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.
Highlights
The most well understood mechanism for doping is the integer charge transfer mechanism, in which an electron is transferred from the highest occupied molecular orbital (HOMO) of the electron donating semiconductor to the lowest unoccupied molecular orbital (LUMO) of the electron accepting dopant. As such there is a general requirement for matching of the ionisation energy of an electron donor and the electron affinity of the electron acceptor. This is challenging for the n-doping of Organic semiconductors (OSCs), since to match the LUMO of the host molecules, the HOMO of the dopant has to roughly lie above the range of −3.5 to −4.5 eV, which can make the dopant unstable under atmospheric conditions.[6]
A few stable n-type dopants have been reported via mechanisms other than integer charge transfer.[13,14,15,16,17,18,19,20,21,22,23,24,25]
Since n-type dopants designed according to the traditional integer charge transfer principle often encounter air stability problems due to their small ionisation potentials, the development of new n-doping motifs is highly desirable to improve the charge transport properties of electron transport semiconductors
Summary
Organic semiconductors (OSCs) for organic thin film transistors (OTFTs) have been extensively studied for the development of high-performance next-generation plastic electronics, aiming at flexible, large-area and low-cost devices through processing from solution.[1,2,3,4,5] Similar to their inorganic counterparts, control of the optoelectronic properties of OSCs by the addition of dopants has proved to be an effective and important strategy in their application and optimisation.[6,7,8,9,10] When the basic principle of doping is fulfilled, that is, the introduction of dopant molecules results in charge transfer with the host and the generation of additional mobile charge carriers, the critical properties of devices can become controllable by tuning the amount of dopant. The most well understood mechanism for doping is the integer charge transfer mechanism, in which an electron is transferred from the highest occupied molecular orbital (HOMO) of the electron donating semiconductor to the lowest unoccupied molecular orbital (LUMO) of the electron accepting dopant (for p-doping, and vice versa for n-doping). As such there is a general requirement for matching of the ionisation energy of an electron donor and the electron affinity of the electron acceptor. This difficulty in designing stable dopants has somewhat limited the development of n-doping of OSCs.[11,12] a few stable n-type dopants have been reported via mechanisms other than integer charge transfer.[13,14,15,16,17,18,19,20,21,22,23,24,25]
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