Impact of Phonons on Charge and Heat Transport in Organic Semiconductors

Charge transport in molecular crystals is reduced by the dynamic disorder induced by thermal fluctuations. This implies that the key parameters controlling charge transport at the molecular level (in particular the electronic couplings also referred to as transfer integrals) strongly fluctuate over time under the influence of intermolecular vibrations (phonons). Such thermal effects can be grasped from molecular dynamics simulations by extracting snapshots along the trajectory and by tracking the time evolution of relevant parameters. Nevertheless, such simulations do not tell us about the nature of the phonons contributing the most to the fluctuations of the transfer integrals, which would be of prime interest to design new strategies to freeze such detrimental degrees of freedom. In this context, I will first describe in this talk our recent efforts in this direction aiming at assessing the individual role of phonons as well as the possible impact of phonon dispersion. Interestingly, the same phonons are involved in heat transport so that their impact on the thermal conductivity can be also inferred. This is of high relevance to the field of organic thermoelectrics requiring strategies to minimize the thermal conductivity. The second part of the talk will describe some recent simulations of heat transport in molecular crystals, pointing to strong anharmonic effects for some structures leading to strongly localized phonons.