Chasing the "killer" phonon mode for the rational design of low disorder, high mobility molecular semiconductors

Schweicher, Guillaume and D'Avino, Gabriele and Ruggiero, Michael T. and Harkin, David J. and Broch, Katharina and Venkateshvaran, Deepak and Liu, Guoming and Richard, Audrey and Ruzié, Christian and Armstrong, Jeff and Kennedy, Alan R. and Shankland, Kenneth and Takimiya, Kazuo and Geerts, Yves H. and Zeitler, J. Axel and Fratini, Simone and Sirringhaus, Henning (2019) Chasing the "killer" phonon mode for the rational design of low disorder, high mobility molecular semiconductors. Advanced Materials, 31 (43). 1902407. ISSN 1521-4095

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    Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron – phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high mobility molecular semiconductors, we have combined state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron phonon coupling constants with experimental measurements of the lowfrequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way we have been able to identify the long-axis sliding motion as a 'killer' phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, we propose a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high mobility molecular semiconductors.