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-1H-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 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 (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 cm². 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.