Photochemical origins of burn-in degradation in small molecular weight organic photovoltaic cells

Using a combination of Fourier transform infrared (FTIR) spectroscopy and physics-based degradation models, we find that the early aging of small molecular weight organic photovoltaic (OPV) cells is due to photochemical degradation of the C-60 acceptor layer. Planar and mixed boron subphthalocyanine chloride (SubPc)-donor/C-60-acceptor heterojunctions show significant changes in their IR absorption spectra after aging under illumination, which we find is due to exciton-mediated photo-oligomerization of C-60. The consequent decrease in exciton diffusion length in the C-60 layer results from a decreased exciton lifetime for C-60 oligomers (e.g. C-120 and C-180) compared to that of the monomer. The model describes the short-circuit current reduction in planar SubPc/C-60 OPV cells during early aging, and explains the lack of degradation in analogous mixed heterojunctions.