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

Poor absorption below the bandgap

solar spectrum

Si solar cell

Eg

Indirect bandgapSemiconductor (Si):poor absorption justbelow the bandgap thick cell required

Solution: light trapping

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

fsubs fsubs

fair

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

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

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 %

(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

Metal nanoparticle scattering

Cross section > 1 All light captured and scattered into substrate (=AR coating)

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

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)

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)

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

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

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

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

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

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

Appl. Phys. Lett. 93, 191113 (2008)

For details/referencesvisit: www.erbium.nl

VACANCIES in nano-photovoltaicssee: www.amolf.nl

• 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

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)

Tadeoff between cross section and incoupling

Optics Express (2008), in press

Point dipole