Effects of Dispersion Forces on Structure and Photoinduced Charge Separation in Organic Photovoltaics

We present a theoretical study on the role of van der Waals (vdW) interactions on the structure and, as a consequence, the photoinduced charge separation (CS) of a series of dimer complexes formed by the polymer P3HT and the fullerene derivative PCBM. CS rate constants for P3HT/PCBM dimer structures in which vdW interactions are taken into account agree well with experimental data. Without proper treatment of vdW interactions during geometry optimizations, the predicted CS rates can be too low by up to 3 orders of magnitude. These variations in computed CS rates are not due to changes in the Gibbs energy for CS. Instead, the electronic coupling increases by up to 2 orders of magnitude for structures obtained with dispersion-corrected density functionals that lead to deformations in the P3HT oligomer with pronounced π-π stacking interactions with PCBM.