Wavelength-selective thermal extraction for higher efficiency and power density
thermophotovoltaics
Zoila Jurado
, Junlong Kou
, Seyedeh Mahsa Kamali
, Andrei Faraon
, and
Austin J. Minnich
Citation:
Journal of Applied Physics
124
, 183105 (2018); doi: 10.1063/1.5049733
View online:
https://doi.org/10.1063/1.5049733
View Table of Contents:
http://aip.scitation.org/toc/jap/124/18
Published by the
American Institute of Physics
Wavelength-selective thermal extraction for higher ef
fi
ciency and power
density thermophotovoltaics
Zoila Jurado,
a)
Junlong Kou,
a)
Seyedeh Mahsa Kamali, Andrei Faraon,
and Austin J. Minnich
b)
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California
91125, USA
(Received 25 July 2018; accepted 24 October 2018; published online 13 November 2018)
Thermophotovoltaics have long been of interest as an energy conversion technology but suffer from
low power density and low ef
fi
ciency. Structured emitters designed to alter the emission spectrum
and increase the ef
fi
ciency are not stable at the necessary high emitter temperatures and also reduce
the power density. Here, we propose a wavelength-selective thermal extraction device that mitigates
these challenges and demonstrate a transfer-printing process needed to fabricate the device. The
device consists of a ZnS solid hemisphere with a patterned thin
fi
lm optical
fi
lter that passively
increases the far-
fi
eld radiated
fl
ux from an emitter within a wavelength band near the bandgap of a
photovoltaic cell. Crucially, the device does not need to be in physical contact with the emitter and
thus can be maintained at a lower temperature, circumventing the thermal stability challenge. Our
work helps one to address long-standing issues with applications of thermophotovoltaics.
Published
by AIP Publishing.
https://doi.org/10.1063/1.5049733
I. INTRODUCTION
Thermophotovoltaics (TPV) convert heat in the form
of electromagnetic radiation directly to electrical energy
using a photovoltaic cell (PV),
1
and they have long been of
interest due to the possibility to recover energy from waste
heat sources as well as their potential to break the
Shockley-Queisser limit.
2
–
4
The majority of losses in a pho-
tovoltaic cell (PV) are due to the lack of absorption or ther-
malization.
5
TPV could reduce these losses provided the
emitter spectrum can be altered to emit within a narrow band
near the bandgap energy of a PV cell.
5
–
11
Numerous works have thus focused on schemes to realize
selective emission of the emitted radiation or selective absorp-
tion of the incoming radiation combined with photon recycling.
For instance, spectral
fi
ltering by the PV cell itself has recently
been identi
fi
ed as a promising approach to realize
.
50% ef
fi
-
cient TPV systems, but it requires high back-re
fl
ector re
fl
ectiv-
ity and external quantum ef
fi
ciency of the PV cell.
12
The
luminescent bands of rare earth oxides have been proposed for
selective emission.
10
However, the spectral emittance and ef
fi
-
ciency of these emitters are sensitive to temperature, and the
materials may lack the necessary thermal stability. Additionally,
rare earth oxides may have a narrow bandwidth resulting in a
small overall power density.
9
,
13
,
14
Other efforts have focused on using patterned emitters
such as photonic crystals. Metallic photonic crystals have been
reported with a modi
fi
ed spectral emissivity compared to
unstructured metals.
7
,
15
–
17
Refractory metallic ceramics pat-
terned into two-dimensional and three-dimensional inverse
opal structures have demonstrated selective thermal emission
along with the moderate thermal stability up to 1126 K.
8
,
11
Unfortunately, the structures degraded after several hours at
temperatures exceeding 1300 K. Other methods use external
fi
lters such as Bragg stacks to re
fl
ect low energy photons, but
this
fi
ltering reduces the power density.
18
TheuseoftheBragg
stacks as an interference
fi
lter between the blackbody emitter
and PV cell to tailor emission also leads to peaks in the emis-
sion spectrum outside the desired bandpass region.
19
Chase
and Joseph
20
and Moller
et al
.
21
fi
rst proposed using a cross-
mesh array in thin metallic
fi
lmsasabandpass
fi
lter for the
infrared region and found that the wavelength peak re
fl
ection is
determined by the dimensions of the cross. Further contribu-
tions by Morgan
et al
. reported the design and fabrication of
nanoscale cross-patterned optical
fi
lters.
