1.27
Luminescent Solar Concentrator
JC Goldschmidt, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany
© 2012 Elsevier Ltd. All rights reserved.
1.27.1
Introduction
1.27.2
Theory of Luminescent Solar Concentrators
1.27.2.1
The Factors that Determine the Efficiency of Luminescent Concentrator Systems
1.27.2.2
Thermodynamic Efficiency Limits
1.27.2.3
Thermodynamic Models of the Luminescent Concentrator
1.27.2.4
Ray Tracing Simulations of Luminescent Concentrators
1.27.3
Materials for Luminescent Solar Concentrators
1.27.3.1
Organic Dyes
1.27.3.2
Inorganic Luminescent Materials
1.27.4
Luminescent Solar Concentrator System Designs and Achieved Results
1.27.4.1
System Designs
1.27.4.2
Achieved System Efficiencies
1.27.5
The Future Development of Luminescent Solar Concentrators
1.27.5.1
Extending the Used Spectral Range into the IR
1.27.5.2
Controlling Escape Cone Losses
1.27.5.2.1
Photonic structures for increased efficiencies
1.27.5.2.2
Controlling the angular emission
1.27.6
Conclusion
Acknowledgments
References
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1.27.1 Introduction
In a luminescent collector, a luminescent material embedded in a transparent matrix absorbs sunlight and emits radiation at a
different wavelength than the incident one. Total internal reflection traps most of the emitted light and guides it to the edges of the
collector (Figure 1). This underlying principle was first used in scintillation counters [1, 2]. A geometric concentration is achieved if
the area of the edges is smaller than the illuminated front surface of the collector, that is, when the area from which light is collected
is larger than the area from which light is emitted.
When solar cells are optically coupled to the edges, they can convert the guided light into electricity. The application to
concentrate solar radiation onto a solar cell was proposed in the late 1970s [3, 4]. If the solar cell is illuminated with a higher
intensity than it would be in direct sunlight, a real concentration is achieved. For real concentration, high geometric concentration,
as well as high collection efficiency, is necessary. Probably, the most outstanding feature of luminescent solar concentrators is their
ability to concentrate both direct and diffuse radiation. This ability is a great advantage for the application of luminescent
concentrators in temperate climates, such as in middle Europe, or in indoor applications with relatively high fractions of diffuse
radiation. Additionally, luminescent concentrators do not require tracking systems that follow the path of the sun, in contrast to
concentrator systems that use lenses or mirrors in combination with trackers. This facilitates, for instance, the integration of
luminescent concentrators in buildings.
Luminescent concentrators were investigated intensively in the early 1980s, for example, [5, 6]. Research at that time aimed at
cutting costs by using the concentrator to reduce the need for expensive solar cells. However, several problems led to reduced
research interest. First, the used organic dyes had only relatively narrow absorption bands. Second, although the organic dyes
showed high quantum efficiencies, defined as the ratio between absorbed and emitted photons, above 95% in the visible range of
the spectrum, quantum efficiencies remained at 50% and lower in the infrared (IR) because of fundamental physical reasons.
Furthermore, the dyes that were sensitive in the IR were unstable under long-term illumination. Reabsorption of the emitted light
due to overlapping absorption and emission spectra further reduced efficiencies [5]. A fundamental problem was the escape cone of
the internal reflection, which caused losses of at least 26%. Finally, efficient solar cells of which the spectral response matched the
emission spectra of the dyes were hardly available.
After more than 20 years of progress in the development of solar cells and luminescent materials, and with new concepts,
several groups such as those of References 7–25 are currently reinvestigating the potential of luminescent concentrators. High
efficiencies have been achieved [19, 25, 26] and there has also been considerable progress in the understanding and theoretical
description, for example, [22, 24, 26–28]. However, efficiencies are still too low and system sizes too small for a commercial
application.
Comprehensive Renewable Energy, Volume 1
doi:10.1016/B978-0-08-087872-0.00132-3
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E1 Escape cone
Radiation
θc
Solar
cell
E2
Collector
plate
Reabsorption and
emission
Solar
cell
Dye
Figure 1 Principle of a luminescent concentrator. A luminescent material (dye) in a matrix absorbs incoming sunlight (E1) and emits radiation with a
different energy (E2). Total internal reflection traps most of the emitted light and guides it to solar cells optically coupled to the edges. Emitted light that
impinges on the internal surface with an angle steeper than the critical angle θc is lost due to the escape cone of total internal reflection. A part of the
emitted light is also reabsorbed, which can be followed by reemission.
The concept of luminescent concentrators has been given many different names; among them are fluorescent collectors
[4], quantum-dot solar concentrator [20–22, 29], and organic solar concentrator [30]. In this work, I will use the term
‘luminescent concentrator’ for the overall concept. For clarity, ‘luminescent collector’ will refer to the collector plate without
attached solar cells and ‘luminescent concentrator system’ will refer to a system constructed from a collector plate with solar
cells attached.
In Section 1.27.2, I will give an overview of the theoretical understanding on luminescent concentrator. I will start with
identifying the factors that determine luminescent concentrator system efficiencies. I will then shortly review theoretical models
and simulation approaches.
In principle, all kinds of luminescent materials can be used in a luminescent concentrator: fluorescent materials that show a
Stokes shift of the emission to longer wavelengths; phosphorescent materials; upconverters that emit one high-energy photon after
the absorption of at least two low-energy photons; and quantum-cutting materials that emit two low-energy photons after the
absorption of one high-energy photon. In Section 1.27.3, I will review which materials are investigated for their application in
luminescent concentrators.
Many different system configurations have been proposed for luminescent concentrators. Variations include deposition of the
luminescent material in a film on a transparent slab [31], stacking of different luminescent collectors [4], cylindrical collectors [32],
solar cells coupled to the bottom of the collector [13, 15], and many more. The most important system configurations and results of
their experimental investigation will be presented in Section 1.27.4. In Section 1.27.5, I will discuss the possibilities for the future
research on luminescent concentrators.
