Impact of the Edges of a Backside Diffusive Reflector on the External Quantum Efficiency of Luminescent Solar Concentrators: Experimental and Computational Approach

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Luminescent Solar Concentrators (LSCs) have been proposed in the 1970s as cheap planar concentrators for residential applications and nowadays represent a novel idea with excellent perspectives for building integration photovoltaics. The interest in LSCs has increased in the last years, due to improved stability of luminescent dyes, the introduction of quantum dots and nanorods and the overall reported increase in module efficiency. Computational methods have been suddenly applied as an important tool for the description of light dynamics in LSCs. With “raytracing methods” light is described as particle-like (photons) and each particle is tracked. It is precious tool for the description of absorption/reemission events, refraction and internal reflection in LSCs. It is also a very useful approach for the description of LSC edge effects, which may be well described by means of basic geometrical optics and are the subject of this work. The impact of scattering layers on the backside of LSCs is analysed in detail both experimentally and computationally. Results give evidence of the non-wavelength dependent impact of backside diffusers to the external quantum efficiency of LSCs and thus to their overall performance. A possible design of LSC as smart windows in photovoltaic facades is also suggested, where the benefits of the edge effects described are taken into account.

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_American Journal of Engineering and Applied Sciences_

Original Research Paper

# Impact of the Edges of a Backside Diffusive Reflector on the External Quantum Efficiency of Luminescent Solar Concentrators: Experimental and Computational Approach

1Mauro Pravettoni, 1Cristina S. Polo Lòpez and 2Robert P. Kenny

_1Institute of Applied Sustainability to the Built Environment,_
_University of Applied Sciences and Arts of Southern Switzerland (SUPSI), Canobbio, Switzerland_
_2Institute for Energy and Transport, DG Joint Research Centre, European Commission, Ispra, Italy_

_Article history_ Abstract: Luminescent Solar Concentrators (LSCs) have been
Received: 13-01-2016 proposed in the 1970s as cheap planar concentrators for residential
Revised: 15-01-2016 applications and nowadays represent a novel idea with excellent
Accepted: 15-01-2016

perspectives for building integration photovoltaics. The interest in
LSCs has increased in the last years, due to improved stability of

Corresponding Author:

luminescent dyes, the introduction of quantum dots and nanorods and

Mauro Pravettoni
Institute of Applied Sustainability the overall reported increase in module efficiency. Computational
to the Built Environment, methods have been suddenly applied as an important tool for the
University of Applied Sciences description of light dynamics in LSCs. With “raytracing methods”
and Arts of Southern Switzerland light is described as particle-like (photons) and each particle is
(SUPSI), Canobbio, Switzerland tracked. It is precious tool for the
Email: mauro.pravettoni@supsi.ch

perspectives for building integration photovoltaics. The interest in
LSCs has increased in the last years, due to improved stability of

Corresponding Author:

luminescent dyes, the introduction of quantum dots and nanorods and

LSCs. It is also a very useful approach for the description of LSC
edge effects, which may be well described by means of basic
geometrical optics and are the subject of this work. The impact of
scattering layers on the backside of LSCs is analysed in detail both
experimentally and computationally. Results give evidence of the
non-wavelength dependent impact of backside diffusers to the
external quantum efficiency of LSCs and thus to their overall
performance. A possible design of LSC as smart windows in
photovoltaic facades is also suggested, where the benefits of the edge
effects described are taken into account.

Keywords: Photovoltaics, Luminescent Solar Concentrators, Monte
Carlo Modelling

Introduction cell area and the receiver area. In principle, the

Luminescent Solar Concentrators (LSCs) are static
planar photovoltaic (PV) concentrators and have been
proposed by Weber and Lambe (1976) and by
Goetzberger and Gruebel (1977). In LSCs a refractive
material (typically glass or PMMA matrix) is filled with
fluorescent particles, either dyes or quantum dots
(Barnham _et al_ ., 2000), that absorb photons at certain
wavelengths and re-emit them at higher wavelengths.
A fraction of re-emitted light (here simply referred to
as _luminescent light_ ) is transmitted to the PV cell at
edge of the concentrator by total internal reflection,
with a gain factor _G_ given by the ratio between the PV

cell area and the receiver area. In principle, the
spectral range of the luminescent light can be
controlled by selecting appropriate luminescent
species and the efficiency may be enhanced by
reflectors at the rear surface.

