ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 91 (2007) 1599–1610
www.elsevier.com/locate/solmat
Improving solar cell efficiency using photonic band-gap materials
Marian Florescua,b,, Hwang Leea, Irina Puscasuc, Martin Prallec, Lucia Florescua,b,
David Z. Tingb, Jonathan P. Dowlinga
a
Department of Physics and Astronomy, Hearne Institute for Theoretical Physics, Louisiana State University, 202 Nicholson Hall,
Baton Rouge, LA 70803, USA
b
Jet Propulsion Laboratory, California Institute of Technology, Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
c
Ion Optics Inc., 411 Waverley Oaks Rd. Suite 144, Waltham, MA 02452, USA
Received 31 October 2006; received in revised form 2 May 2007; accepted 2 May 2007
Available online 29 June 2007
Abstract
The potential of using photonic crystal structures for realizing highly efficient and reliable solar-cell devices is presented. We show that
due their ability to modify the spectral and angular characteristics of thermal radiation, photonic crystals emerge as one of the leading
candidates for frequency- and angular-selective radiating elements in thermophotovoltaic devices. We show that employing photonic
crystal-based angle- and frequency-selective absorbers facilitates a strong enhancement of the conversion efficiency of solar cell devices
without using concentrators.
r 2007 Elsevier B.V. All rights reserved.
Keywords: Photonic band-gap materials; Thermophotovoltaics; Solar cells
1. Introduction
Photovoltaic (PV) solar energy conversion systems (or
solar cells) are the most widely used power systems.
However, these devices suffer of very low conversion
efficiency. This is due to the wavelength mismatch between
the narrow wavelength band associated with the semiconductor energy gap and the broad band of the (blackbody)
emission curve of the Sun. The power loss is associated
with both long-wavelength photons that do not have
enough energy to excite electron–hole pairs across the
energy gap (leading to a 24% loss in silicon, for instance)
and short-wavelength photons that excite pairs with energy
above the gap, which thereby waste the extra kinetic energy
as heat (giving a 32% loss in silicon). The efficiency of the
thermophotovoltaic (TPV) system may be increased by
recycling the photons with frequency larger than the solar
cell band-gap frequency, by using a spectrally dependent
Corresponding author. Jet Propulsion Laboratory, California Institute
of Technology, Mail Stop T1714 106, 4800 Oak Grove Drive, Pasadena,
CA 91109, USA.
E-mail address: [email protected] (M. Florescu).
0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2007.05.001
coupling between the absorber and the cell (Fig. 1).
However, any approach to solar-cell efficiency improvement that does not address this fundamental wavelengthband mismatch, can achieve at most around 30% efficiency
[1]. Moreover, this can be achieved only for concentrated
radiation, which requires an additional optical device,
which is not desirable in applications where the mass is a
critical concern.
This article outlines novel approaches to the design of
highly efficient solar cells using photonic band-gap (PBG)
materials [2,3]. These are a new class of periodic materials
that allow precise control of all electromagnetic wave
properties [4–6]. A PBG occurs in a periodic dielectric or
metallic media, similarly to the electronic band gap in
semiconductor crystals. In the spectral range of the PBG,
the electromagnetic radiation light cannot propagate. The
ability to tailor the properties of the electromagnetic
radiation in a prescribed manner through the engineering
of the photonic dispersion relation enables the design of
systems that accurately control the emission and absorption of light. This gives rises to new phenomena including
the inhibition and enhancement of the spontaneous
emission [3], strong localization of light [2], formation of
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Fig. 1. Schematic of a TPV energy conversion scheme. An intermediate
absorber is heated by the Sun’s thermal radiation. The photovoltaic (PV)
cell is illuminated by radiation from emitter transmitted by a filter.
atom–photon bound states [7], quantum interference
effects in spontaneous emission [8], single atom and
collective atomic switching behavior by coherent resonant
pumping, and atomic inversion without fluctuations [9].
These remarkable phenomena have attracted a considerable interest for important technological applications, such
as low-threshold micro-lasers [10,11], ultra-fast all-optical
switches, and micro-transistors [12–14].
