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Dual waveguide patterned luminescent solar concentrators
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Solar Energy 95 (2013) 216–223 www.elsevier.com/locate/solener
Dual waveguide patterned luminescent solar concentrators Peter T.M. Albers, Cees W.M. Bastiaansen, Michael G. Debije ⇑ Chemical Engineering & Chemistry, Functional Organic Materials & Devices, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands Received 13 March 2013; received in revised form 3 June 2013; accepted 11 June 2013 Available online 17 July 2013 Communicated by: Associate Editor J.-L. Scartezzini
Abstract Re-absorption of dye-emitted light is a primary loss mechanism in luminescent solar concentrator waveguides. Previous work has demonstrated patterning the waveguide surface with dyes will increase transport efficiency at the cost of total light absorption. This work demonstrates a patterned double waveguide system employing patterning offset between the plates which allows complete surface coverage but can generate an edge emission greater than a standard thin-film fully covered luminescent solar concentrator of the same total thickness, and is effective for all incident angles of light. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Luminescent solar concentrator; Patterned waveguide; Fluorescence
1. Introduction Luminescent solar concentrators (LSCs) have been proposed as an alternative device for generating electricity from sunlight which could be well-suited for use in urban settings (Debije and Verbunt, 2012; Weber and Lambe, 1976). The standard LSC is quite simple: normally it is a plastic sheet which acts as a waveguide doped with or topped by luminescent materials. Incident sunlight is absorbed by the dye material and emitted at longer wavelengths, a significant fraction being trapped inside the plate by total internal reflection. To the edge(s) of the plate photovoltaic (PV) cells are attached for conversion of the light into electrical current. The LSC could be ideal for functioning in the built environment because they may be of any color and shape, simplifying architectural integration, and they work equally well in direct and indirect sunlight. The LSC has not yet found widespread use due to excessive losses that limit their light conversion efficiency. In ⇑ Corresponding author. Tel.: +31 40 247 5881.
E-mail address: [email protected] (M.G. Debije). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.06.014
particular, re-absorption of light emitted by one dye molecule by a subsequent dye molecule because of the limited Stokes shifts of the dyes is a primary loss mechanism (Earp et al., 2011; Olson et al., 1981; Wilson et al., 2010). The losses are a result of non-unity fluorescence quantum yield (FQY) of the luminophores, and significant probability that the re-emitted light is directed in such a way as to escape the top or bottom surfaces of the waveguide (Debije et al., 2008). One way to limit the number of re-absorption events is using patterned waveguides which reduce the probability that emitted light encounters another dye molecule in its path towards the edge of the plate. It was shown previously that the emission efficiency increased significantly by reducing the surface area covered by the dye structures from 100% to 20%, and the efficiency was relatively insensitive to the shape of the pattern (Tsoi et al., 2010, Tsoi 2012). However, such a system produced less total emission than a fully-covered waveguide because there was a significant reduction of absorbed light energy, as much of the incident light simply passed through the blank regions of the waveguide.
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In this work, we describe using patterned double waveguides to effectively collect incident light. A comparison between several systems is made, with the conclusion that it is possible to outperform a standard fully-covered LSC while maintaining a device that should perform similarly in indirect and direct lighting. 2. Experimental The LSCs used in this project consisted of fluorescent organic dye doped (patterned) films coated on top of polymer waveguides. Two different fluorescent dyes were used to dope the films: perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylic acid-bis-(20 -60 diisopropylanilide) (Lumogen F Red305, BASF), referred to as Red305 and perylene1,7,8,12-tetrachloro-3,4,9,10 tetracarboxylic acid-bis-(20 60 diisopropylanilide) (Lumogen F Yellow 083, BASF), referred to as Yell083. The organic dyes were dissolved in a matrix of 5:1 weight mixture of di-pentaerythriol pentacrylate (DPPA, Polyscience Inc.) and methylmethacrylate (MMA, Aldrich), the latter used to reduce viscosity for easier mixing, with 0.3 w% 1-hydroxycyclohexyl phenyl ketone (as photoinitiator) (Irgacure 184, Ciba Specialty Chemicals). Fully covered film LSCs were produced on either 50 50 3 mm3 or 50 50 6 mm3 PMMA waveguides (Plano Plastics). 0.2 w% Red305 was added to the acrylate mixture and applied on the PMMA waveguides at 65 °C by manual bar-coating, resulting in a wet film with layer thickness of approximately 80 lm. The wet films were photopolymerized in a nitrogen environment at RT by illuminating them for 135 s with a high-intensity UV lamp (OmniCure S-2000 XL UV spot curing lamp). Two fully covered film 50 50 3 mm3 LSCs were produced with an average peak absorbance of 0.87 ± 0.04 and five fully covered films 50 50 6 mm3 LSCs were produced with an average peak absorbance of 0.83 ± 0.04 measured using a Shimadzu UV-3102 spectrophotometer. Fig. 1 schematically depicts the patterned dye waveguides containing Red305. Patterned LSCs based on Red305 were produced on 50 50 3 mm3 PMMA waveguides using identical dye/ acrylate mixtures and application procedure. To produce the patterned film LSCs, a mask consisting of equally spaced lines covering 20%, 50% and 80% of the waveguide surface was placed above the wet film which was photopolymerized by illuminating for 15 s with the high-intensity UV lamp through this mask. After exposure, the samples
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were developed in ethanol for 40 s at room temperature under continuous agitation to etch away the non-polymerized material on the waveguide. After removing from the ethanol, the LSCs were illuminated for another 120 s with the high-intensity UV lamp in nitrogen to ensure complete crosslinking. The average peak absorbance of individual lines in the patterned LSCs ranged from 0.80–0.84. LSCs with a dye pattern on either side of the same waveguide, called ‘double patterned’, were produced on 50 50 6 mm3 PMMA waveguides. These LSCs consisted of offset patterned films on both the top and bottom sides of the waveguide. Two versions of this LSC type were produced: (1) waveguides with Red305 doped patterned film on each side of the waveguide and (2) waveguides with one Red305 doped patterned film on top and one complementary Yell083 doped film on the bottom. Production of the Yell083 doped patterns was similar to the Red305: 0.05 w% Yell083 was added to the matrix-mixture and applied on the PMMA waveguides at 65 °C and bar-coated by hand using bars of various wire diameters to obtain structure thicknesses of 50, 75 and 150 lm. The average thickness of the Red305 structures was measured to be 75.3 ± 4.2 lm using a Fogale Zoomsurf 3D optical profiler. Such high structures resulted in obvious emission losses from the edges of the features. To make a smooth surface and maintain waveguiding properties, an additional coating layer using the matrix without fluorescent dyes was applied by bar-coating the matrix-mixture at 65 °C on the patterned film to fill the gaps between the structures. The LSCs were transferred to a nitrogen environment and photopolymerized by illuminating them for 180 s with a Philips Original Home Solaria type HB 172. Applying the additional gap-filling coating resulted in a slightly ‘waved’ surface with a total combined thickness of 124.3 ± 18.6 lm. The additional coating prevented light losses from the edges of the pattern structures themselves: edge output increased by 20% (40.6 ± 0.9 mW to 48.0 ± 1.3 mW) upon pattern filling in this way. The light emitted from the edges of the LSCs was recorded using an integrating sphere equipped with a Labsphere LPS 100-0260 light detector array (see Fig. 2 for a depiction of the measurement setup). A sample holder that could be rotated around its axis was used for angular dependency measurements. The samples were illuminated with collimated light from a 300 W solar simulator (Lot Oriel) equipped with a filter to approximate the 1.5 AM solar spectrum at a distance of 15 cm with an integrated
Fig. 1. A schematic depiction of the three different LSC types studied in this project. Top left: a fully covered film LSC. Top right: a patterned film LSC. Bottom: a double side patterned film LSC.
