Organic solar cells working under continuous light irradiation are prone to photo-induced degradation. Photostabilities of benzodithiophene-alt-2-decyltetradecyl substituted isoindigo (PBDTI-DT) copolymer in solution, and as pristine film, and a PBDTI-DT:PC60BM bulk hetero-junction (BHJ) film were investigated for more than 70 h under simulated AM 1.5 solar irradiation. The photodegradation kinetics studied in standalone polymer chains were found to be fast due to the absence of intermolecular interaction while the inter-chain interaction in the polymer films kept the backbone intact against light-induced degradation. Further addition of PC60BM in the polymer made the BHJ film more stable as PC60BM serves as photoprotective layer and radical scavenger. This conclusion was supported by the similar XRD traces of PBDTIDT: PC60BM fillm during degradation while the traces of the pristine film indicated bleaching and shifted with increasing photon stress. In addition, the absorption of PBDTI-DT:PC60BM and pristine PBDTI-DT films were reduced to 80 and 56%, respectively, after 70 h of photo-degradation.

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