Light trapping with particle plasmons

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Light trapping with particle plasmons
Kylie Catchpole1,2, Fiona Beck2 and Albert
Polman1 1Center for Nanophotonics, FOM
Institute AMOLF Amsterdam, The Netherlands 2Austr
alian National University Canberra, Australia
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Poor absorption below the bandgap
Indirect bandgap Semiconductor (Si) poor
absorption just below the bandgap ? thick cell
required
Eg
solar spectrum
Si solar cell
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Solution light trapping

• Goal
• Increased efficiency (IR response)
• and/or
• Reduced thickness (cost)

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Plasmon-enhanced photocurrent 5 examples
Stuart and Hall, APL 69, 2327 (1996)
Derkacs et al., APL 89, 93103 (2006)
SOI
a-Si
Nakayama et al., APL 93, 121904 (2008)
Pillai et al., JAP 101, 93105 (2007)
GaAs
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Plasmon-enhanced photocurrent 5 examples
Stuart and Hall, APL 69, 2327 (1996)
Derkacs et al., APL 89, 93103 (2006)
SOI
a-Si
What are the physical principles and limitations
Nakayama et al., APL 93, 121904 (2008)
Pillai et al., JAP 101, 93105 (2007)
GaAs
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Light scattering
Rayleigh scattering from point dipole
Scattering from point dipole above a substrate
p
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Preferential scattering into high-index substrate
96
See, e.g. J. Mertz, JOSA-B 17, 1906 (2000)
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Metal nanoparticle scattering
Scattering vs Ohmic lossesAlbedo ? 1 for D gt
100 nm
Resonant scattering
Ag
Albedo
Absorption r3 Scattering r6
Plasmon resonance ? -2?m(?)
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Metal nanoparticle scattering
All light captured and scattered into substrate
(AR coating)
Cross section gt 1
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Resonance tunable by dielectric environment
Ag, D100 nm
Si (n3.5)
Si3N4 (n2.00)
Q
Q
O
D
D
H
Optics Express (2008), in press
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From point dipole to particle plasmon
96
FDTD calculations
Fraction scattered into substrate highest for
cylinder hemisphereStrongest near-field
coupling Tradeoff larger size ? larger albedo
but lower coupling
Appl. Phys. Lett. 93, 191113 (2008)
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Maximum path length enhancement
Geometric series
Highest path length enhancement for cylinder and
hemisphere
Appl. Phys. Lett. 93, 191113 (2008)
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Scattering cross-section with dielectric spacer
sscat normalized to particle area
Larger spacing Interference in driving
field But lower coupling fraction ( local
density of states variation modifies albedo)
Q
30 nm
tot
D
sub
10 nm
Appl. Phys. Lett. 93, 191113 (2008)
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Ag nanoparticle formation on SiO2/Si3N4/TiO2 on Si
Thermal evaporation of 14 nm Ag
300 C anneal
Thermal SiO2 dave 135 nmf 26n1.46
LPCVD Si3N4 dave 220 nm f 28n2.00
APCVD TiO2 dave 215 nm f 30n2.50
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Optical absorption (1-R-T) in Si wafers
Integrating sphere
SiO2Si3N4 TiO2
30 nm
100 µm
c-Si
c-Si
SiO2
Si3N4
Ref.
Ref.
TiO2
TiO2
Strongly enhanced near-IR absorption egineered by
dielectric spacer
Si3N4
AR effect, interference for shorter wavelength
redshift
SiO2
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Photocurrent, external quantum efficiency
Si3N4
TiO2
front
SiO2
front
front
back
back
back
Red-shifted EQE enhancement with refractive index
of underlying dielectric Decrease at short
wavelength due to phase shift Small
increase at long wavelength for TiO2
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Relative photocurrent, EQE enhancement
TiO2 coated Si EQE enhancement 2.7 fold at ?
1050 nm
Note particle size and distribution are not
optimized
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Design principles for plasmon-enhanced solar cells
1) Metal nanoparticles ?scat gt 1 2) Coverage
10-20 required 3) Dgt100 nm ? albedo gt 0.95
i.e. Ohmic losses lt 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
optimization Include inter-particle coupling
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Appl. Phys. Lett. 93, 191113 (2008)
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Substrate Conformal Imprint Lithography
Full-wafer soft nano-imprint

• Flexible rubber on thin glass
• Conform to substrate bow and roughness
• No stamp damage due to particles

Marc Verschuuren, Hans van SprangSpring MRS
2007, 1002-N03-05
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Angular dependence of scattered light
Lambertian dav2
Dipole dav1.5
Increased power around critical angle for dipole
compared to isotropic Lambertian less
oblique path
K.R Catchpole and A. Polman, APL (2008)
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Tadeoff between cross section and incoupling
Point dipole
Optics Express (2008), in press