Bright small molecular white organic light-emitting devices with two emission zones

Bright multilayer organic light-emitting devices (OLEDs) containing both perylene-doped ${\text{[4,4}}^{\prime }$ -bis(9-carbazolyl) biphenyl (CBP)] and [4-(dicyano-methylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran (DCM1)-doped tris-(8-hydroxy quinoline) Al $({\mathrm{Alq}}_3\mathrm{)]}$ are described. The electroluminescence spectra consist of blue and green bands at 453, 487, and 524 nm due to perylene and a red band at 600 nm due to DCM1, although these two emitting layers are separated by a layer of [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (Bu-PBD)]. As the thickness of the hole-injecting ${\mathrm{[N,N}}^{\prime }$ -diphenyl-${\mathrm{N,N}}^{\prime }$ -bis(1-naphthyl-phenyl)-1, 1′-biphenyl-${\text{4,4}}^{\prime }$ -diamine (NPB)] and perylene:CBP layers increases and that of the ${\mathrm{DCM1:Alq}}_3$ layer decreases the perylene emission intensifies strongly relative to the DCM1 emission. For 350 Å thick NPB, 350 Å thick perylene:CBP, 100 Å Bu-PBD, and 150 Å ${\mathrm{DCM1:Alq}}_3,$ the brightness reaches 3750 Cd/m² at 20 V, the efficiency is 2.4 Cd/A at 19 V, and the (x,y) CIE coordinates are well within the white region. However, as the bias is increased the intensity of the perylene emission increases relative to the DCM1 emission. Both the layer thickness and bias dependence are believed to result from changes in the recombination zone and in the field- and cathode-mirror-induced quenching of DCM1 singlet excitons.