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The morphology of polyfluorene: fullerene blend films for photovoltaic applications
Karlstad University, Faculty of Technology and Science, Department of Mechanical and Materials Engineering.
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 [en]
Materials Science, Organic Photovoltaics, Polymer Solar Cells
National Category
Materials Engineering
Research subject
Materials Engineering
Identifiers
URN: urn:nbn:se:kau:diva-7950ISBN: 978-91-7063-370-6 (print)OAI: oai:DiVA.org:kau-7950DiVA: diva2:430532
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
List of papers
1. Tuning the Vertical Phase Separation in Polyfluorene:Fullerene Blend Films by Polymer Functionalization
Open this publication in new window or tab >>Tuning the Vertical Phase Separation in Polyfluorene:Fullerene Blend Films by Polymer Functionalization
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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
Keyword
morphology; polymer−fullerene interaction; solar cell
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-7817 (URN)10.1021/cm1021596 (DOI)000290063600004 ()
Funder
Swedish Research Council
Available from: 2011-06-27 Created: 2011-06-27 Last updated: 2017-12-06Bibliographically approved
2. Molecular orientation and composition at the surface of APFO3:PCBM blend films
Open this publication in new window or tab >>Molecular orientation and composition at the surface of APFO3:PCBM blend films
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2012 (English)In: Hybrid and Organics Photovoltaics Conference: Uppsala, Sweden, 2012 / [ed] Anders Hagfeldt, SEFIN, Castelló (Spain), 2012, 278- p.Conference paper, Poster (with or without abstract) (Refereed)
Place, publisher, year, edition, pages
SEFIN, Castelló (Spain): , 2012
Series
Nanoenergy Series, Vol. 9
Keyword
polymer, solar cell, photovoltaics
National Category
Materials Chemistry Condensed Matter Physics
Research subject
Materials Science; Physics; Chemistry
Identifiers
urn:nbn:se:kau:diva-29032 (URN)978-84-940189-0-9 (ISBN)
Conference
4th international Conference on Hybrid and Organic Photovoltaics, HOPV12, Uppsala, 6-9 May 2012
Funder
Swedish Research Council, 2010-4155
Available from: 2013-09-13 Created: 2013-09-13 Last updated: 2017-12-06Bibliographically approved

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Anselmo, Ana Sofia

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