We report the synthesis of an aromatic amine-containing polymer (PDPAT-12) and the use of this polymer as an effective additive to an ambipolar polymer semiconductor (PIBDFBT-e37) to realize unipolar n-type charge transport in organic thin film transistors (OTFTs). With 10 wt% PDPAT-12 addition to PIBDFBT-e37, the OTFTs achieved unipolar n-type charge transport with electron mobility of up to 0.42 cm² V⁻¹ s⁻¹ in nitrogen and 0.19 cm² V⁻¹ s⁻¹ in air. Moreover, the polymer blend demonstrated improved stability towards ambient air (retaining 50% of its initial electron mobility after exposed in air for 28 days) than the pristine PIBDFBT-e37 (retaining 13% of its initial electron mobility under the same conditions) and the PIBDFBT-e37:PEI (polyethylenimine) blend, which lost transistor behaviour almost instantly once exposed to ambient air. Our results show that the use of this aromatic amine-containing polymer as an additive can convert an ambipolar polymer into an n-type unipolar polymer in OTFTs with much improved air stability.