Toward high-energy-density, high-efficiency, and
moderate-temperature chip-scale thermophotovoltaics
Walker R. Chana,b, Peter Bermela,b,c,d, Robert C. N. Pilawa-Podgurskie, Christopher H. Martonf, Klavs F. Jensenf,
Jay J. Senkevichb, John D. Joannopoulosa,b,c,d,1, Marin Solja? ci? ca,b,c,d, and Ivan Celanovicb,1
aResearch Laboratory of Electronics,bInstitute for Soldier Nanotechnologies,cDepartment of Physics,dCenter for Materials Science and Engineering,
andfDepartment of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; andeDepartment of Electrical
and Computer Engineering, University of Illinois at Urbana–Champaign, Urbana, IL 61801
Contributed by John D. Joannopoulos, January 24, 2013 (sent for review November 27, 2012)
and small-scale portable power generation is addressed here
using a distinctive thermophotovoltaic energy conversion mecha-
nism and chip-based system design, which we name the micro-
thermophotovoltaic (μTPV) generator. The approach is predicted to
be capable of up to 32% efficient heat-to-electricity conversion
within a millimeter-scale form factor. Although considerable tech-
nological barriers need to be overcome to reach full performance,
we have performed a robust experimental demonstration that val-
idates the theoretical framework and the key system components.
Even with a much-simplified μTPV system design with theoretical
efficiency prediction of 2.7%, we experimentally demonstrate
2.5% efficiency. The μTPV experimental system that was built
and tested comprises a silicon propane microcombustor, an inte-
grated high-temperature photonic crystal selective thermal emit-
ter, four 0.55-eV GaInAsSb thermophotovoltaicdiodes,andanultra-
verter. The system was demonstrated to operate up to 800 °C (silicon
generating 344 mW of electric power over a 1-cm2area.
catalytic combustion|micro generator|thermal radiation
clean, quiet, and portable high-energy-density, compact power
sources. Although batteries offer a well-known solution, limits on
the chemistry developed to date constrain the energy density
to ∼0.2 kWh/kg, whereas many hydrocarbon fuels have energy
densities closer to 12 kWh/kg. The fundamental question is, How
converted into electricity in a millimeter-scale system? Indeed, it
is difficult to tap the full potential of hydrocarbon fuels on a small
scale. However, their high energy density allows even relatively
inefficientgenerators tobecompetitivewith batteries. To this end,
as mechanical heat engines (1), fuel cells (2, 3), thermoelectrics
(4, 5), and thermophotovoltaics (TPVs) (6, 7).
TPVs present an extremely appealing approach for small-scale
power sources due to the combination of high power density lim-
ited ultimately by Planck blackbody emission, multifuel operation
due to the ease of generating heat, and a fully static conversion
process. Small-scale TPVs have yet to be demonstrated and are
particularly challenging because of the need to develop strong
synergistic interactions between chemical, thermal, optical, and
electrical domains, which in turn give rise to requirements for ex-
treme materials performance and subsystems synchronization.
In this work, we present a proof of concept microthermovoltaic
(μTPV) system, shown in Fig. 1, that validates the theoretical
foundation and paves the way toward a new breed of ultra-high-
energy-density, high-efficiency, propane-fueled, chip-scale power
sources. Specifically, our system comprises a catalytic microcom-
bustor with a high-temperature photonic crystal for efficient
conversion of heat into spectrally confined thermal radiation,
optically coupled to low-bandgap photovoltaic (PV) diodes that
are electrically interfaced with a unique ultra-low-power, on-chip
power electronics converter, providing an optimal interface to
external electrical loads.
Each of these four key components can be optimized for maxi-
mum performance with various degrees of complexity and diffi-
culty. For simplicity, we shall begin our discussion with a simple
and easilyrealizable μTPVmaterials system. A general theoretical
formalism is then introduced that can accurately model these
components, explore how they can work together, and provide
optimized design within a constrained geometric and materials
space. This also helps provide experimental validation of this ap-
proach. Indeed, as we shall see, the predicted efficiency of this
simple μTPV system is 2.7%, whereas our results give 2.5%. Fi-
nally, the theoretical framework will be used to present a detailed
optimized design using advanced material systems, system geom-
etry, and PV cells. This optimized theoretical design boasts effi-
ciencies that exceed 30% heat-to-electricity conversion.
