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Chenchen Yang and Richard R. Lunt*
surfaces into energy harvesting resources.
This new approach to visibly transparent
LSCs was developed by exploiting the
nature of excitonic semiconductors to optimize the use of this light-exposed surface
area without compromising its existing
functionality, thereby avoiding the aesthetic tradeoffs that can hinder adoption.
Recent work has demonstrated devices
that selectively harvest different parts of
the solar spectrum including i) ultraviolet
(UV) absorption, with massive downconverted luminescence deeper into the NIR
(>400 nm)[4] and ii) near-infrared (NIR)
absorption, with even deeper luminescence in the NIR.[3] This work has been
subsequently followed by a number of
other groups.[8–12] In contrast to transparent photovoltaics, which have also
been developed to exploit this untapped
area,[13–15] this new concentrator approach
offers an alternative pathway whereby energy is transported
optically, offering a simple route to large-area manufacturing,
high defect tolerance, angle independent performance, and
low-level energy cost. In this progress report we discuss the
theoretical limits of transparent LSCs, idealized configurations,
scalability, and properties of enabling materials.
Visibly transparent solar harvesting surfaces are an exciting new approach to
harvesting solar energy around buildings and mobile electronics to improve
their efficiency and autonomy without impacting their appearance. The
recent demonstration of ultraviolet (UV) and near-infrared (NIR) selective
light harvesters have enabled transparent luminescent solar concentrators
(LSCs) that boast unparalleled scalability, flexibility, aesthetic quality, and
affordability. Consequently, the question of the efficiency limits has emerged
in these new systems. In this perspective, the theoretical efficiency limits of
these concentrator systems are reviewed and practical considerations are
outlined to approach these limits. For UV and UV/NIR selective harvesting
single-junction transparent LSCs are constrained thermodynamically to 21%,
which increases to over 35% in fully transparent multipanel devices with light
trapping. In deriving these limits, key material and engineering challenges
are identified to fully optimize transparent solar concentrators that can push
them towards commercial reality.
1. Introduction
The primary aim for the development of the luminescent
solar concentrator (LSC) has been to provide a route to lower
solar module costs.[1] While record LSC efficiencies of 7.1%
have been demonstrated,[2] efforts to translate this technology
to the building envelope have been limited due to the visual
impact and performance, hindering the ability to take advantage of materials, installation, framing, and maintenance of the
existing building envelope. Color-tinted windows are acceptable
in certain applications, for example in the MUSAC museum
in Spain, and can add aesthetic quality in colorful construction
exteriors. However, the majority of windows and displays in
daily use require high transmission and low tinting to enable
wider adoption. Recent demonstrations of clear transparent
LSCs have further reinvigorated research into these systems.[3–7]
By converting windows, architectural glazing, and displays
into LSC waveguides, it is possible to transform these passive
C. Yang, Prof. R. R. Lunt
Department of Chemical Engineering and
Material Science Michigan State University
East Lansing MI 48824, USA
E-mail: [email protected]
Prof. R. R. Lunt
Department of Physics and Astronomy
Michigan State University
East Lansing MI 48824, USA
DOI: 10.1002/adom.201600851
Adv. Optical Mater. 2017, 1600851
2. Operating Principles of LSCs
Luminescent solar concentrators operate based on the following mechanisms (see Figure 1a): a portion of solar spectrum is absorbed in a luminescent dye (or “luminophore”)[16]
embedded in a transparent waveguide. That absorbed solar
energy is then re-emitted at another wavelength isotropically in
all directions within the waveguide, provided the oscillators are
not preferentially oriented. Due to the index of refraction difference between the waveguide and the ambient environment the
re-emitted photons are predominately trapped by total internal
reflection, causing them to be directed towards the waveguide
edges where they can be converted to electrical power in an
edge mounted photovoltaic cell. The overall system power conversion efficiency, ηLSC, is then given by:
*
*
ηLSC = ηOpt ⋅ηPV
= (1 − R )⋅ηAbs ⋅ηPL ⋅ηTrap ⋅ηRA ⋅ηPV
(1)
where ηOpt is the optical efficiency (number of photons reaching
the waveguide edge)/(number of photons incident on the waveguide), R is the front face reflection, ηAbs is the solar spectrum
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Limits of Visibly Transparent Luminescent Solar
Concentrators
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Richard R. Lunt is the
Johansen Crosby Endowed
Associate Professor at
Michigan State University in
the Departments of Chemical
Engineering & Materials
Science and Physics. He
earned his B.S. from the
University of Delaware and
his Ph.D. from Princeton
University. He then worked
as a post-doctoral researcher
at MIT. Currently, his group focuses on understanding and
exploiting excitonic photophysics and molecular crystal
growth to develop unique thin-film optoelectronic devices.
