The central core size effect in quinoxaline-based non-fullerene acceptors for high VOC organic solar cells

For organic solar cells (OSCs), obtaining a high open circuit voltage (V-OC) is often accompanied by the sacrifice of the circuit current density (J(SC)) and filling factor (FF), and it is difficult to strike a balance between V-OC and J(SC) x FF. The trade-off of these parameters is often the critical factor limiting the improvement of the power conversion efficiency (PCE). Extended backbone conjugation and side chain engineering of non-fullerene acceptors (NFAs) are effective strategies to optimize the performance of OSCs. Herein, based on the quinoxaline central core and branched alkyl chains at the beta position of the thiophene unit, we designed and synthesized three NFAs with different sized cores. Interestingly, Qx-BO-3 with a smaller central core showed better planarity and more appropriate crystallinity. As a result, PM6:Qx-BO-3-based devices obtained more suitable phase separation, more efficient exciton dissociation, and charge transport properties. Therefore, the OSCs based on PM6:Qx-BO-3 yielded an outstanding PCE of 17.03%, significantly higher than the devices based on PM6:Qx-BO-1 (10.57%) and PM6:Qx-BO-2 (11.34%) although the latter two devices have lower V-OC losses. These results indicated that fine-tuning the central core size can effectively optimize the molecular geometry of NFAs and the film morphology of OSCs. This work provides an effective method for designing high-performance NFA-OSCs with high V(OC)s.

Automated summary

For organic solar cells, balancing open circuit voltage and circuit current density is challenging. This study focuses on optimizing the performance of organic solar cells by extending backbone conjugation and engineering side chains of non-fullerene acceptors. The researchers designed and synthesized non-fullerene acceptors with different sized cores and found that a smaller central core resulted in improved planarity and crystallinity, leading to higher power conversion efficiency. This work provides an effective method for designing high-performance non-fullerene acceptor-based solar cells.