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For a fair comparison, it is necessary to keep the total thickness of the waveguides the same for both the single and dual waveguide systems. Thus, if the fully-covered waveguide is 6 mm thick, the dual waveguide system should consist of two waveguides each 3 mm in thickness.
Fig. 2. Schematic depicting the setup for the edge emission measurements.
power between 350 and 750 nm of 879.3 ± 5.2 mW. The power emitted by each of the four edges of the LSCs was calculated by integrating the emission spectrum from that edge between 350 nm to 750 nm. The total power emitted was calculated by taking the sum of the power emitted by each edge of the LSC. Measurements were performed using two different backgrounds placed below the LSCs using an air gap: either a black absorbing background made from a piece of black cardboard or a white scattering background was made by spray painting a piece of white cardboard (Montana Gold matte white acrylic). The total power efficiency (gtot) of a LSC was calculated using Eq. (1), where Pout is the calculated total power emitted from the edge of the LSC and Pincident is the total incident power on the LSC. Pincident was calculated by integrating the measured solar simulator spectrum from 350 to 750 nm and multiplying this output with the illuminated area of the LSCs. gtot ¼
P out P incident
Fig. 3. Edge emission spectra of 0.2% Red305 waveguides using a white scattering background of fully covered film LSCs with geometries: 50 50 3 mm3 (black) and 50 50 6 mm3 (red). Inset: photographs of fully covered film LSCs, top: a 50 50 3 mm3 LSC, bottom: a 50 50 6 mm3 LSC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The average peak absorbance, average integrated edge output from 350 to 750 nm (Pout) and total efficiency (gtot) of fully covered film 50 50 mm2 LSCs with different thicknesses using a scattering background. Waveguide thickness (mm)
Average peak absorbance (a.u)
Pout (mW)
gtot (%)
3 6
0.87 ± 0.04 0.84 ± 0.04
131.9 ± 1.4 143.0 ± 2.8
15.0 ± 0.2 16.3 ± 0.3
ð1Þ
3. Results and discussion The goal of this study is to identify a patterned LSC system that can outperform a fully covered LSC acting on its own. To accomplish this, we propose stacking two patterned waveguides on top of each other, to maintain total absorption of the system while simultaneously taking advantage of reduction of reabsorption in the patterned waveguides. The dye we have chosen to study is the same used in the previous work on patterned systems, namely Lumogen Red305, a commercially available perylene dye which has been often used in LSC work due to its relatively broad absorption range, near unity FQY, good photostability and proven solubility in acrylate matrices (Baumberg et al., 2001; Desmet et al., 2012; Goldschmidt et al., 2009; Slooff et al., 2008; Wilson et al., 2010).
Fig. 4. Total efficiency plotted against area coverage of a patterned film 50 50 3 mm3 LSC by using a black absorbing background (black circles) and a white scattering background (gray circles). Error bars are smaller than the size of the symbols. Inset: Photograph of patterned film 50 50 3 mm3 LSCs with varying area coverage (20%, 50% and 80%) including an additional gap-filling coating.
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Fig. 5. Normalized absorbance spectra (solid lines) and emission spectra (dotted lines) of Yell083 (gray) and Red305 (black) in ethanol at low concentration.
Fig. 6. Depiction of the functionality of the dual-dye patterned waveguide and photograph of a double side patterned film 50 50 6 mm3 LSC (RY75).
Table 2 The average pattern thickness and average peak absorbance of double side patterned film 50 50 6 mm3 LSCs with an area coverage of 50% per pattern. The given LSC type name refers to the layer thickness of the Yell083 pattern.
Top pattern Bottom pattern Top pattern Bottom pattern Top pattern Bottom pattern
Dye
Average thickness (lm)
Average peak absorbance (a.u.)
LSC type name
Red305 Yell083
75.3 ± 4.2 48.2 ± 1.0
0.82 ± 0.07 0.17 ± 0.01
R-Y50
Red305 Yell083
75.3 ± 4.2 73.6 ± 2.1
0.88 ± 0.04 0.44 ± 0.02
R-Y75
Red305 Yell083
75.3 ± 4.2 156.5 ± 3.5
0.88 ± 0.04 0.71 ± 0.04
R-Y155
Fig. 3 shows the emission spectrum of 6 mm thick LSCs coated with a thin solid acrylate layer containing the Red305 dye compared to the emission spectrum of the 3 mm thick LSC similarly coated. The blue shifted shoulder of the spectrum of the 6 mm waveguide demonstrates that
more photons with higher energy are capable of reaching the LSC edges, indicating less photons encountered another luminophore. The average integrated output from 350 to 750 nm (Pout) and the total efficiency (gtot) of the fully covered films are given in Table 1. Patterned film LSCs were produced with equally spaced lines that covered 20%, 50% or 80% of the 50 50 3 mm3 LSC top surface. The average total emission efficiencies of patterned film 50 50 3 mm3 LSCs using both a black absorbing and white scattering background are given in Fig. 4. As expected, the LSCs with smaller area coverage benefited more (relatively) from a scattering background than LSCs with larger area coverage (Debije et al., 2009; Roncali and Garnier, 1984). There was a spectral red shift in the emission light with increased surface coverage of the LSCs, similar to that seen previously (Tsoi et al., 2010). A first option to improve light collection in a single plate is to add an offset patterned film doped with a different luminophore. This new luminophore ideally would absorb light that otherwise would be lost where the areas were clear, and emit within the absorption band of the Red305 dye. Furthermore, the emission of the Red305 should be outside the absorption of the spacefilling dye. The dye chosen for this purpose was Lumogen Yellow083. The absorption and emission spectra for both dyes are shown in Fig. 5. A schematic depiction of this LSC system using the yellow/green emitting luminophore (Yell083) and the red emitting luminophore (Red305) is given in Fig. 6. By making use of this offset pattern, light that otherwise would be lost through the bottom surface has now a chance to get absorbed by the additional (yellow1) pattern. Photons emitted by the yellow pattern have a high probability of being re-absorbed upon encountering the red pattern.