The modifications of the spontaneous emission rate of
atoms inside the photonic crystal structure determine,
in turn, important alterations of thermal radiative processes. Thermal radiation is just spontaneous emission
thermally driven and in thermal equilibrium with its
material surroundings. In 1999, Cornelius and Dowling
suggested the use of PBG materials for the modification of
thermal emission [15]. They explored two alternative
approaches: a method based on a passive lossless PBG
thin-film coating over the absorber, and an approach which
uses an active PBG material made out of an absorptive
medium. Thermal emission modification has been experimentally demonstrated in 2000, using a thin slab of 3D
photonic crystal on a silicon substrate [16]. Pralle et al.
demonstrated a thermally excited, narrow-band, midinfrared source using a PBG technique [17]. Recently,
researchers at Sandia Labs demonstrated a high-efficiency
TPV system using tungsten photonic crystals [18–20].
These studies suggest that by optimizing the coupling of
the multi-mode radiation field of a PBG material and a
spatially extended collection of atomic or electronic
emitters, it is possible to achieve dramatic modifications
of Planck’s blackbody radiation spectrum [15,21]. In
the PBG spectral range the thermal emission of radiation
is strongly suppressed, whereas for specific frequencies in
the allowed photonic bands, that correspond to transmission resonances of the photonic crystal, the thermal
emission of radiation is resonantly enhanced up to the
black-body limit.
The ability of the photonic crystals to funnel the thermal
radiation into a prescribed spectral range is illustrated in
Fig. 2, which shows a comparison between the intensity
emitted by a photonic crystal sample when electrically
heated, which reaches a temperature of 420 when the
electrically heated with an input power of 135 mW (black
curve), and two blackbody systems, one kept at the same
temperature as the photonic crystal at the expense of using
a higher input power (315 mW) and a second one exposed
at the same input power as the photonic crystal sample, but
having a lower temperature ð273:4 Þ. We notice in the case
of the photonic crystal sample that by eliminating the
emission in certain frequency bands (corresponding to the
spectral range of the PBG), the emission is enhanced in the
spectral region corresponding to the allowed bands and,
with the same input power, the photonic crystal reaches a
higher temperature than a blackbody. This is solely due to
the funneling of the thermal radiation from the forbidden
spectral range (the orange area in Fig. 2) into the allowed
spectral range (the brown area in Fig. 2). Therefore, the
heated photonic crystal emitter achieves thermal equilibrium at a higher temperature than would otherwise be
possible. These facts suggests the possibility to leverage the
funneling properties of photonic crystals to improve the
spectral coupling of an emitter into the acceptance band of
a PV cell.
Fig. 2. Spectral funneling of the thermal radiation by photonic crystals.
By designing a photonic band gap in prescribed frequency region of the
photonic crystal emission spectrum, the structure becomes unable to
radiate at these frequencies and the corresponding energy is re-radiated in
the allowed spectral range. As a consequence, the intensity of the
blackbody emission at these frequencies increases, and the photonic
crystal emitter radiates the same power as it would a blackbody
maintained at a higher temperature.
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We present a design of highly efficient solar cells using
PBG materials as intermediary between the Sun and the PV
cells. We predict limiting conversion efficiency of around
60%. We propose two approaches to achieve this. The first
approach is to couple broadband solar radiation into a
PBG material, engineered to re-emit the solar radiation
into a narrow frequency band corresponding to the
semiconductor energy gap. In this way, power loss due to
photons of wavelength too much above or below the gap is
eliminated. Another approach is intended to eliminate the
roadblocks in the design of TPV systems based on nonconcentrated radiation, and makes use of a photonic
crystal-based angle-selective absorber. The selective absorber has the property of absorbing only certain parts of the
whole solar spectrum. If the absorber can absorb solar
radiation whose frequency is above the solar cell band-gap
frequency, the TPV efficiency of 45% can be achieved by
using non-concentrated radiation (maximum of the dashed
curve in Fig. 4). In this case, additional spectral filters are
needed in front of the absorber. Here we show a specially
designed photonic crystal that exhibits both angular and
spectral selectivity in absorption and emission. Also,
experimental studies show that the photonic crystalenhanced (PCE) infrared emitters enhance the wall plug
conversion efficiency of MWIR solar cells relative to
blackbody broad band sources.
