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The Photooxidation of PC60BM: New Insights from Spectroscopy
KTH Royal Institute of Technology.ORCID iD: 0000-0003-1671-8298
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).ORCID iD: 0000-0002-7533-4860
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).ORCID iD: 0000-0002-4745-1074
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In the quest towards more durable solution-processed solar cells, the stability of the active layer materials under operation conditions is important. While lifetimes of several years have been demonstrated for encapsulated organic solar cells, it is generally known that degradation events can be accounted for by air components (O2 and/or water vapour) leaking into the cell through a non-ideal sealing. Here we present a fundamental study of intentional photo-degradation of the electron-acceptor PC60BM ([6,6]-phenyl-C61-butyric acid methyl ester) in air, with the purpose of improving the understanding of the electronic effects of fullerene photo-oxidation. We have studied spincoated thin films of PC60BM by X-ray Photoelectron Spectroscopy, Near-edge X-ray Absorption Fine Structure spectroscopy, and Fourier Transform Infrared Spectroscopy, before and after exposing them to simulated sunlight in air. The changes observed in the spectra obtained by these complementary methods were compared with calculated spectra of a large set of possible oxidation products of PC60BM where oxygen atoms have been attached to the C60 cage. The best fit with experimental IR spectra of photodegraded PC60BM films was obtained for a linear combination of calculated spectra for two degradation products, a dicarbonyl and an anhydride, both with open cages with 58 carbon atoms, and the pristine PC60BM molecule. From this comparison, we conclude that the conjugation of the fullerene cage is disturbed by the formation of several carbonyl-based derivatives on the C60 cage, accompanied by a transition from sp2 to sp3-hybridized carbon. The π* resonance in the C1s NEXAFS spectrum was found to be a very sensitive probe for small changes to the fullerene cage, and FT-IR was needed in combination with O1s NEXAFS, to identify the oxidation products.

Keywords [en]
photodegradation, fullerene, density functional theory, NEXAFS, XPS, FTIR
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kau:diva-75121OAI: oai:DiVA.org:kau-75121DiVA, id: diva2:1358191
Note

Manuscript

Available from: 2019-10-07 Created: 2019-10-07 Last updated: 2019-10-08
In thesis
1. Probing the effects of photodegradation of acceptor materials in polymer solar cells: bulk, surface, and molecular level
Open this publication in new window or tab >>Probing the effects of photodegradation of acceptor materials in polymer solar cells: bulk, surface, and molecular level
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer solar cells (PSC) have reached record power conversion efficiencies of over 15%. The operational lifetime of PSCs, however, has to increase for their use in large area outdoor applications. In this work, a set of spectroscopic techniques (UV-vis, FTIR, NEXAFS, XPS) was used to study the impact of exposure to light and air (photo-oxidation) on the photoactive layer and its components. We focused on the electron acceptor components: the fullerene derivatives, PC60BM and PC70BM, and the polymer N2200. A comparative study of photo-oxidized PC60BM and PC70BM thin films by UV-vis and FTIR spectroscopy has shown that both materials undergo similar photochemical transformation, with the process being faster in PC60BM, due to the greater curvature of the C60 cage. Comparing experimental FTIR, XPS and NEXAFS spectra of the photo-oxidized PC60BM thin films with the calculated spectra for a large variety of photo-oxidation products, it was found that dicarbonyl and anhydride groups attach to the C60 cage during photo-oxidation. The study of photo-oxidized TQ1:PC70BM blend films by spectroscopic and J-V measurements shows that deterioration of the charge transport in PC70BM is the major contributor to the device performance degradation. Kelvin Probe measurements demonstrated that the charge transport deterioration was due to upward band bending and gap states being formed on the surface of photo-oxidized PC70BM. The TQ1:PC70BM blends films were further studied by AFM-IR in order to determine the lateral distribution of pristine components, as well as the photo-oxidation products. It was found that anhydride oxidation products of PC70BM are equally distributed over the blend film surface. The PC70BM is replaced with the polymer N2200 in the blend with TQ1. The photostability in air of the blend and its neat components was studied by UV-vis and FTIR spectroscopy. The spectra show that thermal annealing improves the photostability in air of both components.

Abstract [en]

Increase of the global energy demand and the climate change are two factors motivating the study and use of renewable energy sources, such as the solar energy. Organic photovoltaics (OPV) is a technology that uses organic molecules to convert solar energy into electricity. These organic molecules can be kept in ink form, allowing OPV device manufacture via coating, and ultimately roll-to-roll printing techniques, resulting in inexpensive, light weight, portable, and mechanically flexible sources of electricity. OPV devices have reached over 15% in power conversion efficiency, but their operational lifetime has to increase.

In this work, the photostability of the active layer in organic solar cells and its molecular components was studied by a variety of spectroscopy, microscopy and electrical characterization techniques, with focus on the chemical changes that these materials undergo during exposure to light and air. The aim was to determine the relation between materials’ degradation and the device performance degradation.

Place, publisher, year, edition, pages
Karlstads universitet, 2019. p. 59
Series
Karlstad University Studies, ISSN 1403-8099 ; 2019:30
Keywords
photovoltaics, polymer solar cell, conjugated polymers, fullerene, photo-oxidation, spectroscopy
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-75093 (URN)978-91-7867-054-3 (ISBN)978-91-7867-064-2 (ISBN)
Public defence
2019-11-29, 21A 342 Eva Erikssonsalen, 13:15 (English)
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Supervisors
Available from: 2019-11-05 Created: 2019-10-08 Last updated: 2019-11-05Bibliographically approved

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Ericsson, LeifBlazinic, VanjaHansson, RickardMoons, Ellen

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