Photoelectrochemical response and photoconductivity of poly(3-methylthiophene)

We report on the photoelectrochemical response and photoconductivity of poly(3-methylthiophene), PMeT, films deposited on different substrates. The frequency dependency of the modulated photoconductivity of PMeT ( ρ=3.6×10 3 Ω cm) is sublinear indicating that the measured photoconductivity is not due to a thermal effect caused by sample heating. An interesting feature was the larger backwall photoconductivity (with the laser beam reaching the film from the substrate side), when compared to photoconductivity in the normal, frontwall illumination. This result is an indication for heterogeneity in the deposited film. The photoelectrochemical response of PMeT films in contact with the electrolyte was studied by spectral response and by the time dependence of the photoelectrochemical response under irradiation with the Ar + laser light (close to the maximum of the spectral photoresponse), with light intensity varying from 10 −5 W cm −2 to 30 mW cm −2. From that experiment we can infer that charge trapping occurs mainly in the polymer surface in contact with the electrolyte. Another experiment was performed with two different light beams, one from a lamp plus monochromator, chopped and at low intensity and another from an Ar + laser, continuous and at high intensity. When the laser beam incides from the electrolyte side, i.e. when light is absorbed in the space-charge layer, we observe a strong drop in the photocurrent. We suppose that the same space-charge layer trapping phenomenon is responsible for the drop in the photocurrent.