Simple Silicon μTPV
The starting point for any portable TPV system is a compact
mechanism for generating heat. Our basic μTPV, shown in Fig. 1,
catalytically combusts hydrocarbon fuel (i.e., propane) inside
a catalyst-coated Si microchannel structure. The microreactor is
designed to minimize nonradiative thermal losses by suspending it
with glass capillary tubes that double as fluidic connections and
vacuum packaging. In this design we use pure oxygen, instead of
air, for the chemical reaction, to simplify the test and character-
ization. Thermal energy generated from chemical reaction inside
the reactor heats up the Si and is consequently converted into
spectrally confined radiative heat by a one-dimensional (1D) Si/
To convert thermal radiation into electricity, we use low-
grown by metalorganic vapor phase epitaxy (MOVPE), with
a bandgap of 0.547 eV. They consist of a 1-μm n-GaInAsSb base,
a 4-μm p-GaInAsSb emitter, an AlGaAsSb window layer, and
design and performance parameters are given in refs. 8–10.
In the final conversion step, the raw output of the TPV cell is
dynamically converted into useful current and voltage levels via
a low-power power electronics converter known as the maximum
power-point tracker (MPPT). This step is important because the
generator operating conditions (i.e., incident irradiation and the
cell junction temperature) can vary over time and must therefore
be continuously tracked to ensure that maximum power is
extracted from the cells at all times. The extraordinarily small
Author contributions: W.R.C., P.B., R.C.N.P.-P., C.H.M., K.F.J., J.J.S., J.D.J., M.S., and I.C.
designed research, performed research, analyzed data, and wrote the paper.
The authors declare no conflict of interest.
Freely available online through the PNAS open access option.
1To whom correspondence may be addressed. E-mail: firstname.lastname@example.org or email@example.com.
| April 2, 2013
| vol. 110
| no. 14
size of the overall system in this work makes it particularly dif-
ficult to ensure perfect TPV cell-current matching, owing to
cavity reflections and uneven irradiation across the different
cells. Unfortunately, any irradiation mismatch between cells will
limit the current through a series-connected string of cells to that
of the weakest cell, causing a lower output power than what is
theoretically possible. Ref. 11 describes a method that employs
distributed intelligent power electronics to mitigate the effects of
cell mismatch. By using multiple low-power, low-voltage MPPTs,
we individually control each section of the cell array to operate
at its most efficient point. The distributed MPPT architecture
optimizes the electrical output of the TPV cells in real time to
continuously track the maximum power point, irrespective of
uneven irradiation and any changes in the heat-source operation.
Converging the four coupled energy conversion technologies
(microreactor, selective thermal emitter, low-bandgap PV diode,
and low-power power electronics converter) and achieving high
overall system performance requires accurate system-level mod-
eling (including ab initio calculations for each component), fol-
lowed by multidimensional constrained global optimization of the
design parameters. We modeled the microreactor with custom
heat transfer code incorporating conduction, convection, and ra-
diative heat transfer between surfaces and into the ambient. We
account for radiation from the two sides facing the cells and the
the hot exhaust gases. We use the transfer matrix method (12, 13)
tocalculate theangular-dependentemissivity fora givenstructure,
as discussed in ref. 14. Then we use ray optics to accurately in-
corporate multiple scattering effects, as discussed in ref. 15. Our
heat transfer and diode modeling codes discussed elsewhere were
used to calculate electrical power. The operating temperature is
calculated using a self-consistent nonlinear Newton–Raphson
thermal radiationof photons below the electronic bandgap of the
TPV diode. Recent literature points to the potential for spectral
shaping and photon recycling to dramatically reduce the losses
associated with below-bandgap photons (14, 16–25), enabled by
photonic crystals (26, 27). To explore some design limits on the
assuming maximally efficient recuperators and a view factor of
100%. They consist of a baseline design with a homogeneous
crystal design. The photonic crystal emitter shapes the radiated
spectrum, maximizing the efficiency by suppressing unconvertible
radiation. In addition, the photonic crystal (PhC) emitter enables
the use of a lower-temperature (700–1,100 °C) emitter, resulting in
reduced thermal stresses, a larger spectrum of available materials,
and better material stability, thus extending the possible design
space. The photonic crystal design parameters were chosen via
constrained global optimization of the figure of merit, as described
in ref. 14. The results that show output power as a function ofinput
of microreactor surface temperature is given in Fig. 2B. It can be
the 1D metallic photonic crystal exceeds that of the uniform grey-
body Si emitter by approximately a factor of 3, as shown in Fig. 2.
