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Donor-acceptor polymer complex formation in solution confirmed by spectroscopy and atomic-scale modelling
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences (from 2013).ORCID iD: 0000-0003-2995-3692
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).ORCID iD: 0000-0003-0377-3669
Chalmers University of Technology, Sweden.
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
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2023 (English)In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 11, no 27, p. 9316-9326Article in journal (Refereed) Published
Abstract [en]

In all-polymer solar cells, high performance is attributed to the fine-grained morphology of the film in the active layer. However, the mechanism by which this fine-grained morphology is achieved remains unknown. Polymeric non-fullerene acceptors have the potential to restrict the self-aggregation, typical of non-fullerene small molecule acceptors. Here we employed a blend of the polymeric acceptor PF5-Y5 and the donor polymer PBDB-T to investigate the balance between molecular interactions in solution. Temperature-dependent absorption spectra show evidence of temperature-induced disaggregation of both donor and acceptor polymers, where the donor polymer disaggregation depends on the solvent polarity. Concentration-dependent fluorescence spectra of blend solutions display blue-shifted acceptor emission upon dilution, similar to that observed in acceptor solutions, and a decreased tendency for charge transfer from donor to acceptor upon dilution. Excitation spectra of dilute blend solutions contain an increased contribution to the long-wavelength acceptor emission, as compared to pure acceptor solutions, from a chromophore that absorbs in a region where the donor does not absorb. These observations can be explained by donor-acceptor complexation in dilute blend solutions, that is stabilized in more polar solvents. Moreover, the near IR-region of the absorption spectrum could be matched with the calculated electronic excitations of donor-acceptor complexes of PBDB-T and PF5-Y5 oligomers. The results corroborate that the interaction between segments of the donor and acceptor polymer chains favours the formation of donor-acceptor charge transfer complexes, stabilized by hybridization of the molecular orbitals, which reduces the electronic energy. The proposed donor-acceptor complex formation competes with the donor and acceptor self-aggregation and is influenced by the solvent environment. These pre-formed donor-acceptor complexes in low-concentration solutions can be expected to have important consequences on the film morphology of all-polymer blends. The results from this joint experimental-theoretical spectroscopy study provide insights that can guide the design of compatible donor and acceptor polymers for future high-performance organic solar cells.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2023. Vol. 11, no 27, p. 9316-9326
National Category
Polymer Chemistry Theoretical Chemistry
Research subject
Chemistry; Materials Science
Identifiers
URN: urn:nbn:se:kau:diva-94224DOI: 10.1039/d1tc03853bISI: 001019691000001Scopus ID: 2-s2.0-85164140693OAI: oai:DiVA.org:kau-94224DiVA, id: diva2:1749619
Funder
Knut and Alice Wallenberg Foundation, 2016.0059Swedish Energy Agency, 48598-1Swedish National Space Board, 174/19 and 137/21Swedish Research Council, 2014-05984
Note

This paper was included as a manuscript in Ishita Jalan's PhD thesis entitled "Solution Chemistry and Morphological Properties for Organic Solar Cells: Exploring Alternative Solvents Using Microgravity and Modelling as Tools", 2023:13.

Available from: 2023-04-10 Created: 2023-04-10 Last updated: 2023-08-09Bibliographically approved
In thesis
1. Solution Chemistry and Morphological Properties for Organic Solar Cells: Exploring Alternative Solvents Using Microgravity and Modelling as Tools
Open this publication in new window or tab >>Solution Chemistry and Morphological Properties for Organic Solar Cells: Exploring Alternative Solvents Using Microgravity and Modelling as Tools
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic photovoltaics (OPVs) have the advantage of the accessibility of energy for all, due to facile and low-cost processing, with its low energy payback time compared to other technologies, therefore promising applications. Research and development have led to power conversion efficiencies of nearly 20% and now catching up to their inorganic counterparts. To enhance the efficiency even further, it is crucial to get an insight into the correlation between the active layer's morphology and the device's performance as well as how to control the morphology of the active layer.

This thesis focuses on a molecular understanding of the morphology formation in a thin film of a polymer blend for OPVs. By using Hansen solubility parameters (HSP) and solution chemistry, the thermodynamics of the phase separation of conjugated polymers, both in solution and thin films, is investigated. Furthermore, to get a deeper understanding of the phase separation between the polymers in the active layer, films were prepared under microgravity conditions, as the phase separation is slowed down under such conditions. Atomic force microscopy combined with infrared spectroscopy was used to characterize the morphology of the dry film.

Our results show that understanding solvent-solute and solute-solute interactions is key to comprehending morphology formation. Moreover, HSP proves to be a valuable tool for the initial screening of alternative solvents and solvent blends for more environmentally friendly processing and upscaling. It was found that microgravity conditions provide a tool to study the early stages of phase separation, as well as facilitate the study of the dependence of the morphology on the thicknesses of the film. Additional research is needed to separate the complex effects of gravity fluctuations and to eliminate uncertainty concerning the complete drying of the film under the microgravity phase.

Abstract [en]

Organic photovoltaics have the advantage of the accessibility of energy for all, due to facile and low-cost processing and therefore promising applications. Research and development have led to power conversion efficiencies of nearly 20% and now catching up to their inorganic counterparts. It is crucial to understand the correlation between the active layer's morphology and the device's performance.

This thesis focuses on a molecular understanding of the morphology formation in a thin polymer film for application in OPVs. Hansen solubility parameters (HSP) and solution chemistry, the thermodynamics of phase-separation of two photo-active polymers, both in solution and films, were investigated. To probe the phase separation in the active layer, films were made under microgravity conditions, as the separation is slowed down under such conditions. Atomic force microscopy combined with infrared spectroscopy was used to characterize the dry film.

Our results show that understanding the solution chemistry is key to comprehending the morphology formation. It is shown that HSP is useful for the initial screening of more environmental-friendly solvents and solvent blends for processing. It is found that microgravity conditions provide means to study the early stages of phase separation.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2023. p. 116
Series
Karlstad University Studies, ISSN 1403-8099 ; 2023:13
Keywords
solution chemistry, green solvents, microgravity, OPV, HSP
National Category
Physical Chemistry
Research subject
Chemistry - Physical Chemistry
Identifiers
urn:nbn:se:kau:diva-94228 (URN)978-91-7867-368-1 (ISBN)978-91-7867-369-8 (ISBN)
Public defence
2023-05-29, 1B309 (Sjöströmsalen), 14:00 (English)
Opponent
Supervisors
Available from: 2023-05-05 Created: 2023-04-11 Last updated: 2023-05-24Bibliographically approved

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Jalan, IshitaMarchiori, CleberAraujo, Moysesvan Stam, JanMoons, Ellen

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