Physical
Sciences Inc.
VG14-148
High-temperature Selective Emitter for
Thermophotovoltaic Energy Conversion
David Woolf and Joel Hensley
Physical Sciences Inc., Andover, MA
Jeff Cederberg and Eric A. Shaner
Sandia National Laboratories
OSA Incubator on the Fundamental Limits of
Optical Energy Conversion
12-14 November 2014
Acknowledgement of Support and Disclaimer
This material is based upon work supported by the Office of Naval Research under Contract Number_N00017-13-P-1190. Any opinions, findings and conclusions
or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research.
Physical Sciences Inc.
20 New England Business Center
Andover, MA 01810
0
Physical Sciences Inc. ‒ Who we are
Physical Sciences Inc.
VG14-148 -1

A growing 41 year-old company of 180
talented scientists, engineers and
administrative personnel

PSI is headquartered in Andover, MA, with
operations in Bedford, MA; Dayton, OH;
Lanham, MD; Princeton, NJ and Pleasanton,
CA

PSI companies FY2014 revenues of >$40M

Q-Peak manufactures lasers and optical
devices

Research Support Instruments supports
space ops

Faraday Technology develops industrial
processes

Multiple commercial spin-outs

PSI is a 100% employee owned company
 Significant efforts in
developing photonicsbased technologies and
devices
 Sensors: RMLD, TLDAS,
QCL systems
 AIRIS, LIDAR
 Thermophotovoltaics
Thermophotovoltaics Overview
Physical Sciences Inc.
Emitter
Heat in
Blackbody,
greybody, modified
emissive surface
Concentrated
solar energy,
combustion
source
1980s
–
Very high
temperature
emitters
• Rare earth
oxides,
SiC, etc
1990s
–
Radiation
Low bandgap
materials
• Ge
• InGaAs,
Sb-based
materials
PV Cell
Electrical
Power OUT
Dielectric filters, Silicon
Plasma (TCO Germanium,
filters) III-Vs
2000s
–
Filter
VG14-148 -2
Breakthrough in
spectrally selective
materials
• Plasmonics
• Metamaterials
Now
 Can we make a selective emitter that:
 Survives T > 1300 K
 Survives repeated thermal cycling
 Operates in ambient atmosphere
 Has non-directional (Lambertian)
emission
 Matches well with PV EQE
TPV Energy Conversion: Model
Physical Sciences Inc.
VG14-148 -3
 Assume InGaAs 0.6 eV TPV cell, 1300K blackbody
Black Body Power Spectrum
2ℎ𝑐 2
1
𝑃= 5
ℎ𝑐
𝜆
𝑒 𝜆𝑘𝐵 𝑇 − 1
Black Body Photon Density
Spectrum
2𝑐
1
𝑛𝐵𝑏 = 4 ℎ𝑐
𝜆 𝜆𝑘 𝑇
𝑒 𝐵 −1
PV Cell Fill Factor
 Most power, photons are below band-gap
– useless if transmitted, increases TPV heating if
absorbed
 Predicts hTPV = 8.25%, Pout = 1.28 W/cm2
 Can see hTPV as PPV/Prad
𝐼𝑆𝐶
𝐼𝑚
𝐹𝐹 =
𝑉𝑚
𝐼𝑚 𝑉𝑚
𝐼𝑆𝐶 𝑉𝑂𝐶
𝑉𝑂𝐶
Model with Ideal Selective Emitter
Physical Sciences Inc.
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 Only emits where EQE of TPV cell is near unity
Prad = emitter spectrum x
blackbody power
spectrum
Pout = emitter spectrum x
blackbody photon
density (norm) x EQE
hTPV = magenta area / cyan
area
 Model predicts hTPV = 39.2% at 1300 K
– 475% increase in efficiency compared to black-body radiation
– Want actual emitter to approximate this ideal emitter
Selective Emitter Design
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 Metamaterial emitter consists of
a thin-film Pt cross above a Pt
backplane
– Sapphire substrate, Al2O3 spacer
– Al2O3 and Pt:
• Stable in atmosphere
• Matched CTE up to ~ 1500 K
 Used Lumerical FDTD to
determine geometric parameters:
– Spacer (h ≈ 90 nm)
– Pt cross (t ≈ 45 nm)
– p ≈ 550 nm, w ≈ 275 nm, l ≈ 200 nm
t
h
Al203
Pt
p
w
l
h
 Fabricated via e-beam
lithography + e-beam
evaporation
Woolf et al., APL105, 081110
Fabricated Structures