1.27.2 Theory of Luminescent Solar Concentrators
1.27.2.1
The Factors that Determine the Efficiency of Luminescent Concentrator Systems
Several factors determine the efficiency of a luminescent concentrator system. Most of these factors are wavelength dependent. By
integrating over the respective relevant spectrum, a description of the overall system efficiency with a set of efficiencies for individual
processes is possible [33, 34]. Following their representation, the important parameters are the following:
ηtrans,front transmission of the front surface with respect to the solar spectrum
absorption efficiency of the luminescent material due to its absorption spectrum with respect to the transmitted solar
ηabs
spectrum
QE
Quantum efficiency of the luminescent material
ηStoke
‘Stokes efficiency’; (1 − ηStoke) is the energy loss due to the Stokes shift
fraction of the emitted light that is trapped by total internal reflection
ηtrap
ηreabs
efficiency of light guiding limited by self-absorption of luminescent material; (1−ηreabs) is the energy loss due to
reabsorption
‘matrix efficiency’; (1 − ηmat) is the loss caused by scattering or absorption in the matrix
ηmat
ηtref
efficiency of light guiding by total internal reflection
ηcoup
efficiency of the optical coupling of solar cell and luminescent collector
efficiency of the solar cell under illumination with the edge emission of the luminescent collector
ηcell
The overall system efficiency ηsystem can be calculated from the single parameters via
ηsystem ¼ ηtrans ηabs QEηStoke ηtrap ηreabs ηmat ηtref ηcoup ηcell
½1
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589
Several aspects are of importance for the different efficiencies:
– The transmission of the front surface is determined by its reflection R(λinc), where λinc is the incident wavelength. This is usually
the Fresnel reflection, which is
nðλinc Þ−1 2
RðλÞ ¼
½2
nðλinc Þ þ 1
–
–
–
–
for normal incidence and the surface between a medium with refractive index of 1 and a medium with refractive index n(λinc). The
reflection of typical materials is in the range of 4% for one surface. Special layers or structures applied to the front can decrease or
increase reflection. Interestingly, an antireflection coating that reduces the Fresnel reflection does not affect the total internal
reflection. This can be understood by considering that total internal reflection is an effect strongly linked to refraction. Total
internal reflection occurs when the light from inside the high-index material impinges on the surface with an angle sufficiently
shallow that the light would be refracted back into the medium again. As the antireflection coating does not change the refraction,
total internal reflection is not affected either.
The absorption spectrum Abs(λinc) determines the absorption efficiency. A large fraction of the solar spectrum is lost, because
many luminescent materials absorb only a narrow spectral region. The absorption range of typical fluorescent organic dyes is only
about 200 nm in width.
The quantum efficiency QE of the luminescent material is defined as the ratio of the number of emitted photons to the number of
the absorbed photons. For organic dyes, the luminescent quantum efficiency can exceed 95%.
The energy of the emitted photons is usually different from the energy of the absorbed photons. For most luminescent materials,
a Stokes shift to lower energy occurs. This means that the emitted photons possess less energy than the absorbed ones. Therefore,
the wavelength of the emitted photons λemit is different from the wavelength of the incident photons λinc. As we will see in Section
1.27.2.2, this Stokes shift is of critical importance to the ability of the luminescent concentrator to concentrate light.
The luminescent material emits light isotropically in a first approximation. All light that impinges on the internal surface with an
angle smaller than the critical angle θc(λemit) leaves the collector and is lost (Figure 1). The critical angle is given by
1
θc ðλÞ ¼ arcsin
½3
nðλemit Þ
This effect is also called the escape cone of total internal reflection. The light that impinges with greater angles is totally internally
reflected. Integration gives a fraction
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ηtrap ðλÞ ¼ 1−nðλemit Þ− 2
½4
–
–
–
–
–
of the emitted photon flux that is trapped in the collector [35]. For polymethyl methacrylate (PMMA) with n = 1.5, this results in a
trapped fraction of around 74%, which means that a fraction of 26% is lost after every emission process. The 26% ratio accounts
for the losses through both surfaces. An attached mirror does not change this number, as with a mirror the light leaves the
collector through the front surface after being reflected.
The absorption spectrum and the emission spectrum overlap. For principal reasons, absorption must be possible in the spectral
region where emission occurs. Therefore, part of the emitted light is reabsorbed. Again, the energy loss due to a quantum
efficiency smaller than 1 occurs, and again radiation is lost into the escape cone.
Realistic matrix materials are not perfectly transparent. They absorb light and they scatter light, so it leaves the collector.
Total internal reflection is a loss-free process. However, the surface of the luminescent collector is not perfect. Minor roughness at
the surface causes light to leave the collector, because locally the light hits the uneven surface with a steep angle. Fingerprints and
scratches can seriously harm the efficiency of light guiding.
The luminescent collector and the solar cell have to be optically coupled. Otherwise, reflection losses occur at the interface
between collector and air and again at the interface between air and solar cell. However, the optical coupling can also cause losses:
light can be scattered away from the solar cell or parasitic absorption can occur.
Finally, the solar cell has to convert the radiation it receives from the collector into electricity. Again, a whole set of parameters
determine this process, ranging from reflection and transparency losses to thermalization and electrical losses.
This description is not very relevant for actually calculating the efficiency of luminescent concentrator systems, because some of the
involved efficiencies are neither easy to calculate nor directly accessible by measurement. Nevertheless, this description illustrates
very well the effects that affect the efficiency of luminescent concentrator systems.
1.27.2.2
Thermodynamic Efficiency Limits
The ability to as well concentrate diffuse radiation sets the luminescent concentrator apart from all other types of concentrators.
