Many metal oxides investigated for solar photocatalysis or photoelectrochemistry have band gaps that are too wide to absorb a sufficient portion of the solar spectrum. Doping with impurity ions has been extensively explored as a strategy to sensitize such oxides to visible light, but the electronic structures of the resulting materials are frequently complex and poorly understood. Here, we report a detailed photoconductivity investigation of the wide-gap II-VI semiconductor ZnO doped with Co${}^{2+}$ (Zn${}_{1-x}$Co${}_x$O), which responds to visible light in photoelectrochemical and photoconductivity experiments and thus represents a well-defined model system for understanding dopant-sensitized oxides. Variable-temperature scanning photoconductivity measurements have been performed on Zn${}_{1-x}$Co${}_x$O epitaxial films to examine the relationship between dopant concentration ($x$) and visible-light photoconductivity, with particular focus on mid-gap intra-d-shell (d-d) photoactivity. Excitation into the intense ${}^4$T${}_1$(P) d-d band at ∼2.0 eV (620 nm) leads to Co${}^{2+/3+}$ ionization with a quantum efficiency that increases with decreasing cobalt concentration and increasing sample temperature. Both spontaneous and thermally assisted ionization from the Co${}^{2+}$ d-d excited state are found to become less effective as $x$ is increased, attributed to an increasing conduction-band-edge potential. These trends counter the increasing light absorption with increasing $x$, explaining the experimental maximum in external photon-to-current conversion efficiencies at values well below the solid solubility of Co${}^{2+}$ in ZnO.

• Received 3 May 2011

DOI:https://doi.org/10.1103/PhysRevB.84.125203