Controlling Polymorphism in Poly(3-Hexylthiophene) through Addition of Ferrocene for Enhanced Charge Mobilities in Thin-Film Transistors

Crystalline organic molecules often exhibit the ability to assemble into multiple crystal structures depending on the processing conditions. Exploiting this polymorphism to optimize molecular orbital overlap between adjacent molecules in the unit lattice is an effective method for improving charge transport within the material. In this study, grazing incident X‐ray diffraction was employed to demonstrate the formation of tighter π‐π stacking poly(3‐hexylthiophene‐2,5‐diyl) polymorphs in films spin coated from ferrocene‐containing solutions. As a result, the addition of ferrocene to casting solutions yields thin‐film transistors which exhibit approximately three times higher source‐drain currents and charge mobilities than neat polymer devices. Nevertheless, XPS depth profiling and NMR analyses of the active layer reveal that all ferrocene is removed during the spin coating process, which may be an essential factor to achieve high mobilities. Such insights gleaned from ferrocene/poly(3‐hexylthiophene‐2,5‐diyl) mixtures can serve as a template for selection and optimization of other small molecule/polymer systems with greater baseline charge mobilities. The electrical performance of poly(3‐hexylthiophene) thin‐film transistors increases with addition of ferrocene. Intermediate‐to‐high concentrations of ferrocene included in poly(3‐hexylthiophene) spin casting solutions produces transistors demonstrating roughly three times higher source‐drain current and charge mobility than neat components. Interactions between the polymer and metallocene lead to the formation of polymorphs with closer π‐stacking that decreases the charge hopping distance.