Ladder-Type Nonacyclic Arene Bis(thieno[3,2-b]thieno)cyclopentafluorene as a Promising Building Block for Non-Fullerene Acceptors

The ladder-type nonacyclic arene (bis(thieno[3,2-b]thieno)cyclopentafluorene (BTTF)) has been designed and synthesized through fusing thienothiophenes with the fluorene core from the synthon of dimethyl 9,9-dioctyl-2,7-bis(thieno[3,2-b]thiophen-2-yl)fluorene-3,6-dicarboxylate. With BTTF as the central donor unit, a novel acceptor-donor-acceptor (A-D-A) type non-fullerene small-molecule acceptor (BTTFIC) was prepared with 1,1-dicyanomethylene-3-indanones (IC) as the peripheral acceptor units. The energy level of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of BTTFIC locate at -5.56 and -3.95eV, respectively, presenting a low optical band gap of 1.58eV. Encouragingly, polymer solar cells based on the blends of BTTFIC with both the representative wide- and low-bandgap polymer donors (PBDB-T, 1.82eV. PTB7-Th, 1.58eV) offer power conversion efficiencies over 8% (8.78 +/- 0.18% for PBDB-T:BTTFIC and 8.18 +/- 0.29% for PTB7-Th:BTTFIC). These results highlight the advantage of ladder-type BTTF on the preparation of nonfullerene acceptors with extended conjugated backbones.