suppressing reabsorption.[1,17] The light trapping efficiency
can be approximated for simple waveguides with isotropic
2
emitters as η Trap = 1 − 1/nSub
and R = (nSub − 1)2/(nSub + 1)2
excited at normal incidence (the only case considered here),
where nSub is the index of refraction of the waveguiding substrate (see Figure 2b). The absorption efficiency is defined as
ηAbs =
EGLum
∫
∞
AM1.5G( λ ) ⋅ A( λ ) dλ /
300 nm
Figure 1. a) Schematic showing a transparent luminescent solar concentrator (LSC) that selectively harvests ultraviolet (UV) and near-infrared
(NIR) light while passing visible light. b) Typical absorption and emission
spectra of luminophores for transparent LSCs highlight two key parameters: the Stokes or downconversion shift, S, and the width of the luminescent peak emission, W.
absorption efficiency of the luminophore, ηPL is the luminescence efficiency of the luminophore, ηTrap is the photon trapping (or waveguiding) efficiency, and ηRA is the efficiency of
∫
AM 1.5G( λ ) dλ , where A(λ)
300 nm
is the absorption spectrum of the luminophore, E GLum is the
bandgap of the luminophore (in nm), and AM1.5G is the air
mass 1.5 global solar photon flux spectrum. The efficiency of
the edge mounted photovoltaic cell, ηPV (reported for illumination under AM1.5G) must be normalized by its solar spectrum
absorption efficiency and quantum efficiency at the luminophore wavelength to account for monochromatic conversion as:
 ηPV (AM1.5G) 
*
ηPV
=  PV
⋅
 ηAbs (AM1.5G) 
∫η
(λ ) PL (λ ) d λ
∫ PL (λ ) d λ
EQE
(2)
where ηEQE is the external quantum efficiency of the PV as a
function of wavelength, PL (λ) is the luminophore photoluminescence emission spectrum as a function of
PV
wavelength, and ηAbs
( AM 1.5G) is the absorption efficiency of the PV material (rather
than the luminophore). The thermodynamic
*
limiting monochromatic ηPV
is shown in
Figure 2a where this scaled efficiency only
accounts for the open circuit voltage (VOC)
and fill factor (FF) PV losses as a function
of bandgap. Because of the light intensity
*
dependence of ηPV
, this correction will typically be dependent on the geometrical gain
of the collector but is not considered here.
Thermodynamic limiting PV efficiencies
for edge mounted cells are derived based on
Figure 2. a) Theoretical (black line) and best-performance solar cell efficiency (data points) as a
the Shockley–Queisser (SQ) detailed balance
function of bandgap under AM1.5G (ηPV) and normalized by the AM1.5G absorption efficiency limit.[18–20]
(ηPV/ηAbs(AM1.5G), gold line). This efficiency represents the limits of the PV efficiency as a
Accounting for multiple reabsorption
function of only FF and VOC losses. b) Reflection (red, dashed line) and trapping efficiency (grey,
and
emission events, the efficiency of supdashed line) as a function of the index of refraction of the substrate (nSub) for simple optical
waveguiding. The maximum of the product of the reflection losses and trapping efficiency pressing reabsorption are then defined
as:[21]
(blacked line) is at nSub = 2.0 for simple optical waveguiding of isotropic emitters.
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1 − ηRAP
1 − ηRAPηPLηTrap
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ηRA =
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(3)
where the reabsorption probability, ηRAP, is integrated over all
emission angles and the absorptive path length is corrected for
each take-off angle in a rectilinear system as:[21]
ηRAP =
∞
π /2
π /4
0
θcrit
− π /4
∫ d λ ∫ dθ ∫



Lt
sin (θ ) PL ( λ ) 1 − exp − ε ( λ ) C
 d φ
2t0 sin(θ ) cos(φ ) 


∞
π /2
π /4
0
θcrit
− π /4
∫ d λ ∫ dθ ∫
(4)
sin (θ ) PL ( λ ) d φ
The critical angle (emission cone) is θcrit = sin− 1(1/nSub), ε
is the molar absorptivity, C is the luminophore concentration,
L is the plate length, θ is azimuth relative to the normal of the
LSC plane, t is the thickness of the film with the luminophore
coated on the front surface of the waveguide, t0 is the waveguide thickness, and φ is the in-plane rotation angle. In the
case of the luminophore being embedded directly in the matrix
(without an additional film), t = t0 (as drawn in Figure 1a). The
reabsorption losses then depend on plate length, PL efficiency,
waveguiding efficiency, absorption coefficient, and degree of
spectral overlap.
