Size- and orientation-dependent photovoltaic properties of ZnO nanorods

ZnO nanorod arrays on an ITO substrate and nanorod powder have been prepared via a chemical method in aqueous solution at low temperature. Two dimensions of composite nanorods in the arrays were obtained by controlling the reaction time. SEM, XRD, UV-vis transmission, and PL measurements have been utilized to characterize the samples. The surface photovoltage (SPV) spectra of the three samples have been comparatively investigated by a lock-in amplifier with dc bias and Kelvin probe (KP) based measurements. The kinetic features of SPV responses are interpreted in terms of ac SPV phase spectra and SPV transients on a KP. We demonstrate that the photovoltaic properties of ZnO nanorods not only depend on the rod size, but also rely on the crystallographic orientation. The mechanisms therein have been discussed in detail. Our results could lead to better understanding of the photovoltaic properties in ZnO nanostructures.