Molecular doping is an important strategy to improve the charge transport properties of organic semiconductors in various electronic devices. Compared to p-type dopants, the development of n-type dopants is especially challenging due to poor dopand stability against atmospheric conditions. In this article, we reprot the n-doping of the milestone naththalenediimide-based conjugated polymers P(NDI2OD-T2) in organic thin film transistor devices by introducing soluble anion dopants which resulted in the formation of stable radical anions in thin films, as confirmed by EPR spectroscopy. By tuning the dopant concentration via simple solution mixing, the transistor parameters could be readily controlled.