Finally, the maximum concentration ratio (incident photon
energy)/(photon energy redirected through a single aperture) has
been derived previously for luminescent solar concentrators
with a detailed balance assuming typical incident illumination
around 1 sun as:[20,22]
G=
E − E2 
E 23
exp  1

 kT 
E 13
(5)
where E1 is the energy at the edge of the absorption, E2 is the
energy of the emission onset. G describes the maximum concentration achievable considering the entropy balance between
absorption-emission, and the energy distribution for a boson
gas (i.e., photons) – that is, there is an entropy cost to concentrating diffuse light. This concentration is fundamentally
limited by the constraint that the concentrated light cannot
effectively exceed the source temperature (it is not possible
to radiate more energy than the black body source at equilibrium).[22] In Equation (5), (E1 – E2) is proportional to the Stokes
shift, S, defined in Figure 1b (so that E1 − E2 = S · hc/(λ1λ2) or
sometimes defined as (v1 – v2) and a system with 4-sided apertures would have an additional factor of 4 in the denominator
that acts to reduce the level of light concentration. For reference, the maximum concentration for diffuse concentrators
without spectral shifting is G = (n2/n1)2.
3. Ideal Transparent Luminescent Solar
Concentrator, Case I
Considering the processes described by Equation (1), it is clear
that any power producing LSC is fundamentally limited by the
PV efficiency of the edge-mounted photovoltaic cells. Thus, this
discussion necessarily begins with the Shockley–Queisser (SQ)
single-junction PV limit.[18] In Figure 1b, typical emission and
absorption shapes and characteristics of a luminophores are
Adv. Optical Mater. 2017, 1600851
Figure 3. a) Schematic of idealized absorption and emission characteristics (W → 0, S → 0) for selectively harvesting luminophores for theoretically-limited transparent LSCs with single waveguide cells. Configurations
for both UV-only and UV+NIR harvesting are drawn. b) Corresponding
efficiency limits for the profiles in (a) as a function of the luminophore
bandgap and degree of visible transparency (AVT). Note that the limit for
AVT = 0 is just that of the Shockley–Queisser theoretical single-junction
PV limit.
provided. Two key parameters are highlighted: the Stokes shift
(or downshift), S, defined as the difference between the absorption and emission peaks; and the emission full width at half the
maximum, W.[23]
We begin by analyzing the limiting cases, where the emission
and absorption characteristics of ideal luminophores are shown
schematically in Figure 3a. Using this as a starting point, the
limits of transparent luminescent solar concentrators (TLSCs)
are derived as a function of key parameters outlined below. The
AM1.5G solar spectrum is integrated as a function of band gap
outside the visible spectrum. As described in previous work, the
color rendering index (CRI) and average visible transparency
(AVT) can be used to determine the visible spectrum range to
avoid; the range of 435–675 nm provides CRI > 95 and AVT >
99.5%.[14] As a result, we utilize this definition (435–675 nm)
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as the visible range in this work. Although the human eye can
sense the light from 390 to 435 nm and from 675 to 720 nm if
bright enough, these ranges do not contribute significantly to
our overall perception of color.[14]
The ideal transparent solar concentrator then requires the
following conditions: 1) the PL efficiency of the luminophore
is unity; 2) the PV mounted around the edge has an efficiency
and quantum efficiency equal to the Shockley–Queisser limit;
3) there are no reflection losses into the luminescent absorber
or at the waveguide edge; 4) there are no waveguiding losses
(perfect light trapping); 5) there is no overlap between the emission and absorption (no reabsorption losses – this also plays an
important role in the light concentration and scaling); 5) there
is no diverging intensity dependence from the ideal limits of
the edge mounted PV; 6) the absorption edges are sharp cutoffs around the visible spectrum; and 7) the emission width
(W) is perfectly narrow. Given these restrictions, the theoretical limits of transparent luminescent solar concentrators are
then described Equation (1), which reduces to ηOpt = ηAbs and
*
ηLSC = ηAbs (E GLum ) ⋅ ηPV
(E GLum ) = ηPV (E GLum ) where all the incident
photons absorbed by the luminophore are subsequently converted to the same number of photons absorbed by the edgemounted PV cell. In the absence of any concentrating effects,
the single-junction thermodynamic efficiency limits are shown
in Figure 3b. Although these ideal requirements represent a
significant challenge, it is conceivable that all of the criteria outlined above can be satisfied (or nearly so) to meet this limit.
