A small molecule acceptor with a heptacyclic benzodi(thienocyclopentafuran) central unit achieving 13.4% efficiency in polymer solar cells with low energy loss

Abstract

Optimizing the molecular backbone is one of the most important strategies for improving the photovoltaic performance of acceptor–donor–acceptor type small molecule acceptor (SMA) materials. Herein, two new SMA materials, namely BFHIC-4F and BFHIC, were designed and synthesized based on a new donor unit of heptacyclic benzodi(thienocyclopentafuran) (BFH) with benzodifuran (BDF) as a core and alkyl-thiophene as conjugated side chains. The materials show a broad absorption, deep highest occupied molecular orbital energy level, and good molecular arrangement compared to the analogues with benzodithiophene (BDT) as a core. BFHIC-4F exhibits a stronger crystallinity, ordered intermolecular π–π stacking, and enhanced photovoltaic performance than BFHIC. The polymer solar cells (PSCs) based on PM6:BFHIC-4F show an impressive power conversion efficiency (PCE) of 13.4%, with a high V_oc of 0.87 V, high J_sc of 22.8 mA cm⁻², and FF of 67.7% due to the influence of fluorine atoms. Notably, the PM6:BFHIC-4F-based and PM6:BFHIC-based devices display a small E_loss of 0.58 eV and 0.52 eV, respectively, indicating a more effective exciton dissociation, charge collection efficiency, and photo-induced charge transfer between PM6 and BFH-based acceptor materials. These results demonstrate that the BFH central unit is a promising building block in the SMA materials, and the device based on the PM6:BFHIC-4F blend film is a potential candidate for high-performance PSCs in the future.