Measured surface loss from luminescent solar concentrator waveguides
Michael G. Debije1*, Paul P. C. Verbunt1, Brenda C. Rowan2, Bryce S. Richards2 and Theo L. Hoeks1
1. Polymer Technology, Department Chemical Engineering and Chemistry, Eindhoven University of
Technology, 5600 MB Eindhoven, NL
2. School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, Eh14 4AS, United
* author for correspondence: email@example.com
The surface and edge emissions from dye-filled and dye-topped polycarbonate and PMMA
luminescent solar concentrators (LSCs) were measured. We demonstrate that about 40-50% of the
absorbed light energy (and 50-70% of the photons) is lost through the top and bottom surfaces of the
filled waveguide. In most cases the escape cone losses are greater at the top than the bottom surface.
OCIS codes: 350.6050, 350.5500, 310.2785, 260.2510, 230.7390, 160.5470
Luminescent solar concentrators (LSC) are attractive as possible replacements for large-area
silicon-based solar panels in the built environment, owing to their promise of lower production costs
Applied Optics (2008) 47(36), 6763–-6768
and suitability for building-integrated photovoltaics. Research on the LSC slowed in the mid-1980’s
due to two main factors. The first was a concern over the stability of the dyes used. At that time,
lifetimes of fluorescent dye materials were measured in days to weeks under solar illumination . In
the last quarter century there has been a tremendous improvement in the performance and lifetimes of
fluorescent dye molecules and these materials now have lifetimes of years to decades . In addition,
materials such as quantum dots  and rare earth materials  offer alternatives which may provide
improved stability. As we believe this concern over dye stability and efficiency has been mostly
alleviated, we now focus on the second major factor: losses from the waveguide itself.
In the past, most research on losses in the LSC has focused on minimizing the internal losses
due to dye re-absorptions (see for example ). Aside from emphasis for producing optically smooth
surfaces  and the use of higher or variable refractive index waveguides , the losses through the
surfaces (top or top and bottom) of the waveguides have been accepted as unavoidable. Most
theoretical simulations of the LSC describe the fluorescent dye system as an isotropic absorber and
emitter, and it becomes a simple calculation to determines that roughly 25% of light energy will be lost
via the ‘escape cone’ of the assumed n=1.5 waveguide) [8-10]. However, the dyes in general are
neither isotropic absorbers nor emitters, but are functional dipoles with more directed absorption and
emission (see for example [11-15]). Also, the majority of the input light is typically incident upon the
waveguide from one surface alone (i.e. the top surface). In such a situation, the dye molecules lying
with their dipoles directed parallel to the incoming electric field of the sunlight will tend to absorb
more light than those aligned perpendicular to the incoming light E-field . In addition, the dye
system will emit in a similar direction, resulting in an emission with an oblong profile rather than
spherical, with a significant fraction of emitted light directed towards the surface [14,15]. In addition
to this initial loss, there will be further surface losses from light re-absorbed and re-emitted by
subsequent dye molecules, a result of the limited Stokes shift of the dyes, a feature rarely considered in
theoretical calculations (for example, ). In our laboratory, we have been researching various
methods to reduce or eliminate these surface losses, including the application of organic semi-
transparent reflectors  and the unidirectional alignment of the emitting dye molecules .
The purpose of this paper is the direct measurement of the amount of light emitted from the
surface of two waveguide configurations: one with dye material dispersed throughout the bulk of the
sample, and the other with thin dye-doped layers on the surface of clear waveguides. We offer an
explanation for the differences in top/bottom and total emissions noted at high and low doping
A number of filled waveguides were produced by Sabic Innovative Plastics (Bergen op Zoom,
NL) by injection molding of polycarbonate (PC) (n=1.586 at 587 nm) with various concentrations of
the fluorescent dye Lumogen® F Red305 (hereafter referred to as Red305 - BASF) or
polymethylmethacrylate (PMMA) (n=1.49) with one concentration of Red305 into plates 50 x 50 x 3
mm. Surface-topped waveguides were produced by spin casting solutions of Red305 in PC from THF
or Red305 in a pentaacrylate (Polysciences, Inc.)/MMA (Aldrich) 80:20 blend containing 1%
photoinitiator (Irgacure 184, Ciba) on top of either clear PC (Sabic Innovative Plastics) or PMMA
(Plano Plastics) plates at 1000 rpm for 30 seconds. The pentaacrylate systems were subsequently
exposed to ultraviolet light in a N2 atmosphere to crosslink the system. The thicknesses of the dye
layers were measured by a Zoomsurf 3D interferometer (Fogale), and were about 3 µm for the PC
layers, and 15-30 µm for the pentaacrylate/MMA layers.
