light trapping with particle plasmons kylie catchpole 1,2, fiona beck 2 and albert polman 1 1 center...
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Light trapping with particle plasmons
Kylie Catchpole1,2, Fiona Beck2 and Albert Polman1
1Center for Nanophotonics, FOM Institute AMOLFAmsterdam, The Netherlands
2Australian National UniversityCanberra, Australia
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Poor absorption below the bandgap
solar spectrum
Si solar cell
Eg
Indirect bandgapSemiconductor (Si):poor absorption justbelow the bandgap thick cell required
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Solution: light trapping
Goal:• Increased efficiency (IR response) and/or • Reduced thickness (=cost)
fsubs fsubs
fair
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Plasmon-enhanced photocurrent: 5 examples
Nakayama et al., APL 93, 121904 (2008)
GaAs
Stuart and Hall, APL 69, 2327 (1996)
SOI
Derkacs et al., APL 89, 93103 (2006)
a-Si
SiSOI
Pillai et al., JAP 101, 93105 (2007)
Schaadt et al., APL 86, 63106 (2005)
Si
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Plasmon-enhanced photocurrent: 5 examples
Nakayama et al., APL 93, 121904 (2008)
GaAs
Stuart and Hall, APL 69, 2327 (1996)
SOI
Derkacs et al., APL 89, 93103 (2006)
a-Si
SiSOI
Pillai et al., JAP 101, 93105 (2007)
Schaadt et al., APL 86, 63106 (2005)
Si
What are the physical principlesand limitations
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Light scattering
2
2
032
42 sin
32 rc
pI s
E
p
p p
Rayleigh scatteringfrom point dipole
Scattering from point dipoleabove a substrate
Preferentialscatteringinto high-indexsubstrate
See, e.g.: J. Mertz, JOSA-B 17, 1906 (2000)
4 %
96 %
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(a)
(b)
0 50 100 1500,0
0,2
0,4
0,6
0,8
1,0
Material: Ag (Palik)
F
Sphere diameter (nm)
0
50
100
150
TO
T, D
IP /
R
RE
FAbsorption ~ r3
Scattering ~ r6
Metal nanoparticle scattering
Scattering vs Ohmic losses
Albedo 1 for D > 100 nm
Ag
304
2m
m
R
Resonant scattering
Plasmon resonance: = -2m()
Alb
edo
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Metal nanoparticle scattering
Cross section > 1 All light captured and scattered into substrate (=AR coating)
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Resonance tunable by dielectric environment
Ag, D=100 nm
Si3N4 (n=2.00) Si (n=3.5)
DQDQ O
H
Optics Express (2008), in press
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From point dipole to particle plasmon
500 550 600 650 700 750 8000
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Fra
ctio
n s
catte
red
into
su
bst
rate
dipolecylinderhemispheresphere 100nmsphere 150nm
500 550 600 650 700 750 8000
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Fra
ctio
n s
catte
red
into
su
bst
rate
dipolecylinderhemispheresphere 100nmsphere 150nm
Fraction scattered into substrate highest for cylinder & hemisphere:Strongest near-field coupling
Tradeoff: larger size larger albedo but lower coupling
96 %
0
FDTD calculations
Appl. Phys. Lett. 93, 191113 (2008)
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Maximum path length enhancement
Highest path length enhancement for cylinder and hemisphere
Geometric series
fsubs fsubs
fair
Appl. Phys. Lett. 93, 191113 (2008)
0.6 0.7 0.8 0.9 11
10
100
fraction into substrate
ma
xim
um
pa
th le
ng
th e
nh
an
cem
en
t
sphere 150nm
sphere 100nm
cylinder
hemisphere
Lambertian
horizontal dipole
Fraction scattered into substrate
Path
length
enhance
ment
30 x
(A=0.95)
(A=0.90)
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Scattering cross-section with dielectric spacer
σscat normalized to particle area
Larger spacing:
Interference in driving field
But: lower coupling fraction
(+ local density of states variation modifies albedo)
500 600 700 800 900 10000
2
4
6
8
10
12
14
wavelength (nm)
Qsc
at, Q
subs
30nm
10nm
30 nm
10 nm
D
Q
Appl. Phys. Lett. 93, 191113 (2008)
tot
sub
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Thermal SiO2
dave= 135 nmf = 26%n=1.46
Ag nanoparticle formation on SiO2/Si3N4/TiO2 on Si
LPCVD Si3N4
dave= 220 nm f = 28%n=2.00
APCVD TiO2
dave= 215 nm f = 30%n=2.50
Thermal evaporation of 14 nm Ag + 300 °C anneal
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c-Si c-Si100 μm
Integratingsphere
30 nmSiO2
Si3N4
TiO2
Optical absorption (1-R-T) in Si wafers
Si3N4
TiO2
SiO2
Si3N4
TiO2
SiO2
Ref. Ref.
Strongly enhancednear-IR absorptionegineered by dielectric spacer
AR effect, interferencefor shorter wavelength+ redshift
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Photocurrent, external quantum efficiency
Red-shifted EQE enhancement with refractive index of underlying dielectricDecrease at short wavelength due to phase shift Small increase at long wavelength for TiO2
Si3N4 TiO2SiO2front front
frontback
back
back
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Relative photocurrent, EQE enhancement
Si3N4
TiO2
SiO2
Si3N4
TiO2
SiO2
frontback
TiO2 coated Si:EQE enhancement 2.7 foldat λ = 1050 nm
Note: particle size and distributionare not optimized
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Design principles for plasmon-enhanced solar cells
1) Metal nanoparticles scat > 1 2) Coverage ~ 10-20 % required 3) D>100 nm albedo > 0.95 i.e. Ohmic losses < 5% 4) Angular distribution (=path length) increased 5) Coupling fraction f = 0.96 for point dipole 6) f reduces for larger particle size 7) scat increases with spacer thickness 8) f decreases with spacer thickness
Design parameter optimizationInclude: inter-particle coupling
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Appl. Phys. Lett. 93, 191113 (2008)
For details/referencesvisit: www.erbium.nl
VACANCIES in nano-photovoltaicssee: www.amolf.nl
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• Flexible rubber on thin glass• Conform to substrate bow and roughness• No stamp damage due to particles
PDMS Stamp
Thin glassPDMS stamp (6”) on 200 µm AF-45 glass
1 m
Full-wafer soft nano-imprint
Marc Verschuuren, Hans van SprangSpring MRS 2007, 1002-N03-05
Substrate Conformal Imprint Lithography
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Angular dependence of scattered light
Increased power around critical angle for dipole compared to isotropic Lambertian less oblique path
fair
W dav
Dipole dav~1.5
Lambertian dav=2
K.R Catchpole and A. Polman, APL (2008)
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Tadeoff between cross section and incoupling
Optics Express (2008), in press
Point dipole