Prolonging Exciton Diffusion Length via Modulating Aggregation Structures for Binary Organic Photovoltaics Approaching 20% Certified Efficiency

Abstract The performance of flexible all‐polymer organic photovoltaics (OPVs) constrained by low short‐circuit current density ( J SC ) and fill factor (FF), resulting in diminished power conversion efficiency (PCE) and compromised mechanical stability. Enhancing the exciton diffusion length ( L D ) is pivotal for improving device parameters, including PCE. However, the underlying mechanisms governing exciton diffusion dynamics, influenced by the aggregation structure of conjugated polymers, remain insufficiently understood. This study employs molecular dynamics simulations to calculate the interchain free energy distribution [Δ G (r)] and strategically modulates the aggregation behavior of the polymer donor PM6 by controlling its molecular weight (MW). Medium‐MW PM6 demonstrates optimized aggregation behavior, leading to extended L D and precisely tuned fluid mechanics, which facilitate the formation of a pseudo‐planar heterojunction (PPHJ) active layer. These advancements enable PPHJ‐based all‐polymer flexible devices to achieve a PCE of 18.01%, with notable improvements in J SC and FF, and retain 90.4% of their initial efficiency after 2000 bending cycles. Encouraged by these advantages, a record‐breaking efficiency of 20.0% (certified 19.68%) was achieved for eco‐friendly, printed OPVs (PM6//L8‐BO) with small‐area devices (0.0621 cm 2 ), whereas large‐area modules (23.60 cm 2 ) reached an efficiency of 15.60%.