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The effect of a scattering layer on the edge output of a luminescent solar concentrator
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 1345–1350
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
The effect of a scattering layer on the edge output of a luminescent solar concentrator Michael G. Debije a,, Jean-Pierre Teunissen c, Maud J. Kastelijn c, Paul P.C. Verbunt a, Cees W.M. Bastiaansen a,b a
Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom c Polymer Technology Group Eindhoven, PO Box 6284, 5600HG Eindhoven, The Netherlands b
a r t i c l e in fo
abstract
Article history: Received 10 November 2008 Accepted 9 February 2009 Available online 10 March 2009
The effect of adding white scattering layers to the bottom side of luminescent solar concentrator waveguides is evaluated. It is determined that adding a rear scatterer separated from the waveguide by an air gap results in a large increase of energy output from the waveguides, and this enhancement persists over long (430 cm) distances, although the magnitude of the enhancement decreases with distance. An attached scatterer resulted in the greatest improvement of light output for short (6 cm) distances, but actually reduced edge emissions over longer distances. We provide estimates for the relative contribution of dye-emitted light and scattered light to the total waveguide emission, as well as distinguishing between the contributions of direct and indirect scattering of light to the total output as a function of dye content of the waveguides. & 2009 Elsevier B.V. All rights reserved.
Keywords: Luminescent solar concentrator Scatterer Photovoltaic Silver mirror
1. Background Luminescent solar concentrators (LSCs) were first suggested in the 1970s [1–4] as systems that could reduce the cost of solar energy generation. The LSC usually consists of a plastic or glass waveguide filled or topped by organic or inorganic [5] fluorescent dye molecules. The dye molecules absorb a fraction of the incident sunlight and fluoresce at a longer wavelength. A fraction of the emission light is trapped by total internal reflection within the waveguide. At one or more edges of the waveguide one then places a photovoltaic cell for conversion of the concentrated light into electricity. Metallic mirrors have been applied to the bottom of luminescent solar concentrators to improve the absorption of the dye layer by allowing a ‘second chance’ for absorption of light that passes through the dye layer [2]. However, metallic mirrors are not 100% reflective, and even the best silver mirrors result in losses of 5% of energy per light-ray encounter [6]. Light travelling along the long, thin waveguide will encounter the rear (mirrored) surface many times before the light reaches the edge: at 5% losses, the light energy remaining after just 10 reflections will be reduced by 40%. Another option for performance enhancement of the LSCs is the use of an opaque white scattering layer at the rear side of the
Corresponding author. Tel.: +31 40 247 5881; fax: +31 40 243 6999.
E-mail address: [email protected] (M.G. Debije). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.02.013
device [7–12]. In this application, the scatterer has multiple purposes: first, it will return light that is not absorbed by the dye back through the dye layer, enhancing the absorption of the system. Second, the scatterer can redirect light that would not normally be absorbed (i.e. outside the absorption range of the dye) towards the exit face, and thus to the photovoltaic cell. It is the goal of this paper to gain insight into the fractional contributions of the scatterer and dye to the final output of the LSC, and the size/dimension effects of a scattering layer both attached to the waveguide and separated from the waveguide by an air gap.
2. Experimental To study the long-range effects of a scatterer, a polycarbonate sheet waveguide containing 35 ppm of Lumogen Red305 dye (BASF) was provided by Sabic Innovative Plastics (absorbance of 0.5). This was cut into a 30 cm strip about 5 cm wide and 3 mm thick. This waveguide was placed such that one end of the waveguide was inserted into an integrating sphere equipped with an SLMS LED 1050 light detection array (Labsphere). A floodlight source (Massive) was used to illuminate the sample in a 5 5 cm2 area by placing it directly atop the waveguide, and the source moved along the rod. Emission intensities were integrated from 575 to 750 nm (corresponding to the emission spectrum of the dye). The sample was studied with a black background and with white scattering layers, both separate (i.e., with an air gap
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between the waveguide and scattering layer) and optically attached. The separate white backgrounds were made by spray painting a cardboard sheet with matte white paint (Marabu). Attached white scattering backgrounds were created by spray painting the rear of the polycarbonate waveguides directly with matte white paint, and acted essentially as Lambertian reflector. To gain insight into the short-range effects of a scatterer, we investigated seven injection molded 50 50 3 mm polycarbonate plates (Sabic Innovative Plastics) containing various concentrations of Lumogen Red305 dye (BASF). The absorbance spectra of the waveguides were measured using an UV–vis spectrophotometer (Shimadzu UV-3102) and ranged from 0.07 to 5.0 absorbance units (AU) at peak absorbance. Attached and separate white scattering backgrounds were created as described above. Black (absorbing) background was made by spray painting a cardboard sheet with matte black paint (Marabu). A light source (300 W solar simulator with filters to approximate the 1.5 air mass (AM) (global) solar spectrum (Lot-Oriel)) illuminated the top of the waveguide, and emission from edge of the sample was determined by placing one end of the waveguide adjacent to the entry port of an integrating sphere equipped with an SLMS LED 1050 light detection array (Labsphere). Waveguide emission intensities were determined by integrating the output spectra from 350 to 750 nm. To obtain some finer details of the effect of the scatterer at short ranges, a multicrystalline silicon photovoltaic cell with an efficiency around 18% (Voc 0.62 V, FF 0.80, 36.5 mA/cm2, produced by NaRec) was attached to one side of a 5 5 cm Red305 waveguide with peak absorbance 0.8 using an epoxy glue (Bison). The sample was illuminated by a 3 mm diameter light spot, and the output current of the photovoltaic was recorded. The light source was moved to a new position, and the output current was again recorded. In total, 36 data points were measured. A white background was placed behind the waveguide and the results of the 36 points were again recorded. Finally, the rear of the waveguide was sprayed with white paint, and the results were again recorded. The power efficiency was then calculated from the measured current and the characteristics of the photovoltaic cell as described above.
attached scatterer compared to the output of the same sample with a separate black background. The emissions from small, 5 5 cm waveguides with different optical densities exposed to a light source that illuminated the entire surface area were also recorded with two different backgrounds (see Fig. 2). The outputs of the waveguides with the separate scatterer are considerably higher than those with black backgrounds for the entire absorption range. At lower absorbance the output was more than tripled. At the highest absorbance, the emission power was still improved by 12%.