1.1. Thermal emission control
From the foundations of quantum mechanics, it is
known that atomic oscillators in thermal equilibrium with
photon heat bath at temperature T have an average energy
at frequency o given by
eðo; TÞ ¼
_o
,
expð_o=kB TÞ 1
(1)
where _ is the Dirac constant and kB is the Boltzmann
constant. The energy density per unit frequency, then, can
be written as
uðo; TÞ ¼ rðoÞeðo; TÞ,
(2)
where rðoÞ is the electromagnetic density of modes. For
free space, the density of modes has the form
2o2
,
(3)
pc3
such that the radiant power then takes he usual form of
Planck’s law
rFS ðoÞ ¼
1
o2
_o
pBB ðo; TÞ ¼ cuðo; TÞ ¼
.
3
_o=k
B T1
4
2pc e
(4)
This suggests that since the density of electromagnetic
modes is altered in a photonic crystal, the radiant power
can also be altered.
The ability of the photonic crystal to change the spectral
properties of the emitted and absorbed electromagnetic
radiation can be illustrated considering a photonic crystal
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coating over a many-wavelength-thick substrate. The
radiation is emitted from the substrate and passes through
the passive photonic crystal filter and then is emitted into
vacuum. The absorbance A is given by energy conservation, namely, A þ T þ R ¼ 1, where R and T are
reflectance and transmittance, respectively. The absorbance
is unity if the source is a perfect blackbody. Finding the
absorbance is equivalent to finding the thermal emittance
E, since using Kirchhoff’s second law, the ratio of the
thermal emittance to the absorbance is the same, independent of the nature of the material. Consequently, it is
possible to then compute E by matrix transfer techniques
[15,22]. Once E is obtained, multiplication by the Planck
power spectrum gives the power spectrum of the PBG
emitter pTH ðo; TÞ in terms of the emittance EðoÞ and
blackbody spectrum pBB ðo; TÞ, as given by
pTH ðo; TÞ ¼ EðoÞpBB ðo; TÞ.
(5)
Therefore, the thermal radiant power in a photonic crystal
can be controlled by altering the thermal emittance.
2. Photonic crystal-based solar TPV: concepts and designs
2.1. TPV conversion efficiency
The conversion efficiency of a TPV solar system is
determined by both the absorption efficiency of the
intermediate absorber and the cell conversion efficiency.
Let us first examine the absorption efficiency of the
intermediate absorber. The incident power density is
related to the spectral power density defined as
Z
PS ¼ do_obS ðoÞ,
(6)
where
FS
o2
(7)
4p3 c2 e_o=kB T S 1
is the spectral photon flux. Here, T S is the temperature of
the Sun and F S is a geometric factor, equal to 2:16 105 p
for non-concentrated light (determined by the radius of the
Sun and the distance between the Sun and the Earth), and
p for the full concentration. This leads to the Stefan–
Boltzmann’s law
FA
PS ¼
(8)
sT 4S .
p
bS ðoÞ ¼
The intermediate absorber loses its energy by emitting
radiation with the rate ½F A =psT 4A , where the geometric
factor F A is equal to p since the absorber emits in all
directions. Hence the net gain of the absorber is
FS
FA
Pnet ¼
(9)
sT 4S sT 4A .
p
p
In what regards the cell conversion efficiency, assuming
that the spectral filter allows only the radiation with
frequencies bigger than the gap frequency oG , and the
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recombination loss is all radiative, the open-circuit
assumption may be used for estimation of the maximum
conversion efficiency. Under this assumption, we have
_o
_o Dm
¼
,
kB T A
kB T C
(10)
from the generalized Planck’s law [23]. Here, T C is the
temperature of the cell and Dm is the chemical potential.
The efficiency of an electron–hole pair to generate electrical
energy is then given by the chemical potential divided by
the photon energy as Dm=_o ¼ 1 ½T C =T A , which is the
Carnot efficiency. The actual working situation is a slight
deviation from the open-circuit condition, such that this
expression of efficiency still holds (Fig. 3).
Combining the two contributions, we have the efficiency
of the TPV conversion system as
!
F A T 4A
TC
1
ZPV ¼ 1 .
(11)
F S T 4S
TA
2.2. Non-concentrated radiation: frequency- and angleselective absorber
An increased efficiency of a TPV system may be obtained
mainly by recycling of photons of frequency larger than the
solar cell band-gap frequency by using a spectral filter
between the absorber and the cell. The combined system of
the absorber and the filter can be called a selective emitter.
However, such a high efficiency can be achieved only for
concentrated radiation, which requires additional optical
devices, not desirable for instance for space applications,
where mass is of critical concern.
In order to design a high-efficiency TPV system using
non-concentrated radiation, we have introduced an
Fig. 3. Schematic of the photonic crystal-based TPV energy conversion.
