Revisiting electronic couplings and incoherent hopping models for electron transport in crystalline C-60 at ambient temperatures

We assess the validity of incoherent hopping models that have previously been used to describe electron transport in crystalline C-60 at room temperature. To this end we present new density functional theory based calculations of the electron transfer parameter defining these models. Specifically, we report electronic coupling matrix elements for several ten thousand configurations that are thermally accessible to the C-60 molecules through rotational diffusion around their lattice sites. We find that the root-mean-square fluctuations of the electronic coupling matrix element (11 meV) are almost as large as the average value (14 meV) and that the distribution is well approximated by a Gaussian. Importantly, due to the small reorganisation energy of the C-60 dimer (approximate to 0.1 eV), the ET is almost activationless for the majority of configurations. Yet, for a small but significant fraction of orientations the coupling is so strong compared to reorganisation energy that no charge-localised state exists, a situation that is aggravated if zero-point motion of the nuclei is taken into account. The present calculations indicate that standard hopping models do not provide a sound description of electron transport in C-60, which might be the case for many other organics as well, and that approaches are needed that solve the electron dynamics directly.