White organic light-emitting diodes with high efficiency and stable color coordinates

We report the fabrication and the characterization of white organic light-emitting diodes that exhibit high efficiency and very stable color coordinates over the wide range of bias voltages. The blue-emitting layer of 1,4-bis(2,2-diphenyl vinyl)benzene (DPVBi) is sandwiched between the red-emitting layers in which red fluorescent dyes of 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-8-yl)vinyl]-4H-pyran) (DCM2) are doped into the hole-transporting layer of 4,4'bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (alpha-NPD) and the electron-transporting layer of tris(8-hydroxyquinoline) aluminum (Alq(3)). The device structure is ITO/PEDOT:PSS/alpha-NPD(50 nm)/alpha-NPD:DCM2 (5 nm, 0.2 %)/DPVBi(10 mn)/Alq(3):DCM2(5 nm, 0.2 %)/Alq(3)(40 nm)/LiF(0.5 nm)/Al. The partial energy transfer from the blue layer to the nearby red layers results in white light emission with the stable color coordinates of (0.36, 0.37). The device shows the luminous efficiency of about 3.6 lm/W at 100 cd/m(2) and the maximum luminance of 40,650 cd/m(2) at the bias of 12 V.