Disentangling degradation of non-fullerene organic solar cells operated in air

Stability concerns of organic solar cells in ambient air remain a major roadblock to the widespread exploitation of this technology, while the underlying degradation mechanisms remain elusive. Here, we disentangle the roles of the key atmospheric species, oxygen and water, in the photodegradation of the most popular PM6:Y6 system. Water causes an illumination-driven twisting of the PM6 backbone while leaving Y6 largely unaffected. Oxygen attacks both PM6 and Y6, where we identify the oxidation of the PM6 benzo[1,2- b :4,5- b ']dithiophene (BDT) unit as the key process for device degradation by disrupting the π-conjugated electronic structure. Further, we show that the formation of either Y6 excitons or charge-transfer excitons is sufficient to initiate PM6 oxidation. Combined with chemical-probe evidence, these results identify exciton-mediated singlet-oxygen generation as the most critical degradation pathway limiting the stability of organic solar cells operated in air.