Structural Effects of Gating Poly(3‐hexylthiophene) through an Ionic Liquid

Abstract

Ionic liquids are increasingly employed as dielectrics to generate high charge densities and enable low‐voltage operation with organic semiconductors. However, effects on structure and morphology of the active material are not fully known, particularly for permeable semiconductors such as conjugated polymers, in which ions from the ionic liquid can enter and electrochemically dope the semicrystalline film. To understand when ions enter, where they go, and how they affect the film, thin films of the archetypal semiconducting polymer, poly(3‐hexylthiophene), are electrochemically doped with 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide, the archetypal ionic liquid. High‐resolution, ex situ X‐ray diffraction measurements and complete pole figures reveal changes with applied voltage, cycling, and frequency in lattice spacing, crystallite orientation, and crystallinity in the bulk and at the buried interface. Dopant ions penetrate the film and enter the crystallites at sufficiently high voltages and low frequencies. Upon infiltrating crystallites, ions permanently expand lamellar stacking and contract pi‐stacking. Cycling amplifies these effects, but higher frequencies mitigate the expansion of bulk crystallites as ions are hindered from entering crystallites. This mechanistic understanding of the structural effects of ion penetration will help develop models of the frequency and voltage impedance response of electrochemically doped conjugated polymers and advance electronic applications.