Improving Molecular Arrangement and Alleviating Nonradiative Energy Loss Using a Chlorinated Pyrido[3,4-b]Quinoxaline-Core-Based Acceptor for High-Performance Organic Solar Cells

The electron-deficient A1 unit in A-DA1D-A structured acceptors is critical for optimizing the efficiency of organic solar cells (OSCs). Drawing inspiration from the high performance of the previously reported pyrido[2,3-b]quinoxaline-core acceptors, Py6, an isomer of Py1 is designed with a repositioned pyridine nitrogen atom, and further modified it by chlorinating Py6 to create Py7. Theoretical calculations show that chlorine incorporation strengthens intermolecular non-covalent interactions and promotes the tighter molecular stacking, as confirmed by grazing-incidence wide-angle X-ray scattering. Consequently, D18/Py7 device delivers the enhanced fill factor and short-circuit current density, compared to D18/Py1 and D18/Py6 device. Notably, D18/Py7 device also yields a higher open-circuit voltage of 0.871 V, significantly outperforming Py1 (0.764 V) and Py6 (0.723 V), due to the low nonradiative energy losses. Further studies reveal that introducing Cl directs hole density toward the central pyrido[3,4-b]quinoxaline unit and decreases the charge transfer state ratio of D18/acceptor. This prompts triplet-to-singlet conversion and reduces non-radiative recombination losses. Additionally, using a mutual donor-acceptor dilution strategy, the (D18:1wt.% Py7)/(Py7:1wt.% D18) device achieves an impressive efficiency of 19.60%. This work emphasizes the great potential of the Py-series acceptors and demonstrates that chlorine incorporation effectively reduces non-radiative losses.