Oxygen-dependent photophysics and photochemistry of prototypical compounds for organic photovoltaics: inhibiting degradation initiated by singlet oxygen at a molecular level

Photo-initiated, oxygen-mediated degradation of the molecules in the active layer of organic photovoltaic, OPV, devices currently limits advances in the development of solar cells. To address this problem systematically and at a molecular level, it is informative to quantify the kinetics of the pertinent processes, both in solution phase and in solid films. To this end, we examined the oxygen-dependent photophysics and photochemistry of selected functionalized fullerenes, thiophene derivatives, and a subphthalocyanine commonly used in OPV devices. We find that the photosensitized production of singlet molecular oxygen, O-2(a(1)?(g)), by these molecules is a key step in the degradation process. We demonstrate that the addition of either ?-carotene or astaxanthin as antioxidants can inhibit degradation by a combination of three processes: (a) deactivation of O-2(a(1)?(g)) to the oxygen ground state, O-2(X-3?(?)(g)), (b) quenching of the O-2(a(1)?(g)) precursor, and (c) sacrificial reactions of the carotenoid with free radicals formed in the photo-initiated reactions. For OPV systems in which reaction with O-2(a(1)?(g)) contributes to the degradation, the first two of these processes are desired and should have appreciable impact in prolonging the longevity of OPV devices because they do not result in a chemical change of the system.