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

Abstract 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.