THE LUMINESCENT CONCENTRATOR: STABILITY ISSUES L.H. Slooff, 1 T. Budel, 1 A. R. Burgers, 1 N.J. Bakker, 1 A. Büchtemann 2 , R. Danz, 2 T. Meyer, 3 A. Meyer 3 1 Energy research Centre of the Netherlands, P.O.Box 1, 1755 ZG Petten, The Netherlands, tel: +31 224 564314, fax: +31 224 56 8214, email: slooff@ecn.nl 2 Fraunhofer-Institute for Applied Polymer Research, Geiselbergstr.69, D-14476 Golm, Tel.: +49 331/568-1918, Fax: +49 331/568-3910 3 Solaronix SA, Rue de l'Ouriette 129, CH-1170 Aubonne / Switzerland, Tel. +41 21 821 22 80, Fax +41 21 821 22 89 4 Faculty of Science, Organic Chemistry and Catalysis, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands. Fax: +31 30 2523615; Tel: +31 30 2539444 ABSTRACT: One of the major challenges in the research on luminescent concentrators is the lifetime of the luminescent polymer plates. There are some commercial plates available, but data on lifetime are very limited, especially when dedicated to applications like the luminescent concentrator. In this paper we report stability experiments on luminescent concentrator plates, aged under continuous white light illumination, outdoor conditions and high intensity monochromatic illumination. The results show that the lifetime strongly depends on the organic luminescent dye in the plate. The best materials exhibit an initial decrease in performance of about 20% and then remain more or less stable. It is shown that the degradation is not caused by UV illumination. Keywords: stability, luminescent concentrator 1 INTRODUCTION An interesting approach for conversion of light to electricity is the luminescent concentrator (LC). [1,2,3] The device allows concentration of light entering through a large front surface onto a small photo active area. A fundamental difference of the luminescent concentrator compared to other concentrator approaches is that it concentrates diffuse light as well as direct light. Tracking of the sun is not necessary. The motivation for pursuing the luminescent solar concentrator as with any concentrators is that if the receiver can be made much cheaper than solar cells, the LSC might offer a cost benefit. The LC consists of a transparent matrix material, usually a flat plate, with solar cells connected to one or more sides. The transparent matrix contains luminescent particles such as organic dyes or quantum dots that absorb part of the incident solar spectrum. Part of the light emitted by the luminescent particles is guided towards the solar cells by total internal reflection. The LC has an important advantage over geometric concentrators as both direct and diffuse sunlight is collected, making solar tracking unnecessary, making them especially interesting for building integration applications. Early studies on the LC resulted in a power conversion efficiency (η) of 4% for a stack of two LCs connected to GaAs cells[4]. The results were limited due to the low luminescence quantum efficiency of the available organic dyes and a large overlap between absorption and emission, causing re-absorption losses. Since then, organic dyes with improved stability and efficiency have become available. This, together with the availability of semiconductor quantum dots with a small overlap between absorption and emission, opens up the way to LCs with better efficiencies and stabilities. For this reason several groups are currently active in the development of LCs using improved luminescent dyes[5,6], rare-earth complexes[7], and semiconductor quantum dots[8,9]. Apart from the power conversion efficiency, a huge challenge for the LC is the stability. There are some manufacturers of fluorescent polymer sheets that give lifetime numbers. Cyro[10], for example sells fluorescent plates that have weatherability numbers of 3-5 years. The Italian company Madreperla mentions similar numbers in a private communication. These numbers are comparable with the best literature values mentioned in LSC applications.[11,12] The origin of the limited stability is degradation resulting from a combination of light and additives of the polymer matrix as well as remaining monomers in the matrix.[11,13,14]. In this paper we study the stability of LCs under continuous white light illumination, outdoor conditions and high intensity monochromatic illumination. Both the absorption as well as the external quantum efficiency of the LCs is studied as a function of illumination time. 2 EXPERIMENTAL Our luminescent concentrator (LC) consists of a clear PMMA matrix doped with one ore more dyes. The LC plates that are used in this study are made at the Fraunhofer institute for Applied Polymer Research (FhG- IAP) and consist of a transparent polymer, Plexit55, of which the monomer is commercially available. The luminescent dye is added to the monomer solution and filled into moulds. Azoisobutyronitrile was used as the initiator in the polymerization at a concentration of 0.05%. Homogeneous, bubble-free plates were obtained after thermal polymerization. After the polymerisation, plates were removed from the mould and the sides were polished to obtain good, optically flat surfaces. For the experiments described in this paper, the dyes Fluorescence Yellow CRS 040 (CRS040) from Radiant Colour and Lumogen F RED305 (RED305) from BASF were used. The 5 mm thick LC plates were doped with