A graded doping technique was presented to fabricate high brightness and high efficiency OLEDs, in which a copper phthalocyanine(CuPc) film acts as buffer layer (alpha) -naphthylphenybiphenyl amine(NPB) film as hole-transport layer and a tris(8-hydroxyquinolinolate)aluminum(Alq3) film as the electron-transport layer.The luminescent layer consists of the mixture of NPB,Alq3 and an emitting dopant 5,6,11,12-petraphenylnaphthacene(Rubrene), where the concentration of NPB raised while the concentration of Alq3 was declined gradually in the deposition process. The graded doping device exhibited the maximum emission of 50000cd/m2 at 35v and the maximum efficiency of about 8cd/A at 9v, respectively, which have been improved by four times in comparison with the conventional doped devices.
A bilayer is used as an electrode for organic light-emitting devices. The bilayer consists of a Sn layer adjacent to Alq layer and an Al outerlayer. The effects of a controlled Sn buffer layer on the behavior of ITO/CuPc/NPB/Alq/Sn/Al light-emitting devices are described. It is found that both electron injection efficiency and electroluminescence are significantly enhanced when the thickness of Sn layer is suitable. The enhancement is believed to be due to increased electron density of Sn layer near the Sn/Al interface .