1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
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P.T.M. Albers et al. / Solar Energy 95 (2013) 216–223 Table 3 The average integrated edge output from 350 to 750 nm (Pout) and total efficiency (gtot) of double side patterned film 50 50 6 mm3 LSCs. The data of a 50 50 6 mm3 LSC with only the Red305 doped pattern on the top surface is also given as R-blank. The data are given for both a black absorbing background and a white scattering background. LSC type
Pout (mW) Absorbing background
gtot (%) Absorbing background
Pout (mW) Scattering background
gtot (%) Scattering background
R-blank R-Y50 R-Y75 R-Y155
53.3 ± 0.5 59.3 ± 1.5 70.7 ± 1.4 76.5 ± 1.6
6.1 ± 0.1 6.7 ± 0.2 8.0 ± 0.2 8.7 ± 0.2
91.4 ± 0.8 101.0 ± 1.4 118.8 ± 2.1 122.6 ± 1.6
10.4 ± 0.1 11.5 ± 0.2 13.5 ± 0.3 13.9 ± 0.2
Fig. 7. (left) Photograph of a double side patterned 50 50 6 mm3 LSC with a Red305 pattern and a gap-filling coating on both sides. (right) Photograph of a 50–50% stacked patterned film LSC using dual 50 50 3 mm3 waveguide, including a gap-filling coating on each pattern.
Due to restricted overlap between its emission band and the absorption band of the Yell083 pattern, photons emitted by this Red305 pattern will not be absorbed in the Yell083 regions, and so maintain the benefits of patterning. Each pattern covered 50% of their respective surface. The thickness of the Red305 doped films was kept constant, while the thickness of the Yell083 doped film was varied between approximately 50 and 155 lm; in this way, the absorption could be varied without changing the dye concentration. Combining the Red305 doped patterns with complementary Yell083 patterns on the opposite face
of a LSC resulted in three different LSC types, named for the thickness of the Yell083 layer: (1) R-Y50, (2) R-Y75 and (3) R-Y155. See Table 2 for the average layer thickness and peak absorbance of each of the five lines in the patterns, together with the name of each type of double side patterned film LSCs. The total emission of each type double side patterned film LSC was determined using both a black absorbing background and a white scattering background. The output and total efficiency of these LSCs are given in Table 3.
Fig. 8. Schematic depiction of a stack of two complementary patterned waveguides. The waveguides are separated by an air gap which preserves the waveguiding properties of each separate waveguide. By applying a complementary pattern on each waveguide the overall absorbing area will be the same as that of a fully covered film LSC. The dimensions in the picture are exaggerated.
Fig. 9. The total efficiency of the 50 50 3 mm3 stacked patterned film LSCs using a scattering background plotted against the surface coverage of the top waveguide. The red line represents the total efficiency of fully covered 50 50 6 mm3 LSC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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This data shows a modest output increase upon increasing the thickness of the Yell083 pattern. Light conversion would be improved when the emission band of the first luminophore has a larger overlap with the absorption band of the second luminophore to increase the probability that the emitted light is absorbed. The overlap between the emission band of Yell083 and the absorption band of Red305 is reasonable, but not optimal, and less overlap of the emission of the Red305 would be preferred, as stated earlier. A double sided pattern as shown in Fig. 6 could be modified to use the Red305 dye on both surfaces, rather than using the Yell083: a photograph of such a LSC is shown in the left image of Fig. 7. The emission of such a double side patterned film 50 50 6 mm3 with average peak absorbance of 0.87 ± 0.07 on both sides on a scattering background was 146.2 ± 3.1 mW, resulting in a gtot of 16.6 ± 0.4%, higher than the Red305/Yell083 system (as
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expected), and slightly higher than from a fully-covered film. An alternative flat system could make use of a stack of two separate patterned film waveguides (see Figs. 7b and 8). The total absorbing area will be identical to that of a fully covered film LSC while reducing the re-absorption probability within each separate waveguide. Patterned 50 50 3 mm3 LSCs were overlaid to form a stack with a total coverage of 100% of the illuminated LSC area. The total efficiency of the stacked patterned film LSCs is plotted against the surface coverage of the top waveguide in Fig. 9. The red line in the graph represents the total efficiency of fully covered film LSCs with similar dimensions as the stacked patterned film LSCs. The graph shows a stacked patterned film LSC outperformed a fully covered LSC when the area coverage of the top waveguide was between 20% and 90%, with a maximum efficiency around 50% coverage of 17.4%; this is compared to 16.3% for the fully
Table 4 Overview of efficiencies of the 50 50 6 mm3 LSCs described in this work. LSC type
Example
Total efficiency (%) (power out/power incident) with a scattering background
Fully covered film LSC
16.3
Patterned film LSC (50% coverage)
10.4
Double side patterned film LSC (R-Y150)
13.9
Stacked patterned film LSC (50–50%)
17.4
Stacked patterned film LSC with one double side patterned waveguide on bottom (50%-R-Y75)
18.8
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covered film LSC on a 6 mm waveguide (see Table 4). Thus, dividing the total absorbing area over two complementary patterned waveguides benefits from reduced reabsorption losses and increases the total efficiency of the LSC. Further increasing the amount of separate waveguides was not considered because higher reflection losses due to additional surfaces will likely counteract efficiency gains from reducing the re-absorption losses. The higher efficiency of stacked patterned film waveguides compared to fully covered LSCs is not restricted to light incident perpendicular to the LSC surface. The dependency on the angular incidence of light on the LSC output for a fully covered film 50 50 6 mm3 LSC and a 50–50% stacked patterned LSC was measured using a white scattering background. The angle of incident light was varied between ±60o with respect to the normal of the LSCs with the lengths of the line pattern oriented both parallel perpendicular to the entry port of the integrating sphere. The results are given in Fig. 10. The measured edge outputs (the inset of the figure) were adjusted for the reduced illumination area of the films at larger angles by dividing the measured output by the cosine of the angle of deviation of the rotated sample, taking zero as the situation where the waveguide normal is parallel to the incidence of the collimated light from the solar simulator sample. It can be seen from the graph that the total output of the stacked patterned LSCs from all four edges is higher than that of a fully covered film LSC for each angle of incident light. Absorption increases as the pathlength through the dye increases for both samples at higher angles. We noted the emission from the edges parallel and perpendicular to the line patterns differed slightly, with more light emitted from the parallel edges, similar to previous results (Tsoi et al., 2010).
