Análisis de tráfico de voz

Many losses and especially the escape cone losses depend on the average path the emitted light has to travel until it reaches a solar cell at the edge of the collector plate. Therefore, it is very interesting to investigate the effect of the size of the fluorescent concentrator on the collection efficiency, especially in conjunction with photonic structures. To investigate the size dependency, I used the system described in the previous section with the 2 x 6 cm2 _{fluorescent concentrator. A white BaSO}

4 reflector

was placed beneath the concentrator. The illuminated area was then reduced stepwise with the help of black blinds, placed on the fluorescent concentrator (Fig. 4.67) and an IV-measurement was performed for each area size.

Fig. 4.67: Sketch of the measurement setup for the investigation of the size effects. The illuminated area was reduced in 1 cm steps with the help of black blinds, placed on the fluorescent concentrator. The area was reduced starting from the side opposite the solar cell.

Fig. 4.68 displays the open circuit voltage VOC and the short circuit current ISC. The

error bars indicate the standard deviation in repeated measurements. When the illu- minated area is increased, more light is collected. Therefore, the total short circuit current increases, and as a consequence the open circuit voltage as well. However, the short circuit current density JSC, calculated by dividing the short circuit current by the

particular illuminated area, decreases with increasing area (see Fig. 4.69). The probability of a photon to reach the solar cell at the edge is much lower, if it stems from an absorption/emission process further away from the solar cell. The reason is that parasitic absorption, re-absorption and emission events and the associated escape cone losses are becoming more likely with an increasing path length of the photon.

4.5 Fluorescent concentrator systems

Fig. 4.68: Open circuit voltage VOC and short circuit current ISC of the fluorescent

concentrator system depending on the illuminated area. With increasing illuminated area, more light is collected and reaches the solar cell at the edge, so the total short circuit current ISC increases. As a consequence, the

voltage increases as well.

Fig. 4.69: The short circuit current density JSC depending on the illuminated area.

With increasing area, the short circuit current density drops. The reason is that the probability to reach the solar cell at the edge decreases for the photons, which stem from absorption/emission processes further away from the solar cell.

Fig. 4.70 shows the dependence of the overall system efficiency on the illuminated area. Additionally, the experiment was repeated with a black material and with the photonic structure from mso-Jena under the fluorescent concentrator and with and without the photonic structure on top. The efficiency drops in all cases with increasing area, because the relative effect of the voltage increase is smaller than the relative decrease of the current density. This can be understood as the voltage only increases logarithmically with the current, while the losses should follow some kind of exponential dependence on the average path length, which increases roughly linearly with the area in our configuration. However, the effect of the decreasing efficiency is slightly exaggerated. In an optimized fluorescent concentrator system, one would choose more quadratic geometric dimensions and would place solar cells on all edges of the concentrator. This would reduce the average path length of the light and therefore decrease the path length associated losses.

Fig. 4.70: System efficiency in dependence of the illuminated area. Efficiencies were measured with different bottom reflectors placed under the system and with and without a photonic structure on top. For all systems, efficiency decreases with increasing illuminated area. The error bars indicate the standard deviation in repeated measurements. For these small sizes, a white bottom reflector yields the highest efficiencies.

The differences between the different bottom and top configurations are very enlightening. For all sizes, the efficiencies are the lowest for the case where there is a black bottom material. With a black bottom, all light that passes the fluorescent

4.5 Fluorescent concentrator systems

concentrator or leaves it in the bottom direction after emission is absorbed and lost. In contrast the photonic structure reflects in the emission range of the dye. If it is placed under the concentrator it increases the efficiency, because it reflects emitted light back into the concentrator. It is important to note that all the light entering the concentrator from the outside is refracted in a way that it would leave the concentrator already after one pass to the opposite surface. So to stay in the concentrator or to become useful it has to be reabsorbed, scattered or absorbed by the solar cell.

Adding the photonic structure on top of the concentrator does not increase the efficiency for the investigated sizes, but reduces it for the small areas. For larger areas, there are no significant differences between the results with and without a photonic structure on top. This is true both for the case with a photonic structure at the bottom and with the white bottom reflector. To understand this, it is helpful to look in more detail into how the white bottom reflector increases the efficiency.

Fig. 4.71: Comparison between an EQE measurement with a white bottom reflector and with a black bottom for a system with a fluorescent concentrator 2 x 2 cm2, 3 mm thick and four parallel interconnected GaInP solar cells at the edges. The illumination spot was located in the center of the concentrator. One can see that the white bottom reflector increases the efficiency in the absorption region of the dye. Especially in the weaker absorbing region a second chance to be absorbed after a reflection from the bottom increases the light collection.

Fig. 4.71 shows a comparison between an EQE measurement with a white bottom reflector and with a black bottom. Because the white bottom strongly reflects over a broad spectral range, it gives the light a second chance to be absorbed which lies in the absorption range of the dye, but has passed the fluorescent concentrator without being absorbed. Additionally, light outside the absorption range of the dye but within the usable wavelength range of the solar cell is also reflected and therefore has the chance to reach the solar cell directly or by scattering events.

