A new strategy to develop simple and efficient monochrome and white organic light-emitting diodes

How to adopt simpler device structure and preparation process to achieve high device performance are the continuous efforts of researchers for promoting the industrialization development of white organic light-emitting diodes (OLEDs) lighting technology. In view of this, vertical stacked multiple interfacial exciplexes with thermally activated delayed fluorescence (TADF) character are demonstrated in the same device for the first time by cleverly arranging donor and acceptor layers alternately and precisely controlling the thickness of donor and acceptor layers. Inserting blue phosphorescent ultra-thin emitting layers (Ph-UEMLs) (FIrpic, 0.40 nm) into the interfaces of mCP/PO-T2T-forming multi-interfacial exciplex, the doping-free blue phosphorescent OLED achieves the maximum external quantum efficiency (EQE) of 28.15 %, and still maintain at 23.40 % at a luminance of 1000 cd/m2, which is among the best performance for FIrpic-based doping-free devices reported in the literatures. And it is testified that this scheme is universal for structuring other color phosphorescent OLEDs. On this basis, the doping-free white OLEDs are developed by simply incorporating complementary Ph-UEMLs with different order of arrangement and film thickness into the interfaces of above TADF multi-interfacial exciplex. The resulting two-, three-, four-color white OLEDs realize the maximum EQE reaching 21.33 %, 25.38 %, and 24.15 %, respectively. Meanwhile, all white OLEDs also exhibit high color stability, and achieve color rendering index exceeding 80 for optimized three, four-color white devices, indicating a huge potential application. The underlying physical mechanisms for proposed monochrome and white OLEDs are investigated and analyzed in detail. We believe this work offers a new strategy to develop simple and efficient monochrome and white OLEDs via doping-free technology, is of great significance.