High efficiency organic red electrophosphorescence devices with changing thickness of the emitting layer
发光层厚度变化的高效红色有机电致磷光器件

Devices using red phosphorescent dye as dopant with exciton blocking layer were constructed. The device structure is ITO/CuPc/NPB/TPBi:Btp-2Ir(acac)/TPBi/Alq/LiF/Al. The EL spectra, luminance-current, efficiency-current characteristics of the device have been investigated. At the CIE coordinates x=0.62,y=0.35,efficiency 2.43cd /A has been achieved. At 20mA/cm~2 and 400mA/cm~2, luminance is 431cd/m~2 and 4798 cd/m~2, respectively. The effects of the emitting layer thickness on efficiency and EL spectra of the device have been studied also. With the decrease of the emitting layer thickness, efficiency decreases and the blue band of the El spectra increases. For the thickness d<15nm, the efficiency significantly decreases. This is mainly because of pronounced triplet-Triplet (T-T) annihilation. Another factor which reduces the efficiency and impairs the color is incomplete capture of holes channeling from Btp-2Ir(acac) HOMO energy level. For the thickness d>20nm, the decrease of the efficiency is attributed to the added invalid emitting-layer and more annihilation centers.