Persistent photoexcitation effect on the poly(3-hexylthiophene) film: Impedance measurement and modeling

We report on the equivalent circuit modeling of the relaxation behavior of an optically excited thick poly(3-hexylthiophene) (P3HT) film by means of impedance spectroscopy. Fabricated metal-semiconductor-metal devices with Au electrodes showed a nearly perfect ohmic behavior under ambient conditions. Impedance measurements on illuminated P3HT device showed a dramatical decrease of the impedance modulus under illumination and very slow relaxation to the initial state. Impedance-frequency data obtained during relaxation could not be explained by a simple parallel resistance-capacitance circuit but it could be best fitted by incorporating a constant-phase element instead of a normal capacitance. By observing the variation of the circuit parameters, it is found that the relaxation process is dominated by slow recombination (elimination) of the excess photogenerated carriers, which is confirmed by the time-varying photoconductivity of the device.