For example, all reflections could be eliminated with the use
of antireflection coatings; perfect waveguiding achieved with
tailored and selectively reflecting photonic mirrors; luminophores with near unity ηPL and emission widths <5 nm have
been demonstrated; and PV cells are now closely approaching
the SQ limit.
In this idealized case there are multiple ways to configure
selective UV harvesting for transparent devices. Provided the
emission is perfectly narrow and non-overlapping with the
absorption, the UV-only TLSC efficiency is plotted in Figure 4,
yielding a maximum efficiency of 6.9% with an ideal cutoff at
435 nm. We reiterate that the formation of color in these systems stems from both absorptive filtering and any luminescence in the visible range. It is therefore necessary to design
emitters with either emission with S = 0 and W = 0 (e.g.,
PL(λ) = δ(λ − λEm)), or large downshifted emission past the
visible spectra into the near-infrared. The latter case is advantageous for both reducing the formation of colorful glow and
eliminating reabsorption losses over large areas. This larger
downshift, while beneficial for scaling, also results in drop
of the UV-only efficiency limit to 3.7% due to lower voltage
from the edge mounted PV cells. However, because the downshift required in this scenario is so large, it is possible to
consider the application of multiexciton generation (MEG)
mechanisms[24–30] or singlet fission (SF) to overcome some of
this thermal loss (See Figure 4a). In singlet fission, one high
energy singlet excited state can decay to form two triplet excitons and is well known to be nearly 100% efficient in many
molecule systems.[24,31–35] This process is thermodynamically
endothermic if the triplet energy is less than or equal to the
singlet energy, providing the minimum emission wavelength
(870 nm), and corresponding PV bandgap, requirement. In this
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Figure 4. a) Schematic and b) corresponding efficiency limits as function of luminophore bandgap for two alternative UV-only harvesting
transparent LSCs designs. The first configuration harvests UV light and
downconverts this energy into the NIR to prevent coloring from escape
cone luminescence. The second configuration uses a multiexciton (MEG)
UV harvesting luminophore with emission at half of the energy of the
UV cutoff to enable emission of two photons from one high energy UV
photon. Note that these two configurations place varying restrictions on
the maximum VOC and FF of the edge-mounted PV. b) The case of UV
harvesting and UV emission with no spectral overlapping is also included.
case the UV-only efficiency limits (with emission in the NIR
and SF) increases from 3.7% to 5.6% as shown in Figure 4b.
Thus, there is a surprising amount of potential in harvesting
the UV-only portion of the solar spectrum, particularly if the
full photon energy can be fully exploited.
For the case of combined UV/NIR harvesting, the entire spectrum of incident UV and NIR photons would ideally be selectively absorbed. Then the same number of UV/NIR photo­ns is
re-emitted via photoluminescence resulting in the generation
of equal number of electrons. Hence, the relationship between
absorption (A) and transmission (T) of TLSC A(λ) + T(λ) =
100% for UV/NIR wavelengths becomes EQE(λ) + T(λ) =
100%. Since the emission edge borders the absorption edge with
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4. Transparent Luminescent Solar
Concentrators with Finite Width and
Stokes Shift Emitters, Case II
Figure 5. a) Schematic of idealized absorption and emission characteristics (W → 0, S → 0)
for selectively harvesting luminophores for theoretically-limited transparent LSCs with multiple
(multijunction) waveguide cells. b) Corresponding efficiency limits for the profiles in (a) as a
function of the waveguide cells and degree of visible transparency (AVT) for series-matched
currents.
ideally no overlap between emission and absorption the ideal
bandgap for the edge mounted PV can be selected resulting in
no voltage loss in this edge mounted PV. As a result, the thermodyanic efficiency limits for TLSC are essentially identical to
theoretical limits for transparent photovoltaics. With increasing
transparency from 0% to 100% in visible range, the maximum
efficiency of TLSC is modestly reduced from 33.1% (SQ limit
of opaque photovoltaics) to 20.6% (thermodynamic efficiency of
fully transparent LSCs). TLSCs are then just another version of
a transparent solar cell which shift the solar energy conversion
optically to waveguide edges.
