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

ABSTRACT

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, O2(a1Δg), by these molecules is a key step in the degradation process. We demonstrate that the addition of either ß-carotene or astaxanthin as antioxidants can inhibit degradation by a combination of three processes (a) deactivation of O2(a1Δg) to the oxygen ground state, O2(X3Σg -), (b) quenching of the O2(a1Δg) precursor, and (c) sacrificial reactions of the carotenoid with free radicals formed in the photo-initiated reactions. For OPV systems in which reaction with O2(a1Δg) 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.