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Photodegradation in air of the active layer components in a thiophene-quinoxaline copolymer:fullerene solar cell
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Interact)
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics.
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics.
Humboldt univ. .
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 16, 11132-11138 p.Article 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.

Place, publisher, year, edition, pages
2016. Vol. 18, no 16, 11132-11138 p.
Keyword [en]
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: urn:nbn:se:kau:diva-37835DOI: 10.1039/c5cp07752dISI: 000374786300053OAI: oai:DiVA.org:kau-37835DiVA: diva2:851270
Available from: 2015-09-04 Created: 2015-09-03 Last updated: 2017-02-16Bibliographically approved
In thesis
1. Morphology and material stability in polymer solar cells
Open this publication in new window or tab >>Morphology and material stability in polymer solar cells
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer solar cells are promising in that they are inexpensive to produce, and due to their mechanical flexibility have the potential for use in applications not possible for more traditional types of solar cells. The performance of polymer solar cells depends strongly on the distribution of electron donor and acceptor material in the active layer. Understanding the connection between morphology and performance as well as how to control the morphology, is therefore of great importance. Furthermore, improving the lifetime of polymer solar cells has become at least as important as improving the efficiency.

 

In this thesis, the relation between morphology and solar cell performance is studied, and the material stability for blend films of the thiophene-quinoxaline copolymer TQ1 and the fullerene derivatives PCBM and PC70BM. Atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) are used to investigate the lateral morphology, secondary ion mass spectrometry (SIMS) to measure the vertical morphology and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to determine the surface composition. Lateral phase-separated domains are observed whose size is correlated to the solar cell performance, while the observed TQ1 surface enrichment does not affect the performance. Changes to the unoccupied molecular orbitals as a result of illumination in ambient air are observed by NEXAFS spectroscopy for PCBM, but not for TQ1. The NEXAFS spectrum of PCBM in a blend with TQ1 changes more than that of pristine PCBM. Solar cells in which the active layer has been illuminated in air prior to the deposition of the top electrode exhibit greatly reduced electrical performance. The valence band and absorption spectrum of TQ1 is affected by illumination in air, but the effects are not large enough to account for losses in solar cell performance, which are mainly attributed to PCBM degradation at the active layer surface.

Abstract [en]

The performance of polymer solar cells depends strongly on the distribution of electron donor and acceptor material in the active layer. Understanding the connection between morphology and performance as well as how to control the morphology, is therefore of great importance. Furthermore, improving the lifetime has become at least as important as improving the efficiency for polymer solar cells to become a viable technology.

 

In this work, the relation between morphology and solar cell performance is studied as well as the material stability for polymer:fullerene blend films. A combination of microscopic and spectroscopic methods is used to investigate the lateral and vertical morphology as well as the surface composition. Lateral phase-separated domains are observed whose size is correlated to the solar cell performance, while the observed surface enrichment of polymer does not affect the performance. Changes to the unoccupied molecular states as a result of illumination in ambient air are observed for the fullerene, but not for the polymer, and fullerenes in a blend change more than pristine fullerenes. Solar cells in which the active layer has been illuminated exhibit greatly reduced electrical performance, mainly attributed to fullerene degradation at the active layer surface.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2015. 51 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2015:44
Keyword
polymer solar cell, photovoltaics, morphology, photo-degradation, conjugated polymer, fullerene, synchroton-based techniques
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-37843 (URN)978-91-7063-662-2 (ISBN)
Presentation
2015-10-16, Eva Erikssonsalen, 21A 342, Karlstad universitet, 10:15 (English)
Opponent
Supervisors
Note

Paper 2 ingick som manuskript i avhandlingen. Nu publicerad. 

Available from: 2015-09-25 Created: 2015-09-04 Last updated: 2016-06-03Bibliographically approved
2. Materials and Device Engineering for Efficient and Stable Polymer Solar Cells
Open this publication in new window or tab >>Materials and Device Engineering for Efficient and Stable Polymer Solar Cells
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. 75 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2017:2
Keyword
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:nbn:se:kau:diva-47257 (URN)978-91-7063-736-0 (ISBN)978-91-7063-739-1 (ISBN)
Public defence
2017-02-03, 21A342, Karlstads universitet, Karlstad, 13:15 (English)
Opponent
Supervisors
Available from: 2017-01-13 Created: 2016-11-23 Last updated: 2017-01-13Bibliographically approved

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