Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
DOI: https://doi.org/10.29363/nanoge.hopv.2022.177
Publication date: 20th April 2022
Organic photovoltaics (OPV) have attracted considerable attention in recent years due to their flexibility, versatility, and solution processability. The polymer PffBT4T-C9C13 (poly[(5,6-difluoro-2,1,3-benzothiadiazole-4,7-diyl)[3,3′′′-bis(2-decyltetradecyl)[2,2′:5′,2′′:5′′,2′′ -quaterthiophene]-5,5′′′-diyl]]) produces organic solar cells of >11% efficiency with fullerenes. We present a comprehensive morphology and spectroscopy study of this polymer and its blends, focusing on atomic force microscopy, x-ray diffraction, and transient absorption spectroscopy on microsecond timescales. Unusually, fullerene-induced ordering is observed, with the polymer/fullerene blend displaying a greater crystallinity compared to the pristine polymer. This was correlated with the appearance of bimodal polarons: fast-decaying polarons in the pristine amorphous polymer domains and trapped polarons localized in the fullerene-induced ordering (crystalline) domains. The lifetime of the trapped polaron was significantly enhanced upon thermal annealing, and the complex relationship observed between lifetime and film crystallinity suggests a contribution from trap states at the interfaces between ordered and disordered domains that lead to inhibited recombination. PffBT4T-C9C13 shows clear evidence of polymer triplet formation, which is the first time triplets have been identified in PffBT4T-based blends. As the newly designed non-fullerene acceptors (NFAs), there is a wide range of options to match with polymer and cover a broader spectra range than the fullerenes. In the PffBT4T-C9C13: NFAs’ blends, different types of interfacial triplet have been detected. Triplet states play a key role in OPVs, which are normally regarded as an energy loss mechanism. Nonetheless, triples have also been discovered in high-performance polymer/fullerene and non-fullerene acceptors blends. As such, it is important to elucidate triplet pathways in organic solar cells, in particular how they affect charge photogeneration and recombination. In this talk, I will discuss the role of triplet in PffBT4T-C9C13 fullerene and NFAs blend films. In this study, we remark upon the complex relationship between morphology and photophysics. This relationship will open the door to the synthesis of new molecules to control the blend morphology and thus optimize organic photovoltaic performance.
I would like to acknowledge support from my supervisor Dr. Tracey M Clarke and EPSRC project EP/N026411/1.
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