Cristalli fotonici:
semiconduttori di luce
Lucio Claudio Andreani
Dipartimento di Fisica “Alessandro Volta”,
Università di Pavia
http://fisicavolta.unipv.it
http://fisicavolta.unipv.it/nanophotonics
XCVI Congresso Nazionale della Società Italiana di
Fisica, Bologna, 20-24/09/2010
OUTLINE
¾
Introduction: photonic crystals in nature and in lab
¾
Control of light propagation
¾
Control of light emission
¾
Active control of light: all-optical switching, nonlinear optics
¾
Photonic nanostructures for photovoltaics
¾
Conclusions
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Generalities
Photonic crystals=dielectric materials, whose dielectric function is
periodic in one, two or three dimensions
¾
The periodicity of the dielectric function leads to multiple Bragg
reflections and to the formation of forbidden frequency regions for light
propagation
¾
⇒ photonic band gap
ε1=13
ε2=1
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
The starting year (conventional): 1987
¾
1D photonic crystals or Bragg reflectors: well known since long
Lord Rayleigh (1887), Born and Wolf, …
¾
2D PhCs: Bragg coupling in waveguides, coupled mode theory (Yariv ~ 1977)
¾
General concept, 3D photonic crystals:
Eli Yablonovitch, PRL 58, 2059
(1987): a full 3D photonic band gap
can be used to suppress spontaneous
emission and to reduce the threshold
of semiconductor lasers
Sajeev John, PRL 58, 2486
(1987): disorder close to the
edge of a photonic band gap can
be used to produce strong
(Anderson) localization of light
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Complete photonic band gap in 3D:
the diamond lattice of dielectric spheres
kz
U
K
ε=13
L
Γ
ky
X
W
kx
K.M. Ho, C.T. Chan and C.M. Soukoulis, Phys. Rev. Lett. 65, 3152 (1990)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Photonic crystals: in nature
opal
Butterfly (Morpho Rhetenor)
Peacock
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Photonic crystals: in the lab
2D: macroporous silicon
3D: artificial opal
Top view
Period a=2 µm
Side view
Etch depth ~ 40 µm
(L. Pavesi et al., Trento)
direct opal
inverse opal
(Y. Vlasov et al)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Photonic crystal fibers
Light guiding by out-of-plane propagation in a 2D photonic crystal
“… a next-generation, radically improved version of a
well-established and highly successful technology”
[Philip Russel, Science 299, 358 (2003)]
(a) Solid-core PCF: guiding by
total internal reflection
(c) Birefringent PCF
(d) Small-core PCF with ultra-high
nonlinearity
Photograph
of
supercontinuum
generation in photonic crystal fiber.
Pulse parameters: 5 nJ, 100 fs, 75
MHz, λ=850 nm.
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Photonic crystal slabs: Silicon-on-Insulator (SOI)
(D. Peyrade, M. Belotti and Y. Chen, LPN Marcoussis)
ePMMA
Patterning
SILICON
e-beam lithography system
(50keV JEOL 5D2U )
SILICON DIOXIDE
Resist
development
METAL MASK
Metal deposition
Lift-off (metal mask)
Ni 30nm
Γ−X
Si
Resist removal
SiO2
Anisotropic etching of
Silicon top layer
Reactive ion
etching
Schematic of the
sample preparation
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Waveguide-embedded photonic crystals (Si-based)
One-dimensional
Two-dimensional
membrane
with line-defects
with ‘point ’-defects
membrane
Light propagation is controlled by
• photonic lattice in the 2D (xy) plane: Bragg reflections
• Slab confinemend in the vertical (z) direction: total internal reflection
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
2D PhC slabs for optical interconnects
and for integrated photonic devices
(M. Agio, C.M. Soukoulis: e.m. wave propagation by FDTD simulations)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
