Photostability of benzodithiophene based polymer: effect of PC 60 BM and intermolecular interactions

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:PC 60 BM 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 PC 60 BM in the polymer made the BHJ film more stable as PC 60 BM serves as photoprotective layer and radical scavenger. This conclusion was supported by the similar XRD traces of PBDTI-DT:PC 60 BM film during degradation while the traces of the pristine film indicated bleaching and shifted with increasing photon stress. In addition, the absorption of PBDTI-DT:PC 60 BM and pristine PBDTI-DT films were reduced to 80 and 56%, respectively, after 70 h of photo-degradation.