Since two stacked complementary Red305 patterned waveguides can outperform a fully covered film LSC, and knowing addition of a Yell083 doped pattern to a single patterned Red305 doped film LSC will increase the output of the patterned film LSC, it was likely that applying a Yell083 doped film on the bottom of a patterned waveguide in the stack will increase the total output of the stacked patterned film LSC. A stack consisting of a 50% covered patterned film waveguide and a double side patterned film 50 50 3 mm3 waveguide with one 0.2 w% Red305 doped pattern on top and a 0.05 w% Yell083 doped pattern with a thickness of approximately 74 lm and peak absorbance of 0.44 on the bottom was studied. The two waveguides were placed with the double side patterned waveguide at the bottom of the stack. The total efficiency of the stack increased from 17.4% to 18.8% (see Fig. 11 and Table 4). Naturally, it is of interest to compare the performance of the dual waveguide system with the lens/pattern system described previously (Tsoi et al., 2013). The differences in absorbance between LSCs produced in this work and previously (Tsoi et al., 2010, 2013) complicate the comparison. The current LSCs used a Red305 dye concentration of 0.2% and layer thicknesses of 75 lm and absorbances of 0.8–1.3, compared to dye concentrations of 0.5% and thicknesses closer to 20 lm for the samples from Tsoi et al., 2013 and absorbance closer to 0.4. The apparent efficiencies in Tsoi et al. are lower than in this work, at least partially due to the low absorbance these systems. To compare performance, we have used the results of the fully covered waveguides of both studies to scale the LSCs produced in the earlier work. The two 100% covered samples have the same efficiency when the results of the samples from Tsoi et al. are multiplied by the factor 1.5
Fig. 10. Angular dependence of the average total emission of a 50–50% stacked patterned film LSC (red) and a fully covered film LSC (black) using a white scattering background adjusted for changes in exposure area. Lines connecting the data points are added to aid the eye. (Inset) Raw data before adjusting for the changing illumination area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Stacks of two Red305 doped complementary patterned waveguides outperformed a fully covered LSC when the surface area coverage of the top waveguide is between 20% and 90%. A surface area coverage division of 50% on both waveguides gave the maximum total efficiency of 17.4% and was able to outperform the fully covered LSC for all angles of incident light (measured up to ±60°). Applying an additional Yell083 doped pattern on the bottom of the bottom waveguide of the stack increased the total efficiency of the stacked LSC to 18.8%, the highest we measured. Acknowledgement
Fig. 11. Edge emission spectra of a 50–50% stacked patterned LSCs using only Red305 (black) and of stacked LSCs consisting of a 50% patterned Red305 50 50 3 mm3 waveguide on top of a 50 50 3 mm3 double side patterned waveguide using Red305 and Yell083 (red line). The patterns contained 0.2 w% Red305 or 0.05 w% Yell083. Spectra were measured a white scattering background. Inset: Photographs of stack of 50% pattern waveguide on top of double side patterned waveguide on the bottom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(the earlier paper reported total efficiency of 10.7% for their fully-covered waveguide). The relation of emission output to absorbance is not strictly linear as we have assumed for this comparison; such an assumption somewhat overestimates the performance of the samples from Tsoi et al. Extrapolating the patterned film LSC with lenses from the reported value of 11.8% total efficiency (Tsoi et al., 2013) results in an estimated result of 17.9% total efficiency, very comparable to current results for the dual waveguide patterned system (see Table 4). In contrast to the dual waveguide systems, the patterned waveguide using lenses showed reduced performance of the system at steeper angles (Tsoi et al., 2013): the lenses were designed only to accept light within ±30°. 4. Conclusions This work has investigated the possibility of increasing the total efficiency of a luminescent solar concentrator by making use of the reduced re-absorption probability of patterned film waveguides. The addition of an offset Yell083 doped pattern to a Red305 patterned waveguide increased the total efficiency from 10.4% to 13.9%. Although this shows that using a Yell083 pattern to emit light to the Red305 pattern that otherwise would be lost through the clear region of the patterned waveguide works, this LSC system could not outperform a fully covered Red305 film LSC (which has a total efficiency of 16.3%). As one might expect, making use of Yell085 to increase the amount of Red305 absorbable light is not sufficient to compensate the light loss by lowering the Red305 doped absorbing area of the LSC, but it did demonstrate multicolored waveguides are possible, and that such systems can improve performance over a simple patterned waveguide.
M.D. acknowledges the support provided by STW Vidi Grant 07940. References Baumberg, I., Berezin, O., Drabkin, A., Gorelik, B., Kogan, L., Voskobojnik, M., Zaidman, M., 2001. Effect of polymer matrix on photo-stability of photo-luminescent dyes in multi-layer polymeric structures. Polymer Degradation and Stability 73, 403–410. Debije, M.G., Verbunt, P.P.C., 2012. Thirty years of luminescent solar concentrator research: solar energy for the built environment. Advanced Energy Materials 2, 12–35. Debije, M.G., Verbunt, P.P.C., Rowan, B.C., Richards, B.S., Hoeks, T., 2008. Measured surface loss from luminescent solar concentrator waveguides. Applied Optics 47, 6763–6768. Debije, M.G., Teunissen, J.-P., Kastelijn, M.J., Verbunt, P.P.C., Bastiaansen, C.W.M., 2009. The effect of a scattering layer on the edge output of a luminescent solar concentrator. Solar Energy Materials Solar Cells 93, 1345–1350. Desmet, L., Ras, A.J.M., De Boer, D.K.G., Debije, M.G., 2012. Monocrystalline silicon photovoltaic luminescent solar concentrator with 4.2% power conversion efficiency. Optics Letters 37, 3087–3089. Earp, A.A., Franklin, J.B., Smith, G.B., 2011. Absorption tails and extinction in luminescent solar concentrators. Solar Energy Materials and Solar Cells 95, 1157–1162. Goldschmidt, J.C., Peters, M., Bo¨sch, A., Helmers, H., Dimroth, F., Glunz, S.W., Willeke, G.P., 2009. Increasing the efficiency of fluorescent concentrator systems. Solar Energy Materials and Solar Cells 93, 176–182. Olson, R.W., Loring, R.F., Fayer, M.D., 1981. Luminescent solar concentrators and the reabsorption problem. Applied Optics 20, 2934–2940. Roncali, J., Garnier, F., 1984. New luminescent back reflectors for the improvement of the spectral response and efficiency of luminescent solar concentrators. Solar Cells 13, 133–143. Slooff, L.H., Bende, E.E., Burgers, A.R., Budel, T., Pravettoni, M., Kenny, R.P., Dunlop, E.D., Bu¨chtemann, A., 2008. A luminescent solar concentrator with 71% power conversion efficiency. Physica Status Solidi – R 2, 257–259. Tsoi, S., 2012. Structured luminescent solar energy concentrators. PhD Thesis, Eindhoven University of Technology, Eindhoven, pp. 160. Tsoi, S., Broer, D.J., Bastiaansen, C.W.M., Debije, M.G., 2010. Patterned dye structure limit reabsorption in luminescent solar concentrators. Optics Express 18, A536–A543. Tsoi, S., Broer, D.J., Bastiaansen, C.W.M., Debije, M.G., 2013. Using lenses to improve the output of a patterned luminescent solar concentrator. Advanced Energy Materials 3, 337–341. Weber, W.H., Lambe, J., 1976. Luminescent greenhouse collector for solar radiation. Applied Optics 15, 2299–2300. Wilson, L.R., Rowan, B.C., Robertson, N., Moudam, O., Jones, A.C., Richards, B.S., 2010. Characterization and reduction of reabsorption losses in luminescent solar concentrators. Applied Optics 49, 1651– 1661.