The interest in LSCs has increased in the last
years, due to improved stability of luminescent dyes
(Rowan _et al_ ., 2008), the introduction of quantum dots
and nanorods (Bose _et al_ ., 2008) and the overall
reported increase in module efficiency (Slooff _et al_ .,
2006; Currie _et al_ ., 2008); Goldschmidt _et al_ ., 2009).
Debije and Verbunt (2011), Farrell and Yoshida (2012)
and previously Van Sark _et al_ . (2008) described the most
recent results in LCS development and characterization.

Creative Commons Attribution (CC-BY) 3.0 license.

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

_LSC as a Concentrating Photovoltaic Building_
_Integration Resource_

Conventional concentrating photovoltaic (CPV)
systems require one or two axis solar tracking to collect
the direct light. Under highly concentrated light, the PV
cell reaches also high temperatures and active cooling is
required to avoid performance degradation. Both these
aspects, in addition to a bulkier module compared to
traditional nonconcentrating flat panel PV modules, are
particularly challenging in view of the integration of
CPV into buildings and as a consequence CPV has found
applications only in large solar farms to date.

Today, residential construction is called to make a
major contribution to reducing energy waste,
environmental sustainability and comfort. New standards
are required by the industry to develop more efficient
buildings. Among these, the European Directive 31 of
2010 that will impose for 2020 to all buildings
constructed in Europe to self-produce through all the
renewable energy needed to cover consumption towards
net zero energy buildings. To achieve these goals in the
near future, it will be extremely helpful to use not only
the roofs to integrate PV systems but also facades.

Solar transmission through window glazing affects
not only the air-conditioning load into a building, but
also the thermal and visual comfort. Glazing with
advanced thermal and optical properties for energy
conservation and aesthetic purposes, such as low
emissivity glass and switchable glazing, are already
commercially available. The most common in buildingintegrated PV are the so-called “see-through” and “lightthrough” solar systems. Silicon solar cells and thin film
solar cells are used today for semitransparent glasses for
windows. Semi transparency in traditional building
integrated PV can create a pleasant environment
allowing natural light to enter the building.

Heat dissipation is another crucial problem. The
houses are often built with materials which do not
allow the heat containment and therefore require a
great deal of energy to maintain a correct internal
environmental condition.

Lighting and thermal environment comfort within
these areas using PV glass and its incidence in the
global energy building consumption is still under
investigation (Polo Lopez _et al_ ., 2012). It is not
known yet what effect transparent solar building
integrated PV systems may cause to the sensory and
psychological perception of people.

LSC panels represent a novel idea to get
reasonable efficiencies with no-tracking CPV, with
excellent prospects for cost reduction. In this study
investigation in the LSC design is presented to help
increasing performance, while maintaining good
architectural integration perspectives as smart

windows. LSCs collect diffuse light more efficiently
than any other PV technology (Pravettoni _et al_ .,
2009a) and the module efficiency can be increased
with backside diffusive reflectors.

The LCS systems are also typically transparent to the
infrared radiation, thus avoiding the PV cell overheating.
By contrast LCS elements are coloured according to the
luminescent dye in use: this aspect makes LSCs
especially attractive from an aesthetic point of view but
should be studied to determine the impact that various
colours have on the human mind.

_Computational_ _Methods_ _and_ _Pre-Normative_
_Electrical Characterization of LSCs_

Computational methods have been applied as an
important tool for the description of light dynamics in
LSCs since the 1980s (Carrascosa _et al_ ., 1983). There are
basically two different approaches:

The thermodynamic approach (Yablonovitch, 1980;
Chatten _et al_ ., 2005), based on the detailed balance
principle and describing the radiative energy transfer
between mesh points in the concentrator plate

The “ray-tracing method”, in which light is
described as particle-like (photons) and each particle is
tracked. Ray-tracing method is usually a Monte Carlo
code, where a random number generator is used to set
the photons in their configurational phase-space. The
ray-tracing approach is important for the description of
absorption/re-emission events, refraction and internal
reflection in LSCs. It is also a very useful tool for the
description of LSC edge effects (Sidrach de Cardona _et_
_al_ ., 1985; Pravettoni _et al_ ., 2008).

