In this report, the hole-only device was fabricated in order to study the carrier mobility of N, N'-diphenyl-N,N' bis(1,1'-biphenyl)-4,4'-diamine through the space charge limited current model. Then the capacitance spectrum and the corresponding imaginary part of impedance spectrum weremeasured. When the frequency-dependent mathematical physics equations were directly used to fit the capacitance spectrum, it was found that there was obviously error in the curve between the range of omega similar to tau(-1)(t). Considering the distribution of the carrier transit time, two models, the exponential distribution and the Gaussian distribution, were added to fit the experimental curve. Through such distribution functions, the curve could be more satisfactorily to be fitted. And from the logic thinking, we think the Gaussian distribution is more reasonable and more close to the real transit time distribution. Further, we used the imaginary part of the impedance spectra for research and also achieved satisfactory results. Finally, the four kinds of carrier mobility tested by the above four research methods were all found to be in accordance with the Poole-Frenkel model, with only small difference in the values. (C) 2014 Elsevier B. V. All rights reserved.