Luminescent Solar Concentrators A review of
recent results
Wilfried G.J.H.M. van Sark,
1,*
Keith W.J. Barnham,
2
Lenneke H. Slooff,
3

Amanda J. Chatten,
2
Andreas B chtemann,
4
Andreas Meyer,
5
Sarah J. McCormack,
6

Rolf Koole,
7
Daniel J. Farrell,
2
Rahul Bose,
2
Evert E. Bende,
3
Antonius R. Burgers,
3

Tristram Budel,
3
Jana Quilitz,
4
Manus Kennedy
6
, Toby Meyer,
5
C. De Mello DonegÆ,
7

Andries Meijerink,
7
Daniel Vanmaekelbergh
7

1
Science, Technology and Society, Copernicus Institute of Sustainable Development and Innovation,
Utrecht University, Heidelberglaan 2, 3584 CS Utrecht, the Netherlands
2
Department of Physics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom
3
ECN Solar Energy, P.O. Box 1, 1755 ZG Petten, the Netherlands
4
Fraunhofer Institute for Applied Polymer Research (IAP), Geiselbergstra e 69, 14476 Potsdam, Germany
5
Solaronix SA, Rue de l Ouriette 129, 1170 Aubonne, Switzerland
6
Focas Institute, School of Physics, Dublin Institute of Technology, Kevin St, Dublin 8, Ireland
7
Chemistry of Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University,
Princetonplein 1,3584 CC Utrecht, the Netherlands
*
Corresponding author: w.g.j.h.m.vansark@uu.nl
Abstract: Luminescent solar concentrators (LSCs) generally consist of
transparent polymer sheets doped with luminescent species. Incident
sunlight is absorbed by the luminescent species and emitted with high
quantum efficiency, such that emitted light is trapped in the sheet and
travels to the edges where it can be collected by solar cells. LSCs offer
potentially lower cost per Wp. This paper reviews results mainly obtained
within the framework of the Fullspectrum project. Two modeling
approaches are presented, i.e., a thermodynamic and a ray-trace one, as well
as experimental results, with a focus on LSC stability.
2008 Optical Society of America
OCIS codes: (130.7405) Wavelength conversion devices; (040.5350) Photovoltaic; (130.0250)
Optoelectronics
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1. Introduction
Attaining higher conversion efficiencies at lower costs is the key driver in photovoltaics (PV)
research and development and has been so for many decades. Today, the prices of PV
modules are too high and widespread deployment of PV technology still needs financial
support schemes, such as investment subsidies or feed-in tariffs, the latter being also a quality
assurance check. Nevertheless, over the past 10 years the PV industry is experiencing average
annual growth rates of 40% [1]. To reach lower cost per installed capacity (
/W), several
routes are being pursued, all more or less directed towards a better use of the complete solar
spectrum, and they are being referred to as Next or Third Generation PV [2,3]. Examples are
intermediate band-gap cells [4], quantum dot concentrators [5] and down- and up-converters
[6,7]. Conversion of the incident solar spectrum to monochromatic light would greatly
increase the efficiency of solar cells. Down conversion was suggested in the late 1970s to be
used in so-called luminescent solar concentrators (LSC), also referred to as fluorescent
concentrators. To these LSCs solar cell(s) were attached [8]. LSCs consist of a highly
transparent plastic, in which luminescent species, originally organic dye molecules, are
dispersed, see Fig. 1. These dyes absorb incident light and isotropically emit it at a red-shifted
wavelength, with high quantum efficiency. Internal reflection ensures collection of part of the
emitted light in the solar cells at the side(s) of the plastic body. The energy of the emitted
photons ideally is only somewhat larger than the band gap of the attached solar cells, to ensure
near-unity conversion efficiency.
LSCs were developed as an alternative approach to lower the costs of PV. As both direct
and diffuse light is concentrated by a factor of 5-10, without the need for expensive tracking,
smaller silicon (or other) solar cells can be used. As the cost of the transparent plastic is
expected to be (much) lower than the area cost of the solar cell the cost per Watt-peak is lower
compared to the cost of a planar silicon solar cell. Also, LSCs are of special interest for
building integrated PV applications.
The development of the LSC was initially limited by the performance of the luminescent
dyes available some decades ago. Nevertheless, efficiencies of up to 4% have been reported
for a stack of two plates (40×40×0.3 cm
3
), one being coupled to a GaAs solar cell, and the
other to a Si solar cell [9]. Particular problems were the poor stability of the dyes under solar
irradiation and the large re-absorption losses owing to significant overlap of the absorption
and emission. Within the Fullspectrum project [10] the performance of both quantum dots
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21775
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Fig. 1. Schematic 3D view of a luminescent concentrator. AM1.5 light is incident from the top.
The light is absorbed by a luminescent particle. The luminescence from the particle is randomly
emitted. Part of the emission falls within the escape cone (determined by the angle (θ
c
)) and is
lost from the luminescent concentrator at the surfaces (1). The other part of the luminescence is
guided to the solar cell by total internal reflection (2).