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VG14-148 -6
Fabrication procedure
Sapphire Wafer
150 nm
400 nm
l
p
250 nm
600 nm
250 nm
E-beam evaporate
Pt and Al2O3
Optical image of
Fabricated Structure
Spin lift-off resist
and e-beam resist
500 𝜇𝑚
Write pattern
w
Develop
e-beam resist
Deposit Pt
– Higher order absorption
resonances give each array
distinct color
300 nm
Undercut
lift-off resist
Remove resists
Woolf et al., APL105, 081110
Fabricated Structures
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VG14-148 -7
Fabrication procedure
Sapphire Wafer
SEM image of
Fabricated Structure
E-beam evaporate
Pt and Al2O3
Spin lift-off resist
and e-beam resist
Write pattern
Develop
e-beam resist
Undercut
lift-off resist
– SEM has resolution of ~ 20 nm
Deposit Pt
Remove resists
Woolf et al., APL105, 081110
Physical Sciences Inc.
Thermal Testing at 1300 K
SEM Images
VG14-148 -8
 Heat sample in RTA in 1 atm of Argon, hold for 2 min
Before Heating
After Heating
 Pt/AlO thin films survive (no delamination)
 Metal pattern on surface deforms
– Due to interfacial stress
Woolf et al., APL105, 081110
Physical Sciences Inc.
Thermal Testing at 1300 K
Optical Images
VG14-148 -9
 Heat sample in RTA in 1 atm of Argon, hold for 2 min
Before Heating
After Heating
 Visible frequency color change indicates morphological pattern
change
Woolf et al., APL105, 081110
Physical Sciences Inc.
Spectral Emission
after Heating at 1300K
VG14-148 -10
Heat cycle at 1300 K
2 min cycle
10 min cycle
Pre-heat cycle
 Spectral shift happens in first 2 minutes then remains static
through additional heating cycles
 Could redesign emitter to optimize post-anneal geometry
– Lose some tuning parameters (cross to square shape)
– Absorption feature narrows (not good for matching TPV EQE)
Woolf et al., APL105, 081110
Emitter Stabilization
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 Use encapsulation to stabilize
cross pattern
 Deposit ~150 nm of Al2O3 using
Atomic Layer Deposition (ALD)
on top of structure
– ALD chosen because it is more
conformal than sputtering
– Encapsulating material same as
dielectric spacer
Pre-thermal cycling
Post-thermal cycling
• more thermally stable
configuration for micro-structures
500 nm
Woolf et al., APL105, 081110
Physical Sciences Inc.
Emitter Encapsulation
Optical Images
VG14-148 -12
Before
heating
After 2 min
at 1000°C
After 2 + 5 min
heating cycles
After 2+5+5 min
heating cycles
Woolf et al., APL105, 081110
Physical Sciences Inc.
Encapsulated Emitter
Thermal testing at 1300 K
VG14-148 -13
 Minimal spectral
effect due to heating
– Slight shift in
spectrum in first
heating cycle
• Densification of
Al2O3
– Remains constant
through 2, 5, 5 minute
thermal cycles
Heat cycle at 1300 K
2 + 5 + 5 min cycles
2 min cycle
Pre-heat cycle
 Encapsulation layer broadens resonance
What is the expected TPV power and efficiency using this emitter?
Woolf et al., APL105, 081110
Selective Emitter Predicted Performance
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VG14-148 -14
 Selective emitter boosts
TPV conversion efficiency
to 22% at 1300 K
 from 8.5% with no
selective emitter
 1.2 W/cm2 out
 27% at 1500 K
 3 W/cm2 out
 With cold side filter,
efficiency can be
improved to ~40%
Woolf et al., APL105, 081110
Selective Emitter Predicted Performance
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 Selective emitter boosts
TPV conversion efficiency
to 22% at 1300 K
 from 8.5% with no
selective emitter
 1.2 W/cm2 out
 27% at 1500 K
 3 W/cm2 out
 With cold side filter,
efficiency can be
improved to ~40%

