Photonic Crystals:
Shaping the Flow of Thermal Radiation
Ivan Čelanović
Massachusetts Institute of Technology
Cambridge, MA 02139
Overview:
• Thermophotovoltaic (TPV) power generation
• Photonic crystals, design through periodicity
• Tailoring electronic- and photonic bandgap properties:
a path towards record efficiencies
• Photovoltaic module: design and characterization
• TPV system design challenges
• Quasi-coherent thermal radiation via photonic crystals
Thermophotovoltaic power generation:
basic ideas and concepts
Thermo-photo-voltaic conversion
TPV power conversion describes the direct conversion of thermal
radiation into electricity.
Photons
Brief History
1956
- Dr. H. Kolm / Dr. P. Aigrain independently propose TPV power conversion concept
1970’s
- Loss of interest in TPV due to low efficiencies
1990’s
- Advancements in microfabrication technology allow for production of low-bandgap diodes, opening the
door for more efficient TPV
1994
- First NREL Conference on TPV Generation of Electricity
2000’s
-Photonic crystals for thermal radiation control
Basic TPV energy conversion diagram
Blackbody
Radiation
Cell Surface
Reflection
N
GaSb
Heat
Emitter
P
Waste Heat
+
– Pout
PV vs. TPV
PV (Solar Cells) TPV
Properties:
Sensitivity Range
Source
Source
Temperature
Distance from
Source
Visible and NIR
NIR and IR
Sun
Thermal emitter
Over 5000K (sun’s surface) 1000-1500K
Over 90 million miles
Energy reflected
Lost to atmosphere
from cell surface
µm to cm
Recycled to the emitter
Courtesy of DOE/NREL, Credit - Beck Energy.
TPV Technologies and applications
applications
AA radioisotope TPV battery:
~10 mWe
30 years life time
24% efficiency
micro-TPV power generator (propane/butane operated)
1 We
15% efficiency
Plutonium pellet
Courtesy of Klavs Jensen. Used with permission.
Photo courtesy of LLNL.
concentrator
Solar cell
absorber/photonic crystal
Solar TPV
sun radiation
18 W/kg, (PuO2 fuel)
for 30 years
24% efficient
TPhC
Radioisotope TPV power system for
deep space and Mars missions
Images courtesy of NASA.
Photo courtesy of Sandia National Labs.
Thermophotovoltaics: converting thermal radiation into
electricity, with no moving parts
good
photons
thermal emitter
filter
PV diode
bad photons
Heat sink
Heat
Waste
Input
Heat
Si (1.23 eV)
InGaAsSb (0.53 eV)
InGaAs (0.6 eV)
GaSb (0.72 eV)
Photonic Crystals: shaping thermal radiation
1
0.9
Normalized radiated power
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
wavelength [ µm]
6
7
8
TPV Technology roadmap: the time is now
now
Si and Ge
PV
diode
III-V’s (GaSb, InGaAs, GaInAsSb)
rare earth oxides
Spectral
control
Dielectric stack
Photonic
filters
Crystals
JX
Thermo Power…
System
design
1950’s
1960’s
1970’s
1980’s
1990’s
2000’s
Photonic crystals, design through periodicity
Photonic crystals are periodical structures with
1D, 2D or 3D periodicity
1-D Periodicity
ε ( x , y , z ) = ε ( x + λx , y , z )
2-D Periodicity
ε ( x , y , z ) = ε ( x + λx , y + λ y , z )
3-D Periodicity
ε ( x, y , z ) = ε ( x + λx , y + λ y , z + λz )
Metamaterial:
Low
Lo w inindex
dex
o f re
fra c tio n
of
refraction
Allowed
Forbidden
Allowed
frequency
High
index
Hig h in
dex
o f refraction
re fra c tio n
of
Frequency
optical properties determined from its nano-
structure
(rather than its composition)
3D photonic crystal:
a “semiconductor for photons”
Refs:
E.Yablonovitch, Phys. Rev. Lett. 58, 2059, (1987).
S.John, Phys. Rev. Lett. 58, 2486, (1987).
wavevector
Controlling density of photonic states
⇓
⇓
controlling thermal emission spectrum