Solar Energy 95 (2013) 216–223 www.elsevier.com/locate/solener
Dual waveguide patterned luminescent solar concentrators Peter T.M. Albers, Cees W.M. Bastiaansen, Michael G. Debije ⇑ Chemical Engineering & Chemistry, Functional Organic Materials & Devices, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands Received 13 March 2013; received in revised form 3 June 2013; accepted 11 June 2013 Available online 17 July 2013 Communicated by: Associate Editor J.-L. Scartezzini
Abstract Re-absorption of dye-emitted light is a primary loss mechanism in luminescent solar concentrator waveguides. Previous work has demonstrated patterning the waveguide surface with dyes will increase transport efficiency at the cost of total light absorption. This work demonstrates a patterned double waveguide system employing patterning offset between the plates which allows complete surface coverage but can generate an edge emission greater than a standard thin-film fully covered luminescent solar concentrator of the same total thickness, and is effective for all incident angles of light. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Luminescent solar concentrator; Patterned waveguide; Fluorescence
1. Introduction Luminescent solar concentrators (LSCs) have been proposed as an alternative device for generating electricity from sunlight which could be well-suited for use in urban settings (Debije and Verbunt, 2012; Weber and Lambe, 1976). The standard LSC is quite simple: normally it is a plastic sheet which acts as a waveguide doped with or topped by luminescent materials. Incident sunlight is absorbed by the dye material and emitted at longer wavelengths, a significant fraction being trapped inside the plate by total internal reflection. To the edge(s) of the plate photovoltaic (PV) cells are attached for conversion of the light into electrical current. The LSC could be ideal for functioning in the built environment because they may be of any color and shape, simplifying architectural integration, and they work equally well in direct and indirect sunlight. The LSC has not yet found widespread use due to excessive losses that limit their light conversion efficiency. In ⇑ Corresponding author. Tel.: +31 40 247 5881.
E-mail address: [email protected] (M.G. Debije). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.06.014
particular, re-absorption of light emitted by one dye molecule by a subsequent dye molecule because of the limited Stokes shifts of the dyes is a primary loss mechanism (Earp et al., 2011; Olson et al., 1981; Wilson et al., 2010). The losses are a result of non-unity fluorescence quantum yield (FQY) of the luminophores, and significant probability that the re-emitted light is directed in such a way as to escape the top or bottom surfaces of the waveguide (Debije et al., 2008). One way to limit the number of re-absorption events is using patterned waveguides which reduce the probability that emitted light encounters another dye molecule in its path towards the edge of the plate. It was shown previously that the emission efficiency increased significantly by reducing the surface area covered by the dye structures from 100% to 20%, and the efficiency was relatively insensitive to the shape of the pattern (Tsoi et al., 2010, Tsoi 2012). However, such a system produced less total emission than a fully-covered waveguide because there was a significant reduction of absorbed light energy, as much of the incident light simply passed through the blank regions of the waveguide.
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In this work, we describe using patterned double waveguides to effectively collect incident light. A comparison between several systems is made, with the conclusion that it is possible to outperform a standard fully-covered LSC while maintaining a device that should perform similarly in indirect and direct lighting. 2. Experimental The LSCs used in this project consisted of fluorescent organic dye doped (patterned) films coated on top of polymer waveguides. Two different fluorescent dyes were used to dope the films: perylene-1,7,8,12-tetraphenoxy-3,4,9,10 tetracarboxylic acid-bis-(20 -60 diisopropylanilide) (Lumogen F Red305, BASF), referred to as Red305 and perylene1,7,8,12-tetrachloro-3,4,9,10 tetracarboxylic acid-bis-(20 60 diisopropylanilide) (Lumogen F Yellow 083, BASF), referred to as Yell083. The organic dyes were dissolved in a matrix of 5:1 weight mixture of di-pentaerythriol pentacrylate (DPPA, Polyscience Inc.) and methylmethacrylate (MMA, Aldrich), the latter used to reduce viscosity for easier mixing, with 0.3 w% 1-hydroxycyclohexyl phenyl ketone (as photoinitiator) (Irgacure 184, Ciba Specialty Chemicals). Fully covered film LSCs were produced on either 50 50 3 mm3 or 50 50 6 mm3 PMMA waveguides (Plano Plastics). 0.2 w% Red305 was added to the acrylate mixture and applied on the PMMA waveguides at 65 °C by manual bar-coating, resulting in a wet film with layer thickness of approximately 80 lm. The wet films were photopolymerized in a nitrogen environment at RT by illuminating them for 135 s with a high-intensity UV lamp (OmniCure S-2000 XL UV spot curing lamp). Two fully covered film 50 50 3 mm3 LSCs were produced with an average peak absorbance of 0.87 ± 0.04 and five fully covered films 50 50 6 mm3 LSCs were produced with an average peak absorbance of 0.83 ± 0.04 measured using a Shimadzu UV-3102 spectrophotometer. Fig. 1 schematically depicts the patterned dye waveguides containing Red305. Patterned LSCs based on Red305 were produced on 50 50 3 mm3 PMMA waveguides using identical dye/ acrylate mixtures and application procedure. To produce the patterned film LSCs, a mask consisting of equally spaced lines covering 20%, 50% and 80% of the waveguide surface was placed above the wet film which was photopolymerized by illuminating for 15 s with the high-intensity UV lamp through this mask. After exposure, the samples
