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Optical modeling of a folded organic solar cell
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology. (BiOrgel)
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics . Linköping University, The Institute of Technology. (BiOrgel)
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics . Linköping University, The Institute of Technology. (BiOrgel)
2008 (English)In: Journal of Applied Physics, ISSN 0021-8979, 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.

Place, publisher, year, edition, pages
2008. Vol. 103, no 9, 094520- p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-17205DOI: 10.1063/1.2917062OAI: oai:DiVA.org:liu-17205DiVA: diva2:207342
Available from: 2009-03-10 Created: 2009-03-10 Last updated: 2011-12-13Bibliographically approved
In thesis
1. Light Trapping and Alternative Electrodes for Organic Photovoltaic Devices
Open this publication in new window or tab >>Light Trapping and Alternative Electrodes for Organic Photovoltaic Devices
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic materials, such as conjugated polymers, have emerged as a promising alternative for the production of inexpensive and flexible photovoltaic cells. As conjugated polymers are soluble, liquid based printing techniques enable production on large scale to a price much lower than that for inorganic based solar cells. Present day state of the art conjugated polymer photovoltaic cells are comprised by blends of a semiconducting polymer and a soluble derivative of fullerene molecules. Such bulk heterojunction solar cells now show power conversion efficiencies of up to 4-6%. The quantum efficiency of thin film organic solar cells is however still limited by several processes, of which the most prominent limitations are the comparatively low mobility and the high level of charge recombination. Hence organic cells do not yet perform as well as their more expensive inorganic counterparts. In order to overcome this present drawback of conjugated polymer photovoltaics, efforts are continuously devoted to developing materials or devices with increased absorption or with better charge carrier transporting properties. The latter can be facilitated by increasing the mobility of the pure material or by introducing beneficial morphology to prevent carrier recombination. Minimizing the active layer film thickness is an alternative route to collect more of the generated free charge carriers. However, a minimum film thickness is always required for sufficient photon absorption.

A further limitation for low cost large scale production has been the dependence on expensive transparent electrodes such as indium tin oxide. The development of cheaper electrodes compatible with fast processing is therefore of high importance.

The primary aim of this work has been to increase the absorption in solar cells made from thin films of organic materials. Device construction, deploying new geometries, and evaluation of different methods to provide for light trapping and photon recycling have been strived for. Different routes to construct and incorporate light trapping structures that enable higher photon absorption in a thinner film are presented. By recycling the reflected photons and enhancing the optical path length within a thinner cell, the absorption rate, as well as the collection of more charge carriers, is provided for. Attempts have been performed by utilizing a range of different structures with feature sizes ranging from nanometers up to centimeters. Surface plasmons, Lambertian scatterers, micro lenses, tandem cells as well as larger folded cell structures have been evaluated. Naturally, some of these methods have turned out to be more successful than others. From this work it can nevertheless be concluded that proper light trapping, in thin films of organic materials for photovoltaic energy conversion, is a technique capable of improving the cell performance.

In addition to the study of light trapping, two new alternative electrodes for polymer photovoltaic devices are suggested and evaluated.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2007. 59 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1174
Keyword
Light trapping, Organic solar cells, Tandem cells, Polymer solar cells, Plastsolceller
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17229 (URN)978-91-7393-924-9 (ISBN)
Public defence
2008-04-18, Sal Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2009-03-26 Created: 2009-03-11 Last updated: 2009-05-15Bibliographically approved
2. Electron tomography and optical modelling for organic solar cells
Open this publication in new window or tab >>Electron tomography and optical modelling for organic solar cells
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
Organic solar cells, Electron tomography, Optical modelling
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-72903 (URN)978-91-7393-007-9 (ISBN)
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

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Andersson, ViktorTvingstedt, KristoferInganäs, Olle

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