LAUSR

Benzene-based 1,1-dicyanomethylene-3-indanone (IC) derivatives have been widely used as the end-group to construct high-performance A-D-A type non-fullerene acceptors (NFAs). Extension of the end-group conjugation of NFAs is a rational strategy to facilitate intermolecular stacking of the end-groups which are responsible for efficient electron transportation. A bicyclic benzothiophene-based end-group acceptor, 2-(3-oxo-2,3-dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ylidene) malononitrile), denoted as -BC was designed and synthesized. Knoevenagel condensation of the unsymmetrical 1,3-diketo-precursor with one equivalent of malononitrile selectively reacts with the keto group attached at the-position of thiophene unit, leading to the isomerically pure benzothiophene-fused -BC. The well-defined -BC with extended conjugation was condensed with three different ladder-type diformylated donors to form three new A-D-A NFAs named as BDCPDT-BC, DTCC-BC and ITBC, respectively. The corresponding IC-based BDCPDT-IC, DTCC-IC and ITIC model compounds were also synthesized for comparison. The incorporation of electron-rich benzothiophene unit in the end-group upshifts the LUMO energy levels of the NFAs, which beneficially enlarges the Voc values. On the other hand, the benzothiophene unit in -BC not also imparts an optical transition in the shorter wavelengths around 340-400 nm for better light harvesting ability but also promotes antiparallel  stacking of the end-groups for efficient electron transport. The OPV devices using BC-based NFAs as the n-type acceptor and PBDB-T as the p-type polymer all showed the improved Voc and Jsc values. The BDCPDT-BC-, and DTCC-BC-based devices exhibited a PCE of 10.82% and 10.74%, respectively, which outperformed the corresponding BDCPDT-IC-, and DTCC-IC-based devices (9.33% and 9.25%). More importantly, the ITBC-based device delivered a highest PCE of 12.07% with a Jsc of 19.90 mA/cm2, a Voc of 0.94 V, and an FF of 64.51%, representing a 14% improvement relative to the traditional ITIC-based device (10.5%).