An intermediate absorber is heated by absorbing thermal radiation. The
photovoltaic (PV) cell is illuminated by radiation from emitter transmitted
by a filter.
angle- and frequency-selective absorber. The selective
absorber has the property of absorbing only certain parts
of the whole solar spectrum. If the absorber can absorb
solar radiation of frequency above the solar cell band-gap
frequency, a TPV efficiency of 45% can be achieved [1] (the
maximum of the dashed curve in Fig. 4). Again, additional
spectral filters are needed in front of the absorber. We show
that a suitably designed photonic crystal can be used as a
selective emitter as well as a selective absorber. If, for
example, we match the band-edge frequency of the
photonic crystal to the semiconductor band-gap frequency,
it is possible to suppress both emission and absorption of
photons of frequency below that of the semiconductor
band-gap. Consequently, the photonic crystal sample plays
simultaneously the role of a selective emitter (with respect
with the cell) and a selective absorber (with respect to
the Sun).
In addition to the frequency-selectivity, thermal emission
of the photonic crystal has angular selectivity as well. The
control over the angular distribution of the emitted
radiation can be extremely for the overall efficiency of
the TPV system. If the solid angle of the emission at the
Sun side can be made very small, it is possible to achieve
the same enhancement of the solar cell efficiency as in
devices using concentrators. In other words, just by
engineering the emission solid angle, the energy conversion
efficiency can be increased without using concentrators.
The radiation concentration in Eq. (11) is mathematically described by the increase of the Sun’s geometric factor
F S . However, a decrease of the absorber’s geometrical
factor F A leads to the same effect. Physically, the decrease
of the absorber’s F A implies that the emission and
absorption of radiation is confined to a certain range of
directions. Fig. 4 shows the TPV efficiency as a function of
the absorber temperature assuming F A =F S ¼ 100 (solid
curve) and F A =F S ¼ 1000 (dashed curve). The TPV
efficiency for 100 reaches up to 68% at about 727 C and
44% at 427 C.
Such a narrowing of absorption angle can be realized by
exploiting the absorption anisotropy of the photonic
crystal. As an illustrative example we consider an inverted
opal photonic crystal consisting of FCC structure of air
spheres in a solid background of silicon. Inverted opal
photonic crystals are ideal for high-quality, large-scale
fabrication of PBG materials with band gaps at micron and
sub-micron wavelengths [24]. In an optimal configuration,
such as the one presented in Fig. 5, the PBG can be as large
as almost 10% of the central frequency. Experimentally, an
artificial inverted opal can be created starting with monodisperse silica spheres with a diameter around 870 nm.
These spheres form a closed-packed FCC lattice by a
process of sedimentation in an aqueous solution of
ethylene glycol. In the second stage, silicon is grown inside
the voids of the opal template by means of chemical vapor
deposition (CVD) using disilane (Si2H6) gas as a precursor.
After disilane is deposited uniformly in the voids, the
crystal is heated to 600 C in order to improve the silicon
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Fig. 4. TPV conversion efficiency as a function of the absorber temperature. The cell temperature is assumed to be 27 C. Solid line is for F A =F S ¼ 100,
and the dashed line is for F A =F S ¼ 1000.
pffiffi
Fig. 5. A close-packed inverted opal structure ðr=a ¼ ð2Þ=4Þ viewed as
sequence of yABCABC yplanes grown along the ½1 1 1 direction. In
each plane, the low-dielectric constant spheres (here, for simplicity, we
assume air spheres) are embedded in a high-dielectric constant host
medium pand
ffiffiffi are sitting on a triangular lattice of lattice constant
axy ¼ a= 2.
crystallization and allow the diffusion of silicon throughout the sample. Finally, the silica template is removed using
controlled fluoride-based etching designed to avoid affecting the silicon backbone, and leaving behind a closedpacked FCC lattice of air spheres in a silicon background
[24]. Photonic crystals are usually characterized by the
dispersion relation (or band structure, representing the
relationship between the frequency and the wave-vector)
and the photonic density of states (the number of available
electromagnetic modes at a specific frequency and at a
location within the photonic crystal) (see Fig. 6), which can
be employed to infer their radiative response.
