High efficiency single dopant white electrophosphorescent light emitting diodesy

Efficient white electrophosphorescence has been achieved with a single emissive dopant. The dopant in these white organic light emitting diodes (WOLEDs) emits simultaneously from monomer and aggregate states, leading to a broad spectrum and high quality white emission. The dopant molecules are based on a series of platinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C²′] β-diketonates. All of the dopant complexes described herein have identical photophysics in dilute solution with structured blue monomer emission (λ_max = 468, 500, 540 nm). A broad orange aggregate emission (λ_max ≈ 580 nm) is also observed, when doped into OLED host materials. The intensity of the orange band increases relative to the blue monomer emission, as the doping level is increased. The ratio of monomer to aggregate emission can be controlled by the doping concentration, the degree of steric bulk on the dopant and by the choice of the host material. A doping concentration for which the monomer and excimer bands are approximately equal gives an emission spectrum closest to standard white illumination sources. WOLEDs have been fabricated with doped CBP and mCP luminescent layers (CBP = N,N′-dicarbazolyl-4,4′-biphenyl, mCP = N,N′-dicarbazolyl-3,5-benzene). The best efficiencies and color stabilities were achieved when an electron/exciton blocking layer (EBL) is inserted into the structure, between the hole transporting layer and doped CBP or mCP layer. The material used for an EBL in these devices was fac-tris(1-phenylpyrazolato-N,C²′)iridium(III). The EBL material effectively prevents electrons and excitons from passing through the emissive layer into the hole transporting NPD layer. CBP based devices gave a peak external quantum efficiency of 3.3 ± 0.3% (7.3 ± 0.7 lm W^-1) at 1 cd m^-2, and 2.3 ± 0.2% (5.2 ± 0.3 lm W^-1) at 500 cd m^-2. mCP based devices gave a peak external quantum efficiency of 6.4% (12.2 lm W^-1, 17.0 cd A^-1), CIE coordinates of 0.36, 0.44 and a CRI of 67 at 1 cd m^-2 (CIE = Commission Internationale de l'Eclairage, CRI = color rendering index). The efficiency of the mCP based device drops to 4.3 ± 0.5% (8.1 ± 0.6 lm W^-1, 11.3 cd A^-1) at 500 cd m^-2, however, the CIE coordinates and CRI remain unchanged.