LSCs represent a challenge for the optical and
electrical characterization, as has been early discussed by
Batchelder _et al_ . (1979; 1981) and Soti _et al_ . (1995).
The impact of scattering layers has been analysed by
Debije _et al_ . (2009). In this study the authors show the
impact on the External Quantum Efficiency (EQE) of a
diffusive reflector (here simply referred to as diffusers)
on the rear side of the LSC. The existing standard for SR
measurements of flatpanel PV is considered as a guide to
the characterization. Several configurations for the
background diffuser are considered. Measurements are
performed by means of dedicated experimental setups in
two laboratories: The European Solar Test Installation
(ESTI), a European centre of reference for standard
measurements in PV where the world record efficient
LSC to date was measured (Green _et al_ ., 2016); and the
Experimental Solid State (EXSS) group laboratory for
quantum efficiency measurements of the Blackett
Laboratory at the Imperial College London (ICL), UK.

Experimental results are compared with the
computational ones from Monte Carlo ray tracing
simulations: As a result, the importance of light
diffusion at the edges of the LSC is highlighted. This

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

result gives new strength towards the development of
LSCs as promising building integration PV elements
for the years to come.

Experimental Setup

The procedure for Spectral Response (SR)
measurements of flat-panel PV devices (cells or
modules) is described by the international standard
IEC 60904-8: the _SR_ (in A W −1 ) is defined as the
following ratio:

_SR_ (λ) = δ _J_ _sc_ (1)
_E_ (λ) _d_ λ

where, δ _Jsc_ (in A m −2 ) is the photogenerated shortcircuit current density from the test device, _E_ ( _λ_ ) is the
incident spectral irradiance (in W m −2 nm −1 ) and
_E_ ( _λ_ ) _dλ_ (in W m −2 ) is the total incident irradiance
between λ − dλ/2 and λ + dλ/2.

_E_ (λ) _d_ λ is detected by a reference cell of known _SR_ ;
_δJsc_ is measured from the test device.

If _Ne_ is the total number of electron-hole pairs
produced and flowing into the external circuit and _N_ λ is
the total number of incident photons at wavelength λ
then the _EQE_ can be defined and calculated as follows:

_EQE_ (λ) = _Ne_ = _hc_ _SR_ (λ) (2)
_N_ λ _q_ λ

where, _h_ = 6.626×10 −34 J s is Planck’s constant, _c_ =
2.998×10 8 m s −1 is the speed of light and _q_ = 1.602×10 −19
C is the elementary charge. EQE is often used instead of
_SR_ in the literature, meaning the probability that an
incident photon at wavelength λ will deliver an electron

hole pair to the external circuit which is collected at the
contacts.

_Setup at the European Solar Test Installation_

The experimental setup in use at ESTI has been
presented already in the literature (Pravettoni _et al_ .,
2009a). A 300 W steady-state xenon lamp provides the
source light that is filtered by up to 80 bandpass filters (8
to 20 nm band width; wavelength range: 300 to 1650
nm). The monochromatic light obtained is chopped,
superimposed to a continuous bias light background and
measured by a digital lock-in amplifier (1 mHz to 102.4
kHz frequency range, 2 nV to 1 V sensitivity). Bias light
is used in order to keep the test device under irradiance
conditions close to the standard operating conditions.

Monochromatic light ±5% uniformity up to 30×30
cm 2 target area is provided by a lens system and allows
simultaneous measurement of a small-size test device
and of a c-Si ESTI reference cell of known absolute SR.
The test device and the reference cell are usually kept at
(25±2) °C temperature by a cooling plate: PV cell in
LSCs cannot be cooled by direct contact with the cooling
plate but has been monitored with a Pt100 sensor at the
backside of the PV cell, showing no significant
temperature increase during the measurements.

