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Electron tomography and optical modelling for organic solar cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic solar cells using carbon based materials have the potential to deliver cheap solar electricity. The aim is to be able to produce solar cells with common printing techniques on flexible substrates, and as organic materials can be made soluble in various solvents, they are well adapted to such techniques. There is a large variation of organic materials produced for solar cells, both small molecules and polymers. Alterations of the molecular structure induce changes of the electrical and optical properties, such as band gap, mobility and light absorption. During the development of organic solar cells, the step of mixing of an electron donor and an electron acceptor caused a leap in power conversion efficiency improvement, due to an enhanced exciton dissociation rate. Top performing organic solar cells now exhibit a power conversion efficiency of over 10%. Currently, a mix of a conjugated polymer, or smaller molecule, and a fullerene derivative are commonly used as electron donor and acceptor. Here, the blend morphology plays an important role. Excitons formed in either of the donor or acceptor phase need to diffuse to the vicinity of the donor-acceptor interface to efficiently dissociate. Exciton diffusion lengths in organic materials are usually in the order of 5-10 nm, so the phases should not be much larger than this, for good exciton quenching. These charges must also be extracted, which implies that a network connected to the electrodes is needed. Consequently, a balance of these demands is important for the production of efficient organic solar cells.

Morphology has been found to have a significant impact on the solar cell behaviour and has thus been widely studied. The aim of this work has been to visualize the morphology of active layers of organic solar cells in three dimensions by the use of electron tomography. The technique has been applied to materials consisting of conjugated polymers blended with fullerene derivatives. Though the contrast in these blends is poor, three-dimensional reconstructions have been produced, showing the phase formation in three dimensions at the scale of a few nanometres. Several material systems have been investigated and preparation techniques compared.

Even if excitons are readily dissociated and paths for charge extraction exist, the low charge mobilities of many materials put a limit on film thickness. Although more light could be absorbed by increased film thickness, performance is hampered due to increased charge recombination. A large amount of light is thus reflected and not used for energy conversion. Much work has been put into increasing the light absorption without hampering the solar cell performance. Aside from improved material properties, various light trapping techniques have been studied. The aim is here to increase the optical path length in the active layer, and in this way improve the absorption without enhanced extinction coefficient.

At much larger dimensions, light trapping in solar cells with folded configuration has been studied by the use of optical modelling. An advantage of these V-cells is that two materials with complementing optical properties may be used together to form a tandem solar cell, which may be connected in either serial or parallel configuration, with maintained light trapping feature. In this work optical absorption in V-cells has been modelled and compared to that of planar ones.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. , 63 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1414
Keyword [en]
Organic solar cells, Electron tomography, Optical modelling
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-72903ISBN: 978-91-7393-007-9 (print)OAI: oai:DiVA.org:liu-72903DiVA: diva2:463576
Public defence
2012-02-03, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2011-12-13 Created: 2011-12-09 Last updated: 2012-01-03Bibliographically approved
List of papers
1. Imaging of the 3D Nanostructure of a Polymer Solar Cell by Electron Tomography
Open this publication in new window or tab >>Imaging of the 3D Nanostructure of a Polymer Solar Cell by Electron Tomography
2009 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 9, no 2, 853-855 p.Article in journal (Refereed) Published
Abstract [en]

Electron tomography has been used for analyzing the active layer in a polymer solar cell, a bulk heterojunction of an alternating copolymer of fluorene and a derivative of fullerene. The method supplies a three-dimensional representation of the morphology of the film, where domains with different scattering properties may be distinguished. The reconstruction shows good contrast between the two phases included in the film and demonstrates that electron tomography is an adequate tool for investigations of the three-dimensional nanostructure of the amorphous materials used in polymer solar cells.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16961 (URN)10.1021/nl803676e (DOI)
Available from: 2009-02-28 Created: 2009-02-27 Last updated: 2017-12-13
2. Nanomorphology of Bulk Heterojunction Organic Solar Cells in 2D and 3D Correlated to Photovoltaic Performance
Open this publication in new window or tab >>Nanomorphology of Bulk Heterojunction Organic Solar Cells in 2D and 3D Correlated to Photovoltaic Performance
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2009 (English)In: Macromolecules, ISSN 0024-9297, E-ISSN 1520-5835, Vol. 42, no 13, 4646-4650 p.Article in journal (Refereed) Published
Abstract [en]