In Fig. 7 we plot the angular dependence of the
absorption for a fully infiltrated inverted opal structure
shown in Fig. 5. Each plot corresponds to different incident
angles for a fixed azimuthal angle. Figs. 8 and 9 are the
enlarged portions of Fig. 7 at the lower band-edge of the
first stop band around oa=2pc ¼ 0:5 and at the higher
band-edge of the second stop band around oa=2pc ¼ 0:8,
respectively. The absorption is enhanced at the band-edge
location. However, in the presence of absorption, as the
incident angle changes, the band edge is not so well defined
anymore. For some directions there are some tails that
enter the gap and the position of the band edge frequency
depends on the specific direction. As a result the absorption
is enhanced considerably, for a given spectral range, for
specific incident directions. This fact opens a novel way to
make an angular-selective PBG absorber, which, in turn,
enables high-efficiency solar energy conversion without
using concentrators.
The increase in the energy conversion efficiency is
determined by the small angle of thermal emission of the
intermediate absorber on the Sun side. For the intermediate absorber, the gain comes from the absorption of solar
radiation and the loss is determined by the emission by the
absorber. As we decrease the solid angle of the emission by
the absorber, we can decrease the loss due to the emission
by the absorber, and effectively increase the net gain. The
so-called minimal emission refers to a situation where the
solid angle of the emission (F A ) is the same as the solid
angle extended by the Sun (F S ). Such an angular selective
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Fig. 6. Band structure and corresponding DOS for a close-packed FCC lattice of air spheres in silicon ðSi ¼ 11:9Þ [22].
Fig. 7. Absorption spectra for the photonic crystal of a fully infiltrated inverted opal structure with different incident angles. The position of the band edge
moves as the direction changes. More detailed view is depicted in Figs. 8 and 9 for the lower band-edge of the first stop band around oa=ð2pcÞ ¼ 0:5 and
the higher band-edge of the second stop band around oa=ð2pcÞ ¼ 0:8, respectively.
absorber, due to the decrease of the radiation loss, gets
much hotter than in the conventional case. A larger value
of temperature difference between the absorber and the PV
cell becomes available, leading to higher Carnot efficiency.
For a small solid angle for absorption such that
F A =F S ¼ 100, the theoretical limit of TPV conversion
efficiency becomes 68% at about 727 1C (see Fig. 4).
However, we can see from Fig. 8 that there are absorption
peaks at different frequencies. Hence, absorption is
enhanced at a different angle of incidence for a different
frequency. In other words, the angular selectivity is
restricted to a small frequency range. This effect might be
useful when applied in the tandem cell configuration. In
realizing an angular selective absorber, the angular
selectivity should cover a wide range of frequencies.
Otherwise, the absorber emits outside of the desired
emission cone with different frequencies.
2.3. Wide-band angular-selective absorber
The best possible absorber would absorb radiation at all
frequencies, but only inside the solid angle subtended by
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Fig. 8. Absorption spectra at the lower band-edge of the first stop band around from Fig. 7.
Fig. 9. Absorption spectra at the higher band-edge of the first stop band around from Fig. 7.
the Sun. Wide-band angular-selectivity can be realized by
designing a photonic crystal absorber such that the
absorption is suppressed at all frequencies for all angles
except inside the desired cone. In other words, all the
frequencies are lying inside the PBG except for one
preferred direction. The effect of such an absorber is
equivalent to using fully concentrated radiation. Since the
angular selectivity requires a sharp cutoff in the emission
solid angle, a one-dimensional defect photonic crystal
structure embedded in the above discussed photonic crystal
is an excellent candidate for a wide-band angular-selective
PBG absorber. We considered a structure with a complete
three-dimensional band gap with one-dimensional absorption characteristics. Such a property can be realized with a
3D–2D–3D [25,26] photonic crystal heterostructure as the
one depicted in Fig. 10, where a waveguide channel is built
in a 2D photonic crystal as a defect mode. The 2D
photonic crystal is, in turn, embedded in a 3D photonic
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Fig. 10. Design of a PBG wide-band angular selective absorber. The
micro-structure consists of a waveguide channel in a 2D photonic crystal,
which is embedded in a 3D photonic crystal. The 1D waveguide is
generated by removing one row of rods in the longitudinal direction. The
3D photonic crystal is assumed to be a woodpile structure [28] that
presents a photonic band gap of about 20% of the mid-gap frequency. In
this example, the 2D photonic crystal consists of square rods of width
a2D =a ¼ 0:3. The width and the height of the stacking rods in the woodpile
structure are a3D =a ¼ 0:25 and h3D =a ¼ 0:3, respectively, where a is the
dielectric lattice constant of the embedding 3D photonic crystal [25,26].
