We have designed a new structure blue emission device with doped Alq_3 of 3% in hole transmission layers of NPB. The CIE coordination of the devices is (0.17,0.19). The maximum electroluminescence efficiency is 4.1 cd/A at 11V, the brightness is 118.8 cd/m^2 at 7 V, and the maximum brightness is 10770 cd/m^2 at 13V.
The calculation of the external quantum efficiency(η_ ext ) of organic light-emitting devices(OLEDs) is presented theoretically. The basic idea is to obtain η_ ext by calculating the fraction of the generated light that can escape through the top surface and the average transmittance over the escape cone. During the calculation, the effects of the interference, absorption and multiple reflections are neglected. Then a result of a typical 8-tris-hydroxyquinoline aluminum(Alq_ 3 )-based OLEDs on a planar glass substrate is obtained. And a twice increase in η_ ext can be achieved by using a high-index-glass substrate with an antireflection coating(SiO_ 2 ) on substrate surface.