Fig. 4.72: EQE measurements of the 2 x 6 cm2 fluorescent concentrator system with white bottom reflector in different distances to the solar cell. This measurement helps to understand the effect of the white bottom reflector placed under the fluorescent concentrator. The EQE increases significantly with decreasing distance to the solar cell. It increases also in the wavelength range above 550 nm in which the fluorescent concentrator hardly absorbs. This is a clear hint that light is directly reflected onto the solar cell. For small distances this is a quite significant contribution to the overall light collection.

Fig. 4.72 displays external quantum efficiency measurements of the described 2 x 6 cm2 _{system with white bottom reflector for a variation of the distance between}

the illumination spot (3 mm diameter) and the solar cell. With smaller distances efficiency also increases strongly in the spectral region above 550 nm. In this region the dye does not absorb. This finding supports the explanation that light is reflected directly to the solar cell from the bottom reflector. As we can see, this absorption-less

4.5 Fluorescent concentrator systems

light collection contributes significantly to the overall light collection for small areas. Since the reflection of the photonic structure is designed to be high in the emission region of the dye, which is exactly the spectral range above 600 nm, it prevents the light in that region from entering the system and to be directly led to the solar cell by reflection from the bottom. Under these circumstances, although the photonic structure increases the collection of the emitted light, the overall system performance decreases for small areas in comparison to the white bottom reflector alone.

The effect of the absorption-less light collection is present in the system that combined different materials and spectrally matched solar cells, which has been investigated in section 4.5.2, as well. This system reached an efficiency of 6.9% and it would have been great to increase this efficiency even further by applying a photonic structure. Unfortunately, the application of a specially adapted photonic structure did not increase efficiency, because especially the system with the GaAs solar cells attached profited significantly from the absorption-less light collection. Therefore, until now the integration of all features of the advanced fluorescent concentrator system design into

one system has not been successful.

However, the effect of the area close to the solar cell is of less relevance for larger systems. Furthermore, the losses due to re-absorption and emission become more important with increasing size of the fluorescent concentrators. That is, the beneficial effect of the photonic structures should be more pronounced for bigger systems. Accordingly, we have seen in the previous section that the photonic structure increased the efficiency of a larger system with 5 x 10 cm2 _{area by 20% relative. Fig. 4.73 shows}

the spatially resolved light collection efficiency as it was measured with a Light Beam Induced Current (LBIC) setup on this system with a photonic structure on top. One can see that the collection efficiency is highest close to the solar cell. The efficiency drops with increasing distance to the solar cell and closer to the solar cell free edges. Close to the edges the probability for emitted light to hit the edge surface with an angle smaller than the critical angle of total internal reflection is higher for simple geometrical reasons. If the light leaves the collector at the edges, not all the light is reflected back because of the imperfect reflection of the white reflectors. Interestingly, the collection efficiency increases as well close to the edge opposite the solar cell. This effect was observed in different systems of varying sizes. Therefore it can not be considered a simple measurement artifact. Very likely, light outside the absorption range of the dye is somehow redirected to the solar cell by the bottom and the edge reflector, or by the actual edge of the collector. However, a precise explanation is yet to be developed.

Fig. 4.73: Light Beam Induced Current (LBIC) scan of the 10 x 5 cm2 sample described above. A photonic structure was placed on top of the fluorescent concentrator during the measurement. A white reflector made from PTFE was placed at the bottom and the edges without solar cells. The edge with the attached solar cell is located at the right in this picture. Not the full collector area was scanned to avoid contact of the scanning head with the wiring of the system. One can see that the collection efficiency is highest close to the solar cell. The efficiency drops with increasing distance to the solar cell and closer to the edges.

Fig. 4.74: Averaged linescans in x-direction from an LBIC scan with and without photonic structure. Close to the solar cell the efficiency is lower with the photonic structure, because it reduces the effectiveness of the bottom reflector for small distances as discussed extensively above. However, over most of the fluorescent concentrator collection efficiency is significantly higher with a photonic structure, resulting in a relative efficiency increase of 20%.

4.5 Fluorescent concentrator systems

Fig. 4.74 compares the averaged linescans in the x-direction of the LBIC scan shown in Fig. 4.73 and of a scan without photonic structure. The average was taken from 1.25 to 2.5 cm in the y-direction. Close to the solar cell the efficiency is lower with the photonic structure. This is the result of the reflection of light that would have reached the solar cell by scattering as discussed extensively above. However, over most of the fluorescent concentrator, collection efficiency is significantly higher with a photonic structure, resulting in the relative efficiency increase of 20%. This is a clear demonstration of how photonic structures can help to increase the collection efficiencies of larger fluorescent concentrator systems.