1D defect in the 3D PBG can support a single waveguide
mode, which experiences a sharp cutoff in the gap of a 3D
photonic crystal, as shown in Fig. 11. In this case, the subgap generated by the waveguide channel has a true onedimensional character, since there is only one direction
available for wave propagation. The sharp cut-off of the
guided mode at the Brillouin zone boundary gives rise to a
low-group velocity do=dk ! 0, which combined with the
one-dimensional character of the system leads to a
divergent density of states (DOS): rðoÞ / dk=do ! 1.
For an infinite structure, there is a physical square-root
singularity in the photonic DOS near the cutoff of the
waveguide modes [27]. For a finite structure, the divergence
is removed by the finite-size effects, but the strong variation
with frequency of the photonic DOS remains.
The dispersion relation for the PBG hetero-structure
described in Fig. 10 presented in Fig. 11 indicates that
unidirectional light absorption can be achieved for a
relatively broad spectral range. Thus, the photonic crystal
heterostructure will operate as a frequency- and angleselective absorber.
Furthermore, we envisage a hybrid scheme for the
intermediate absorber as depicted in Fig. 12. It consists of a
3D–2D–3D photonic crystal architecture acting as a
frequency and angle-selective absorber on the Sun side,
and a 3D photonic crystal structure acting as a frequencyselective emitter on the cell side. The absorber and the
emitter systems are in thermal contact and reach thermal
equilibrium. We note that the absorber and emitter systems
are macroscopic objects that exchange energy not only by
radiative means, but also through direct thermal contact
(exchange of vibrational excitations or phonons). As a
result of the thermal contact, the photonic-crystal emitter
reaches the same temperature as the absorber, and the
thermal energy absorbed is subsequently funneled into a
narrow spectral range by the PBG emitter. We also point
out, that due to the angle-selective character of the
absorber system, the thermal equilibrium temperature of
the whole device it will be much higher than the one of a
conventional absorber system.
3. Experimental demonstration of PCE infrared emitters for
efficient TPV applications
Fig. 11. Schematic dispersion relation of the PBG hetero-structure
described in Fig. 10 for propagation along the waveguide direction
(w ¼ o=2pc). By removing one row of rods, the linear defect supports a
single waveguide mode. By appropriately choosing unit cell size, the mode
will experience a sharp cutoff in the spectral region around the desired
frequency [25].
crystal. The electromagnetic field is confined vertically by
the 3D structure and in-plane by the stop gap of the 2D
photonic crystal. By tuning the characteristics of the microstructure (geometry and index of refraction contrast), the
The development of a solar-cell device based on the
frequency and angular control of the emission and
absorption of thermal radiation in a photonic crystal is a
complex and laborious process. Such a device will
incorporate all the advances in current solar-cell technology, and then selectively replace components in the solarcell architecture by their photonic crystal engineered
counterparts. Similar to the conventional solar cell design,
the incident solar power will be absorbed by an absorber
system. As we have shown in the previous section, the
absorber system may consist of a photonic crystal
architecture, engineered such that it has an enhanced
absorption coefficient along the direction of the incident
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Fig. 12. A hybrid scheme for the intermediate absorber in TPV solar energy conversion. The inverted opal structure is used for the frequency-selective
emitter on the PV cell side. On the other hand, the 1D waveguide in a 3D woodpile structure provides the angular-selective absorber.
solar radiation. The emitter system consists of a different
photonic crystal architecture, engineered such that it
presents an enhanced emission coefficient for a spectral
range that matches the photocell semiconductor band gap.
In our experimental study, we have focused only on a
specific problem: the improvement of the emitter efficiency
using a photonic crystal-based emitter system. In order to
simplify the analysis, we have used a low-temperature TPV
energy conversion experimental set-up. In TPV, an
incandescent radiator illuminates a PV cell that converts
radiant heat to electrical energy. These systems are an
attractive long-term power source since, in principle, they
can achieve conversion efficiencies considerably higher
than the 6–8% capabilities of current thermoelectric
generators. The crucial problem for this technology is
matching the emission spectrum of the radiator to the band
gap of the photocell. Approaches to this problem include
radiators with strong (ionic) emission lines and reflective
filters between the radiator and photocell. In this work we
explore an alternate radiator concept a PCE narrow band
incandescent emitter tuned for peak emission near the band
gap of the photocell. Our results show the feasibility of
fabricating rugged emitters with tunable wavelengths
through control of surface geometry. Furthermore, when
radiation from this photonic crystal was shown onto a midIR PV device, we observe significant wall-plug efficiency
improvements (45%) relative to a broad blackbody source.
