Polymorph Transformation of Pentacene Thin Films Associated with Crystal-to-Liquid Crystal Transitions of Buffer Layers

Abstract

A buried liquid crystal (LC) buffer layer is examined as a thermally switchable interface for post-deposition irreversible polymorph transformation in pentacene thin films. Variable-temperature 2D grazing-incidence X-ray diffraction (2D-GIXD) reveals that pentacene on bare SiO₂/Si remains predominantly in the thin-film phase after thermal cycling, whereas pentacene on Ph-BTBT-C10 undergoes an irreversible transformation from the thin-film phase to the Campbell phase, coupled with the onset of the buffer crystal-to-LC transition. The extent of transformation is found to depend on the interfacial configuration of the pentacene/Ph-BTBT-C10 bilayer, i.e., whether Ph-BTBT-C10 is used as the underlayer or overlayer. Separate measurements on additional LC buffer layers reveal a certain association between pentacene transformation and buffer layer LC transition, indicating that buried LC interfaces may provide a means of facilitating post-deposition polymorph conversion in organic semiconductor thin films.