(QDs) and organic dyes are being evaluated as the luminescent species in the LSC. The
important characteristics of organic dyes are that they: (i) can provide extremely high
luminescence quantum efficiency (LQE) (near unity), (ii) are available in a wide range of
colours and, (iii) new molecular species are now available with better re-absorption properties
that may also provide the necessary UV stability. QDs have advantages over dyes in that: (i)
their absorption spectra are far broader, extending into the UV, (ii), their absorption properties
may be tuned simply by the choice of nanocrystal size, and (iii) they are inherently more
stable than organic dyes. Moreover, (iv) there is a further advantage in that the red-shift
between absorption and luminescence is quantitatively related to the spread of QD sizes,
which may be determined during the growth process, providing an additional strategy for
minimising losses due to re-absorption [5]. However, as yet QDs can only provide reasonable
LQE: a LQE > 0.8 has been reported for core-shell QDs [11].
This paper reviews recent results in LSC development, mainly obtained within the
framework of the Fullspectrum project [10]. These encompass modeling and experimental
work, as well as stability issues.
2. Modeling
Several groups have reported on the modeling of the LSC [12-18] Principally two different
approaches are used, a detailed balance model which is based on the radiative energy transfer
between mesh points in the concentrator plate, and a ray-tracing model in which every
incoming photon is tracked and its fate determined. In the following these two approaches are
detailed.
2.1 Thermodynamic modeling
Self-consistent 3D thermodynamic models for planar LSCs [13,19], modules [20] and stacks
[12] have been developed which show excellent agreement with experiments on test devices.
Detailed balance arguments relate the absorbed light to the emission using 3D fluxes and the
thermodynamic models were derived by performing a Schwartzchild-Milne [21] type
sampling of Chandrasekhar s radiative transfer equation [22], integrating the resulting
differential equations over the volume of the concentrator and applying appropriate reflection
boundary conditions. The resulting integral equations are applied over a mesh sampling the
concentrator volume such that a realistic representation of the continuous media emerges. The
thermodynamic approach provides equations from which the photon chemical potential as a
function of position within the concentrator may be determined by iteration. An optimal, self-
consistent linearization of the depth dependence of the chemical potential for a single planar
1
a
2
1
Fr
Ri
Re
AM1.5
2
Ө
c
solar cell
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21776
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020
40
60
80
100
120
140
Control Control +
3M
mirror
Coating
Top
Coating
Top + 3M
mirror
Coating
Bottom
J
S
C

/

m
A

m
-
2
Model
Experiment

Fig. 4. Measured and predicted short-circuit currents, J
SC
, for the red dye doped test LSC.

The model results for the dyes agreed with the measurements using the known LQE of
0.95 for both dyes giving confidence in the fitted LQEs of 0.45 and 0.50 for the red and
yellow QDs respectively [24]. The results show that the thermodynamic approach can predict
both the room temperature red-shift and the total flux escaping each surface of the LSC
providing a tool for its optimisation.
2.2 Ray trace modeling
Ray-tracing for LSCs uses basic ray-tracing principles, which means that a ray, which
represents light of a certain wavelength travelling in a certain direction, is traced until it leaves
the system e.g. by absorption or reflection at the interface. The model applies statistical
averaging of the absorption, leading to a much reduced computation time, compared with
modeling of individual luminescent particles or dyes, such as in the work of Gallagher [17].
The main extension to the standard ray-tracing model is the handling of the absorption and
emission by the luminescent species in the LSC. The 3-D ray-tracing program described here
takes these absorption and emission characteristics into account [26].


The model is well able to explain experimental results on practical LSC devices of
reflection and transmission measurements, as well as LSC photo response [26]. Also, a
parameter study has been performed to find attainable LSC efficiencies. The parameters
studied were 1) mirror configuration; 2) polymer material properties; 3) solar cell type; 4) dye
type. In addition, the major loss factors could be determined.
2.2.1 Mirror configuration
The basic configuration for the modeling in this section consists of a planar 5x5 cm
2