To completely reduce the energy losses associated with
thermal relaxation, ideal multipanel (or multijunction) LSC
devices are also considered.[23,36] It is important to appreciate
that the narrow band emission of the luminophore requires
edge mounting of single junction photovoltaics (as opposed to
ultra-high efficiency multijunctions) as it will be particularly
difficult to current or voltage match all of the PV subcells with
essentially only monochromatic light. However, it is still possible to form multpanel LSC devices where each subpanel is
separated by an air gap or photonic mirror, and harvests light
from a particular section of the solar spectrum, with a PV
bandgap matched to the emitter in each subpanel. We analyze
this configuration in Figure 5a, again in the absence of waveguiding losses and assuming all subcells are photocurrent
matched. The efficiency then increases to 36.6% for fully transparent multipanel devices (See Figure 5b).
In all of these idealized cases, we further consider the light
concentration limitation imposed by Equation (5) based on the
Heaviside and delta function approximations of the absorption and emission profiles, respectively. This leads to E1 = E2
and results in a maximum concentration of G = 1. This means
that for the idealized profile in Figure 3a, no additional light
concentration above the incoming solar flux intensity would
be possible, thus limiting the size and geometry constraints
of such an ideal system. However, this entropy cost is not so
severe and small Stokes shifts of even 45 meV (in Equation (5))
can lead to G > 5 (a value typical of the best demonstrated
LSCs[17] with only a slight reduction (<5% at the peak) to the
limiting efficiency curves in Figure 3b. For reference, a Stokes
shift of 100 meV leads to G > 35 across the spectral range
above 0.9 eV, much greater than has been demonstrated in
Adv. Optical Mater. 2017, 1600851
We next turn our attention to the practical
considerations of the performance of TLSCs.
This discussion becomes dependent on a
greater number of parameters and thus follows a discussion for a number of cases as
the idealized parameters are relaxed.[23]
This discussion is extended to less ideal systems where emitters have a fixed finite emission width (W) with a varied Stokes
shift (S), and vice versa. This case is one of the first key areas of
loss as it confines the solar spectral range that can be harvested
for a given PV bandgap and dictates the voltage loss required in
selecting a particular PV cell bandgap. For example, given a particular molecule emitter with a defined absorption, the smaller
the W, the higher voltage PV that can be selected. For reference,
a range of emission widths have been demonstrated for organic
molecules (30–100 nm),[21,37–40] nanoclusters (50–300 nm),[4]
J-aggregates (15–40 nm),[41,42] nanocrystals (15–30 nm),[43] rareearth ions (<5–20 nm),[44,45] and single diameter carbon nanotubes (8–15 nm),[46] suggesting that such losses can, in fact,
be minimized. Nonetheless, for the most efficient emitters,
this value of W is typically around 20–100 nm. As shown in
Figure 6a, finite values of W and S require a smaller bandgap at
the edge-mounted PV: E GPV ≤ [E GLum − (S + W )] . Consequently, the
output PV voltage (open-circuit voltage) will become lower. As
W + S increase from 0 to 160 nm, the corresponding efficiency
limit will decrease from 20.6% to 16.8% due to this “voltage
loss” (see Figure 6b). In practical TLSC design, since there will
be at least some overlap between absorption and emission,
large S with small W is preferable than large W with small S in
scaling up, which will be discussed in detail below.
5. Practical Transparent Luminescent Solar
Concentrators, Case III
For the practical efficiency limit, we then apply more realistic
limitations to Equation (1). For example, we use the practical
reflection and waveguiding optimization (see Figure 2b) where
the optimum of the product of the reflection and waveguiding
efficiencies of simple waveguides leads to (1 − R) · ηtrap = 0.77
for a substrate with nsub = 2.0. It is reiterated that this can,
in theory, be improved to 1.0 with higher complexity optical
designs that also add considerable cost, e.g. with antireflection coatings[47] and distributed Bragg reflectors (DBRs, also
known as 1D photonic mirrors) with tunable stop bands.[48,49]
Given that a number of photovoltaic systems are now closely
approaching the SQ single junction limit (e.g. Si and GaAs, see
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actual LSC systems due to other parasitic
reabsorption losses. Thus this entropy driven
limitation to scalability quickly becomes negligible when further considering the more
practical scenarios below.
