We proposed a novel material named sodium hypochlorite (NaClO) solution as a source of activation for amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs). We reduced the activation temperature from 300 to 150 °C using NaClO solution (concentration: 50%) and obtained satisfactory electrical characteristics of a-IGZO TFTs. The field-effect mobility, threshold voltage, on/off ratio, subthreshold swing, and threshold voltage (V_th) shift under negative bias illumination stress (V_G = −20 V and V_D = 10.1 V for 10,000 s) of NaClO (50%)-activated a-IGZO TFTs were 10.41 cm²/V·s, 1.51 V, 2.78 × 10⁸, 0.37 V/dec, and −5.43 V, respectively. Also, the V_th shifts of the NaClO (50%)-activated a-IGZO TFTs (150 °C) under the positive bias stress test decreased from 5.01 to 1.87 V (V_G = 20 V and V_D = 10.1 V for 10,000 s) compared with that of only-annealed (300 °C) a-IGZO TFTs. Also, the mechanism of NaClO activation for a-IGZO TFTs is clarified through photo-assisted oxygen radical (POR) and heat-driven oxygen radical (HOR) effects. The POR and HOR effects generated the reactive oxygen species (ROS) from NaClO solution (50%), which activated a-IGZO TFTs at a low temperature (150 °C). When the NaClO solution (50%) was exposed to external energy, it generated ROS such as hydroxyl radicals (OH^•), hydroperoxyl radicals (HO₂^•), and oxygen radicals (O^•), which promoted the formation of strong metal–oxide bonds in a-IGZO TFTs. Furthermore, NaClO solution (50%) was applied to a-IGZO TFTs on a flexible polyimide substrate and electrohydrodynamic printing process for selective deposition.