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Nanophotonic light trapping for high efficiency solar cells
Kylie CatchpoleCentre for Sustainable Energy Systems, Research School of EngineeringAustralian National University, Canberra.
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Why nanophotonics?
Thin film eg. c-Si, a-Si
mc-Si
Quantum dot Organic
c-SiApplications of nanophotonics
?
New types of solar cells are hard to texture or very thin
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Nanophotonics for solar cells
Localized surface plasmons
≈ (wavelength scale)
<< (sub-wavelength): effective medium
>> : geometrical optics
10m
Scattering back reflectors
Diffraction gratings
Catchpole et al. (2011) MRS Bulletin 2011; 36(6) : 461-467
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Nanophotonics for light trapping
Mokkapati and Catchpole, Journal of Applied Physics - Focused Review 112, 101101 (2012)
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Progress for crystalline silicon
Mokkapati and Catchpole, Journal of Applied Physics - Focused Review 112, 101101 (2012)
Open symbols – theory
Closed symbols - experiment
Nano-cones (Wang et al. Nano Lett. 2012)
Skewed pyramids (Chong et al. J. Opt. 2012)
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Snow Globe Coating
A. Basch, F.J. Beck, T. Söderström, S. Varlamov, K.R. Catchpole, Progress in Photovoltaics, 2012
0
0.2
0.4
0.6
0.8
1
Wavelength [nm]
R
SG coating
paint2
paint1
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Silver particles and Snow Globe Coating
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Snow Globe Coating combined with Plasmonic Nanoparticles
Plain 4.0mA/cm2
Snow Globe 8.0mA/cm2
coated with plamonic particles 100% increase in Jsc
300 600 900 12000
0.1
0.2
0.3
plainSG-coated SP
Wavelength [nm]
EQE
A. Basch, F.J. Beck, T. Söderström, S. Varlamov, K.R. Catchpole, Appl. Phys. Lett. 2012
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TiO2 diffraction gratings
0 200 400 600100
300
500
700
Time (s)
eff ( s
)
Light off
Light on
Light induced passivation gives lifetimes of 700µs Barbé et al. Progress in Photovoltaics, 20(2), 143 (2011)
Wang et al. Progress in Photovoltaics (2012), DOI: 10.1002/pip.2294
PassivationLight trapping
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Plasmonic enhancementFar-field (Scattering):
Near-field:
• Strong local field enhancement - very thin absorbers• Increased optical local density of states• Parasitic absorption
• Scattering/absorption cross-sections• Diffraction efficiency• Mode coupling/light trapping
Catchpole & Polman, Opt. Express 2008, Atwater & Polman, Nature Mat. 2010.
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Metallic perfect absorbers I
C. M. Watts et al., Advanced Mater. 24, OP98-OP120 (2012).
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Metallic perfect absorbers II
a) Metamaterial (t, d << l)• Effective medium (e,m)• Impedance matched to free space
b) Resonant cavity (t~ /4)l
c) Plasmonic grating (d~l)• Coupling to SPPs
t
d
dielectricmetal
metal
C. M. Watts et al., Advanced Mater. 24, OP98-OP120 (2012).
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Extremely thin absorber cells
Conventional ETA SC• Large absorption volume• Short carrier path length• Transparent transport layers • Solution processed• Uniformity/infiltration issues
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Extremely thin absorber cells
Conventional ETA SC• Large absorption volume• Short carrier path length• Transparent transport layers • Solution processed• Uniformity/infiltration issues
Planar ETA SC• Local field-enhancement• Reduced surface area
(recombination?)• Physical layer deposition
(sputtering/evaporation)
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Ultrathin absorber geometry
AIR
SUPERSTRATE (n = 3.6)
Silver stripe, 100nm wide, 25nm high
F.J. Beck et al., Opt. Express 19, A146-A156 (2011).
