Efficient Nanoscale Exciton Transport in Non-fullerene Organic Solar Cells Enables Reduced Bimolecular Recombination of Free Charges.

The highest efficiency organic photovoltaic (OPV)-based solar cells, made from blends of electron donating and accepting organic semiconductors, are often characterized by strongly reduced (non-Langevin) bimolecular recombination. Although the origins of the reduced recombination are debated, mechanisms related to the charge-transfer (CT) state and free-carrier encounter dynamics controlled by the size of donor and acceptor domains have been proposed as underlying factors. In this communication we report a novel photoluminescence-based probe to accurately quantify the donor-acceptor domain size in OPV blends. Specifically, we measure the domain size in high-efficiency non-fullerene (NFA) acceptor systems and compare them to a conventional fullerene. We find that the NFA-based blends form larger domains but that the expected reductions in bi-molecular recombination attributed to the enhanced domain sizes are too small to account for the observed reduction factors. Further, we show that the reduction of bimolecular recombination is correlated to enhanced exciton dynamics within the NFA domains. This indicates that the processes responsible for efficient exciton transport also enable strongly non-Langevin recombination in high efficiency NFA-based solar cells with low energy offsets. This article is protected by copyright. All rights reserved.