ZnO-based photo-thin film transistors with enhanced photoresponse were developed using transparent conductive oxide contacts. Changing the electrode from opaque Mo to transparent In-Zn-O increases the photocurrent by five orders of magnitude. By changing the opacity of each source and drain electrode, we could observe how the photoresponse is affected. We deduce that the photocurrent generation mechanism is based on an energy band change due to the photon irradiation. More importantly, we reveal that the photocurrent is determined by the energy barrier of injected electrons at the interface between the source electrode and the active layer.