Systems utilizing only geometrical optics cannot concentrate diffuse light. For a discussion of this difference, the concept of étendue
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and its links with entropy are very helpful. For light incident from a cone, with θinc being half of the opening angle, and a flat, not
tilted illuminated area Ainc, the étendue ε can be calculated to be
ε ¼ π sin 2 θinc Ainc
½5
From the definition and this result, the étendue can be understood as a measure of how ‘spread out’ a light beam is in terms of
angular divergence and illuminated or emitting area. The étendue can be calculated for an emitted beam as well. In this case, the area
of the emitting surface and the opening angle of the emission cone must be applied in the above relation.
The étendue is closely linked to entropy. If an optical system increases the étendue, entropy is generated. Following Markvart in
Reference 36, if εinc is the étendue of the incident beam and εemit the étendue of the emitted beam, the entropy per photon σ that is
generated is
σ ¼ kB ln
εemit
εinc
½6
with kB is the Boltzmann constant. A conservative system does not generate entropy, so in a conservative system, the étendue is
constant. If there are no other sources of entropy, the étendue cannot be reduced, because the entropy cannot decrease. That is, any
concentration with geometrical optics that decreases the illuminated area must increase the angular divergence. Because for diffuse
radiation, angular divergence is already at its maximum, diffuse radiation cannot be concentrated with a system using only
geometrical optics.
As a luminescent concentrator is able to concentrate diffuse radiation, there must be another source of entropy. This source of
entropy can be found in the Stokes shift. The dissipation of part of the excitation energy as heat generates entropy and leads to
photon emission at longer wavelengths. Therefore, the extent of the Stokes shift determines the maximum concentration that is
achievable with a luminescent concentrator.
This relationship between Stokes shift and concentration has been theoretically described in Reference 37. The theoretical model
considers two distinct photon fields: one that is incident on the luminescent collector and one that is emitted from the collector. The
entropy change Δσ1 associated with the loss of a photon from the incident Bose field is
8π n2 ν2inc
½7
Δσ1 ¼ − kB ln 1 þ 2
c Fp;ν;inc
where Fp,v,inc is the flux of photons (i.e., photons per unit time) per unit area, per unit bandwidth, and per 4π solid angle of the
incident field. The other parameters are the frequency of the photon νinc, the speed of light c, and the refractive index n [37].
The emission of a photon with the frequency νemit increases the entropy of the emitted field. Additionally, due to the Stokes shift,
the energy h (νinc − νemit) is dissipated as heat at the ambient temperature T. The entropy generated from these two processes is
8πn2 ν2emit
hðνinc − νemit Þ
þ
½8
Δσ2 ¼ kB ln 1 þ 2
T
c Fp;ν;emit
with the parameters defined for the emission field corresponding to the parameters of the incident field.
According to the second law of thermodynamics, it must hold
Δσ 1 þ Δσ 2 ≥0
½9
In the argument of the logarithm in eqns [7] and [8], the ‘1’ can be neglected under illumination with sunlight and for frequencies
in the visible spectral range. With this approximation and the eqns [7]–[9], the concentration ratio C can be calculated to be
Fp;ν;emit ν2emit
hðνinc − νemit Þ
½10
C :¼
≤ 2 exp
kB T
Fp;ν;inc
νinc
Figure 2 illustrates these results. The maximum possible concentration has been calculated with eqn [10] for three different
wavelengths of the incident light. It becomes obvious that from an entropic point of view, higher concentrations can be achieved for
shorter wavelengths than for longer wavelengths. For short wavelengths, the maximum concentration is very high and constitutes no
practical limit. For longer wavelengths, however, a sufficiently large Stokes shift is necessary in order to avoid limitations for
principal reasons.
The fact that there is a maximum concentration has one more consequence: when the maximum concentration is reached,
increasing the collector area will not increase the output at the edges of the concentrator. Already before the maximum concentra­
tion is reached, increasing the collector area of a large concentrator will not increase the output in the same way as increasing the
area of smaller collector. As a consequence, the light collection efficiency of luminescent collectors decreases with increasing size.
1.27.2.3
Thermodynamic Models of the Luminescent Concentrator
The picture of an incident and an emitted light field that was introduced by Yablonovitch in Reference 37 has been subsequently
developed into a thermodynamic model of luminescent concentrators, for example, [20–22, 29, 38]. These models were success­
fully used to describe luminescent concentrators based on luminescent quantum dots. At a microscopic level, further developments
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591
Maximum concentration
106
Maximum concentration
under illumination at
250 nm
550 nm
1000 nm
105
104
103
102
101
100
0
20
40
60
80
100
Stokes shift (nm)
Figure 2 Illustration of the maximum concentration from an entropic point of view. For short wavelengths, very high concentrations are theoretically
possible. For longer wavelengths, a large Stokes shift is necessary to avoid limitations.
were presented in Reference 39. At this point, I would like to present a phenomenological thermodynamic model, which brings
together the main ideas from different theoretical discussions and offers valuable insight into the working principles of luminescent
concentrators.
If one considers a conventional luminescent collector with dye molecules embedded in a transparent matrix, the incident light
field with the intensity Binc excites the ensemble of luminescent molecules in the collector out of equilibrium with the ambient
temperature T. Because of the fast thermal equilibration among the vibrational substates of the electronically excited state, the
electrons cool down very fast to the ambient temperature. But as the molecule remains nonetheless in an electronically excited
state, the electrons have a chemical potential µ > 0, just as in an illuminated semiconductor. The chemical potential is a measure
of how many luminescent molecules are excited. The emission of the ensemble of the luminescent molecules is described by the
generalized Planck’s law [37, 40]. The number of emitted photons per time, per area, per unit solid angle, and per frequency
interval Bp,v,emit(νemit,T,μ) is
Bp;ν;emit ðνemit ; T; μÞ ¼
2νemit 2 n2
αðνemit Þ
c2
1
hνemit − μ
exp
−1
kB T
½11
where α(νemit) is the absorption coefficient, which is equal to the emission coefficient following Kirchhoff’s law.
