September 6, 2011
Journal Article

Visible-light photoconductivity of Zn1-xCoxO and its dependence on Co2+ concentration

Abstract

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 Co2+ (Zn1-xCoxO), 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 high-structural-quality Zn1-xCoxO epitaxial films to examine the relationship between dopant concentration (x) and visible-light photoconductivity, with particular focus on mid-gap d-d photoactivity. Excitation into the intense 4T1(P) d-d band at ~2.0 eV (620 nm) leads to Co2+/3+ ionization with a quantum efficiency that increases with decreasing cobalt concentration and increasing sample temperature. Both spontaneous and thermally assisted ionization from the Co2+ 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 Co2+ in ZnO.

Revised: August 15, 2014 | Published: September 6, 2011

Citation

Johnson C.A., A. Cohn, T.C. Kaspar, S.A. Chambers, G. Salley, and D.R. Gamelin. 2011. Visible-light photoconductivity of Zn1-xCoxO and its dependence on Co2+ concentration. Physical Review B 84, no. 12:Article No. 125203. PNNL-SA-79985. doi:10.1103/PhysRevB.84.125203