Smart Optical Materials by Sol-Gel Method
by R. Reisfeld
Dept. of Inorganic and Analytical Chemistry , The Hebrew University, Jerusalem 91904, Israel

Tunable Laser samples prepared by the sol-gel method 

Luminescent solar concentrators obtained using sol-gel process

The world conventional energy supplies, which are based mainly on readily available fossil fuel source, are diminishing rapidly. The main approach to the energy crisis, nuclear fission, is raising a great deal of hope but its practicality has still to be demonstrated. There is no doubt that solar energy, which is clean and non-hazardous, could contribute considerably to a solution of the energy problem if appropriate methods were developed to collect, concentrate, store and convert solar radiation which is diffuse and intrinsically intermittent. 



Owing to the original efforts of the National Aeronautics and Space Administration to supply electric current from silicon photovoltaic (PV) cells to space vehicle, such devices are now available at a price of several US dollars per watt of power. At present large scale solar cell arrays are operating in inaccessible locations distant from conventional electricity plants. Previous estimates of price decrease to $1-$2/W which were obtained by making comparisons with the aluminium or electronic computers industry, may be slightly optimistic as the difficulties of preparing inexpensive silicon with a high photoelectric yield can not be removed by increased production. One way of lowering the price of PV electricity is to concentrate the solar radiation, particularly that part which is most efficient in PV energy conversion, on high efficiency solar cells. Although expensive their amount and cost can be considerably diminished by using concentrated solar light on their small areas. The light emitted as fluorescence form the edges of the concentrator can be matched to about 50% efficiency of solar cells.

The operation of a Luminescent Solar Concentrators (LSC) is based on absorption of solar radiation in a collector containing a fluorescent species in which the emission bands have little or no overlap with the absorption bands. The fluorescence emission is trapped by total internal reflection and concentrated at the edges of the collector which is usually a thin glass plate. 

LCS advantages over conventional solar concentrators

LSC’s have the following advantages over conventional solar concentrators They collect both direct and diffuse light; there is a good heat dissipation of non-utilized energy by the large area of the collector plate in contact with air so that essentially "cold light" reaches the PV cells; tracking the sum is unnecessary; the luminescent species can be chosen to allow matching of the concentrated light to the maximum sensitivity of the PV cells. The main advantage is that the large area to be covered by the solar cell is reduced to the area of the edges.

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. A transparent plate doped by fluorescent species absorbs in the visible (solar part of the spectrum). The resulting high yield luminescence should then be evolved at the longer wavelengths part of the spectrum. About 75%-80% of the luminescence is trapped by total internal reflection in the plate having a refractive index about 1.5. Repeated reflections of the fluorescent light carry the radiation to the edges of the plate where it is emerges in the concentrated form. The concentration factor is proportional to the ratio of the surface of the plate to its edges and the optical efficiency of the plate. Photovoltaic cells can be coupled to the edges and receive the concentrated light. Such an arrangement should decrease substantially the amount of photovoltaic cells need to produce a given amount of electricity and thus reduce the cost of the system of photovoltaic cells. 

Click here to see the parameters determining the optical plate efficiency and the factors affecting it

While a large number of papers have been published about luminescent plates in which the dye is incorporated in the entire bulk of the plate, the configuration in which the plate is covered by a thin film incorporating the colorant deposited in close contact with the plate is relatively new. The advantage of doped thin sol-gel films having optical contact with the transparent plate is that the luminescence emitted from the thin film is trapped in the plate which parasitic losses due to self-absorption and scattering from impurities can be greatly reduced as compared to bulk doped plates

As an example of such a system we may examine rhodamine 6G incorporated in a sol-gel film. Based on the experimental data of absorption and emission of rhodamine 6G in sol-gel glass, its quantum efficiency of 0.95, molar extinction coefficient of 82000, and the overlap of absorption of Rh6G with the solar spectrum using a 50 micron thick film deposited on a plate having refractive index of 1.51, Monte-Carlo computations were performed to model optical efficiency of a plate having dimensions of 1 m2. The optical efficiencies of such plates were found approximately 15%. 
While Rh6G is not stable enough to be used in practical device for LSCs it can serve as a model for computation in sol-gel glasses. Much better results can be obtained with the photostable perylimide dyes, provided they can be introduced into the sol-gel system.

In a recent work we have indeed been able to introduce the dyes into a composite polymer/so-gel glass system and into a glass using the sol-gel procedure. A combination of two dyes increases the overlap of absorption with the solar spectrum followed by an increase in optical efficiency.

The main requirements for LSC are their efficiency, photostability and ease of fabrication. This has been achieved here by deposition of organically modified sol-gel films doped by photostable perylimide dyes on glass substrate. The absorption spectra of these dyes extends from 420 to 620 nm covering the visible part of the solar spectrum and the emission is between 550 and 750 nm, close to the optimum response of silicon and germanium arsenide solar cells. The efficiency of such type of collector was calculated from the absorption coefficients, quantum efficiency of the fluorescence and the overlap between emission and absorption spectra, by the method of Monte-Carlo and found to be close to 20%. Optimum concentrations are shown to be strongly dependent on the extent of overlap between the absorption and the emission spectra, which also appears to be the limiting factor in respect to the efficiency of the Concentrator.



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