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The effect of photonic structures on the light guiding efficiency of fluorescent concentrators
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10.1063/1.2996081
/content/aip/journal/jap/105/1/10.1063/1.2996081
http://aip.metastore.ingenta.com/content/aip/journal/jap/105/1/10.1063/1.2996081

Figures

Image of FIG. 1.
FIG. 1.

Working principle of the fluorescent concentrator. The fluorescent concentrator consists of a transparent matrix material in which a fluorescent dye is included. Light in a certain spectral range is absorbed by the dye and reemitted at a higher wavelength (Stokes shift). Because of TIR a part of the light is internally transported to the edges of the concentrator, where it is used by solar cells. In this sketch the solar cell is shown only at one side of the concentrator, typically all four sides are covered.

Image of FIG. 2.
FIG. 2.

Sketch of the main loss mechanism of the fluorescent concentrator. For the light emitted by the dye, there are two possibilities. Either it is emitted into an angle greater than the one of TIR, than it transported to the edges of the concentrator (I) or it is emitted into an angle smaller than the one of TIR. In this case the light is lost for the concentrator (II).

Image of FIG. 3.
FIG. 3.

Measured absorption (left) and photoluminescence (right) of the fluorescent concentrators used in this study. The spectra derived from measurements of the fluorescent concentrators differ from those of the dye because of reabsorption events and absorption in the matrix material. The true characteristics of the dyes are difficult to measure. The following considerations have been made on the basis of these data.

Image of FIG. 4.
FIG. 4.

Refractive index profile of an optimized rugate filter (left) and simulated spectral reflectance characteristic of this filter (right). The filter has been designed for the application on the fluorescent concentrator with the dye JMC 4. The refractive index is varied sinusoidally between and with a period length of 160 nm and 60 periods. The filter is optimized by a Gaussian modulation and an additional index matching to the adjoined materials. For the simulation the method of characteristic matrices has been used (Ref. 11).

Image of FIG. 5.
FIG. 5.

Measured spectral reflectance characteristic of a rugate filter fabricated at Fraunhofer IST. The filter was optimized for the application on a fluorescent concentrator with the dye BA 856. It shows a very high reflectance in the photoluminescence range of the dye. Unfortunately a lot of unwanted reflections occur in the absorption range of the dye that is the cause for losses.

Image of FIG. 6.
FIG. 6.

Measured spectral reflectance characteristic of a band edge filter fabricated at MSO-Jena for the application on a fluorescent concentrator with the dye JMC 4. The absorption and the photoluminescence of the dye are also shown. The filter shows a very high reflectance between and a very high transmittance for . The reflectance for all other wavelengths was left undefined.

Image of FIG. 7.
FIG. 7.

Opal structure (left) and simulated spectral reflectance characteristic of an opal designed for the application on a fluorescent concentrator with the dye JMC 4 (right). The absorption and the photoluminescence of the dye are also shown. The opal consists of spheres with a refractive index of and a diameter of . It is made of 21 layers of spheres. The reflectance in the peak region is in the absorption range of the dye, the average transmittance is . On the right hand side the structure of the opal is sketched. For the simulation of the opal the rigorous coupled wave analysis (RCWA) method has been used (Refs. 12 and 13).

Image of FIG. 8.
FIG. 8.

Measured spectral reflectance characteristic of an opal designed for the application on a fluorescent concentrator with the dye JMC 4 (left). The absorption and the photoluminescence of the dye are also shown. The parameters are the same as for the one shown in Fig. 7 apart from the number of layers of spheres. The opal used for this measurement consists of 10 layers of spheres. No thicker opals with a sufficient quality have been produced yet. The reflectance peak is slightly shifted from the simulated one because of uncertainties in the sphere diameter occurring during the manufacturing of the spheres. A variation in the sphere diameter in the opal results in a reduced peak width. The reflectance in the peak region is in the absorption range of the dye, the average transmittance is between . Reflections here are caused by scattering.

Image of FIG. 9.
FIG. 9.

Geometrical considerations to determine the average distance from a point to the sides of a fluorescent concentrator. Each point sees the four sides of the concentrator under four different angles . The distance to a certain side must be determined for each side individually and in the corresponding angular range. The example shows the needed parameters for the right side of the fluorescent concentrator.

Image of FIG. 10.
FIG. 10.

Map of the average distance for a photon to reach the edge of the fluorescent concentrator. The side length of the concentrator is normalized to 1. The average pathlength is therefore in terms of the side length.

Image of FIG. 11.
FIG. 11.

Map of the fraction of photons in the loss cone reaching the side of the concentrator when a reflective filter is used. The reflectance of the filter is , the geometrical ratio is , and the critical angle is (corresponding to PMMA with ). Depending on the position of the emission between 30% and 80% of the photons are gained by the filter.

Image of FIG. 12.
FIG. 12.

Fraction of photons in the loss cone that reach the side of the concentrator in dependence of the reflectance of the filter and the critical angle . The geometrical ratio is corresponding to our experimental setup. The critical angle is defined through the refractive index contrast between the matrix material and air. It can be altered by destroying the conditions for TIR. For a critical angle of only the reflection of the filter is used for the transport of the photons to the edges.

Image of FIG. 13.
FIG. 13.

Fraction of photons in the loss cone that reach the side of the concentrator in dependence of the reflectance of the filter. The critical angle is corresponding to a matrix material with a refractive index (PMMA).The geometrical ratio is corresponding to our experimental setup.

Image of FIG. 14.
FIG. 14.

Reflectance of the filter required for a fraction of the photons to be guided to the sides in dependence of the critical angle . The geometrical ratio is . Common materials of fluorescent concentrators have a refractive index of . The corresponding critical angle is . For this case a reflectance of is required.

Image of FIG. 15.
FIG. 15.

Fraction of photons in the loss cone that are guided to the sides in dependence of the reflectance of the filter and the geometrical ratio. The geometrical ratio is given logarithmically to the basis of 10. The critical angle is corresponding to PMMA. Assuming a constant thickness of the fluorescent concentrator the demands on the reflectance of the filter are increasing with an increasing area. The system for testing had a geometrical ratio of corresponding to a value of 0.85 in this figure. Commercial use would require a factor of around , corresponding to a value of 2 in this figure. Much less light is transported to the edges in that case, so that the demands on the filter are much higher.

Tables

Generic image for table
Table I.

Fraction of light gained and lost because of the optically uncoupled filter. The total gain is calculated with Eq. (17) and a fraction of light in the loss cone . Only the simulated rugate filter and the band edge filter have a positive effect.

Generic image for table
Table II.

Fraction of light gained and lost because of the filters if no TIR occurs and all light is transported to the edges because of the filters. The total gain is calculated with Eq. (17) and a fraction of light in the loss cone . If the TIR was perfect, 74% of the light would be guided to the sides. The last line gives the light guiding efficiency of the filter compared to these 74%.

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/content/aip/journal/jap/105/1/10.1063/1.2996081
2009-01-13
2014-04-16
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The effect of photonic structures on the light guiding efficiency of fluorescent concentrators
http://aip.metastore.ingenta.com/content/aip/journal/jap/105/1/10.1063/1.2996081
10.1063/1.2996081
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