With the intrinsic merits of low cost, tunable transparency and color, flexibility, nontoxicity and lightweight, organic solar cells (OSCs) have been regarded as a promising photon-to-current conversion technology in the last few decades. The central unit (benzo[c][1,2,5]thiadiazole) in Y6 series of molecules plays a determining role in their unique intermolecular packing for a three-dimensionally (3D) network, largely endowing their OSCs with so far the best power conversion efficiencies (PCEs) and also largely suppressed energy losses (E_loss). Despite its vital role in molecular packing, very few explorations for central unit have been conducted due to possibly the constructing challenge of central heterocyclic units.

Recently, Yongsheng Chen’s group designed and synthesized a highly efficient acceptor-donor-acceptor (A-D-A) type electron acceptor, CH17, featured with a prominent π extension in both directions of the central and end units with respect to Y6 series. An excellent PCE of 17.84% is achieved with PM6 as the donor in a binary device compared with a PCE of 16.27% for the controlled Y6 device. Furthermore, a further improved PCE of 18.13% is achieved by CH17-based ternary single-junction OSCs along with a markedly reduced Eloss of 0.49 eV and larger open-circuit voltage (Voc) of 0.89 V, compared with that (16.27% of PCE, 0.85 V of Voc, and 0.53 eV of E_loss) of the control device using Y6. This significantly improved photovoltaic performance caused by molecular multiple conjugation extension, especially through the largely unexplored central unit, indicates that there is still much room to further enhance OSC performance by addressing the most important issue for OSC, i.e, the smaller Voc caused by larger E_loss, through engineering molecular packing by designing/tuning molecule more dedicatedly. Relevant achievements were published in Sci. China Chem., 2022, DOI: 10.1007/s11426-022-1393-1.

Molecular structures and photophysical properties.