luminescent concentrator with mirrors on the three side facets and a mc-Si cell on the
remaining side facet as well as a mirror at the bottom (see Fig. 1). The concentrator consists of
a PMMA plate (refractive index n=1.49, absorption 1.5 m
-1
) doped with two luminescent
dyes, CRS040 from Bayer and Lumogen F Red 305 from BASF, with a FQE’s of 95% [27].
With the ray-tracing model the efficiency of this basic configuration was determined to be
2.45%.
Next, the mirror configuration was varied, using a realistic FQE of 95% for both dyes.
Direct mirrors or a mirror with an air-gap between the mirror and the LSC were modeled, as
well as different reflectivity values and specular or Lambertian mirror types. Without a side
mirror, rays within the escape cone are leaving the LSC at the side; rays outside the escape
cone are subject to total internal reflection, see Fig. 5(a). When a direct mirror is applied, the
total internal reflection would disappear, and all rays would reflect with the reflection
coefficient of the mirror, see Fig. 5(b). For the rays outside the escape cone this leads to a
reduction in the reflection, and thus lower power conversion efficiency. The use of an air-gap
mirror , i.e., an air-gap between the mirror and the LSC, combines total internal reflection
with reflection of the escaping rays, see Fig. 5(c). The results of the ray-tracing calculations
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21779
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2.2.3 Solar cell optimization
One major reason for the lower efficiency compared to the work of Zastrow [29] is that the
solar cells that are used up to now are not optimized for the emission-spectrum of the dye: mc-
Si solar cells absorb all the light up to about 1100 nm, but the dyes used in these LSCs emit
around 650 nm. It would therefore be beneficial to use a solar cell with a larger bandgap,
because these deliver the same current as the mc-Si cell, but at larger V
oc
s. In Table 1
estimates are listed for attainable efficiencies η for similar concentrator plates with different
solar cells attached. Replacing the mc-Si cell by a GaAs cell or a InGaP cell, will increase η
from 3.8% to 6.5 and 9.1%, respectively (based on V
oc
(FF) values of 0.58 V (0.83), 1.00 V
(0.83), and 1.38 V (0.84), for mc-Si, GaAs, InGaP, respectively). Thus, the use of GaAs or
InGaP cells will result in higher efficiencies, but these cells are more expensive. A cost
calculation must be performed to determine if the combination of the luminescent
concentrator with this type of cells is an interesting alternative to mc-Si based solar
technology.
Table 1. Calculated efficiencies (in %) for the LSC based on experimentally determined parameters and subsequently
using optimized parameters based on realistic estimates
mc-Si GaAs InGaP parameters
2.4 4.2 5.9 fixed mirrors, 85% reflectivity, dyes with 95% FQE
2.9 5.1 7.1
97% reflectivity air-gap mirrors on sides, and 97% reflectivity
Lambertian mirror at bottom
3.4 5.9 8.3 reduce background absorption of polymer matrix from 1.5 m
-1
to 10
-3
m
-1

3.8 6.5 9.1 increase of refractive index from 1.49 to 1.7

2.2.4 Extending the spectral sensitivity range
Instead of using an optimized solar cell, a dye could be added that absorbs into the infrared.
However, such dyes have not yet been developed, i.e. their FQEs are too low for use in an
LSC, while absorption spectra are suitable. Zastrow [29] already addressed this in the 1980s,
and argued that the C-H bond vibration becomes resonant with the luminescent transition in
the dye, providing a non-radiative pathway for the excitation resulting in quenching of the
luminescence. Nevertheless, despite the low FQE, the IR dyes can improve the efficiency
when used in stacked concentrators [30]. Ray-tracing calculations show that adding an IR dye
with an FQE of 50% to a plate containing both Red305 and CRS040 dye, results in a
reduction in efficiency from 3.8 % to 2.3%, see Table 2. This is due to the fact that the
emission from the Red305 dye is now absorbed by the low FQE IR dye, thereby increasing
the re-absorption losses substantially. However, if the IR dye is present in a separate LSC
plate below the plate with the Red305 and CRS040 dyes, the efficiency can increase by about
20% compared to a single plate containing the Red305 and CRS040 dyes, but at the expense
of increased material and solar cell costs. Putting the IR dye containing plate at the top is less
efficient, due to the filtering effect of the top plate: photons that are otherwise efficiently
converted by the CRS040 and Red305 dyes are absorbed by the IR dye.
Table 2. Ray-tracing results for a stack of two LSC plates with a low FQE IR dye in one plate and the
Red305+CRS040 dyes in the other, compared to all dyes in a single LSC plate
dye // cell combination efficiency (%)
single plate Red305+CRS040 // 1 c-Si solar cell 3.8
single plate Red305+CRS040+IR dye // 1 c-Si solar cell 2.3
stack Red305+CRS040 top/IR dye bottom // 2 c-Si solar cells 4.5
stack IR dye top/Red305+CRS040 bottom // 2 c-Si solar cells 4.3
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If an IR dye with an FQE of 95% could be synthesised, this dye combined with the
CRS040 and Red305 dyes in a single LSC plate would lead to an increased efficiency of
5.4%. When comparing this efficiency with the efficiency that can be achieved by using GaAs
or InGaP cells it can be concluded that the use of a different solar cell is more beneficial than
the use of a high FQE IR dye. However, as these cells are more expensive, cost calculations
must be performed to see which option is most favourable.
2.2.5 Major losses
The LSC with two dyes and an η of 3.8% as mentioned above has an EQE of 50-60% in the
absorption range of the dyes used. The remaining part of the light is mainly lost at the top
surface. It is clear that further improvement of the LSC must be achieved by reducing the top
escape losses. One way of doing this is to use special top mirrors that transmit light in the dye
absorption range and reflect light in its emission range, so-called wavelength selective
mirrors. The mirror should be transparent in the region where the dye is absorbing and highly
reflecting in the range where the dye is emitting. The absorption and emission spectra of the
dye are shown in Fig. 7(a), together with the desired transmission and reflection spectrum of
the mirror. As described earlier one way to realize such a mirror is by applying selectively-
reflective chiral nematic (cholesteric) liquid crystal (LSC) layer(s) [25]. In Fig. 7(b) the
optical properties of cholesteric layers are illustrated.
Fig. 7. (a) Absorption and emission spectra of the Red 305 dye, together with the desired
transmission and reflection spectrum of the selective mirror; (b) reflection of a cholesteric layer
for different angles of incidence. With increasing angle of incidence the centre wavelength of
the high reflection region moves to shorter wavelengths.

