Novel light management for ultrathin solar cells

Novel light management for ultrathin solar cells

Novel light management for
ultrathin solar cells
Ruud E.I. Schropp
Energy research Centre of The Netherlands & TU/e Eindhoven
INC10
May 15, 2014 Gaithersburg MD, USA
www.ecn.nl
2
Partners in Solliance
(www.solliance.eu)
• ECN, the leading energy research institute in the Netherlands; thin film research group in Eindhoven
• TNO, the leading Dutch institute for applied scientific research
• Holst Center, a joint research initiative of imec and TNO (and ECN for PV); roll‐
to‐roll processes, mainly oriented towards OPV on flexible substrates
• TU/e, Eindhoven University of Technology, fundamental and technological research in PV
• imec, a world‐leading research institute in nanoelectronics, based in Flanders, participates with its OPV and CIGS PV research groups
• Forschungszentrum Jülich, is one of the large interdisciplinary research centres
in Europe. The Photovoltaics section of the Institute of Energy and Climate Research (IEK‐5) belongs to the world leading research institutions in the range of thin‐film photovoltaics based on amorphous and nanochrystalline
silicon.
3
Outline
• Key Challenges in Thin Film Photovoltaics
• Light trapping (c.q. “concentration”)
• Utilization of nanostructure in Photovoltaics:
– Conventional “random” textures and designed textures
– Nanorod, nanowire structures
• Conclusion
4
15‐5‐2014
5
Status of thin film solar cells
(laboratory ~ 1 cm2)
2014/05
• Thin film Si:
– Single junction: µc‐Si:H 10.7% (EPFL Neuchâtel); a‐Si:H 10.1% (Oerlikon/TEL Solar)
– Double junction (tandem): a‐Si:H/µc‐Si:H 12.3% (Kaneka); 12.3% (Oerlikon Solar)
– Triple junction: a‐Si:H/µc‐Si:H/µc‐Si:H 13.44% efficiency (LG Electronics)
• CIGS:
– Single junction: 20.8% (rigid; ZSW); 20.4% (flexible; EMPA)
• CdTe: – Single junction: 20.4% (First Solar & GE Global Research partnership)
• Perovskite
– Single junction 17.9% (KRICT)
• OPV: – Double junction 12.0% small molecule cell (Heliatek)
– Single junction 11.1% Mitsubishi Chemical
– Double junction: 10.6% UCLA‐Sumitomo Chemical
6
Installed PV capacity - world
7
Installed PV capacity - Netherlands
2013:
700 MWp
(0.5% of total
electricity
consumption)
8
PV system prices
(Germany)
9
Grid integration
Germany explores (and shifts) the frontiers
M. Lippert,
SAFT
Source: Fraunhofer ISE
(2013)
10
Commercial module efficiencies (selection)
wafer Si IBC
wafer Si IBC
wafer Si IBC
wafer Si HIT
wafer Si mono
wafer Si multi
CIGS
tf a/µcSi
M.J. de Wild‐Scholten
SmartGreenScans
(June 2013)
tf aSi
CdTe
Thinner is beautiful
1. (local) concentration of light  Better ratio of photocurrent over dark current  higher Voc’s*
Concentration can be achieved
in two ways
• Optical concentration outside the cell
• Concentration of light inside the cell by internal light trapping structures, such as diffractive and plasmonic structures.
In both cases:
IL photogenerated current
I0 reverse saturation current
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Thinner is beautiful
1. Better ratio of photocurrent over dark current 
higher Voc’s*
2. Collection of carriers also improves  higher FF’s
3. Better stability and thus higher stabilized efficiency (if Jsc is maintained).
4. Better cycle time (lower cost of ownership)
5. Lower materials consumption – further cost reduction.
R.E.I. Schropp
Key Challenges
•
reduce cost (by
thinning down)
higher efficiency
•
materials resources
•
Light trapping
Better spectral
matching (multijunction, photon
manipulation)
use only abundant, environmentally
benign materials
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Abundancy table
Various random light-scattering
morphologies (front TCO)
Asahi U Type
LPCVD ZnO
PVD ZnO+ Wet etch
Front
TCO
Random (bottom‐up)
morphologies
Haze
Challenge: increase the haze at long wavelengths,
and scatter light into large angles.
M. Zeman et al. , MRS Proc. Ser. (2010)
Scalable top down
technology:
Nanoimprint Lithography
Nanoimprint Lithography (NIL)
• Nano‐imprinting offers great perspectives to implement sub‐micron light management structures in solar cells in a cost effective manner.
• Can periodic textures outperform random textures? Periodic texture ‐
Random texture (ZnO)
Solution growth (ZnO)
