We investigated solution-processed indium-zinc oxide (IZO) thin-film transistors (TFTs) by inserting a 2-(4- biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) buffer layer. This buffer layer efficiently tuned the energy level between the semiconducting oxide channel and metal electrode by increasing charge extraction, thereby enhancing the overall device performance: the IZO TFT with embedded PBD layer (thickness: 5 nm; width: 2,000 μm; length: 200 μm) exhibited a field-effect mobility of 1.31 cm²V^-1s^-1, threshold voltage of 0.12 V, subthreshold swing of 0.87 V decade^-1, and on/off current ratio of 9.28×10⁵.