On the influence of the photo-oxidation of P3HT on the conductivity of photoactive film of P3HT:PCBM bulk heterojunctions

Abstract

Previously, we reported that the photo-oxidation of P3HT:PCBM was shown to induce an increase of the electric resistance at the active layer in the solar cells, resulting in a drop in the device performances. Meanwhile, fragments resulting from the partial molecular scission of P3HT chains were also detected by analyzing the produced materials using MALDI-TOF-MS. However, the way the photo-oxidation of these active materials actually impacts on the electric resistance of the resulting device yet remains to be investigated. In this work, we perform Conductive-AFM (C-AFM) and Space Charge Limited Current (SCLC) measurements on the photo-oxidized P3HT based films; the results indicate that the conductivity of P3HT decreases upon irradiation, mainly due to a decrease of the hole mobility. Even though the number average molecular weight of the P3HT only modestly decreases during the photo-oxidation, UV–vis spectra indicate a significant decrease of the long-range ordering of the P3HT chains. These results inferred that P3HT crystal formation may partially be prevented by the oxidized molecular fragments of reduced molecular weight, resulting in the decrease of P3HT hole mobility, hence leading to the observed resistance increase in these photovoltaic active layers.