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) cm(2)V(-1)s(-1) in the 2-7wt.% 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 (similar to 10(-3) cm(2)V(-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. (C) 2009 Elsevier B.V. All rights reserved.