Exciton Self-Splitting: One More Reason for Poor Photovoltaic Performance of Non-Fullerene Acceptors

Novel A-D-A (acceptor–donor–acceptor)-type molecules were synthesized and tested in organic photovoltaics (OPV) devices. For a pristine film of compound 1b with a 2,2′-(naphtho[2,3-b]thiophene-4,9-diylidene)dipropanedinitrile A unit and carbazole-based donor D unit, efficient exciton splitting by intermolecular electron transfer was proved. The observation of the out-of-phase electron spin echo in the pristine 1b film unambiguously testifies to a high yield of charge-transfer state formation. Despite this, the yield of free charge generation in pristine 1b is low due to the fast geminate and non-geminate recombination. This process is detrimental for OPV performance when the compound capable of exciton self-splitting is used as an acceptor component of the bulk heterojunction (BHJ) active layer because of the fast charge recombination within this component. Exciton self-splitting can be of general significance for push–pull OPV acceptors or donors in bulk heterojunctions, although it can be masked by other photophysical processes in the BHJ active layer. This is the reason why molecules with a strong intermolecular charge-transfer band are not suitable components of the active layer of efficient OPV devices.