Advantages of admittance spectroscopy over time-of-flight technique for studying dispersive charge transport in an organic semiconductor

We show that admittance spectroscopy (AS) is a better technique than time of flight (TOF) to study the charge transport properties in dispersive materials. The hole transport properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'-diamine (NPB) doped with different traps were evaluated by AS and TOF techniques. It was found that both techniques can show clear signals for measuring the mobility of NPB doped with shallow traps. When NPB was doped with deep traps, the AS signals were still clear for mobility extraction. In sharp contrast, the TOF transients become featureless and the carrier transit time cannot be determined. The validity of AS in mobility determination was demonstrated by comparing the extracted AS to TOF mobilities. Generally, the hole mobilities extracted by these two techniques were in excellent agreement. In addition, we will demonstrate that AS can be employed to measure carrier dispersion. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3251409]