 hω

u (ω , T ) = N (ω ) ∗
 hω

k
T

e
− 1 
B
density of
photonic
modes
energy in each
photonic mode
frequency
energy
density
wavevector
Photonic crystals are analogous to semiconductors
Face center cubic lattice
E
E
allowed
states
Conduction
band
forbidden
states
electronic
bandgap
Eg
allowed
states
k
valence
band
k
Naturally occurring photonic crystals:
Butterfly wings
Opal
Photo by Megan McCarty at Wikimedia Commons.
Images removed due to copyright restrictions.
Please see: http://www.tils-ttr.org/photos/Mitoura-gryMDneo.jpg
http://www.tils-ttr.org/photos/Mitoura-gryMVneo.jpg
Fig. 11 in Ghiradella, Helen. "Light and Color on the Wing: Structural Colors in Butterflies and Moths."
Applied Optics 30 (1991): 3492-3500.
Fig. S1a, S2, and S4a in Vukusic, Pete, and Ian Hooper. "Directionally Controlled Fluorescence Emission in Butterflies."
Science 310 (November 18, 2005): 1151.
Fig. 3 in Pendry, J. B. "Photonic Gap Materials." Current Science 76 (May 25, 1999): 1311-1316.
P. Vukusic, I. Hooper, “Directionally controlled fluorescence emission in butterflies,” Science, vol. 310, pp. 1151
Tailoring electronic- and photonic bandgap properties:
a path towards record efficiencies
Photonic crystal as omnidirectional mirror
Selective emitter
Front-side filter
Waste
Heat
Heat
A
B
C
1D Si/SiO2 photonic crystals exhibit omni-directional bandgap
0
filter PV diode Heat sink
thermal emitter
Heat
Waste
1 2
n PV
ε1 ε2 ε3
εn εPV
TPV
θ1
Heat
εo
y
z
0
Projected photonic band diagram
ligh
t li n
e
0.7
0.
7
vac
uum
Angular frequency ω (2πc/a)
0.6
0.
6
0.5
0.
5
0.4
0.
4
0.3
0.
3
Si
C
li
t li
gh
ne
Δω gN
0.2
0.
2
0.1
0.
1
TM modes
TE modes
0
1
0.8
0.6
0.4
0.2
0.
2
0
0.2
0.4
Parallel wave vector ky (2π/a)
(2π/a)
0.6
0.8
1
0
0.2
0.4
0.6
Reflectance
0.8
1
Spectral characterization of 1D photonic crystal
1
m
measured
easured
measured
m
easured
simulated
simulated
0.9
0.8
Transmittance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
500 nm
LPCVD*
TEM cross section of
grown
quarter-wave stack filter with half-layer at the top
0
1
1.5
2
2.5
3
Wavelength (µm)
Si = lighter layers (170nm)
SiO2 = darker layers (390nm)
Image removed due to copyright restrictions.
Please see Fig. S2 in Vukusic, Pete, and Ian Hooper.
"Directionally Controlled Fluorescence Emission in Butterflies."
Science 310 (November 18, 2005): 1151.
Front side PhC designs, 0.72 eV, 0.6 eV, 0.52 eV
11
1D
1D&&plasma
plasma0.52
0.52eV
eV
1D
1Dw/plasma
w/plasma0.6
0.6eV
eV
1DSi/SiO
1DSi/SiO2 0.72
0.72eV
eV
0.9
0.9
2
0.8
0.8
Transmittance
Transmittance
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
00
11
1.5
1.5
22
2.5
2.5
33
3.5
3.5
44
4.5
4.5
55
wavelength
wavelength[µ
[µm]
m]
BB
0
1 2
n PV
BB
0
1 2
εBB
εo
ε1 ε2 ε3
y
Lo
n PV
θBB
θBB
εBB
εo
y
Lo
ε1 ε2 ε3
εn εPV
εplasma
εn εPV
z
z
1D Si/SiO2 photonic crystals: quarter-wave based stack and genetic
algorithm optimized stack as a spectral control tool
Quarter-wave photonic crystal
1
2
10 PV
TPV
(a)
Transmittance
1
0.8
0.6
ε1 ε2
0.4
1
2
3
4
5
Wavelength [µm]
6
7
8
ε1
ε2 εPV
half layer
Genetic algorithm optimized stack
1
Transmittance
ε2
0.2
0
0
(b)
ε1
0.8
1
2
10 PV
0.6
0.4
0.2
0
0
1
2
3
4
5
Wavelength [µm]
6
7
8
ε1 ε2
ε1
ε2
ε1
ε2 εPV
TPV
Spectral characterization of fabricated 1D photonic crystal
1
0.8
0.6
Reflectance
0.2
0
0.8
a)
TE 20°
TE 30°
TE 40°
TE 50°
0.4
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
