Intrinsic Interfacial Energy Alignment Enables ETL‐Free Organic Solar Cells

ABSTRACT

Organic solar cells (OSCs) commonly rely on low‐work‐function electron transport layers (ETLs) to optimize energetic alignment at the active layer/cathode interface. However, most ETLs exhibit strong thickness sensitivity and poor environmental stability, which increases fabrication complexity and compromises long‐term stability. In this work, we reveal that molecular orientation can strongly modulate the interfacial energetics of conjugated small‐molecule donors and thereby enable efficient ETL‐free operation. Using the small‐molecule donor BTR‐Cl, we identify a pronounced orientation‐dependent shift of the integer charge transfer level that results in Fermi‐level pinning at the active layer/cathode interface. This intrinsic energetic alignment allows ohmic contact formation with Ag electrodes without the need for an ETL. Leveraging this mechanism, we incorporate BTR‐Cl into the benchmark polymer donor: small‐molecule acceptor (PM6:Y6) system to form a ternary blend and fabricate the best‐performing ETL‐free OSCs with an efficiency of 18.6% and significantly improved operational stability. These results clarify the interfacial physics governing ETL‐free OSCs and provide a general strategy for simplifying device architecture while maintaining high efficiency and stability, paving the way toward the commercialization of OSC technology.