increasing fluorescent concentrator light collection efficiency by

Increasing Fluorescent Concentrator Light Collection Efficiency byRestricting the Angular Emission Characteristic of the Incorporated

Luminescent Material - the “Nano-Fluko” Concept

J.C. Goldschmidt* , M. Peters , J. Gutmann , L. Steidl , R. Zentel , B. Bläsi , M. Hermlea a a b b a a

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germanya

Institut für Organische Chemie, Universität Mainz, Duesberweg 10-14, 55128 Mainz, Germanyb

ABSTRACT

Fluorescent concentrators concentrate both diffuse and direct radiation without requiring tracking of the sun. Influorescent concentrators, luminescent materials embedded in a transparent matrix absorb sunlight and emit radiationwith a different wavelength. Total internal reflection traps the emitted light and guides it to solar cells attached to theconcentrator’s edges. The escape cone of total internal reflection, however, limits the light collection efficiency.Spectrally selective photonic structures, which transmit light in the ab sorption range of the luminescent material andreflect the emitted light, reduce these losses. In this paper, we review different realizations of such structures and showth at they increase collection efficiency by 20%. However, light emitted into steep angles in respect to the front surface,which would be lost without the photonic structures, has a very long effective path inside the concentrator until it reachesa solar cell. Therefore it suffers from path length dependent losses. We discuss how emission into the unfavorabledirections can be suppressed by integrating the luminescent material into photonic structures, thus reducing these losses.We present possible realizations both for the concentrator design and for the solar cells used in such systems.

Keywords: Solar energy, Photovoltaics, Concentrators, Fluorescent concentrators, photonic crystals

1. INTRODUCTION

Fluorescent concentrators are a special type of light concentrating device. The underlying principle was first used inscintillation countersand then their application to concentrate solar radiation was proposed in the late 1970s. In a1, 2 3, 4fluorescent collector, a luminescent material embedded in a transparent matrix absorbs sunlight and emits radiation witha different wavelen gth. Total internal reflection traps most of the emitted light and guides it to the edges of the collector(Figure 1). Solar cells op tically coup led to the edges convert this light into electricity.

Fluorescent concentrators are able to concentrate both direct and diffuse radiation. A geometric concentration isachieved, if the area of the solar cell at the edges is smaller than the illuminated front surface of the collector, i.e. whenth e area from which light is collected is larger than the solar cell area. The ability to concentrate diffuse radiationpresents a great advantage for the application of fluorescent concentrators in temperate climates, such as in middleEurope, or in indoor applications with relatively high fractions of diffuse radiation. Additionally, fluorescentconcentrators do not require tracking systems that follow the path of the sun, in contrast to concentrator systems that uselenses or mirrors. This facilitates, for instance, the integration of fluorescent concentrators in buildings.

Fluorescent concen trators were investigated intensively in the early 1980s. Research at that time aimed at cutting5, 6costs by using the concentrator to reduce the need for expensive solar cells. After 20 years of progress in thedevelopment of solar cells and luminescent materials, and with new concepts, several groups such as those of Refs.7-2 7are currently reinvestigating the potential of fluorescent concentrators. High efficiencies have been achievedand7, 1 0, 24there has been also considerable progress in the understanding and theoretical description, e.g.. However,7 , 8, 12, 27, 28efficiencies are still too low and system sizes too small for a co mmercial application.

*[email protected], Tel +49 761 4588 5475, www.ise.fraunhofer.de

Photonics for Solar Energy Systems III, edited by Ralf B. Wehrspohn, Andreas Gombert, Proc. of SPIEVol. 7725, 77250S · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.854278

Proc. of SPIE Vol. 7725 77250S-1

Figure 1: Working principle of a fluorescent concentrator (lef). A luminescent material in a matrix absorbs incomingsunlight (E ) and emits radiation with a different energy (E ). Total internal reflection traps most of the emitted light

1 2and guides it to solar cells optically coupled to the edges. Emitted light that impinges on the internal surface with anangle steeper than the critical angle is lost due to the escape cone of total internal reflection. A part of the emitted

clight is also re-absorbed, which can be followed by re-emission. On the right, it is shown how the escape cone lossescan be reduced: a selective reflector, realized as a photonic structure, acts as a band stop reflection filter. It allows lightin the absorption range of the dyes to enter the collectors, but reflects light in the emission range.

