Novel Insight into Thermally Activated Absorption, Charge-Transfer and Polaron Relaxation in Molecular Semiconductors from Theory and Experiment

Charge transport in molecular materials is governed by strong electron-phonon coupling and polaronic effects. Such polaronic effects manifest in structural relaxation of molecules upon charging, which is quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor−acceptor interfaces.
By comparing simulations and experiments, we show in this presentation that the line shape of the CT absorption bands is governed by temperature-activated intramolecular vibrations.[1] This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 donor:C60 CT complexes correlate with values calculated within density functional theory. Still the residual deviations between experiment and simple models are even more interesting indicating the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

[1] K. Vandewal et al. J. Am. Chem. Soc. 139, 1699 (2017)