The phosphorescence polymer LEDs (PhPLEDs) with the structure of ITO/PEDOT:PSS/PVK:Ir(ppy)<sub>3</sub>/TPBI/LiF/Al
were fabricated and investigated on the electrical and optical properties at variations doping concentrations (0.5wt% to
2.5wt%) of Ir(ppy)<sub>3</sub>. PVK(poly-vinylcarbazole) and Ir(ppy)<sub>3</sub>[tris(2-phenylpyridine)iridium(III)] polymers were used as
the host and guest materials for the emission layer. TPBI was introduced to improve the light efficiency of the devices.
The maximum luminance and current density for PhPLED were about 8600 cd/m<sup>2</sup> and 155 mA/cm<sup>2</sup> at 7 V, when the
TPBI layer was introduced between emission and cathode electrode film. The emission spectrum and the CIE color
coordinator were about 512nm and x, y = 0.28, 0.63, respectively, showing green color.
Device performances of green phosphorescent organic light-emitting diodes with triplet mixed host emitting layer were
correlated with energy levels and composition of host materials. Two hole transport type host materials, (4,4'-N,N'-
dicarbazole)biphenyl(CBP) and 4,4',4"-tris(N-carbazolyl)triphenylamine(TCTA), were combined with two electron
transport type host materials, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene(TPBI) and PH1. Maximum quantum
efficiency was obtained in 5:5 mixed host in the case of TCTA:TPBI and TCTA:PH1, while CBP:PH1 showed the best
performances in 9:1 mixed host. Quantum efficiency of green mixed host devices could be improved by more than 50 %
compared with that of corresponding single host devices. Effect of dopant energy levels on device performances of
triplet mixed host devices was also investigated and optimum composition for high efficiency depended on energy levels
on dopant materials.