Comparison of the carrier mobility, unipolar conduction, and light emitting characteristics of phosphorescent host–dopant system

We have investigated the charge carrier transport mobility and hole-only current behavior of phosphorescent host–dopant mixture, and evaluated the efficiency and lifetime behavior of organic light emitting device (OLED) containing the corresponding light emitting host–dopant system. The carrier drift mobilities of the phosphorescent host, 4,4′-N,N′-dicarbazole-biphenyl (CBP), doped with green-emitting fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) or bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetyl-acetonate) (Btp2Ir(acac)) red-emitting dopants, were directly investigated with time of flight (TOF) photoconductivity method. The resolved electron mobility of phosphorescent host–dopant layer by TOF-PC method showed the significant reduction at CBP:(Btp2Ir(acac)) layer, which can be explained by the electron trapping. Measured hole-only current data also shows the reduction, which is more significant at CBP:(Btp2Ir(acac)) as expected from larger energy level offset. The efficiency, spectral emitting properties, and device stability of phosphorescent OLEDs with identical host–dopant layer were evaluated. Compared with CBP:(Btp2Ir(acac)), device with CBP:Ir(ppy)3 emitter shows the spectral response and half-lifetime less dependent upon the hole/exciton blocking layer and its thickness. Such device data was well-correlated with probed TOF and hole current behavior.