Vibrational mode contribution to the dielectric permittivity of disordered small-molecule organic semiconductors

The functioning of organic optoelectronic devices such as organic light-emitting diodes (OLEDs) is determined in part by the dielectric permittivity $\ensuremath{\epsilon}$ of the organic materials used, at frequencies that vary from quasistatic to the optical range. The difference between the dielectric constants at these extremes of the frequency scale is due to contributions of vibrational and (for some materials) dipole rotational modes and can depend on the detailed molecular packing. Studies of these contributions are therefore expected to sensitively probe differences in thin-film structures that affect their long-term stability. The absolute value of the dielectric constant affects key processes, such as charge transport, exciton generation, and exciton dissociation. As a first step toward disentangling the various contributions to $\ensuremath{\epsilon}$, we present in this paper the results of first-principles calculations of the vibrational mode contribution to $\ensuremath{\epsilon}$ for a large number of small-molecule organic semiconducting materials that are relevant to OLEDs. We find that this contribution is significant for molecules with polar groups and strongly infrared-active vibrational modes, but also for molecules without such groups but with very-low-frequency vibrational modes, below $\ensuremath{\sim}2$ THz ($\ensuremath{\sim}10$ meV). A comparison with available experimental data reveals good overall agreement concerning the order of magnitude of this contribution, but also indicates the need for detailed material-specific studies of the sensitivity to the thin-film structure.