Photostability of benzodithiophene based polymer: effect of PC₆₀BM and intermolecular interactions

Abstract

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.

Effect of an ambient environment on light-induced degradation of organic solar cells based on a benzodithiophene–quinoxaline copolymer in air

“…The contribution of PC 70 BM to exciton formation is negligible due to the low photon flux of solar irradiation below 450 nm, indicating that its main function in the bulk heterojunction film is as an electron transport material. 11,12,20…”

“…The electrochemical band gap E el g = E HOMO − E LUMO of PBDTQ-4O is larger than that of its optical band gap deduced from the absorption λ onset , and this can be attributed to electrons and holes in a conjugated polymer remaining electrostatically bound in the exited states after photon absorption. 20…”

“…37 However, a higher PC 70 BM is beneficial to mask the device from photo-induced degradation. 12,20…”

“…18,19 However, light irradiation is one of the unavoidable factors driving OSC device degradation by causing various morphological and molecular changes such as active layer material rearrangement, cross linking, and chain scission. 17,20…”

“…18,19 However, light irradiation is one of the unavoidable factors driving OSC device degradation by causing various morphological and molecular changes such as active layer material rearrangement, cross linking, and chain scission. 17,20 For conjugated polymers, a number of studies that compare the photo-stability of diverse materials in air by observing their absorption spectra have deepened our understanding of their inherent stability. 12,20 Findings of such studies revealed that their photochemical stability and the rate of degradation of polymers are dependent on their chemical bond, electronic properties related to their reactivity with light and oxygen, HOMO levels, structural conformation, intra-and inter-chain interactions of the chains, and solubility and processibility.…”