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Figure 6. a) Schematic and b) corresponding efficiency limits as function of luminophore bandgap, idealized Stokes shift (S), and idealized emission
width (W). Note that as W or S increases, the corresponding PV bandgap and voltage must decrease, thus reducing the efficiency limit. In this idealized configuration S and W are essentially equivalent. In practice, there is a distinct advantage to small W and large S in reducing reabsorption losses
and increasing scalability since there is typically at least some spectral overlap. c) Corresponding efficiency limits as function of luminophore bandgap,
idealized Stokes shift (S), and idealized emission width (W) with practical light trapping considerations.
Figure 2a),[50] and the current trajectory of improvement will be
limited over the next few decades we can use the current state
of the art in lab-scale performance as a practical module limit
for the foreseeable future. We then fix the quantum efficiency
of the PV cell in Equation (2) to ηEQE = 95% since this is among
the highest average quantum efficiencies reported for a labscale single junction cell (i.e., GaAs). Similarly, we can assume
a factor of 0.9 reduction in the monochromatic PV efficiency
considering the slight divergence of FF and VOC from the SQ
limit. Combining all the PV losses leads to an overall reduction
of 85.5% of the SQ limit (33.1%) of the PV cell, which is close
to the record single junction cell reported for GaAs. Finally, we
include finite widths to the emission and Stokes shift as variables in Figure 6c. For W = 80 nm and S = 80 nm (S + W =
160 nm), this results in a practical efficiency limit ≈11% for an
emitter bandgap of 1.12 eV, The practical efficiency limits track
surprisingly close to that for transparent photovoltaics which
was estimated to be 11%, also with a bandgap of 1.1 eV.[14]
6. Practical Consideration for Scaling of
Transparent Luminescent Solar Concentrators
In reality, reabsorption losses are one of the most significant
sources of efficiency and scaling reduction. To explore this effect,
we integrate Equations (3) and (4) numerically as a function of
length, luminophore quantum yield, and Stokes shift to evaluate
practical transparent LSC system efficiencies.[39,51,52] Light
emitted by the luminophore in the waveguide must traverse the
length of the waveguide before being reabsorbed by the luminophore or waveguide to reach the solar cell and produce power.
These losses are critically dependent on the quantum yield of
the luminophore, the waveguiding efficiency (ηTrap), the overlap
(or Stokes shift, S) of the luminophore emission-absorption, and
the overall waveguide dimensions (L). Waveguide roughness and
optical transparency also play an important role as waveguides
are scaled to around square meters areas, where both can act
as reabsorption or scattering losses. Absorption coefficients of
<0.01 cm–1 are ideal for eliminating waveguide absorption in the
NIR and are in this range for many polymer based waveguides
(e.g., polymethmethracalate, polycarbonate, etc.).
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Even for luminophores with 100% quantum yield, reabsorption losses can become dominant for luminophores with
small Stokes shift in large waveguides with only modest waveguiding efficiency since each absorption/emission event leads
to a reduction of photon flux through cone emission from the
front of the waveguide that effectively act as scattering events
(see Figure 7e), unless the waveguiding efficiency is 100%.
These cone losses for each reabsorption/emission event for
perfect emitters (luminophore with 100% quantum yield)
still follows (ηTrap)N, where N is the number of reabsorption/
scattering events and this behavior is already captured in
Equation (4). For example, for an ideal TLSC with ηPL = 1 and
nSub = 1.5 (ηTrap = 0.745), over 90% of the photons would be
lost via the escape cones after just 8 scattering (or re-emission)
events. If the quantum efficiency of the luminophore is less
than 100%, the photon flux reduction will be more significant
after each scattering event. Shown in Figure 7a–c is the scaling
for a practical case accounting for reabsorption losses given
the spectral characteristics shown. For reference, the case of
both ηTrap = ηPL = 1 is also plotted, leading to ηOpt = (1–R) at
all plate lengths. We note that the shape of the roll-off at large
plate lengths stems entirely from ηRA so that the limit of the
size of the LSC depends only on ηRA. Considering the data in
Figure 7d, a reasonable target for the practical performance and
scalability would be to target larger Stokes shifts in the range
100 nm < S < 160 nm combined with emission widths that are
as narrow as possible. Figure 7a also highlights that the Stokes
shift is just as important as ηPL, if not more so, particularly for
the largest area scaling. This can be seen in any of the three
subplots, where for a substrate index of 1.5 and 1 m2 plate size
the larger Stokes of 150nm combined with the lower ηPL of 75%
results in 25% higher optical efficiency than for system with a
Stokes shift of 75 nm combined with a ηPL of 100%. Below, we
review key representative emitter materials applied in luminescent solar concentrators (See Figure 8a–e).
