Storage-period dependent bias stress instability of sol-gel processed amorphous indium zinc oxide (a-InZnO) thin-film transistors (TFTs) was analyzed for more than 90 storage days. Two different solvents, 2-methoxyethnanol and acetyl acetone, were used to investigate the effect of solvent on bias-stress instability. The threshold voltage was shifted to the positive direction under positive gate-bias stress in both devices after 15 days, which is typically observed for conventional amorphous based TFTs. However, after 90 days, the TFTs became stable independent of the chemical compositions of the solvents, indicating that there is a common mechanism for solution processed a-InZnO TFTs concerning stability improvement after long storage periods. It is suggested that out-diffusion of excess oxygen in the channel region is the origin of the stable operation of a-InZnO TFTs after long storage periods. (C) 2015 Elsevier B.V. All rights reserved.