22
The decreased power density caused by the narrowing of
the emission wavelength spectrum for many of the schemes
described above impedes applications.
6
,
23
In principle, this
limitation can be overcome by increasing the area of the
emitter. Although structured emitters have been realized on a
wafer-scale,
24
generally large emitter areas exacerbate fabri-
cation and stability challenges. Utilizing PV cells with
smaller bandgaps can in principle increase the maximum
power density, but the ef
fi
ciency of these cells decreases pre-
cipitously as the bandgap decreases.
25
This trade-off is a fun-
damental challenge for the TPV technology.
Therefore, numerous practical challenges complicate the
application of TPV. It is clear that a device that increases
power density, enables spectral
fi
ltering of radiation, and
does not suffer from thermal stability challenges would play
an important role in furthering TPV. Recent studies suggest
that such a device is possible.
26
–
28
These works reported a
passive thermal extraction device consisting of a transparent
hemisphere placed in optical contact, but not physical contact,
with the emitter. This device enables an enhancement in
radiated
fl
ux by up to a factor of
n
2
, where
n
is the refractive
a)
Z. Jurado and J. Kou contributed equally to this work.
b)
Author to whom correspondence should be addressed: aminnich@
caltech.edu
JOURNAL OF APPLIED PHYSICS
124
, 183105 (2018)
0021-8979/2018/124(18)/183105/5/$30.00
124
, 183105-1
Published by AIP Publishing.
index of the emitter.
29
–
32
However, the device was not capable
of spectrally
fi
ltering the emitted radiation and thus thermaliza-
tion losses and losses due to the lack of absorption would still
occur in a TPV system despite the increased power density.
Here, we propose a passive selective thermal extraction
device that addresses the power density, spectral
fi
ltering,
and thermal stability challenges of TPV, and we further dem-
onstrate a transfer-printing process needed to fabricate the
device. The passive device consists of an infrared-transparent
hemisphere with a patterned thin metal
fi
lm optical
fi
lter
that transmits light only around a selected wavelength; other
wavelengths are re
fl
ected back to the emitter for re-absorp-
tion and re-emission. Within the transmitted wavelength
band, the radiated
fl
ux can be enhanced by up to
n
2
, where
n
is the refractive index of the emitting medium. The device
can be maintained at a lower temperature than the emitter
using a thermal reservoir as the device need only be in
optical contact with the emitter. We show that the transfer-
printing process preserves key features of the optical
transmission spectrum of the
fi
lter. Future work will focus on
fabricating the entire device and testing its ability to enhance
the ef
fi
ciency of TPV.
II. METHODS
The unpatterned thermal extraction device is a ZnS hemi-
sphere from Crystran Ltd., UK. ZnS transmits radiation between
0.37 and 13
:
5
μ
m and possesses a refractive index of 2.36 in
the infrared wavelengths.
33
The spectral
fi
lter consists of a thin
Au
fi
lm of 50 nm thickness etched with cross patterns that are
placed over the hemisphere. The crosses have geometrical
dimensions depicted in Fig.
1
, designed such that photons at
the bandgap energy of InGaAsSb, 0.55 eV (2
:
25
μ
m), transmit
through the
fi
lter.
7
The symmetry of the cross pattern allows
for both polarizations of light to be
fi
ltered.
The optical
fi
lter is fabricated in two steps. The
fi
rst
step is illustrated in Figs.
2(a)
–
2(c)
. First, a 300 nm sacri
fi
-
cial layer of Ge is deposited on a 500
μ
m thick doped Si
wafer.
34
Next, electron-beam lithography is used to create a
positive, elevated pattern in photoresist ZEP520A.
Subsequently, 3 nm of Ti, serving as an adhesion layer, and
50 nm of Au are deposited on the wafer using physical
vapor deposition. The second step, Figs.
2(d)
–
2(f)
, focuses
on the preservation and isolation of the pattern Au layer to
obtain the desired optical
fi
lter. Prior to lift-off, a layer of a
self-assembled monolayer (SAM) is vapor-deposited on the
wafer to promote the adhesion of Au.
35
This process is per-
formed by suspending the wafer, as in Fig.
2(c)
,over
200
μ
l of (3-mercaptopropyl)trimethoxysilane (MPTMS) in
low vacuum (3.1 kPa) for
.