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were developed in ethanol for 40 s at room temperature under continuous agitation to etch away the non-polymerized material on the waveguide. After removing from the ethanol, the LSCs were illuminated for another 120 s with the high-intensity UV lamp in nitrogen to ensure complete crosslinking. The average peak absorbance of individual lines in the patterned LSCs ranged from 0.80–0.84. LSCs with a dye pattern on either side of the same waveguide, called ‘double patterned’, were produced on 50 50 6 mm3 PMMA waveguides. These LSCs consisted of offset patterned films on both the top and bottom sides of the waveguide. Two versions of this LSC type were produced: (1) waveguides with Red305 doped patterned film on each side of the waveguide and (2) waveguides with one Red305 doped patterned film on top and one complementary Yell083 doped film on the bottom. Production of the Yell083 doped patterns was similar to the Red305: 0.05 w% Yell083 was added to the matrix-mixture and applied on the PMMA waveguides at 65 °C and bar-coated by hand using bars of various wire diameters to obtain structure thicknesses of 50, 75 and 150 lm. The average thickness of the Red305 structures was measured to be 75.3 ± 4.2 lm using a Fogale Zoomsurf 3D optical profiler. Such high structures resulted in obvious emission losses from the edges of the features. To make a smooth surface and maintain waveguiding properties, an additional coating layer using the matrix without fluorescent dyes was applied by bar-coating the matrix-mixture at 65 °C on the patterned film to fill the gaps between the structures. The LSCs were transferred to a nitrogen environment and photopolymerized by illuminating them for 180 s with a Philips Original Home Solaria type HB 172. Applying the additional gap-filling coating resulted in a slightly ‘waved’ surface with a total combined thickness of 124.3 ± 18.6 lm. The additional coating prevented light losses from the edges of the pattern structures themselves: edge output increased by 20% (40.6 ± 0.9 mW to 48.0 ± 1.3 mW) upon pattern filling in this way. The light emitted from the edges of the LSCs was recorded using an integrating sphere equipped with a Labsphere LPS 100-0260 light detector array (see Fig. 2 for a depiction of the measurement setup). A sample holder that could be rotated around its axis was used for angular dependency measurements. The samples were illuminated with collimated light from a 300 W solar simulator (Lot Oriel) equipped with a filter to approximate the 1.5 AM solar spectrum at a distance of 15 cm with an integrated
Fig. 1. A schematic depiction of the three different LSC types studied in this project. Top left: a fully covered film LSC. Top right: a patterned film LSC. Bottom: a double side patterned film LSC.
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For a fair comparison, it is necessary to keep the total thickness of the waveguides the same for both the single and dual waveguide systems. Thus, if the fully-covered waveguide is 6 mm thick, the dual waveguide system should consist of two waveguides each 3 mm in thickness.
Fig. 2. Schematic depicting the setup for the edge emission measurements.
power between 350 and 750 nm of 879.3 ± 5.2 mW. The power emitted by each of the four edges of the LSCs was calculated by integrating the emission spectrum from that edge between 350 nm to 750 nm. The total power emitted was calculated by taking the sum of the power emitted by each edge of the LSC. Measurements were performed using two different backgrounds placed below the LSCs using an air gap: either a black absorbing background made from a piece of black cardboard or a white scattering background was made by spray painting a piece of white cardboard (Montana Gold matte white acrylic). The total power efficiency (gtot) of a LSC was calculated using Eq. (1), where Pout is the calculated total power emitted from the edge of the LSC and Pincident is the total incident power on the LSC. Pincident was calculated by integrating the measured solar simulator spectrum from 350 to 750 nm and multiplying this output with the illuminated area of the LSCs. gtot ¼
P out P incident
Fig. 3. Edge emission spectra of 0.2% Red305 waveguides using a white scattering background of fully covered film LSCs with geometries: 50 50 3 mm3 (black) and 50 50 6 mm3 (red). Inset: photographs of fully covered film LSCs, top: a 50 50 3 mm3 LSC, bottom: a 50 50 6 mm3 LSC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The average peak absorbance, average integrated edge output from 350 to 750 nm (Pout) and total efficiency (gtot) of fully covered film 50 50 mm2 LSCs with different thicknesses using a scattering background. Waveguide thickness (mm)
Average peak absorbance (a.u)
Pout (mW)
gtot (%)
3 6
0.87 ± 0.04 0.84 ± 0.04
131.9 ± 1.4 143.0 ± 2.8
15.0 ± 0.2 16.3 ± 0.3
ð1Þ
3. Results and discussion The goal of this study is to identify a patterned LSC system that can outperform a fully covered LSC acting on its own. To accomplish this, we propose stacking two patterned waveguides on top of each other, to maintain total absorption of the system while simultaneously taking advantage of reduction of reabsorption in the patterned waveguides. The dye we have chosen to study is the same used in the previous work on patterned systems, namely Lumogen Red305, a commercially available perylene dye which has been often used in LSC work due to its relatively broad absorption range, near unity FQY, good photostability and proven solubility in acrylate matrices (Baumberg et al., 2001; Desmet et al., 2012; Goldschmidt et al., 2009; Slooff et al., 2008; Wilson et al., 2010).
Fig. 4. Total efficiency plotted against area coverage of a patterned film 50 50 3 mm3 LSC by using a black absorbing background (black circles) and a white scattering background (gray circles). Error bars are smaller than the size of the symbols. Inset: Photograph of patterned film 50 50 3 mm3 LSCs with varying area coverage (20%, 50% and 80%) including an additional gap-filling coating.
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Fig. 5. Normalized absorbance spectra (solid lines) and emission spectra (dotted lines) of Yell083 (gray) and Red305 (black) in ethanol at low concentration.
Fig. 6. Depiction of the functionality of the dual-dye patterned waveguide and photograph of a double side patterned film 50 50 6 mm3 LSC (RY75).
Table 2 The average pattern thickness and average peak absorbance of double side patterned film 50 50 6 mm3 LSCs with an area coverage of 50% per pattern. The given LSC type name refers to the layer thickness of the Yell083 pattern.
Top pattern Bottom pattern Top pattern Bottom pattern Top pattern Bottom pattern
Dye
Average thickness (lm)
Average peak absorbance (a.u.)
