Bottom-gate bottom contact amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) were fabricated using solution processing. The nonpassivated a-IGZO-TFTs exhibited a significant threshold voltage shift, large hysteresis in thecurrent–voltage characteristics, and degradation in the subthreshold swing. TFTs’ performance degradation is due to the interaction of its back channel with adsorbed oxygen and water molecules in the ambient air. The hydroxy-phenyl zinc porphyrin (ZnP) self-assembled monolayer (SAM) was formed on TFTs’ back channelusing a low-cost solution-based approach to passivate TFTs back channel. The passivated a-IGZO-TFTs exhibited enhanced electrical characteristics and improved stabilitycompared with nonpassivated TFTs. Passivated TFTs presented significantly reduced hysteresis up to 87% and subthreshold slope degradation up to 71%. The passivated TFTs showed excellent stability under positive (20 V) and negative (-20 V) bias stress conducted for 5000 s.