Part of the emitted light is lost due to the escape cone of total internal reflection, but most of the light is trapped and guided in
the collector to its edges. As a consequence, the molecules are illuminated by not only the incident field but also the emitted and
trapped light. The higher the combined intensity Bint is at a point of the collector, the higher is the chemical potential, and in turn
also the emission of light. The chemical potential is not constant throughout the collector. For instance, close to the front surface,
the chemical potential is higher because the luminescent molecules are excited from the full incident field. Further away from the
surface, part of the incident light has been absorbed and therefore intensity is lower (Figure 3).
This picture can explain why there is a maximum possible concentration. The larger the collector is, the more the photons from
the incident field are collected. Thus, the intensity of the trapped light field that travels toward the edges also increases. This increases
the chemical potential, and consequently the emission of light as well. The maximum concentration is reached when the chemical
potential has become so high that the emitted light lost in the escape cone equals the incident field. The limit obtained from this
consideration is stricter than the limit presented in eqn [10] [37].
The link of the maximum concentration with the Stokes shift and the problem of reabsorption can be understood considering a
simple model system that features an absorption region and an emission region (see Figure 4) [13]. The absorption coefficient αabs
in the absorption region is much higher than that in the emission region with an absorption coefficient αemit.
As said before, the absorption and emissions coefficients are equal as described by Kirchhoff’s law. In spite of αabs > αemit, the
emission in the emission region is much larger than that in the absorption region because of the energy dependency of the
generalized Planck’s law, which states that in this regime, the emission at lower energies is considerably more likely than that at
higher energies. Hence, the larger the Stokes shift, that is, the bigger the energy difference E2 − E1, the less frequent the emission in
the absorption range ‘relative’ to the emission in the emission range. With less light being emitted in the absorption range,
reabsorption becomes less likely. Because each reabsorption and reemission again causes escape cone losses, with less reabsorption,
the escape cone losses are reduced as well. Less escape cone losses mean that a higher internally guided field and a higher chemical
potential are possible until the emitted light lost in the escape cone equals the incident field. As a consequence, a higher maximum
concentration is possible.
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Binc
μ (Bint)
Bemit (μ)
Bint
Figure 3 Illustration of the main ideas of the thermodynamic model. Incident radiation with the intensity Binc excites the ensemble of luminescent
molecules in the luminescent collector. The fraction of excited molecules is described by the chemical potential µ of the molecule ensemble. The
luminescent molecules emit radiation with the intensity Bemit, which depends on the chemical potential. The trapped fraction of the emitted light and the
incident light combines to the internal intensity Bint. This internal intensity again determines the chemical potential. As the internal intensity is not constant
throughout the collector, the chemical potential varies as well.
Absorption coeff.
Reflection band
αabs
αemit
Emission
Energy
E1
E2
Energy
Figure 4 Idealized model of the absorption and emission characteristics of a luminescent concentrator [13]. In the absorption region, the absorption
coefficient αabs is high, while in the emission region, the coefficient αemit is much smaller. Following Kirchhoff’s law, the emission coefficient equals the
absorption coefficient. Nevertheless, emission in the emission region is much higher, because the generalized Planck’s law favors emissions at lower
energies. A band-stop reflection filter that reflects in the emission region can increase efficiency and the maximum possible concentration considerably.
Because absorption and emission are linked by Kirchhoff’s law, it is not possible to eliminate reabsorption entirely. Additionally,
without reabsorption, the excitation of the molecules would be completely independent of the emitted light. This would allow for
an infinite concentration, which is a clear contradiction of the second law of thermodynamics. However, it is possible to reduce the
escape cone losses and therefore to increase the maximum possible concentration with the addition of a band-stop filter. The
band-stop filter should reflect in the emission range but should transmit in the absorption range. The desirable reflection band is
sketched in Figure 4. Like this, only the small amount of light emitted in the absorption region can be subjected to escape cone
losses. Again, this means that a higher internally guided field and a higher chemical potential are possible until the emitted light lost
in the escape cone equals the incident field. In Reference 13 it was shown that the maximum efficiency of a luminescent
concentrator system with such a band-stop filter equals the Shockley–Queisser limit of a solar cell with a bandgap similar to that
of the cutoff wavelength E2 of the band-stop filter.
1.27.2.4
Ray Tracing Simulations of Luminescent Concentrators
For complex geometries with different materials, imperfect surfaces, scattering, etc., most thermodynamic models were not
sufficient. Therefore, several works have investigated luminescent concentrators using Monte Carlo methods and ray tracing. One
of the first works is the dissertation of Heidler [41]. Heidler modeled absorption of the dye, isotropic emission, total internal
reflection, and reabsorption. The model was even capable of simulating a diffuse-back reflector and stack configurations. Simulated
collection efficiencies exceeded experimental data by 15% on average due to the idealized conditions of the model. However, good
agreement between the simulated and measured edge emission spectrum was achieved. Recently, new attempts for simulating
luminescent concentrators have been made [13, 14, 42–45]. Kennedy et al. [43, 44] use a simple model describing absorption of the
dye, emission, total internal reflection, and reabsorption to calculate the relative Jsc of solar cells coupled to one edge of the collector
and to predict the emitted spectrum leaving at the bottom of the collector. The Jsc values were overestimated by about 10%, but
Luminescent Solar Concentrator
593
again the emitted spectra agreed well with predictions. Burgers et al. [45] used a quite similar model. He determined relevant
parameters, such as the dye concentration or the quantum efficiency, by fitting the model to measured data. He also included
mirrors at the collector edges. With his model, he achieved good agreement between external quantum efficiency (EQE) measure­
ments and the simulation-based predictions; a review was presented in Reference 8. In References 13 and 14, a highly idealized
model was presented, which for the first time included a photonic structure. This idealized model has then been further developed
by Prönneke et al. [46] to describe more realistic systems [46]. In References 26 and 47, a model was presented that was tested
against spectrally resolved experimental data, such as reflection and transmission spectra as well was the emission spectra from
different surfaces of the collector. The shape of the photoluminescence spectrum and the angular distribution of the emitted light
proved to be very important input parameters. With the model it was also possible to reproduce the angular distribution of the light
leaving the collector at the edges. In References 26 and 48, the effect of a dependence of the photoluminescence spectrum on the
excitation wavelength was investigated. In Reference 48, the impact of reabsorption was examined. It was found that reduced
reabsorption due to a larger Stokes shift can overcompensate a lower quantum efficiency of the absorption/emission process in
respect of the overall system efficiency.
