Simultaneous measurement of carrier density and mobility of organic semiconductors using capacitance techniques

We present a method to measure both the majority carrier density and mobility in organic semiconductors from the voltage and frequency dependence of capacitance (C-V-f). Poly(3-hexylthiophene) (P3HT) is used as the prototypical material. The carrier density, and its spatial distribution in a planar device structure, is obtained from a subset of the C-V-f data by conventional capacitance-voltage analysis. We show that the validity of the carrier density extraction depends critically on the measurement frequency. Namely, one should make sure that the measurement frequency is lower than the modified dielectric relaxation frequency, which is characteristically low in organic semiconductors due to their low carrier mobility. Our method further exploits the voltage dependence of the modified dielectric relaxation frequency to measure the conductivity and carrier mobility. This mobility extraction method requires no complex fitting or simulation. Nor does it assume any particular dispersive model of mobility a priori. The carrier density, mobility, and conductivity of P3HT all increase with temperature from 250 to 300 K. The activation energies of mobility and conductivity are 0.15 ± 0.01 and 0.24 ± 0.03 eV, respectively.