Barrier-Free Charge Separation Enabled by Electronic Polarization in High-Efficiency Non-Fullerene Organic Solar Cells.

The separation of charge-transfer (CT) states into free charges at the donor/acceptor (D/A) interfaces plays a central role for organic solar cells (OSCs). Because of strong Coulomb attraction, the separation mechanisms are elusive, particularly for the high-efficiency non-fullerene (NF) OSCs with low exciton-dissociation driving forces. Here, we demonstrate that the Coulomb barriers can be substantially overcome by electronic polarization for OSCs based on a series of A-D-A acceptors (ITIC, IT-4F, and Y6). In contrast to fullerene-based D/A heterojunctions, the polarization energies for both donor holes and acceptor electrons are remarkably increased from the interfaces to pure regions in the NF heterojunctions owing to strong stabilization on electrons but destabilization on holes by electrostatic interactions in the A-D-A acceptors. Especially, upon incorporating fluorine substituents and electron-poor cores to ITIC, the increased polarization energies can completely compensate the Coulomb attraction in the IT-4F and Y6 based Heterojunctions, leading to barrierless charge separation.