¾
Introduction: photonic crystals in nature and in lab
¾
Control of light propagation
¾
Control of light emission
¾
Active control of light: all-optical switching, nonlinear optics
¾
Photonic nanostructures for photovoltaics
¾
Conclusions
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Linear waveguides in 2D PhC slabs
a
w
Single missing row of holes in
triangular lattice, channel thickness
(W1 waveguide): w= w0 ≡ 3a
Group velocity: vg=dω/dk=c/ng
Group-velocity dispersion: dng/dω
0.34
Gap-guided mode: much higher losses
Frequency ωa/2πc
Index-guided mode
• nearly linear dispersion, vg=c/n
Æ very low losses just below light line
• close to band edge ka=π: vgÆ0
Æ slow light (very dispersive)
0.32
0.30
0.28
0.26
0.24
0.0
Slow
light
even
odd
0.2
0.4
0.6
0.8
1.0
Wavevector ka/π
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Defect-mode dispersion from
Fabry-Pérot interference
Narrower interference fringes close to band edge
Æ increased group index, slow wave
M. Notomi et al., PRL 87, 253907 (2001)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Angle-resolved micro-reflectance and
Attenuated Total Reflectance (ATR)
Micro-reflectometer+FT spectrometer
Spectral range 4 µm–200 nm, ∆θ=0.5°
Spot diameter ∼100 µm at λ=1.5 µm
ATR cell with Silicon hemisphere
Æ coupling to evanescent modes
⇒dispersion of photonic bands in a plane parallel to the crystal surface
(2D, 3D photonic crystals and PhC slabs)
M. Galli, D. Bajoni et al., DFAV-University of Pavia
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Defect-mode dispersion in W1 and W1.5 waveguides
by Attenuated Total Reflectance
0 .9 2
Energy (eV)
0 .9 0
H
z
E
y
k
W 1 .0
0 .8 8
0 .8 6
0 .8 4
0 .8 2
σ k z= − 1
0 .8 0
σ k z= + 1
0 .7 8
θ
0 .9 2
Energy (eV)
0 .9 0
x
t
φ
M. Galli et al., Phys. Rev. B 72, 125322 (2005)
W 1 .5
0 .8 8
0 .8 6
0 .8 4
0 .8 2
0 .8 0
0 .7 8
0 .0
0 .5
k xa / π
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
1 .0
Dispersion-engineered PhC waveguides
s2
s1
Control of dispersion, group velocity
and GVD by geometry optimization
Æ flat-band slow light, low GVD
Petrov et al. APL 85, 4866 (2004)
Li et al. Opt. Expr. 16, 6227 (2008)
W1: s1=s2=0
s1=−0.13a, s2=0
W1
s1=−0.13a
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Disorder-induced scattering
Random variations of hole diameters (and/or hole positions,
micro-roughness, etc) produce two kinds of elastic scattering:
Out-of plane scattering (3D)
Scattering into counterpropagating mode Æ backscattering (2D)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Losses for increasing degree of disorder
0.34
∆r/a=0.005
∆r/a=0.01
Frequency (ωa/2πc)
0.33
0.32
∆r/a=0.005
∆r/a=0.01
∆r/a=0.02
∆r/a=0.04
∆r/a=0.08
0.31
0.30
∆r/a=0.02
∆r/a=0.04
∆r/a=0.08
0.29
0.28
0.27
0.26
0.0 0.2 0.4 0.6 0.8 1.0
Wavevector (ka/π)
-6
10
-5
-4
10
10
Im(ωa/2πc)
-3
-4
10 10
-3
10
-2
-1
10
10
Loss*a (dB)
0
10
The losses show a quadratic behavior as
a function of the disorder parameter…
typical of Rayleigh scattering
D. Gerace et al., Opt. Lett. 29, 1897 (2004)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Losses in Silicon PhC waveguides
W1 waveguide on Si membrane
∆r=1.7 nm
W1 with oxide clad (Fox, Flowable Oxide)
∆r=1.4 nm
Good agreement between expt and theory, surface roughness is reduced with
oxide clad. Losses are comparable at the same group-index values.
T. P. White et al., Opt. Expr. 16, 17076 (2008)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
PhC and strip waveguides:
the competing systems
W1 waveguide on a Si membrane
Single-mode strip waveguide on
SOI with 450x220 nm cross section
Loss ≅ 3÷8 dB/cm
(Krauss, Notomi, Vlasov et al.)
Loss ≅ 3-4 dB/cm
(Vlasov et al; De La Rue et al.)
