Interfacial Potential Compensation for HOMO Alignment in Ternary Organic Solar Cells.

Conventional energetic design rules for ternary organic solar cells (OSCs) often overlook interfacial potential shifts, leading to an incomplete understanding of energy-level alignment (ELA) based on the idealized vacuum-level assumption. In this work, we reveal that these hidden potential steps, together with the formation of an alloy donor morphology, are the key to unlocking high-performance ternary blends. By utilizing the monolayer-by-monolayer Langmuir-Schaefer method combined with photoelectron spectroscopy, we directly characterized the ELA at each donor and acceptor interface within the ternary blends. We show that the two donors, possessing complementary absorption profiles, exhibit synchronized contact-induced potential steps at their respective interfaces with the acceptor. This potential compensation precisely offsets the intrinsic HOMO offsets measured in neat films, resulting in a fully aligned and unified HOMO-level landscape across the ternary blend. This configuration provides the physical solution to reconcile efficient charge generation with a maximized photovoltaic gap. Our findings highlight that integrating morphological alloying with synchronized potential compensation provides a robust framework for understanding and rationally engineering the complex electronic landscapes of multi-component OSCs.