Evaluating Charge Carrier Transport and Surface States in CuFeO2 Photocathodes

Interest in delafossite (CuFeO2) as a candidate p-type photocathode for photoelectrochemical (PEC) solar fuel production has recently been increasing, mainly due to its excellent stability in aqueous environments and favorable light absorption properties. However, its PEC performance has remained poor for reasons that have not yet been specifically determined. Herein, we report a detailed investigation on sol-gel-processed CuFeO2 with a range of spectroscopic, PEC, and microscopy techniques aimed at unraveling the material properties governing photogenerated charge carrier harvesting in this v. An analysis of the bulk transport properties using microwave conductivity measurements reveals a good charge carrier mobility (0.2 cm(2) V-1 s(-1)) and a relatively long lifetime (200 ns) for photogenerated charge carriers. Conversely, systematic PEC measurements with varied redox systems reveal the existence of a high density of surface states (10(14) cm(-2)) positioned 0.35 eV above the conduction band, inducing Fermi level pinning at the semiconductor-liquid junction. X-ray photoelectron spectroscopy suggests the presence of a thin layer of metal hydroxide at the surface of the material. These surface states were found to behave as electron traps, correlated with an inversion of polarity at the surface of the semiconductor, and thereby promoting charge recombination and limiting the photovoltage developed at the junction. These findings suggest that if the detrimental effects of the surface states can be eliminated, CuFeO2 would provide a sufficiently high photovoltage to be combined with other solution-processed and stable photoanodes into an easily scalable tandem PEC cell.