1
0.8
0.6
0.2
0
0.8
b)
TM 20°
TM 30°
TM 40°
TM 50°
0.4
1
1.2
1.4
1.6
1.8
Wavelength (µm)
2
2.2
2.4
2.6
Improving the spectral efficiency via selective thermal emission
Selective emitter
Front-side filter
Waste
Heat
Heat
A
B
C
But remember thermal emitter is really hot! (up to 1500K)
Refractory metals have high melting temperature, especially tungsten, and that is why
it has been used for incandescent light bulbs ever since
William D. Coolidge, invented the process for producing the
ductile tungsten in 1909 that revolutionized light bulbs
and X-ray tubes. His first light bulb was named “Mazda”
Images removed due to copyright restrictions.Please see:
http://www.harvardsquarelibrary.org/unitarians/images/coolidge4.jpg
http://www.harvardsquarelibrary.org/unitarians/images/coolidge10.jpg
http://www.harvardsquarelibrary.org/unitarians/images/coolidge11.jpg
http://www.harvardsquarelibrary.org/unitarians/images/coolidge12.jpg
http://www.harvardsquarelibrary.org/unitarians/coolidge.html
Adding an array of resonant cavities in tungsten can help us
tailor the emittance
Lorentz-Drude model for tungsten
ω2
ω1
2D W PhC as selective thermal emitter:
11
meas.
meas.2D
2D W
WPhC
PhC (r=450nm
(r=450nmd=560nm)
d=560nm)
sim.
sim.2D
2D W
WPhC
PhC (r=450nm
(r=450nmd=560nm)
d=560nm)
meas.
meas.2D
2D W
WPhC
PhC (r=440nm
(r=440nmd=315nm)
d=315nm)
sim.
2D
W
PhC
(r=440nm
sim. 2D W PhC (r=440nmd=315nm)
d=315nm)
meas.
meas.2D
2D W
WPhC
PhC (r=390nm
(r=390nmd=600nm)
d=600nm)
sim.
2D
W
PhC
(r=390nm
sim. 2D W PhC (r=390nmd=600nm)
d=600nm)
meas.
meas.flat
flattungsten
tungsten
sim.
flat
sim. flattungsten
tungsten
0.9
0.9
0.8
0.8
Emittance
Emittance
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
00
11
1.5
1.5
22
wavelength
wavelength [[µµm]
m]
2.5
2.5
33
2D W PhC exhibits tunable cut-off and resonant enhancement
Fabrication process improvements
• Old
• New
Fabrication Process
Laser Interference
Lithography
Development
Soft-mask etch
Hard-mask etch
ARC = Anti-Reflective Coating
Soft-mask removal
Tungsten etch
Hard-mask removal
Tailoring electronic- and photonic bandgap properties:
a path towards record efficiencies
GaSb and GaInAsSb diode comparison
0.5
1
0.45
External Quantum Efficiency
Quantum efficiency
GaSb
0.4
V oc [ V ]
Voc (V)
0.35
0.3
0.25
GaInAsSb
0.2
GaInAsSb EQE
GaSb EQE
0.9
0.15
0.8
0.7
0.6
0.5
GaSb
0.4
0.3
GaInAsSb
0.2
0.1
0.1
-4
10
0
-3
10
-2
-1
10
10
Jsc [A/cm2]
Isc (A/cm2)
0
10
1
10
1
1.5
2
wavelength [µm]
2.5
Wavelength (µ
µm)
3
Tuning the PhC and PV diode bandgaps: GaSb (0.72 eV) and
GaInAsSb (0.52 eV)
GaInAsSb
PhC
PhCTransmittance
Transmittance
GaSb
GaSbQE
QE
0.9
0.9
0.8
0.8
EQE, Transmittance
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
ηηspectral
spectral
TT>Eg
>Eg
0.3
0.3
0.2
0.2
== 41
41 %
%
== 70
70 %
%
0.1
0.1
00
11
1.5
1.5
22
2.5
2.5
wavelength
wavelength[µ
[µm]
m]
33
3.5
3.5
Quantum efficiency,
Transmittance
EQE, Transmittance
11
EQE,Transmittance
Transmittance
EQE,
Quantum efficiency, Transmittance
GaSb
44
11
PhC
PhCTransmittance
Transmittance
InGaAsSb
InGaAsSbQE
QE
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
ηηspectral
spectral
TT>Eg
>Eg
0.3
0.3
0.2
0.2
== 80
80 %
%
== 79
79 %
%
0.1
0.1
00
11
1.5
1.5
22
2.5
2.5
Wavelength
Wavelength((µµm)
m)
33
3.5
3.5
44
Photonic crystals tailoring photonic- a
nd
an
d
electronic bandgaps
bandgaps
GaSb EQE,
