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Influence of solvents and substrates on the morphology and the performance of low-bandgap polyfluorene:PCBM photovoltaic devices
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. (Materialfysik)
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science. (Fysik)
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. (Materialvetenskap)ORCID iD: 0000-0002-1609-8909
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2006 (English)In: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, Vol. 6192, p. 61921X-Article in journal (Refereed) Published
Abstract [en]

Spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are used as the active material in polymer photovoltaic cells. Such blends are known for their tendency to phase separate during film formation. Tuning the morphology of the blend in a controlled way is one possible road towards higher efficiency. We studied the effect of adding chlorobenzene to chloroform-based blend solutions before spin-coating on the conversion efficiency of APFO-3:PCBM photodiodes, and related that to the lateral and vertical morphology of thin films of the blend. The lateral morphology is imaged by atomic force microscopy (AFM) and the vertical compositional profile is obtained by dynamic secondary ion mass spectrometry (SIMS). The profiles reveal compositional variations consisting of multilayers of alternating polymer-rich and PCBM-rich domains in the blend film spin-coated from chloroform. The vertical compositional variations are caused by surface-directed spinodal waves and are frozen in during the rapid evaporation of a highly volatile solvent. With addition of the low-vapour pressure solvent chlorobenzene, a more homogeneous vertical composition is found. The conversion efficiency for solar cells of this blend was found to be optimal for chloroform:chlorobenzene mixtures with a volume-ratio of 80:1. We have also investigated the role of the substrate on the morphology. We found that blend films spin-coated from chloroform solutions on PEDOT:PSS-coated ITO show a similar compositional structure as the films on silicon, and that changing the substrate from silicon to gold only affects the vertical phase separation in a region close to the substrate interface

Place, publisher, year, edition, pages
Cardiff: SPIE - International Society for Optical Engineering, 2006. Vol. 6192, p. 61921X-
Keywords [en]
Atomic force microscopy ; Coating ; Gold ; Interfaces ; Mass spectrometry ; Multilayers ; Photodiodes ; Polymers ; Roads ; Silicon
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:kau:diva-1937DOI: 10.1117/12.662857OAI: oai:DiVA.org:kau-1937DiVA, id: diva2:5083
Conference
Conference on Organic Optoelectronics and Photonics II, Photonics Europe, Apr. 03-06, 2006, Strasbourg, France.
Note

editors: Heremans, P.L., Muccini, M. and Meulenkamp, E.A. 

Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Thin films of polyfluorene:fullerene blends - Morphology and its role in solar cell performance
Open this publication in new window or tab >>Thin films of polyfluorene:fullerene blends - Morphology and its role in solar cell performance
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The sun provides us daily with large quantities of energy in the form of light. With the world’s increasing demand of electrical energy the prospect of converting this solar light into electricity is highly tempting. In the strive towards mass-production and low cost solar cells, new types of solar cells are being developed, e.g. solar cells completely based on organic molecules and polymers. These materials offer a promising potential of low cost and large scale manufacturing and have the additional advantage that they can be produced on flexible and light weight substrate which opens for new and innovating application areas, e.g. integration with paper or textiles, or as building materials. In polymer solar cells a combination of two materials are used, an electron donor and an electron acceptor. The three dimensional distribution of the donor and acceptor in the active layer of the device, i.e. the morphology, is known to have larger influence of the solar cell performance. For the optimal morphology there is a trade-off between sometimes conflicting criteria for the various steps of the energy conversion process. The dissociation of photogenerated excitons takes place at an interface between the donor and acceptor materials. Therefore an efficient generation of charges requires a large interface between the two components. However, for charge transport and collection at the electrodes, continuous pathways for the charges to the electrodes are required.

In this thesis, results from morphology studies by atomic force microscopy (AFM) and dynamic secondary ion mass spectrometry (SIMS) of spin-coated blend and bilayer thin films of polyfluorene co-polymers, especially poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4´,7´-di-2-thienyl-2´,1´,3´-benzothiadiazole)] APFO-3, and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are presented. It is shown that by varying the blend ratio, the spin.-coating solvent, and/or the substrate, different morphologies can be obtained, e.g. diffuse bilayer structures, spontaneously formed multilayer structures and homogeneous blends. The connection between these different morphologies and the performance of solar cells is also analysed. The results indicate that nano-scale engineering of the morphology in the active layer may be an important factor in the optimization of the performance of polymer solar cells.

Place, publisher, year, edition, pages
Fakulteten för teknik- och naturvetenskap, 2007
Series
Karlstad University Studies, ISSN 1403-8099 ; 2007:43
Keywords
Polymer Solar Cells, Polymer Blends, Morphology, Polyfluorene, PCBM, AFM, SIMS, Spin-Coating, Surface Directed Spinodal Decomposition
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-1243 (URN)978-91-7063-147-4 (ISBN)
Public defence
2007-12-07, 1B 364 (Frödingsalen), Karlstads universitet, Karlstad, 13:15
Opponent
Supervisors
Available from: 2007-11-20 Created: 2007-11-20

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Magnusson, KjellMoons, Ellen

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