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Materials aspects in spin-coated films for polymer photovoltaics
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics.
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 [en]
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: urn:nbn:se:kau:diva-16107ISBN: 978-91-7063-475-8 (print)OAI: oai:DiVA.org:kau-16107DiVA: diva2:575870
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
List of papers
1. Molecular Orientation and Composition at the Surface of Spin-Coated Polyfluorene:Fullerene Blend Films
Open this publication in new window or tab >>Molecular Orientation and Composition at the Surface of Spin-Coated Polyfluorene:Fullerene Blend Films
2013 (English)In: Journal of Polymer Science Part B: Polymer Physics, ISSN 0887-6266, E-ISSN 1099-0488, Vol. 51, no 3, 176-182 p.Article in journal (Refereed) Published
Abstract [en]

The surface composition in spin-coated films of polyfluorene:fullerene blends was determined quantitatively by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. By comparing partial and total electron yield spectra, we found vertical compositional differences in the surface region. Furthermore, the orientation of the polymer chains was investigated by variable-angle NEXAFS. Blend films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole] with [6,6]-phenyl-C61-butyric acid methyl ester in two different blend ratios were studied. Results showed polymer enrichment of the surfaces for films with a polymer:fullerene weight ratio of 20:80 and of 50:50, spin-coated from both chlorobenzene and chloroform solutions. The angular dependence of the NEXAFS spectra of the pure polymer films showed a preferential plane-on orientation, which was slightly stronger in the subsurface region than at the surface. In blend films, this orientational preference was less pronounced and the difference between surface and subsurface vanished

Place, publisher, year, edition, pages
Hoboken, New Jersey: John Wiley & Sons, 2013
Keyword
polymer solar cell, bulk heterojunction, phase separation, morphology, miscibility, near-edge X-ray absorption fine structure
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-16005 (URN)10.1002/polb.23198 (DOI)000312728600002 ()
Available from: 2012-11-30 Created: 2012-11-30 Last updated: 2017-12-06Bibliographically approved
2. 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
Show others...
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: 2017-12-11Bibliographically approved
3. Polyfluorene copolymers with functionalized side chains: Opto-electronic properties and solar cell performance
Open this publication in new window or tab >>Polyfluorene copolymers with functionalized side chains: Opto-electronic properties and solar cell performance
Show others...
2012 (English)Manuscript (preprint) (Other academic)
Keyword
conjugated polymers, photovoltaics, bulk heterojunction, acceptor-donor interaction, HOMO/LUMO levels, I-V characteristics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kau:diva-16108 (URN)
Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2017-12-06Bibliographically approved
4. Near-edge X-ray Absorption Fine Structure Study of the C60-derivative PCBM
Open this publication in new window or tab >>Near-edge X-ray Absorption Fine Structure Study of the C60-derivative PCBM
Show others...
2013 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 568-569, 130-134 p.Article in journal (Refereed) Published
Abstract [en]

The fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester plays a key role for electron transport in polymer solar cells. We have studied the unoccupied molecular orbitals of PCBM by near edge X-ray absorption fine structure spectroscopy and were able to assign the main resonances to molecular moieties by comparison with calculated sum spectra of individual carbons. We analyzed specifically the origin of the high-energy shoulder to the first π-resonance and identified contributions from the lowest-energy transition of a specific carbon in the phenyl and from transitions to higher unoccupied orbitals of the unmodified carbons in the C60-cage.

Place, publisher, year, edition, pages
Amsterdam: , 2013
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-16110 (URN)10.1016/j.cplett.2013.03.031 (DOI)000318320300025 ()
Note

(included in PhD thesis: Material aspects in spin-coated films for polymer photovoltaics, Ana Sofia Anselmo)

Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2017-12-06Bibliographically approved
5. Photodegradation of the electronic structure of PCBM and C60 films in air
Open this publication in new window or tab >>Photodegradation of the electronic structure of PCBM and C60 films in air
2016 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 652, 220-224 p.Article in journal (Refereed) Published
Abstract [en]

Fullerenes are common electron acceptors in organic solar cells. Here the photostability in air of the electronic structures of spin-coated PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and evaporated C60 films are studied using ultraviolet photoelectron spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. After exposing these materials in air to simulated sunlight, the filled and empty molecular orbitals are strongly altered, indicating that the conjugated π-system of the C60-cage has degraded. Even a few minutes in normal lab light induces changes. These results stress the importance of protecting fullerene-based films from light and air during processing, operation, and storage.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Degradation; Fullerene; Photostability; Photovoltaics; NEXAFS
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-16111 (URN)10.1016/j.cplett.2016.04.003 (DOI)000377204100042 ()
Note

The article was still in manuscript form in anna Anselmos thesis. It had then the title: Light-induced modification of the electronic structure of PCBM and C60 films

Available from: 2012-12-07 Created: 2012-12-07 Last updated: 2017-12-06Bibliographically approved

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