Study of carrier mobility of N,N′-diphenyl-N,N′bis(1,1′- biphenyl)-4,4′-diamine (NPB) by transmission line model of impedance spectroscopy

As a powerful method for electrical measurement, impedance spectroscopy and admittance spectroscopy methods began to receive more and more attention in organic electronics research scholars. It demonstrates outstanding advantages especially in the measurement of the mobility of the charge carriers. In this paper, the hole mobility of N,N′-diphenyl-N,N′bis(1,1′- biphenyl)-4,4′-diamine (NPB) was studied by the transmission line model based on impedance spectroscopy. According to energy level of the materials of each layer, a hole-only current device with single-layer structure of indium-tin-oxide(ITO)/NPB/Ag was designed and fabricated, and its Nyquist diagram was measured at different biased voltage. The corresponding transmission line model was proposed according to the device structure and the materials, which was used to the fitting procedure to get the transfer time of the carrier. At last, the carrier mobility was obtained from the transfer time. The results showed that the hole mobility of NPB obtained by transmission line model was in line with the Poole-Freckle model. The zero-field mobility and the pre-exponential factor was further achieved to be 3.9 × 10^{- 5} cm²·V^{- 1}·s^{- 1} and 6.8 × 10^{- 3}(V/cm)^{- 1/2}, respectively. Moreover, the method can also be easily used for the study of the electron transport properties of the organic semiconductor.