Ultraviolet (UV) ozone passivation of the metal/dielectric interface was investigated in HfO2-based devices. It is found that the passivation of Al gate reduces the gate leakage current by two orders of magnitude and increases the breakdown field strength by 14%. A thicker wide-band gap Al2O3 interlayer formed on the Al gate during UV ozone treatment improves the interface quality and suppresses the leakage associated with the high-k material. Copper phthalocyanine-based organic thin-film transistors with HfO2 as gate dielectric were fabricated on glass. UV ozone passivated devices exhibited a low threshold voltage of -0.29 V and a low subthreshold slope of 0.38 V/decade, demonstrating the advantage of UV ozone passivation.