The short-circuit current value _δJsc_ in Equation 1 is
measured over a calibrated shunt resistor. The resistance
value is set in order to maintain the test device close to
short-circuit. Measurement uncertainties associated with
the expetimental setup described has been discussed in
detail by Mullejans _et al_ . (2009).

_Setup at Blackett Laboratory_

Figure 1 shows a picture of the experimental setup in
use at the EXSS group at ICL for SR measurements of
PV devices.

Fig. 1. SR measurement setup at the QE laboratory (EXSS group, Blackett Laboratory, ICL)

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

Here the monochromatic light is produced by a
monochromator, chopped and collimated onto the test
device. In order to drive the pointlike monochromatic
beam to the test LSC, a 90° beam reflector is used. The
test LSC can be moved on a mechanical XY stage with
micrometric control.

No bias light is used: Lock-in technique reduces the
signal-to-noise ratio. A calibrated c-Si reference cell is
used to detect the monochromatic beam intensity as
above. No simultaneous measurement of the test device
and of the reference cell is possible: Temporal stability
of the monochromatic source is periodically checked.
Since no bias light is used, which is the main responsible
of the test device heating, temperature control is almost
unnecessary and is not performed at ICL.

With respect of ESTI’s setup described above, light
can here be directed with high precision to several points
of the receiver and the impact of the backside diffusive
reflector can be analysed in detail.

Results

Table 1 lists two LSC modules that have been
tested and characterized. Both devices have been
assembled by ECN, the independent research institute
for renewable energy of the Netherlands, in
cooperation with the Fraunhofer Institute of Applied
Polymer Research (Fraunhofer-IAP), a German
research centre pioneering studies in polymer
materials. One (LM701) or four (LM704) 5×50 mm 2
PV cells are placed on the edges of 5×5×0.5 cm 3
Poly(Methyl Methacrylate) (PMMA) matrices. Mirror
tapes by 3M (97% reflectivity) are placed on the free
edges of LM701. Cells are connected with index
matched PE 399 KrystalFlex film.

been tested. In all the measurements reported, the
backside diffuser is a sheet of white paper of different
covering area and shapes, placed over a black cloth
used as absorber. The diffuser is not glued to the LSC
backside: A thin air gap is therefore assumed between
the LSC and the diffuser. Figure 2a-d show the four
configurations tested on LM701: The backside
diffuser covers the rear side of the LSC only partially,
near the edge where the GaAs cell is placed.

LM704 has been tested with the various configurations
shown in Fig. 2e-h. Similarly to the LM701 case, different
diffuser areas have been employed: In this case the
diffuser is a frame of different thickness.

Results of EQE measurements on LM701 are shown
in Fig. 3a. The chart highlights the two absorption peaks
at 440 nm (coumarin) and at 578 nm (perylene).

Luminescent emission cuts off at 630 nm: The
shoulder of non-zero EQE between 600 and 900 nm
arises from the unabsorbed light, diffused at the matrixto-air interfaces into the GaAs cell, which has its
bandgap cutoff at 900 nm.

The larger overall EQE has been measured with the
full diffuser covering the entire backside surface, while
the smaller EQE when no diffuser is used, as expected.

assembled by ECN, the independent research institute

Similar results are shown in Fig. 3c (LM704).

for renewable energy of the Netherlands, in

The different response to the presence of different

cooperation with the Fraunhofer Institute of Applied

diffusers on LM701 and LM704 is highlighted in Fig.