1.1 Escape cone losses

One fundamental problem limitin g the collection efficiency of fluorescent concentrators is the escape cone of totalinternal reflection. All light that impinges on the internal surface with an angle smaller than the critical angle ( )

c e m i t

leaves the collector and is lost (Figure 1 left). The critical angle is given by

1() () . (1)= arcsinc n

emi t

Integration gives a fraction

() ( )= 1- n - 2 (2)t r ap e mi t

of the emitted photon flux that is trapped in the collector . For PMMA (Polymethylmethacrylate) with approx. n= 1.5,2 9

this results in a trapped fraction of around 74%, which means that a fraction of around 26% is lost after every emissionprocess. The 26% account for the losses through both surfaces. An attached mirror does not change this number, as witha mirror the light leaves the collector through the front surface after being reflected. The loss of around 26% does notonly occur once, but after every re-absorption and subsequent re-emission.

On the other hand , the Stokes shift between absorption and emission opens the opportunity to reduce these lossessignificantly: a selective reflector, which transmits the light in the absorp tion range of the luminescent material andreflects the emitted light, would trap nearly all the emitted light inside the collector . The concept is illustrated in Figure30

1 on the right. There are several ways to realize such selective reflectors. For instance, in hot mirrors were proposed to17

serve as selective reflectors and in photonic structures. In the fist part of the paper, we will show experimental results18

on how different photonic structures affect the efficiency of fluorescent concentrator systems.

Even with photonic structures, however, light emitted into the former escape cone is more frequently subject to lossevents. This light is emitted into steep angles in respect to the front surface. Therefore it has a very long effective pathinside the concentrator until it reaches a solar cell. In consequence, it suffers quite strongly from path length dependentlosses, such as re-absorption by the dye, absorption in the matrix, scattering etc. In the second part of the paper, wediscuss integration of the luminescent material into photonic structures that suppress emission into the unfavorabledirections to reduce these losses. We present possible realizations both for th e concentrator design as well as for the solarcells that are to be used in such systems.


2. REDUCING ESCAPE CONE LOSSES WITH PHOTONIC STRUCTURES

2.1 Multilayer system

To reduce the escape cone losses, a selective reflector is necessary that shows a high transmission in the absorption rangeof the luminescent material and high reflection in the emission range. A possible realization of such a selective reflectoris a so-called Rugate filter. It features a continuously varying refractive index profile that results in a single reflectionpeak. However, some unwanted side lobes remain. Optimized Rugate filters show only one single reflection peak for acertain wavelength and almost no other reflectio ns. In this work, such optimized Rugate filters are used, which were

an d they were optimized forproduced at the company mso-jena by Ion-Assisted-Deposition (IAD. Their size is 5x5cmth e used dye material. The filters had an antireflection coating adapted to the absorptio n range of the used dye. The usedmaterial is denoted BA241 and was develop during the first research period in the 1980s The reflection of the filter,the absorption and the photoluminescence of the BA241fluorescent concentrator are shown in Figure 2. The filtertransmits the light in the absorption range of the dye and it reflects the emitted light and therefore has exactly the desired

properties.

Figure 2: Reflection spectrum of the used photonic structure and the absorption and photoluminescence of the fluorescentconcentrator the filter was designed for. The reflection of the structure very nicely fits the emission peak of the dye inthe concentrator.

To investigate how the filter increases the light guiding efficiency of the concentrators we attached a 21 x 3 mm GaInP2

solar cell on one rim of a 3 mm thick fluorescent concentrator with a size of 2 x 6 cm (the cell was attached to the 2 cmrim). A white BaSO bottom reflector was placed under the system and the EQE of the system was measured, with and

4

with out the filter on top. During the EQE measurement, the system was illuminated with a 3 mm wide spot in 1 cmdistance to the solar cell. Figure 3 sh ows the comparison of the two measurements and additionally the reflection of thefilter. Obviously the filter reduces the efficiency in the region where it is reflective, which is the case for the wan tedreflection above 550 nm and also for the unwanted reflection below 380 nm. On the other hand, the filter increasesefficiency significantly over a broad spectral range, because it traps the emitted light.

