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Luminescent solar concentrators

Luminescent solar concentrators (LSCs) are luminescent plates either totally impregnated by luminescent species or fluorescent thin films on transparent plates. They absorb solar light which is converted to fluorescence guided to plate edges where it emerges in a concentrated form. The concentration factor is directly proportional to the plate surface and inversely proportional to the plate edges. Such arrangement allows to use small amounts of solar cells as a result of concentration of fluorescent light. The fluorescent concentrator is able to concentrate both direct and diffuse light which is a specially important in cloudy days. The theoretical explanation of the concentration performance and the recent advances in the field can be found at the

The theory of LSC, which is based on internal reflection of fluorescent light which is subsequently concentrated at the edges, has been discussed in detail for inorganic materials and organic dyes incorporated in bulk polymers  For optimum performance a transparent plates doped by fluorescent species should absorb the major part of solar spectrum. The resulting luminescence emitted at the longer wavelength part of the spectrum should have high yield. Repeated reflections of the fluorescent light in a transparent matrix should carry the radiation to the edges of the plate where the light will emerge in a concentrated form. The concentration factor is the ratio of the plate surface to the plate edge. Theoretically about 75–80 % of the luminescence would be trapped by total internal reflection in the plate having a refractive index of about 1.51. Photovoltaic cells can be coupled to the edges and receive the concentrated light. Such an arrangement should substantially decrease the amount of photovoltaic cells needed to produce a given amount of electricity and thus reduce the cost of the system of photovoltaic electricity. If all these ideal conditions could be realized our calculations show that the collecting efficiency, which is the amount of energy reaching the photovoltaic cell divided by the energy falling on the plate should be about 20 %. This result is obtained by taking into account the overall absorption of the colorants in the plate, their fluorescent efficiency, the trapping efficiency (depending of the refractive index of the plate medium), and the Stock efficiency (which is the ratio of the average energy emitted to the average energy absorbed). So far the efficiency of LSC was never above 7%, one of the main reasons for the relatively low efficiency is the self-absorption of the luminescent dyes as a result of overlapping of the absorption and luminescence of the dyes. Another difficulty is the escaping of the luminescence emitted beyond the critical angle. The advantages and disadvantages of the different colorants: organic dyes vs. rare earth elements and other inorganic species, trapping media glass or polymer, balks vs. thin films deposited on transparent plates have to be considered. Use of photonic systems as a band gaps preventing the escape of the trapped radiation is recommended. Using materials in which the absorption of light and the emission arise from different electronic levels prevents self-absorption. Utilizing solar cells with their sensitivity matching maximum emission of the colorants is beneficial. A possible solution to the problems is a luminescent material in which the excitation and emission arise from different electronic levels. Interaction of the colorants with surface plasmons increase their transition probabilities from the ground to the excited state and efficiency of absorbed solar light.