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.
With the increasing performance of organic solar cells (OSCs), the design of stable donor materials is of paramount importance. In this work, we synthesised a simple alternating copolymer with thiophene and BDT (P3T- BDT) units to understand the impact of incorporation of BDT on the thermal stability of the commonly used thiophene-based OSCs. OSCs fabricated with P3T-BDT:PC71BM and P3HT:PC71BM with 1:4 ratios displayed a different degradation kinetics despite their similar initial power conversion efficiency (PCE). The PCE of P3HT-based device degraded by half while the P3T-BDT- based device lost only 15% of its initial PCE after thermally being degraded at 85 C for 48 h mainly due to the decrease in Jsc. Thermally-induced degradation of P3HT caused chain breakage while the P3T-BDT remained intact. The surface morphology of P3HT:PC71BM film exhibited a large increase in roughness in the first few hours leading to a loss of more 40% of its initial PCE while the P3T-BDT based OSC had a similar roughness throughout the accelerated ageing. This is also confirmed by the growth of the PL intensity of P3HT:PC71BM while P3T-BDT:PC71BM remained the same confirming a suppression of demixing of the donor and acceptor in the latter. In summary, the addition a BDT unit in the thiophene-based homopolymer was found to improve the thermal stability thiophene-based OSCs.
The type of aggregation in conjugated polymers determines their use in electronic devices. H-type aggregates are most suitable for solar cell applications, while J-type aggregates are recommended for light-emitting diodes. In this work, we used three methods to determine the type of aggregates in a benzodithiophene–isoindigo-based (PBDTI-DT) copolymer, namely, Huang–Rhys factor evolution with temperature, Franck–Condon analysis, and relative quantum yield (QY) calculation. All three methods indicate that both aggregation types are present, and the QY calculation clearly indicates that H-aggregates are more dominant. Time-dependent density functional theory was used to identify the two absorption bands of PBDTI-DT as local π − π* and intramolecular charge-transfer transitions.
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