The excitonic nature of organic dyes stems from their
π-conjugated molecular structure, enabling tunable and selective harvesting in various parts of the solar spectrum that is
key for selective NIR harvesting. The most common organic
dyes for LSC include rhodamines,[53–59] coumarins,[59–62]
lumogens,[63–65] cyanine salts[3] and perylenebismides
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Figure 7. Practical optical efficiency (ηOpt) as a function of PL efficiency (ηPL), Stokes shift (S), and LSC length for substrate index (nSub) of a) 1.5,
b) 2.0, c) 4.0 given d) the the more realistic spectral overlap profiles and e) device schematic emphasizing reabsorption events. Note that the varying
degrees of spectral overlap are highlighted in d) with the grey shaded area associated with each S. The situation of perfect light trapping is also shown
for reference (e.g. using wavelength selective mirrors), the optical efficiency is still reduced by (1–R) and which shows perfect scaling only when the
PL efficiency is unity.
derivatives,[66–69] etc. (see Table 1). While the Stokes shifts of
organic dyes are usually small (<100 nm) recent efforts have
resulted in S > 100 nm for both fluorescent and phosphorescent
emitters. Another challenge is that the quantum yields of emitters in NIR range are typically lower (<50%).[3] Nonetheless,
selective NIR harvesting TLSCs have been demonstrated based
Figure 8. Representative absorption and emission spectra (normalized) of typical emitters applied in LSCs and TLSCs. a) cyanine salt.[3] b) CdSe/CdS
quantum dot.[11] c) (TBA)2Mo6Cl14 nanoclusters.[4] d) Eu(TTA)3(TTPO)2 rare-earth ion complex.[76] e) single-walled carbon nanotube.[92] f) Emission
linewidth comparison of different emitter species.[3,4,11,46,76,92] Note that small W (<5 nm), and a range of S (5–500 nm) have been reported.
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Table 1. Representative summary of the absorption λmax, emission λmax and quantum yield of typical luminophores and dye emitters used in luminescent solar concentrators (LSC) and transparent LSCs.
Luminophore
Absorption
λmax (nm)
Emission
λmax (nm)
Quantum Yield
(%)
Visibly Colored
Reference
Rhodamine 6G
528
558
95
Yes
[53]
Lumogen F Red 305
578
613
98
Yes
[94]
Perylene Derivative
577
674
70
Yes
[77]
Cyanine Salt
742
772
20–30
No
[3]
CdSe/CdS QDs
<500
650
86
Yes
[11]
Mn2+ :ZnSe/ZnS NCs
400
600
37
Yes*
[9]
CuInSexS2–x/ZnS QDs
500
960
40
Yes
[12]
CdSe/CdxPb1–xS QDs
460
625
40
Yes
[75]
(TBA)2Mo6Cl14 NCs
325
750
75
No
[4]
Eu(TTA)3(TPPO)2
577
674
70
Yes*
[76]
*Molecules that are visibly colored due to emission peaked in the visible spectrum.
on variations of these cyanine salts with system efficiency
of 0.4% efficiency combined with an AVT of 86% and little
tinting.[3] Looking forward, these NIR-selective TLSCs show
potential for efficiencies up to 10% as described in the efficiency
limits above. Thus, while there are a number of challenges in
designing these materials to selectively harvest invisible photons
with high quantum yield and Stokes shift, organic chemistry
affords massive design space to solve many, if not all, of these
challenges.
Another class of emitters are quantum dots and nanocrystals.[70–72] By controlling the reaction condition, the variation of
their particle sizes, structures and compositions results in the
tunability of both absorption and emission spectra. However,
the limited oscillator strength of these materials near their
bandgap typically prevents their use as NIR selective harvester,
making them more suitable for selective UV harvesting or semitransparent (colored or opaque) applications. Currently, several
strategies have been developed to increase the Stokes shift.
