In the process of inkjet-printed zinc tin oxide thin-film transistor, the effect of metallic interlayer underneath of source and drain electrode was investigated. The reason for the improved electrical properties with thin molybdenum oxide (MoO_3) layer was due to the chemically intermixed state of metallic interlayer, aluminum source and drain, and oxide semiconductor together. The atomic configuration of three Mo 3d3 and 3d5 doublets, three different Al 2p core levels, two Sn 3d5, and four different types of oxygen O 1s in the interfaces among those layers was confirmed by X-ray photospectroscopy.