Lowing the energy loss of organic solar cells by molecular packing engineering via multiple molecular conjugation extension

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 organic solar cells (OSCs) with so far the best power conversion efficiencies (PCEs) and also largely suppressed energy losses (Eloss). 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. Herein, a highly efficient acceptor-donor-acceptor (A-D-A) type electron acceptor, CH17, has been designed and constructed, featured with a prominent π extension in both directions of the central and end units with respect to {bfY6} series. Such a multiple and much enhanced conjugation extension in CH17 enables a much more effective and compact 3D molecular packing compared with that of Y6 supported by X-ray single crystal and other analysis, mainly caused by a newly observed distinctive dual “end unit to central unit” packing mode. This much favorable molecular packing, also kept in its blends with donor materials, leads a larger electron and hole transfer integrals and hence much improved charge transport, and reduced energetic disorders in CH17 blends. More importantly, the observed upshifted charge transfer (CT) state of CH17 blends compared with that of Y6, due to its increased molecular conjugation extension in both directions, further enhances the hybridization between its CT and local exciton (LE) states, resulting in higher luminescence efficiency, much suppressed non-radiative recombination loss and smaller Eloss with respect to that of Y6. Consequently, 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 Eloss) 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 Eloss, through engineering molecular packing by designing/tuning molecule more dedicatedly.