Intense research activities are currently being undertaken on organic light-emitting devices.Because of their potential applications in flat panel displays and solid state lightening.Light-emitting materials for these devices can be divided into two general classes

those are fluorescent and phosphorescent materials. Phosphorescent light emitting devices are particularly interesting for being able to utilize all the injected charges for emission.Heavy-metal complexes in which strong spin-orbit coupling leads to singlet-triplet state mixing can result in high efficiency electrophosphorescence in organic light-emitting devices(OLEDs).In this report

devices using red phosphorescent dye Ir(pig)2(acac) as dopant and BAlq as host are fabricated.The structure of the device is ITO/NPB(30 nm)/Ir(piq)

2

(acac):BAlq(25 nm)/BCP(13 nm)/Alq

3

(35 nm)/LiF(1 nm)/Al(1000 nm). All of the films are fabricated by thermal deposition way onto a precleaned ITO-coated glass substrate (sheet resistance 50 Ω/□) at a background pressure

<

2×10

-4

Pa. The deposition appartus is Edward Auto-500 Thermal Evaporation Coating System.The thinkness of the deposited films is monitored by an oscillating quartz thickness monitor.The electroluminescence(EL) spectra

brightness and CIE coordinates of the devices are recorded with Spectra Scan PR-650(Photo Research).The brightness-current density

voltage versus current density are measured by a OLEDs testing system

comprosing of a 2400 Sourcemeter(Keithley) and a 485 Picoammeter(Keithley).All the measurements are carried out at room temperature.We change the doping concentration in the host material.When the doping concentration reaches 8%

the commission internationale de’Eclairage (CIE) coordinates of the device are (

x

=0.67

y

=0.32)

which are close to meet video display standards.The device reaches the highest brightness 9 380 cd/m

2

at voltage 16 V. The external quantum efficiency reaches 5.7% at the current density 5.45 mA/cm

2

.Because of the short phosphorescent life of the Ir(piq)

2

(acac)

the external quautum efficiency is still as high as 4.7% at the current density of 100 mA/cm

2

.Futher study shows that both the short range Dexter energy transfer mechanism and the direct dopant dye trap exist simultaneously.