Morphology and photochemical stability of P3HT:PCBM active layers of organic solar cells

We report on the effects of continuous UV-visible light illumination at 60 degrees C in the absence of oxygen on P3HT:PCBM blend films commonly used active layer of bulk heterojunction solar cells. A full description of the behavior of P3HT: PCBM blend films either unconfined or confined by an Al cathode and a PEDOT:PSS layer during annealing treatment and irradiation is provided. We also focused on the impact of the P3HT type on the photostability. It was shown that the microstructure of P3HT dramatically influenced the photodegradation process of P3HT thin films deposited on inert substrate. The rate of photodegradation process was decreased when P3HT was blended with PCBM. It was shown that the photostability of the active layer was not influenced by a PEDOT-PSS sub-layer. Solar cells were then fabricated from high-regioregular P3HT. Many large PCBM crystals were observed in P3HT: PCBM blend films and it was shown that the top surface of the active layer in contact with the Al cathode was nearly entirely composed of P3HT. Both results account for the low performances of the devices. Finally, photo-aging experiments provoked a rapid failure of the Al cathodes which was tentatively attributed to an increase of internal strain within the devices.