We have developed a photonic crystal structure that acts
as a selective emitter, preferentially emitting light in a
narrow band when heated. With a narrow emission line,
yet broader than current technology, these materials can
emit greater energy in the selective band at lower
temperatures. In Fig. 13 we show a scanning electron
micro-graph of the PCE emitter surface. The initial
development of PCE emitter surfaces is presented in
Ref. [29]. The PCE technology was then integrated with
MEMS technology, yielding discrete silicon-based narrowband infrared light sources as shown in Fig. 13. The
fabrication of this device uses traditional photo-lithography processing. More specifically the photonic crystal is
fabricated by depositing metal onto an oxide-coated silicon
wafer. Using photo-lithography, the photonic crystal holes
are patterned onto the surface, and then using dry etching
techniques (reactive ion etching), the holes are drilled into
the substrate. When heated the surface emits a narrow peak
of infrared light with a center wavelength commensurate
with photonic crystal lattice spacing (in this case 4:2 mm), as
shown in Fig. 14. Peak wavelength and width do not
change with temperature variation. The peak of the
emission curve will lie on the blackbody emission curve
for the measured sample temperature. The most important
point for application to TPV is that infrared emission at
short and long wavelengths relative to the central peak are
dramatically suppressed. Out-of-band emission is emissivity limited to 10% of the blackbody curve at the same
temperature. As a result, these emitters have demonstrated
unprecedented infrared emission efficiency with total wall
plug efficiencies approaching 20% in band. This should
translate to improved TPV efficiency and it was this
experiment that was the focus of this feasibility study.
In order to develop an efficient TPV system, it is
necessary to first select the most efficient solar cell device
and then tune the emission spectrum of the hot source to
the optimum conversion efficiency wavelength of that solar
cell. Here, we optimized the solar-cell device to already
available PCE emitters. We selected a PV device based on
HgCdTe (MCT) manufactured by Vigo Inc. This device
was doped with Zn to maximize efficiency from 4 to 5 mm in
the infrared. As a solar cell, this device would be very
inefficient, but it was chosen to demonstrate the potential
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Fig. 13. PCE MEMS infrared emitter vacuum packaged in a standard lead less chip carrier. The device shows unparalleled wall plug efficiency (larger
10%) in the mid infrared spectrum (MWIR 3–5 mm). This is two orders of magnitude more efficient than IR light emitting diodes highlighting the
advantages of photonic crystal emitter enhancement.
Fig. 14. Infrared emission spectrum of the MEMS PCE emitter driven at
0.130 mW of input power. Inband the emission reaches the blackbody
emission, but out of band the emission is suppressed dramatically. The
blackbody spectrum (red) is at the same temperature as the PCE. The
spikes in emission below 2 mm are noise in the measurement.
Fig. 15. Spectral responsivity of the photovoltaic detector. Above 6 mm
the solar cell is not longer effective. The wavelength at peak responsivity is
between 4–4:5 mm, well aligned with the peak emission of the PCE emitter
shown in Fig. 14.
of PCE TPV. Initial characterization of the photonic
crystal emitter device was carried out using a calibrated
blackbody reference on a Nicolet Nexus 670 FTIR,
equipped with an external emission port, following
procedures outlined elsewhere [30]. The spectral characteristics are shown in Fig. 14. The integrated power in the
2–6 mm band is 30 mW of infrared light, resulting in a wall
plug efficiency of 23.5%. The spectral characteristics of the
Vigo middle wavelength infrared (MWIR) PV detector
were provided by the manufacture and shown in Fig. 15.
