Here, we report the persistent photoconductance (PPC) and transient photo-response characteristics of photochemically activated and thermally annealed indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs). From various analyses on the time-variant photo-current change in IGZO TFTs, it was found that photochemically activated IGZO TFTs exhibited significantly weak PPC behavior and rapid photo-decaying characteristics compared to thermally annealed IGZO TFTs. It is claimed that such behaviors of photochemically activated IGZO TFTs are due to the low activation energy for the neutralization of ionized oxygen vacancies, which can be beneficial in achieving highly photo-stable electronic devices. Furthermore, we report the influence of pulsed gate bias on the photo-decaying characteristics of photochemically activated and thermally annealed IGZO TFTs.