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Improved Color Purity of Blue Organic Light-Emitting Diodes with Double Hole-Transporting Structure.

A blue organic light-emitting diode with a double hole-transporting (DHT) structure has been developed. The device, ITO/N,N'-Bis (naphthalene-1-yl)-N,N'-bis(phenyl)-benzidine/4,4'-Bis (N-carbazolyl)-1,1'-biphenyl (CBP)/4,4'-Bis(2,2-diphenylethen-yl) diphenyl/CBP/tris(8-hydroxyquinolate)aluminum/lithium fluoride/aluminum, consists of a hole-transporting layer, a DHT structure layer and an electron-transporting layer. The blue color purity is improved by modulation the thickness of CBP layer. When the thicknesses of left CBP and right CBP are respectively 8 nm and 2 nm, the more pure blue coordinates (0.155, 0.079) are obtained at 9 V, which are very close to the national television system committee standard blue coordinate (0.14, 0.08). Furthermore the current density, brightness and the luminous efficiency of device with the left CBP of 8 nm and the right CBP of 2 nm are respectively 144.7 mA/cm2, 1065 cd/m2 and 0.93 cd/A at voltage of 9 V.