated air mass 1.5 G illumination, and stable operation over 100 hours. Furthermore, we will share recent research outcomes that elucidate the significance of facet-dependent defects in their contributions to photoelectrochemical applications.