Isomeric effect of fluorene-based fused-ring electron acceptors to achieve high-efficiency organic solar cells

Acceptor-donor-acceptor (A-D-A) non-fullerene electron acceptors (NFEAs) using ladder-type donor structures have become the dominant n-type materials for achieving high-efficiency OSCs. In this work, two isomeric fluorene-based ladder-type structures FCTT (TT-C-F-C-TT) and FTCT (T-C-TFT-C-T) have been designed and synthesized. These two isomeric donors with the different fused-ring arrangement, molecular geometry, and side-chain placement were end-capped with the FIC acceptors to form two NFEAs FCTT-FIC and FTCT-FIC isomeric materials. Compared to FTCT-FIC using the thiophene (T)-terminal donor, FCTT-FIC with the thienothiophene (TT)-terminal donor has more evenly distributed side chains on both sides of the backbone and less steric hindrance near the FIC acceptors, which enables stronger antiparallel π-π packing among the end-groups to create a channel for efficient electron transport, as evidenced by the thin-film GIWAXS measurements. FCTT-FIC displayed a larger optical bandgap and deeper-lying energy levels than its FTCT-FIC isomer. Compared to the PBDB-T:FTCT-FIC device, the PBDB-T:FCTT-FIC device showed a higher PCE of 10.32% with an enhanced J_sc of 19.63 mA cm^-2 and an FF of 69.14%. A PM6:FCTT-FIC device using PM6 as a p-type polymer achieved the highest PCE of 12.23%. By introducing PC₇₁BM as the second acceptor to enhance the absorption at shorter wavelengths, optimize the morphology and facilitate electron transport, the ternary-blend PM6:FCTT-FIC:PC₇₁BM (1 : 1 : 0.5 in wt%) device yielded the highest PCE of 13.37% with a V_oc of 0.92 V, a higher J_sc of 19.86 mA cm^-2, and an FF of 73.2%. This result demonstrated that the TT-terminal ladder-type donor is generally a better molecular design than the corresponding T-terminal ladder-type isomer for the development of new A-D-A NFEAs.