Hybrid white quantum dot–organic light-emitting diodes with highly stable CIEx,y coordinates by the introduction of n-type modulation and multi-stacked hole transporting layer

Abstract

Extremely stable white emission out of a hybrid white quantum dot-organic light-emitting diode (WQD-OLED) was achieved by developing a novel concept of device architecture. The new inverted device structure employs a thermally-evaporated red phosphorescent emitting layer (EML) with an n-type modulation and a multi-stacked hole transporting layer (HTL) on the top of solution-processed ZnO nanoparticles for an electron transporting layer and blue and green QD-mixed EML. The multi-stacked HTL, tris(4-carbazoyl-9-ylphenyl)amine (TCTA)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), and the n-type modulation layer, 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) successfully balanced low-mobility holes with high-mobility electrons, and uniformly distributed the charges across the blue/green QD and red EMLs. In particular, the role of the n-type modulation layer was comprehensively analyzed with impedance spectroscopy and verified to minimize the undesired Auger recombination by excess charges and broaden the recombination zone. Consequently, highly stable white emission (0.32 ± 0.02, 0.31 ± 0.02) in CIE_xy color coordinates over 4 V operating voltage range (or two decades of current density), 3.92% external quantum efficiency, and 3.83 h device lifetime at 500 cd m⁻² up to 50% of the initial luminance (LT₅₀) were simultaneously obtained.