Although phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLED) exhibit high efficiency and long lifetime for red and green emission, blue OLED is still a bottleneck. In mass production, triplet-triplet emission (TTA) OLED is the main stream for reasonable lifetime together with limited efficiency. To improve the efficiency of blue TTA-OLED, a bilayer emitting layer (EML) was employed. Compared to single-EML device, external quantum efficiency of the bilayer OLED increased from 9.4% to 13.0%, which mainly resulted from the increase of delayed emission from 15.0% to 37% with enhanced TTA process.
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