Impact of Aggregation on the Photochemistry of Fullerene Films: Correlating Stability to Triplet Exciton Kinetics

The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PC₆₁BM blend films, dispersed with polystyrene, are found to be more susceptible to photobleaching in air than the more aggregated neat films. This enhanced photobleaching correlated with increased oxygen quenching of PC₆₁BM triplet states and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation, suggesting PC₆₁BM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PC₆₁BM films were observed to undergo photo-oxidation in air for even modest (≤40 min) irradiation times, degrading electron mobility substantially, indicative of electron trap formation. This conclusion is supported by observation of red shifts in photo- and electro-luminescence with photo-oxidation, shown to be in agreement with time-dependent density functional theory calculations of defect generation. These results provide important implications on the environmental stability of PC₆₁BM-based films and devices.