Fine-tuning of the charge carrier polarity in organic transistors is an important step toward high-performance organic complementary circuits and related devices. Here, three new semiconducting polymers, namely, pDPF-DTF2, pDPSe-DTF2, and pDPPy-DTF2, are designed and synthesized using furan, selenophene, and pyridine flanking group-based diketopyrrolopyrrole cores, respectively. Upon evaluating their electrical properties in transistor devices, the best performance has been achieved for pDPSe-DTF2 with the highest and average hole mobility of 1.51 and 1.22 cm(2) V-1 s(-1), respectively. Most intriguingly, a clear charge-carrier-polarity change is observed when the devices are measured under vacuum. The pDPF-DTF2 polymer exhibits a balanced ambipolar performance with the mu(h)/mu(e) ratio of 1.9, whereas pDPSe-DTF2 exhibits p-type dominated charge carrier transport properties with the mu(h)/mu(e) ratio of 26.7. Such a charge carrier transport change is due to the strong electron-donating nature of the selenophene. Furthermore, pDPPy-DTF2 with electron-withdrawing pyridine flanking units demonstrates unipolar n-type charge transport properties with an electron mobility as high as 0.20 cm(2) V-1 s(-1). Overall, this study demonstrates a simple yet effective approach to switch the charge carrier polarity in transistors by varying the electron affinity of flanking groups of the diketopyrrolopyrrole unit.