2 h. Using Remover PG, the
electron-beam resist can be removed, leaving the pattern in
the optical
fi
lter.
The next step is to remove the
fi
lter from the Si substrate
using transfer printing.
34
Polydimethylsiloxane (PDMS) is
spin-coated on a fabricated optical
fi
lter at 2000 rpm for 60 s
and baked at 80
C for 1 h. This
70
μ
m PDMS layer adds
structural stability without compromising the
fl
exibility of the
optical
fi
lter. The 0.36 cm
2
sample is then immersed in a wet
solution of NH
3
OH, H
2
O
2
, and deionized water (2:10:5) for
approximately one week. The solution etches the sacri
fi
cial
layer of Ge, allowing the Au/PDMS
fi
lm to detach from the
substrate and resulting in a robust and
fl
exible optical
fi
lter.
Finally, the sample can be placed onto a desired substrate. In
this work, we demonstrate the transfer printing process by
transferring the
fi
lter to various substrates and characterize the
optical properties of the
fi
lter using Fourier-transform infrared
spectroscopy (FTIR). All FTIR measurements were taken
using a continuum microscope (Spectra-tech Inc. In
fi
nity
Re
fl
achromat) to measure specular re
fl
ection on a
fl
at surface.
In future work, we will measure the spectral transmittance of
the
fi
lter on a hemisphere and test the device
’
s ability to
simultaneously spectrally
fi
lter and increase the radiated
fl
ux.
III. RESULTS
An SEM image of the
fi
nal fabricated optical
fi
lter on Si
is given in Fig.
3(a)
. The image shows the cross pattern with
well-de
fi
ned edges and good periodicity. Figure
3(b)
shows
an SEM image of the
fi
lter after transfer printing onto
another
fl
at substrate. The
fi
lter remains mostly intact; the
inset image shows that the cross voids generally remain as in
the original pattern although with some alteration.
We next proceed to examine the optical properties of the
fi
lter before and after transfer printing. First, the re
fl
ectance
of the
fi
lter was calculated using COMSOL with a unit cell
of the selective thermal extraction device depicted in Fig.
1
.
Spectral re
fl
ection versus wavelength, shown in Fig.
4(a)
,
shows a minimum re
fl
ection around 2
μ
m, intended to corre-
spond to the bandgap of an InGaAsSb PV cell.
36
The simu-
lated results show that the designed pattern has
98
:
5%
FIG. 1. Schematic cross section of the
selective thermal extraction device con-
sisting of a solid ZnS hemisphere
covered with an Au optical
fi
lter with a
cross-void pattern. The PDMS layer is
used for the transfer printing process.
A PV cell would surround the device
in a TPV system. (b) Schematic of
cross pattern used to spectrally
fi
lter
the outgoing radiation. (c) Geometrical
parameters of the cross pattern; leg
width (W) of 100 nm and total length
(L) of 600 nm.
183105-2 Jurado
etal.
J. Appl. Phys.
124
, 183105 (2018)
re
fl
ection outside the bandpass region and
1
:
5% loss due
to absorption and transmission. Within the transmission
band, the design re
fl
ects
0
:
02% of the emitted radiation.
We can also calculate the transmission of a unit cell of the
selective thermal extraction device. The transmission versus
wavelength, illustrated in the inset of Fig.
4(a)
, validates that
the
fi
lter does transmit light in the low re
fl
ectance region.
Within the transmission band, the designed pattern has
90
:
6% transmission. Based on the simulated results, we
can also determine that
10% of the emitted radiation is
expected to be absorbed by the
fi
lter between 1.7 and 3.0
μ
m
and
2
:
5% outside this band.
We then measured the spectral re
fl
ection of the optical
fi
lter before transfer printing using FTIR, as shown in
Fig.
4(b)
. The qualitative shape of the measured re
fl
ectance
of the
fi
lter on ZnS is in reasonable agreement with the
simulated results with a minimum re
fl
ection occurring at
1
:
8
μ
m. The red shift between the actual and simulated
spectra is due to differences in the design and actual cross
dimensions. The FTIR measurement of spectral re
fl
ectance
was normalized to that of a solid Au mirror that was used as
the background measurement.
