Liquid-like lattice vibrations in organic semiconductors

Organic semiconductor materials have drawn a lot of interest in the last few decades due to their applications in the (opto)electronic industry such as for solar cells, LEDs and FETs. Unlike inorganic semiconductors, organic solids are held together by two types of interactions: intramolecular covalent bonds and intermolecular van der Waals (vdW) forces. Phonons (dispersed quasi-particles of a vibration) in organic materials include mainly intermolecular vibrations. At finite temperatures, phonons dominate charge carriers (i.e. electrons and holes) scattering in semiconductors and therefore, determine their charge transport properties. Due to the relatively weak vdW interaction, these vibrations are in the low-frequency range.

Standard semiconductor theories are developed by assuming that the atomic/molecular motion is harmonic. Thus, they do not incorporate anharmonic effects. Since organic semiconductors have weak intermolecular interactions, the anharmonic components of their low-frequency vibrations are expected to be significant. Therefore, understanding the low-frequency vibrations of organic semiconductors is crucial for determining their transport properties.

In my talk, I will show how I elucidate the anharmonic nature of the low-frequency vibrations of anthracene and pentacene, by using temperature-dependent low-frequency Raman polarization-orientation measurements and first-principles calculations. The results show that specific lattice modes in anthracene and pentacene single crystals gradually lose their polarization dependence as the temperature is increased, which makes them resemble a vibration of a liquid. This finding indicates a symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Additionally, pentacene shows a never detected before subtle phase transition between 80-150~K, indicated by a change in the vibrational symmetry of one of the lattice modes.