Photo-degradation in air of spin-coated PC60BM and PC70BM films

The fullerene derivatives PC60BM and PC70BM are widely used as electron accepting components in the active layer of polymer solar cells. Here we compare their photochemical stability by exposing thin films of PC60BM and PC70BM to simulated sunlight in ambient air for up to 47 h, and study changes in their UV-vis and FT-IR spectra. We quantify the photo-degradation by tracking the development of oxidation products in the transmission FT-IR spectra. Results indicate that PC60BM photodegrades faster than PC70BM. The rate of photo-oxidation of the thin films is dependent on the rate of oxygen diffusion in to the film and on the photo-oxidation rate of a single molecule. Both factors are dependent on the nature of the fullerene cage. The faster photo-oxidation of PC60BM than of PC70BM is in agreement with its slightly lower density and its higher reactivity. The use of PC70BM in solar cells is advantageous not only because of its absorption spectrum, but also because of its higher stability.