Determining Which Phonons Affect Dynamic Disorder and Charge Mobility in Molecular Crystals
- 1Department of Materials Science and Engineering
- 2University of California Davis
- 3Department of Chemical Engineering
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.
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BibTeX
@misc{chiang2026determining,
title = {{Determining Which Phonons Affect Dynamic Disorder and Charge Mobility in Molecular Crystals}},
author = {Chen-Wei Chiang and Toulik Maitra and Jeremy K. Mason and Adam J. Moulé},
howpublished = {ChemRxiv},
year = {2026},
month = sep,
doi = {10.26434/chemrxiv.15009367/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15009367/v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - Determining Which Phonons Affect Dynamic Disorder and Charge Mobility in Molecular Crystals
AU - Chen-Wei Chiang
AU - Toulik Maitra
AU - Jeremy K. Mason
AU - Adam J. Moulé
PY - 2026
DA - 2026/09/23
PB - ChemRxiv
DO - 10.26434/chemrxiv.15009367/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15009367/v1
AB - 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.
ER -