Application of admittance spectroscopy to evaluate carrier mobility in organic charge transport materials

We examine the feasibility of admittance spectroscopy (AS) and susceptance analysis in the determination of the charge-carrier mobility in an organic material. The complex admittance of the material is analyzed as a function of frequency in AS. We found that the susceptance, which is the imaginary part of the complex admittance, is related to the carrier transport properties of the materials. A plot of the computer-simulated negative differential susceptance versus frequency yields a maximum at a frequency $τr−1$⁠. The position of the maximum $τr−1$ is related to the average carrier transit time $τdc$ by $τdc=0.56τr$⁠. Thus, knowledge of $τr$ can be used to determine the carrier mobility in the material. Devices with the structure ITO/4,$4′,4″$ -tris[$N$⁠, -(3-methylphenyl)-$N$-phenylamino] triphenylamine/Ag have been designed to investigate the validity of the susceptance analysis in the hole mobility determination. The hole mobilities were measured both as functions of the electric field and the temperature. The hole mobility data extracted by susceptance analysis were in excellent agreement with those independently obtained from time-of-flight (TOF) measurements. Using the temperature dependence results, we further analyzed the mobility data by the Gaussian disorder model (GDM). The GDM disorder parameters are also in good agreement with those determined from TOF.