Abstract We investigate the photostability of organic photovoltaic (OPV) cell active layers comprised of the archetype donor, boron-subphthalocyanine chloride (SubPc), and fullerene acceptors, aged under either AM1.5G illumination or in the dark, and in either air or inert atmosphere. Under long-term exposure to light, we observe significant photobleaching and crystallization of SubPc. Mixing SubPc with C 60 as is commonly done in high efficiency small molecule OPVs, the crystallite formation is inhibited and the bleaching is suppressed due to a significantly reduced exciton lifetime in the blends. Furthermore, the spectral dependence of the degradation suggests that photo-dimerization of C 60 is an important factor leading to burn-in loss in efficiency previously reported in SubPc/C 60 OPVs. The existence of dimerization is supported by Fourier transform infrared spectroscopy data taken both before and after exposure to light. Increasing the fraction of SubPc in a SubPc:C 60 blend leads to a decrease in the rate of film degradation, providing further evidence for C 60 dimerization. Due to its reduced tendency for photo-dimerization, C 70 is more stable than C 60 when used in small molecule OPVs.