Minimal benefit from using more
exotic TPV materials due to
worsening dark current, EQE
Woolf et al., APL105, 081110
Large-area Emitter Fabrication
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 E-beam lithography is not scalable
– Nano-imprint, interference lithography
• Not mature
– Stepper projection lithography
• Commercially viable
 Fabrication steps using deep UV stepper photolithography
Mask
Prepared
Substrate
Lift Off
Spin on
Antireflection
+ Photoresist
Deposit Metal
UV expose
Develop
– Resolution limit ~ 200nm
– compared to ~20nm resolution
for e-beam used in P1 base
 Need to verify that performance
still okay with 10x resolution
Large-area Fabrication via Stepper Lithography
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 Preliminary demonstration of
large-area fabrication using
conventional lithography
methods
 Puck geometry can produce
spectra equivalent to cross
geometry spectra
600 nm
Pt
330nm
Al203
Conclusions and Outlook
Physical Sciences Inc.
VG14-148 -18
 Fabricated a heterogeneous metasurface capable of surviving
repeated temperature cycling to 1300 K
 Measured metasurface reflectivity, used to estimate thermal-toelectrical energy conversion efficiency
 Demonstrated large scale fabrication using conventional
lithography
 Suitable for TPV or Solar TPV applications
 TPV is rapidly maturing due to innovations in high-temperature
emitters
– Applications in remote energy generation and combined heat and power
Physical
Sciences Inc.
VG14-148
Thank you.
Questions?
Physical Sciences Inc.
20 New England Business Center
Andover, MA 01810
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Physical
Sciences Inc.
VG14-148
Backup Slides
Physical Sciences Inc.
20 New England Business Center
Andover, MA 01810
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What We Do
Physical Sciences Inc.
VG14-148 -21

Applied research and development for all major
agencies of the U.S. government
– ~ 60% FY 10 revenue

Technology transition and product development for
government and industrial customers
– ~ 15% FY 10 revenue

Pre-production manufacturing process development
– ~ 5% FY 10 revenue

Components, systems, and instrumentation for
industry and government sales
– ~20% FY 10 revenue

Technology and product licensing to strategic
partners and spin-outs for high-volume commercial
markets
– ~ 2% FY 10 revenue from royalties
TPV Converter Cell Model Concept
Physical Sciences Inc.
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TPV cell
Reflected
power
>BG
𝑃𝑟𝑎𝑑
Heat
Source
Reradiated
power
Radiated
power from
combustion
Electrical
Losses
<BG
Reradiated
power
 Goal of TPV model is to calculate:
– TPV efficiency:
𝑃𝑜𝑢𝑡
Below bandgap
absorption
Selective
Emitter
– Electrical output power:
Generated
power
𝑃𝑜𝑢𝑡
𝑃
𝜂 𝑇𝑃𝑉 = 𝑃𝑜𝑢𝑡
𝑟𝑎𝑑
𝑁
– TPV spectral efficiency: 𝜂𝑠𝑝𝑒𝑐 = 𝑁 𝑎𝑏𝑠
𝑟𝑎𝑑
 Not included in model:
– Temperature rise of TPV
(assume perfect heatsinking)
– Above bandgap thermalization
– Below bandgap absorption
– Electrical losses
Experimental Results - Fabrication
Physical Sciences Inc.
Fabrication process
Sapphire Wafer
E-beam evaporate
Pt and Al2O3
Spin lift-off resist
and e-beam
photoresist
Write pattern
Develop
e-beam photoresist
Undercut
lift-off resist
Deposit Pt
Remove resists
using acetone
SEM Image
Motivation
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 Need a higher energy-density source for remote energy
generation
Energy Density (MJ/kg)
1000
26.4 44.4
100
10
1
0.288
48
53.6
0.875
0.1
 Combined heat and power (CHP) potential.
 10% total efficiency TPV beats battery by factor
142
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Experiments vs Theory
Physical Sciences Inc.
 Measurements taken using FTIR
– Unity absorption
on resonance
– FWHM ~ 1um
– Tunable
– Matches
simulations
Acknowledgements
Physical Sciences Inc.
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Team
PSI
 Dr. David Woolf
 Dr. Joel Hensley
Sandia
 Dr. Eric Shaner
 Dr. Jeff Cederberg
 Albert Grine
 Don Bethke
Funding
ONR N00014-13-P-1190