 Two examples of periodic textures
20
1. Light trapping by surface plasmons
• Design of plasmonic nanoparticles patterns
for optimum waveguiding.
Fraction scattered into substrate highest for
cylinder & hemisphere:strongest near-field
coupling
Ohmic damping V, scattering V2,
Tradeoff: larger size  more scattering,
but lower coupling
Atwater & Polman, Nature Mater. 9, 205 (2010)
metal nanostructures
controllably couple
light to
in-plane waveguide
modes
Substrate Conformal Imprint Lithography
PDMS Stamp
Thin glass
PDMS stamp (6”) is bonded on 200 µm AF-45
glass
1 m
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 Sprang
Spring MRS 2007, 1002-N03-05
Electromagnetic Simulation for Generation Rate Calculations
simulation
FDTD simulations at discrete wavelengths
across the spectrum.
Assume that one photon absorbed
produces one electron:
AM1.5g Spectrum
for SR: generation rate at each λ
for Jsc: solar spectrum weighting
around each simulated point
V. E. Ferry, et al. Opt. Express, 18, A237-A245 (2010).
Vivian Ferry
24
Comparing Random and Designed Surfaces
= 500 nm
= 670 nm
1016
Nanopatterned
500 nm
= 500 nm
= 670 nm
1014
1013
Photon Flux (cm-3 sec-1)
1015
1012
Asahi
Controlled spatial curvature of metal layer is
critical to avoid losses and increase
V. E. Ferry, et al. IEEE PVSC Proceedings (2010).
photocurrent
Vivian Ferry
25
Periodic arrays
Penrose
Replicated Asahi
Pseudo-random
2D FT
Designed
classes of nanoparticles
Design constraints:
- 200 nm, 290 nm, 310 nm
diameter
- Ag particles should not
touch
- Same packing fraction
as the 400 nm periodic
pattern
Rounded metal
nanostructures:
Less parasitic absorption
Cell Fabrication
90 nm active layer
9.5%
ITO
a-Si:H
ZnO:Al
500 nm
Ag
SiO2 sol-gel
Cells deposit conformally
over nanopatterned back
contacts: best pseudorandom pattern and periodic
pattern have similar
efficiencies
Vivian Ferry
27
Broadband Light Trapping: Experiment vs. Simulation
Experiment: 90 nm i-layers
Mie
Simulation FDTD
waveguide modes
- Significant broadband photocurrent enhancement
- Good agreement between simulation and
experiment Vivian Ferry
28
2. Light trapping by scalable periodic
texture (with µc-Si rather than a-Si cells)
c-Si nip cells on steel foil with various back contacts:
1. Flat back contact
2. BC with 2D periodic texture
3. BC with random texture
(replica of SnO2:F)
(EU FP-7 Energy 2009 – 241477 project
“Silicon-Light”), Coordinator W. Soppe
(ECN).
www.silicon-light.eu
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
Light management with R2RR Nano-Imprint
Lithography
1. Modeling 2. Mastering
3. Tooling
4. Imprinting
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Single junction c-Si cells:
Jsc
Periodic textures can
outperform random textures
31
Single junction µc-Si cells (750 nm):
spectral response
32
Single junction uc-Si cells:
Fill Factor
Higher FF for periodic texture!
33
Lower FF on random textures
due to shunting
R.E.I. Schropp et al. Journal of Crystal Growth 311, 760 (2009)
Microcracks
34
Conformal growth without
microcrack formation on periodic
texture
Dark field TEM (one selected diffraction direction)
Bright field TEM
ITO
c Si
ZnO
Ag
HR-TEM by Martial Duchamp (FZ Jűlich)
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11% efficiency reference cell produced on nanotextured metal foil in a roll-to-roll process
350 nm top cell; 1000 nm
bottom cell
Best cell on CB‐R
Best cell on CB‐WR
Jsc top cell Jsc bottom cell FF [%] Efficiency Voc
[mV] [mA/cm2] [mA/cm2]
[%]
1381
11,84
12,31
67,0
11,0
1366
11,90
13,24
65,7
10,7
ECN labs @ Eindhoven 36
Roughness of which layer is
important?
90 nm active layer
ITO
a-Si:H
ZnO:Al
500 nm
Ag
SiO2 sol-gel
Where should the roughness be?
AZO
80 or 500 nm AZO
Ag
Asahi
Which roughness is important: that of the ZnO or that of the Ag?
Claire van Lare,
Albert Polman
15‐5‐2014
Frank Lenzmann
38
Periodic structures - simulations
• No structure in AZO 
poor light trapping
Claire van Lare
Periodic structures - simulations
• No structure in AZO 
poor light trapping
• Flat Ag can be better
than structured Ag
Claire van Lare
Absorption in Ag layer
• Structuring Ag leads
to increased parasitic
absorption
Claire van Lare
Photonic “roughness” rather
than geometric roughness?
15‐5‐2014
43
FLiSS made by polishing down

