Photonics for High-efficiency Crystalline Silicon
Solar Cells
Stefan Glunz
Fraunhofer Institute for
Solar Energy Systems ISE
Workshop “Nanophotonics essential ingredient for efficient
and cost-effective solar cells?”
EU-PVSEC, Paris, Sept. 2013
© Fraunhofer ISE
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Plasmonic structure
 Diffractive optics
 Using the full spectrum
 Up-conversion
2
© Fraunhofer ISE, S. W.Glunz, September 2013
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Plasmonic structure
 Diffractive optics
 Using the full spectrum
 Up-conversion
3
© Fraunhofer ISE, S. W.Glunz, September 2013
Shockley-Queisser-Limit
Maximum efficiency as function of bandgap
 Detailed balance between
sun (temp. T1) and solar cells
(temp. T2)
0.35
 Maximum efficiency ~ 33%
0.30
 High non-absorption for
high band gap energy
Efficiency
 High thermalization for low
band gap energy
GaAs
0.20
mc-Si
InP
CdTe
0.15
 Actual record efficiency of
GaAs closer to limit than
Silicon
0.00
0.5
© Fraunhofer ISE, S. W.Glunz, September 2013
Record Efficiencies
c-Si
0.10
4
Shockley-Queisser
0.25
 Silicon and GaAs are close to
the optimum band gap
 Why?
Non-absorption
Thermalization
0.05
a-Si
GaSb
AlGaAs
Ge
1.0
1.5
EG [eV]
2.0
2.5
Shockley-Queisser-Limit
Other recombination channels
 Ideal solar cell has only radiative recombination
(Shockley and Queisser, J. Appl. Phys. 1961)
 But silicon is an indirect semiconductor
therefore direct recombination has no high
probability (three particle process)
Radiative recombination in an
indirect semiconductor
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© Fraunhofer ISE, S. W.Glunz, September 2013
Shockley-Queisser-Limit
Other recombination channels
 Ideal solar cell has only radiative recombination
(Shockley and Queisser, J. Appl. Phys. 1961)
 But silicon is an indirect semiconductor
therefore direct recombination has no high
probability (three particle process)
 In silicon solar cells Auger recombination
(non-radiative) is limiting loss mechanism.
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© Fraunhofer ISE, S. W.Glunz, September 2013
Auger recombination
process in an indirect
semiconductor
What is the maximum efficiency?
Including Auger recombination
 Shockley, Queisser (1961):
Efficiency limit of solar cell
made of one material =
33% (AM1.5)
 Theoretical efficiency for
silicon = 29.4%1
 Best silicon cell = 25%
 85% of theoretical
efficiency!
1Richter,
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© Fraunhofer ISE, S. W.Glunz, September 2013
Hermle, Glunz et al., IEEE J. Photovolt. (2013)
What is the maximum efficiency?
Including Auger recombination
 Theoretical efficiency for
silicon = 29.4%1
 Best silicon cell = 25%
 85% of theoretical
efficiency!
Shockley-Queisser
0.35
Record Efficiencies
0.30
Efficiency
 Shockley, Queisser (1961):
Efficiency limit of solar cell
made of one material =
33% (AM1.5)
GaAs
0.25
c-Si
0.20
mc-Si
InP
CdTe
0.15
0.10
0.05
0.00
0.5
a-Si
GaSb
Ge
1.0
1.5
EG [eV]
8
© Fraunhofer ISE, S. W.Glunz, September 2013
AlGaAs
2.0
2.5
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Plasmonic structure
 Diffractive optics
 Using the full spectrum
 Up-conversion
9
© Fraunhofer ISE, S. W.Glunz, September 2013
Light absorption
 Indirect semiconductors
as silicon exhibit a low
absorption for photons
with energies around
the band gap energy
IR
P. Würfel, Physik der Solarzellen
10
© Fraunhofer ISE, S. W.Glunz, September 2013
UV
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Plasmonic structure
 Diffractive optics
 Using the full spectrum
 Up-conversion
11
© Fraunhofer ISE, S. W.Glunz, September 2013
Light trapping in thin silicon solar cells
 Reduction of direct
reflection due to multiple
chances to enter the cell
 Oblique path of light
increases optical thickness
D. Kray, M. Hermle, and S. W. Glunz, Progress in
Photovoltaics 16 (2008)
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© Fraunhofer ISE, S. W.Glunz, September 2013
Light trapping in thin silicon solar cells
Perfect scattering
 Goetzberger,
IEEE-PVSC 1981:
Perfect rear scatterer
(Lambertian)
 Yablonovitch,
J. Opt. Soc. 1982:
 Absorption enhancement
limit 4n2
J. Eisenlohr, Diploma Thesis, Uni Freiburg (2012)
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© Fraunhofer ISE, S. W.Glunz, September 2013
High-efficiency cells with dielectrically passivated rear
Rear reflector
 Optimized Cell with improved
optical and electrical
performance
 Rear passivation layer serves as
internal reflector (low n)
- +
Passivation Layer
(SiO2, Al2O3/SiNX)
Reduced
Contact Area
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© Fraunhofer ISE, S. W.Glunz, September 2013
- +
Cell parameters as a function of thickness
21.5
21.0
Diffusion length
(Material quality)
High
(L>> 250 μm)
Medium (L 250 μm)
Low
(L<< 250 μm)
 [%]
20.5
20.0
19.5
19.0
18.5
18.0
0
50
100
150
200
250
Thickness [µm]
Thin cells can improve electrical characteristics (carrier confinement)
But: Light trapping has to be excellent!
