Extremely low-efficiency roll-off and extended lifetime in hybrid WOLEDs based on a TADF forming co-host interlayer

White organic light-emitting diodes (WOLEDs) employing ultrathin emitting layers (UEMLs) have received more attention due to their simple properties. However, UEMLs-based WOLEDs still suffer from severe efficiency roll-off and poor operational lifetime. Herein, we propose an innovative interlayer strategy to construct high-performance WOLEDs comprising blue fluorescent and orange phosphorescent UEMLs. Utilizing what we believe to be a novel TADF-based co-host interlayer, the resulting device achieves extremely low-efficiency roll-off of only 1.7% and a superior T 50 operational lifetime of 3319.2 h at 1000 cd/m 2 , among the best values reported so far for hybrid WOLEDs. It also shows excellently stable spectra and a maximum power efficiency of 54.7 lm/W. The high performance should be attributed to more balanced carrier transport as well as efficient Förster energy transfer. We believe our findings present a new strategy to develop high-performance UEMLs-based WOLEDs.