Modulating Molecular Electrostatic Potential Unlocks Efficient Exciton Dissociation in A-D-A-type Narrow Bandgap Acceptors-based Organic Solar Cells

Abstract

Multithiophene-based fused-ring electron acceptors (MFREAs) have demonstrated remarkable performance not only in organic solar cells, but also in other optoelectronic devices owing to their extended near-infrared absorption. However, an insufficient electrostatic potential (ESP) difference with the donor results in a weak intermolecular electric field (IEF), which leads to low exciton dissociation efficiency. In addition, the narrow bandgap causes substantial energy loss (E_loss), thereby restricting the open-circuit voltage. To address these problems, we developed two novel MFREAs, 6TFIC-C11-4F and 6TFIC-C11-4Cl, which were developed from the typical molecule 6TIC-4F via central-core side-chain fluorination and terminal chlorination. Although core fluorination slightly attenuated the intramolecular charge transfer (ICT) effect, this strategy significantly increased the overall average ESP value, strengthening the donor-acceptor IEF, thereby facilitating exciton dissociation. Terminal chlorination enhanced the ICT effect, resulting in a red-shifted absorption spectrum and stronger IEF. Notably, 6TFIC-C11-4F exhibited increased intermolecular interactions and reduced π-π stacking distances, leading to better charge transport. As a result, the PM6:6TFIC-C11-4F device achieved a decent power conversion efficiency of 13.32%, significantly outperforming the PM6:6TIC-4F device (10.52%). In addition, the PM6:6TF-IC-C11-4Cl device demonstrated reduced E_loss and a higher short-circuit current density, validating the potential of central-core side-chain fluorination and terminal chlorination in designing high-performance MFREAs.