Polymer Research (Fraunhofer-IAP), a German

3b and Fig. 3d, where the normalized EQE is plotted

research centre pioneering studies in polymer
materials. One (LM701) or four (LM704) 5×50 mm 2 as a function of the fraction of the diffuser area
PV cells are placed on the edges of 5×5×0.5 cm 3 coverage, at four different wavelengths: At 450 and

550 nm (near the peaks of luminescent absorptions),

Poly(Methyl Methacrylate) (PMMA) matrices. Mirror

at 483 nm (between the peaks) and at 850 nm (where

tapes by 3M (97% reflectivity) are placed on the free

noluminescence occurs). As a result, the EQE at

edges of LM701. Cells are connected with index
matched PE 399 KrystalFlex film. wavelengths where luminescence occurs is shown to

Fluorescent dyes in use are indicated and specified increase linearly with the fraction of area coverage. At
in Table 2. Lumogen F Red305 is a perylene colour by 850 nm, the EQE increase is sharp and, especially in
BASF; yellow CRS040 is a coumarin by Radiant LM704, nearly constant when any diffuser is present,
Color. In LM704 PV cells may be connected either in while significantly decreases when no diffuser is used
series or in parallel. (65% EQE increase at 850 nm on LM704 between the

At the rear side of the collector, diffusive “no diffuser” and the “full area diffuser” case; less
reflectors of different materials, size and shape have than 5% only increase at peak of irradiances).

Table 1. Test devices (Slooff _et al_ ., 2006)
ESTI label Dim (cm 3 ) Matrix Dye Cell type No. cell s Gain _G_
LM701 5×5×0.5 PMMA Yellow CRS 040 (0.003%) GaAs 1 10.0
Lumogen F Red305 (0.01%)
LM704 5×5×0.5 PMMA Yellow CRS 040 (0.003%) GaA s 4 2.5
Lumogen F Red305 (0.01%)

Table 2. Luminescent dyes in use: Nominal values given in the cited references
Peak (nm)
--------------------------------------------Fluorescent dye Chemical Producer Absorption Emission
Lumogen F Red 305 Perylene BASF 578 613
Yellow CRS 040* Coumarin Radiant Color 440 506
*Later on renamed Yellow CFS 002

Fluorescent dyes in use are indicated and specified
in Table 2. Lumogen F Red305 is a perylene colour by
BASF; yellow CRS040 is a coumarin by Radiant
Color. In LM704 PV cells may be connected either in
series or in parallel.

At the rear side of the collector, diffusive
reflectors of different materials, size and shape have

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

(a) (b)

(e) (f)

(f) (h)

Fig. 2. Test devices with various backside configurations: (a-d) LM701 and (e-h) LM704

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

(a) (b)

Fig. 3. EQE results at ESTI. (a) LM701 as a function of wavelength; (b) LM701 a function of reflector area. (c) LM704 as a function

of wavelength; (d) LM704 as a function of reflector area

Fig. 4. EQE results at Blackett Laboratory: LM701 as a function of wavelength

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

(a) (b)

Fig. 5. EQE results at Blackett Laboratory: (a) As a function of the position, at various wavelengths; (b) Percentage distribution of

EQE over LM701 receiving area

Figure 4 shows results of EQE measurements of
LM701 on the experimental setup at ICL described
above. EQE measurements have been performed in 25
points, equally distributed on a 5×5 square matrix. A
white backside diffuser has been placed on the rear
side of the LSC. Figure 5a shows the measured
average EQE over the five measurements on a single
row at position _x_ (mirror: _x_ = 0 cm; GaAs cell: _x_ = 5
cm) and as a function of _x_ .

Figure 5b shows on a grayscale colormap, the impact
of each of the 25 test positions to the overall integrated
EQE (white is where local integrated EQE is maximum,
black where it is minimum). All charts show that
monochromatic light between _x_ = 1 and _x_ = 2 cm and in
the middle of the LSC gives less contribution to the
global EQE, while a bigger contribution is given in the
LSC region closest to the PV cell.

_The Effect of the Aperture Area_

In Equation 1 it is important to define the appropriate
aperture area of the LSC module in the calculation of
δ _Jsc_ . Several different area values can be considered for
the LSCs listed in Table 1, whether or not the thin glue
index matched layer connecting the PMMA matrix to the
PV cells and the cells themselves are taken into account
(Fig. 6a). Accurate area measurements have been
performed at ESTI and are reported in the caption of Fig. 6.