Fig. 1. Edge output of strip waveguides with a black (diamonds) and white (square) background separated by an air gap, and an integrated white background (triangle). Lines have been added to aid the eye.
3. Results The long-range effects were determined by comparing the integrated output emission from all the measured strip samples. The samples were illuminated locally and the light source moved further away from the emission edge to determine the effect of distance on the output of the waveguide. The results may be seen in Fig. 1 as a function of distance of the light source from the emission end of the waveguide. The sample with the separate black background gave the lowest output when the light source was centered less than 5 cm from the emitting end of the waveguide. The output of the waveguide with the separate scatterer was 50% higher than the black background at this short range (4 cm), and showed enhanced power output in comparison to all other systems all the way out to 30 cm, but the enhancement was reduced to 37% at this distance. The largest output at short distance was measured in the waveguide with the attached scatterer: the output more than doubled over the sample with the black background for input light at 4 cm. However, the positive influence of the scatterer in this system has a very limited range. After 8 cm, the performance of the waveguide with the attached scatterer is significantly worse than the waveguide with the black background, and at 30 cm there is a 66% reduction of light output in the sample with
Fig. 2. Output of a series of 5 5 0.3 cm waveguides doped with Red305 dye to different optical densities. The waveguides were with absorbing black (diamond) and white scattering (square) backgrounds separated by air gaps.
Fig. 3. Emission spectra of a filled Red305 waveguide of peak absorbance 0.8 with a separate black (solid black line), separate silver mirror (solid gray line), separate white (dotted black line) background, and with an integrated silver mirror (dotted gray line).
ARTICLE IN PRESS M.G. Debije et al. / Solar Energy Materials & Solar Cells 93 (2009) 1345–1350
To compare the effect of adding a white scatterer and a silver mirror, edge emissions of a 5 5 cm waveguide with separate black backgrounds, white backgrounds, and silver mirrors were recorded, as well as the output of the same sample with a 200 nm silver mirror sputtered (integrated) to the bottom side. The output spectra of the samples may be seen in Fig. 3. The output of the LSC with attached mirror was practically identical to the LSC using a separate black background. Separating the mirror from the waveguide demonstrated improved performance, but the separate scatterer generated the highest integrated power output. The response of the waveguides exposed to smaller (point) sources was also determined, and a map of the output response as a function of illumination position was made (see Fig. 4). In total there were 36 illumination positions recorded, and the output from an attached photovoltaic cell was recorded for each
Fig. 4. Output (efficiencies in percentage, derived from output current and PV characteristics described in the Experimental section) from a 5 5 cm waveguide with an attached photovoltaic cell (shown as a black bar on the left side of the waveguides in the illustration) as a function of illumination position for a dyefilled waveguide (top), the same waveguide with a white scatterer separated by an air gap (middle) and for an integrated white scatterer (bottom). The darker cells indicate regions of the waveguide under partial shading from support clamps used during measurements.
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illumination position. The sample with the attached scatterer dramatically outperformed the separate scatterer at short distances, up to around 2 cm, but performed significantly worse beyond this distance.
4. Discussion As suggested in Section 1, the use of a white scattering background holds an advantage over using a silver mirror. The attached mirror has minimal effect: while it improves the absorption of the dye by allowing a second passage of unabsorbed light through the waveguide, the losses incurred by the absorptive nature of the mirror negate these benefits (see Fig. 3). Even the mirror separated from the waveguide by an air gap performs significantly less than the separate white scattering layer. This is because the mirror may only be effective for wavelengths of light that may be absorbed by the dye: reflected light of wavelengths outside the absorption band cannot be absorbed by the dye, and will simply leave the surface. The scattering layer not only will return the unabsorbed light within the absorption band of the dye as the mirror, but also changes the direction of incoming light of wavelengths both within and outside the absorption band of the dye. Some of these scattered rays will be directed so as to exit the emission edge of the waveguide. The differences between the scattering waveguide systems are illustrated in Fig. 5. As described above, the output of the waveguide with the separate black background was the lowest at short distances. This might be expected, as any light that passes through the waveguide without being absorbed by the dye will be absorbed by the black background. When the black background is replaced by an optically attached white background, there is a sizable increase in the output of the waveguide at a short distance from the emission edge. Light that passes through the waveguide
Fig. 5. A diagram comparing the three different waveguiding systems. (a) A separate black background. Waveguided light may continue unimpeded. Incoming light that is not absorbed will be absorbed by the black background, and is lost. (b) A separate white background. Waveguided light may continue unimpeded. Incoming light that is not absorbed is scattered, with a portion re-entering the waveguide to be either re-absorbed, directed into the waveguide, or directed back out the input surface. (c) An attached white scatterer. Waveguided light may be scattered upon encountering the bottom surface. Incoming light that is not absorbed is scattered, with a portion re-entering the waveguide to be either reabsorbed, directed into the waveguide, or directed back out the input surface.