As can be seen, the transmission shows the desired low transmission in the dye emission
range, but the transmission band is rather small and shows interference fringes. Furthermore,
the transmission spectrum depends on the angle of incidence. This will have a large impact on
the application of such a mirror in the LSC. The emission by the dye is more or less random
and will thus arrive at the top interface under various angles of incidence. So a ray at a
wavelength of 600 nm at normal incidence will not be reflected, as the center wavelength of
the cholesteric mirror at this wavelength is 680 nm, but would it have an angle of 40
o
it would
be reflected, see Fig. 8(a). To determine the effect of such a cholesteric mirror, the reflection
and transmission characteristics of Fig. 7(b) were implemented in the ray-tracing program
[31] Calculations were performed for an LSC with and without a cholesteric mirror. Fig. 8(b)
shows the current from the attached solar cell, as calculated with the ray-tracing program for
different center wavelengths of the cholesteric mirror. The calculations were done for the
configuration with a diffuse air-gap mirror at the bottom and for a direct 100% reflecting
specular mirror at the bottom. From Fig. 8(b) it is clear that the center wavelength of the
cholesteric mirror needs to be red-shifted substantially for optimal performance. Although the
peak emission wavelength of the dye is about 600 nm, the optimal center wavelength of the
0.4 0.6 0.8
0
20
40
60
80
100


R
,
T

(
%
)
n
o
r
m
a
l
i
s
e
d

a
b
s
o
r
p
t
i
o
n

a
n
d

e
m
i
s
s
i
o
n
Wavelength (µm)
dye absorption
filter transmission
filter reflection
dye emission
400 500 600 700 800
0
25
50
75
100
60
angle of
incidence
0
10
20
30
40
50
60


T
r
a
n
m
i
s
s
i
o
n
Wavelength (nm)
0
(a) (b)
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21782
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008

Fig. 8. (a) Schematic view of the different regions the emitted fluorescence of the dye is facing
when a cholesteric mirror with a center wavelength of 680 nm is applied. Light that falls
perpendicularly on the top surface faces a reflector with a center wavelength of 680 nm,
whereas light that falls in at roughly 45
o
faces a reflector with a center wavelength of 600 nm.
At even larger angles total internal reflection will take place. (b) Calculated short circuit current
from the mc-Si cell connected to the LSC plate as a function of the center wavelength of the
cholesteric top mirror as compared to the situation where no top mirror is applied. Calculations
are shown for a diffuse air-gap bottom mirror, and a direct specular bottom mirror (100% spec).

