This study investigated the correlations between the microstructure and electrical properties of regioregular poly(3-hexylthiophene) (rr-P3HT) blended with polystyrene, poly(vinyl phenol), or poly(methyl methacrylate) (PMMA) thin-film transistors (TFTs). Compared with pure rr-P3HT TFT, the blended rr-P3HT TFTs exhibited superior characteristics such as higher on/off current ratio of approximately 104, lower leakage current of<10(-11)A, smaller sub-threshold swing of approximately 2.09 V/dec, and higher mobility of approximately 8.18 x 10(-3) cm(2)/Vs. The suppression of the leakage current of the sub-threshold and off regimes may be attributed to the enhanced oxygen/humidity resistance of blended rr-P3HT TFTs. The evaluation of the physical properties showed that polymer insulators positively contributed to the morphology, molecular orientation, and effective conjugated length of the rr-P3HT film, thereby enhancing the device characteristics. The hybrid Al2O3/PMMA gate insulator was also utilized to apply low-voltage TFTs with rr-P3HT/polymer blends and yielded a low operation voltage of -2V. (C) 2017 Elsevier B.V. All rights reserved.