One promising approach is the use of core/shell structured
nanocrystals, forming quasi-type II heterojunction where the
shell has larger energy bandgap and acts as a light absorbing
antenna that transfers the energy to the core crystal as the
light emitter.[11,51,73–75] Impurity-doping and alloying are both
alternative approaches to tackle the reabsorption problem. For
example, in Mn-doped ZnSe/ZnS core/shell NCs, small amount
(0.1–1%) of Mn2+ introduces new localized excited energy states
within the bandgap of ZnSe resulting in exhibit enhanced
downshifted luminescence.[9] Ternary or quaternary compound
semiconductors are also introduced in core/shell QDs, such as
CdSe/CdxPb1–xS [73] and CuInSexSe2–x/ZnS QDs.[12]
Among quantom dot emitters, a new class of emitter materials has been reported recently based on 0D nanoclusters and
nanocluster nanocrystals for these applications. Such nanoclusters
include hexanuclear metal halide salts A2B6X14 (e.g. A = K, B = Mo,
W, and X = Cl, I, Br). The (TBA)2Mo6Cl14 nanocluster, for example,
was shown to selectively absorb the UV portion of the solar spectrum (300–430 nm) and emit in the NIR (centered at 750 nm) with
a massive phosphorescent downshift of around 400 nm that was
one of the first examples to eliminate reabsorption losses.[4] Since
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both the absorption and emission are outside the visible range,
these materials provide the highest level of aesthetic quality with
a reported average visible transmittance over 85% (compared to
typical glass with 92%) combined with a device efficiency of 0.44%
that was predicted to exceed 1% at larger scales.
Rare-earth ions and ion-complexes are also an important
class of potential emitters for LSCs and TLSCs.[76,77] Historically, these have been used in fluorescent lighting and
downconverting phosphors.[44,45,78–81] In LSC applications,
organic emitters such as europium tris(2-thenoyl trifluoro
acetonate)-di(triphenylphosphine oxide (Eu(TTA)3(TPPO)2)
have been demonstrated, where light is absorbed by the central
organic ligand, energy is transferred to Eu3+ and then emitted
as photons by the Eu3+ ion.[76] There have also been a number
of demonstrations of rare-earth ions (e.g., Ce3+, Eu2+, Eu3+,
Sm2+, and Tm2+)[82–85] directly embedded into a variety inorganic hosts. While the UV absorption combined with large
luminescent downshifts (>200 nm) and sharp emission peaks
(<20 nm) makes Eu(TTA)3(TPPO)2 and other ions potentially
suitable for the UV LSC, the emission spectrum shown to date
peak in the visible range that results in luminescence based
tinting combined with narrow absorption bands. Moving forward, efforts are needed to further increase the emission outside of the visible range and expand the absorption bands to
make them more compelling for TLSC applications.
Single-walled carbon nanotubes (SWCNTs) consist of 1D
rolled graphene sheets of varying tube diameter and chirality.
SWCNTs exhibit both metallic and semiconducting characteristics, which strongly depend on their (n, m) chiral indexes
and diameters.[86,87] While these materials are notably difficult
to sort (metallic, semiconducing, diameter, chirality, etc.) and
to process at large scale,[88,89] they have attracted tremendous
attention due to their outstanding optoelectronic properties.[90]
Semiconducting SWCNTs have direct bandgap, and many
diameters/chiralities exhibit tunable photoluminescence in the
NIR range (wavelengths of emission peaks spanning from 1.0
to 1.6 µm), which makes them promising TLSCs emitter candidates.[46,87] Recently, several methods including oxygen atom
doping and sp3 defect doping have been developed to enhance
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7. Conclusion
In this progress report, we review the theoretical limits and
identify key parameters to the development of transparent
luminescent solar concentrators, which provide new opportunities for solar deployment that are a compliment to traditional solar modules. The theoretical and practical efficiency
limits track surprisingly close to those for transparent photovoltaics. It now becomes a material development challenge
to create emitters with high quantum yield, optimal bandgap,
narrow emission, large Stokes shifts, and wavelength selectivity. Provided these properties are met, highly efficient and
exceptionally low cost transparent solar harvesting systems
can be developed and deployed that approach these limits.
Ultimately, this report provides guidelines for the development of ideal materials and properties for the realization of
the full potential of this new clear luminescent solar concentrator architecture.
Acknowledgements
Financial support for this work was provided in part by Michigan State
University startup funds and the National Science Foundation (CAREER
award, CBET-1254662). The authors thank Yimu Zhao and Miles C. Barr
for useful discussions.
Received: October 15, 2016
Revised: November 27, 2016
Published online:
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