2D ZnO:Ga grating filled with n-type nc-Si:H

Voc with FLiSS
Voc with flat reference substrate
Chemical mechanical
polishing
H. Sai et al. Applied Physics Letters 98, 113502 (2011)
Plasmonic Flat Scattering Surfaces:
Lourens van Dijk
Simulation of Absorption enhancement
Scattering
Design
Flat
Lourens van Dijk
Absorption
Si
SiO2
Ag
Nanocylinder or nanowire
Yinghuan Kuang, et al., IOP Reports on Progress
in Physics, 2013 (accepted for publication)
Avoiding the trade-off
between “optically thick” and
“electronically thin”
Light direction
Traditional textured thin film
silicon solar cells
Current
direction
Glass
TCO
Si p-i-n
ZnO
Ag/Al
Trade-off:
Optically thick vs Electrically thin
Nanostructured three
dimensional (nano-3D)
solar cells
Yinghuan Kuang, et al., IOP Reports on Progress in
Physics, 2013 (accepted for publication)
R.E.I. Schropp
50
Avoiding the trade-off
between “optically thick” and
“electronically thin”
Light direction
Current
direction
+-
SUPER scattering!
Nanostructured three
dimensional (nano-3D)
solar cells
Yinghuan Kuang, et al., IOP Reports on Progress in
Physics, 2013 (accepted for publication)
R.E.I. Schropp
51
Avoiding the trade-off
Light direction
Elongated nanostructures:
“Nanorod solar cell”



Current
direction
Orthogonalize charge carrier
path and photon path
Super scattering (between
nanorods)
Anti-reflection texture
Yinghuan Kuang, et al., Journal of Non-Crystalline Solids,
358, 17, 2209-2213 (2012)
Challenges
 Conformal coverage (internal shunting
paths  low FF).
 Surface and interface recombination 
lowR.E.I.
Voc Schropp
Nanostructured three
dimensional (nano-3D)
solar cells
Yinghuan Kuang, et al., IOP Reports on Progress in
Physics, 2013 (accepted for publication)
52
Conformal n-i-p coverage by Hot Wire CVD;
“nano-3D” cells
•
•
•
•
•
•
ITO and grid
p‐layer (PECVD, B(CH3)3)
i‐layer (HWCVD, 25 nm)
n‐layer (PECVD, PH3)
ZnO:Al (38 nm)
Ag (20 nm)
nano‐3D cells
- Thicknesses determined by HRSEM
- For optimal conformal coverage, essential to use HWCVD
Yinghuan Kuang et al., Appl. Phys. Lett. 98 (2011) 113111. doi:10.1063/1.3567527
Synthesis of ZnO nanorods by chemical bath deposition
(Zn(CH3COO)2·2H2O)
+
C6H12N4
Hot Wire CVD Set-up at Utrecht University in Eindhoven
Two Ø0.5 mm Ta filaments
Substrate to filaments distance:
55 mm.
Distance between filaments: 40
mm.
Temp. at filaments: ~1750°C
Temp. at substrate: ~200°C
5
Introduction: Hot wire CVD for conformal coverage on
high aspect ratio structures
Qi Wang et al., NREL and Applied
Materials Inc.,
Appl. Phys. Lett. 84 (2004)
338
Makiko Kitazoe, Shuuji Osono, Hiromi Itoh, Shin
Asari, Kazuya Saito and Masahiro Hayama
ULVAC, Japan,
3rd Hot-Wire CVD Conference, Utrecht
Y. Kuang et al., Appl. Phys. Lett. 98,
113111 (2011).
500 nm
4
Cell results
• Flat, 75 nm
• Asahi texture, 75 nm
• Nanorod cell, 25 nm
8.3 mA/cm2 from a 25‐nm layer!
 Nanorod type solar cell with 25 nm i-layer has higher Jsc
than a flat or even a textured solar cell with 75 nm i-layer;
slight trade off with Voc.
Yinghuan Kuang et al., Appl. Phys. Lett. 98 (2011) 113111. doi:10.1063/1.3567527
ZnO nanorod substrate for thin film solar cells
rms:68
Nanorod+Ag+ZnO
Shorter rods
to prevent Voc loss
rms:37
rms:13
Quite different from
Asahi‐U
Asahi U-type +Ag+ZnO
7/14
Latest results
Cell
type
i-layer
Jsc (mA/cm2)
thickness (nm)
Flat
3D100
Flat
3D200
0.8
9.1
13.3
11.5
15.0
100
100
200
200
Voc
(V)
FF
η
(%)
0.89
0.86
0.89
0.85
0.70
0.62
0.63
0.65
5.7
7.1
6.4
8.4
200 nm
EQE
0.6
0.4
Flat
NR
0.2
0.0
400
500
600
700
800
Wavelength (nm)
Yinghuan Kuang et al., J. Non‐Cryst. Solids 358 (17) (2012) 2209‐2213
10/14
Conclusions
• New insight in scattering back reflectors for thin film solar
cells
• Plasmonic and dielectric scatterers have different
effects.
• Index contrast for geometrically flat scattering.
• These concepts are of interest to all thin film solar
cells. Large area manufacturing methods need to be
developed.
•Nanowire/nanorod enhancement schemes can reduce
cost of photovoltaically generated kWh’s.
• Approaches for non-lithographic random nanorod-type
solar cells are very promising.
Acknowledgments
Yinghuan Kuang, Lourens van Dijk,
Pim Veldhuizen, Marcel Di Vece,
Jatin K. Rath
W. Soppe,
Albert Polman
M. Dörenkämper,
Claire van Lare
N.J. Bakker
C.H.M. van der Werf
15-5-2014
Chinese
62
Scholarship
Yinghuan Kuang Lourens van Dijk
Council