high-efficiency thin film nano-structured multi-junction ...€¦ · photon management in thin film...
TRANSCRIPT
High-efficiency thin film
nano-structured multi-junction solar
cells James S. Harris (PI) (Co-PIs-Mark Brongersma, Yi Cui, Shanhui Fan)
Stanford University
GCEP Research Symposium 2013 Stanford, CA October 9, 2013
2
Single-crystalline
Thin film III-V PV
cSi
Cost per area
Efficiency
40%
30%
20%
10%
Solar Cell Efficiency and Cost
glass
Konarka
(~1 $/W)
(<<1 $/W)
(>>1 $/W)
CPV
High efficiency
Low cost
Higher efficiency reduces entire system cost (BOS)
3
Lowering Cell Cost--Ultra-Thin Films
Substrate 30%
Epitaxial growth
50%
Processing 20%
Estimated cost distribution
for III-V solar cells
Scaled processing (3X)
Substrate recycling (10X)
Ultra-thin films decrease material cost
Increased throughput decreases capital cost (10X)
3% 10%
6%
81% Substrate
Epitaxial growth
Processing
Save cost Cost savings
4
Outline
Photon management in thin film solar cells
• Key to cost reduction and efficiency improvement
• Enhanced optical absorption by nano-structuring
Nanostructured III-V solar cells
• Conventional designs
GaAs nano-junction cell performance and challenges
• New designs for high efficiency nanostructured solar cells
• Dielectric nanostructure window layer solar cell
Ultra-thin film Si cells for tandem junction cells
Conclusions
5
Benefits of Photon Management
-30
-20
-10
0
10
20
0 0.2 0.4 0.6 0.8 1
Cu
rre
nt
de
nsi
ty (
mA
/cm
2)
Voltage (V)
Carrier
confinement
Photon Management
Black: Thin film GaAs cell
Blue: Conventional GaAs cell
Red: Ultra-thin film GaAs cell
with photon management
Photon management is crucial for ultra-thin film solar cells
Y. Kang et al. 39th
PVSC, 2013
6
2Ω/square at 90% transmittance and 10Ω/square at 95% transmittance
H. Wu, D. Kong, S. Fan, Y. Cui: Nature
Nanotechnology 8, 421 (2013)
Metal Nanowire Transparent Conducting Electrodes
7
Meso- and Nano- Wire Transparent Electrodes
P. Hsu, S. Wang, Y. Cui et. al. Nature
Communication 4: 2522, 2013.
Meso-scale metal wires
reduce sheet resistance
by at least one order of
magnitude with the same
transmittance.
8
Enhancing Open Circuit Voltage with Nanophotonic Design
S. Sandhu, Z. Yu, S. Fan, Optics Express 21, 1209 (2013);
44nm GaAs
bulk GaAs
A. Niv et al PRL (2012);
9
Model cell: 1 m thick cSi cell Light absorption vs angle & wavelength
Wave optics regime: Optimizing absorption = Optimizing resonance excitation
1: Local (Mie) resonance 2:Fabry-Perot Resonance 3:Guided mode resonance 4:Diffracted resonance
Z. Yu et al., PNAS 107, 17491, (2010).
Total absorption = Aggregate of contributions of all ‘narrow’ resonances
Nanostructure Light Trapping
10
Outline
Photon management in thin film solar cells
• Key to cost reduction and efficiency improvement
• Enhanced optical absorption by nano-structuring
Nanostructured III-V solar cells
• Conventional designs
GaAs nano-junction cell performance and challenges
• New designs for high efficiency nanostructured solar cells
• Dielectric nanostructure window layer solar cell
Ultra-thin film Si cells for tandem junction cells
Conclusions
11
Prior Nanostructured Solar Cells
Back reflector
Hsu et al. Adv. Energy. Mat. 2012
Light trapping absorbers
Oh et al. Nat. Nanotechnol. 2012
Antireflection
Xi et al. Nat. Photonics, 2007
D. Liang, et.al. Adv. Energy Mat., 2012
Nano-structured, flexible
GaAs thin film
12
Nanostructured p-n Junctions
B. M. Kayes, et al. J. Appl. Phys. 97, 114302 (2005)
Nanostructured p-n junction:
1) Enhance absorption by antireflection and light trapping
2) Decouple the absorption length and carrier transport
13
-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6-5
0
5
10
15
20
25
Voltage (V)
Curr
ent D
ensity (
mA
/cm
2)
Planar
Nano-pyramid
Nano-dome
Challenges for III-V Nano-
structured Solar Cells
200nm
200 nm Cell Jsc (mA/cm2) Voc (V) Fill Factor Eff. (%)
Planar 5.1 0.49 0.57 1.44
Nano-dome 7.5 0.35 0.48 1.21
Nano-pyramid 18.5 0.32 0.28 1.67
Nano-wire * 15.5 0.20 0.27 0.83
Planar and nano-pyramid cells with
200nm thick p-n junction
200nm
Nanostructured GaAs cells enhance Jsc, however degrade Voc and F.F.
