Deep-level defect states in sputtered ZnO thin-film transistors were investigated as a function of oxygen partial pressure during sputtering growth. Photo-induced threshold voltage-shift measurements under monochromatic illumination were used to characterize the deep-level defect distribution. Intrinsically, the defect states of oxygen vacancies were ionized to V-o(+) and V-o(2+) while the photon energy was absorbed within the bandgap, resulting in the shift of threshold voltage. Extracted deep-level defect distribution from this shift of threshold voltage was clearly confirmed in the range of 1.8-2.1 eV below the conduction band minimum and this region was suppressed with increasing oxygen partial pressure. These deep-level defect states can have a detrimental effect on device performance, such as threshold voltage shift and photo-induced leakage current. The photo instability of the devices occurred under visible light due to the photo-ionization of deep-level trapped charges associated with oxygen vacancies. (C) 2016 Elsevier B.V. All rights reserved.