Isomerically Pure Benzothiophene-Incorporated Acceptor: Achieving Improved V_oc and J_sc of Nonfullerene Organic Solar Cells via End Group Manipulation

Benzene-based 1,1-dicyanomethylene-3-indanone (IC) derivatives have been widely utilized as the end-group to construct acceptor–donor–acceptor type nonfullerene acceptors (A–D–A type NFAs). The extension of the end-group conjugation of nonfullerene acceptors (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 α-BC was designed and synthesized. The Knoevenagel condensation of the unsymmetrical 1,3-diketo-precursor with one equivalent of malononitrile selectively reacts with the keto group attached at the α-position of the thiophene unit, leading to the isomerically pure benzothiophene-fused α-BC. The well-defined α-BC with extended conjugation was condensed with three different ladder-type diformylated donors to form three new A–D–A NFAs named 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 the electron-rich benzothiophene unit in the end-group upshifts the lowest unoccupied molecular orbital energy levels of the NFAs, which beneficially enlarges the Voc values. On the other hand, the benzothiophene unit in α-BC not also imparts an optical transition in the shorter wavelengths around 340–400 nm for a better light harvesting ability but also promotes the antiparallel π–π stacking of the end-groups for efficient electron transport. The organic photovoltaic cell devices using a PBDB-T polymer and BC-based NFAs all showed the improved Voc and Jsc values. The BDCPDT-BC- and DTCC-BC-based devices exhibited a power conversion efficiency (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 the highest PCE of 12.07% with a Jsc of 19.90 mA/cm2, a Voc of 0.94 V, and an fill factor of 64.51%, representing a 14% improvement relative to the traditional ITIC-based device (10.05%).