EQE,1D
1D PhC
PhC Transmittance,
Transmittance,2D
2D PhC
PhC Emittance
Emittance
GaSb
Tuning the PhC and PV diode bandgaps: GaSb (0.72 eV)
11
2D
2DPhC
PhCEmittance
Emittance
1D
1DPhC
PhCTransmittance
Transmittance
GaSb
GaSbEQE
EQE
0.9
0.9
Selective
emitter
Front-side PV
diode
filter
0.8
0.8
0.7
0.7
Waste
Heat
0.6
0.6
Heat
0.5
0.5
0.4
0.4
0.3
0.3
Electricity out
0.2
0.2
0.1
0.1
00
11
A
1.5
1.5
22
2.5
2.5
33
3.5
3.5
B
44
Wavelength
Wavelength((µµm)
m)
Spectral efficiency
1D PhC and 2D W
PhC
93 %
Above bandgap
transmittance
70 %
C
Photovoltaic module:
design and characterization
Simple TPV diode model
GaInAsSb diode characterization cont’d
Packaged Cells
External Quantum Efficiency
0.4
100
99-017-08
00-202-18
01-471-15
01-471-16
99-017-08
00-202-18
01-471-15
01-471-16
90
0.35
80
70
V oc (V)
EQE (Percent)
0.3
0.25
60
50
40
30
20
0.2
10
0.15
-2
10
0
-1
10
Jsc (A/cm2)
0
10
1
1.2
1.4
1.6
1.8
Wavelength (µm)
2
2.2
2.4
2.6
GaInAsSb diode characterization
99-017-08
00-202-18
0.35
0.35
Unpackaged
Packaged
AR Coated
0.3
Voc (V)
Voc (V)
0.3
0.25
0.2
0.15
-2
10
0.25
0.2
-1
0.15
-2
10
0
10
10
Jsc (A/cm2)
01-471-15
0
10
01-471-16
0.35
0.3
0.3
V oc (V)
V oc (V)
-1
10
Jsc (A/cm2)
0.35
0.25
0.2
0.15
-2
10
Unpackaged
Packaged
AR Coated
0.25
0.2
-1
0
10
10
2
Jsc (A/cm )
0.15
-2
10
-1
0
10
10
2
Jsc (A/cm )
MIT µ-T
PV Generator Prro
ojje
ec
t
TPV
ct
Key innovations in: photonic crystals,
crystals,
MEMs reactors, power electronics, PV
PV
2D tungsten photonic crystal
1D photonic crystal
Power electronics
Low-bandgap PV cells
Si micro-fabricated reactor
Photonic crystals tailoring photonic- a
nd
an
d
electronic bandgaps
bandgaps
Robust, integrated catalytic
catalytic
micro-rreactor
eactor d
es
n
de
siig
gn
Integrated power electronics controller
controller
single chip
integrated MPPT
Quasi-coherent thermal emission via photonic crystals
•Vertical-cavity resonant thermal emitter
•2D PhC slab resonant thermal emission
Broad-band spectral control
1
1
0.8
0.8
flat W
2D W PhC (r=440 nm,d=315 nm)
2D W PhC (r=390 nm,d=560 nm)
2D W PhC (r=440 nm, d=560 nm)
0.7
0.7
Generation 1
measured
simulated
0.9
Emittance
Reflectance
Reflectance
0.9
0.6
0.5
0.4
0.3
0.6
0.5
0.4
0.3
0.2
0.2
0.1
0.1
0
1
1.5
2
2.5
Wavelength (µm)
Narrow-band spectral control
z
θ
ε0
εH
εH
εL
εH
εL
εH
L0
εcavity
εM
y
Generation 2
3
0
1
1.5
2
2.5
3
3.5
Vertical cavity resonant thermal emitter is highlydirectional, quasi-coherent radiation source
z
θ
ε0
εH
εH
εL
εH
εL
εH
L0
εcavity
εM
y
Tungsten
cavity
mirror
Vertical cavity resonant thermal emitter:
narrow-band, highly directional and
Quasi-coherent thermal emission via photonic crystals
•Vertical-cavity resonant thermal emitter
•2D PhC slab resonant thermal emission
Thermal
Thermal
Thermal
Thermal
Black/Gray-
Body Physics
Ref: Max Planck, Annalen der Physik, 4, 553, (1901).
Modes of a 2D PhC slab
|Hz|
odd guided resonance
z
even guided resonance mode
|Hz|
y
x
Fano resonances of a 2D PhC slab
z
y
x
Ref: S. Fan and J. D. Joannopoulos, Phys. Rev. B 65, 235112 (2002).
Thermal emittance of a 2D PhC slab
z
y
x
Thermal Emittance
Im(ε)≈0.005
Ref: D.Chan, I.Celanovic, J.D.Joannopoulos, and M.Soljačić, submitted for publication.
z
y
x
Thermal Emittance
θ
θ increases
Thermal Emittance
Dependence on angle of observation
θ increases
Analytical understanding of Fano resonances
⇒
aPhC
2
=
2
Q ABS QRAD
2
 ω

