Conjugated polymer-based organic thin film transistors (OTFTs) have received tremendous attention due to their potential applications. In addition to their high performances, air stability is also essential for application and another main property that OTFTs have. In this paper, three aza-heterocycle (BABDF)-based polymers were designed and synthesized using strong donor thiophene-vinylene-thiophene (TVT), weak donor thiophene-cyanovinylene-thiophene (TCNT), and weak acceptor dithiazole (TZ) as co units. The lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) energy levels were effectively lowered by introducing TCNT and TZ units, especially for PBABDF-TZ, for which the too much deep LUMO/HOMO energy levels of -4.28/-6.06 eV were obtained. These levels are low enough for air-stable electron transport and large enough for the hole injection barriers in OTFTs. Consequently, the unencapsulated bottom-gate/top-contact (BG/TC) devices exhibited unipolar electron transport under air conditions. Furthermore, these devices had high air stability and maintained unipolar electron transport with a mobility of up to 0.01 cm(2) V-1 s(-1) during the one-year characterization period. Very low LUMO and HOMO levels were necessary for electron transport and the hole barriers, respectively, and both were important for long-term, air-stable n-channel polymer transistors.