Coherence and Electron Transport in Pi-Stacked Acceptor-Donor-Acceptor Molecular Triads

There has been a concerted effort to design non-fullerene acceptors (NFAs) for organic photovoltaic devices, in a large part, due to the processability and stability problems of fullerenes, but concomitantly there is a desire to further understand ultrafast charge generation in donor-acceptor organic molecules. Recently, covalently bonded molecular triads have shown substantial increases in device efficiency when blended with mid-gap polymers. There has been some advances in understanding the details of electron transport with NFA molecules, but there has yet to be quantitative investigation into the relationship of molecular organization in NFA device with its transport capabilities. Here, we present a detailed study of NFA aggregates that show slipped-stacked geometries and strong binding energies between the intermolecular layers. A consequence of strongly coupled LUMO orbitals is intermolecular delocalization leading to transport coherence. We find that for some slip-stacked arrangements favor coherent transport, while others do not, and we propose design strategies to get NFA packings with strong electronic couplings.