high-temperature selective emitter for thermophotovoltaic ... · 2 o 3 spacer – al 2 o 3 and pt:...

Physical

Sciences Inc.

Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

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

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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.

Physical Sciences Inc. ‒ Who we are

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

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Significant efforts in

developing photonics-

based technologies and

devices

Sensors: RMLD, TLDAS,

QCL systems

AIRIS, LIDAR

Thermophotovoltaics


Thermophotovoltaics Overview

1980s

– Very high

temperature

emitters

• Rare earth

oxides,

SiC, etc

Emitter PV Cell Heat in Electrical

Power OUT Radiation

Concentrated

solar energy,

combustion

source

Blackbody,

greybody, modified

emissive surface

Silicon

Germanium,

III-Vs

1990s

– Low bandgap

materials

• Ge

• InGaAs,

Sb-based

materials

2000s

– 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

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Filt

er

Dielectric filters,

Plasma (TCO

filters)


TPV Energy Conversion: Model

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Assume InGaAs 0.6 eV TPV cell, 1300K blackbody

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

Black Body Power Spectrum

𝑃 =2ℎ𝑐2

𝜆5

1

𝑒ℎ𝑐

𝜆𝑘𝐵𝑇 − 1

Black Body Photon Density

Spectrum

𝑛𝐵𝑏 =2𝑐

𝜆4

1

𝑒ℎ𝑐

𝜆𝑘𝐵𝑇 − 1

𝐼𝑆𝐶

𝐼𝑚

𝑉𝑂𝐶 𝑉𝑚

𝐹𝐹 =𝐼𝑚𝑉𝑚

𝐼𝑆𝐶𝑉𝑂𝐶

PV Cell Fill Factor


Model with Ideal Selective Emitter

Prad = emitter spectrum x

blackbody power

spectrum

Pout = emitter spectrum x

blackbody photon

density (norm) x EQE

hTPV = magenta area / cyan

area

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Only emits where EQE of TPV cell is near unity

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


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

Fabricated via e-beam

lithography + e-beam

evaporation

Selective Emitter Design

Pt

Al203 h

t

p

h

w

l

Woolf et al., APL105, 081110

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Fabricated Structures

Fabrication procedure Optical image of

Fabricated Structure

– Higher order absorption

resonances give each array

distinct color


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400 nm p 600 nm

25

0 n

m

w 3

00

nm

150 nm l 250 nm

500 𝜇𝑚

Sapphire Wafer

E-beam evaporate

Pt and Al2O3

Spin lift-off resist

and e-beam resist

Develop

e-beam resist

Undercut

lift-off resist

Deposit Pt

Write pattern

Remove resists


Fabricated Structures

Fabrication procedure SEM image of

Fabricated Structure

– SEM has resolution of ~ 20 nm


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Sapphire Wafer

E-beam evaporate

Pt and Al2O3


and e-beam resist

Develop

e-beam resist

Undercut

lift-off resist

Deposit Pt

Write pattern

Remove resists


Thermal Testing at 1300 K

SEM Images

Heat sample in RTA in 1 atm of Argon, hold for 2 min

Pt/AlO thin films survive (no delamination)

Metal pattern on surface deforms

– Due to interfacial stress

Before Heating After Heating


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Thermal Testing at 1300 K

Optical Images

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


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Spectral Emission

after Heating at 1300K

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)

Heat cycle at 1300 K 2 min cycle

10 min cycle

Pre-heat cycle


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Emitter Stabilization

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

• more thermally stable

configuration for micro-structures


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500 nm

Pre-thermal cycling

Post-thermal cycling


Emitter Encapsulation

Optical Images

Before

heating

After 2 min

at 1000°C

After 2 + 5 min

heating cycles

After 2+5+5 min

heating cycles


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Encapsulated Emitter

Thermal testing at 1300 K

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

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Encapsulation layer broadens resonance

What is the expected TPV power and efficiency using this emitter?

Heat cycle at 1300 K

2 + 5 + 5 min cycles

2 min cycle

Pre-heat cycle



Selective Emitter Predicted Performance

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%


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Selective Emitter Predicted Performance

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%


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Minimal benefit from using more

exotic TPV materials due to

worsening dark current, EQE


E-beam lithography is not scalable

– Nano-imprint, interference lithography

• Not mature

– Stepper projection lithography

• Commercially viable

Fabrication steps using deep UV stepper photolithography

Large-area Emitter Fabrication

Prepared Substrate

Spin on Antireflection + Photoresist

UV expose

DevelopDeposit MetalLift Off

Mask

– Resolution limit ~ 200nm

– compared to ~20nm resolution

for e-beam used in P1 base

Need to verify that performance

still okay with 10x resolution

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Large-area Fabrication via Stepper Lithography

330nm

Pt

Al203

600 nm

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Preliminary demonstration of

large-area fabrication using

conventional lithography

methods

Puck geometry can produce

spectra equivalent to cross

geometry spectra


Conclusions and Outlook

Fabricated a heterogeneous metasurface capable of surviving

repeated temperature cycling to 1300 K

Measured metasurface reflectivity, used to estimate thermal-to-

electrical 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

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Physical

Sciences Inc.


Thank you.

Questions?

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19

Physical

Sciences Inc.


Backup Slides

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What We Do

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


Pre-production manufacturing process development


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

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TPV Converter Cell Model Concept

𝑃𝑟𝑎𝑑

Heat Source

Radiated power from combustion

Selective Emitter

Reradiated power

Reflected power

Below bandgap absorption

Reradiated power

TPV cell Generated

power

𝑃𝑜𝑢𝑡

>BG

<BG

Electrical Losses

Not included in model:

– Temperature rise of TPV

(assume perfect heatsinking)

– Above bandgap thermalization

– Below bandgap absorption

– Electrical losses

Goal of TPV model is to calculate:

– Electrical output power: 𝑃𝑜𝑢𝑡

– TPV efficiency: 𝜂𝑇𝑃𝑉 =𝑃𝑜𝑢𝑡

𝑃𝑟𝑎𝑑

– TPV spectral efficiency: 𝜂𝑠𝑝𝑒𝑐 =𝑁𝑎𝑏𝑠

𝑁𝑟𝑎𝑑

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Experimental Results - Fabrication

Fabrication process SEM Image

Sapphire Wafer

E-beam evaporate

Pt and Al2O3


and e-beam

photoresist

Develop

e-beam photoresist

Undercut

lift-off resist

Deposit Pt

Write pattern

Remove resists

using acetone


Motivation

Need a higher energy-density source for remote energy

generation

Combined heat and power (CHP) potential.

10% total efficiency TPV beats battery by factor

0.1

1

10

100

1000

0.288 0.875

26.4 44.4 48 53.6 142

Energy Density (MJ/kg)

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1 2 3 4 5

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6 7 8 9 10

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11 12 13 14 15

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16 17 18 19 20

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23 21 22 24 25

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Experiments vs Theory

Measurements taken using FTIR

– Unity absorption

on resonance

– FWHM ~ 1um

– Tunable

– Matches

simulations


Acknowledgements

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

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high-temperature selective emitter for thermophotovoltaic ... · 2 o 3 spacer – al 2 o 3 and pt:...

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