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Sökning: WFRF:(Tvingstedt Kristofer)

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21.
  • Tvingstedt, Kristofer, 1976-, et al. (författare)
  • Light Confinement in Thin Film Organic Photovoltaic cells
  • 2006
  • Ingår i: Photonics Europe, Strasbourg. - : SPIE.
  • Konferensbidrag (refereegranskat)abstract
    • 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.
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22.
  • Tvingstedt, Kristofer, 1976- (författare)
  • Light Trapping and Alternative Electrodes for Organic Photovoltaic Devices
  • 2007
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • 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.
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23.
  • Tvingstedt, Kristofer, et al. (författare)
  • Light trapping with total internal reflection and transparent electrodes in organic photovoltaic devices
  • 2012
  • Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 101:16
  • Tidskriftsartikel (refereegranskat)abstract
    • Herein, we demonstrate a method to build highly efficient light trapping structures for printed organic solar cells and modules, compatible with roll to roll manufacturing. Echelle grating structures in combination with semitransparent electrodes allow for efficient light trapping via means of total internal reflection. With this method, we demonstrate an increased cell photocurrent response up to 24%, compared to a standard cell configuration with a planar reflector. The demonstrated light trapping approach is expected to be even more useful for photovoltaic modules, where light hitting "dead areas" in between the sub-cells comprising the module will now be utilized.
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24.
  • Tvingstedt, Kristofer, et al. (författare)
  • On the Dissociation Efficiency of Charge Transfer Excitons and Frenkel Excitons in Organic Solar Cells: A Luminescence Quenching Study
  • 2010
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society. - 1932-7447 .- 1932-7455. ; 114:49, s. 21824-21832
  • Tidskriftsartikel (refereegranskat)abstract
    • The field dependence of photocurrent found in many organic solar cells is a significant and detrimental setback for internal quantum efficiency. In this work we study the important contribution to this field dependence due to the dissociation efficiency of the weakly bound interfacial charge transfer (CT) state, crucial for organic bulk heterojunction solar cells. Three different donor polymers and two different acceptors are examined, and their respective dissociation characteristics are evaluated by photoluminescence (PL) quenching, both for Frenkel excitons and for the intermolecular charge transfer excitons. We observe that while the field-dependent photocurrent for pure polymers does correlate well with quenching efficiency, the CT exciton quenching from the blend generally displays a less pronounced correlation with extracted photocurrent. We further note that while the electroluminescence and photoluminescence of the pure polymer are identical, we observe a red shift for the blend electroluminescence. This indicates that lower energetic states, not visible in PL, are available in the blend. The emissive state of the blends probed by PL is therefore proposed to originate from sites that are involved in photocurrent generation to a lesser extent.
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25.
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26.
  • Tvingstedt, Kristofer, et al. (författare)
  • Surface plasmon increased absorption in polymer photovoltaic cells
  • 2007
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:11, s. 113514 -
  • Tidskriftsartikel (refereegranskat)abstract
    • 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.
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27.
  • Tvingstedt, Kristofer, et al. (författare)
  • Trapping light with micro lenses in thin film organic photovoltaic cells
  • 2008
  • Ingår i: Optics Express. - 1094-4087. ; 16:26, s. 21608-21615
  • Tidskriftsartikel (refereegranskat)abstract
    • 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%.
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28.
  • Vandewal, Koen, et al. (författare)
  • Charge-Transfer States and Upper Limit of the Open-Circuit Voltage in Polymer: Fullerene Organic Solar Cells
  • 2010
  • Ingår i: IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS. - : Institute of Electrical and Electronics Engineers (IEEE). - 1077-260X. ; 16:6, s. 1676-1684
  • Tidskriftsartikel (refereegranskat)abstract
    • The power conversion efficiency of polymer: fullerene bulk heterojunction solar cells depends on the generated photocurrent and photovoltage. Here we show, using the thermodynamic theory of detailed balance, that the photovoltage in particular is limited by the presence of polymer: fullerene material interaction, resulting in the formation of a weak donor-acceptor charge transfer complex (CTC). Excited CTCs, or charge transfer (CT) states, are visible in highly sensitive measurements of the absorption and photovoltaic action spectrum, or in photoluminescence and electroluminescence measurements. It is shown that photovoltaic and electroluminescent actions of the polymer: fullerene CTC are related by a reciprocity relation. This relation reproduces the measured open-circuit voltage (V-oc) of the photovoltaic device under solar conditions. Also, the temperature and illumination intensity dependence of V-oc is reproduced by the theory. Assuming perfect conditions for charge generation and recombination, a maximum obtainable V-oc value in function of polymer: fullerene CTC properties is derived.
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29.
  • Vandewal, Koen, et al. (författare)
  • Charge Transfer States in Organic Donor-Acceptor Solar Cells
  • 2011
  • Ingår i: Semiconductors and semimetals. - : Academic Press. - 0080-8784. ; 85, s. 261-295
  • Tidskriftsartikel (refereegranskat)abstract
    • For an efficient conversion of photons to electrons by organic materials used for photovoltaic applications, the presence of a material interface between an electron-donating and electron-accepting material is crucial. This chapter deals with the interfacial charge transfer states formed at such interfaces under solar illumination. Absorption of long-wavelength light, with energy lower than the optical gap of both donor and acceptor results in the direct formation of these charge transfer states. Decay of CT states to the ground state will result in weak light emission. Both CT absorption and emission will be linked to photovoltaic performance. The role of the CT state in determining the open-circuit voltage is discussed in detail. We will also elaborate on the efficiency of dissociation and photocurrent generation from thermally relaxed CT states. Based on thermodynamical considerations and in the absence of nonradiative recombination, upper limits for the efficiency of organic solar cells based on donor–acceptor interfaces are derived and possible improvements and future research directions are indicated.
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30.
  • Vandewal, Koen, et al. (författare)
  • On the origin of the open-circuit voltage of polymer-fullerene solar cells
  • 2009
  • Ingår i: NATURE MATERIALS. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 8:11, s. 904-909
  • Tidskriftsartikel (refereegranskat)abstract
    • The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (V-oc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and V-oc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that V-oc is determined by the formation of these states. Our work further suggests alternative pathways to improve V-oc of donor-acceptor devices.
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