A non-fullerene acceptor based on alkylphenyl substituted benzodithiophene for high efficiency polymer solar cells with a small voltage loss and excellent stability

In this work, a new non-fullerene small molecule acceptor (NF-SMA) named BP-4F , based on benzo[1,2- b :4,5- b ′]di(cyclopenta[2,1- b :3,4- b ′]dithiophene) with 4-(2-ethylhexyl)phenyl conjugated side chains (BDTP) as an electron-donating core, flanked with a strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1 H -inden-1-ylidene)-malononitrile (2FIC) unit, is designed and synthesized for polymer solar cell applications. BP-4F exhibits strong absorption in the 550 to 830 nm region with a narrow optical band gap of 1.49 eV, suitable energy levels with a lowest unoccupied molecular orbital (LUMO) of −3.90 eV and an effective electron mobility of 2.10 × 10 −4 cm 2 V −1 s −1 . When blended with the wide bandgap polymer PM6 as the active layer, the polymer solar cells (PSCs) achieve an average power conversion efficiency (PCE) of 13.9% with an energy loss ( E loss ) as low as 0.59 eV, which is of benefit to overcome the trade-off between J sc and V oc . Furthermore, the BP-4F -based PSCs achieve an excellent PCE of 12.3% with a device area of 1.10 cm 2 . Notably, the devices show an excellent storage stability and photo-stability by retaining nearly 90% of the initial PCE in air under dark conditions and 93.5% in a glovebox under continuous illumination for 720 hours, respectively. These results indicate that BP-4F is an effective electron acceptor for high efficiency and stable polymer solar cells. A new small molecule acceptor named BP-4F is developed. The optimal PSC based PM6:BP-4F achieves an excellent average PCE of 13.9% with an E loss of 0.59 eV and outstanding stability.