15
© Fraunhofer ISE, S. W.Glunz, September 2013
Cell parameters as a function of thickness
 Measurement can be
described by parameters
extracted from test
structures
2
Short-circuit current Jsc [mA/cm ]
 Short-circuit current as a
function of cell thickness
of a PERC cell on
high-quality material
40
39
38
37
Measurement
Simulation
36
0
50
100
150
200
250
Cell Thickness W [µm]
Thin cells can improve electrical characteristics (carrier confinement)
But: Light trapping has to be excellent!
16
© Fraunhofer ISE, S. W.Glunz, September 2013
Ultra-thin high-efficiency cells
 20.2% on 37 μm thin
wafer using LFC
technology
 750 mV with HIT cell
structure by
Sanyo/Panasonic on
98 μm thickness
S.W. Glunz, Sol. Energy and Solar Cells 90 (2006)
17
© Fraunhofer ISE, S. W.Glunz, September 2013
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Plasmonic structure
 Diffractive optics
 Using the full spectrum
 Up-conversion
18
© Fraunhofer ISE, S. W.Glunz, September 2013
Improving light absorption
Plasmonics
 Scattering at metal
nanoparticles at
front
 longer effective
path length
 Metal nanoparticles  Corrugated
embedded in
semiconductor/metal
semiconductor
back surface
 Near–field
 longer effective
excitation of carriers
path length
Atwater and Polman, Nature Materials 9 (2010)
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© Fraunhofer ISE, S. W.Glunz, September 2013
Improving light absorption
Plasmonics
 Plasmonic on rear side:
Only small wavelength range has
to be considered
 Less parasitic absorption in Ag
particles possible
 Separation of Ag particles from
semiconductor by thin
passivation layer (e.g. Al2O3)
 Better electrical performance
 Uniform size distribution crucial
Jüchter et al., EUPVSEC (2013)
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© Fraunhofer ISE, S. W.Glunz, September 2013
Fabrication of Nanoparticles
Interference Lithography, Imprint, Metallization, Lift-off
 Interference lithography masters
up to m2 possible
 Uniform distribution of particle size achieved
 Proof of concept at cell level under investigation
~31 nm
~150 nm
Jüchter et al., EUPVSEC (2013)
21
© Fraunhofer ISE, S. W.Glunz, September 2013
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Plasmonic structures
 Diffractive optics
 Using the full spectrum
 Up-conversion
22
© Fraunhofer ISE, S. W.Glunz, September 2013
Increasing light absorption
Diffractive rear surfaces
 Heine and Morf,
Applied Optics 34 (1995)
 Diffractive optics
 But: Structuring of silicon can
increase surface
recombination
Heine and Morf, Applied Optics 34 (1995)
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© Fraunhofer ISE, S. W.Glunz, September 2013
Increasing light absorption
Diffractive rear surfaces
 Diffractive allows longwavelength light to be
redirect in very shallow angles
incoming light
emitter
 Optical properties decoupled
from rear surface passivation
diffracted light
 Optically rough,
Electrically flat
passivation
 Important especially for very
thin cells
diffractive rear side structure
© Fraunhofer ISE, S. W.Glunz, September 2013
base
rear side metal
Janz et al., EU-PVSEC 2009
Voisin et al., EU-PVSEC 2009
24
antireflection layer
Increasing light absorption
Diffractive rear surfaces
 Optimization using rigorous coupled wave analysis (RCWA1) simulation
Simulated photo current densities
2
photo current density [mA/cm ]
45
40
AM1.5g (280-1200nm)
35
front: planar (no ARC) - rear: mirror
front: planar (DARC) - rear: mirror
30
25
front: planar (no ARC) - rear: perfect scatterer
front: planar (DARC) - rear: perfect scatterer
20
15
10
front: inverted pyramids with SARC - rear: mirror
front: planar (DARC) - rear: sphere grating
1
10
cell thickness [m]
1RCWA
P. Lalanne, Reticolo 2D
25
© Fraunhofer ISE, S. W.Glunz, September 2013
100
Increasing light absorption
Diffractive rear surfaces
 Realization by self-organized
opaline structures
 Hexagonal photonic structure
 Deposition of ALD Al2O3
layer (surface passivation)
 Spin Coating of spherical
nanoparticles (SiO2)
Eisenlohr et al., EU-PVSEC 2011
26