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Results
Absorbing layer:• n = 3.6• a = 3.4 x 104 cm-1
~1.8%
5nm
ETM
Numerical simulations: COMSOL (FEM)TM polarization
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Results
~4%
5nm
Superstraten = 3.5
~1.8%
Single passOn superstrate
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Results
~16%
5nm
Superstraten = 3.5
120nm
25nm Ag
~4%~1.8%
Single passOn superstrate
Grating
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Results
Single passOn superstrate
GratingGrating+ mirror
5nm
Superstraten = 3.5
120nm
25nm Ag
~4%~1.8%
~16%
90%
Wang, White & Catchpole, IEEE Photonics Journal 2013.
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Results
98%
5nm
Superstraten = 3.5
120nm
25nm
90%
~4%~1.8%
~16%Single pass
On superstrateGratingGrating+ mirror
Total absorption
Wang, White & Catchpole, IEEE Photonics Journal 2013.
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Results
5nm
Superstraten = 3.5
120nm
25nm
Single passOn superstrate
GratingGrating+ mirror
Total absorption
Wang, White & Catchpole, IEEE Photonics Journal 2013.
53x increase inabsorption
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Results: angular dependence
Angle and polarization averaged path length enhancement = 28Compared to 2D Lambertian limit n 11
Wang, White & Catchpole, IEEE Photonics Journal 2013.
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Crystalline-Si tandems
High efficiency c-Si:• UNSW PERL Cell (1998): h = 25% (4cm2)• Sunpower (2010): 24.2% (155cm2)• Panasonic (2014): 25.6% (143cm2)
Low-cost thin film• Bandgap ~1.7eV• Cheap • Earth-abundantEfficiencies 25-30%?
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Crystalline-Si Tandems
Nanotechnology 19, 245201 (2008).
Janz et al., EU PVSEC (2013).
Sunshot projects (Next Generation PV II): • III-V Nanowires on c-Si• CdSe on c-Si
• Organic/c-Si tandem: Energy Environ. Sci., 5, 9173 (2012).
• Nature, 501, 395 (2013): “perovskite cells have now achieved a performance that is sufficient to increase the absolute efficiency of high-efficiency crystalline silicon cells”
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Tandem solar cells
• How good does the top cell need to be?• Material requirements
• Bandgap• Diffusion length• Luminescence efficiency
• Optical requirements• Low parasitic α• Minimal transparent conductor loss• Wavelength selective light trapping
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How good does a top cell need to be?
1.5 1.6 1.7 1.8 1.9Top cell bandgap (eV)
0
50
100
150
2.0Bot
tom
cel
l pow
er(W
/m2 )
T
l
AM1.5G (1000W/m2)
h = 25% c-Si PERL Cell
Jsc = 42.7mA/cm2
Voc = 706mVFF=0.828
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How good does a top cell need to be?
1.5 1.6 1.7 1.8 1.9 2.00
5
10
15
20
25
Top cell bandgap (eV)
Req
uire
d to
p ce
ll ef
ficie
ncy
(%)
h = 27.5%
h = 30%
1.5 1.6 1.7 1.8 1.9Top cell bandgap (eV)
0
50
100
150
2.0Bot
tom
cel
l pow
er(W
/m2 )
h = 25% (breakeven efficiency)
T
l
AM1.5G (1000W/m2)
h = 25% c-Si PERL Cell
Jsc = 42.7mA/cm2
Voc = 706mVFF=0.828
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How good does a top cell need to be?
1.5 1.6 1.7 1.8 1.9Top cell bandgap (eV)
0
50
100
150
2.0Bot
tom
cel
l pow
er(W
/m2 )
T
l
AM1.5G (1000W/m2)
h = 25% c-Si PERL Cell
Jsc = 42.7mA/cm2
Voc = 706mVFF=0.828
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Four-terminal tandem model
Bottom cell: c-Si PERL (25%)
J0 ~ 49 fA/cm2
FF = 82.8%
Top cell (p-i-n):
Bandgap Eg (direct)
Absorption
Diffusion length Ld => carrier collection [1]
Luminescence efficiency => F Voc [2]
FF = 0.8
Strong absorbers (a0~104cm-1)Short diffusion lengths ~200nm
[1] Taretto, Appl. Phys. A 77, 865 (2003)[2] Smestad, Solar Energy Mat. Solar Cells 25 51 (1992)
White, Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: 10.1109/JPHOTOV.2013.2283342
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Results: Eg and F dependence
Ld = 100 nma0 = 104 cm-1
External luminescence efficiency:[Green, Prog. Photovolt: Res. Appl. 20, 472 (2012)]
GaAs: > 0.2 c-Si: ~6 x 10-3 CIGS: 10-3 a-Si: 10-7~10-5
F = 10-5
White. Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: 10.1109/JPHOTOV.2013.2283342
• Light trapping can increase efficiency by 3% absolute.• Optimum bandgap increases as decreases to offset Voc loss.
