[Effects of PBD on the luminescence properties of PVK: Ir(ppy)3]

The effects of PBD on the luminescence properties of PVK : Ir(ppy)3 codoped with PBD with lower concentration were investigated. Two kinds of devices with the structures of ITO/PVK : Ir(ppy)3/PBD/Al and ITO/PVK : Ir(ppy)3 : PBD/ Al were fabricated. PVK emission appears in the electroluminescence (EL) spectra of ITO/PVK : Ir(ppy)3/PBD/Al. The reason is that the Forster energy transfer is not efficient enough in the emission layer, and then the emission layer was doped with PBD. A set of devices were fabricated with the configuration of ITO/PVK : Ir(ppy)3 : PBD/BCP/Al. The doping weight ratio of PVK : Ir(ppy)3 was fixed as 100 : 1, and the weight ratio of PBD changed from 100 : 0 (PVK : PBD) to 100 : 20. The photoluminescence (PL) spectra and electroluminescence (EL) spectra of these devices were measured. Through the analysis of their luminescence, it was found that the brightness of these devices codoped with PBD is improved. The brightness of these devices increases with increasing PBD doping ratio as the ratio does not exceed 100:10 (PVK : PBD). The PBD codoping enhances the injection and transportation of electron, resulting in the enhanced carrier recombination probability. If the doping ratio of PBD exceeds 100 : 10 (PVK : PBD), the brightness of the device decreases. There are two reasons. One is that the superfluous PBD causes the carriers to be imbalanced and induces a larger leakage current. The other is that the superfluous PBD may block the hoping of holes or electrons between the PVK chains. The brightness is the strongest when the weight ratio of PVK to PBD is 100 : 10.