New designs required to improve Voc and F.F. * Czaban et al. Nano. Letter 2008
14
Metal Contact Problems and New Nano-Cell Design
Metal Mesa grid design eliminates shunts
Shunts at valleys Add Insulation layer,
still some shunts
Planar junction/mesa
separates contact
from nanostructures
15
Outline
Photon management in thin film solar cells
• Key to cost reduction and efficiency improvement
• Enhanced optical absorption by nano-structuring
Nanostructured III-V solar cells
• Conventional designs
GaAs nano-junction cell performance and challenges
• New designs for high efficiency nanostructured solar cells
• Dielectric nanostructure window layer solar cell
Ultra-thin film Si cells for tandem junction cells
Conclusions
16
Nano-Pyramid Cell Design
Replace nanocone GaAs with nanocone window (AlGaAs, InGaP)
A heterojunction window
in III-V solar cell
• Window layer has wide bandgap --- transparent to most light
• Window layer repels minority carriers back to the junction
• Transport Electron-hole pairs in nanocones to p-n junction
p-GaAs 0.3 um
n-GaAs 2.0 um
Metal
p-AlGaAs 1.0 um
n-AlGaAs 0.1 um
17
Nano-Pyramid Window Solar Cell
Nanocone window
p Al0.8Ga0.2As
p GaAs
n GaAs
Metal mesa
D. Liang et al. Nano Letters 2013
18
Nano-Pyramid Absorption
Reflection < 3% from 450nm to 850nm
Nanocone window
Planar window
500nm Completely black
even under 1-sun
illumination
D. Liang et al. Nano Letters 2013
19
Performance: Measured J-V (1 sun)
Voc (V) Jsc (mA/cm2) Fill Factor Efficiency(%)
Planar window 0.979 21.23 63.1 13.1
Nanocone window 0.982 24.40 71.0 17.0
Improvement 0.3% 15% 13% 30%
0
5
10
15
20
25
0 0.2 0.4 0.6 0.8 1
J (
mA
/cm
2)
Voltage (V)
500nm
Nanocone window
Planar window
D. Liang et al. Nano Letters 2013
20
0
0.5
1
Ba
nd
ga
p O
ffs
et
(eV
) E
g-V
oc
Best Voc in nanostructure-based solar cells
1.12 1.42 1.49 1.75
c-Si GaAs CdTe a-Si
Bandgap Offset (Eg - qVoc)
Mariani et.al. Nano.Lett. 2011
Fan et al. Nat. Mat. 2009 Hsu et al. Adv. Energy. Mat.2012
Zhu et al. Nano. Lett.2010
Czaban et al. Nano.Lett.2008
Oh et al. Nat. Nanotechnol. 2012
Putnam et al. Energy Environ. Sci. 2010
Our work Nov 2012
Our work May 2012
Zhao et al. Appl. Phys. Lett. 1998
Kayes et al. 37th PVSC. 2011
21
Benefits of Nano-Pyramid Window
Enhanced light absorption and carrier collection from nanocone window
---- 15% improvement in Jsc (24.4 mA/cm2)
High-quality and low-area junction minimize J0
----- high Voc (1.003 V)
Metal mesa contact improves shunt and series resistance
----- good FF (71%)
----- 30% enhancement in efficiency (17%)
22
0 20 40 60 800
0.1
0.2
0.3
0.4
Incident Angle (degree)
Re
flecta
nce
(A
.U.)
