The role of phase impurities and lattice defects on the electron dynamics and photochemistry of CuFeO₂ solar photocathodes

CuFeO₂ is a promising photocathode for H₂ evolution and CO₂ reduction reactions. To better understand the complex defect chemistry and role of impurity phases in this material and their effect on the photochemical performance, we employ visible light transient absorption spectroscopy and density functional theory (DFT) calculations to investigate the electron dynamics in electrochemically deposited Cu-Fe oxide thin films. Kinetic analysis of carrier lifetime shows a fast, sub-ps contribution to relaxation followed by persistence of a long-lived state to time delays greater than 2 ns. Increasing amplitude of the long-lived state is shown to correlate with the rate of fast initial relaxation, and this is explained in terms of a competition between charge carrier trapping and charge separation. Charge separation in CuFeO₂ occurs via hole thermalization from O 2p to Cu 3d valence band states leading to segregation of electrons and holes across layers in the CuFeO₂ lattice. Correlation between transient absorption measurements and DFT calculations suggest that Cu vacancies enhance photochemical performance by facilitating charge separation kinetics. In contrast, O interstitials are predicted to switch the relative positions of O 2p and Cu 3d valence band states, which would inhibit charge separation by inter-band hole thermalization. Finally, we find no evidence for electron injection from CuFeO₂ to CuO suggesting that charge separation at this heterostructure interface does not play a role in the carrier lifetime or photochemical performance of the catalysts studied here.