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Results: Ld and a0 dependence
Eg = 1.95eVa0 = 104cm-1
F = 10-8 (qVoc = 0.6Eg)Ld = 35 nm
F = 10-6
Ld = 100 nm
F = 10-5
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Optical losses
TCO
TCO
Zeng et al. Adv. Mater. 22, 4484-4488 (2010).
Req
uire
d t
op c
ell e
ffici
ency
(%
)
1.5 1.6 1.7 1.8 1.9 2.00
5
10
15
20
25
Top cell bandgap (eV)
htandem = 25%
htandem = 30%
10% parasitic loss in top cell20% parasitic loss in top cell
Parasitic absorption >20% makes reaching 30% practically impossible
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Sub-bandgap absorption
400 600 800 1000 120010
1
102
103
104
105
106
Wavelength (nm)
a (c
m-1
)
a-Si:H
CIS
Sb2S3
CZTS
10 100 10000
2
4
6
8
10
12
14
16
18
Top cell thickness (nm)C
urr
en
t lo
st f
rom
bo
tto
m c
ell
(mA
/cm
2)
Sb2S3 (Eg=1.73eV)
CZTS (Eg=1.5eV)
CIS (Eg=1.5eV)
a-Si:H (Eg=1.7eV)
Cell thickness is limited by sub-bandgap loss
CZTS
Sb2S3
a-Si:H CIS
Parasitic absorption of perovskite is very low (similar to a-Si:H) – much more promising than CZTS.
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Light trapping
Any light trapping must be wavelength selective
0.220.240.250.260.270.28
0.29
0.3
0.3
0.31
0.31
c (nm)
Rb
to
c
Tandem
efficiency
400 600 800 10000
0.2
0.4
0.6
0.8
1
0.2
0.22
0.24
0.26
0.28
0.3
0.320.318
selective trapping broadband trapping
no light trapping
Broadband trapping for the top cell is detrimental to total efficiency
After Green (2002) Prog Photovolt. Res. Appl. (10) 252, to include transparency in the rear reflector
Lal, White & Catchpole, submitted
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Tandem cells on Si • Gaining 5% efficiency on c-Si requires very good top cells
• How to get there:– III-V or perovskites on Si– Identify new materials: Eg, Ld, F
– High bandgap (Eg~1.95eV @ F =10-5)
– Minimize sub-bandgap absorption– Minimize TCO absorption– Wavelength-selective light trapping
Top cell bandgap htandem = 25% htandem = 30%
1.5eV htop > 17% htop > 22%
1.7eV htop > 12% htop >17%
2eV htop > 9% htop > 14%
White, Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: 10.1109/JPHOTOV.2013.2283342
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Conventional absorption measurement :
36
Electron-hole pairs
Quantifying light trapping
900 950 1000 1050 1100 1150 12000
20
40
60
80
100
Abs
orpt
ion
(%)
wavelength
900 950 1000 1050 1100 1150 12000
20
40
60
80
100
AFC(ћω) Free carrier absorption& Parasitic absorption
ABB(ћω) for band to band transition
Absorptance = 100% - Reflectance - Transmission
Conventional absorption measurement makes it hard to identify best light trapping structures.
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“A good solar cell makes a good LED and a great LED makes a great solar cell”
37
EmissionAbsorptionhigher energy state
lower energy state
ΔEhν hν
Quantifying light trapping
Luminescence
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900 1000 1100 1200 1300 1400
PL
Inte
nsity
(A
.U.)
