Dispersion effect of chlorination-induced intermolecular stacking optimization of small-molecule acceptors for high-performance organic solar cells

Introduction of a chlorine substituent facilitates improvement in intermolecular interactions, but the intrinsic mechanism is unexplored. We demonstrated the significance of a polarizable dispersion effect of a chlorine substituent in optimizing the intermolecular π–π stacking of a small-molecule acceptor. Chlorination conducted on a small molecule could enhance the polarizability along the direction parallel to the conjugated backbone, favoring dispersion-dominated molecular face-to-face packing. As a result, the chlorinated derivatives QX-Cl and QX-2Cl demonstrated more than two-times higher electron mobility compared with that of the chlorine-free analogue QX. Conversely, monochlorination conducted on the QX molecule improved the J-aggregation characteristic, resulting in the highest extinction coefficient among the three analogues. Therefore, the D18:QX-Cl-based device yielded a power-conversion efficiency (PCE) of 19.53%, with an exceptional fill factor (FF) of 82.88%, which was higher than that of the device based on D18:QX (PCE = 18.28%, FF = 79.91%). Using QX-Cl as a guest acceptor for the D18:N3 system to optimize molecular packing and phase separation morphology, the ternary device demonstrated a notable PCE of 20.41%, which was a significant improvement with regard to the PCE of 19.12% for the 18:N3-based binary device.