High Efficiency Triple-Junction Solar Cells Employing Biomimetic Antireflective Structures
M.Y. Chiu, C.-H. Chang, F.-Y. Chang, and Peichen Yu,
Green Photonics Laboratory Department of Photonics
National Chiao-Tung University, Hsinchu, Taiwanhttp://www.ieo.nctu.edu.tw/gpl/
ERATO Symposium, Tohoku Univ. Japan 2/16
Outline
IntroductionBiomimetics “Moth-eye” principle
Device Fabrication ProcessPolystyrene nanosphere lithography
Optical and Photovoltaic CharacteristicsReflectance Engineering via RCWA*Summary
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*RCWA: rigorous coupled-wave analysis
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Self-cleaning abilities of a lotus leave:
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Biomimetics
http://spie.org/x33323.xml?ArticleID=x33323
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Colors of butterfly wings:
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Biomimetics
http://www.imtek.de/
Man-made polymer photonic crystals
http://www.science.org.au/
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The moth-eye principle : broadband and omni-directional AR
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Biomimetic Antireflective Structures
http://tywkiwdbi.blogspot.com/1608/11/scanning-electron-micrographs.html
Si Polymer
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Sub-wavelength structure (SWS)
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“Moth-eye” principle
λ
air
semiconductor
nair
n
ns
neffGraded-index
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Graded refractive index profile
Introduction
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air
semiconductor
Multi-layer ARC:•Material selection for different refractive indices•Thermal constant mismatch that change mechanical and optical properties
Biomimetic ARC:•Single layer SWS•Mechanically and optically robust and durable•Profile control possible
nair
n
ns
*ARC: antireflective coating
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Triple-junction solar cell
InGaP
GaAs
Ge
GaAs
Ge
Power conversion Efficiency ~40%
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Ga0.5In0.5P/GaAs/Ge Triple-junction solar cell Broadband absorption (300nm ~1800nm)Very thin thickness (~ a few micrometers)
Surface Recombination =>SWS fabricated on SiNx passivation layer
Current Matching => Reflectance engineering
Triple-junction solar cell with SWS
λInGaP
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GaAs
Ge
GaAs
Ge
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Polystyrene Nanosphere LithographyRequirements for substrate Hydrophilic surface Homogeneous chemical propertyFlat and clean surface
Spin Coating:1.Scan speed2.PS solution concentration
Substrate
Poly Styrene (PS) sphere
4” wafer10 μm
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ERATO Symposium, Tohoku Univ. Japan 11/16
200 nm1 μm
Profile Control via RIE
Sacrificial mask for reactive ion etching (RIE)
~100 nm-thick SiNx was kept for passivation
TJ waferTJ wafer
SiNx (n~1.8)
Polystyrene
TJ wafer
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200 nm
1-step etching 2-step etching
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Reflectance spectra (measured by an integrating sphere)
SWS shows a much flatter spectrum, particularly in UV and IR.Reflectance of SWS can be further improved by choosing a
passivation material with a higher refractive index than SiNx, ~1.8.SWS designed to enhance the spectral response of the current
limited junction
Optical Characterization
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400 600 800 1000 1200 1400 16000.0
0.5
1.0
1.5
SL-ARC SWS AM1.5D
Irrad
ianc
e (W
m-2nm
-1)
Wavelength (nm)
0
20
40
60
80
100
Ref
lect
ivei
ty (%
)
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Device Fabrication Flow
pattern ohmic GaAs 1um SiNx deposition
metal evaporation
middle cell: GaAs
top cell: Ga0.5In0.5P
bottom cell: Ge
cap GaAsfront contact
rear contact
RIE etching for SWS
3J wafer
spin on PS spheres
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ERATO Symposium, Tohoku Univ. Japan 14/16
Device Characterization
Current-Voltage measurement
AR condition w/o ARC SL ARC SWS
Voc (V) 2.51 2.48 2.52
Jsc (mA/cm2) 9.36 11.37 11.62FF(%) 84.98 86.42 86.42
Efficiency (%) 19.93 24.41 25.26
Jsc is increased by 24.2% and 2.2%, compared to those without ARC and with SLARC, respectively.
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0.0 0.5 1.0 1.5 2.0 2.50
2
4
6
8
10
12
14
SWS SL-ARC no ARC
Cur
rent
Den
sity
(mA
/cm
2 )Voltage (V)
No ARC
SL-ARC
SWS
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Modeling parameters
Rigorous Coupled Wave Analysis
SWS AR 7x7 hexagonalSiNx parabola arrayPeriodicity~600 nm
Height ~ 900nmpassivation SiNx 100 nm
Window layer 50% AlInP 50 nmTop cell 50%GaInP 500 nm
1. Al0.5In0.5P and Ga0.5In0.5P n,k mismatch.2. Only top cell is included.
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Summary
We have successfully fabricated SiNx-based SWS for a Ga0.5In0.5P/GaAs/Ge triple-junction solar cell utilizing the polystyrene nanosphere lithography.
PCE and Jsc of triple-junction solar cell were enhanced due to the absorption improvement of the GaAs mid-cell.
The angular response of photocurrent nearly follows the cosine law and demonstrates the omnidirectionality of SWS .
An RCWA approach enables the reflectance engineering for Jscoptimization of tandem cells with the SWS.
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ERATO Symposium, Tohoku Univ. Japan 17/16
Thanks for your attention!!
Green Photonics Lab. at NCTU