The absorbance for all samples was determined using a Shimadzu UV-3102
spectrophotometer: the reported optical density (OD) values in this paper refer to the absorbance at the
peak of the main absorption band.
Bottom surface emissions (that is, emission from the surface opposite the incident light source)
were determined by placing the 5 x 5 cm waveguides against the entry port of an integrating sphere
equipped with a SLMS LED 1050 light detection array (Labsphere), with a blank waveguide used as
the reference – see Figure 1 for a depiction of the experimental setup. The samples were exposed to the
light of a 300 W solar simulator with filters to approximate the 1.5AM (global) solar spectrum (Lot-
Oriel). To reduce the background spectra from the source light, a stack of cholesteric filters centered at
670, 710, and 750 nm were placed between the source and the sample to filter out these longer
wavelengths. The illumination area was limited to an approximately 2 cm diameter spot in the
Top surface loss measurements (that is, from the surface being illuminated by the light source)
were derived from emission data taken using an Autronic DMS 703 (Melchers GmbH) together with a
CCD-Spect-2 array detector (CCD-Camera). The LSC samples were placed in a custom-made sample
holder and exposed to a roughly uniform light source located at a distance of about 11 cm. Output
spectra were recorded for surface emissions from 30º to 70º from the normal of the waveguide surface
for both sides of the waveguide. See Figure 2 for a depiction of the experimental setup. The integrated
outputs were determined for both ‘top’ and ‘bottom’ sides of the waveguide, and a ratio of top/bottom
emission was obtained. This ratio was compared to the absolute measurements taken for the ‘bottom’
surface from the integrating sphere, and from this the ‘top’ emission was calculated.
Edge emissions from the waveguides were determined by placing the samples on a horizontal
stage with only one edge of the waveguide entering the integrating sphere. Illumination was over the
whole surface by the solar simulator, and the quantity of energy absorbed was determined from the
measured absorption spectrum of the waveguide and the emission spectra of the light source.
A representative absorption and edge emission spectra for a dye filled waveguide is shown
below in Figure 3. Emission from the edge of all the samples were recorded, and the total power
output of all the samples as a function of absorbance are shown in Figure 4.
Filled waveguide surface emissions were determined by placing the 5 x 5 cm waveguides
against the opening of the integrating sphere, and illuminating with a solar simulator. Using a blank
waveguide, we could determine the amount of light transmitted and reflected by the waveguide itself.
Absorption was essentially zero through this narrow width of the waveguide, and the reflection from
PMMA was measured to be around 8%, and from PC about 11.5% in the visible range.
Samples containing Red305 dye were then placed in the measurement position, and the results
recorded. Representative spectra of the blank and a filled waveguide may be seen in Figure 5a. By
subtracting the results of the blank measurement from the results of a filled waveguide, we obtained the
spectra seen in Figure 5b. The fraction of light emitted from the surface was determined by integrating
the emission spectra (the light emitted into the sphere from the surface of the waveguide, or the positive
region of the curves in Figure 5b, roughly 600nm out to our cutoff of 750 nm, depicted as region II)
and dividing by the light absorbed by the dye (the negative region of the spectra of Figure 5b, roughly
400 - 600 nm and depicted as region I). The integrations are seen in Figure 6.
Measurements of the surface-topped samples were done in a similar manner, with the dye layer
situated on the top side of the waveguide, facing the light source. For all but the lowest absorbance, the
emissions measured with the dye layer on top and with the dye layer at the back were within 5%.
Measurements using the Autronic system (described previously in the Experimental section and
Figure 2) were used to establish the ratio of top to bottom emission from the waveguide surfaces. Once
the ratio was determined, the absolute emission from the top surface was calculated given this ratio and
the measured value of bottom emission. The Table gives numerical loss ratios for the filled
polycarbonate samples; the ratios of the other samples are depicted in Figure 7.