SLARC
DLARC
Si3N
4 Nano
Nanostructured Dielectric Window
0.4 0.6 0.80
0.5
1
Wavelength ( m)
Re
flection
(A
.U.)
Planar
n=1.9
n=2.0
n=2.1
n=2.2
Reflection < 10%
Transparent, large band gap (5.5eV)
Refractive index ~2 and tunable
Widely used as anti-reflective coating
MgF2
SiO2
Si3N4
ZnO
ZnS TiO2
Al2O3
MgO
3
4
5
6
7
8
9
10
11
1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
Ban
d g
ap
(e
V)
Refractive index
0 20 40 60 800
0.05
0.1
0.15
0.2
Incident Angle (degree)
Re
flecta
nce
(A
.U.)
DLARC
Si3N
4 Nano
AlGaAs Nano
23
0.3 0.4 0.5 0.6 0.7 0.8 0.90
0.2
0.4
0.6
0.8
1
Wavelength ( m)
Absorp
tion (
A.U
.)
Si3N
4 window
Planar
Ultra-Thin Nano-Pyramid Cells
Strong light trapping, 88% absorption enhancement
Guided lateral propagating mode
Jsc = 28.55 mA/cm2
Jsc = 15.18 mA/cm2
Absorption in 200nm GaAs slab
24
Outline
Photon management in thin film solar cells
• Key to cost reduction and efficiency improvement
• Enhanced optical absorption by nano-structuring
Nanostructured III-V solar cells
• Conventional designs
GaAs nano-junction cell performance and challenges
• New designs for high efficiency nanostructured solar cells
• Dielectric nanostructure window layer solar cell
Ultra-thin film Si cells for tandem junction cells
Conclusions
25
Nano-Pyramid Si Cell Structure
Si3N4 window
Si Cell
n-type 0.3um
p-type 1.7um
1 x 1019
p-substrate 1 x 1015
[intrinsic Si] 0.9um
3 x 1018
Metal
contact
Y. Kang et al. to be published, 2013
26
0
5
10
15
20
25
30
0 0.2 0.4 0.6
Cu
rre
nt
den
sit
y
(mA
/cm
^2
)
Voltage (V)
Si3N4 nano window
Si3N4 SL ARC
w/o Si3N4
J-V Characteristics
Voc (V) Jsc (mA/cm^2) Eff. (%) F.F. (%)
Si3N4 Nano window 0.57 28.15 11.44 71.26
Si3N4 SL ARC 0.56 26.11 10.22 69.51
W/O Si3N4 0.54 21.32 8.08 69.32
Si3N4 nano-cone
enhances
Jsc by 32%
Eff. by 43%
Y. Kang et al. to be published, 2013
27
External Quantum Efficiency
0
10
20
30
40
50
60
70
80
90
400 500 600 700 800 900 1000 1100
EQ
E (
%)
Wavelength (nm)
Si3N4 nano window
Si3N4 SL ARC
w/o Si3N4
• In the visible light region, EQE is enhanced by over 30%;
• In the near infrared region, EQE is enhanced by ~15%,
due to the low absorption in thin film. Y. Kang et al. to be published, 2013
28
Summary
Demonstrated Highest Efficiency Nano-structured III-V solar cell
• Nano-scale photon management enables ultra-thin film solar cells
to achieve high efficiency
• The nano-window, planar junction, mesa contact design
simultaneously improves junction quality, light absorption, carrier
collection and eliminates shunting defects
• The AlGaAs nano-cone window solar cell showed and enhanced
efficiency of 17%
• Demonstrated highest transparency nano-metal contact grid
• Demonstrated thin film lift-off and flexible GaAs cell
• The Si3N4 nanostructure window design reduces the optical losses
in window layer and enhances the efficiency by 43%
29
Acknowledgements
Thank You
STUDENTS and POSTDOCS
Brongersma Group Fan Group
Dianmin Lin Sunil Sandhu
Cui Group Harris Group
Hui Wu Yangsen Kang
Desheng Kong Yusi Chen
Po-Chun Hsu Yijie Huo
Shuang Wang
Corporate Collaborators Research Support
Solar Junction
Solexel
OEpic