Highly efficient and color-stable hybrid warm white organic light-emitting diodes using a blue material with thermally activated delayed fluorescence

Abstract

Hybrid white organic light-emitting diodes (WOLEDs) often undergo triplet energy loss through the triplet state of the blue fluorophors. Here, blue fluorophors with thermally activated delayed fluorescence (TADF) are introduced to solve this problem. The triplet excitons formed on blue TADF fluorophors can be harvested by either energy transfer to the low-lying triplet states of the phosphor or thermal upconversion to the emissive singlet states, eliminating the energy loss. Moreover, device structures are wisely designed to take full advantages of the charge trapping ability of the TADF dopant, 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene, achieving a color-stable warm white emission. Remarkably, a maximum forward viewing external quantum efficiency (EQE) of 22.5% and a maximum forward viewing power efficiency (PE) of 47.6 lm W⁻¹ are achieved. These values are among the highest reported for hybrid WOLEDs and even comparable to full-phosphorescent ones, demonstrating that the strategy reported here is promising for OLED lighting.