© Fraunhofer ISE, S. W.Glunz, September 2013
Increasing light absorption
Diffractive rear surfaces
 Realization by self-organized
opaline structures
 Hexagonal photonic structure
 Deposition of ALD Al2O3
layer (surface passivation)
 Spin Coating of spherical
nanoparticles (SiO2)
 Structure filled up using
ALD of TiO2
(alternative SolGel)
Eisenlohr et al., EUPVSEC (2013)
27
© Fraunhofer ISE, S. W.Glunz, September 2013
 Realization by self-organized
opaline structures
 Hexagonal photonic structure
 Deposition of ALD Al2O3
layer (surface passivation)
 Spin Coating of spherical
nanoparticles (SiO2)
 Structure filled up using
ALD of TiO2
(alternative SolGel)
absorption enhancement A-Aref[%]
Increasing light absorption
Diffractive rear surfaces
20
15
10
5
0
-5
700
wafer thickness 100 m, measured
wafer thickness 250 m, measured
800
900
1000
wavelength [nm]
 Current gain 1.5 mA/cm2
 Surface passivation < 10 cm/s
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© Fraunhofer ISE, S. W.Glunz, September 2013
1100
Eisenlohr et al., EUPVSEC (2013)
1200
1300
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Plasmonic structures
 Diffractive optics
 Using the full spectrum
 Up-conversion
29
© Fraunhofer ISE, S. W.Glunz, September 2013
Specifications for solar cell upconverter devices
 Bifacial solar cell
 Large transmittance of subband-gap photons
 Large EQE of UC photons from
the rear
 Upconverter
 Large absorptance
 Large upconversion quantum
yield (UCQY)
 Broad absorption range
 Er3+ based upconverter
 Shockley-Queisser limit enhanced
from 30% to 40%1
1T.
Trupke, et al., Sol. Energy Mater. Sol. Cells 90, 3327 (2006)
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© Fraunhofer ISE, S. W.Glunz, September 2013
Symbolic example
Upconversion (UC)
Er3+ based upconverter
Laser
1523 nm
 Energy transfer most efficient
UC process (ETU)
 Subsequent absorption of
photons (GSA, ESA)
 UC luminescence due to
spontaneous emission (SPE)
 Multi-phonon relaxation
(MPR)
F. Auzel, Chemical Review 104, 139-73 (2004)
K. W. Krämer, et al., Chemistry of Materials 16(7), 1244-51 (2004)
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© Fraunhofer ISE, S. W.Glunz, September 2013
Solar cell upconverter device
Short-circuit current density
 Monochromatic laser excitation
 1.79% at 1000 W/m2 (0.179 cm2/W)
 Broad-band excitation
 0.77% at 1063 W/m2 (0.072 cm2/W)
 2.2 mA/cm2 under 78 suns
 Solar concentrator
 13.3 mA/cm2 under 207 suns
 Efficiency increase of 0.19%
1S.
Fischer et al., submitted to SOLMAT (2013)
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© Fraunhofer ISE, S. W.Glunz, September 2013
Conclusion
Q: “Nanophotonics - essential ingredient for efficient and
cost-effective solar cells?”
A: Yes, but …
 Don´t deteriorate the surfaces electrically!
 Structures should be applicable and not only
scientifically sexy!
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© Fraunhofer ISE, S. W.Glunz, September 2013
Acknowledgements
My sincere thanks
 to my all my coworkers at Fraunhofer ISE especially to
Johannes Eisenlohr, Stefan Fischer, Benedikt Bläsi, Jan-Christoph
Goldschmidt
 for the fruitful and pleasant national and international cooperation
 for the budget from industry, German Government and European
Commission
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© Fraunhofer ISE, S. W.Glunz, September 2013
AGENDA
 Crystalline Silicon: How to handle an indirect semiconductor?
 Theoretical efficiency limit
 Light absorption
 Increasing light absorption: Classical approaches
 Pyramids
 Internal reflection layers
 Increasing light absorption: New concepts
 Diffractive optics
 Plasmonic structure
 Using the full spectrum
 Up-conversion
 Spectral splitting
35
© Fraunhofer ISE, S. W.Glunz, September 2013
Using the full spectrum
Spectral splitting
 Spectral splitting of the spectrum
using beam splitters
 Different cells adapted to each part
of the spectrum
 Realized system: Efficiency 32.1%
B. Mitchell et al., Progress in Photovoltaics 19 (2011)
36
© Fraunhofer ISE, S. W.Glunz, September 2013