LSC type name
Red305 Yell083
75.3 ± 4.2 48.2 ± 1.0
0.82 ± 0.07 0.17 ± 0.01
R-Y50
Red305 Yell083
75.3 ± 4.2 73.6 ± 2.1
0.88 ± 0.04 0.44 ± 0.02
R-Y75
Red305 Yell083
75.3 ± 4.2 156.5 ± 3.5
0.88 ± 0.04 0.71 ± 0.04
R-Y155
Fig. 3 shows the emission spectrum of 6 mm thick LSCs coated with a thin solid acrylate layer containing the Red305 dye compared to the emission spectrum of the 3 mm thick LSC similarly coated. The blue shifted shoulder of the spectrum of the 6 mm waveguide demonstrates that
more photons with higher energy are capable of reaching the LSC edges, indicating less photons encountered another luminophore. The average integrated output from 350 to 750 nm (Pout) and the total efficiency (gtot) of the fully covered films are given in Table 1. Patterned film LSCs were produced with equally spaced lines that covered 20%, 50% or 80% of the 50 50 3 mm3 LSC top surface. The average total emission efficiencies of patterned film 50 50 3 mm3 LSCs using both a black absorbing and white scattering background are given in Fig. 4. As expected, the LSCs with smaller area coverage benefited more (relatively) from a scattering background than LSCs with larger area coverage (Debije et al., 2009; Roncali and Garnier, 1984). There was a spectral red shift in the emission light with increased surface coverage of the LSCs, similar to that seen previously (Tsoi et al., 2010). A first option to improve light collection in a single plate is to add an offset patterned film doped with a different luminophore. This new luminophore ideally would absorb light that otherwise would be lost where the areas were clear, and emit within the absorption band of the Red305 dye. Furthermore, the emission of the Red305 should be outside the absorption of the spacefilling dye. The dye chosen for this purpose was Lumogen Yellow083. The absorption and emission spectra for both dyes are shown in Fig. 5. A schematic depiction of this LSC system using the yellow/green emitting luminophore (Yell083) and the red emitting luminophore (Red305) is given in Fig. 6. By making use of this offset pattern, light that otherwise would be lost through the bottom surface has now a chance to get absorbed by the additional (yellow1) pattern. Photons emitted by the yellow pattern have a high probability of being re-absorbed upon encountering the red pattern.
1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
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P.T.M. Albers et al. / Solar Energy 95 (2013) 216–223 Table 3 The average integrated edge output from 350 to 750 nm (Pout) and total efficiency (gtot) of double side patterned film 50 50 6 mm3 LSCs. The data of a 50 50 6 mm3 LSC with only the Red305 doped pattern on the top surface is also given as R-blank. The data are given for both a black absorbing background and a white scattering background. LSC type
Pout (mW) Absorbing background
gtot (%) Absorbing background
Pout (mW) Scattering background
gtot (%) Scattering background
R-blank R-Y50 R-Y75 R-Y155
53.3 ± 0.5 59.3 ± 1.5 70.7 ± 1.4 76.5 ± 1.6
6.1 ± 0.1 6.7 ± 0.2 8.0 ± 0.2 8.7 ± 0.2
91.4 ± 0.8 101.0 ± 1.4 118.8 ± 2.1 122.6 ± 1.6
10.4 ± 0.1 11.5 ± 0.2 13.5 ± 0.3 13.9 ± 0.2
Fig. 7. (left) Photograph of a double side patterned 50 50 6 mm3 LSC with a Red305 pattern and a gap-filling coating on both sides. (right) Photograph of a 50–50% stacked patterned film LSC using dual 50 50 3 mm3 waveguide, including a gap-filling coating on each pattern.
Due to restricted overlap between its emission band and the absorption band of the Yell083 pattern, photons emitted by this Red305 pattern will not be absorbed in the Yell083 regions, and so maintain the benefits of patterning. Each pattern covered 50% of their respective surface. The thickness of the Red305 doped films was kept constant, while the thickness of the Yell083 doped film was varied between approximately 50 and 155 lm; in this way, the absorption could be varied without changing the dye concentration. Combining the Red305 doped patterns with complementary Yell083 patterns on the opposite face
of a LSC resulted in three different LSC types, named for the thickness of the Yell083 layer: (1) R-Y50, (2) R-Y75 and (3) R-Y155. See Table 2 for the average layer thickness and peak absorbance of each of the five lines in the patterns, together with the name of each type of double side patterned film LSCs. The total emission of each type double side patterned film LSC was determined using both a black absorbing background and a white scattering background. The output and total efficiency of these LSCs are given in Table 3.
Fig. 8. Schematic depiction of a stack of two complementary patterned waveguides. The waveguides are separated by an air gap which preserves the waveguiding properties of each separate waveguide. By applying a complementary pattern on each waveguide the overall absorbing area will be the same as that of a fully covered film LSC. The dimensions in the picture are exaggerated.