Next, we suspended the transfer printed optical
fi
lter by
placing it over a microscope stage with an aperture and mea-
sured re
fl
ectivity versus wavelength. The wavelength at
which the minimum re
fl
ection occurs remains nearly the
same. The overall selectivity of the transfer printed
fi
lter
appears to be reduced as indicated by the decrease in re
fl
ec-
tion outside the pass band. However, the actual overall re
fl
ec-
tance is likely larger than measured here as the FTIR only
measures the specular re
fl
ection; scattered light is not mea-
sured. In the actual device, the scattered light would still be
returned to the emitter for absorption and re-emission.
The
fi
lter can be transfer printed to a spherical surface
due to mechanical properties that are similar to those of poly-
ethylene
fi
lms. The
fi
lter coats the 5 mm diameter ZnS hemi-
sphere conformally although with wrinkles that are expected
when a planar surface is placed on a spherical object. Due to
experimental challenges associated with measuring optical
properties of the curved surface, we were unable to obtain
optical spectra of this device; this effort will be the subject of
future work.
IV. DISCUSSION
We now examine the capability of the device to address
the challenges of TPV. First, we note that the thermal extrac-
tion device will increase the power density of a TPV system
by a factor of
n
2
. Using the ZnS hemisphere with
n
¼
2
:
36,
the device would yield around a factor of 5.6 increase in
power density compared to that without thermal extraction.
Second, thermal stability is no longer a requirement
because the device is in optical contact, not in physical
contact, with the emitter. As the
fi
lter does absorb some frac-
tion of the radiation, we perform a simple estimation of the
expected temperature rise of the device using the known
thermal conductivity of ZnS and the heat input. The
fi
lter
’
s
average absorption over the blackbody spectrum is 3.48%,
and so the total absorbed radiation from an emitter at 1500 K
is 55
:
6 kWm
2
. Therefore, for a hemisphere with a radius of
FIG. 2. Schematic illustration of the fabrication process of the optical
fi
lter. (a) Deposition of a 300 nm sacri
fi
cial layer of Ge on a Si wafer. (b) Spin-coating of
positive electron beam resist and patterning using electron beam lithography. (c) Deposition of a 3 nm of Ti adhesion layer and a 50 nm layer of Au. (d)
Vapor-deposited MPTMS [(3-mercaptopropyl)trimethoxysilane] surface modi
fi
cation to promote adhesion between Au and PDMS layers. (e) Lift-off to create
the Au optical
fi
lter and removal of any remaining resist. (f) Spin-coating of a PDMS layer to stabilize the Au pattern prior to the removal of the Au optical
fi
lter from the fabrication substrate. After this step (not shown), transfer printing is used to transfer the Au/PDMS
fi
lm to another substrate.
FIG. 3. SEM images of the Au optical
fi
lter, before (a) and after (b) the
transfer printing process. Inset: zoomed image of the transfer printed
fi
lter.
The primary features of the pattern are maintained through the transfer print-
ing process.
183105-3 Jurado
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J. Appl. Phys.
124
, 183105 (2018)
2.5 mm, the device will absorb
2
:
2 W. The edges of the
fl
at
surface of the hemisphere are assumed to be maintained at a
given temperature. Using a thermal resistor model at steady
state, we estimate the temperature difference between the
fi
lter and the bottom edge of the device. Taking the distance
between hot and cold as
L
r
and an approximate cross-
sectional area as
A
c
π
r
2
, we estimate the peak temperature
rise on the order of 1 K, demonstrating that the device can be
maintained at a different, lower temperature than the emitter
using a thermal reservoir.
Finally, we use the simulated optical re
fl
ectivity and
transmission curves to determine the ideal increase in the
ef
fi
ciency and power density. Figure
5
illustrates the distribu-
tion of spectral irradiance of a blackbody at
T
¼
1500 K to
the far-
fi
eld between 0 and 10
μ
m. Without the spectral
fi
lter,
the total irradiance is 1
:
60 MWm
2
, including the
n
2
factor
due to the hemisphere. In the presence of the optical
fi
lter,
we calculate that 72.2% of the emitted
fl
ux will be re
fl
ected
back to the emitter to be re-absorbed, meaning 1
:
15 MWm
2
will be recovered. From the re
fl
ectance curves in Fig.
