Impedance spectroscopy measurements of charge carrier mobility in 4,4'-N,N'-dicarbazole-biphenyl thin films doped with tris(2-phenylpyridine) iridium

The charge carrier mobility of green phosphorescent emissive layers, tris(2-phenylpyridine) iridium [Ir(ppy)3]-doped 4,4'-N,N'-dicarbazole-biphenyl (CBP) thin films, has been determined using impedance spectroscopy (IS) measurements. The theoretical basis of mobility measurement by IS rests on a theory for single-injection space-charge limited current. The hole mobilities of the Ir(ppy)3-doped CBP thin films were measured to be 10− 10-10− 8 cm2V− 1 s− 1 in the 2-7 wt.% Ir(ppy)3-doped CBP from the frequency dependence of both conductance and capacitance. These hole mobility values are much lower than those of the undoped CBP thin films (~ 10− 3 cm2V− 1 s− 1) because the Ir(ppy)3 molecules act as trapping centers in the CBP host matrix. These mobility measurements in the Ir(ppy)3-doped CBP thin films provide insight into the hole injection process.