Marginal solvents preferentially improve the molecular order of thin polythiophene films

Abstract

The crystalline order within π-conjugated polymer films prepared using solution processing methods determines the electrical properties of the film. A channel's morphology is particularly important to device performance. The molecular order and morphology within a channel region near the underlying active layer have not yet been examined systematically. Here, we characterize the crystal order homogeneity as a function of the solvent penetration depth after applying simple solvent post-treatment. The morphological, optical, and electrical properties of poly(3-hexylthiophene) (P3HT) films could be profoundly improved by casting the films in methylene chloride solutions. The impact of the solvent application was most pronounced in the thin P3HT films, especially in the center of the film. During solvent casting, the central region of the film was exposed to methylene chloride for a longer period of time than the edge region of the film, thereby producing a thinner and more ordered film structure in the central region. Concomitant with the improved order, the charge carrier transport in the resulting field-effect transistors increased.