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without being absorbed by the dye will encounter the white scattering layer, resulting in two possible results. First, light scattered at an angle close to the normal will re-enter the waveguide, and there is a subsequent opportunity for the dye to absorb this light, effectively doubling the pathlength through the waveguide. Second, light scattered at more oblique angles has the possibility of being directed towards the emission edge of the waveguide via total internal reflection, and allows the collection of light that is outside the absorption range of the dyes. Fig. 6 gives an excellent representation of the effects of the scatterers. These graphs depict the external quantum efficiency, a measure of the number of electrons generated in the external circuit as a function of number of absorbed photons, determined for the waveguide/PV system as a function of illumination wavelength (see Section 2). For shorter distances (illumination 0.5 cm from the attached PV cell), with no background on the waveguide, one can see there is essentially no contribution to the cell current from excitation wavelengths beyond about 620 nm, as one would expect as the dye has no absorption in this wavelength region. However, by adding a rear scatterer, either separated by an air gap or integrated, one can see a sizeable contribution of light beyond 620 nm, especially in the case of the integrated scatterer.
0.30 0.25 0.20 0.15
External Quantum Efficiency
0.10 0.05 0.00 0.30
ET ¼ Ed þ Es þ Ei
(1)
Ed is determined by integrating the emission spectra of the dyefilled waveguides measured on black absorbing backgrounds. Obtaining the scattering contributions Es and Ei is more involved. The spectrum of a polycarbonate waveguide containing no dye placed on top of a white scatterer was measured. This spectrum was added to the spectra of the dye-filled waveguides on the black background, resulting in an ‘interaction-free’ summation spectrum. The ‘summation spectrum’ can be described as
0.25 0.20 0.15 0.10
Esum ¼ Ed þ Eis
0.05 0.00 400
This situation changes considerably for the longer-distance measurement (illumination 4.5 cm from the photovoltaic cell). The contribution from the dye itself is basically eliminated in the sample with the integrated scatterer: practically all the dyeemitted light has been scattered out of the waveguide after only 4.5 cm. The waveguide with scatterer separated from the waveguide by the air gap demonstrates improved light transport to the PV over all wavelengths: dye-emitted light remains in the waveguiding mode, plus there is the addition of in-scattered light and enhanced absorption of shorter wavelengths as well. A comparison of Fig. 1 (illumination of a 5 5 cm2 area of a long waveguide) and Fig. 4 (point illuminations of a small waveguide) indicates that the effect of the scattering layer has a decreasing benefit as incident light strikes farther from the emission edge. To maintain the benefits of a scattering layer while minimizing its deficiencies, it may be desirable to design a LSC system wherein the scattering layer is restricted to cover an area around the edge of the waveguide close to where the photovoltaic is attached, perhaps less than 4–8 cm in width. An extension of the beneficial effect can be achieved beyond 30 cm by incorporating an air gap or other low-index material between the scatterer and the waveguide. To better understand the impact of the scatterer, we determine the relative contributions to the total emission of the waveguide (ET) of the dye-emitted light resulting from first-pass absorption of the incoming light (Ed), the light directly scattered that does not interact with the dye (Es), and the scattered light that reaches the edge via the dye molecule, which we label indirectly scattered light (Ei). Ei is actually made up of more than one term. The first includes the light that initially passed unabsorbed through the dye, but upon return via the scattering layer gets absorbed and emitted so as to escape the edge. The second term is the light absorbed initially by the dye that was emitted by the dye towards the bottom that normally would exit the waveguide by this surface, but now is scattered back into the waveguide, either to exit the emission edge or to be re-absorbed and subsequently reemitted to escape the emission edge. There are even higher-order events, such as dye-emitted light re-entering via the scatterer that is re-absorbed and again emitted by the dye. We will, however, present the result of Ei as a single value, rather than trying to differentiate between these separate events. Thus, the total output of the waveguides may be described by
500
600
700
800
Wavelength (nm) Fig. 6. Plot of measured external quantum efficiency of the waveguide as a function of wavelength for an illumination point far from (4.5 cm from the PV cell, top graph) and near to (0.5 cm from the PV cell, bottom graph) the attached photovoltaic cell for the filled waveguide (black line), with a separate white (dotted black line) and integrated white (dark gray line) scatterers.
(2)
where Eis is the spectrum of the isolated scatterer and Ed the spectrum of the dye. This ‘summation spectrum’ could then be compared to the spectrum measured of the same dye-filled waveguide on top of a white scatterer. The difference in these two spectra, the ideal ‘summation spectrum’ and the measured spectrum, will provide us the details of the interaction of the scatterer with the dye-filled waveguide. It can be written as ET Esum ¼ Es þ Ei Eis
(3)
The results of two such spectral subtractions may be seen in Fig. 7. There are two regions in these calculated spectra, labeled in the figure as I and II.
ARTICLE IN PRESS M.G. Debije et al. / Solar Energy Materials & Solar Cells 93 (2009) 1345–1350
Fig. 7. Resultant spectra obtained by the subtraction of the ideal ‘summation spectrum’ from the actual measured spectrum for waveguides of peak dye absorbance of 0.09 (black line) and 1.07 (gray line). The negative area of the resultant spectra are labeled Region I and it indicates the effect of absorption by the dye on the direct scattering and the positive area of the resultant spectra are labeled Region II and it indicates the effect of indirect scattering.
