Asymmetrified Benzothiadiazole‐Based Solid Additives Enable All‐Polymer Solar Cells with Efficiency Over 19%

Disordered polymer chain entanglements within all‐polymer blends limit the formation of optimal donor‐acceptor phase separation. Therefore, developing effective methods to regulate morphology evolution is crucial for achieving optimal morphological features in all‐polymer organic solar cells (APSCs). In this study, two isomers, 4,5‐difluorobenzo‐c‐1,2,5‐thiadiazole (SF‐1) and 5,6‐difluorobenzo‐c‐1,2,5‐thiadiazole (SF‐2), were designed as solid additives based on the widely‐used electron‐deficient benzothiadiazole unit in nonfullerene acceptors. The incorporation of SF‐1 or SF‐2 into PM6:PY‐DT blend induces stronger molecular packing via molecular interaction, leading to the formation of continuous interpenetrated networks with suitable phase‐separation and vertical distribution. Furthermore, after treatment with SF‐1 and SF‐2, the exciton diffusion lengths for PY‐DT films are extended to over 40 nm, favoring exciton diffusion and charge transport. The asymmetrical SF‐2, characterized by an enhanced dipole moment, increases the power conversion efficiency (PCE) of PM6:PY‐DT‐based device to 18.83% due to stronger electrostatic interactions. Moreover, a ternary device strategy boosts the PCE of SF‐2‐treated APSC to over 19%. This work not only demonstrates one of the best performances of APSCs but also offers an effective approach to manipulate the morphology of all‐polymer blends using rational‐designed solid additives.