Discovery of “killer” phonon mode informs the search for new organic semiconductors
27 Oct 2020





​Relative displacements of neighbouring molecules associated with specific modes that contribute strongly to the energetic disorder.


The electronic properties and the environmental and operational stability of organic semiconductors (OSCs) has improved considerably over the last ten years. The goal remains to find new materials that have higher carrier mobilities as this will enable faster operation and lower power consumption for use in advanced liquid crystal and organic light‐emitting diode displays.

For high carrier mobility, the thermal molecular disorder needs to be minimised, as should the sensitivity of the carrier motion to this disorder. The sensitivity can be tuned through engineering of the molecular packing, but little is known about whether it is possible to influence the thermal disorder by molecular design. This results from the complexity of the unit cell of molecular crystals, typically containing on the order of 1000 atoms.

The vibration of these crystals can align in many different ways to produce hundreds of possible phonon modes, which can couple to the charge motion and contribute to thermal disorder. This study used terahertz time‐domain spectroscopy and inelastic neutron scattering on TOSCA, coupled with computational modelling, to assess the contributions that individual modes make to the total thermal disorder in the OSCs pentacene, rubrene and a group of thienoacenes.

The researchers wanted to discover whether all the modes make important contributions to the disorder, or if there were specific “killer” modes that are responsible for the majority. Their results revealed that this was the case: a single sliding mode appears to be a “killer” mode: responsible for a large fraction of the total coupling. By considering the structural properties of the molecule that influences this sliding mode, the group’s work will inform the quest for higher mobility semiconductors.

Instrument: Tosca

Related publicationChasing the “Killer” Phonon Mode for the Rational Design of Low‐Disorder, High‐Mobility Molecular Semiconductors Advanced Materials31, 43, 19024072019


Funding: EPSRC, Royal Society, German Research Foundation, European Research Council, ARCHER UK National Supercomputing Service, Belgian National Fund for Scientific Research.

AuthorsG Schweicher (University of Cambridge), G D'Avino (Institut Néel‐CNRS and Université Grenoble Alpes), MT Ruggiero , DJ Harkin, K Broch, D Venkateshvaran, G Liu (University of Cambridge), A Richard, C Ruzié (Université Libre de Bruxelles), J Armstrong (ISIS), AR Kennedy (University of Strathclyde), K Shankland (University of Reading), K Takimiya (RIKEN Center for Emergent Matter Science), YH Geerts (Université Libre de Bruxelles), JA Zeitler (University of Cambridge), S Fratini (Institut Néel‐CNRS and Université Grenoble Alpes), H Sirringhaus  (University of Cambridge). 

Contact: Gianchandani, Shikha (STFC,RAL,ISIS)