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Materials and Device Engineering for Efficient and Stable Polymer Solar Cells
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics.ORCID iD: 0000-0002-4745-1074
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer solar cells form a promising technology for converting sunlight into electricity, and have reached record efficiencies over 10% and lifetimes of several years. The performance of polymer solar cells depends strongly on the distribution of electron donor and acceptor materials in the active layer. To achieve longer lifetimes, degradation processes in the materials have to be understood. In this thesis, a set of complementary spectroscopy and microscopy techniques, among which soft X-ray techniques have been used to determine the morphology of polymer:fullerene based active layers. We have found that the morphology of TQ1:PC70BM films is strongly influenced by the processing solvent and the use of solvent additives. We have also found, by using soft X-ray techniques, that not only the light-absorbing polymer TQ1, but also the fullerene is susceptible to photo-degradation in air. Moreover, the fullerene degradation is accelerated in the presence of the polymer. Additionally, this thesis addresses the role of the interfacial layers for device performance and stability. The commonly used hole transport material PEDOT:PSS has the advantage of being solution processable at room temperature, but this layer is also known to contribute to the device degradation. We have found that low-temperature processed NiOx is a promising alternative to PEDOT:PSS, leading to improved device performance. Even for encapsulated polymer solar cells, some photo-induced degradation of the electrical performance is observed and is found to depend on the nature of the hole transport material. We found a better initial stability for solar cells with MoO3 hole transport layers than with PEDOT:PSS. In the pursuit of understanding the initial decrease in electrical performance of PEDOT:PSS-based devices, simulations were performed, from which a number of degradation sources could be excluded.

Abstract [en]

With the increasing global demand for energy, solar cells provide a clean method for converting the abundant sunlight to electricity. Polymer solar cells can be made from a large variety of light-harvesting and electrically conducting molecules and are inexpensive to produce. They have additional advantages, like their mechanical flexibility and low weight, which opens opportunities for novel applications. In order for polymer solar cells to be more competitive, however, both the power conversion efficiencies and lifetimes need to further improve. One way to achieve this is to optimize the morphology of the active layer. The active layer of a polymer solar cell consists of electron donating and electron accepting molecules whose distribution in the bulk of the film is a major factor that determines the solar cell performance.

This thesis presents the use of complementary spectroscopy and microscopy methods to probe the local composition in the active layer of polymer solar cells. The stability of the active layer is studied and the interplay between the photo-degradation of the donor and acceptor molecules is investigated. Additionally, this thesis addresses how the interfacial layers between the active layer and the electrodes can influence device performance and stability.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2017. , p. 75
Series
Karlstad University Studies, ISSN 1403-8099 ; 2017:2
Keywords [en]
polymer solar cell, photovoltaics, morphology, photo-degradation, conjugated polymer, fullerene, synchroton-based techniques, hole transport layers
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:kau:diva-47257ISBN: 978-91-7063-736-0 (print)ISBN: 978-91-7063-739-1 (electronic)OAI: oai:DiVA.org:kau-47257DiVA, id: diva2:1049534
Public defence
2017-02-03, 21A342, Karlstads universitet, Karlstad, 13:15 (English)
Opponent
Supervisors
Note

I publikationen felaktigt ISBN 978-91-7063-739-1

Available from: 2017-01-13 Created: 2016-11-23 Last updated: 2018-06-20Bibliographically approved
List of papers
1. Vertical and lateral morphology effects on solar cell performance for a thiophene–quinoxaline copolymer:PC70BM blend
Open this publication in new window or tab >>Vertical and lateral morphology effects on solar cell performance for a thiophene–quinoxaline copolymer:PC70BM blend
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2015 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, p. 6970-6979Article in journal (Refereed) Published
Keywords
NEXAFS, SIMS, solar cell, organic photovoltaics, solvent additives
National Category
Condensed Matter Physics
Research subject
Physics; Materials Science
Identifiers
urn:nbn:se:kau:diva-35418 (URN)10.1039/c5ta00683j (DOI)000351552300034 ()
Funder
Swedish Research Council, 2010-4155Göran Gustafsson Foundation for Research in Natural Sciences and Medicine
Available from: 2015-03-22 Created: 2015-03-22 Last updated: 2018-06-20Bibliographically approved
2. Photodegradation in air of the active layer components in a thiophene-quinoxaline copolymer:fullerene solar cell
Open this publication in new window or tab >>Photodegradation in air of the active layer components in a thiophene-quinoxaline copolymer:fullerene solar cell
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 16, p. 11132-11138Article in journal (Refereed) Published
Abstract [en]

We have studied the photo-degradation in air of a blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), and how the photo-degradation affects the solar cell performance. Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, changes to the electronic structure of TQ1 and PCBM caused by illumination in ambient air are investigated and compared between the pristine materials and the blend. The NEXAFS spectra show that the unoccupied molecular orbitals of TQ1 are not significantly changed by the exposure of pristine TQ1 to light in air, whereas those of PCBM are severely affected as a result of photo-induced degradation of PCBM. Furthermore, the photo-degradation of PCBM is accelerated by blending it with TQ1. While the NEXAFS spectrum of TQ1 remains unchanged upon illumination in air, its valence band spectrum shows that the occupied molecular orbitals are weakly affected. Yet, UV-Vis absorption spectra demonstrate photo-bleaching of TQ1, which is attenuated in the presence of PCBM in blend films. Illumination of the active layer of TQ1: PCBM solar cells prior to cathode deposition causes severe losses in electrical performance.

Keywords
polymer solar cells, photodegradation, fullerene, near-edge X-ray absorption fine structure
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-37835 (URN)10.1039/c5cp07752d (DOI)000374786300053 ()
Available from: 2015-09-04 Created: 2015-09-03 Last updated: 2018-06-20Bibliographically approved
3. Low temperature processed NiOx hole transport layers for efficient polymer solar cells
Open this publication in new window or tab >>Low temperature processed NiOx hole transport layers for efficient polymer solar cells
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2017 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 44, p. 59-66Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
OPV Nickel oxide Hole injection layer UV-ozone Kelvin probe XPS
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-47258 (URN)10.1016/j.orgel.2017.01.040 (DOI)000397441800009 ()
Funder
Swedish Energy Agency, 38327-2Swedish Research Council, 2015-03778Göran Gustafsson Foundation for Research in Natural Sciences and Medicine
Available from: 2016-11-23 Created: 2016-11-23 Last updated: 2017-12-14Bibliographically approved
4. Opportunities and challenges in probing local composition of organic material blends for photovoltaics
Open this publication in new window or tab >>Opportunities and challenges in probing local composition of organic material blends for photovoltaics
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2017 (English)In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 32, no 10, p. 1982-1992Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Materials Research Society, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kau:diva-47259 (URN)10.1557/jmr.2017.7 (DOI)000402284600014 ()
Funder
Swedish Energy Agency, 38327-1Swedish Research Council, 2015-03778Göran Gustafsson Foundation for Research in Natural Sciences and Medicine
Available from: 2016-11-23 Created: 2016-11-23 Last updated: 2018-01-05Bibliographically approved
5. The role of the hole transport layer in the initial photo-degradation of PCDTBT: PC70BM solar cells
Open this publication in new window or tab >>The role of the hole transport layer in the initial photo-degradation of PCDTBT: PC70BM solar cells
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-47260 (URN)
Funder
Swedish Energy Agency, 38327-1Swedish Research Council, 2015-03778Göran Gustafsson Foundation for Research in Natural Sciences and Medicine
Available from: 2016-11-23 Created: 2016-11-23 Last updated: 2018-06-27Bibliographically approved

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