Ultrathin film heterojunctions by combining solution processing and sublimation for ambipolar organic field-effect transistors

Ambipolar organic field-effect transistors based on binary blends of n- and p-type semiconductors frequently suffer from low or unbalanced electron and hole transport. Uncontrolled phase separation and intermixing of donor and acceptor phases in such thin film blends lead to formation of bulk heterojunctions that lack sufficient percolation paths to ensure effective transport of free charge carriers of both signs. In this work, we present a novel approach to improve the functionality of ambipolar transistors based on a common conjugated polymer, namely poly(3-hexylthiophene) (P3HT), and a small molecular naphthalene derivative (NDI-C9). We studied the correlation between the morphology and the performance of P3HT and NDI-C9 blend transistors with bulk, bilayer and structured bulk-like heterostructures. The bulk heterostructure was obtained from solution, while the bilayer and ordered bulk-like morphologies were produced by a combination of solution and vacuum deposition. The best ambipolar performance with balanced electron and hole transport was found for the bulk-like heterojunction structures of NDI-C9 evaporated onto the P3HT fiber network spin-cast from a pre-aggregated toluene solution.