This study examined the use of a simple sol-gel method for synthesizing indium zinc oxide (IZO) films for use as semiconductor channel layers in thin-film transistors (TFTs) fabricated on flexible polyimide substrates. The performance of the flexible IZO-based TFTs was investigated, and the entire process was conducted at a low temperature to ensure the flexibility of the devices. First, transistors were fabricated on a flexible substrate by using silicon nitride and silicon dioxide as dielectric layers to investigate the effect on the on/off current ratios and electronic mobility of the devices. Second, the dielectric layer thickness was increased to enhance device performance, and the devices were finally subjected to bending tests to confirm their flexibility and stability for flexible transistor applications. The study successfully increased the dielectric layer thickness to improve flexible IZO-based TFT performance.