 1
1
− 1 + 
+
4
Q ABS

 QRAD
 ω FANO
Q ABS =



2
εR
σε I
Q ABS = QRAD ⇒ a PhC
MAX
= 50%
Thermal Emittance
Rules for designing thermal emission
z
y
x
ωEMIT(θ
θ):
• slab thickness
• Re(ε)
• lattice constant
ΓEMIT ⇔ QRAD:
• “size” of holes
Peak emission ⇔ QABS:
• Im(ε)
aPhC
2
=
2
Q ABS QRAD
2
 ω


1
1
4
− 1 +

+
Q
ABS

QRAD
 ω FANO

2



An example of thermal design
z
x
QRAD=370
QRAD=2000
Thermal
y
Quasi-coherent thermal radiation: summary and
opportunities
•PhC’s offer unprecedented opportunities for tailoring
thermal emission spectra
• Highly anomalous thermal spectra can be obtained
• Even dynamical tuning of spectra is possible
•Research in the combined near-field and quasi-coherent PhC
radiation is opening up new frontiers
• Possible applications include: masking thermal targets,
coherent thermal sources, high-efficiency TPV generation,
chemical sensing, etc.
MIT OpenCourseWare
http://ocw.mit.edu
2.997 Direct Solar/Thermal to Electrical Energy Conversion Technologies
Fall 2009
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.