Polarity Switching in Organic Electronic Devices via Terminal Substitution of Active-Layer Molecules

Organic electronic devices often suffer from poor charge injection limiting their performance. Specifically, high-performance electronic devices usually need Ohmic contacts, but it is not easy to realize them in junctions of the organic semiconductor with the electrodes because of the contact problems. In this work, polarity switching in organic field-effect transistors (OFETs) that is independent of the electrode work function is demonstrated─the switching of charge injection from the p-type to ambipolar and to the n-type─via modification of the donor/acceptor character of the molecular terminal substituents. By using three thiophene–phenylene co-oligomers with the same conjugated core, similar crystal packings, but different terminal substituents (methyl, trimethylsilyl, and trifluoromethyl), the polarity switching in both thin-film and single-crystal OFETs is demonstrated. The ultraviolet photoelectron spectroscopy studies and electronic structure calculations justify a definitive role of the interface dipole stemming from the terminal groups in controlling the heights of charge injection barriers and hence in charge injection into the OFET active layer. The results obtained are expected to facilitate rational design of organic semiconductors for high-performance electronic devices.