Performance depends on
• losses with optimum design
• suitability for slow-light applications
• figure of merit of resonators (Q, Q/V, …)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
¾
Introduction: photonic crystals in nature and in lab
¾
Control of light propagation
¾
Control of light emission
¾
Active control of light: all-optical switching, nonlinear optics
¾
Photonic nanostructures for photovoltaics
¾
Conclusions
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Redistribution of spontaneous
emission in a patterned slab
External Emission
(very weak)
Guided Emission
Waveguide
Leaky Modes
Guided Modes
Diffraction Grating or Photonic Crystal slab
Theoretical proposal of using quasi-guided modes for enhancing surface
emission in a PhC slab:
S. Fan et al., Phys. Rev. Lett. 78, 3294 (1997)
Æ application to photonic crystal LEDs and lasers
Enhancement of emission implies enhancement of absortion
Æ application to light harvesting in photovoltaic cells
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Enhancement of Er3+ vertical emission by 2D PhC
Si
SRO:Er
Si
Si
0.4
Reflectance (arb. units)
SiO2
PL intensity (arb. units)
0.5
0.3
PhC, a=1210 nm
PhC, a=1070 nm
0.2
Unpatterned SOI
0.1
(x 5)
0.0
0.72
0.76
0.80
0.84
0.88
Energy (eV)
Emission enhancement > 150-fold
SRO=Silicon-Rich Oxide (SiOx)
M. Galli et al. APL 88, 251114 (2006)
Æ SiO2 +Si nanocrystals
Collab. with F. Priolo et al., MATIS-CNR and University of Catania & ST Micro
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Integrated intensity: angular dependence
Integrated Intensity (arb. units)
40
PhC, a=1070 nm
PhC, a=1210 nm
Unpatterned SOI
35
30
25
20
15
10
5
1
0
2
4
6
8
10
12
14
16
Angle (deg)
Most of the emission is in a narrow cone around normal incidence.
Forbidden mode has higher emission intensity (except at θ=0).
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Integrated light-emitting devices:
towards PhC silicon LED
Electrical contacts
n+ poly Si
SiOSi
n+ poly
x
+ Si
pSiO
x
p+ Si
SiO
2
0.2 µm
50 nm
A. Irrera et al., Nanotechnology 17, 1248 (2006)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Nanocavities in photonic crystal slabs
L3 cavity
Heterostructure cavity
T. Akahane, T. Asano, B.S. Song, and S. Noda, Nature 425, 944 (2003);
B.S. Song, S. Noda, T. Asano, and T. Akahane, Nature Mat. 4, 207 (2005).
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Channel-drop filter in heterostructure cavity
Q=600,000 Æ Q=2·106
S. Noda et al. (2005-2007)
Very high-Q cavities
Æ control of emission
Æ low-threshold lasing
Æ all-optical switching
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Low-T emission of Er3+ in Si PhC cavities
2000
PL intensity
1.54 µm emission
1500
SOI:Er
T = 10 K
λexc = 532 nm
P = 1 mW
1000
500
0
1520
1530
1540
1550
1560
Wavelength (nm)
L3 cavity mode resonant with
Er3+ emission by means of
lithographic and gas tuning
PL intensity (arb. units)
2.0
1.6
a=429 nm
∆x/a=0.16
T=6 K
λexc= 532 nm
P=300 µW
a=429 nm
∆x/a=0.16
T=6 K
λexc= 532 nm
P=300 µW
1.2
0.8
0.4
0.0
1536
1538
1540
1542
Wavelength (nm)
1536
1538
1540
1542
Wavelength (nm)
collab. Catania, Pavia, Univ. St Andrews, ILN Grenoble – EraNet LECSIN
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Spontaneous emission control in PhC slabs
Purcell effect (1946):
enhancement of SE
rate
of
a
dipole
transition
interacting
with a high-Q cavity
mode
Γµ
3
3 ⎛ λ ⎞ Qµ
=
≡ FP
⎟
2 ⎜
Γ0 4π ⎝ n ⎠ Vµ
Qµ=mode Q-factor
Vµ=mode volume
SE enhancement and Purcell effect (InGaAs):
W. Chang et al., PRL 96, 117401 (2006)
SE suppression (InGaAsP): S. Noda et, J. Opt. A 8, S131 (2006)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
PhC nanocavities with quantum dots
GaAs membrane, d=126 nm
L3 photonic crystal nanocavity
d
Self-assembed InAs quantum dots
S. Strauf et al., PRL 96, 127404 (2006)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Low-threshold lasing
β=
spontaneous emission intensity in lasing mode
total emitted intensity
Thanks to the high-Q nanocavity, ~85% of the light is emitted
into the lasing mode Æ low-threshold behavior (124 nW)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
QW excitons + photonic modes Æ PhC polaritons
resonant light scattering
polariton
splitting
D. Bajoni, M. Galli, J. Bloch et al. PRB 80, 201308 (2009)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Quantum nature of a strongly coupled single
quantum dot – PhC nanocavity system
Strong quantum correlations Æ
photon antibunching in time domain
K Hennessy , A. Badolato et al.,
Nature 445, 896 (2007)
See also Yoshie et al., Nature
432, 200 (2004)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
¾
Introduction: photonic crystals in nature and in lab
¾
Control of light propagation
¾
Control of light emission
¾
Active control of light: all-optical switching, nonlinear optics
¾
Photonic nanostructures for photovoltaics
¾
Conclusions
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
All-optical switching on a Silicon chip
Optical bistability
Optical transistor operation
Mechanism for nonlinearity: E-field is enhanced
by high-Q cavity Æ two-photon absorption
Æ free carriers modify the refractive index
M. Notomi et al., Opt. Expr. 13, 2678 (2005)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
PhC cavity
Signal Intensity (a.u.)
