Fluorene-fused ladder-type non-fullerene small molecule acceptors for high-performance polymer solar cells

In this work, we designed and synthesized two new small molecule acceptors (SMAs), namely FTTCN and FTTCN-M, structured with a coplanar central core based on a fluorene-fused ladder-type donor and electron-withdrawing end-capping groups (CPTCN and CPTCN-M). Compared to FTTCN-M, FTTCN exhibits slightly redshifted absorption, higher electron mobility and a more favorable morphology of the blend film. FTTCN-based polymer solar cells (PSCs) paired with p-type polymer PBDB-T achieved a power conversion efficiency (PCE) of 10.56%, with an open-circuit voltage (V-OC) of 0.90 V, a short-circuit current density (J(SC)) of 15.89 mA cm(-2) and a fill factor (FF) of 73.82%, which is slightly higher than that of FTTCN-M-based devices (a PCE of 10.08% alone with a V-OC of 0.93 V, a J(SC) of 15.19 mA cm(-2) and a FF of 71.37%). To the best of our knowledge, the FF of 73.82% is the highest value for fluorene-based PSCs reported to date. The higher J(SC) and FF of the FTTCN-based device could be attributed to the lower charge recombination, more balanced carrier mobility, and more favorable surface morphology of the PBDB-T:FTTCN blend film than those of the PBDB-T:FTTCN-M blend film. This work revealed that a fluorene-fused ladder-type donor exhibits great potential in constructing high-performance non-fullerene SMAs and optimizing end-capping groups.