Tailoring Molecular Orientation with a Polymer Additive Enables Bilayer Organic Solar Cells with 20.2% Efficiency

Abstract Tuning the molecular packing from the edge‐on to the preferred face‐on orientation is beneficial for improving vertical charge transport and the photovoltaic performance in organic solar cells. However, achieving precise control over this structural transition remains a significant challenge due to the complex processing conditions. Herein, a trace amount of the polymer donor PTO2 as an additive, effectively inducing a preferential face‐on molecular orientation in the acceptor phase is incorporated. This strategy enhanced exciton dissociation efficiency, improved charge carrier extraction, reduced trap density, and subsequently achieved a fill factor (FF) nearing 80%, results in a power conversion efficiency (PCE) of 20.2%, the highest reported for bilayer OSCs. Remarkably, the PTO2‐driven molecular orientation strategy maintains consistent efficacy across diverse donor–acceptor systems, highlighting its broad applicability. This approach offers a comprehensive insight for the effective modulation of NFA molecular orientation, paving a practical approach for high‐performance bilayer OSCs.