cholesterics for normal incidence is 710 nm. Furthermore, the effect of the cholesteric mirror
is much larger, 12%, in the case of a direct 100% reflecting specular mirror than for the
diffuse air-gap bottom mirror, 3%. The reason for this must be found in the different angular
dependence of the reflected rays. In the case of a diffuse air-gap mirror, the randomly emitted
luminescence is scattered at the backside mirror and a fraction of this scattered light will be
transported directly to the solar cell, without reaching the top surface. This fraction will not
benefit from the application of a cholesteric top mirror. With a direct specular mirror, the
randomly emitted luminescence will be specular reflected and will thus remain randomly
oriented. As a result, part of the emission will reflect at the backside and escape from the top
side, if emitted within the escape cone. For this reason, the performance of a direct specular
bottom mirror is lower than for a diffuse air-gap mirror. However, by applying a cholesteric
mirror, now the escaping light will be reflected and the light can reach the solar cell.
The subject of selective mirrors has recently been addressed by several other groups
[24,32,33] and is clearly an important topic in the study of LSCs. Escape cone losses through
the top surface of the LSC may also be reduced by exploiting directional emission within the
LSC. Organic dye molecules can be aligned using liquid crystalline host materials [34], and
nanorods self-align at high concentrations and may also be aligned by stretching polymer
films. The dye alignment must be chosen such that the peak in the emission direction falls
outside the escape cone of the LSC. However, this will also reduce the absorption in the
waveguide for perpendicular incidence light. Thus, it is expected that the dye concentration
must be increased. On the other hand, Debije et al. [34] suggest to use a high concentration
dye layer with a transparent waveguide on top. In this configuration, the emitted light will
mainly pass through the transparent waveguide. After reflection at the top surface it will have
an almost 90 degrees angle with respect to the dyes and reabsorption losses will thus be
strongly reduced. This results in a larger fraction of the luminescent light being guided in the
direction of the attached solar cell.
2.3 Comparison of modeling approaches
The modeling methods described above should yield preferably identical results, when
modeling identical LSCs. The thermodynamic approach requires a minimum of input data and
is quick to run, but it is limited to rectangular flat plate LSCs homogeneously doped with a
600 650 700 750 800
90
100
110
120
No cholesteric
Cholesteric
No cholesteric (100% spec.)
Cholesteric (100% spec.)


I
s
c

(
m
A
)
Center Wavelength (nm)
(a) (b)
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21783
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008

single luminescent species. The ray-trace approach is more flexible allowing multiple dopants,
thin-films and different geometries to be investigated. In order to check the validity of both
model approaches, four Plexit slabs, of different sizes, containing a Bayer Fluorescent Red
Coumarin dye or a Fluorescent yellow Coumarin dye were fabricated [12], and modeled. The
dimensions of each slab are given in Table 3. The quantum yield

of both dyes was determined
to be 0.95. To measure the electrical output of each, the slabs were illuminated at normal
incidence by an Oriel fibre-optic lamp. A 2.65×x2.65 mm Siemens Si photodetector was
utilised to obtain short circuit current (J
sc
) values at the edge of each slab. Ray-trace modeling
and thermodynamic modeling, as described above, were used to predict the photon count and
luminescence spectrum escaping at the edge where the photodetector was placed. The
photodetector spectral response and its angle dependent reflectivity were used with the
predicted photon count to obtain the predicted J
sc
of the four LSC devices, as given in Table 3.
Clearly, there is a high level of agreement between the predictions and observed values.
Despite the many differing processes involved in each modeling approach, there is very good
agreement between both techniques. The results show that both thermodynamic and ray-trace
modeling provide useful tools for optimizing LSC devices and predicting their electrical
output.
Table 3. Measured and predicted short-circuit current densities (J
sc
) of the four LSC devices

2.4 Device geometry effects
The effect of varying the device geometry on LSC performance can be analyzed using ray-
trace modeling. Square, right-angled triangular and hexagonal quantum dot doped luminescent
solar concentrators (QDSCs) of increasing top surface apertures (A
conc
) were considered.
Concentration ratios (C) were predicted for increasing A
conc
of each geometry type, as detailed
in [35]. Figure 9 shows that a hexagonal geometry attains the highest C for the range of A
conc



Fig. 9. Predicted concentration ratios (C) for devices of varying geometry and top surface
aperture (A
conc
). The device thickness was fixed at 0.3 cm.
slab dimensions (cm
3
)
measured
J
sc

(mA/m
2
)
predicted J
sc

(mA/m
2
)
Thermodynamic
predicted
J
sc
(mA/m
2
)
Ray-trace
Red large
4.78×1.7×0.255
53.2 – 2.0 51.6 51.9
Red small
1.93×0.994 ×0.25
22.5 – 2.0 23.9 24.9
Yellow large
4.78×1.78×0.269
10.4 – 2.0 10.2 9.3
Yellow small
2.26×1.0×0.27
5.2 – 2.0 5.0 5.0
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21784
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008

Fig. 10. Predicted relative costs per unit power output for square, triangular and hexagonal
QDSC geometries of increasing top surface aperture. The predictions for square QDSCs with
PV attached at all four sides [35] are also shown.