1.27.3 Materials for Luminescent Solar Concentrators
1.27.3.1
Organic Dyes
Organic dyes were the dominant luminescent material in the first research campaign in the 1980s [5, 6, 31, 33, 49–52]. They were
applied both distributed in a transparent matrix material and as a thin layer on a transparent slab of material. The research in that
time resulted in luminescent dyes with high luminescent quantum efficiencies above 95% and good stability. Figure 5 shows a
photograph of a selection of luminescent concentrator materials produced during that time. Today, these luminescent dyes are
commercially available and therefore also used in recent works [16, 18, 19, 53, 54]. Until now, the highest reported efficiencies [19, 25]
were reached with systems based on organic dyes.
However, high quantum efficiency is achieved only in the visible range of the spectrum, while efficiency remains low in the IR. In
Reference 33, it is shown that these low quantum efficiencies have fundamental reasons that are difficult to overcome. The main
reason is that the energy difference between the excited electronic states moves closer to the energy of vibrational transitions within
the molecule, which facilitates nonradiative transitions. Nevertheless, also in recent works, organic dyes are being developed that
extend the efficiently used spectral range to longer wavelength. For example, in Reference 55, the synthesis of a dye based on a
perylene perinone is described, which extends the absorption wavelength range by more than 50 nm in comparison to the
perylene-based dye Lumogen Red 305, which is very frequently used in luminescent concentrators. This extended absorption
allows for the collection of potentially 25% more photons at a reasonable luminescent quantum yield and photostability.
Another problem of the organic dyes is the large overlap between absorption and emission spectra. This results in reabsorption
and reemission with the associated losses. Research has therefore been conducted to increase the Stokes shift of the organic dyes.
One option is to use energy transfer from one absorbing dye to another emitting dye [30, 56]. In Reference 57, hybrid dyes were
investigated, which were composed of organic antenna and inorganic emitting ions to achieve large Stokes Shifts. Another option is
to use phosphorescence instead of fluorescence [30]. Phosphorescence is associated with a larger Stokes shift, but also with lower
quantum efficiency. There have also been attempts to increase overall efficiency by bringing metal nanoparticles close to the dyes, in
order to increase absorption and luminescence due to plasmonic resonances [58, 59].
Figure 5 A selection of luminescent concentrator materials based on organic dyes that were produced during the first research campaign in the 1980s at
Fraunhofer ISE [5, 6, 33, 49, 51] that are still among the most efficient luminescent concentrator materials. The luminescent collectors consist of PMMA
doped with organic dyes produced from BASF. The used dyes are perylene derivates. The precise chemical structures, however, were not published by
BASF.
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1.27.3.2
Technology
Inorganic Luminescent Materials
Because of the instability of organic materials, especially under ultraviolet radiation, inorganic materials have been investigated as
well. Promising inorganic materials are glasses and glass ceramics doped with rare-earth ions like Nd3+ and Yb3+, or other metal ions
like Cr3+ [60–63]. The advantages to these approaches are high stability and a high refraction index of the glasses, which increases
the trapped fraction of light. One big disadvantage is the narrow absorption bands of the luminescent materials. Additionally, these
material systems turned out to be quite complex and costly to fabricate.
With the development of nanotechnology, luminescent nanocrystalline quantum dots (NQDs) have become of interest for
luminescent concentrators. The most frequently used materials are CdS, CdSe, and ZnS quantum dots [20, 21, 64–68]. One big
advantage of the NQDs is that absorption and emission properties can be tuned by the size and composition of the nanocrystals.
Additionally, the NQD features a broad absorption range. However, the achieved quantum efficiencies are lower than those of
organic dyes, especially if the NQDs are incorporated into polymer matrixes. In addition, the low Stokes shift presently prohibits
reaching high efficiencies; NQDs with larger Stokes shifts are under development (nanorods [68] and type II heteronanocrystals
[69]), thus potentially leading to high light collection efficiencies. There are also hybrid approaches; for example, in Reference 70,
Er-doped WO6 nanocrystals in composite telluride glasses are investigated.
1.27.4 Luminescent Solar Concentrator System Designs and Achieved Results
1.27.4.1
System Designs
Absorption
Resp. emission
Many system designs have been proposed for efficient and economic luminescent concentrator systems. Probably, the most
fundamental one was the concept to stack several collector plates [4]. With different dyes in each plate, different parts of the
spectrum can be utilized (see Figure 6). At each luminescent collector, a solar cell can be attached, which is optimized for the
spectrum emitted from the collector. With this spectrum splitting, high efficiencies can be achieved in principle. The stack design
with the matched solar cells at the edges provides a high degree of freedom for cell interconnection. Therefore, there is no forced
series connection like in tandem cell concepts, which causes current limitation problems. Additionally, no tunnel diodes are
necessary.
Figure 6 shows mirrors at some of the edges of the collector plate as well. If some edges are not covered with solar cells, but with
reflectors, the geometric concentration is increased. This can be beneficial for the costs of the luminescent concentrator system, as solar
cells are usually the most expensive component of the system. However, the reflection on mirrors is not free of losses. Therefore, it
should be kept to a minimum and the emitted light should reach the solar cells with as few reflections on mirrors as possible. For this
purpose, an isosceles and rectangular triangular shape of the luminescent collector is beneficial [4]. With solar cells at the hypotenuse
and the two other sides covered with mirrors, only two reflections are necessary at most until the emitted light hits a solar cell.
