Studying Polymers for Organic Solar Cells via Transient Absorption and Raman Spectroscopy

Hristo Gonev^a, Junjun Guo^a, Jose Marin-Beloqui^b, Katherine Holt^a, Tracey Clarke^a

^aDepartment of Chemistry, University College London, London, UK

^bDepartment of Physical Chemistry, University of Malaga, Blvrd Louis Pasteur 31, 29010, Malaga, Spain

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)

London, United Kingdom, 2023 June 12th - 14th

Organizers: Tracey Clarke, James Durrant and Trystan Watson

Poster, Hristo Gonev, 235
Publication date: 30th March 2023

Replacing an alkoxy group with a thiophene one has been found to lead to an increase in the efficiency of polymers used as donors in organic solar cells (OSCs). Learning more about the fundamental processes occurring in these materials would enable better informed decision making in the synthesis and selection of polymers. This report looks at two pairs of materials, where in each pair one polymer has a thiophene group (PBDB-T, PTB-7Th) where the other possesses an alkoxy one (PBDB, PTB-7). A further comparison is made between pristine films and blends with either a fullerene (PC₆₀BM) or a non-fullerene (ITIC-Th) acceptor. First, transient absorption spectroscopy (TAS) is used to show that in the PBDB/PBDB-T pair of polymers the blend with ITIC-Th produces more charges than the fullerene blend, while the opposite is true for the PTB-7/PTB-7Th system. Furthermore, when sensitised with PtOEP both PBDB-T and PTB-7Th exhibit triplet species that are not apparent in the pristine spectra or the other blends. Then, resonance Raman spectroscopy data is used to show that a few C = C stretches shift to lower wavenumbers in the thiophene-containing polymers, which is associated with a strengthening of the intramolecular coupling, linked to improved efficiency. Furthermore, ITIC-Th is found to promote degradation along the PBDB main chain, while enhancing absorbance in PTB-7 and PTB-7Th. Finally, the backbone BDT moiety in all polymers is observed to degrade in an oxygen environment, meaning that it is a relatively unstable part of the molecules.

References:

[1] Kim S., Rashid M.A.M., Ko T., Ahn K., Shin Y. et al. New Insights into the Photodegradation Mechanism of the PTB7-Th Film: Photooxidation of π-Conjugated Backbone upon Sunlight Illumination. J. Phys. Chem. C. 2020;124:2762-2770.

[2] Awartani O. M., Gautam B., Zhao W., Younts R., Hou J. et al. Polymer non-fullerene solar cells of vastly different efficiencies for minor side-chain modification: impact of charge transfer, carrier lifetime, morphology and mobility. J. Mater. Chem. A, 2018; 6, 12484

[3] Ye L., Jiao X., Zhou M., Zhang S., Yao H. Manipulating Aggregation and Molecular Orientation in All-Polymer Photovoltaic Cells. Adv. Mater. 2015, 27, 6046-6054.

Acknowledgements:

We are grateful to Dr Daniel Congrave from the Bronstein group at the University of Cambridge for synthesising and kindly providing the four polymers for this study.

We would also like to acknowledge support from the Engineering and Physical Sciences Research Council [grant numbers EP/N509577/1, EP/ T517793/1].

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