Polythiophene–perylene diimide heterojunction field-effect transistors

Abstract

Thin film field-effect transistors based on binary blends of poly(3-hexylthiophene) (P3HT) and two perylene diimide (PDI) derivatives with different alkyl substituents have been investigated in terms of device performance, microstructure and molecular organization on the surface. For the same blend ratios the PDIs phase separate differently due to solubility variation. Blends with a horizontal phase separation between the donor and acceptor show ambipolar behavior due to well defined homogenous pathways for both charge carriers. In this layer arrangement the polymer is located near the dielectric interface, while the PDI molecules crystallize on top of the film. Interestingly, the electron mobility is improved by a few orders of magnitude in comparison to the pure acceptor. This increase is attributed to the altered microstructure of PDI in the blends. Layers in which the PDI crystals are embedded within the polymer matrix and are not interconnected with each other lead only to hole transport in the transistor. For one blend ratio, the hole mobility improves by one order of magnitude compared to pure P3HT as a result of the reorganization of the polymer in the blend layer. This study provides new insights into the role of microstructure and molecular organization in the charge carrier transport in heterojunction field-effect transistors for the development of high-performance future devices.