Doping in the Mixed Layer to Achieve High Brightness and Efficiency Organic Light Emitting Devices

Doping in the mixed layer was introduced to fabricate high brightness and high efficiency organic light-emitting devices, in which a copper phthalocyanine (CuPc) film acts as the buffer layer, a naphthylphenybiphenyl amine (NPB) film as the 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 (to be called the mixed layer), and an emitting dopant 5,6,11,12-petraphenylnaphthacene (rubrene), where the concentration of NPB declined and the concentration of Alq₃ was increased gradually in the deposition process. Adopting this doping mixed layer, the device exhibites the maximum emission of 49300cd/m² at 35V and the maximum efficiency of 7.96cd/A at 10.5V, respectively, which have been improved by twice in comparison with the conventional doped devices. We attribute this improvement to the effective confinement of carriers in the mixed layer, which leads to the increase of the recombination efficiency of carriers.