We also realized a system with a 5 mm thick, 5 x 10 cm fluorescent concentrator to which one GaInP solar cell was2

coupled with silico ne. The solar cell had an active area of 5 x 49 mm . Hence the relation between illuminated2

fluorescent concentrator area and solar cell area constitutes a geometric concentration ratio of 20x. The so lar cell had anefficiency of 16.7% under AM1.5g illumination. White PTFE served as bottom reflector and also as reflector at theedges that were not covered by solar cells. Without the filter this system had an efficiency of 2.6±0.1% (uncertainty isfor relative comparison) in reference to the 50 cm area of the system. The filter increased the efficiency to 3.1±0.1%,2

which constitutes an efficiency increase of around 20% relative. With the achieved efficiency of 3.1% and theconcentration ratio of 20, the realized fluorescent concentrator produces about 3.7 times more energy than the GaInPsolar cell had produced on its own.


Figure 3: External Quantum Efficiency (EQE) measurement of a system with a GaInP solar cell attached to a fluorescentcollector of 3 mm thickness made from BA241 under which a BaSO bottom reflector was placed with and without a

4photonic structure on top of the collector. Additionally the reflection of the photonic structure is shown. The efficiencyis increased significantly over a broad spectral range, because more emitted light is trapped and guided to the sides.

Figure 4 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 closeto the solar cell. The efficiency drops with increasing distance to the solar cell and closer to the solar cell free edges.Interestingly, the collection efficiency increases as well close to the edge opposite the solar cell. This effect was observedin different systems of varying sizes. Therefore it cannot be considered a simple measurement artifact. Very likely, lightoutside the absorption range of the dye is somehow redirected to the solar cell by the bottom and the edge reflector, or bythe actual edge of the collector. However, a precise explanation is yet to be developed.

Figure 4: Light Beam Induced Current (LBIC) scan of the 10 x 5 cm sample described above. A photonic structure was2

placed on top of the fluorescent concentrator during the measurement. A white reflector made from PTFE was placed atthe 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 seethat the collection efficiency is highest close to the solar cell. The efficiency drops with increasing distance to the solarcell and closer to the edges.


Figure 5: Averaged linescans in x-direction from an LBIC scan with and without photonic structure. Close to the solar cellthe efficiency is lower with the photonic structure, because it reduces the effectiveness of the bottom reflector for smalldistances. Over most of the fluorescent concentrator, however, collection efficiency is significantly higher with aphotonic structure, resulting in a relative efficiency increase of 20%.

Figure 5 compares the averaged linescans in the x-direction of the LBIC scan shown in Figure 4 and of a scan withoutphotonic structure. The average was taken from 1.25 to 2.5 cm in the y-direction. Close to the solar cell the efficiency islo wer with the photonic structure. The reason is that light, which enters the collector close to the solar cell, can bereflected directly to the solar cell by the bottom reflector. This absorption-less light collection works as well for the lightoutside the absorption range of the dye, which is still in the spectral region that can be used by the solar cell. This lightoutside the absorption range, however, is blocked by the photonic structure. Nevertheless, over most of the fluorescentconcentrator, collection efficiency is significantly higher with a photonic structure, resulting in the relative efficiencyincrease of 20%. This is a clear demonstration of how photonic structures can help to increase the collection efficienciesof larger fluorescent concentrator systems.

2.2 Opaline systems

The efficiency increase of 20% is already a great success since it shows that photonic structures reduce the escape conelo sses sign ificantly. However, the used filter is a multilayer system and therefore costly to produce. Three-dimensionalphotonic structures are a potential alternative to the presented multilayer systems. A special three-dimensional photonicstructure is the opal. The opal has the advantage that it can be produced by a dip -coating process utilizing self-organization of mono-disperse PMMA beads (Figure 6) . This is a potentially low-cost process that could be applied on32

large area concentrators.

Producing opaline films of high quality directly on the PMMA o f the fluorescent collector is difficult, since the PMMAsurface is hydrophobic. However, the surface can be made hydrophilic by an oxygen plasma treatment. An alternativeapproach is producing the opaline film on a sacrifice layer and transferring the film to the fluorescent concentrator.

