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

TL;DR

This study identifies the dominant phonon mode affecting charge transport in high mobility organic semiconductors, providing insights for designing materials with lower disorder and higher mobility.

Contribution

It combines advanced quantum simulations with experimental techniques to pinpoint the key phonon mode influencing electron-phonon coupling in organic semiconductors.

Findings

Long-axis sliding motion is the primary phonon mode contributing to thermal disorder.

This mode accounts for over 80% of the total thermal disorder in some molecules.

The insights enable rational design guidelines for high mobility molecular semiconductors.

Abstract

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, state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron phonon coupling constants are combined with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way, the long-axis sliding motion is identified as a killer phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high mobility molecular semiconductors is suggested.