A small molecular electron acceptor based on asymmetric hexacyclic core of thieno[1,2-b]indaceno[5,6-b']thienothiophene for efficient fullerene-free polymer solar cells.

A novel A-D-A (acceptor-donor-acceptor) type non-fullerene small molecule, A201, consisting of an asymmetric thieno[1,2-b]indaceno[5,6-b']thienothiophene (TITT) unit as middle D part and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) groups as end-capped A parts was designed and synthesized. The asymmetric TITT building block showed a higher dipole moment of 0.85 Debye (1 Debye = 3.33564 × 10 cm) compared with the symmetric analogues of indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) of 0.098 and 0.13 Debye, respectively. The solution-processed bulk heterojunction solar cells using a benzotriazole (BTA)-based polymer of J71 as donor and A201 as acceptor, showed a power conversion efficiency (PCE) of 9.36% with an open-circuit voltage (V) of 0.88 V, a short-circuit current (J) of 13.15 mA cm, and a fill factor (FF) of 0.67, under the illumination of AM 1.5G at 100 mW cm. The high PCE of this material combination could be attributed to its broad absorption spectrum and the high hole mobility (μ) and electron mobility (μ) of 9.56 × 10 and 5.17 × 10 cm V s, respectively. These results indicate that the asymmetric electron-donating segments are promising to construct A-D-A type small molecular acceptors, which could largely enhance the diversity of building blocks to design photovoltaic materials.