4(a)
,it
is also seen that 24.4% of the re
fl
ected light has energy
exceeding that of the TPV cell bandgap.
We use these values to calculate the increase in the
ef
fi
ciency of a TPV device that incorporates the
fi
lter.
We assume that the solar cell has a hemispherical shape
surrounding the thermal extraction device. Using the
Shockley-Queisser analysis
2
and following the notation from
Ref.
35
, the solar cell ef
fi
ciency (
η
sc
) is given by
2
,
37
η
sc
¼
U
(
T
,
E
g
)
v
(
T
,
E
g
)
m
(
V
op
)
:
(1)
The
fi
rst term in Eq.
(1)
is the ultimate ef
fi
ciency of a photo-
electric device with a single bandgap. It assumes that all
photons with
E
E
g
produce one electron-hole pair with
voltage of
V
g
¼
E
g
=
q
. In our calculations, we take the trans-
mitted photons to be equivalent to the emitted photons.
Therefore, the emitted photons with the spectral
fi
lter are
equal to the transmission curve, Fig.
4(a)
inset, multiplied by
the blackbody distribution at 1500 K. The second term in
Eq.
(1)
originates from the inequality of the open-circuit
voltage and the bandgap voltage. We assume an ideal solar
cell with a hemispherical geometry for both the emitter and
the cell such that the non-ideality factor is equal to that of an
ideal solar cell system with planar geometry. Finally, the
third term in Eq.
(1)
is the impedance matching factor.
Using Eq.
(1)
, we calculated the ef
fi
ciency under three
conditions: (1)
T
¼
6000 K,
E
g
¼
1
:
1 eV without the spectral
fi
lter; (2)
T
¼
1500 K,
E
g
¼
0
:
55 eV without the spectral
fi
lter,
and (3)
T
¼
1500 K,
E
g
¼
0
:
55 eV with the spectral
fi
lter.
The theoretical ef
fi
ciencies are 40.6%, 24.4%, and 51.2%,
respectively. Therefore, the ideal spectral
fi
lter will increase the
maximum ef
fi
ciency from 24.4% to 51.2% while simultaneously
increasing the power density by
5 as mentioned previously.
FIG. 4. The (a) simulated and (b) experimental re
fl
ectance versus wavelength of the Au
fi
lter depicted in Fig.
1
. Inset: simulated transmission versus wave-
length. The unit cell is composed of the 50 nm Au cross-void pattern and an approximately 70
μ
m thick PDMS layer. The measured re
fl
ectance of the
fi
lter
prior to lift-off (black) is in reasonable agreement with the simulated results. The measured re
fl
ectance of the
fi
lter after transfer printing (blue) maintains a
minimum at a wavelength of 2
μ
m. The re
fl
ectance for wavelengths outside the bandpass is decreased, which may be due to diffuse re
fl
ection of the light that is
not measured in the FTIR.
FIG. 5. The calculated distribution of the
fi
ltered spectral irradiance of a
blackbody at
T
¼
1500 K to the far-
fi
eld between 0 and 10
μ
m, based on the
simulated re
fl
ection and transmission curves shown in Fig.
4(a)
. The total
absorbed, transmitted, and re
fl
ected energy are indicated by black, turquoise,
and teal, respectively.
183105-4 Jurado
etal.
J. Appl. Phys.
124
, 183105 (2018)
V. SUMMARY
In summary, we propose a selective thermal extraction
device that addresses several of the key challenges of TPV
and demonstrates a transfer printing process that is necessary
to fabricate the device. The device has the potential to
achieve simultaneous spectral
fi
ltering and enhancement of
radiative
fl
ux beyond the apparent blackbody limit of the
emitter using thermal extraction. The thermal stability chal-
lenge is eliminated as the device need not be in physical
contact with the emitter. Our work helps one to address chal-
lenges associated with the application of the TPV technology.
ACKNOWLEDGMENTS
This work is part of the
“
Light-Material Interactions in
Energy Conversion
”
Energy Frontier Research Center funded
by the U.S. Department of Energy, Of
fi
ce of Science, Of
fi
ce
of Basic Energy Sciences under Award No. DE-SC0001293.
The authors would also like to recognize the Kavli
NanoScience Institute at Caltech and Professor George R.
Rossman and Dr. Alireza Ghaffari of Caltech for the use of
their facilities for fabrication and testing.
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