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Light Trapping and Alternative Electrodes for Organic Photovoltaic Devices
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics . Linköping University, The Institute of Technology.
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 [en]
Light trapping, Organic solar cells, Tandem cells, Polymer solar cells
Keyword [sv]
Plastsolceller
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-17229ISBN: 978-91-7393-924-9 (print)OAI: oai:DiVA.org:liu-17229DiVA: diva2:207592
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
List of papers
1. Light Confinement in Thin Film Organic Photovoltaic cells
Open this publication in new window or tab >>Light Confinement in Thin Film Organic Photovoltaic cells
2006 (English)In: Photonics Europe, Strasbourg, 2006, Vol. 6197Conference paper, Published paper (Refereed)
Abstract [en]

Microstructuring of polymer surfaces on optical spacers allows formation of reflective light traps. Such flexible reflectors can be combined with flexible polymer solar cells. We have demonstrated enhanced absorption using Lambertian and regular light reflectors, demonstrated via luminescence from fluorescent layers. Such light traps are suitable to use in combination with polymer solar cells incorporating transparent electrodes. The possibility to enhance the concentration of excited states and photogenerated charges through light trapping also helps to increase charge carrier mobility. These experimental results indicate that light confinement may be an alternative approach for boosting the efficiency of thin film conjugated polymer photovoltaics.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17206 (URN)10.1117/12.662794 (DOI)
Available from: 2009-03-10 Created: 2009-03-10 Last updated: 2009-06-05Bibliographically approved
2. Surface plasmon increased absorption in polymer photovoltaic cells
Open this publication in new window or tab >>Surface plasmon increased absorption in polymer photovoltaic cells
Show others...
2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, no 11, 113514 - p.Article in journal (Refereed) Published
Abstract [en]

The authors demonstrate the triggering of surface plasmons at the interface of a metal grating and a photovoltaic bulk heterojunction blend of alternating polyfluorenes and a fullerene derivative. An increased absorption originating from surface plasmon resonances is confirmed by experimental reflection studies and theoretical modeling. Plasmonic resonances are further confirmed to influence the extracted photocurrent from devices. More current is generated at the wavelength position of the plasmon resonance peak. High conductivity polymer electrodes are used to build inverted sandwich structures with top anode and bottom metal grating, facilitating for triggering and characterization of the surface plasmon effects.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14621 (URN)10.1063/1.2782910 (DOI)
Available from: 2007-08-30 Created: 2007-08-30 Last updated: 2017-12-13Bibliographically approved
3. Trapping light with micro lenses in thin film organic photovoltaic cells
Open this publication in new window or tab >>Trapping light with micro lenses in thin film organic photovoltaic cells
2008 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 16, no 26, 21608-21615 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a novel light trapping configuration based on an array of micro lenses in conjunction with a self aligned array of micro apertures located in a highly reflecting mirror. When locating the light trapping element, that displays strong directional asymmetric transmission, in front of thin film organic photovoltaic cells, an increase in cell absorption is obtained. By recycling reflected photons that otherwise would be lost, thinner films with more beneficial electrical properties can effectively be deployed. The light trapping element enhances the absorption rate of the solar cell and increases the photocurrent by as much as 25%.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-16406 (URN)10.1364/OE.16.021608 (DOI)
Note
On the day of the defence date the status of this article was: In Manuscript.Available from: 2009-01-23 Created: 2009-01-23 Last updated: 2017-12-14Bibliographically approved
4. Folded reflective tandem polymer solar cell doubles efficiency
Open this publication in new window or tab >>Folded reflective tandem polymer solar cell doubles efficiency
2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, no 12, 123514- p.Article in journal (Refereed) Published
Abstract [en]

Conjugated polymers are promising materials for the production of inexpensive and flexible photovoltaic cells. Organic materials display tunable optical absorption within a large spectral range. This enables the construction of organic tandem photovoltaic cells. The authors here demonstrate a reflective tandem cell where single cells are reflecting the nonabsorbed light upon another adjacent cell. By folding two planar but spectrally different cells toward each other, spectral broadening and light trapping are combined to give an enhancement of power conversion efficiency of a factor of 1.8±0.3.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17204 (URN)10.1063/1.2789393 (DOI)
Available from: 2009-03-10 Created: 2009-03-10 Last updated: 2017-12-13Bibliographically approved
5. 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
6. Transparent polymer cathode for organic photovoltaic devices
Open this publication in new window or tab >>Transparent polymer cathode for organic photovoltaic devices
Show others...
2006 (English)In: Synthetic metals, ISSN 0379-6779, E-ISSN 1879-3290, Vol. 156, no 16-17, 1102-1107 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a prototype solar cell with a transparent polymer cathode, and indium-tin-oxide (ITO)/poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT:PSS) anode. As an active layer, thin film of a bulk heterojunction of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2thienyl-2′,1′3′-benzothiadiazole)] (APFO-3) and an electron acceptor molecule [6] and [6]-phenyl-C61-butyric acid methyl ester (PCBM) (1:4 wt.) was sandwiched between the two transparent polymer electrodes. The cathode is another form of PEDOT formed by vapor phase polymerised PEDOT (VPP PEDOT) of conductivity 102–103 S/cm. The cathode is supported on an elastomeric substrate, and forms a conformal contact to the APFO-3/PCBM blend. Transparent solar cells are useful for building multilayer and tandem solar cells.

Keyword
PEDOT; Vapor phase polymerization; VPP–PEDOT; Soft contact lamination; All-plastic solar cell; Ultraviolet photoelectron spectroscopy
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14223 (URN)10.1016/j.synthmet.2006.07.006 (DOI)
Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2017-12-13Bibliographically approved
7. Electrode grids for ITO-free organic photovoltaic devices
Open this publication in new window or tab >>Electrode grids for ITO-free organic photovoltaic devices
2007 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 19, no 19, 2893-2897 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Wiley InterScience, 2007
Keyword
Conjugated polymers, Electrodes, Organic electronics, Photovoltaic devices, Solar cells
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-17203 (URN)10.1002/adma.200602561 (DOI)
Available from: 2009-03-10 Created: 2009-03-10 Last updated: 2017-12-13Bibliographically approved

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Tvingstedt, Kristofer

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