Several samples were measured for output from both faces by repositioning of the waveguide
with respect to the incoming light in the Autronic setup, and the differences were determined to be less
We may derive some general conclusions regarding the edge emission from waveguides made
from PMMA and PC in bulk and as thin films on clear waveguides by considering Figure 4. The edge
emissions are essentially identical for bulk and thin-film waveguides of PC and PMMA up to OD ~
0.75 (about 85% absorption at the peak). At higher OD values, the filled waveguides demonstrate
higher efficiencies. This can be expected owing to the limited solubility of the Red305 dye in the thin
(3 µm for polycarbonate, ~20 µm for the pentaacrylate/MMA blend) dye layers. At the high
concentrations necessary to achieve >85% absorption, the dye materials start to self-associate, resulting
in the formation of crystallites and the quenching of fluorescence.
In looking at the Table and Figure 6, it becomes apparent that there is a considerable loss of
light from the surfaces of the filled waveguides, and this loss is constant at around 40% except in the
lightly doped waveguide. This translates to a loss of 52% of the photons. At low optical densities, the
energy losses are greater than 50%, which translates into 71% photon loss.
The differences in light emission between the top and bottom surfaces illustrated in Figure 7
may be understood by considering that light absorption through the sample will show a significant
gradient, with a greater fraction of light absorbed near the top surface in a heavily dye-doped
waveguide. This will result in an emission gradient, with more of the light being emitted near the top
surface; thus, more light will escape from the top, as that light emitted towards the bottom has a
significantly increased chance of being re-absorbed before escape. In the lightly doped waveguide,
more light will penetrate deeper into the waveguide before absorption, leading to a relative increase in
light escape fraction out the bottom surface, as light traveling towards the top surface will have an
increased chance of re-absorption.
One may calculate the emission profile of an isotropic collection of dye illuminated from an
arbitrary direction [using 14, 15]; see Figure 8. Using this profile, we may determine the fraction of the
emission light that will be directed into the waveguiding mode for any given input light angle, and the
fraction directed so as to escape the surface of the waveguide, unless otherwise reabsorbed and
redirected by subsequent emission. The results of this calculation are seen in Figure 9. This latter
Figure may be used to simulate the effect of solar variation during the day as the sun crosses the sky.
In the extreme cases of incoming light directed at 75° with respect to the normal of the waveguide
surface, there is a calculated reduction of surface-emitted light of 25%, but in absolute terms is still
50% of the emitted photons.
For the case of light incident perpendicular to the waveguide surface, simulated results suggest
65% of the photons initially emitted by the dyes will be directed towards the top and bottom in such a
way as to permit their escape from the waveguide if not reabsorbed. Our measured values tally
between 52 and 71%, depending on the optical density. This suggests there is a considerable amount of
reabsorption of surface-directed light in the high optical density waveguides, and that there is very little
in the low optical density waveguides.
Surface losses are somewhat reduced in the surface-coated waveguide systems (Figure 6).
However, the internal losses of the thin waveguide systems are much higher, and the edge output at
higher absorbancies is also reduced (Figure 4). This is at least partly due to the reduced dye-dye
spacing necessary in these thin layers (3 to 30 µm as opposed to 3 mm for the filled waveguides),
which result in enhanced intermolecular interactions and consequent increased internal light losses.
A series of simulations using the RAYLENE software  give some insight into the
measurements. Calculations were made for the number of interactions each photon emitted by the dye
undergoes in the filled PC samples before escaping the waveguide or being lost. For an OD of 4
(99.99% absorption at the peak) each emitted ray undergoes an average 1.8 dye encounters. At an OD
of 1, this becomes about 1.5 encounters, while at 0.08 it is about 0.4 encounters (see Figure 10). Thus,
it is clear to see at lower ODs, there is a factor four less encounters per ray in escaping the waveguide,
which corresponds to an increased surface losses in the low absorbance region of Figure 6 compared to
the higher absorbance region.
The significant energy losses through the surface of the waveguide have been accepted as a
matter of course in the past, with more than 20% leaving the top surface and 40% overall (equivalent to
more than 50% of the photons). Losses through the bottom may be reduced by the application of
mirrors or scatterers, for example, but a fraction of this light will itself be redirected out the top surface
again. Our lab has set up several research programs designed to reduce or eliminate these surface
losses, and to promote enhancement of edge emissions and the overall efficiency of the LSC system.