In conventional flat panel modules, the area
definition usually affects only the module efficiency
calculation, since the area between cells is typically not
active and does not contribute to the output power.
However it may play an important role in certain
devices, where it is proven that light diffuses from the
inactive area between cells to the cells themselves. This
is definitely the case of LSCs, where the glue layer
diffuse light to the PV cell and the cell itself might

intercept direct light other than the one emitted by
luminescent dyes.

As a result, when the device is characterized it is
important to define the aperture area, eventually through
the application of a mask. Figure 6b shows three different
current-voltage characteristic curves at 1000 W m −2 and
the calculated efficiencies on LM704, with no backside
diffuser and with the three possible area definitions
specified in the caption of Fig. 6a: (a) PMMA matrix only,
giving 4.9% module efficiency; (b) including the thin glue
layer, 5.1%; and (c) including cell thickness, 5.2%.

_Computational Model_

To better focus on the results in section above, the
impact of a backside diffusive reflector was investigated
with a monochromatic, non-wavelength dependent
Monte Carlo ray-tracing code based on classical optics.

_Description_

In the ray-tracing approach, luminescent light is
modelled by means of a set of _N_ monochromatic,
unpolarized, non-interacting particles in a rectangular
box with the dimensions of the testing LSCs. At start,
particles randomly move in the matrix; when reaching
the matrixto- air interfaces, particles undergo the laws of
classical optics: Total internal reflection or refraction.
Fresnel reflection is also taken into account: Particles in
the escape cone can be reflected according to their
reflectance which is calculated from the reflection
coefficients in the unpolarized approximation. Secondary
absorption is neglected.

Figure 7a shows the choice of coordinate systems: The
origin in set in the middle of an edge (typically where a
PV cell is located); the _z_ -axis is along the edge main axis;
_y_ -axis has the same direction and versus of incoming light
and the x-axis forms a right-handed coordinate system.

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

(a) (b)

Fig. 6. LM704: (a) aperture area measurements performed at ESTI, highlighting the differences between the matrix area (2501.8±0.7

mm 2, back dotted line), the matrix area plus the glue layer (2536.3±0.7 mm 2, red dashed line) and the entire device area,
including cell thickness; (b) comparison of module efficiencies calculated with the three different area definitions

(a) (b)

Fig. 7. Monte Carlo ray-trace modelling. (a) The coordinate system of reference; (b) Four possible events: Total internal reflection

(1), refraction or Fresnel reflection (2), light diffused and lost (3), light diffused and collected (4)

Fig. 8. EQE as a function of the diffuser area (Monte Carlo raytracing method): percentage of photons collected as a function of the

diffuser area

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

(a) (b)

Fig. 9. Possible LSC design as smart windows: (a) The single LSC element (b) an array of LSCs (Pravettoni _et al_ ., 2009b)

The backside diffuser has been modelled by
randomly diffusing back into the LSC matrix each
particle exiting the _y_ >0 interface if −∆ _x_ ≤ _x_ ≤ 0. The size
∆ _x_ of the diffuser can be varied between 0 and the size
of the tested LSC.

_Simulation Results_

Figure 7b shows the four possible events modelled:
(1) Light is total internal reflected; (2) light is refracted
or Fresnel reflected; (3) light is reflected back into the
matrix’s escape cone and then is refracted (or Fresnel
reflected) back from _y_ < 0 (secondary absorption is
neglected); (4) light reflected back by the diffuser
intercepts the index matched PV cell at the _x_ = 0
interface and is collected.

The code counts the fraction of particles collected at
_x_ = 0 and those lost form the _y_ - 0 and _y_ < 0 interfaces.

Figure 8 shows the total number of collected particles
in normalized units as a function of the ratio of area
covered by the diffuser.

According to the basic ray-tracing approach, less than
3% more particles are collected by the diffuser and this
contribution is almost totally due to the presence of the
diffuser at the edges where PV cells are placed. This
result gives qualitative confirmation of the results shown
in the previous sections.

Discussion

Both experimental and computational results
presented in the previous sections help to investigate on
the impact of a backside diffusive reflector to the
performance of a LSC of the given size.

