Phosphate Ester‐Modified Acceptor Additives Enable Concurrent Vertical Morphology and Interfacial Engineering for Organic Solar Cells Approaching 21% Efficiency

Organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) have progressed rapidly, yet further gains are constrained by coupled challenges in vertical morphology control and energy alignment at the acceptor-cathode interface. Here, a molecular engineering strategy is presented that installs strongly polar phosphate ester groups onto the inner alkyl chains of the benchmark NFA L8-BO, yielding two derivatives-1POE and 2POE. Employed as non-volatile solid additives during layer-by-layer processing, these molecules induce vertical composition redistribution to form a graded donor-acceptor-additive architecture. The resulting vertical profiling strengthens intermolecular interactions, raises surface energy, and drives additive accumulation near the top interface, thereby improving interfacial energetics and facilitating electron extraction. Consequently, devices incorporating 2 wt.% 1POE or 2POE deliver power conversion efficiencies (PCEs) of 19.87% and 19.28%, respectively, versus 18.83% for controls, alongside enhanced operational stability. The strategy shows strong universality across multiple blends, achieving a PCE of 20.90% in a D18/L8-BO:BTP-eC9FCl ternary system. These results demonstrate that precise phosphate ester-based additive design enables concurrent optimization of vertical phase distribution and interfacial energetics, offering a practical route to high-efficiency, stable OSCs.