Hole mobility and conductance of iridium complex doped NPB emission layers for organic light-emitting diodes

•Hole transport properties of Ir(ppy)3 doped NPB films was investigated by admittance spectroscopy.•The conductance increases nearly linearly with the voltage bias above the turn-on voltage when doping concentration smaller than or equal to 8.0 wt%.•The mobility decrease, instead of increase, near exponentially with the square root of the electric field. Iridium complexes are widely used in dye phosphorescent guest-host systems which usually as emission layer of efficient organic light-emitting diodes. Phosphorescent organic light-emitting diodes (OLEDs) with tris(2-phenylpyridine) iridium [Ir(ppy)3] as emissive dopant exhibited high internal quantum efficiency. N, N-diphenyl-N, N-bis(1-naphthylphenyl)-1,1-biphenyl-4,4-diamine (NPB) is a good candidate of host for guest–host system in OLEDs. It is significant that understanding and engineering of charge transport in Ir(ppy)3 doped NPB films to design and optimize OLEDs, even organic lasers. In this paper, hole mobility and conductance of Ir(ppy)3 doped NPB films were investigated by admittance spectroscopy. The results show that for a given applied voltage, with the doping concentration increasing, the conductance first increase reaching a maximum then decrease, which indicate that light doping of NPB with Ir(ppy)3 weakly improves, while heavy doping deteriorates the conductivity. We found that light doping of NPB with Ir(ppy)3 almost do not alter the hole mobility, which coincides with the previous theory. It is concluded that for films thinner than 300 nm, the electric field dependence of hole mobility is negative, that is, the mobility decrease, rather than increase, near exponentially with the square root of the electric field.