Comprehensive Modelling Study of Singlet Exciton Diffusion in Donor-Acceptor Dyads: When Small Changes in Chemical Structure Matter

We compare two small π-conjugated Donor-Bridge-Acceptor organic molecules differing mostly by the number of thiophene rings in their bridging motifs (1 ring in 1; 2 rings in 2) and for which significantly different photovoltaic efficiencies have been reported in fullerene-based bulk heterojunction organic solar cells. The focus here is on the use of fully atomistic modelling approaches to assess the origin for the reported differences in singlet exciton diffusion, held responsible for the improved photovoltaic response of 1 compared to 2. By combining Force Field Molecular Dynamics and Micro Electrostatic schemes to Time Dependent Density Functional Theory and Kinetic Monte Carlo simulations, we dissect the nature of the lowest electronic excitations in amorphous thin films of these molecules and model the transport of singlet excitons across their broadly disordered energy landscapes. In addition to a longer excited-state lifetime associated with a more pronounced intramolecular charge-transfer character, our calculations reveal that singlet excitons in 1 are capable to funnel through long-distance hopping pathways, presumably as a result of the less anisotropic shape of the molecule favouring long-range 3D transport.