The role of solvents in the microstructure and charge transport of semiconducting polymer films prepared at the air–liquid interface

Abstract

Charge transport in semiconducting polymers (SCPs) is strongly influenced by their film microstructure. Fabrication of SCP floating films at the air–liquid interface offers numerous advantages over conventional solution processing. The casting solvent plays a crucial role in forming film microstructures, which has been extensively studied for various fabrication methods. However, floating films of SCPs are generally processed with a single solvent, and the role of their boiling point (T_b) or solubility of individual solvents (or blends) has never been considered. In this report, we have systematically studied the effect of different solvents and blends on the film microstructure and field-effect transistor (FET) mobility of three well-known SCPs, PQT, PBTTT and P(NDI2OD-T2). We used the unidirectional floating film transfer method (UFTM) and interfacial spreading to fabricate large-area oriented and isotropic films, respectively. The detailed analysis of the film microstructure, macroscopic orientation and FET mobility of the films of these SCPs processed with different solvents has been studied. Our results clarify that a low T_b solvent (<90 °C) is a prerequisite for obtaining uniform large-area oriented films, and films processed with high T_b solvents are isotropic. As expected, the effect of T_b of the solvent on the orientation of SCPs and FET mobility is different for various SCPs under consideration. Instead, the variation of FET mobility is linked to the optical dichroic ratio (polymer orientation), interchain ordering in films and the presence of fiber-like aggregates.