When the emission spectrum is multiplied by the responsivity we get the response curve for both the blackbody and
the PCE emitter shown in Fig. 16. Integrating and
multiplying by detector area we get 9.8 and 13.4 mV for
the PCE emitter and the blackbody, respectively. This
agrees well with the measured value of output voltage from
the detector of 9.6 and 12.8 mV for the PCE emitter and a
blackbody (with equal aperture), respectively. Solar cell
output power was then measured by varying the resistive
load and measuring the current and voltage. This is shown
in Fig. 17. Converting to output power, we find that for the
blackbody we measure 0:56 mW and the PCE emitter yields
0:32 mW (Fig. 18) . The absolute magnitude of these
numbers is very low, which is expected because this solar
cell device is very inefficient. However, we can compare the
relative efficiencies of the PCE emitter and the blackbody
reference. Interestingly, the PCE emitter only requires
130 mW of input power verses the blackbody (normalized
for area), which requires 315 mW. Therefore, the total wall
plug conversion efficiency of the PCE emitter is 2:46 106
and only 1:68 106 for the blackbody. The PCE emitter
resulted in a net improvement of 46% over the broadband
blackbody source. The improved performance of the PCE
emitter is derived from the narrow emission band. Instead
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M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610
Fig. 16. The spectral response curve for the system. The red curve is the
blackbody and the black is the PCE emitter.
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Fig. 18. Power versus voltage for the TPV solar cell setup. Peak power of
0.32 and 0.56 mW are observed the PCE emitter (black) and blackbody
(red), respectively.
Therefore, much of the blackbody emission profile can be
converted to electricity. In contrast, higher efficiency solar
cells, like polysilicon, have much narrower spectral bandwidth. For these detectors, the narrow emission from
photonic crystal surfaces will be dramatically enhanced.
We anticipate two to three times improvements are possible
with photonic crystal enhancement.
4. Conclusions
Fig. 17. Current–Voltage characteristics for emitter-solar cell system. The
total output power is higher for the blackbody (in red) versus the PCE
emitter (black) as evidenced by the higher red curve. However, the PCE
emitter required significantly less input power.
of emitting photons at all wavelengths, thereby wasting
optical power in spectral bands where the solar cell cannot
convert them, the PCE emitter concentrates all of the
optical power into a narrow band commensurate with the
absorption wavelength of the solar cell. As equal power is
dumped into the PCE emitter, it achieves a higher
temperature than the blackbody and yields more output
in the spectral band of interest. This effect will be greatly
enhanced at higher emitter temperatures where radiative
power dominates.
The efficiency enhancement of photonic crystal emitters
can be compounded with a narrow spectral absorption
solar cell. The Vigo MCT PV device has a very broad
spectral responsivity (Fig. 15) from 0.8 to 5:5 mm wavelength.
We have shown that the ability to control the thermal
emission and absorption of radiation in a photonic crystal
enables the realization of high-efficiency solar cells. We
have combined predictive modeling, micro-fabrication, and
optical measurements to provide a basis for understanding
and controlling the thermal emission and absorption of
radiation in complex photonic structures and to design
novel solar cell devices. We have demonstrated that the
thermal emission in photonic crystal is characterized by
spectral- and angular selectivity. The spectral selectivity
plays an important role in eliminating wavelength-band
mismatch between the semiconductor energy gap and
blackbody emission, affecting the efficiency of solar cells,
and may lead to a significant increase in the solar cell
efficiency. On the other hand, the use of angle-selective
absorbers based on suitably designed photonic crystal
structures opens new avenues for realizing high-efficiency
TPV systems without concentrators.
The current state-of-the-art thermal shields use multilayer devices or textured surfaces to reduce the impact of
the thermal radiation on thermal sensitive devices. They
offer little control over frequency, and, typically, require
mechanical operations to achieve a limited control over the
angular distribution of the absorbed radiation. The hybrid
photonic crystal architecture we have proposed are
frequency- and angle-selective and allows a precise control
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M. Florescu et al. / Solar Energy Materials & Solar Cells 91 (2007) 1599–1610
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of the absorption and emission of thermal radiation. By
infiltration such structures with liquid crystals and by the
application of an external electric field, the resulting device
can be tuned without the use of any mechanical operation.
On the other hand, the capability to control the thermal
emission and absorption of radiation in a photonic crystal
may also find many technological applications in the field
of thermally pumped optical devices such as lasers and
tunable infrared emitters.
Acknowledgments
Part of this work was performed at the Jet Propulsion
Laboratory, California Institute of Technology, under a
grant from the National Aeronautics and Space Administration. MF, HL, and JPD would like to acknowledge the
Hearne Institute for Theoretical Physics, the Disruptive
Technologies Office, the Army Research Office, and the
Louisiana State University Board of Regents LINK
Program for support.
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