Abstract
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 dopant stability against atmospheric conditions. In this article, we report the n-doping of the milestone naphthalenediimide-based conjugated polymer P(NDI2OD-T2) in organic thin film transistor devices by soluble anion dopants. The addition of the dopants 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. Hence the contact resistance between the electrodes and the semiconducting polymer could be significantly reduced, which resulted in the transistor behaviour approaching the desirable gate voltage-independent model. Reduced hysteresis was also observed, thanks to the trap filling by the dopant. Under optimal doping concentrations the channel on-current was increased several fold whilst the on/off ratio was simultaneously increased by around one order of magnitude. Hence doping with soluble organic salts appears to be a promising route to improve the charge transport properties of n-type organic semiconductors. Organic electronics: Conjugated polymer doped by fluoride anionFluoride anions have been confirmed to be capable of n-dope the organic semiconductors and thus provide more possibilities in molecular doping. A collaborative team led by Martin Heeney from Imperial College London shows an effective n-doping effect in P(NDI2OD-T2) polymer by the Lewis basic anion fluoride. The formation of radical anions is confirmed by electron paramagnetic resonance and absorption spectroscopy. Furthermore, the doping effect is verified with the improved characteristics of the thin film transistors such as reduced injection barrier, contact resistance and hysteresis. This anion doping method can be generalised to other halide anions, thus overcoming the problem of poor stability for the conventional 'charge transfer' doping. It opens up new opportunities for simple solution processed doping strategies and optimisation of organic electronic devices.