Table 1 Absolute and relative contributions of dye emission and direct and indirect scattering to the total output of the waveguides as a function of absorbance. Absorbance
0.09 0.25 0.52 1.07 1.70 5
Absolute contribution (mW)
Relative contribution (%)
Dye
Direct
Indirect
Dye
Direct
Indirect
4.01 12.8 23.2 36.9 44.9 52.7
4.3 3.7 3.4 3.1 2.9 2.6
3.8 7.6 12.5 11.4 9.0 3.5
33 53 59 72 79 90
36 15 9 6 5 4
31 32 32 22 16 6
Region I (with negative energies) is a measure of the absorption of the directly scattered light by the dye molecules (the removal of light from the isolated scatterer spectrum, described as Eis above). By adding Region I to the spectrum obtained from measurements of the blank waveguide plus the scatterer, we obtain an estimate of the light that reaches the output edge of the waveguide that has come directly from the scatterer, and not been absorbed by the dye. Region II (with positive energies) is the additional light obtained via emission of the dye after adding the scatterer, so then light that initially passed through the waveguide unabsorbed, but was subsequently absorbed when returned through the waveguide via the scatterer, or light that had been emitted by the dye initially in such a direction as to escape the bottom surface, but that has returned after encountering the scatterer. By integrating over Region II, we obtain an estimate of the amount of indirectly scattered light that reaches the output edge. The results of these calculations are presented in Table 1. The absolute contribution of the directly scattered light will be somewhat higher than reported, since in this work the cutoff wavelength for integrating the energy was 750 nm, due to the noise in the source light above this wavelength. Thus, there will be a small contribution to the emission signal for directly scattered light for wavelengths from 750 to 1100 nm (the practical range for use of a silicon photovoltaic) that is not accounted for here. The Red305 dye used in these experiments has a limited Stokes shift, and there is a degree of overlap between the absorption and emission spectra of the dye. Thus, Regions I and II in Fig. 7 are not completely independent of one another: there is some direct
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scatter contributing to Region II and some indirect scatter contributing to Region I, and so the absolute numbers should be approached with a degree of caution. Nevertheless, we believe these numbers give a good estimate of the relative contribution of the dye and scatterer to the total output of the LSC systems, and furthermore we are able to provide an additional estimate of the relative contributions of direct and indirect scattering to the total output. The relative contribution of directly scattered light to the total edge emission decreases with increase in dye content of the waveguides. The absolute amount of light directly scattered so as to reach the emission edge is a combination of scattered light with wavelengths outside the absorption band of the dye (a constant value) and with wavelengths within the absorption band of the dye that are not absorbed, which is a function of the dye content. As the amount of light redirected to the waveguide edge via the absorption and emission of dye increases the relative contribution to the total emission by the direct scattering of light decreases. Thus, increasing dye content or broadening the absorption band of the dye materials will reduce the direct effect of a scattering layer. The indirectly scattered light plays a pronounced role until the higher dye concentration is reached, when there is little unabsorbed incident light within the absorption range of the dye reaching the scatterer. The indirect scatter contribution to the edge-emission spectra at the high dye contents is primarily restricted to the reflection of dye-emitted light that was directed as to exit the bottom surface, but has been scattered to the emission edge.
5. Conclusions The incorporation of a scattering layer to the rear of a luminescent solar concentrator can have significant positive impact on the energy output of the LSC system. The physical distance over which the scatterer provides a positive effect depends whether or not the waveguide is attached to (shorter range) or separated by an air gap (longer range) from the scatterer. In LSCs with edge dimensions larger than about 5 cm, the optimal use of scatterers will involve the inclusion of an air gap or other low-index spacer between the waveguide and the scatterer, and the relative contribution of the scatterer to the total output of the LSC system will decrease with increasing dye content and broadened dye-absorption range.
Acknowledgements The authors would like to thank T. Hoeks from Sabic Innovative Plastics, for providing the waveguides used in these experiments. MD would like to acknowledge the support of STW VIDI Grant 07940. References [1] W.H. Weber, J. Lambe, Luminescant greenhouse collector for solar radiation, Appl. Opt. 15 (10) (1976) 2299–2300. [2] A. Goetzberger, W. Greubel, Solar energy conversion with fluorescent collectors, Appl. Phys. 14 (1977) 123–139. [3] J.A. Levitt, W.H. Weber, Materials for luminescent greenhouse solar collectors, Appl. Opt. 16 (10) (1977) 2684–2689. [4] B.A. Swartz, T. Cole, A.H. Zewail, Photon trapping and energy transfer in multiple-dye plastic matrices: an efficient solar-energy concentrator, Opt. Lett. 1 (2) (1977) 73–75. [5] A.J. Chatten, K.W.J. Barnham, B.F. Buxton, N.J. Ekins-Daukes, M.A. Malik, A new approach to modelling quantum dot concentrators, Sol. Energy Mater. Sol. Cells 75 (2003) 363–371.
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[6] E.T. de la Perrelle, T.S. Moss, H. Herbert, The measurements of absorptivity and reflectivity, Infrared Phys. 3 (1963) 35–43. [7] K. Heidler, Efficiency and concentration ratio measurements of fluorescent solar concentrators using a xenon measurement system, Appl. Opt. 20 (5) (1981) 773–777. [8] J. Mugnier, Y. Dordet, J. Pouget, B. Valeur, A photometric approach of fluorescent solar concentrators. Role of diffuse reflectors and spectral sensitivity of solar cells, Rev. Phys. Appl. 22 (1987) 89–100. [9] A.A. Earp, G.B. Smith, J. Franklin, P.D. Swift, Optimization of a three-colour luminescent solar concentrator daylighting system, Sol. Energy Mater. Sol. Cells 84 (2004) 411–426.
