Determining Which Phonons Affect Dynamic Disorder and Charge Mobility in Molecular Crystals
Year: 2026
Type: preprint
Abstract: Organic semiconductors (OSCs), and especially molecular crystals, exhibit unique structural complexity and strong coupling between electronic and vibrational degrees of freedom. The strong electron-phonon coupling (EPC) localizes the charge carriers and limits the charge transport rate as a consequence of the dynamic disorder (σij). In principal, it should be possible to engineer new molecular crystals with reduced dynamic disorder, but determining which phonons contribute the most to dynamic disorder and what are the corresponding effects on charge transport has been elusive. Here, we propose the use of singular value decomposition (SVD) to study phonon modes within molecular crystals by transforming the original phonon modes (OMs) into a reduced set of ranked projected modes (PMs) that affect the EPC. The SVD phonon analysis reduces the number of modes to consider for phonon engineering while retaining the essential physical information which is validated with inelastic neutron scattering. This analysis shows that even the most significant PM for the selected molecules contribute at most 12-45% of the total dynamic disorder. That is, for none of the selected molecules is a single OM responsible for a large proportion of the dynamic disorder. Our results also identify the collective detrimental motions for charge transport by producing atomic displacement visualizations that reveal which atomic and molecular motions contribute to dynamic disorder. Finally, side-chain effects are probed by SVD phonon analysis; these are not usually considered in detail because the electronic coupling is carried exclusively by the conjugated backbone. Our results show that side-chain location and branching can alter the distribution of backbone vibrational states, harden high-energy backbone-involved motions, and couple side-chain motions to backbone dynamics, thereby indirectly affecting the extent of dynamic disorder. All of this supports the conclusion that SVD phonon analysis could be used as a robust phonon engineering tool to eliminate or generate specific features in a phonon spectrum, generating personalized design rules for next-generation OSCs.
Source: ChemRxiv
Institution: University of California, Davis
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7214099601