Fine-tuning energy levels and morphology via fluorination and thermal annealing enables high efficiency non-fullerene acceptor organic solar cells

Non-fullerene acceptors have been utilized to construct efficient organic solar cells (OSCs). In this work, two new molecular acceptors, TPQx-4F and TPQx-6F, with quinoxaline (Qx)-fused ring, were designed and synthesized. The single-junction inverted OSCs device of two NFAs, blended with PM6 as the donor material, were systemically investigated. The introduction of fluorine atoms into Qx-containing fused central core not only downshifted energy levels, but also greatly optimized the morphology of PM6:TPQx-6F blend films for the first time. The power conversion efficiency (PCE) of the PM6:TPQx-4F based devices improved from 6.02% to 7.75% through thermal annealing (TA) treatment. Meanwhile, PM6:TPQx-6F based devices showed a PCE of 12.28%, in which a remarkable PCE of 14.62% was received after TA treatment optimization. The differences performance of devices based PM6:TPQx-4F and PM6:TPQx-6F were mainly attributed to the variation of short-circuit current density (Jsc) and fill factor (FF). Further investigations revealed that PM6:TPQx-6F blend films presented higher electron mobilities, more efficient charge dissociation and uniform nanophase separation than those of TPQx-4F-based devices, giving rise to higher Jsc and FF. In addition, the enhanced Jsc and FF were simultaneously realized by simple TA treatment for TPQx-4F- and TPQx-6F-based devices. This work provides more insight into the underlying reason of fine-tuning energy levels and optimizing active layer morphology on high-performance NFAs based OSCs.