A reflector underneath the collector increases the collection efficiency as well. It reflects transmitted light back into the collector
and creates a second chance for absorption. When a white reflector instead of a mirror is used, light can also be scattered and
redirected toward the solar cells. For both reflectors underneath the collector and mirrors at the edges, it is beneficial to maintain an
air gap between collector and reflector. In this configuration, the reflection of the reflector comes on top of total internal reflection.
However, with an air gap, the diffuse reflector does not change the direction of light emitted into the escape cone to directions that
are subject to total internal reflection. The reason for this is that due to refraction, the light that leaves the collector is already
λ1
S1
C1
λ2 Wavelength λ
S2
Mirrors
C2
λ3
S3
λ
C3
(a)
λ
(b)
Figure 6 Concept of stacked luminescent concentrators, as presented in Reference 4. (a) The different collectors C1−C3 are connected with different
solar cells S1−S3. In each collector, a different dye is incorporated. The absorption and emission (shaded) spectra of the different dyes are shown in
(b). With a proper alignment of the absorption and emission properties, the recycling of photons lost from one collector to another collector is possible. It
is important that an air gap between the different collectors is maintained so that each spectral range of light is guided in one collector by total internal
reflection and does not get lost in adjacent collectors.
Luminescent Solar Concentrator
595
distributed over a complete hemisphere, even before it hits the diffuse reflector. This is not changed by diffuse reflection. So
consequently, when the light enters the collector again, it is refracted into exactly the angles of the escape cone.
Another idea to increase the geometric concentration was proposed in Reference 71. The angular range of the edge emission of
the luminescent concentrator is limited by the critical angle of total internal reflection. Therefore, a further concentration is possible
until the divergence reaches the full hemisphere. Compound parabolic concentrators, which are attached to the edges, are one
possibility for this purpose.
As mentioned before, no luminescent materials that are active in the IR while showing high quantum efficiency, high stability,
and broad absorption have been developed so far. Therefore, a range of designs were proposed to utilize the IR radiation. One
option could be to place a cheap solar cell underneath the collector that utilizes IR radiation [15]. The IR light transmitted through
the collector could be used as well by a thermal collector. It was also suggested to use an upconverter to convert the transmitted
radiation into light that could be collected by the luminescent collector [34]. The transmitted light can also be used to grow plants in
a greenhouse [51]. A complex geometry for building integrated PV was also presented by Chatten et al. in Reference 72, where a
luminescent concentrator is integrated into a blind system. Finally, the application of luminescent concentrators is also discussed in
the context of day lighting, for example, [73], where only the light collection properties are used and no solar cells are involved.
1.27.4.2
Achieved System Efficiencies
Many materials have been investigated for their potential use in luminescent concentrators. However, the number of luminescent
concentrator systems consisting of luminescent collectors with attached solar cells that were tested under standard testing conditions
used for solar cells remained relatively small. The highest efficiencies have been reached based on the combination of different
organic dyes. Already in the first research campaign in the 1980s, Wittwer et al. [51] achieved a conversion efficiency of 4% with a
system that combined two 3 mm-thick plates with different dyes in one stack with GaAs solar cells attached to the edges. The system
was 40 cm 40 cm in size and therefore quite large, so the achieved efficiency can be considered a very good result. The geometric
concentration ratio, that is, the ratio of the illuminated collector area to the solar-cell area, was 16.7. The system produced around 3
times more energy than that the solar cells would have produced if they had been placed directly in the sun.
The highest reported efficiency was achieved by Slooff et al. [19]. In this work, four GaAs were attached to a 5 cm 5 cm
luminescent collector with a thickness of 5 mm. The collector consisted of PMMA. The collector contained two dyes, 0.01 wt.%
Lumogen F Red 305 (Red305) from BASF (a perylene) and 0.003 wt.% Fluorescence Yellow CRS040 (CRS040) from Radiant Color
(a coumarine). At the bottom of the collector, a diffuse reflector was placed. With this configuration, a system efficiency of 7.1% was
achieved. The geometric concentration of this system was 2.5.
While both described systems used two different dyes, only one type of solar cell was used, therefore not fully exploiting the
possibilities of spectrum splitting described in the previous section. In References 26 and 74, a system was investigated that used as
well different types of solar cells, made of GaInP and GaAs. GaInP solar cells have a bandgap of 1.85 eV, which corresponds to a
wavelength of 670 nm. The typical open-circuit voltage (VOC) of a GaInP solar cell is in the region above 1300 mV. The bandgap of
GaAs is at 1.43 eV and the typical VOC is above 1000 mV. The dimensions of the used collector plates were 5 cm 5 cm 0.5 cm.
One plate contained a dye active in the spectral region of 400–550 nm. It was used to illuminate the GaInP solar cells. The second
collector plate contained a dye active between 550 and 650 nm. This one was used to illuminate the GaAs solar cells. To each of the
plates, two solar cells were optically coupled with silicone to adjoining edges. In front of the remaining two edges of each collector
plate, white reflectors made from polytetrafluoroethylene (PTFE) were placed. In this configuration, the geometric concentration
defined as the ratio of the luminescent collector area to the area of the used solar cells is 2.5 as well. At the bottom of the system, a
diffuse reflector was placed. Overall, a system efficiency of 6.9% was achieved. Figure 7 shows the EQE of the two subsystems.
50
EQE (%)
40
GalnP
GaAs
30
20
10
0
300
400
500
600
700
800
900
Wavelength (nm)
Figure 7 EQE measurements of the two subsystems, consisting of two parallel interconnected GaInP solar cells and two parallel interconnected GaAs
solar cells, respectively, coupled to two different luminescent collector plates. The two systems together cover a wide spectral range.