Fig. 9. The total efficiency of the 50 50 3 mm3 stacked patterned film LSCs using a scattering background plotted against the surface coverage of the top waveguide. The red line represents the total efficiency of fully covered 50 50 6 mm3 LSC. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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This data shows a modest output increase upon increasing the thickness of the Yell083 pattern. Light conversion would be improved when the emission band of the first luminophore has a larger overlap with the absorption band of the second luminophore to increase the probability that the emitted light is absorbed. The overlap between the emission band of Yell083 and the absorption band of Red305 is reasonable, but not optimal, and less overlap of the emission of the Red305 would be preferred, as stated earlier. A double sided pattern as shown in Fig. 6 could be modified to use the Red305 dye on both surfaces, rather than using the Yell083: a photograph of such a LSC is shown in the left image of Fig. 7. The emission of such a double side patterned film 50 50 6 mm3 with average peak absorbance of 0.87 ± 0.07 on both sides on a scattering background was 146.2 ± 3.1 mW, resulting in a gtot of 16.6 ± 0.4%, higher than the Red305/Yell083 system (as
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expected), and slightly higher than from a fully-covered film. An alternative flat system could make use of a stack of two separate patterned film waveguides (see Figs. 7b and 8). The total absorbing area will be identical to that of a fully covered film LSC while reducing the re-absorption probability within each separate waveguide. Patterned 50 50 3 mm3 LSCs were overlaid to form a stack with a total coverage of 100% of the illuminated LSC area. The total efficiency of the stacked patterned film LSCs is plotted against the surface coverage of the top waveguide in Fig. 9. The red line in the graph represents the total efficiency of fully covered film LSCs with similar dimensions as the stacked patterned film LSCs. The graph shows a stacked patterned film LSC outperformed a fully covered LSC when the area coverage of the top waveguide was between 20% and 90%, with a maximum efficiency around 50% coverage of 17.4%; this is compared to 16.3% for the fully
Table 4 Overview of efficiencies of the 50 50 6 mm3 LSCs described in this work. LSC type
Example
Total efficiency (%) (power out/power incident) with a scattering background
Fully covered film LSC
16.3
Patterned film LSC (50% coverage)
10.4
Double side patterned film LSC (R-Y150)
13.9
Stacked patterned film LSC (50–50%)
17.4
Stacked patterned film LSC with one double side patterned waveguide on bottom (50%-R-Y75)
18.8
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covered film LSC on a 6 mm waveguide (see Table 4). Thus, dividing the total absorbing area over two complementary patterned waveguides benefits from reduced reabsorption losses and increases the total efficiency of the LSC. Further increasing the amount of separate waveguides was not considered because higher reflection losses due to additional surfaces will likely counteract efficiency gains from reducing the re-absorption losses. The higher efficiency of stacked patterned film waveguides compared to fully covered LSCs is not restricted to light incident perpendicular to the LSC surface. The dependency on the angular incidence of light on the LSC output for a fully covered film 50 50 6 mm3 LSC and a 50–50% stacked patterned LSC was measured using a white scattering background. The angle of incident light was varied between ±60o with respect to the normal of the LSCs with the lengths of the line pattern oriented both parallel perpendicular to the entry port of the integrating sphere. The results are given in Fig. 10. The measured edge outputs (the inset of the figure) were adjusted for the reduced illumination area of the films at larger angles by dividing the measured output by the cosine of the angle of deviation of the rotated sample, taking zero as the situation where the waveguide normal is parallel to the incidence of the collimated light from the solar simulator sample. It can be seen from the graph that the total output of the stacked patterned LSCs from all four edges is higher than that of a fully covered film LSC for each angle of incident light. Absorption increases as the pathlength through the dye increases for both samples at higher angles. We noted the emission from the edges parallel and perpendicular to the line patterns differed slightly, with more light emitted from the parallel edges, similar to previous results (Tsoi et al., 2010).
Since two stacked complementary Red305 patterned waveguides can outperform a fully covered film LSC, and knowing addition of a Yell083 doped pattern to a single patterned Red305 doped film LSC will increase the output of the patterned film LSC, it was likely that applying a Yell083 doped film on the bottom of a patterned waveguide in the stack will increase the total output of the stacked patterned film LSC. A stack consisting of a 50% covered patterned film waveguide and a double side patterned film 50 50 3 mm3 waveguide with one 0.2 w% Red305 doped pattern on top and a 0.05 w% Yell083 doped pattern with a thickness of approximately 74 lm and peak absorbance of 0.44 on the bottom was studied. The two waveguides were placed with the double side patterned waveguide at the bottom of the stack. The total efficiency of the stack increased from 17.4% to 18.8% (see Fig. 11 and Table 4). Naturally, it is of interest to compare the performance of the dual waveguide system with the lens/pattern system described previously (Tsoi et al., 2013). The differences in absorbance between LSCs produced in this work and previously (Tsoi et al., 2010, 2013) complicate the comparison. The current LSCs used a Red305 dye concentration of 0.2% and layer thicknesses of 75 lm and absorbances of 0.8–1.3, compared to dye concentrations of 0.5% and thicknesses closer to 20 lm for the samples from Tsoi et al., 2013 and absorbance closer to 0.4. The apparent efficiencies in Tsoi et al. are lower than in this work, at least partially due to the low absorbance these systems. To compare performance, we have used the results of the fully covered waveguides of both studies to scale the LSCs produced in the earlier work. The two 100% covered samples have the same efficiency when the results of the samples from Tsoi et al. are multiplied by the factor 1.5
Fig. 10. Angular dependence of the average total emission of a 50–50% stacked patterned film LSC (red) and a fully covered film LSC (black) using a white scattering background adjusted for changes in exposure area. Lines connecting the data points are added to aid the eye. (Inset) Raw data before adjusting for the changing illumination area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Stacks of two Red305 doped complementary patterned waveguides outperformed a fully covered LSC when the surface area coverage of the top waveguide is between 20% and 90%. A surface area coverage division of 50% on both waveguides gave the maximum total efficiency of 17.4% and was able to outperform the fully covered LSC for all angles of incident light (measured up to ±60°). Applying an additional Yell083 doped pattern on the bottom of the bottom waveguide of the stack increased the total efficiency of the stacked LSC to 18.8%, the highest we measured. Acknowledgement
Fig. 11. Edge emission spectra of a 50–50% stacked patterned LSCs using only Red305 (black) and of stacked LSCs consisting of a 50% patterned Red305 50 50 3 mm3 waveguide on top of a 50 50 3 mm3 double side patterned waveguide using Red305 and Yell083 (red line). The patterns contained 0.2 w% Red305 or 0.05 w% Yell083. Spectra were measured a white scattering background. Inset: Photographs of stack of 50% pattern waveguide on top of double side patterned waveguide on the bottom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(the earlier paper reported total efficiency of 10.7% for their fully-covered waveguide). The relation of emission output to absorbance is not strictly linear as we have assumed for this comparison; such an assumption somewhat overestimates the performance of the samples from Tsoi et al. Extrapolating the patterned film LSC with lenses from the reported value of 11.8% total efficiency (Tsoi et al., 2013) results in an estimated result of 17.9% total efficiency, very comparable to current results for the dual waveguide patterned system (see Table 4). In contrast to the dual waveguide systems, the patterned waveguide using lenses showed reduced performance of the system at steeper angles (Tsoi et al., 2013): the lenses were designed only to accept light within ±30°. 4. Conclusions This work has investigated the possibility of increasing the total efficiency of a luminescent solar concentrator by making use of the reduced re-absorption probability of patterned film waveguides. The addition of an offset Yell083 doped pattern to a Red305 patterned waveguide increased the total efficiency from 10.4% to 13.9%. Although this shows that using a Yell083 pattern to emit light to the Red305 pattern that otherwise would be lost through the clear region of the patterned waveguide works, this LSC system could not outperform a fully covered Red305 film LSC (which has a total efficiency of 16.3%). As one might expect, making use of Yell085 to increase the amount of Red305 absorbable light is not sufficient to compensate the light loss by lowering the Red305 doped absorbing area of the LSC, but it did demonstrate multicolored waveguides are possible, and that such systems can improve performance over a simple patterned waveguide.
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