Interlayer Sliding Phonon Drives Phase Transition in the Ph-BTBT-10 Organic Semiconductor

In the field of organicelectronics, the semiconductor 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) has become a benchmarkdue to its high charge mobility and chemical stability in thin filmdevices. Its phase diagram is characterized by a crystal phase witha bilayer structure that at high temperature transforms into a SmecticE liquid crystal with monolayer structure. As the charge transportproperties appear to depend on the phase present in the thin film,the transition has been the subject of structural and computationalstudies. Here such a process has been investigated by polarized lowfrequency Raman spectroscopy, selectively probing the intermoleculardynamics of the two phases. The spectroscopic observations demonstratethe key role played by a displacive component of the transition, withthe interpenetration of the crystal bilayers driven by lattice phononmode softening followed by the intralayer rearrangement of the moleculerigid cores into the herringbone motif of the liquid crystal. Themechanism can be related to the effectiveness of thermal annealingto restore the crystal phase in films.