considered. However, for a given A
conc
, each geometry type has a different area of attached
PV (A
pv
). Also, the cost of materials of the LSC plate, although much lower than the cost of
PV (per m
2
), is not negligible. Therefore, to determine the optimum geometry, relative costs
per unit power output were calculated, as detailed in [35], with the relative costs of the LSC
plate and PV factored in. The relative power output of each device is assumed to be
proportional to the product of the concentration ratio attained and A
pv
, in each case. The
results, shown in Fig. 10, indicate that all geometries can attain the same minimum relative
cost per unit power. Under the assumptions made, it is concluded that varying the geometry
type does not offer any significant relative cost reduction, however, the results do show that
the selection of device size is critical for achieving the lowest possible cost per unit power
output.
3. Experimental work
3.1 Preparation of samples
Since acrylic polymers are known to have the potential for high optical transparency,
considerable stability and good mechanical properties, our efforts to make LSCs were focused
on this class of polymers. Two main kinds of samples containing luminescent molecules or
particles were prepared: polymer plates and coatings on substrates.
3.1.1 Plates
Plates were produced by polymerisation of monomers or monomer mixtures in flat cuvettes.
The monomer systems used in most cases were methylmethacrylate (MMA),
methylmethacrylate/2-hydroxyethylmethacrylate (HEMA) with MMA:HEMA=1:1 by weight,
Plexit 55, and mixtures of dodecylmethacrylate(laurylmethacrylate(LMA)) with MMA and
ethylenglycoldimethacrylate (EGDM). The MMA (>99%, Merck Schuchardt) used to make
PMMA plates contained a stabilizer, but better results were obtained using distilled MMA.
The prepolymer Plexit 55 is a commercially available viscous MMA/PMMA mixture
containing 30-40% polymer (R hm GmbH). The purities of the monomers HEMA (Merck
Schuchardt), LMA (Aldrich) and EGDM (Fluka) were 97% , 96% and > 97%, respectively.
The cuvettes consisted of 3-4 mm thick glass plates with an elastic distance holder
between the plates, which were held together by steel clamps. As the polymer sometimes was
sticking strongly to the glass after the polymerization reaction, it may be advantageous to use
glass with a surface coating that diminishes adhesion. Silicon rubber bands of 3mm, 5mm or
8mm thickness or FEP tubes of 3mm, 5mm or 10mm diameter were used as inert distance
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21785
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008

holders. The distance holders as well as the clamps must be elastic to compensate the
considerable shrinkage of the polymerizing material in the cuvette. These elastic properties of
the cuvettes together with certain adhesion between glass and polymer allowed to avoid
undesirable partial separation of the still reacting, solidifying polymer material from the glass
leading otherwise to irregular LSC surfaces.
The reaction mixtures were polymerized by means of thermally or UV initiated
polymerization, respectively. Plexit and MMA were polymerized in most cases thermally
using 0.05% - 0.1% azoisobutyronitrile (AIBN) as initiator, whereas plates on the basis of
MMA/HEMA or other copolymer mixtures were usually made by UV polymerization using in
many cases the liquid initiator Irgacure 1700 (Ciba). Plexit was polymerised for 20 hours in an
oven using a temperature program with a maximum temperature of 70 C. For MMA best
results were achieved for polymerization in a bath with circulating water of 50 C (about 20
hours). The thus thermally produced plates contained still up to 9% monomer, but after a post-
treatment at 110-120 C for some hours no remaining monomer was present, as evidenced by
infrared spectroscopy. In a number of experiments the distilled MMA was prepolymerised
before filling it into the cuvette: the strongly stirred MMA plus 0.1% AIBN initiator was
heated to 95 C with the flask being purged by nitrogen. After ~20 minutes the then viscous
liquid was very rapidly cooled in ice to stop the reaction.
For UV polymerization the cuvettes were irradiated from two sides by UV-A radiating
lamps (band maximum 360nm). The intensity of the irradiation applied was different for the
various mixtures and had to be tested out. For 3mm thick P(MMA/HEMA) plates, e.g., at very
low UV intensity a polymerization time of about 2 hours was sufficient.
The reaction mixtures were prepared by solving the initiator and the dye in the monomer,
if possible, or in a small amount of a solvent compatible with the monomer with this amount
being then added to the monomer. The reaction mixtures were degassed before starting
polymerisation to avoid formation of bubbles in the plates.
Following the above procedure, clear, transparent, bubble-free plates with even surfaces
were obtained with best loss coefficients achieved being in the order of 0.5m
-1
(for plates
without dye). Common plate sizes were, e.g., 10×10 cm
2
and 15×15 cm
2
, with the maximum
size produced being about 50×20 cm
2
.
3.1.2 Coatings
Coatings were prepared by casting solutions of PMMA, ethylmethacrylate polymer (Paraloid
B72, Fa, Dr. Georg Kremer) or cellulosetriacetate (CTA) on glass or Plexiglas substrates of
5×5 cm
2
size. Substrate thicknesses were 1 mm or 3 mm. Suitable solvents had to be chosen
depending on the polymer and the dyes or nanoparticles. Paraloid B72 was dissolved in ethyl-
acetoacetate (AEEE), solvents used for PMMA were ethyl-acetoacetate or CHCl
3,
and CTA
was dissolved in a CHCl
3
/CH
2
Cl
2
mixture. Stock solutions of the polymers (e.g. 10% PMMA/
AEEE) were made to which the luminescent species was added (sometimes after dissolving it
in a small amount of pure solvent) with dispersing it thoroughly by stirring or, if necessary, by
means of ultrasound until the mixture was clear. In most cases definite amounts of solution,
resulting in roughly reproducible layer thicknesses, were poured out onto the substrate and
dried for 12-24 hours at 20 C. The drying procedure had to be tested out for the different
solvents, in some cases it was necessary to slow down the evaporation rate to obtain clear
coatings with an even surface. Remaining solvent was driven out by storage at higher
temperatures (~80 C). Depending on polymer concentration and solution amount used
coatings with thicknesses between < 10 m and several hundred m were prepared.
3.1.3 Luminescent species
As luminescent species a number of organic dyes as well as some types of nanoparticles
(quantum dots, nanorods) were used. An overview of the most important dyes used is given in
table 4. The suppliers of the dyes were BASF (Lumogen dyes), Bayer AG (Macrolex
Fluorescence Red G), Radiant Color N.V. (CRS040) and Lambda Chem GmbH (S13), part of
S13 was a gift from Prof. Langhals (LMU M nich).
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21786
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008

