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Thin films of polyfluorene:fullerene blends - Morphology and its role in solar cell performance
Karlstad University, Faculty of Technology and Science.
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 [en]
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: urn:nbn:se:kau:diva-1243ISBN: 978-91-7063-147-4 (print)OAI: oai:DiVA.org:kau-1243DiVA, id: diva2:5085
Public defence
2007-12-07, 1B 364 (Frödingsalen), Karlstads universitet, Karlstad, 13:15
Opponent
Supervisors
Available from: 2007-11-20 Created: 2007-11-20
List of papers
1. Control of phase separation in blends of polyfluorene (co)polymers and the C60-derivative PCBM
Open this publication in new window or tab >>Control of phase separation in blends of polyfluorene (co)polymers and the C60-derivative PCBM
2005 (English)In: Synthetic metals, ISSN 0379-6779, E-ISSN 1879-3290, Vol. 152, no 1-3, p. 109-112Article in journal (Refereed) Published
Abstract [en]

When creating thin films of polymer blends, interesting morphologies are formed because of phaseseparation. In particular for conjugated polymers, which are used as active material in optoelectronic devices, it is very important to understand the parameters that influence the phaseseparation process and to achieve control over the morphology. The overall goal of this blend morphology study is to contribute to the design of device structures with desired performance.

Here we present results of morphology studies on thin films of polyfluorene-based blends with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The polymers used are poly(9,9-dioctylfluorene) (F8) and four different copolymers of F8. The thin films are spin coated from chloroform solutions onto silicon substrates and their surface morphology is imaged by tapping mode atomic force microscopy (AFM). We observe that the size and the shape of the domains in the film depend on the structure of the polymer. The nature of the monomer that, together with F8, is building the repeating unit in the copolymers has a strong effect on the phaseseparation in the polymer: PCBMblend. Since phaseseparation is influenced by interactions between components of the blend and the solvent, these results indicate that the degree of chemical interaction between polymer, solvent and PCBM, is different for the different blends. For the systems that form larger domains there is a clear correlation between the domain size (area) and the polymer/PCBMblend ratio. We also observe that the spin speed affects the thickness of the films and that the domain size increases with increasing thickness, primarily due to longer drying times

Place, publisher, year, edition, pages
Elsevier, 2005
Keywords
Atomic force microscopy; Spin coating; Morphology; Conjugated polymers; Blend
National Category
Physical Sciences
Identifiers
urn:nbn:se:kau:diva-1934 (URN)10.1016/j.synthmet.2005.07.195 (DOI)
Conference
Proceedings of the International Conference on Science and Technology of Synthetic Metals: Wollongong, Australia, June 28 - July 2, 2004
Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-07Bibliographically approved
2. Multilayer formation in spin-coated thin films of low-bandgap polyfluorene:PCBM blends
Open this publication in new window or tab >>Multilayer formation in spin-coated thin films of low-bandgap polyfluorene:PCBM blends
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2005 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 17, no 50, p. L529-L534Article in journal (Refereed) Published
Abstract [en]

Blends of the low-bandgap polymer 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) were spin-coated from chloroform solution into thin films, which were examined with dynamic secondary ion mass spectrometry. For blends with high PCBM content, the depth profiles show composition waves that were caused by surface-directed phase separation during spin-coating. The formation of such multilayer structures by spontaneous self-stratification is likely to have implications for optimization strategies for the performance of organic solar cells

Place, publisher, year, edition, pages
Philadelphia: Institute of Physics Publishing (IOPP), 2005
Keywords
Soft matter, liquids and polymers, Condensed matter: electrical, magnetic and optical, Surfaces, interfaces and thin films, Nanoscale science and low-D systems
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-1935 (URN)10.1088/0953-8984/17/50/L01 (DOI)
Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-07Bibliographically approved
3. Vertical phase separation in spin-coated films of a low bandgap polyfluorene/PCBM blend: Effects of specific substrate interaction
Open this publication in new window or tab >>Vertical phase separation in spin-coated films of a low bandgap polyfluorene/PCBM blend: Effects of specific substrate interaction
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2007 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 253, no 8, p. 3906-3912Article in journal (Refereed) Published
Abstract [en]

We report on the effect of the substrate on the vertical phase separation in spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-40,70-di-2-thienyl-20,10,30-benzothiadiazole] (APFO-3) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Compositional depth profiles of the films are measured by dynamic secondary ion mass spectrometry (SIMS).We found that changing the substrate from silicon to gold affects the composition profile near the substrate interface. This is caused by a specific interaction between the polymer (APFO-3) and the gold surface, as confirmed by X-ray photoelectron spectroscopy (XPS). The composition profile in the area away from the substrate interface, as well as the enrichment of the free surface with APFO-3, remain however unaffected by the choice of substrate. The vertical composition was also analysed for APFO-3:PCBM films spin-coated on indium tin oxide (ITO) coated with a thin layer of (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2007
Keywords
Conjugated polymer; X-ray photoelectron spectroscopy (XPS); Secondary ion mass spectrometry (SIMS); Spinodal decomposition; Adsorption; Thiophene
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-1936 (URN)10.1016/j.apsusc.2006.08.024 (DOI)
Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-07Bibliographically approved
4. Influence of solvents and substrates on the morphology and the performance of low-bandgap polyfluorene:PCBM photovoltaic devices
Open this publication in new window or tab >>Influence of solvents and substrates on the morphology and the performance of low-bandgap polyfluorene:PCBM photovoltaic devices
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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
Keywords
Atomic force microscopy ; Coating ; Gold ; Interfaces ; Mass spectrometry ; Multilayers ; Photodiodes ; Polymers ; Roads ; Silicon
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-1937 (URN)10.1117/12.662857 (DOI)
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
5. Device Performance of APFO-3/PCBM Solar Cells with controlled morphology
Open this publication in new window or tab >>Device Performance of APFO-3/PCBM Solar Cells with controlled morphology
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(English)Manuscript (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kau:diva-1938 (URN)
Note

Older manuscript version of published paper Björström Svanström, C. M., et al., Device performance of APFO-3/PCBM solar cells with controlled morphology, Adv. Mater. 21, 43 (2009), p.4398-4403. This version features in C.M. Björströms Ph.D. thesis.

Available from: 2007-11-20 Created: 2007-11-20 Last updated: 2017-12-06Bibliographically approved

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