This alone demonstrates the importance of photonic crystals as an
enabling technology for μTPV and TPV systems in general.
As we shall see below, high-temperature material stability and
reactivity of the Si reactor prompt the use of a Si-based photonic
crystal thermal emitter.
Experimental Results for Two Silicon Reactor Designs
The theoretical formalism developed can accurately predict the
losses for a simple μTPV system comprising a greybody Si emitter
operating at ∼700 °C with a view factor of 70% and a GaInAsSb
cell mentioned above, as in ref. 10. Approximately 12% losses
come from heat exhaust in the absence of a recuperator (when
oxygen is used as oxidizer instead of air; with air as oxidizer these
side; 60% from low-energy photons that cannot be converted into
electricity; and 3% from the operation of the thermophotovoltaic
cell, including shadowing, open-circuit voltage degradation, hot-
carrier thermalization, and radiative recombination at the maxi-
mum power point. Thus, this straightforward design will only
results further discussed below, and also shown in Fig. 3. It should
be emphasized that this result is obtained without any need for
is clear that the most important area for improvement is in re-
ducing the energy emitted as low-energy photons via photonic
crystals. Other important changes include adding a recuperator to
recover thermal power from the exhaust and increasing the view
factor to eliminate losses off to the sides.
The theoretical formalism can also be used to further optimize
the parameters of the simple Si μTPV system with the constraint
that only Si and SiO2are used as materials for 1D selective
emitter design on the surface of the reactor. Indeed, a 1D pho-
tonic crystal consisting of five alternating layers of Si and SiO2is
chosen for its ease of fabrication and high-temperature compat-
ibility with the Si microreactors. In Fig. 4 we show the thermal
emission results of 1D photonic crystal consisting of five alter-
nating layers of Si and SiO2, with a total thickness of ∼2 μm. The
materials were chosen for ease of fabrication and compatibility
with the Si microreactor. We measured the thermal emission of
the photonic crystal by electrically heating it and measuring the
emitted spectrum by FTIR spectroscopy. The three spectra shown
in Fig. 4 exhibit good agreement between theory and experiment
a microreactor to catalytically combust propane; the heat is quickly con-
ducted to the photonic crystal emitter at the surface, which selectively emits
thermal photons, across a gap, toward a target TPV cell. The raw electrical
output is then optimized and regulated by our maximum power-point
tracker. (Upper Right) Labeled CAD model of our experimental μTPV system.
(Lower Right) Photograph of our experimental μTPV in operation.
(Left) General operation of a fuel-based μTPV system. It uses
| www.pnas.org/cgi/doi/10.1073/pnas.1301004110Chan et al.
(simulation results took into account temperature-dependent
dielectric functions of Si and SiO2). We used high-performance
TPV cells made from Ga1−xInxAs1−ySby(x = 0.15, y = 0.12), grown
by MOVPE, with a bandgap of 0.547 eV. They consist of a 1-
μm n-GaInAsSb base, a 4-μm p-GaInAsSb emitter, a AlGaAsSb
window layer, and a GaSb contact layer on an n-GaSb substrate.
The measured external quantum efficiency (ratio of electron-hole
generation to incident photons) of the cells with a four-layer an-
tireflective coating is plotted in Fig. 4.
