We investigate the origin of photo-induced electrical instability in hydrogenated amorphous silicon based thin-film transistors (a-Si: H TFTs). Photo instability alone was accompanied by a positive shift in the threshold voltage (V-TH) caused by photo irradiation, and even larger positive or negative shift further exacerbated the instability caused by photo-induced electrical bias stress. Such phenomena can occur as a result of extended charge trapping and/or the creation of defect-states at the semiconductor/dielectric interface or in the gate dielectric. The mechanism for such is difficult to describe through chemical interactions of electron-donating and -withdrawing molecules that exhibit a shift in V-TH in only one direction. We also prove that a transition from an amorphous to a protocrystalline phase improves the photo-induced electrical stability. Such results originate from a reduction in the density of the localized states in protocrystalline-Si: H films relative to that of a-Si: H. We believe that this study provides significant information on the device physics of optoelectronics, which commonly exhibit photo-induced instability and charge transport, as a result of prolonged exposure to photo irradiation. (C) 2015 Elsevier B.V. All rights reserved.