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Tuning the Vertical Phase Separation in Polyfluorene:Fullerene Blend Films by Polymer Functionalization
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science. (Materialfysik)
Department of Chemical and Biological Engineering, Chalmers University of Technology.
Institute of Physics, Jagiellonian University, Poland.
Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Poland.
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2011 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 23, no 9, 2295-2302 p.Article in journal (Refereed) Published
Abstract [en]

Achieving control over the nanomorphology of blend films of the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester, PCBM, with light-absorbing conjugated polymers is an important challenge in the development of efficient solution-processed photovoltaics. Here, three new polyfluorene copolymers are presented, tailored for enhanced miscibility with the fullerene through the introduction of polymer segments with modified side chains, which enhance the polymer’s polar character. The composition of the spincoated polymer:PCBM films is analyzed with dynamic secondary ion mass spectrometry (dSIMS). The dSIMS depth profiles demonstrate compositional variations perpendicular to the surface plane, as a result of vertical phase separation, directed by the substrate. These variations propagate to a higher degree through the film for the polymers with a larger fraction of modified side chains. The surface composition of the films is studied by Near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Quantitative analysis of the NEXAFS spectra through a linear combination fit with the spectra of the pure components yields the surface composition. The resulting blend ratios reveal polymer-enrichment of the film surface for all three blends, which also becomes stronger as the polar character of the polymer increases. Comparison of the NEXAFS spectra collected with two different sampling depths shows that the vertical composition gradient builds up already in the first nanometers underneath the surface of the films. The results obtained with this new series of polymers shed light on the onset of formation of lamellar structures in thin polymer:PCBM films prepared from highly volatile solvents

Place, publisher, year, edition, pages
Washington: ACS Publications , 2011. Vol. 23, no 9, 2295-2302 p.
Keyword [en]
morphology; polymer−fullerene interaction; solar cell
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:kau:diva-7817DOI: 10.1021/cm1021596ISI: 000290063600004OAI: oai:DiVA.org:kau-7817DiVA: diva2:427090
Funder
Swedish Research Council
Available from: 2011-06-27 Created: 2011-06-27 Last updated: 2014-10-28Bibliographically approved
In thesis
1. The morphology of polyfluorene: fullerene blend films for photovoltaic applications
Open this publication in new window or tab >>The morphology of polyfluorene: fullerene blend films for photovoltaic applications
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer photovoltaic systems whose photoactive layer is a blend of a semiconducting polymer with a fullerene derivative in a bulk heterojunction configuration are amongst the most successful organic photovoltaic devices nowadays. The three-dimensional organization in these layers (the morphology) plays a crucial role in the performance of the devices. Detailed characterization of this organization at the nanoscale would provide valuable information for improving future material and architectural design and for device optimization.

In this thesis, the results of morphology studies of blends of several polyfluorene copolymers (APFOs) blended with a fullerene derivative are presented. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was combined with dynamic Secondary Ion Mass Spectrometry (dSIMS) for surface and in-depth characterization of the blend films. NEXAFS was performed using two different electron detection methods, partial (PEY) and total (TEY) electron yield, which provide information from different depth regimes. Quantitative compositional information was obtained by fitting the spectra of the blend films with a linear combination of the spectra of films of the pure components. In blends of APFO3 with PCBM in two different blend ratios (1:1 and 1:4 of polymer:fullerene) NEXAFS data show the existence of compositional gradients in the vertical direction for both blend ratios, with clear polymer enrichment of the free surface. A series of APFOs with systematic changes in the side-chains was studied and it was shown that those small modifications can affect polymer:fullerene interaction and induce vertical phase separation. Polymer-enrichment of the free surface was clearly identified, in accordance with surface energy minimization mechanisms, and a compositional gradient was revealed already in the first few nanometers of the surface of the blend films. dSIMS showed that this vertical phase separation propagates throughout the film. It was possible to determine that as the polar character of the polymer increases, and thus the polymer:fullerene miscibility is improved, the tendency for vertical phase separation becomes stronger.

Place, publisher, year, edition, pages
Karlstad: Karlstad University, 2011. 47 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2011:36
Keyword
Materials Science, Organic Photovoltaics, Polymer Solar Cells
National Category
Materials Engineering
Research subject
Materials Engineering
Identifiers
urn:nbn:se:kau:diva-7950 (URN)978-91-7063-370-6 (ISBN)
Presentation
2011-09-02, 21D 302, Karlstads universitet, Karlstad, 10:15 (Swedish)
Opponent
Note

Paper II was not published at the time of the licentiate defence and had the title: NEXAFS spectroscopy study of the surface composition in APFO3:PCBM blend films

Available from: 2011-08-10 Created: 2011-07-11 Last updated: 2016-08-16Bibliographically approved
2. Materials aspects in spin-coated films for polymer photovoltaics
Open this publication in new window or tab >>Materials aspects in spin-coated films for polymer photovoltaics
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polymer-based photovoltaics have the potential to contribute to boosting photovoltaic energy conversion overall. Besides allowing large-area inexpensive processing, polymeric materials have the added benefit of opening new market applications for photovoltaics due to their low-weight and interesting mechanical properties. The energy conversion efficiency values of polymer photovoltaics have reached new record values over the past years. It is however crucial that stability issues are addressed together with efficiency optimization. Understanding fundamental materials aspects is key in both areas.

In the work presented in this thesis, the morphology of polymer:fullerene films and its influence on device performance was studied, as well as the effect of light exposure on the surface of fullerene films. Several polyfluorene copolymers were used for the morphology studies, where the effects of changing spin-coating solvent and of side chain engineering were investigated with dynamic secondary ion mass spectrometry (dSIMS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Polymer-enriched surfaces were found in all blend films, even in the cases with homogeneous distributions in the bulk. Side chain engineering of the polymer led to gradual changes in the compositional variations perpendicular to the surface, and to slight variations in the photocurrent. The electronic structure of the fullerene derivative PCBM was studied in detail and the spectroscopic fingerprint of the materials was analysed by comparison with theoretically simulated spectra. Photo-stability studies done in air showed that the surface of fullerene films underwent severe damages at the molecular level, which is evident from changes in the valence band and X-ray absorption spectra. These changes were explained by transitions from sp2-type to sp3 hybridization of the carbon atoms in the cage that resulted in the destruction of the fullerene cage.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2013. 88 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2013:3
Keyword
materials science, photovoltaics, conjugated polymer, polymer solar cell, bulk heterojunction, coating, morphology, fullerene, photostability, degradation, X-ray absorption spectroscopy, synchroton-based techniques
National Category
Physical Sciences
Research subject
Materialvetenskap
Identifiers
urn:nbn:se:kau:diva-16107 (URN)978-91-7063-475-8 (ISBN)
Public defence
2013-01-25, 21A342, Karlstad University, SE-651 88, Karlstad, 13:15 (English)
Opponent
Supervisors
Available from: 2013-01-07 Created: 2012-12-07 Last updated: 2013-01-07Bibliographically approved

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Anselmo, Ana SofiaSvensson, Kristervan Stam, JanMoons, Ellen

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