[10] S.T. Bailey, G.E. Lokey, M.S. Hanes, J.D.M. Shearer, J.B. McLafferty, G.T. Beaumont, T.T. Baseler, J.M. Layhue, D.R. Broussard, Y.-Z. Zhang, B.P. Wittmershaus, Optimized excitation energy transfer in a three-dye luminescent solar concentrator, Sol. Energy Mater. Sol. Cells 91 (2007) 67–75. [11] L.H. Slooff, E.E. Bende, A.R. Burgers, T. Budel, M. Pravettoni, R.P. Kenny, E.D. Dunlop, A. Bu¨chtemann, A luminescent solar concentrator with 7.1% power conversion efficiency, Phys. Status Solidi (RRL) 2 (2008) 257–259. [12] J.C. Goldschmidt, M. Peters, A. Bo¨sch, H. Helmers, F. Dimroth, S.W. Glunz, G.P. Willeke, Increasing the efficiency of fluorescent concentrator systems, Sol. Energy Mater. Sol. Cells 93 (2009) 176–182.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
The effect of a scattering layer on the edge output of a luminescent solar concentrator Michael G. Debije a,, Jean-Pierre Teunissen c, Maud J. Kastelijn c, Paul P.C. Verbunt a, Cees W.M. Bastiaansen a,b a
Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom c Polymer Technology Group Eindhoven, PO Box 6284, 5600HG Eindhoven, The Netherlands b
a r t i c l e in fo
abstract
Article history: Received 10 November 2008 Accepted 9 February 2009 Available online 10 March 2009
The effect of adding white scattering layers to the bottom side of luminescent solar concentrator waveguides is evaluated. It is determined that adding a rear scatterer separated from the waveguide by an air gap results in a large increase of energy output from the waveguides, and this enhancement persists over long (430 cm) distances, although the magnitude of the enhancement decreases with distance. An attached scatterer resulted in the greatest improvement of light output for short (6 cm) distances, but actually reduced edge emissions over longer distances. We provide estimates for the relative contribution of dye-emitted light and scattered light to the total waveguide emission, as well as distinguishing between the contributions of direct and indirect scattering of light to the total output as a function of dye content of the waveguides. & 2009 Elsevier B.V. All rights reserved.
Keywords: Luminescent solar concentrator Scatterer Photovoltaic Silver mirror
1. Background Luminescent solar concentrators (LSCs) were first suggested in the 1970s [1–4] as systems that could reduce the cost of solar energy generation. The LSC usually consists of a plastic or glass waveguide filled or topped by organic or inorganic [5] fluorescent dye molecules. The dye molecules absorb a fraction of the incident sunlight and fluoresce at a longer wavelength. A fraction of the emission light is trapped by total internal reflection within the waveguide. At one or more edges of the waveguide one then places a photovoltaic cell for conversion of the concentrated light into electricity. Metallic mirrors have been applied to the bottom of luminescent solar concentrators to improve the absorption of the dye layer by allowing a ‘second chance’ for absorption of light that passes through the dye layer [2]. However, metallic mirrors are not 100% reflective, and even the best silver mirrors result in losses of 5% of energy per light-ray encounter [6]. Light travelling along the long, thin waveguide will encounter the rear (mirrored) surface many times before the light reaches the edge: at 5% losses, the light energy remaining after just 10 reflections will be reduced by 40%. Another option for performance enhancement of the LSCs is the use of an opaque white scattering layer at the rear side of the
Corresponding author. Tel.: +31 40 247 5881; fax: +31 40 243 6999.
E-mail address: [email protected] (M.G. Debije). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.02.013
device [7–12]. In this application, the scatterer has multiple purposes: first, it will return light that is not absorbed by the dye back through the dye layer, enhancing the absorption of the system. Second, the scatterer can redirect light that would not normally be absorbed (i.e. outside the absorption range of the dye) towards the exit face, and thus to the photovoltaic cell. It is the goal of this paper to gain insight into the fractional contributions of the scatterer and dye to the final output of the LSC, and the size/dimension effects of a scattering layer both attached to the waveguide and separated from the waveguide by an air gap.
2. Experimental To study the long-range effects of a scatterer, a polycarbonate sheet waveguide containing 35 ppm of Lumogen Red305 dye (BASF) was provided by Sabic Innovative Plastics (absorbance of 0.5). This was cut into a 30 cm strip about 5 cm wide and 3 mm thick. This waveguide was placed such that one end of the waveguide was inserted into an integrating sphere equipped with an SLMS LED 1050 light detection array (Labsphere). A floodlight source (Massive) was used to illuminate the sample in a 5 5 cm2 area by placing it directly atop the waveguide, and the source moved along the rod. Emission intensities were integrated from 575 to 750 nm (corresponding to the emission spectrum of the dye). The sample was studied with a black background and with white scattering layers, both separate (i.e., with an air gap
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between the waveguide and scattering layer) and optically attached. The separate white backgrounds were made by spray painting a cardboard sheet with matte white paint (Marabu). Attached white scattering backgrounds were created by spray painting the rear of the polycarbonate waveguides directly with matte white paint, and acted essentially as Lambertian reflector. To gain insight into the short-range effects of a scatterer, we investigated seven injection molded 50 50 3 mm polycarbonate plates (Sabic Innovative Plastics) containing various concentrations of Lumogen Red305 dye (BASF). The absorbance spectra of the waveguides were measured using an UV–vis spectrophotometer (Shimadzu UV-3102) and ranged from 0.07 to 5.0 absorbance units (AU) at peak absorbance. Attached and separate white scattering backgrounds were created as described above. Black (absorbing) background was made by spray painting a cardboard sheet with matte black paint (Marabu). A light source (300 W solar simulator with filters to approximate the 1.5 air mass (AM) (global) solar spectrum (Lot-Oriel)) illuminated the top of the waveguide, and emission from edge of the sample was determined by placing one end of the waveguide adjacent to the entry port of an integrating sphere equipped with an SLMS LED 1050 light detection array (Labsphere). Waveguide emission intensities were determined by integrating the output spectra from 350 to 750 nm. To obtain some finer details of the effect of the scatterer at short ranges, a multicrystalline silicon photovoltaic cell with an efficiency around 18% (Voc 0.62 V, FF 0.80, 36.5 mA/cm2, produced by NaRec) was attached to one side of a 5 5 cm Red305 waveguide with peak absorbance 0.8 using an epoxy glue (Bison). The sample was illuminated by a 3 mm diameter light spot, and the output current of the photovoltaic was recorded. The light source was moved to a new position, and the output current was again recorded. In total, 36 data points were measured. A white background was placed behind the waveguide and the results of the 36 points were again recorded. Finally, the rear of the waveguide was sprayed with white paint, and the results were again recorded. The power efficiency was then calculated from the measured current and the characteristics of the photovoltaic cell as described above.
