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 (BDT-P) as an electron-donating core, flanked with the strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)- malononitrile (2FIC) unit, is designed and synthesized for polymer solar cells application. 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 cm2 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 (Eloss) as low as 0.59 eV, which is of benefit to overcome the trade-off between Jsc and Voc. Furthremore, the BP-4F-based PSCs achieve an excellent PCE of 12.3% with a device area of 1.10 cm2. Notably, the devices show an excellent storage stability and photo-stability with retaining near 90% of the initial PCE in air under dark and 93.5% in 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.