Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.241
Publication date: 22nd December 2022
The organic photovoltaics (OPV) field has seen a boost in performance with the introduction of small-molecule electron acceptors, such as Y6, and their copolymer derivatives, leading to power conversion efficiencies above 18% [1]. The device operational lifetime depends critically on the photochemical stability of the materials and the film morphology. Here we report the evolution of optical properties, composition, and energy levels, during one-sun (AM1.5) illumination in air of thin films of the donor polymer PBDB-T, the small-molecule acceptor Y5 [2], its copolymer counterpart PF5-Y5 [3], as well as their blends. UV-Vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Near-edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM) were used to monitor the thin film properties. The absorption spectra show that the PBDB-T polymer and the copolymer PF5-Y5 undergo rapid photobleaching, while the Y5 film spectrum remains, surprisingly, almost intact even after 30 hours of light exposure in air. The corresponding blend films of PBDB-T with Y5 and PF5-Y5 show similar losses in absorption as their components. New carbonyl peaks emerge in the FTIR spectra of PBDBT, PF5-Y5 and blend films, but are absent in those of Y5, indicating that the bulk of the Y5 film is resistant to photooxidation, while the copolymer containing a Y5-moiety undergoes photochemical degradation reactions. The faster photodegradation of PF5-Y5 compared to Y5 raises the question about the role of the copolymer’s BDT moiety in the photooxidation. However, the effect of film packing on the rate of degradation should also be considered. Angle-resolved NEXAFS spectra reveal a stronger linear dichroism in the spin-coated PF5-Y5 films compared to Y5 films, confirming a preferred orientation of the PF5-Y5 polymer backbone, while the random average orientation of Y5 molecules suggests a multi-crystalline Y5 film. Surface analysis by core level XPS indicates, moreover, that the surface of blend films is donor-enriched. These new insights on the effects of intentional photodegradation on donor and acceptor materials properties are expected to contribute to the design of stable acceptors as well as the development long-lived OPV devices.
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