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Thin Film Concepts for Photon Addition Materials
S. Ivanova, F. PellĂ©, R. Esteban, M. Laroche, J.J. Greffet, S. Collin, J.L. Pelouard, J.F. Guillemoles
Upconversion, Rare Earth
Advanced Photovoltaics
Subtopic: New Types of Cells and Modules
Event: 23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain
Session: 1DV.2.56
734 - 736
ISBN: 3-936338-24-8
Paper DOI: 10.4229/23rdEUPVSEC2008-1DV.2.56
0,00 EUR
Document(s): paper


Even in the best state of the art solar cells, a large fraction of the solar energy is left untouched: that of sub-bandgap radiation. It is the aim of the present work to tap this resource by using photon addition (also called upconversion in the literature). In a photon addition type system, a large gap PV cell absorbs the high-energy photons of the solar spectrum whereas the IR photons are converted into higher energy photons and returned to the cell, e.g. using a simple mirror [1,2]. In principle, for an optimized solar cell, the gain can be quite large (up to 63% conversion efficiency under maximal concentration of 6000K black body radiation [3]). This process can be demonstrated using low phonon energy solid state materials (e.g., crystals, glasses or glassceramics) doped with rare earth ions [1,2]: the latter absorb part of the infra red radiation and convert it via internal energy transfer and luminescence in higher energy photons. Fluoride based crystals and vitroceramics, doped with Er and Yb, demonstrating efficient IR to VIS conversion, have been synthesized, and their conversion efficiency has been measured. Efficiencies (i.e., ratio of emitted visible light energy to absorbed IR energy) up to 5% have been obtained. In spite of the fact that the up-conversion processes in rare earth doped materials are extensively studied up to now, there exist a very few publications reporting on the efficiency measurements. Due to the inherent nonlinearity of up-conversion process, such measurements become a rather sophisticated procedure. Characterization of synthesized materials will be presented along with their conversion efficiency measurement. The setup and method will be presented in details. The studied materials demonstrate upconversion efficiencies among the highest published so far. Issue, on which progress has been made in this work, concerns the relatively low absorption coefficient of the lanthanides, which lead to rather low overall efficiency. We show by simulation that, by using special resonant structures, absorptivity of lanthanide doped material could be considerably enhanced to the point that the fabrication of thin film (sub-micrometer) up-converters can be considered.