attached scatterer compared to the output of the same sample with a separate black background. The emissions from small, 5 5 cm waveguides with different optical densities exposed to a light source that illuminated the entire surface area were also recorded with two different backgrounds (see Fig. 2). The outputs of the waveguides with the separate scatterer are considerably higher than those with black backgrounds for the entire absorption range. At lower absorbance the output was more than tripled. At the highest absorbance, the emission power was still improved by 12%.
Fig. 1. Edge output of strip waveguides with a black (diamonds) and white (square) background separated by an air gap, and an integrated white background (triangle). Lines have been added to aid the eye.
3. Results The long-range effects were determined by comparing the integrated output emission from all the measured strip samples. The samples were illuminated locally and the light source moved further away from the emission edge to determine the effect of distance on the output of the waveguide. The results may be seen in Fig. 1 as a function of distance of the light source from the emission end of the waveguide. The sample with the separate black background gave the lowest output when the light source was centered less than 5 cm from the emitting end of the waveguide. The output of the waveguide with the separate scatterer was 50% higher than the black background at this short range (4 cm), and showed enhanced power output in comparison to all other systems all the way out to 30 cm, but the enhancement was reduced to 37% at this distance. The largest output at short distance was measured in the waveguide with the attached scatterer: the output more than doubled over the sample with the black background for input light at 4 cm. However, the positive influence of the scatterer in this system has a very limited range. After 8 cm, the performance of the waveguide with the attached scatterer is significantly worse than the waveguide with the black background, and at 30 cm there is a 66% reduction of light output in the sample with
Fig. 2. Output of a series of 5 5 0.3 cm waveguides doped with Red305 dye to different optical densities. The waveguides were with absorbing black (diamond) and white scattering (square) backgrounds separated by air gaps.
Fig. 3. Emission spectra of a filled Red305 waveguide of peak absorbance 0.8 with a separate black (solid black line), separate silver mirror (solid gray line), separate white (dotted black line) background, and with an integrated silver mirror (dotted gray line).
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To compare the effect of adding a white scatterer and a silver mirror, edge emissions of a 5 5 cm waveguide with separate black backgrounds, white backgrounds, and silver mirrors were recorded, as well as the output of the same sample with a 200 nm silver mirror sputtered (integrated) to the bottom side. The output spectra of the samples may be seen in Fig. 3. The output of the LSC with attached mirror was practically identical to the LSC using a separate black background. Separating the mirror from the waveguide demonstrated improved performance, but the separate scatterer generated the highest integrated power output. The response of the waveguides exposed to smaller (point) sources was also determined, and a map of the output response as a function of illumination position was made (see Fig. 4). In total there were 36 illumination positions recorded, and the output from an attached photovoltaic cell was recorded for each
Fig. 4. Output (efficiencies in percentage, derived from output current and PV characteristics described in the Experimental section) from a 5 5 cm waveguide with an attached photovoltaic cell (shown as a black bar on the left side of the waveguides in the illustration) as a function of illumination position for a dyefilled waveguide (top), the same waveguide with a white scatterer separated by an air gap (middle) and for an integrated white scatterer (bottom). The darker cells indicate regions of the waveguide under partial shading from support clamps used during measurements.
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illumination position. The sample with the attached scatterer dramatically outperformed the separate scatterer at short distances, up to around 2 cm, but performed significantly worse beyond this distance.