Table 4. Overview of used dyes, their absorption and luminescent emission peak wavelengths and luminescent
quantum yields
dye
chemical
structure
absorption
λ
max
(nm)
emission
λ
max
(nm)
quantum
yield (%)
reference
Lumogen F Blue 650 Naphtalimide 377 411 >80 [36]
Lumogen F Violet 570 Naphtalimide 378 413 94 [36]
Lumogen F Yellow 083 Perylene 476 490 91 [36]
Lumogen F Yellow 170 Perylene 505 528 >90 [36]
Lumogen F Orange 240 Perylene 524 539 99 [36]
S13 Perylene 526 534 100 [37]
Lumogen F Red 305 Perylene 578 613 98 [36]
Macrolex Fluorescence
Red G
Coumarin 520 600
>80
87
[26]
[27]
CRS040 (new: CFS002
Yellow)
Coumarin 440 506 98 [27]

A number of tests was performed using two types of commercially available quantum
dots (SD 387, SD 396 from Nanoco), but most work was done with multishell
CdSe/CdS/CdZnS/ZnS quantum dots from Utrecht University [38,39]. Furthermore CdSe/ZnS
nanorods from L. Manna s group [40] were used. The preparation of acrylic polymer plates of
good quality containing nanoparticles by polymerization as described above was more
complicated than found with organic dyes. This is due to the fact that the QDs are passivated
by hydrophobic ligands, which causes the nanoparticles to form turbid dispersions in the
hydrophilic monomers (e.g. MMA) and/or solvents often used. The luminescence of the
hydrophobic nanoparticles in inappropriate, e.g. hydrophilic, media was quenched as a
consequence of the formation of agglomerates and resulting energy transfer between the QDs.
However, when the more hydrophobic monomer LMA was used, clear
nanoparticle/LMA/EGDM/initiator mixtures could be prepared, which were polymerized
successfully. As a result highly transparent and strongly luminescent plates containing QDs or
nanorods with sizes of up to about 5×5×0.4 cm
3
were prepared.
3.2 Dye-doped LSCs
Based on the modeling results presented in Section 2.2, experiments were performed to verify
model predictions. Initially only one luminescent dye, Red305, was used in the LSC. Because
Red305 captures only a small region of the solar spectrum it is combined with the blue
absorbing dye CRS040. This LSC plate was connected to a high efficiency mc-Si cell (18.6%)
using microscope immersion oil; the side mirrors consisted of a 98% reflective visible mirror
foil. Both the higher efficiency of the mc-Si cell and the high reflectivity of the mirror foil
contribute to an increase in the EQE as can be seen in Fig. 11. Adding the CRS040 to the
Red305 dye (black line), gives an additional increase in the EQE and a slight improvement in
the spectral sensitivity around 370 nm. Based on these EQE spectra the AM1.5 short circuit
current can be calculated. The result is shown in Table 5. The LSC devices were also
measured using the ECN solar simulator, resulting in measured AM1.5 efficiencies of 2.4%
for the Red305 doped LSC and 2.7% for the Red305/CRS040 doped LSC. These efficiencies
are comparable to the efficiency obtained with ray-tracing simulation as shown in Table 5.
To verify the effect of a selective top mirror a commercially available dichroic mirror
having the desired characteristics was used instead of a cholesteric mirror, because the latter
was not available. The reflection and transmission spectrum of this mirror (LOT-ORIEL
590FD24-50SX) is shown in Fig. 12. The mirror was employed in combination with an LSC
for different types of attached solar cells, i.e. mc-Si, GaAs, and InGaP. The results of external
quantum efficiency (EQE) measurements are shown in Fig. 13. For all solar cells used the
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21787
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008

