Small reorganization energy acceptors enable low energy losses in non-fullerene organic solar cells

Minimizing the energy loss is of critical importance in the pursuit of attaining high-performance organic solar cells (OSCs). Interestingly, electron-vibration coupling (namely reorganization energy) plays a crucial role in the photo-electric conversion processes. However, a molecular understanding of the relationship between the reorganization energy and the energy loss has rarely been studied. Here, two new acceptors Qx-1 and Qx-2 with quinoxaline (Qx)-containing fused core were designed and synthesized. The results indicate that the reorganization energies of these two acceptors during the photoelectric conversion processes are substantially smaller than the conventional Y6 acceptor, which is beneficial for improving the exciton lifetime and diffusion length, promoting charge transport and reducing the energy loss originating from exciton dissociation and non-radiative recombination. As a result, an outstanding power conversion efficiency (PCE) of 18.2% with high Voc above 0.93 V in the PM6:Qx-2 blend, accompanying a significantly reduced energy loss of 0.48 eV. To the best of our knowledge, the obtained energy loss is the smallest for the binary OSCs with PCEs over 16% reported to date. This work underlines the importance of the reorganization energy in achieving small energy loss in organic active materials and paves a new way to obtain high-performance OSCs.