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 × 10−30 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 (Voc) of 0.88 V, a short-circuit current (Jsc) of 13.15 mA cm−2, and a fill factor (FF) of 0.67, under the illumination of AM 1.5G at 100 mW cm−2. The high PCE of this material combination could be attributed to its broad absorption spectrum and the high hole mobility (μh) and electron mobility (μe) of 9.56 × 10−4 and 5.17 × 10−4 cm2 V−1 s−1, 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.