Control of the nanoscale morphology of the donor-acceptor material blends inorganic solar Cells is critical for optimizing the photovoltaic performances. The influence of intrinsic (acceptor materials) and extrinsic (donor:acceptor weight ratio, substrate, solvent) parameters was investigated, by atomic force microscopy (AFM) and electron tomography (ET), on the nanoscale phase separation of blends of a low-band-gap alternating polyfluorene copolymers (APFO-Green9) with [6,6]-phenyl-C-71-butyric acid methyl ester ([70]PCBM). The photovoltaic performances display an optimal efficiency for the device elaborated with a 1:3 APFO-Green polymer:[70][PCBM weight ratio and spin-coated from chloroform solution. The associated active layer morphology presents small phase-separated domains which is a good balance between as a large interfacial donor-acceptor area and Continuous paths of the donor and acceptor phases to the electrodes.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-20133 (URN)10.1021/ma802457v (DOI)
Available from: 2009-08-31 Created: 2009-08-31 Last updated: 2017-12-13Bibliographically approved
3. The Effect of additive on performance and shelf-stability of HSX-1/PCBM photovoltaic devices
Open this publication in new window or tab >>The Effect of additive on performance and shelf-stability of HSX-1/PCBM photovoltaic devices
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2011 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 12, no 9, 1544-1551 p.Article in journal (Refereed) Published
Abstract [en]

How 1,8-diiodooctane (DIO) enhances performance of polymer solar cells based on polymer HXS-1 and fullerene [6,6]-phenyl C(71)-butyric acid methyl ester (PC(71)BM) from 3.6% to 5.4% is scrutinized with several techniques by comparing devices or blend films spin-coated from dichlorobenzene (DCB) to those from DCB/DIO (97.5:2.5 v/v). Morphology of blend films is examined with atomic force microscopy (AFM), transmission electron microscopy (TEM) and electron tomography (3-D TEM), respectively. Charge generation and recombination is studied with photoluminescence, and charge transport with field effect transistors. The morphology with domain size in 10-20 nm and vertical elongated clusters formed in DIO system is supposed to facilitate charge transport and minimize charge carrier recombination, which are the main reasons for enhancing power conversion efficiency (PCE) from 3.6% (without DIO) to 5.4% (with DIO). Furthermore, a two year inspection shows no significant impact of DIO on the shelf-stability of the solar cells. No visible degradation in the second year indicates that the morphology of the active layers in the devices is relatively stable after initial relaxation in the first year.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2011
Keyword
Polymer solar cell; Additive; Morphology; Shelf-stability; Phase separation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-69770 (URN)10.1016/j.orgel.2011.05.028 (DOI)000292685700013 ()
Note

Original Publication: Weiwei Li, Yi Zhou, Viktor Andersson, Mattias Andersson, Yi Thomann, Clemens Veit, Kristofer Tvingstedt, Ruiping Qin, Zhishan Bo, Olle Inganäs, Uli Wuerfel and Fengling Zhang, The Effect of additive on performance and shelf-stability of HSX-1/PCBM photovoltaic devices, 2011, Organic electronics, (12), 9, 1544-1551. http://dx.doi.org/10.1016/j.orgel.2011.05.028 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/

Available from: 2011-08-10 Created: 2011-08-08 Last updated: 2017-12-08Bibliographically approved
4. Lateral Phase Separation Gradients in Spin-Coated Thin Films of High-Performance Polymer: Fullerene Photovoltaic Blends
Open this publication in new window or tab >>Lateral Phase Separation Gradients in Spin-Coated Thin Films of High-Performance Polymer: Fullerene Photovoltaic Blends
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2011 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 21, no 16, 3169-3175 p.Article in journal (Refereed) Published
Abstract [en]

In this study, it is demonstrated that a finer nanostructure produced under a rapid rate of solvent removal significantly improves charge separation in a high-performance polymer: fullerene bulk-heterojunction blend. During spin-coating, variations in solvent evaporation rate give rise to lateral phase separation gradients with the degree of coarseness decreasing away from the center of rotation. As a result, across spin-coated thin films the photocurrent at the first interference maximum varies as much as 25%, which is much larger than any optical effect. This is investigated by combining information on the surface morphology of the active layer imaged by atomic force microscopy, the 3D nanostructure imaged by electron tomography, film formation during the spin coating process imaged by optical interference and photocurrent generation distribution in devices imaged by a scanning light pulse technique. The observation that the nanostructure of organic photovoltaic blends can strongly vary across spin-coated thin films will aid the design of solvent mixtures suitable for high molecular-weight polymers and of coating techniques amenable to large area processing.