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In all the works [19, 26, 74], it was found that the bottom reflector contributed significantly to the overall system efficiency. This
can be seen as well in Figure 7, where an EQE of more than 5% is visible at wavelengths longer than the active region of the dyes,
which ends at about 650 nm. This effect is expected to decrease for bigger systems.
1.27.5 The Future Development of Luminescent Solar Concentrators
The main task for the future development of luminescent concentrators is to increase system efficiencies based on cheap materials in
reasonably sized systems. When this task is solved successfully, the integration into applications can be undertaken. To increase
system efficiencies, there are two main topics:
• Extending the used spectral range into the IR
• Controlling escape cone losses
1.27.5.1
Extending the Used Spectral Range into the IR
From the EQE measurement presented in Figure 7, it is obvious that in the active region of the dyes, the systems reach already
quite high quantum efficiencies of approximately 45%. As the used solar cells also deliver high voltages and show high fill factors,
the main reason for the low overall efficiency is that only the visible part of the spectrum is used. If one reached a 45% quantum
efficiency also in the range from 650 to 1050 nm, one could expect an extra current density of around 12 mA cm−2. When the
luminescent material emits in the region between 1050 and 1125 nm, the emitted light could be used by a silicon solar cell, which
reaches a maximum power point voltage of around 580 mV. The extra silicon solar-cell luminescent concentrator system then
would have an efficiency of nearly 7%, which would result in an overall system efficiency of close to 14%. Even with only silicon
solar cells attached, which would result in approximately half the efficiency for the higher energy photons because of the lower
voltage of the silicon solar cells, an overall system efficiency of 10% appears to be feasible. However, these are all highly
hypothetical cases, which require a lot of material development for near-infrared (NIR) luminescent materials. The concepts
presented in Section 1.27.3, especially the use of hybrid materials [57, 70] and the use of luminescent semiconductor quantum
dots, might be able to achieve this task. However, the already achieved progress in these fields still remains to be demonstrated at
a system level.
1.27.5.2
Controlling Escape Cone Losses
Besides the losses due to an incomplete utilization of the full solar spectrum, the escape cone of total internal reflection is the most
important loss mechanism. The loss of around 26% does occur not only once but also after every reabsorption and reemission, as
already introduced in Section 1.27.2.1. To reduce the escape cone losses, one can identify two different strategies: first, reabsorption
can be reduced to reduce the number of emission events. The different approaches in the development of luminescent materials
with low reabsorption were already discussed in Section 1.27.3. The second approach is to control the path of the light in the
luminescent collector. In this field, arguably, the biggest progress has been made in the last years.
1.27.5.2.1
Photonic structures for increased efficiencies
The Stokes shift between absorption and emission opens the opportunity to reduce escape cone losses significantly: a selective
reflector, which transmits all the light in the absorption range of the luminescent material and reflects the emitted light, would trap
nearly all the emitted light inside the collector [75]. The concept is illustrated in Figure 8.
In Reference 12, hot mirrors were proposed to serve as selective reflectors and, in Reference 13, photonic structures. A possible
realization of such a selective reflector is a so-called rugate filter. It features a continuously varying refractive index profile that results
in a single reflection peak. However, some unwanted side lobes remain. Optimized rugate filters [76] show only one single
reflection peak for a certain wavelength and almost no other reflections. In Reference 25, it was shown that overall system
efficiencies of luminescent concentrator systems can be increased with the help of such an optimized filter. The investigated system
used a 5 cm 10 cm, 5-mm-thick luminescent collector of PMMA doped with an organic dye. One GaInP solar cell was coupled to
one short edge with silicone. The solar cell had an active area of 5 mm 49 mm. Hence, the ratio of illuminated luminescent
concentrator area and solar-cell area constitutes a geometric concentration ratio of 20 . The solar cell had an efficiency of 16.7%
under AM1.5G illumination. White PTFE served as bottom reflector and also as reflector at the edges, which were not covered by
solar cells. The used photonic structure was produced by the company mso-jena by ion-assisted deposition (IAD) and was tuned for
high reflection in the emission range of the organic dye (see Figure 9). Without this structure, the system had an efficiency of
2.6 0.1% in reference to the 50 cm2 area of the system. The structure increased the efficiency to 3.1 0.1%, which constitutes an
efficiency increase of around 20% relative. Figure 10 shows the result from a light beam-induced current (LBIC) scan of the system,
illustrating how the light collection efficiency is increased over most of the luminescent collector area. With the achieved efficiency
of 3.1% and the concentration ratio of 20, the realized luminescent concentrator produces about 3.7 times more energy than the
GaInP solar cell had produced on its own.
Luminescent Solar Concentrator
Radiation
E1
597
Air gaps
Photonic structure
No loss cone
E2
Solar
cell
Solar
cell
Reabsorption and
emission
Dye
White-bottom reflector
Figure 8 A selective reflector, realized as a photonic structure, reduces the escape cone losses. The photonic structure acts as a band-stop reflection
filter. It allows light in the absorption range of the dyes to enter the collectors but reflects light in the emission range.
100
Absorption BA241
PL emission BA241
Refletion filter
(%)
80
60
40
20
0
350
400
450
500
550
600
Wavelength (nm)
650
700
750
Collection efficiency (a.u)
Figure 9 Reflection spectrum of the used photonic structure and the absorption and photoluminescence of the luminescent concentrator the filter was
designed for. The reflection of the structure very nicely fits the emission peak of the dye in the concentrator.
600
With
Without
Photonic bandstop filter
500
400
300
0
2
4
6
8
Distance from solar cell (cm)
10
Figure 10 Averaged linescans in x-direction from an LBIC scan with and without photonic structure. Close to the solar cell, the efficiency is lower with
the photonic structure, because it reduces the effectiveness of the bottom reflector for small distances. Over most of the luminescent concentrator,
however, collection efficiency is significantly higher with a photonic structure, resulting in a relative efficiency increase of 20%.
