Electron and hole mobility determination in organic layers by analysis of admittance spectroscopy

We present results of a theoretical study on the simultaneous determination of the electron and hole mobilities in organic layers by admittance spectroscopy. By solving the basic semiconductor equations under steady-state and small-signal conditions for a typical system composed of an organic layer sandwiched between two electrodes, we show that it is effectively possible to obtain by admittance measurements on the same organic device, both the electron and hole mobilities. The analysis of the effect of the various parameters characterizing the structure allows determining the conditions for the simultaneous appearance of two resonance peaks in the curve representing the negative differential susceptance $−ΔB$ a as function of the modulation frequency $f$⁠. The frequency position of the two peaks allows us to obtain the transit time of both types of carriers and to deduce the mobility of the electrons and holes. Three major conditions have to be realized: injection of both electrons and holes at the respective electrodes, recombination rates of appropriate strength, and trap state modulation not masking the resonance peak of the slowest carrier.