Strontium Charge‐Generation Assistant Layer for High‐Performance Tandem Organic Light Emitting Diodes

Deep‐LUMO charge‐generation layers (CGLs) have emerged as a robust alternative to doped CGLs for tandem organic light‐emitting diodes (OLEDs), yet their performance remains fundamentally limited by large electron‐injection barriers at the CGL interface. Here, we introduce an ultrathin strontium (Sr) interlayer as a charge‐generation assistant layer (CAL) that simultaneously provides high optical transparency, smooth surface morphology, and effective modulation of interfacial energetics. Approximately 5‐nm Sr films form continuous, highly transparent layers while generating interfacial dipoles and promoting trap‐assisted tunneling (TAT) at the Bphen:Liq/HAT‐CN interface. These combined effects reduce the effective electron‐injection barrier from ∼1.2 to ∼0.3 eV. As a result, the tandem OLED exhibits ideal series turn‐on behavior, balanced exciton recombination, and more than double the luminous efficiency of its single‐unit counterpart, while maintaining comparable power efficiency. In addition, Sr integration narrows the electroluminescence spectrum through an enhanced microcavity effect, thereby improving color purity. Overall, this study identifies Sr as a new and highly effective CAL material that overcomes long‐standing interfacial limitations in dopant‐free tandem OLEDs, providing a practical pathway toward highly efficient and color‐pure next‐generation displays.