400 600 800
0
10
20
30
40
Red 305 + CRS 040
Red 305


E
x
t
e
r
n
a
l

Q
u
a
n
t
u
m

E
f
f
i
c
i
e
n
c
y

(
%
)
Wavelength (nm)

Fig. 11: External quantum efficiency of an LSC with mc-Si solar cell, with one or two dyes
dispersed in the polymer matrix.
Table 5. Calculated AM 1.5 short circuit current (I
sc
) and power conversion
efficiency, together with the measured efficiency using the ECN solar simulator. The
efficiency of the used bare Si cell was 18.6%.
LSC plate
Calculated
Isc
(mA)
Measured
Isc
(mA)
Calculated
efficiency
(%)
Measured
efficiency
(%)
Red305 138 133 2.5 2.4
Red305+CRS040 153 147 2.8 2.7

EQE shows an improvement in the dye absorption range when the mirror is applied (note that
the lower EQE for the InGaP cell is due to a bad electrical contact of the cell). However, the
EQE spectrum without the filter is somewhat broader, indicating that the cut-off wavelength
of the mirror is too blue-shifted, contrary to what could be expected based on the spectra in
Fig. 7, where it seems to be slightly too red. The reason is that this dichroic mirror, similar to
the cholesteric mirror, shows an angle of incidence dependent reflection and transmission
spectrum. As discussed in Section 2.2.5 for the cholesteric mirror, the cut-off wavelength of
the dichroic mirror should be red shifted for optimal performance.
Fig. 12. Absorption and emission spectra of the Red305 dye, together with the transmission and
reflection spectrum of the LOT-ORIEL mirror.

0.4 0.6 0.8
0
20
40
60
80
100


R
,
T

(
%
)
n
o
r
m
a
l
i
s
e
d

a
b
s
o
r
p
t
i
o
n

a
n
d

e
m
i
s
s
i
o
n
Wavelength µm)
dye absorption
mirror transmission
mirror reflection
dye emission
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21788
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008

Fig. 16. (a) Absorbance spectra of CRS040/RED305 doped LSC plates, taken at various
intervals during (a) storage in the dark, (b) continuous illumination under a sulphur lamp.

For comparison an LSC doped with only CRS040 was aged simultaneously. The
absorbance spectrum of this LSC plates reduced in 46 days to about 50% of the initial
absorbance during the ageing. This reduction in absorbance is comparable to the reduction
seen in the plate with both CRS040 and Red305 demonstrating the instability of CRS040.
To study the mechanism of the degradation of the dyes in more detail, monochromatic light
ageing experiments were performed. In these experiments, high intensity LED light was used
to illuminate the LSC plates only in the absorption band of one of the dyes. Figure 16 shows
the effect of illuminating a Red305 doped plate at 470 nm, i.e. within the absorption band of
CRS040, or 589 nm, corresponding to the absorption band of Red305. For both excitation
wavelengths, the absorption of the plate remains unchanged after 631 hours of illumination.
The same experiment was performed on an LSC doped with both Red305 and CRS040. As
can be seen in Fig. 17, illumination at 589 nm does not induce degradation, but illumination at
470 nm induces severe bleaching of the CRS040 absorption band, as was also observed under
white light illumination (see Fig. 16). Contrary to the general understanding that UV light is
responsible for the bleaching of organic dyes, this experiment shows that bleaching of
CRS040 is also induced by 470 nm light.
A series of LSC plates without solar cells was positioned on the roof (facing south at 45°
tilt angle) of the FhG-IAP in Germany for more than 2 years. The plates were fabricated using
different luminescent dyes and matrix material. From time to time the absorption of the plates
Fig. 17. Absorbance spectra of a CRS040/Red305 (dashed lines) doped LSC plate and Red305
only (solid lines) doped LSC plate before and after monochromatic light illumination at (a) 470
nm and (b) 589 nm.
(C) 2008 OSA 22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21791
#99348 - $15.00 USD Received 28 Jul 2008; revised 25 Sep 2008; accepted 26 Sep 2008; published 17 Dec 2008