In Fig. 3 we show our results for the complete system. Our
numerical model predictions are shown as solid lines, and the
corresponding experimental data points are shown as symbols with
the same color as the predictive curve. The excellent point-by-
point match between the two curves obtained without curve fitting
validates our model and provides confidence in our theoretical
formalism. For the experimental results we tested the complete
system by first igniting the microreactor by hydrogen-assisted
combustion of propane in oxygen until a temperature of 400 °C
was reached. Above that temperature, the propane kinetics over
the catalyst were sufficient for autothermal operation, and the
hydrogen flow was shut off. The propane and oxygen flows were
gradually increased in small increments, with 1.5 times the stoi-
chiometric amount of oxygen required for complete com-
bustion. We plot the steady-state electrical power at the maximum
power point versus propane flow in Fig. 3. The propane flow was
converted to watts using the lower heating value. We tested the
system both with a simple Si reactor and a Si reactor with a
photonic crystal. The simple Si-based system (without selective
emitter) generated a maximum 218 mW of electricity with 13.7 W
of propane flow, at an efficiency of 1.6% at 740 °C. Note that at
a lower temperature (or flow) the efficiency is reduced; for ex-
ample, at ∼700 °C the efficiency goes down to 1.2%. The μTPV
system with a 1D Si/SiO2photonic crystal generated 344 mW of
electricity under the same conditions at an efficiency of 2.5%,
whereas the predicted efficiency was 2.7%. Furthermore, accord-
ing to our model, the thermal losses of our system under typical
operating conditions are distributed as follows: 10% exhaust,
40% radiation loss off the edges of the microreactor or the sides
of the vacuum gap, and 50% dissipation in the TPV cells due to
below-bandgap radiation, thermalization, and electrical losses.
The fabrication and testing procedures used to obtain these
results are outlined in Materials and Methods.
In Fig. 5 we show the results for an idealized μTPV system with
perfectly reflective microburner sides, a 2D tungsten PhC slab
consisting of a square lattice of air holes, and multilayer dielectric
filter (14) in front of the PV cell, and a perfect recuperator. In Fig.
5, we plot the efficiency as a function of operating temperature.
The results are very striking: 20% efficiency is possible around
700 °C and over 30% efficiency above 1,000 °C.
To realize a microreactor with metal-coated edges, a high-tem-
perature, highly reflective metallic coating, compatible with a Si
path would be to design and fabricate the microreactor completely
out of metal, thus inherently leading to low-emissivity side walls.
Use of metallic photonic crystals as selective thermal emitters
could possibly be accomplished via either thin-film deposition
Output Power (W)
Input Power (W)
Plain Si emitter
Metal 1D PhC
700 800 9001000
Plain Si emitter
Metal 1D PhC
timized 1D metallodielectric PhC in red. (B) Efficiency as a function of microreactor temperature for μTPV system designs with two types of emitters: Si
greybody in blue, and optimized 1D metallodielectric PhC in red. Note that for a constant input power, the operating temperature of the plain emitter is
lower than that of the 1D PhC. For example, the input power of the plain emitter at 700 °C corresponds to that of the 1D PhC at 830 °C.
(A) Electrical power output as a function of chemical input power of μTPV system designs with two types of emitters: Si greybody in blue and op-
two emitters: bare Si and 1D Si/SiO2photonic crystal. Calculated microreactor
temperatures are labeled.
Theoretical and experimental performance of our TPV system with
Chan et al.PNAS
| April 2, 2013
| vol. 110
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and etching in a Si platform or by building the reactor out of
high-temperature metal. Use of higher-dimensional photonic
crystals (2D and 3D) could be accomplished via layer-by-layer
microfabrication techniques onto a Si or metallic reactor (26).
To incorporate selective filters into a μTPV generator, precise
and robust thin-film deposition and packaging processes will be
needed that will allow for direct deposition of filters onto the
front surface of the PV cell. A smaller distance gap between the
microreactor and the PV cell (close to unity view factor) could be
achieved with a new mechanical design and assembly approach.
Finally, high-efficiency, small-scale recuperators will need to be
integrated into the reactor platform as an additional subsystem.
Conclusion and Summary
In conclusion, with our prototype μTPV system, and accurate
system-level modeling verified by experiment, we have demon-
strated the intriguing potential of μTPV technology. Based on the
experimental fuel flow rate and calculated limit of 32% efficiency
for our millimeter-scale form factor, and adjusting for likely real-
world losses, it should be possible to create compact generators
with electrical outputs from milliwatts to few tens of watts (elec-
trical power) with power densities exceeding 0.5 W/cm2and energy
densities over 3 kWh/kg. Enabling technologies required to
achieve this performance include advanced photonic crystal
fabrication, higher-performance low-bandgap PV cells, vacuum
packaging, and high-efficiency microreactor and recuperator
designs. Indeed, we believe μTPV generators represent a very
attractive alternative to batteries, portable fuel cells, and ther-
moelectric technologies for portable power generation.