4. Discussion As suggested in Section 1, the use of a white scattering background holds an advantage over using a silver mirror. The attached mirror has minimal effect: while it improves the absorption of the dye by allowing a second passage of unabsorbed light through the waveguide, the losses incurred by the absorptive nature of the mirror negate these benefits (see Fig. 3). Even the mirror separated from the waveguide by an air gap performs significantly less than the separate white scattering layer. This is because the mirror may only be effective for wavelengths of light that may be absorbed by the dye: reflected light of wavelengths outside the absorption band cannot be absorbed by the dye, and will simply leave the surface. The scattering layer not only will return the unabsorbed light within the absorption band of the dye as the mirror, but also changes the direction of incoming light of wavelengths both within and outside the absorption band of the dye. Some of these scattered rays will be directed so as to exit the emission edge of the waveguide. The differences between the scattering waveguide systems are illustrated in Fig. 5. As described above, the output of the waveguide with the separate black background was the lowest at short distances. This might be expected, as any light that passes through the waveguide without being absorbed by the dye will be absorbed by the black background. When the black background is replaced by an optically attached white background, there is a sizable increase in the output of the waveguide at a short distance from the emission edge. Light that passes through the waveguide
Fig. 5. A diagram comparing the three different waveguiding systems. (a) A separate black background. Waveguided light may continue unimpeded. Incoming light that is not absorbed will be absorbed by the black background, and is lost. (b) A separate white background. Waveguided light may continue unimpeded. Incoming light that is not absorbed is scattered, with a portion re-entering the waveguide to be either re-absorbed, directed into the waveguide, or directed back out the input surface. (c) An attached white scatterer. Waveguided light may be scattered upon encountering the bottom surface. Incoming light that is not absorbed is scattered, with a portion re-entering the waveguide to be either reabsorbed, directed into the waveguide, or directed back out the input surface.
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without being absorbed by the dye will encounter the white scattering layer, resulting in two possible results. First, light scattered at an angle close to the normal will re-enter the waveguide, and there is a subsequent opportunity for the dye to absorb this light, effectively doubling the pathlength through the waveguide. Second, light scattered at more oblique angles has the possibility of being directed towards the emission edge of the waveguide via total internal reflection, and allows the collection of light that is outside the absorption range of the dyes. Fig. 6 gives an excellent representation of the effects of the scatterers. These graphs depict the external quantum efficiency, a measure of the number of electrons generated in the external circuit as a function of number of absorbed photons, determined for the waveguide/PV system as a function of illumination wavelength (see Section 2). For shorter distances (illumination 0.5 cm from the attached PV cell), with no background on the waveguide, one can see there is essentially no contribution to the cell current from excitation wavelengths beyond about 620 nm, as one would expect as the dye has no absorption in this wavelength region. However, by adding a rear scatterer, either separated by an air gap or integrated, one can see a sizeable contribution of light beyond 620 nm, especially in the case of the integrated scatterer.
0.30 0.25 0.20 0.15
External Quantum Efficiency
0.10 0.05 0.00 0.30
ET ¼ Ed þ Es þ Ei
(1)
Ed is determined by integrating the emission spectra of the dyefilled waveguides measured on black absorbing backgrounds. Obtaining the scattering contributions Es and Ei is more involved. The spectrum of a polycarbonate waveguide containing no dye placed on top of a white scatterer was measured. This spectrum was added to the spectra of the dye-filled waveguides on the black background, resulting in an ‘interaction-free’ summation spectrum. The ‘summation spectrum’ can be described as
0.25 0.20 0.15 0.10
Esum ¼ Ed þ Eis
0.05 0.00 400
This situation changes considerably for the longer-distance measurement (illumination 4.5 cm from the photovoltaic cell). The contribution from the dye itself is basically eliminated in the sample with the integrated scatterer: practically all the dyeemitted light has been scattered out of the waveguide after only 4.5 cm. The waveguide with scatterer separated from the waveguide by the air gap demonstrates improved light transport to the PV over all wavelengths: dye-emitted light remains in the waveguiding mode, plus there is the addition of in-scattered light and enhanced absorption of shorter wavelengths as well. A comparison of Fig. 1 (illumination of a 5 5 cm2 area of a long waveguide) and Fig. 4 (point illuminations of a small waveguide) indicates that the effect of the scattering layer has a decreasing benefit as incident light strikes farther from the emission edge. To maintain the benefits of a scattering layer while minimizing its deficiencies, it may be desirable to design a LSC system wherein the scattering layer is restricted to cover an area around the edge of the waveguide close to where the photovoltaic is attached, perhaps less than 4–8 cm in width. An extension of the beneficial effect can be achieved beyond 30 cm by incorporating an air gap or other low-index material between the scatterer and the waveguide. To better understand the impact of the scatterer, we determine the relative contributions to the total emission of the waveguide (ET) of the dye-emitted light resulting from first-pass absorption of the incoming light (Ed), the light directly scattered that does not interact with the dye (Es), and the scattered light that reaches the edge via the dye molecule, which we label indirectly scattered light (Ei). Ei is actually made up of more than one term. The first includes the light that initially passed unabsorbed through the dye, but upon return via the scattering layer gets absorbed and emitted so as to escape the edge. The second term is the light absorbed initially by the dye that was emitted by the dye towards the bottom that normally would exit the waveguide by this surface, but now is scattered back into the waveguide, either to exit the emission edge or to be re-absorbed and subsequently reemitted to escape the emission edge. There are even higher-order events, such as dye-emitted light re-entering via the scatterer that is re-absorbed and again emitted by the dye. We will, however, present the result of Ei as a single value, rather than trying to differentiate between these separate events. Thus, the total output of the waveguides may be described by
500
600
700
800
Wavelength (nm) Fig. 6. Plot of measured external quantum efficiency of the waveguide as a function of wavelength for an illumination point far from (4.5 cm from the PV cell, top graph) and near to (0.5 cm from the PV cell, bottom graph) the attached photovoltaic cell for the filled waveguide (black line), with a separate white (dotted black line) and integrated white (dark gray line) scatterers.
(2)
where Eis is the spectrum of the isolated scatterer and Ed the spectrum of the dye. This ‘summation spectrum’ could then be compared to the spectrum measured of the same dye-filled waveguide on top of a white scatterer. The difference in these two spectra, the ideal ‘summation spectrum’ and the measured spectrum, will provide us the details of the interaction of the scatterer with the dye-filled waveguide. It can be written as ET Esum ¼ Es þ Ei Eis
(3)
The results of two such spectral subtractions may be seen in Fig. 7. There are two regions in these calculated spectra, labeled in the figure as I and II.
