Proceedings of MATSUS Spring 2025 Conference (MATSUSSpring25)
DOI: https://doi.org/10.29363/nanoge.matsusspring.2025.560
Publication date: 16th December 2024
The organic photovoltaics (OPV) field has seen a boost in performance with the introduction of small-molecule electron acceptors (SMA), such as Y5 and Y6, leading to power conversion efficiencies above 19%[1]. For these SMAs new challenges come about due to their tendency to self-aggregate. One strategy to limit aggregation and promote miscibility with the donor is the use of Y-based copolymers as electron acceptors, yielding high-efficiency all-polymer solar cells.[2] We have shown earlier [3] by temperature-dependent optical spectroscopy of solutions of PBDB-T and the polymer acceptor PF5-Y5 that donor and acceptor interact even at low concentrations.
Here we will first present results from morphological analysis of acceptor films by angle-resolved Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, with focus on the orientation of planar SMAs Y5 and Y6 in pristine spin-coated films. We found that the processing solvent is a key parameter to obtain preferentially face-on orientation. Face-on orientation was also found for films of the Y5-based copolymers PF5-Y5 and PYT. Due to the presence of nitrogen in the acceptor N K-edge NEXAFS spectroscopy could be used to probe the orientation of acceptor molecules in the bulk heterojunction films, where they are blended with the donor polymer PBDB-T, where nitrogen is absent. We find that the surface of these blend films is donor-rich and the polymer acceptors PF5-Y5 and PYT retain the face-on orientation that they have in neat films also in the blend films.[4]
Furthermore, we will present our findings on the photochemical stability of the acceptor materials. We studied the evolution of optical properties, composition, and energy levels, during one-sun (AM1.5) illumination in air of thin films of the small-molecule acceptor Y5 and its copolymer counterparts PF5-Y5 and PYT, along with their blends with PBDB-T.[5] We found that the copolymer PF5-Y5 undergoes rapid photooxidation, while the PYT film degrades much slower and the Y5 film properties remain almost intact, even after 30 hours of light exposure in air. For Y5 molecular packing and aggregation may protect the small molecule film from photodegradation. However, the significantly faster photodegradation of PF5-Y5 compared to PYT indicates that the BDT moiety in PF5-Y5 accelerates its photooxidation. These insights on the effects of intentional photodegradation on materials properties are expected to contribute to the design of stable acceptors for long-lived OPV devices.
The authors acknowledge Zewdneh Genene and Ergang Wang at Chalmers University of Technology for the synthesis of the polymer acceptors. EM is grateful to the Swedish Research Council (grant 2021−04798) and the Swedish Energy Agency (contract 48598-1) for the financial support of the projects. The research conducted at MAX IV is supported by the Swedish Research Council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. The computational part of the work is conducted with the support of the National Academic Infrastructure for Supercomputing in Sweden (NAISS) at the National Supercomputer Centre (NSC) at Linköping University, partially funded by the Swedish Research Council through Grant Agreement Nos. 2022−06725 and 2018−05973.
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