White light emitting devices based on organic multilayer structure

Abstract

We report an efficient white-light-emitting devices using vacuum-deposited film of a hole-transporting layer of 4,4′bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (α-NPD), a bluish-green emitting layer of bis[2-(2-hydroxyphenyl)benzothiazolate] zinc, a Zn(BTZ)2, Zn(BTZ)2 doped with a red fluorescent dye of 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i, j]quinolizin-8-yl)vinyl]-4H-pyran (DCM2), and an electron-transporting layer of tris(8-hydroxyquinoline) aluminum (Alq3). The incomplete energy transfer from Zn(BTZ)2 to DCM2 results in the emission of two complementary colors (blue and orange). Controlling the thickness of a Zn(BTZ)2 layer which acts as a bluish-green emitter and a hole blocking layer and the concentration of DCM2 enables tailoring of the spectral output of the device to have a balanced white emission. A device with the structure of ITO/α-NPD (50 nm)/Zn(BTZ)2 (4 nm)/Zn(BTZ)2:DCM2 (10 nm, 0.8%)/Alq3 (30 nm)/LiF(0.5 nm)/Al shows an external quantum efficiency (QE) of about 0.5%, a luminous efficiency of 0.27 lm/W, and the CIE chromaticity coordinates of (0.36, 0.33) at luminance 100 cd/m2.