A novel 9H-indeno[1,2-b]pyrazine-2,3-dicarbonitrile end group for an efficient non-fullerene small molecule acceptor

For acceptor–donor–acceptor (A–D–A) type non-fullerene small molecule acceptors (NF-SMAs), the electron-deficient active methylene end groups are pivotal for adjusting their properties and photovoltaic performances. However, a limited variety of end groups is not conducive to the full development of organic solar cells (OSCs) based on NF-SMAs. Herein, we developed a novel 9H-indeno[1,2-b]pyrazine-2,3-dicarbonitrile (IPC) end group and applied it into the design and synthesis of NF-SMAs called BTOIPC. BTOIPC with potential intramolecular CN⋯S interactions showed a planar conformation and small torsion angle. The strong electron-withdrawing ability of IPC endowed BTOIPC with a low optical bandgap (Eoptg of 1.45 eV) and suitable HOMO and LUMO energy levels, similar to those of BT-IC. OSCs based on PBDB-T:BTOIPC achieved a power conversion efficiency (PCE) of 9.31% with a fill factor (FF) close to 70% and an energy loss (Eloss) of 0.57 eV, which are comparable with those of the reported BT-IC. The advantageous features of the PBDB-T:BTOIPC bulk-heterojunction layer, such as high and balanced carrier mobility, good exciton dissociation, minor charge recombination and uniform morphology, contributed together to the good performance of IPC-based SMA. These results implied that IPC is a potential end group for NF-SMAs, and the modifiable structure of IPC gives IPC-based SMAs an ability to realize higher PCE in OSCs.