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E1
t≈λ
E2
Figure 11 Conceptual sketch of a ‘Nano-Fluko’. A very thin layer of luminescent material with thickness t in the range of wavelength λ of the emitted light
is placed between two photonic structures, for example, Bragg stacks. The photonic structures transmit light in the absorption range of the luminescent
material with an energy E1. They are reflective in the emission region (E2) of the luminescent material. Because the layer with the luminescent material is so
thin, the photonic structures suppress the emission into unfavorable directions.
The observed efficiency increase of 20% can be already considered as a great success since it shows that photonic structures
reduce the escape cone losses significantly. However, the used filter is a multilayer system and therefore costly to produce.
Three-dimensional (3D) photonic structures are a potential alternative to the presented multilayer systems. A special
three-dimensional photonic structure is the opal. The opal has the advantage that it can be produced by a dip-coating process
utilizing self-organization of monodisperse PMMA beads [77]. This is a potentially low-cost process that could be applied on
large-area concentrators. However, the achieved quality of opaline films is still too low to achieve optical properties that allow for an
increase in efficiency.
Another, probably more promising options are chiral nematic (cholesteric) liquid crystals that act as spectrally selective
mirrors. In Reference 78, such chiral nematic liquid crystals were applied onto luminescent collectors, with a small air gap in
between. It was observed that the highest output is achieved using a scattering background and cholesteric mirror, with a
reflection band significantly redshifted (similar to 150 nm) from the emission peak of the luminescent dye. The use of an air
gap results in light bending away from the waveguide surface normal and, consequently, a redshift of the cholesteric mirrors
is required. Also, the importance of considering the angular dependence of the spectrally selective mirrors was analyzed in
Reference 79. Overall, up to 35% more emitted light exits the luminescent collector edge after application of the cholesteric
mirror.
However, even with a photonic structure, light emitted into the escape cone is more frequently subject to loss events. Because
it is emitted into a steep angle with respect to the front surface, it has a very long effective path until it reaches a solar cell
and therefore suffers more from path length-dependent losses. Hence, it would be very beneficial to suppress emission into these
unfavorable directions completely. This should as well be possible with the help of photonic structures. Already the very
first works on photonic crystals of Bykov [80] and Yablonovitch [81] dealt with influencing emission with photonic structures.
Many papers discussed the possibilities subsequently [82–87]. For influencing the emission of the dye successfully, it is necessary
that the photonic structures are very close to the emission process or that the luminescent material is incorporated into the
photonic structures. For the luminescent concentrator systems, this means that one has to go from the macroscopic design of the
presented systems to a system design in the nanoscale. Possible realizations of such a system denoted ‘Nano-Fluko’ were
suggested in Reference 88. One possible realization consists of a very thin layer of luminescent material between two photonic
structures, for example, rugate filters or Bragg stacks (Figure 11). In such a configuration, the emission of the light would be
restricted to a plane parallel to the photonic structure. Galli et al. [89, 90] showed that the emission of Er 3+ can be strongly
enhanced if it is incorporated in a photonic crystal waveguide and that efficient waveguiding occurs. Therefore, there is first
experimental evidence that such a system can work, and it is an interesting approach to apply this concept to luminescent
concentrators.
However, several layers with the same dye will be needed to achieve sufficient absorption. As the guided light is constraint to very
thin layers, high intensities will occur in these layers. Because of thermodynamic limit for the achievable concentration depending
on the Stokes shift of the used dye, one question is which system sizes can be achieved using this approach until the thermodynamic
limit reduces efficiency.
1.27.5.2.2
Controlling the angular emission
An alternative to selective reflectors is to modify the emission characteristic of the dyes in such a way that emission occurs
predominantly in favorable directions. This can be achieved with an orientation of dye molecules that show a distinct angular
characteristic in their emissions depending on their position. The dye molecules can be aligned accordingly with the help of liquid
crystals [17, 91–93]. The dye alignment has to take place such that the optical transition dipole of the luminescent material (the dye
molecule) is oriented along the luminescent collector surface normal, directing the maximum possible proportion of luminescence
into waveguide modes. In Reference 91, it is reported that up to 30% more light is emitted from the edge of a luminescent collector
due to the dye alignment.
Luminescent Solar Concentrator
599
1.27.6 Conclusion
Luminescent solar concentrators have the fascinating ability to concentrate both direct and diffuse radiation. This ability is directly
related to the Stokes shift that occurs between absorption of incoming light and the subsequent emission. Up to now, system
efficiencies of around 7% have been achieved. The efficiency potential that could be achieved based on commercially attractive
materials such as silicon solar cells is in the range of 10%. To achieve this goal, further progress in the development of luminescent
materials that cover the visible and the near-IR range of the solar spectrum, showing high luminescent quantum efficiencies and low
reabsorption, is necessary. Furthermore, current progress in the research on photonic structures needs to be exploited for its
application in luminescent concentrator systems.
With continuously falling prices for the production of solar cells, however, the cost advantages, which can be achieved by using
cheap concentrators such as luminescent collectors, decrease. Hence, it is unlikely that luminescent concentrators will play an
important role for rooftop or power-plant applications. Nevertheless, the unique optical appearance of luminescent concentrators,
the possibilities to achieve transparency and to combine different colors, the ‘invisibility’ of solar cells integrated into the frames of
modules, etc. offer unique design possibilities and might open the way for luminescent concentrators into building integrated
photovoltaics and also into the power supply of small appliances and portable consumer electronics. Therefore, they could help to
spread photovoltaics into fields that are not accessible by conventional solar-cell module technology.
Acknowledgments
Some of the presented work was supported by the German Research Foundation within the Nanosun (PAK88) project. I would like
to thank all colleagues at Fraunhofer ISE that are or have been involved in the research on luminescent solar concentrators,
especially Marius Peters, Armin Zastrow, Volker Wittwer, and Adolf Goetzberger.
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