Impedance spectroscopy on organic bulk-heterojunction solar cells

We measured the electrical impedance spectra of organic bulk-heterojunction solar cells based on an absorber blend of poly(3-hexylthiophene) and [6,6]-phenyl C-61-butyric acid methyl ester. Comparing the spectra of the non-treated device and after two consecutive treatments with applied forward bias voltage at 110 degrees C, we observed a region in the semiconductor with a low conductivity, which was expanding after the treatments. We concluded that this region is a depletion region at the aluminium contact. This was confirmed by the bias dependence of the impedance spectra.