Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors

Maor Asher, Daniel Angerer, Roman Korobko, Yael Diskin-Posner, David A. Egger, Omer Yaffe

Research output: Contribution to journalArticlepeer-review

22 Scopus citations

Abstract

The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization-orientation (PO) Raman measurements are used to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high-mobility organic semiconductors at room temperature.

Original languageEnglish
Article number1908028
JournalAdvanced Materials
Volume32
Issue number10
DOIs
StatePublished - 1 Mar 2020

Keywords

  • first-principles calculations
  • low-frequency Raman modes
  • oligoacenes
  • polarization-orientation Raman spectroscopy
  • small-molecule organic semiconductors

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