Charge Transfer Complex and Gap States in Yb‐Doped 4,5‐diaza‐9,9‐Spirofluorene: A Key to Efficient n‐Type Charge Generation for Tandem OLEDs

Abstract Organic light‐emitting diodes (OLEDs) offer advantages such as high contrast, vivid colors, and flexible form factors, but their use in demanding IT and automotive displays is limited by short operational lifetimes and low luminous efficiency. Tandem OLEDs, which stack multiple electroluminescent units connected via charge generation layers (CGLs), provide a promising solution. However, the development of efficient and stable n‐type CGLs (nCGLs) remains challenging, particularly due to issues with metal‐doped organic semiconductors. In this study, an effective nCGL system using Ytterbium (Yb)‐doped 4,5‐diaza‐9,9‐spirofluorene (Dasf), a thermally stable host is presented. Optimization identifies 10 wt.% Yb (1:1 molar ratio) as the ideal doping level for peak performance. Photoelectron spectroscopy and density functional theory confirm strong charge‐transfer complex formation between Yb and Dasf's nitrogen atoms. This interaction creates electronic states near the Fermi level through hybridization of Yb 6s orbitals with Dasf's LUMO, enhancing electron injection and transport. OLEDs incorporating this optimized nCGL show a 1.6‐fold improvement in external quantum efficiency and a 1.7‐fold increase in operational lifetime over single‐unit devices. This study highlights Yb‐doped Dasf as a promising nCGL material for high‐performance tandem OLEDs.