Quinoxaline-Containing Nonfullerene Small-Molecule Acceptors with a Linear A2-A1-D-A1-A2 Skeleton for Poly(3-hexylthiophene)-Based Organic Solar Cells.

We used the quinoxaline (Qx) unit to design and synthesize two nonfullerene small-molecule acceptors of Qx1 and Qx1b with an A2-A1-D-A1-A2 skeleton, where indacenodithiophene (IDT), Qx, and rhodanine (R) were adopted as the central donor (D), bridge acceptors (A1), and terminal acceptors (A2), respectively. Qx1 and Qx1b contain different side chains of 4-hexylphenyl and octyl in the central IDT segment to modulate the properties of final small molecules. Both small molecules show good thermal stability, high solubility, and strong and broad absorption spectra with optical band gaps of 1.74 and 1.68 eV, respectively. Qx1 and Qx1b exhibit the complementary absorption spectra with the classic poly(3-hexylthiophene) (P3HT) and the high-lying lowest unoccupied molecular orbital energy levels of -3.60 and -3.66 eV, respectively. Polymer solar cells based on P3HT:Qx1 showed a high open-circuit voltage ( Voc) of 1.00 V and a power conversion efficiency (PCE) of 4.03%, whereas P3HT:Qx1b achieved a Voc of 0.95 V and a PCE of 4.81%. These results demonstrate that the Qx unit is also an effective building block to construct promising n-type nonfullerene small molecules to realize a relatively high Voc and PCE for P3HT-based solar cells.