Among the features being considered for reduction of these surface losses are aligned dye arrays which
will restrict the light emission of the dyes into more defined directions rather than the more random
emission found in isotropic dye systems. A second feature is the selectively-reflecting chiral nematic
(cholesteric) liquid crystal layer (similar in function to the inorganic ‘hot mirrors’ described in ).
The purpose of this latter system is to allow sunlight to reach the fluorescent dyes, but to reflect the
dye-emitted light back into the waveguide, preventing this emitted light from leaving the surface. By
reducing surface and internal losses through selection of higher-quality waveguide host materials and
optimizing the waveguide dimensions, the output of the luminescent solar concentrator could be
significantly increased, an important step in making the LSC a viable competitor to the silicon solar
The surface energy losses from luminescent solar concentrator waveguides is on the order of
40% to more than 50%, translating into a photon loss of 50% to 70%, a considerable amount of light.
The losses from the top surface are greater than the losses from the bottom for more heavily-doped
waveguides. These findings accentuate the need to develop ways of reducing these losses in LSC
devices in order to make them more viable competitors of the standard silicon photovoltaic panel.
The authors would like to acknowledge the support of Altran Technologies Netherlands, and in
particular thank Roel Moonen of Altran for his assistance in the measurements, and Tom Akkermans
from the TU Eindhoven for help in sample preparation. MD would like to acknowledge the support of
STW VIDI grant 07940. BSR and BCR acknowledge the support of EPSRC grant EP/F02763X/1.
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Rotates 30 → 70 º
Rotates 0 → 180º
0 0.51 1.52 2.53 3.54 4.55
Edge Output (mW)
350400 450500550600650700 750800
350 400450500550 600 650 700750800
Total Surface Loss (%)
Top loss / Bottom Loss
-90-70 -50-30 -101030 5070 90
Angle of incident light relative to waveguide normal
Surface loss (% of emitted photons)
# Interactions / Ray
Figure 1. Experimental setup for measurement of the surface losses in the LSC systems. The
measurement of edge emission was accomplished in the same manner, except the waveguide is placed
horizontally, and illumination is incident from above.
Figure 2. Experimental setup for the measurement of top/bottom loss ratios.
Figure 3: Representative absorption (solid line) and emission (dotted lines) spectra of Red305 dye filled
Figure 4: Measured edge emission from polycarbonate (filled symbols) and PMMA (open symbols)
waveguides with dye in the waveguide (squares) and in a thin polycarbonate (triangle) or pentaacrylate
(circle) surface layer.
Figure 5. a) Measured spectrum obtained from illumination of the blank polycarbonate waveguide
(black line) and from the dye-filled polycarbonate waveguide (gray line).
b) Resultant from subtracting the spectrum obtained from blank polycarbonate waveguide from the
emission spectrum of polycarbonate waveguide containing dye (peak absorbance of 2.5). Integration
over region I gives the power absorbed by the dye and integration of region II gives the power emitted
by the dye through the bottom surface.
Figure 6. Measured bottom surface loss from polycarbonate (filled symbols) and PMMA (open
symbols) waveguides with dye in the waveguide (squares) and in a thin polycarbonate (triangle) or
pentaacrylate (circle) surface layer.
Figure 7. Measured ratios of top to bottom surface emissions for dye filled polycarbonate (filled
squares) and dye filled PMMA (filled triangle) waveguides, and for dye-topped polycarbonate
waveguides with Red305 in a polycarbonate (open square) or pentaacrylate (open circles) matrix, and
dye-topped PMMA waveguides with Red305 in a polycarbonate (open diamonds) or pentaacrylate
(open triangles) matrix.
Figure 8. Calculated emission profile for an isotropic dye in a pentaacrylate matrix (n=1.586)
illuminated from directly overhead [14,15].
Figure 9. Calculated percentage of photons emitted outside the waveguiding cone of polycarbonate
(n=1.586) as a function of the incidence angle (with respect to the waveguide normal) of the
Figure 10. Calculated number of interactions per ray passing through the filled waveguides.
Determined using the RAYLENE code.
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Measured bottom loss and derived top losses in energy and photons from the polycarbonate dye-filled