Highly Efficient and Spectrum-Tunable Hybrid Tandem White Organic Light-Emitting Diodes Achieved by Strategic Exciton Management based on Ultrathin Emitting Layer Structures

Highly efficient hybrid white organic light-emitting diodes (WOLEDs) featuring a tunable spectrum are constructed based on a tandem structure composed of a blue thermally activated delayed fluorescent (TADF) unit and a bluish-green TADF/red phosphorescent unit (inserting red ultrathin light-emitting layer (EML) into bluish-green EML). The direct carriers' trapping and multi-channels energy transfer onto red phosphorescent molecules help to enhance red light preferentially, thus realizing re-adjustment of the spectrum with increasing driving voltage. Finally, the corresponding WOLED exhibits the maximum external quantum efficiency (EQE) of 44.26%, along with the shift of correlated color temperature (CCT) from 8945 K at 1000 cd m(-2) to 5836 K at 5000 cd m(-2), and thus to 5078 K at 10 000 cd m(-2), achieving efficient spectrum-tunable cool WOLED. Moreover, by introducing an ultrathin red TADF EML, the obtained WOLED (EQE of 35.67%) achieves obvious variations in chromaticity coordinates and CCT from (0.292, 0.296) and 8469 K at 1000 cd m(-2) to (0.352, 0.355) and 4744 K at 5000 cd m(-2), and thus to (0.389, 0.397) and 3930 K at 10 000 cd m(-2), exhibiting abroad color shift from cool to warm white.