Tetradentate diphenanthroline-based ETMs for highly efficient and stable tandem blue OLEDs

Tandem organic light-emitting diodes (OLEDs) offer high efficiency and extended operational lifetimes, making them strong candidates for next-generation displays. However, their performance is often limited by inefficient electron injections and interfacial degradation arising from alkali-metal n-doped charge generation layers (nCGLs). Despite their widespread use, the instability mechanisms associated with nCGLs remain underexplored. To address this, we developed two diphenanthroline (DPhen) based electron transport materials (ETMs), PA-pPDPhen and PA-mPDPhen, incorporating rigid phenanthrene backbones and nitrogen-rich tetradentate coordination sites. These ETMs exhibit strong binding affinity toward alkali metals, enhancing dopant retention, lowering injection barriers, and suppressing metal ion diffusion. As a result, 2-Tandem blue OLEDs employing these materials achieve external quantum efficiencies (EQE_maxs) over 20.8 %. Notably, the dual role of PA-pPDPhen as CGL and ETM delivers an extended operational lifetime (LT₉₅ of 280 h at the same luminescence), outperforming the benchmark BPhen (165 h). These results underscore the value of rational ligand design for achieving stable, high-performance ETMs in tandem OLEDs.