Place, publisher, year, edition, pages
Wiley-VCH Verlag Berlin, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70526 (URN)10.1002/adfm.201100566 (DOI)000294166200019 ()
Note

Funding Agencies|Swedish Energy Agency||Spanish Ministerio de Ciencia e Innovacion||

Available from: 2011-09-12 Created: 2011-09-12 Last updated: 2017-12-08Bibliographically approved
5. Morphology of organic electronic materials imaged via electron tomography
Open this publication in new window or tab >>Morphology of organic electronic materials imaged via electron tomography
2012 (English)In: Journal of Microscopy, ISSN 0022-2720, E-ISSN 1365-2818, Vol. 247, no 3, 277-287 p.Article in journal (Refereed) Published
Abstract [en]

Several organic materials and blends have been studied with the use of electron tomography. Tomography reconstructions of active layers of organic solar cells, where various preparation techniques have been used, have been analysed and compared to device behaviour. In addition, materials with predefined structures, including contrast enhancing features, have been studied and double tilt data collection has been employed to improve reconstructions. Small changes in preparation procedures may lead to large differences in morphology and device performance, and the results also indicate a complex relation between these.

Place, publisher, year, edition, pages
Wiley, 2012
Keyword
Electron tomography, morphology, organic electronics, organic solar cell
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-72907 (URN)10.1111/j.1365-2818.2012.03643.x (DOI)000307968600008 ()
Note

On the day of the defence date the status of this article was Manuscript.

Available from: 2011-12-09 Created: 2011-12-09 Last updated: 2017-12-08Bibliographically approved
6. Optical modeling of a folded organic solar cell
Open this publication in new window or tab >>Optical modeling of a folded organic solar cell
2008 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 103, no 9, 094520- p.Article in journal (Refereed) Published
Abstract [en]

The optical behavior of a reflective tandem solar cell (V cell) is modeled by means of finite element method (FEM) simulations. The absorption of solar light in the active material as well as in both electrode layers is calculated. The FEM solves the electromagnetic wave equation on the entire defined geometry, resulting in the consideration of interference effects, as well as effects of refraction and reflection. Both single cells and tandem cells are modeled and confirmed to be in accordance with reflectance measurements. Energy dissipation in the active layers is studied as a function of layer thickness and folding angle, and the simulations clearly display the advantage of the light trapping feature of folded cells. This is especially prominent in cells with thinner active layers, where folding induces absorption in the active layer equivalent to that of much thicker cells.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17205 (URN)10.1063/1.2917062 (DOI)
Available from: 2009-03-10 Created: 2009-03-10 Last updated: 2017-12-13Bibliographically approved
7. Comparative study of organic thin film tandem solar cells in alternative geometries
Open this publication in new window or tab >>Comparative study of organic thin film tandem solar cells in alternative geometries
2008 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 104, no 12, 124508- p.Article in journal (Refereed) Published
Abstract [en]

Optical modeling of one folded tandem solar cell and four types of stacked tandem solar cells has been performed using the finite element method and the transfer matrix method for the folded cell and the stacked cells, respectively. The results are analyzed by comparing upper limits for short circuit currents and power conversion efficiencies. In the case of serial connected tandems all of the five cell types may be compared, and we find that the folded cells are comparable to stacked tandem cells in terms of currents and power conversion efficiencies.

Keyword
finite element analysis, power conversion, solar cells, thin film devices
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16513 (URN)10.1063/1.3050346 (DOI)
Available from: 2009-01-30 Created: 2009-01-30 Last updated: 2017-12-14Bibliographically approved
8. Full day modelling of V-shaped organic solar cell
Open this publication in new window or tab >>Full day modelling of V-shaped organic solar cell
2011 (English)In: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257, Vol. 85, no 6, 1257-1263 p.Article in journal (Refereed) Published
Abstract [en]

Folded and planar solar cells are examined with optical simulations, with the finite element method. The maximum photocurrent densities during the full day are compared between cells of different geometries and tilting angles. The change of incident angle and spectrum over time are handled in this analysis. The results show that the light trapping effect of the folded cell makes these cells show higher maximum photocurrent densities than the planar cells during all hours of the day. This is the case for both single and tandem cells. The results also indicate that balancing the currents in the tandem cells by adjusting the active layer thickness may be more cumbersome with the folded tandem cells than the stacked planar cells.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2011
Keyword
Organic solar cell; Tandem cell; Light trapping
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-69881 (URN)10.1016/j.solener.2010.10.017 (DOI)000291832600012 ()
Available from: 2011-08-09 Created: 2011-08-08 Last updated: 2017-12-08
9. An optical spacer is no panacea for light collection in organic solar cells
Open this publication in new window or tab >>An optical spacer is no panacea for light collection in organic solar cells
2009 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 4, 043302- p.Article in journal (Refereed) Published
Abstract [en]

The role of an optical spacer layer has been examined by optical simulations of organic solar cells with various bandgaps. The simulations have been performed with the transfer matrix method and the finite element method. The results show that no beneficial effect can be expected by adding an optical spacer to a solar cell with an already optimized active layer thickness.

Keyword
energy gap, finite element analysis, organic semiconductors, solar cells
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16885 (URN)10.1063/1.3073710 (DOI)
Available from: 2009-02-22 Created: 2009-02-20 Last updated: 2017-12-13Bibliographically approved

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