Materials and Methods
Brandon Blackwell (28)and based onprevious workbythesamegroup (29,30).
The microreactor was a 10- × 10- × 1.3-mm Si slab with a 0.40-mm square ser-
pentinechanneldefined bypotassiumhydroxideetching and Sifusionbonding
using standard microfabrication techniques. The channel was coated with
the walls prevents stable homogeneous combustion (31). Catalytic combustion
offers another benefit: The combustion happens at the channel walls, where
heat is directly conducted through the Si to the selective emitter.
The catalyst was applied as a washcoat after the devices were diced but
before the capillary tubes were attached. We used a 20 wt% suspension of
5 wt% platinum on porous alumina (311324; Sigma-Aldrich) in a 2 wt%
solution of a nitrocellulose in amyl acetate. The suspension was sucked
through the channel. After the solvent dried, a thin coating of catalyst was
glued to the walls by the nitrocellulose, which was subsequently burned out.
Nitrocellulose can decompose to gaseous byproducts without air, which is
unavailable deep in the serpentine structure. The catalyst solution and
loading method were tweaked to constantly deposit about 1 mg of catalyst.
An SEM micrograph of the catalyst is shown in Fig. 1 Inset.
The microreactor was supported by two Pyrex capillary tubes that also
served as fluidic connections to the channel to minimize conductive heat loss.
The two 0.55-mm o.d. × 0.40-mm i.d. × 12-mm-long capillaries were her-
metically sealed to the microreactor with a glass solder (SEM-COM SCC-7).
The powdered solder glass was mixed with a solution of 1 wt% nitrocellu-
lose in isophorone and carefully applied to the joint with a fine wire while
the microreactor and tubes were held in a jig. The ratio of powder to solvent
was determined empirically. The firing cycle was based on the manu-
facturer’s recommendations, the literature (32, 33), a differential scanning
calorimetry analysis of the glass solder, and trial and error. The finished
microreactor and tubes were sealed to the vacuum package shown in Fig. 1
with a polyimide adhesive (Imitec).
The microreactor was initially in a setup similar to that shown in Fig. 1
without cells and with an IR window replacing the top cells. We measured
the temperature of a microreactor without a photonic crystal with an IR
thermometer (G5L; Optris) sensitive to 5-μm thermal radiation that was
calibrated with temperature-indicating lacquer (OmegaLaq). The average
surface temperature reached about 800 °C when burning 10 standard cubic
centimeters per minute (sccm) of propane and 75 sccm of oxygen. The
microreactor with the photonic crystal can reach the same temperature with
less fuel consumption owing to lower heat loss, although IR thermometry
was difficult because of the wavelength-dependent emissivity.
The cutoff in the experimental performance for the measured μTPV
generators in Fig. 3 at 1.1 g/h (8 sccm) propane flow was most likely caused
by softening of the joint between the microreactor and the capillary as it
approaches the transition temperature of the glass solder.
In future work, a joint capable of operating at higher temperatures would
be highly desirable. Additionally, a combustor that burns in air rather than
pure oxygen is necessary, because carrying both the fuel and oxidizer goes
against the goal of high energy density. It should be possible to design
a new microreactor that can reach the necessary temperatures with
propane–air combustion. Although burning with air has the disadvantage of
increasing the exhaust enthalpy loss five times at a given microreactor
surface, of the photonic crystal at 820, 900, and 1,020 °C plotted with solid
and dashed lines, respectively. Simulation results of three graybody emitters
with 0.7 emittance (approximation of Si emittance), at 820, 900, and 1,020 °C
are shown with dotted lines. (Lower) Measured external quantum efficiency
(EQE) for the GaInAsSb PV cell with antireflective coating.