To investigate the effect of the opal on the collection efficiency of the fluorescent concentrator, an opaline film withproperties adapted to the BA241 material was produced on a glass substrate. The diameter of the PMMA beads was256 nm and around 50 layers of beads were deposited on the glass. The reflection of the opal on the glass is shown in

Figure 7 in comparison to the absorption and fluorescence of the dye.The sample was placed on top of a 2 cm x 2 cm sample of BA241 with one GaInP solar cell attached to one edge. Underthe bottom and around the free edges white PTFE reflectors were placed. The efficiency was determined with andwith out the opal on top. Without the photonic structure the efficiency was 3.3%, but dropped with the opal on top to3.0%. The reasons for this drop are that for one there is more than 10% unwanted reflection in the absorption range ofth e dye, wh ich causes sev ere losses. This unwanted reflection is mostly caused by stacking faults that cause scattering.


Second, the reflection peak in the emission region of the dye is not perfectly aligned to the emission spectrum. Moreover,the reflection only peaks at around 70%. Therefo re not all the light emitted into the escape cone is reflected back into theconcentrator

The reflection can be increased with depositing opaline films with more layers of PMMA beads and by reducing thestacking faults. A reduction of stacking fault would result in a reduction of the unwanted reflection as well. However,increasing the number of layers makes a stacking fault-free assembly of the beads less likely. In consequence, progress inth e preparation of the film is necessary, to make it technologically viable to deposit opaline photonic structures on largearea fluorescent concentrators.

Figure 6: SEM image of an opaline film produced by the self-organization of mono-disperse PMMA beads. This film wasproduced on a sacrifice layer so it could be transferred to a fluorescent concentrator later on.

Figure 7:The reflection of an opaline film made of 256 nm PMMA beads in comparison to the absorption and emission of

the fluorescent concentrator. Up to now, the reflection peak of the photonic structure at the emission wavelengths of thedye is not high enough to over-compensate the losses due to the reflection of the photonic structure in the absorptionrange of the dye

2.3 Next steps

To successfully reduce escape cone losses of fluorescent concentrators with photonic structures in a commercialapplication, the most important task is to find a low-cost process to produce high-quality structures. Furthermore, thephotonic structures shou ld be deposited d irectly on to the fluorescent concentrators. This reduces the number ofboundaries and therefore unwanted reflections. With an adequate design, the photonic structures can act as anantireflection coating in the absorp tion range of the dye and as reflector for the emitted radiation at the same time. In thisconfiguration, reflection and escape cone losses are reduced simultaneously. But also even more radical photonicconcepts are possible, as we will discuss in the following section.


3. RESTRICTING THE ANGULAR EMISSION

We have seen before that photonic structures reduce the escape cone losses. However, even with a photonic structure,light emitted into the escape cone is more frequently subject to loss events. Because it is emitted into a steep angle inrespect to the front surface, it has a very long effective path until it reaches a solar cell, and therefore suffers more frompath length dependent losses. Hence, it would be very beneficial to suppress emission into these unfavorable directionscompletely. This shou ld as well be po ssible with the help of photo nic structures. Already the very first works on

and Yablonovitch dealt with influencing emission with photonic structures. Many papersphotonic crystals of Bykov 33 34

discussed the possibilities subsequently . For influencing the emission of the dye su ccessfully, it is necessary that the35 -41

photonic structures are very close to the emission process or that the luminescent material is incorporated into thephotonic structures. For the fluorescent concentrator systems this means that one has to go from the macroscopic designof the presented systems to a system design we denote “Nano-Fluko”.

Figure 8: Conceptual sketch of a “Nano-Fluko”. A very thin layer of luminescent material with thickness t in the range ofwavelength of the emitted light is placed between two photonic structures, e.g. Bragg stacks. The photonic structurestransmit light in the absorption range of the luminescent material with an energy E . They are reflective in the emission1region (E ) of the luminescent material. Because the layer with the luminescent material is so thin, the photonic

2structures suppress the emission into unfavorable directions.