The measured EQE of LSCs with different
configurations of PV cells and diffusers showed that
the impact of the latter can be twofold. As to light at

wavelengths where luminescent species absorb, a
diffusive reflector is shown to increase the absorption
probability, by increasing the optical path of this
fraction of light: Essentially, light that has not been
absorbed while passing once through the LSC, has an
additional chance after diffusion. As a matter of fact, at
those wavelengths EQE seems to increase almost
linearly with the diffuser area.

On the other hand, light at longer wavelengths than the
luminescent cutoff is much more largely affected by the
presence of a diffuser. The experimental results show also
that the EQE at those wavelengths is more than doubled
even when the diffuser is placed only close to the PV cell
and not necessarily covering the entire backside.

The latter experimental result has been confirmed by
the ray-tracing code. Based on the simplest classical
optics approach, the diffuser may recycle the lost
photons back to the interface: From that basic point of
view, almost only the fraction of photons lost close to
the edge where the PV cell is placed can be diffused into
the cell and collected. All other recycled photons are lost
from the opposite side of the LSC: Only particles
diffused far from the PV cell edge at very wide angles
may reach the cell. In the reality, such particles are most
probably scattered and lost before reaching the cell.

Even if results shown in these paper may depend on
the geometry of the LSC, the positive impact of a
backside diffuser at the edges of the LSC is a result of
general validity, being based on the optical properties
described. What has been shown then suggests that, in
view of the application of LSCs as smart windows, a full
backside diffuser may be replaced by a thin diffusive
frame all along the edges where PV cells are located. As
a result the functionality of the window will not be
affected but the LSC performance will benefit
(Pravettoni _et al_ ., 2009b).

Mauro Pravettoni _et al_ . / American Journal of Engineering and Applied Sciences 2016, 9 (1): 53.63
DOI: 10.3844/ajeassp.2016.53.63

In all the possible reflections for an appropriate
integration of LSCs in buildings, other important factors
need to be considered in the future, such perfect thermal
insulation, the quality and quantity of the natural light
income and the possible psychological effects due to
color, for example in the working environments. The
mere of integration of backside diffusers in the window
profile makes the solution presented in this document
extremely versatile and particularly interesting in
building projects, but should take into account the fixing
system, a perfect indoor ventilation with natural air
exchange to the benefit of building’s energy efficiency,
the indoor welfare and air quality. A possible
optimization of this element in the window profile is
shown in Fig. 9: The fixing element ensures both
mechanical stability and thermal and wet insulation but it
also avoids shadowing the active area of the PV cell.

Conclusion

The EQE of LSCs has been analysed with three
different approaches: (a) With the experimental setup in
use for concentional PV cells and minimodules at ESTI;
(b) with the modified experimental setup at ICL,
allowing point-like EQE measurements and (c) with a
basic Monte Carlo ray-tracing method. Results
highlighted the twofold impact of a backside diffuser to
the performance of LSCs: It both increases the
absorption probability of light at wavelengths where
luminescent species absorb and recycles light at
wavelengths where the luminescent species is
transparent. The latter effect is shown to be particularly
important at the edges of LSCs, as the ray-tracing
approach confirms. Adding a thin diffusive frame along
the edges where PV cells are placed may therefore result
in an improvement of the LSC performance, while not
affecting its functionality as a smart window.

Acknowledgement

The authors would like to gratefully acknowledge
Lenneke H. Slooff (ECN, The Netherlands) and Andreas
Buchtemann (Fraunhofer-IAP, Germany) for their kind
cooperation in providing LSC samples to be tested,
Daniel J. Farrell, Amanda J. Chatten, Keith W.J.
Barnham (Imperial College London, UK) and Roberto
Galleano (ESTI, European Commission) for their
valuable help and comments.

Author’s Contributions

Mauro Pravettoni: Conducted the entire research
and develped the experimental tools and computational
activity.

Cristina S. Polo Lòpez: Conducted the review on
LSC applications on building integration.

Robert P. Kenny: Supervised the research programe
and gave important feedback on the research result.

Ethics

This article is original and contains unpublished
material. The corresponding author confirms that all of
the other authors have read and approved the manuscript
and no ethical issues involved.

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