(Upper) Measured and simulated thermal emission, normal to the
700 800 9001000
Metal 2D PhC + filter
timized 2D tungsten PhC.
Theoretical limits on performance of μTPV system designs with op-
| www.pnas.org/cgi/doi/10.1073/pnas.1301004110Chan et al.
temperature, using a recuperator to transfer heat from the exhaust to the
incoming air would bring the losses down to a reasonable level.
Photonic Crystal. The polycrystalline Si and SiO2structure arrived at by the
optimization process was deposited by low-pressure and plasma-enhanced
chemical vapor deposition, respectively, directly on the microreactors. The
bonding but before die sawing. As a result, the edges of the microreactors are
uncoated. A SEM micrograph of the structure is in the upper-left inset of Fig. 1.
We measured the thermal emission of the photonic crystal by electrically
and the optics used to convey the light were calibrated with a standard
blackbody source. The measurement was performed at three different power
levels. The temperature of the sample was estimated from a model because it
shown in Fig. 4. The photonic crystal is able to improve efficiency by sup-
pressing radiation between 2.5 and 5.5 μm. Suppressing this radiation ap-
proximately doubles the efficiency of the system as a whole.
TPV Cells. We modeled our cells with an equivalent circuit model with tem-
perature- and illumination-dependent circuit elements. We fit the equivalent
circuit to the cell’s current voltage data. Then we verified the model by illu-
minating the cell with a calibrated blackbody source and comparing the
measured and calculated performance. The modeling is described in ref. 10.
The cells were mounted to a copper core printed circuit board (Bergquist)
used in the power electronics industry by fluxless indium reflow soldering.
The solder joint was inspected with scanning acoustic microscopy and found
to be free of large voids. The solder joint serves as both a thermal connection
to the substrate and as the negative electrical contact. The positive electrical
contact was made to the bus bar by wire bonding. An SiO2/Ta2O5/Si antire-
flective coating was deposited on the packaged devices. The finished cells
were mounted on temperature-controlled water-cooled blocks above and
below the microreactor in the experimental apparatus in Fig. 1.
Maximum Power-Point Tracker. The prototype MPPT pictured in Fig. 1 was
used to successfully demonstrate the system operation in ref. 11. More recent
work (34) has focused on reducing the overall size of the power electronics to
fit in a small form factor, as well as to increase the electrical conversion ef-
ficiency. Shown in the bottom left corner of Fig. 1 is a die photo of the fully
integrated MPPT developed in a 0.35-μm CMOS process to interface the TPV
system with the load. Table 1 lists the electrical specifications of the MPPT. In
the table, the tracking efficiency is a measure of how close the power con-
verter can operate to the ideal maximum power point.
It should be noted that the low-voltage distributed MPPT architecture de-
veloped for this work is not limited to only this application. Other applications
of this architecture include concentrated solar photovoltaics, thermoelectrics,
and fuel cells.
System Testing. Themicroreactorwasignitedbyhydrogen-assistedcombustion
of propane until a temperature of 400 °C was reached. Above that tempera-
ture, the propane kinetics over the catalyst were sufficient for autothermal
were gradually increased in small increments with our mass flow controllers,
with 1.5 times the stoichiometric amount of oxygen required for complete
combustion. The system was allowed to reach steady state after each flow
increase. At each point, we recorded the input flow and output powers, as
shown in Fig. 3. Output power was found by performing a current-voltage
sweep on a Keithley source meter and calculating the maximum power. Effi-
ciency is defined as the ratio of the input and output powers, where input
power is given by the propane flow rate times the lower heating value.
ACKNOWLEDGMENTS. We thank Dr. W. A. Peters for a critical reading of the
manuscript. This research was supported in part by the US Army Research
Office through the Institute for Soldier Nanotechnologies under Contract
W911NF-07-D-0004. W.R.C., J.J.S., and M.S. were partially supported by the
Massachusetts Institute of Technology Solid State Solar Thermal Energy
Conversion Energy Research Frontier Center of the Department of Energy
under Grant DE-SC0001299.
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Table 1.MPPT specifications
Input voltage range
Output voltage range
Nominal output power
Converter peak efficiency
0.8–1.3 V (1 V nominal)
3.6–4.2 V (4 V nominal)
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