One possible realization of such a “Nano-Fluko” would be a very thin layer of luminescent material between twophotonic structures, e.g. Rugate filters or Bragg stack s (Figure 8). In such a configuration, the emission of the lightwould b e restricted to a plane parallel to the photonic structure. Galli et al. showed that the emission of Er can be3+

strongly enhanced, if it is incorporated in a photonic crystal waveguide and that efficient waveguiding occurs .4 2, 43

Therefore there is first experimental evidence that such a system can work, and it is an interesting approach to apply thisconcept to fluorescent concentrators.

Another realization could be a photonic crystal fiber doped with a luminescent material (Figure 9). However, to designth e photonic structure around the fiber with the right spectral selectivity will be a deman ding task. On the other hand,such a realization would enable very interesting application opportunities. For instance, the fibers could be woven into aflexible fabric with the properties of a fluorescent concentrator.


Figure 9: Alternative realization of a “Nano-Fluko”. The luminescent material is incorporated into a photonic crystal fiber. Ifthe photonic shell is designed with the right spectral selectivity, the fiber could accept light in the absorption range (E )1of the luminescent material from all directions. Emission with energies (E ) would be restricted to the direction of the

2fiber.

Probably a more realistic option is to incorporate the luminescent material d irectly into th e p ho tonic structure. This couldbe done for example with an opaline film made from PMMA beads that incorporate much smaller luminescentnanocrystals (Figure 10). If the optical band-gap in the emission range of the dye is incomplete, emission into certaindirections is allowed and effective light guiding occurs.

Figure 10: Sketch of “Nano-Fluko” realized by incorporation of the luminescent material directly into the photonic structure.This could be done for example with an opaline film made from PMMA beads that incorporate much smallerluminescent nanocrystals. If the optical band-gap in the emission range of the dye is incomplete, emission into certaindirections is allowed and effective light guiding occurs.

For all the suggested options for realization, several layers with the same dye will b e needed to achieve sufficientabsorption. As the guided light is constraint to very thin layers high intensities will occur in these layers. Because ofthermodynamical reasons, there is a limit for which concentration can be achieved with a fluorescent concentratordepending on the Stokes Shift of the used dye . One question is therefore, which system sizes can be achieved using44

th is approach until th e thermodynamic limit reduces efficiency.

Furthermore, attaching solar cells to the thin layers is a challenge. Especially, because preferably different types of solarcells are attached to layers with different dyes. In Figure 11 two different options are shown. One option is to producedifferent types of solar cells on one chip, e.g. with a MOVPE-process from III-V semiconductors. Because the requiredareas will be very small, this could be v iable from a commercial point of view as well. Preferably, contact fingers shouldbe aligned between the light guiding layers such that no reflections losses occur. An alternative option would be tovertically cut through conventional tandem solar cells in a process comparable to that used for sliver cells .4 5


Figure 11: Two options of how a “Nano-Fluko” system complete with solar cells could be realized. To achieve goodabsorption several layers with luminescent materials must be stacked onto each other. The combination of differentmaterials ensures good utilization of the solar spectrum. On the right, the attachment of solar cells made from differentmaterials on one common substrate is shown. The contact fingers should be aligned with the photonic structures so noshading losses occur. On the left, the option is shown to cut a standard tandem solar cell vertically, similar to the slivercell process and to attach such vertical cuts to the edges of a “Nano-Fluko”.

4. SUMMARY

We have investigated, how the escape cone losses of fluorescent concentrators can be reduced with the help of photonicstructures. We showed that a 20% increase in collection efficiency could be achieved with a commercially availablemulti-layer system. Opaline films might be a solution to produce photonic structures on large areas at low costs. For apositive effect on the light collection efficiency, however, further progress in the production of the opaline films isnecessary. We presented different suggestions, of how path-length depend losses can be reduced by restricting theangular emission range of the used dyes by incorporating these dyes into photonic structures.

ACKNOWLEDGEMENTS

The authors would like to thank Armin Bösch, Henning Helmers, and Elisabeth Schäffer for their support and the III-Vgroup at Fraunhofer ISE for the production of solar cells. The presented work was supported by the German ResearchFoundation within the Nanosun (PAK88) project.

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increasing fluorescent concentrator light collection efficiency by

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