Wavelength(nm)
PL Planar PL with Light-trapping
Characterization
Photoluminescence spectra of cell structure with and without light trapping
hν
hν
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39
)(exp))((exp)( 3..
d
kTA
kTCdj VFCF
e
Constant for certain materials at fixed T
Wavelength dependent
T. Trupke et al., Sol. Energ. Mat. Sol. Cells vol 53, (1998)
900 1000 1100 1200 1300 1400
PL
Inte
nsity
(A
.U.)
Wavelength(nm)
PL Planar PL with Light-trapping
Characterization
900 950 1000 1050 1100 1150 12000
10
20
30
40
50
60
70
80
90
100
Abs
orpt
ivity
Wavelength(nm)
Planar with Light-trapping
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• Ag nanoparticles & diffused white coating
Structure 1
Structure design
Structure 2
c-Si
diffused white reflector
• Ag nanoparticles with metal reflector
Ag nanoparticles
c-Si
metal reflector
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41
Plasmonics & DWC on c-Si cell
Structure 1
c-Si
diffused white coating
Dielectric Environment:• Passivation layers PECVD α-Si:HPECVD Si3N4
ALD Al2O3
Nanoparticle size • Ag film thickness:
15nm 21nm 27nm 33nm
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Plasmonics in back contact cell
Structure 2
• 27nm Ag (D=~200nm)
• Capping layer thickness: 60~150nm PECVD Si3N4
c-Si
metal reflector27 nm
1 μm
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950 1000 1050 1100 1150 12000
10
20
30
40
50
60
70
80
90
100
Jlambertian
-JP
Quantifying light-trapping
Absolute Absorption
Spectrum %A
Maximum Possible Photon
Current Jsc
950 1000 1050 1100 1150 12000
10
20
30
40
50
60
70
80
90
100
Lambertian
Light trapping Planar
Abs
orpt
ance
(%
)
Wavelength (nm)
950 1000 1050 1100 1150 12000
10
20
30
40
50
60
70
80
90
100
JAg+BSR
-JP
Fraction of Lambertian Enhancement (FLE)
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Experimental results
60 90 120 150 DWC40
45
50
55
60
42
44
53
49
57
AgNP/Si3N
4
AgNP/DWC
Fra
ctio
n o
f L
am
be
rtia
n E
nh
an
cem
en
t (%
)
Capping Layer Thickness (nm)
C. Barugkin et al., IEEE Journal of Photovoltaics 2013
67% for Inverted Pyramid Texture with PLE=16 (T. Trupke et al., Sol. Energ. Mat. Sol. Cells, 1998)
c-Si
diffused white coating
c-Si
metal reflector
1 2
Light trapping similar to inverted pyramids but applicable to any cell
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Summary
• Metal particles and scattering back reflectors - can give 100% Jsc enhancement.
• Near-field absorption for planar ETA– 90% absorption in 5nm layer
• Defined optical and electrical requirements for high efficiency tandems on Si
• Photoluminescence for quantifying light trapping - 62% of Lambertian increase demonstrated
PTO for conference slide
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Metallic perfect absorbers III
K. Aydin et al., Nature Comms. 2, 517 (2011).
• Can be broadband, angle- and polarization-independent• TPV applications (tunable emissivity)
X. Liu et al., Phys. Rev. Lett. 107, 045901 (2011).
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Absorber layer thicknessM
axim
um
ab
sorp
tan
ce
Absorber thickness (nm)
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Predicted external quantum efficiency
Modelled Jsc: 1000~1200nm
collection η 95%
2.2mA/cm2 2.6mA/cm2
18% enhancement
c-Si
Metal reflector
c-Si
Metal reflector
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Need for light trapping
AM 1.5 solar spectrum and solar radiation absorbed in 2 μm c-Si thin film, assuming single pass
t
optical thickness >> physical thickness ‘t’
glass
Thin semiconductor (few µm)
Thin solar cells an alternative for low cost PV