All-optical switching - towards the optical transistor
0.6
λ=532 nm
pump
0.4
0.2
Pulsed pump beam (Nd:YAG laser) is
focused onto cavity region to excite free
carriers and modity refractive index
Transmission (dB)
0.0
0
ON
-5
-10
ON
λ=1.4 µm
cw-probe
OFF
-15
-5
0
5
10
15
20
Time (ns)
Switching power ~ 140 nW
Switching energy ~ 13 pJ
Switching time <2 ns (expt. limited)
Belotti, Galli et al., Opt. Expr. 16, 11624 (2008) – collab. CNRS-CEA-LETI (Grenoble)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
All-optical switching in Photonic Wire nanocavities
transmission
High-Q, low-volume nanocavity
⇒ fast all-optical switching with
10 dB on/off ratio and delivered
energy of ~120 fJ
Belotti, Galli, Gerace et al., Opt. Expr. 18,
1450 (2010): collab. with Glasgow University
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Resonant scattering on PhC cavities
best result on L7 cavity
RS signal (a.u.)
tilted L3 cavity sample
Scattering of laser light with crossed polarizations Æ low background,
strong resonance at wavelength of PhC cavity mode. Fano lineshapes.
Better detected with 45° tilt of sample Æ breaking of mirror symmetry .
M. McCutcheon et al., APL 87, 221110 (2005); 94, 121106 (2009)
M. Galli et al., APL 94, 071101 (2009)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Far-field optimized PhC cavities for high
coupling efficiency and Q-factor
Coupling efficinecy
Q-factor (x104)
Non-optimized far field
20°
40°
60°
Optimized far field (max at θ=0)
⇒ coupling efficiency to the cavity mode can be raised by an order
of magnitude, while maintaining a high Q-factor
S.L. Portalupi et al., Opt. Expr. 18, 16064 (2010)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Plasmonics
Surface
plasmon
polaritons
(SPPs)=collective excitations of
the electron gas, localized at a
metal surface and propagating
alon the surface itself
Nanofocusing of optical energy in
tapered plasmonic waveguides
M.I. Stockman, PRL 93, 137404 (2004)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
A Hybrid Plasmonic-Photonic Nanodevice
for Label-Free Detection of a Few Molecules
•
•
•
Photonic crystal nanocavity in Si3N4
Ogival Pt/Au nanoantenna with plasmonic resonance
Functionalization and Raman scattering
⇒ nanometric spectroscopic resolution
⇒ few-molecule sensitivity (10-100)
F. De Angelis, M. Patrini, M. Galli, ..., and E. Di Fabrizio, Nano Letters 8, 2321 (2008)
Collaboration with Univ. Magna Graecia and Bionem Lab (Catanzaro)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Nanoscale chemical mapping using 3D adiabatic
compression of surface plasmon polaritons
Field enhancement from adiabatic slowing
down of SPPs. Slow wave again…
De Angelis, Patrini, Galli, Di Fabrizio et al.,
Nature Nanotechnol. 5, 67 (2010)
Collaboration: IIT (Genova), Bionem Lab (CZ),
CBM Area Science Park & TASC Lab (Trieste)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
¾
Introduction: photonic crystals in nature and in lab
¾
Control of light propagation
¾
Control of light emission
¾
Active control of light: all-optical switching, nonlinear optics
¾
Photonic nanostructures for photovoltaics
¾
Conclusions
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
World primary energy demand
World primary energy demand in the reference scenario (Mtoe)
IEA World Energy Outlook, 2008 (n.b. 1 Toe=11.63 MWh)
fossil fuels: 80.9%
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Electrical energy generation, 2006
All data in TWh
World*
EU*
Italy**
Coal
7756
1021
60
Oil
1096
131
35
Gas
3807
682
160
Nuclear
2793
