Modulation of crystallization kinetics using a guest acceptor for high-performance organic solar cells with 19.8% efficiency

Abstract

Controlling the aggregation and crystallization kinetics of donors and acceptors is crucial for forming the desired morphology of the active layer, which heavily determines the power conversion efficiency (PCE) of organic solar cells (OSCs). In particular, modulating the crystallization kinetics of ternary blend films, which are more complex than classic binary systems, is a great challenge. Here, we designed and synthesized two guest acceptors, Qx-8Cl and Qx-5Cl, based on a quinoxaline core with symmetric and asymmetric terminal groups, respectively. The crystallization kinetics of the ternary films, by blending Qx-8Cl and Qx-5Cl into the host system of PM6:BTP-eC9, were monitored using in situ optical spectroscopy. Compared with the symmetric Qx-8Cl, the asymmetric Qx-5Cl effectively balanced the aggregation of the donor and the acceptor, subtly slowing down their intermixing speed and avoiding the limitations imposed by the preformed polymeric donor aggregation framework. Hence, a more desirable intermixed domain and favorable vertical phase separation in the ternary film were obtained, creating an ideal morphology for the active layer. As a result, the ternary OSCs based on PM6:BTP-eC9:Qx-5Cl exhibited an outstanding PCE of 19.83% (one of the highest PCEs), while that of the PM6:BTP-eC9:Qx-8Cl-based device was only 18.81%. Moreover, a 17.3 cm² OSC module based on PM6:BTP-eC9:Qx-5Cl has been fabricated with a remarkable PCE of 14.1%.