Toward High-Performance Pure-Green Tandem Organic Light-Emitting Diodes by Employing Barrier-Free Strategies in the Charge Generation Layer.

Charge generation layers (CGLs) play crucial roles in determining the electroluminescence (EL) performance of tandem organic light-emitting diodes (OLEDs). However, acquiring negligible voltage drops across the CGL unit and high-efficiency multiplications remains challenging. Here, we propose barrier-free strategies to compose a high-performance p-i-n type CGL intermediate by introducing a Yb/HI-9 modification at the heterojunction and a novel n-dopant, Yb:1,3-bis(9-phenyl-1,10-phenanthrolin-2-yl)benzene (mdPPhen), as the n-CGL. Mechanism analysis confirms that the Yb/HI-9 modification efficiently eliminates the intrinsic energy mismatch at the n-CGL/p-CGL interface, while the lowest unoccupied molecular orbital (LUMO) of the n-dopant can be aligned to the electron transport layer of adjacent EL units, creating improved electron transport. Consequently, the resulting 2-EL unit tandem OLEDs exhibited no voltage drops across the CGL intermediate with excellent efficiency multiplications. Notably, the barrier-free CGL strategy worked efficiently in the sensitized fluorescence tandem OLED with low operating voltage, extremely high efficiency (current efficiency of 267.2 cd/A, external quantum efficiency of 62.6%), and operational stability (95% of the initial luminance at 3000 cd/m2, T95 of 4713 h) with pure-green emission (CIEy coordinate of over 0.68), showing the best tandem OLED among reported CGLs to satisfy commercial demands.