Accurate Charge Mobility Simulation and Validation in Organic Semiconductors*

The charge mobility, which is the figure of merit in organic molecular semiconductors, is modulated by the intermolecular dynamic disorder. Knowledge of the full phonon spectrum is essential for accurate calculations of mobility and high mobility molecular design rules. Although phonons can be computed accurately, it is not known which phonons specifically contribute to dynamic disorder. Recently, it was proposed that a single long-axis mode dominates mobility reducing dynamic disorder and that the displacement along the long-axis of the molecule is the only relevant design rule. However, most vibrational spectroscopy techniques under-measure the phonons, and thereby limit the accuracy of calculated phonons.

Here, we show that optical based techniques are inadequate and that inelastic neutron scattering, which measures the full phonon spectrum beyond the gamma point with energies down to the elastic limit, is required. Experimental validation of computed inelastic neutron scattering spectra enables accurate predictions to dynamic disorder. We found that the dynamic disorder in a set of substituted acenes can be miscalculated by up to 24% when comparing the gamma point phonons and the full phonon band spectrum. Even including the dynamic disorder at the gamma point we find that no single phonon mode is responsible for more than 10% of the dynamic disorder. Our results demonstrate the importance of both a thorough phonon calculation with experimental validation and a need to develop appropriate phonon-based design rules that consider the full spectrum of phonon modes.