990
---
Hydro
3035
308
43.4
Renewables (excl. hydro)
433
184
15.3
Biomass and waste
239
93
6.7
Wind
130
82
3.0
59
6
5.5
Solar
4
2
<0.1
Total
18921
3316
314
7.9
18.4
14.8
Geothermal
Energy/person/day (kWh)
*IEA World Energy Outlook, 2008
**Elab. Terna+GSE
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
European Union: the 20-20-20 directive
¾ The European Parliament approved (end 2008) the climateenergy plan, in order to achieve the goals fixed by UE for 2020:
• reduce the emission of greenhouse gases by 20% (with respect to
1990);
• reduce energy consumption by 20% through energy efficiency;
• increase the share of renewable energies to 20% of primary
energy (the national objective for Italy is 17%).
¾ The package includes measures on exchange of emission
quotes (emission trading) and on emission limits for vehicles.
¾ Considering energy consumptions in the 4 sectors (industry,
transport, heating, electricity), the share of renewables in the
production of electrical energy will have to rise to 35-40% at
least (from ~15% in 2006).
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Photovoltaic solar energy
Photovoltaic conversion of solar energy relies on absorption of
light in semiconductor p-n junctions.
The theoretical limit of energy conversion efficiency of single-junction cells
is around 30% (Shockley-Queisser, 1961).
The record value for laboratory cells is 25% (M.A. Green, UNSW, 1999).
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Silicon photovoltaic cells (wafer)
Mono-crystalline silicon
Poly-crystalline silicon
Typical conversion efficiencies: cells 18-20%, modules 12-15%.
Indirect band-gap material Æ thick silicon layers (200-300 micron).
High cost per installed power (~4000-4500 €/kWp)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Thin-film photovoltaic cells
Amorphous silicon
CdTe
AR coat
CIGS – CuInGaSe2
TCO
AR coat
1.5 µm ZnO
50-100 nm n-CdS
300 nm
3-5 µm p-CdTe
passivation layer
metal
2-5 3-5
µm µm
p-CuInGaSe
p-CdTe 2
Mo contact
glass
Trasparent conductor: Indium
Tin Oxide (ITO) or Zinc Oxide
Module efficiency ~ 8%
(a-Si/µc-Si: Æ 10%)
Module efficiency ~ 10%
Module efficiency ~ 12%
All these materials have direct band gap Æ thin layers are sufficient
Lower cost per installed power (~3500-4000 €/kWp)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Cumulative installed PV capacity:
hystorical evolution
Source:
EPIA
MWp
2003-2008
+39%
1998-2003
+24%
The annual growth rate is between 24% and 39%, with a doubling time
between 2 and 3 years (increase by a factor of 10 in 7-10 years)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Actual and planned PV production capacities of
Thin-Film and c-Silicon based solar modules
Crystalline Silicon
Thin Film
Source: JRC EU PV report 2009
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Photovoltaics: the Tera-Watt challenge
• World consumption of electrical energy is ~19’000 TWh/year
• Photovoltaic has to meet the TeraWatt challenge: several TW of
installed power are needed to give an appreciable contribution
• With bulk (wafer) solar cells, about 8 g of Si / Wp are required
⇒ 1 TWp requires 8·1012 g=8·106 tons of solar-grade silicon
• The current annual production of silicon is: ~ 6·106 t metallurgical
~ 8·104 t solar grade
Not physically impossible, but VERY difficult
to achieve several TWp using bulk silicon cells
• The alternative: thin-film silicon solar cells
⇒usage of Silicon is reduced by 2-3 orders of magnitude
⇒ 8·103-8·104 tons of silicon would be sufficient for thin films!
