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

To understand the effect of the photo-oxidized P3HT:PCBM on the resistance increase of the active layer in the solar cells, we focused on changes of the physical properties and structure of P3HT in photo-oxidized P3HT/PCBM blend films. Conductive-AFM (C-AFM) and Space Charge Limited Current (SCLC) measurements suggest that the conductivity decrease along with irradiation derives from a decrease of P3HT hole mobility. Even though the number average molecular weight of the P3HT is slightly decreased by photo-oxidation, the major reason for the decrease of the electrical properties is attributed to a reduction of the crystallinity of P3HT, compared with that of the non-oxidized polymer, as reflected in the UV-vis spectra. These results indicate that P3HT crystal formation may partially be prevented by the photo-oxidized fragments with low molecular weight, resulting in the decrease of P3HT hole mobility and leading to the resistance increase in the organic films.335