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Fig. 7. Resultant spectra obtained by the subtraction of the ideal ‘summation spectrum’ from the actual measured spectrum for waveguides of peak dye absorbance of 0.09 (black line) and 1.07 (gray line). The negative area of the resultant spectra are labeled Region I and it indicates the effect of absorption by the dye on the direct scattering and the positive area of the resultant spectra are labeled Region II and it indicates the effect of indirect scattering.
Table 1 Absolute and relative contributions of dye emission and direct and indirect scattering to the total output of the waveguides as a function of absorbance. Absorbance
0.09 0.25 0.52 1.07 1.70 5
Absolute contribution (mW)
Relative contribution (%)
Dye
Direct
Indirect
Dye
Direct
Indirect
4.01 12.8 23.2 36.9 44.9 52.7
4.3 3.7 3.4 3.1 2.9 2.6
3.8 7.6 12.5 11.4 9.0 3.5
33 53 59 72 79 90
36 15 9 6 5 4
31 32 32 22 16 6
Region I (with negative energies) is a measure of the absorption of the directly scattered light by the dye molecules (the removal of light from the isolated scatterer spectrum, described as Eis above). By adding Region I to the spectrum obtained from measurements of the blank waveguide plus the scatterer, we obtain an estimate of the light that reaches the output edge of the waveguide that has come directly from the scatterer, and not been absorbed by the dye. Region II (with positive energies) is the additional light obtained via emission of the dye after adding the scatterer, so then light that initially passed through the waveguide unabsorbed, but was subsequently absorbed when returned through the waveguide via the scatterer, or light that had been emitted by the dye initially in such a direction as to escape the bottom surface, but that has returned after encountering the scatterer. By integrating over Region II, we obtain an estimate of the amount of indirectly scattered light that reaches the output edge. The results of these calculations are presented in Table 1. The absolute contribution of the directly scattered light will be somewhat higher than reported, since in this work the cutoff wavelength for integrating the energy was 750 nm, due to the noise in the source light above this wavelength. Thus, there will be a small contribution to the emission signal for directly scattered light for wavelengths from 750 to 1100 nm (the practical range for use of a silicon photovoltaic) that is not accounted for here. The Red305 dye used in these experiments has a limited Stokes shift, and there is a degree of overlap between the absorption and emission spectra of the dye. Thus, Regions I and II in Fig. 7 are not completely independent of one another: there is some direct
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scatter contributing to Region II and some indirect scatter contributing to Region I, and so the absolute numbers should be approached with a degree of caution. Nevertheless, we believe these numbers give a good estimate of the relative contribution of the dye and scatterer to the total output of the LSC systems, and furthermore we are able to provide an additional estimate of the relative contributions of direct and indirect scattering to the total output. The relative contribution of directly scattered light to the total edge emission decreases with increase in dye content of the waveguides. The absolute amount of light directly scattered so as to reach the emission edge is a combination of scattered light with wavelengths outside the absorption band of the dye (a constant value) and with wavelengths within the absorption band of the dye that are not absorbed, which is a function of the dye content. As the amount of light redirected to the waveguide edge via the absorption and emission of dye increases the relative contribution to the total emission by the direct scattering of light decreases. Thus, increasing dye content or broadening the absorption band of the dye materials will reduce the direct effect of a scattering layer. The indirectly scattered light plays a pronounced role until the higher dye concentration is reached, when there is little unabsorbed incident light within the absorption range of the dye reaching the scatterer. The indirect scatter contribution to the edge-emission spectra at the high dye contents is primarily restricted to the reflection of dye-emitted light that was directed as to exit the bottom surface, but has been scattered to the emission edge.
5. Conclusions The incorporation of a scattering layer to the rear of a luminescent solar concentrator can have significant positive impact on the energy output of the LSC system. The physical distance over which the scatterer provides a positive effect depends whether or not the waveguide is attached to (shorter range) or separated by an air gap (longer range) from the scatterer. In LSCs with edge dimensions larger than about 5 cm, the optimal use of scatterers will involve the inclusion of an air gap or other low-index spacer between the waveguide and the scatterer, and the relative contribution of the scatterer to the total output of the LSC system will decrease with increasing dye content and broadened dye-absorption range.
Acknowledgements The authors would like to thank T. Hoeks from Sabic Innovative Plastics, for providing the waveguides used in these experiments. MD would like to acknowledge the support of STW VIDI Grant 07940. References [1] W.H. Weber, J. Lambe, Luminescant greenhouse collector for solar radiation, Appl. Opt. 15 (10) (1976) 2299–2300. [2] A. Goetzberger, W. Greubel, Solar energy conversion with fluorescent collectors, Appl. Phys. 14 (1977) 123–139. [3] J.A. Levitt, W.H. Weber, Materials for luminescent greenhouse solar collectors, Appl. Opt. 16 (10) (1977) 2684–2689. [4] B.A. Swartz, T. Cole, A.H. Zewail, Photon trapping and energy transfer in multiple-dye plastic matrices: an efficient solar-energy concentrator, Opt. Lett. 1 (2) (1977) 73–75. [5] A.J. Chatten, K.W.J. Barnham, B.F. Buxton, N.J. Ekins-Daukes, M.A. Malik, A new approach to modelling quantum dot concentrators, Sol. Energy Mater. Sol. Cells 75 (2003) 363–371.
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