(less than current annual production of solar-grade silicon)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Light trapping in PV cells
• The problem: to enhance optical absorption in a thin silicon layer (0.5-2
µm c-Si or 100-300 nm a-Si)
• Traditional light-trapping mechanisms in solar cells are based on
geometrical optics approaches, not suitable in the case of thin film (0.1-4
µm) solar cells: a wave-optics approach is required
Inverted pyramids
Typical dimensions: 10-20 µm
N.b. Light trapping in ray-optics limit:
4n2 increase of optical path for random
diffusion (Yablonovitch limit, 1982)
Photonic lattice
100 nm - 1 µm
Light trapping in wave-optics limit:
unknown, open problem
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Quasi-guided modes: emission vs. absorption
Leaky Modes
Guided Modes
Leaky Modes
Guided Modes
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Light trapping in thin-film PV cells: a hot area
P. Bermel et al., OE 15, 16986 (2007)
Y. Park et al., OE 16, 14312 (2009)
I. Prieto et al., APL 94, 191102 (2009)
M. Kroll et al., phys. stat. sol. (a) 205,
2777 (2008)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Light trapping in thin-film silicon solar
cells with a photonic pattern
Problem: to increase light absorption in thin-film Si-based solar cells
Photonic approach: surface texturing at sub-micrometric scales
300-nm thick
a-Si layer
(1) Si
(2) Si+AR
coat
(3) PhC:
shallow
(4) PhC:
deep
Substantial increase in
short-circuit current by
photonic pattern
S. Zanotto, M. Liscidini and LCA, Optics Express 18, 4260–4274 (2010)
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Results for short-circuit current:
c-Si versus a-Si
(normal incidence, average over polarization)
Relative jsc increase up to 36.5% for c-Si, up to 12.4% for a-Si
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Photonic crystals - “Semiconductors of light”
Can be used to control the propagation of light, also throuh line
defects (linear waveguides)
¾
Can be used to control the emission of light, also through point defects
(nano-cavities): Purcell effect, low-threshold lasing, strong coupling…
¾
Physical properties can be tailored by external fields: all-optical
switching, optical nonlinearities, photonic-plasmonic systems…
¾
Can be used to enhance light trapping in thin-film photovoltaic cells, in
order to increase efficiency and reduce cost for large-scale applications
¾
Light propagation and radiation-matter interaction can be precisely
controlled by designing and fabricating structures with a photonic band
gap, adding line and point defects to create localized photonic modes,
exploiting external fields to tune the physical properties also at ultra-fast
time scales
micro- and optoelectronics ↔ nanophotonics
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia
Thanks to…
The team at University of Pavia:
M. Galli, D. Bajoni, M. Belotti, J.F. Galisteo-Lopez, S. Portalupi,
M. Patrini, F. Marabelli, G. Guizzetti (exp),
D. Gerace, I. Maksymov, M. Liscidini, C. Creatore, M. Agio, S. Zanotto (theo)
The team at LPN-CNRS Marcoussis, France:
D. Peyrade, Y. Chen; I. Sagnes, A. Miard, A. Lemaitre, J. Bloch
The team at University of St. Andrews, U.K:
L. O’ Faolain (W. Whelan-Curtin), T.P. White, J. Li, T.F. Krauss
The team at University of Glasgow, U.K:
A.R.M. Zain, N.P Johnson, M. Sorel, R.M. De La Rue
The team at BIONEM lab, University of Magna Graecia, Catanzaro and IIT:
F. De Angelis, L. Businaro (TASC Trieste), G. Das, E. Di Fabrizio
The team at MATIS-INFM-CNR and University of Catania:
M. Miritello, A. Irrera, A. Canino, R. Lo Savio, F. Iacona, G. Franzò, F. Priolo
Support: MIUR Cofin 2000, 2002, 2004, 2006, FIRB 2001, 2007
INFM PAIS 2001 & PRA 2002
Fondazione CARIPLO 2005, 2007 & Banca del Monte di Lombardia
EU NoE PHOREMOST & COST P11, ERANET
SIF, Bologna, 22/9/2010 - Lucio C. Andreani – Dipartimento di Fisica “A. Volta” – Università di Pavia