Thin film silicon technology - AEIT Sez. di...
Transcript of Thin film silicon technology - AEIT Sez. di...
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Thin film silicon technology
Cosimo Gerardi 3SUN R&D Tech. Coordinator
Outline
• Why thin film Si? • Advantages of Si thin film • Si thin film vs. other thin film • Hydrogenated amorphous silicon • Energy gap / band gap engineering • Tandem junction: amorphous/microcrystalline Si • Triple junction and multiple junctions • Light trapping • Technology roadmap
Solar Cell Technology: Why Thin Film Si?
Solar cell Si raw material Efficiency Peak power Peak power
c-Si 1200-1300 g/m2 16% 160W/m2 0.13W/g
TF-Si 5 g/m2 10% 100W/m2 20W/g
Large area on glass
Flexible plastics
Transparency
Air mass
T. Watanabe, Sharp, “4th Saudi Solar Energy Forum”, May 8-9t , Riyadh Saudi Arabia (2013)
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ized
out
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Temperature (°C)
amorphousc-Si
Temperature coefficient
The output of thin-film silicon solar cell decreases by only 0.23% when temperature increases by one degree, while that of crystalline silicon cell decreases by 0.45%
(Sharp@IEEE-IEDM 2008)
Robust structure: long term stability
T. Watanabe, Sharp, “4th Saudi Solar Energy Forum”, May 8-9t , Riyadh Saudi Arabia (2013)
Structure of glass-glass module
High barrier encapsulant material
a-Si:H distribution of density of allowed energy states for electrons
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Direct optical transitions are not forbidden in amorphous Si
(because of disorder)
Better light absorption than c-Si
Heavily hydrogenated network.
1-10% H2 content
Conventional p - n junction solar cell
• For an abrupt p-n junction with constant doping on each side there are no electric field outside the depletion region.
• Photogenerated carriers in these regions are collected by a diffusion process while in the depletion region by drift
VOC
JSC
Current Density
Voltage
DarkLight
JL JM
VM
e-
n-type layer
h+p-type layer
Metal grid Antireflective layerSunlight
Back Metal Contacthν > EG
EF
Vbi
Electron Hole
EC
EV
Vp
BSFpn
n p p+
Amorphous a-Si:H: p-i-n
i p n • Carriers are photogenerated in the
intrinsic region and collected by drift • p and n doped layers are very thin 15-
20nm to allow all available photons to absorbed by the i-layer.
• The excess doping level induces many defects/traps in n and p layers that recombine the photo-generated carriers
• Typically the p-a-Si:H layer is a p-a-SiC:H layer (Eg~2eV) that
• The I a-Si: layer must be below 300nm to reduce Staebler-Wronsky light induced degradation
Energy gap
Light Eg1 Eg2 Eg3
Absorbed light
hυ>Eg~hυ>>Eg
Thermalization losses
Thermalization losses
(a) (b)Light
Energy=hυ
One junction
Multiple junction
Amorphous Eg=1.7-1.8eV «High» absorption in the green-blue
Microcrystalline Eg=1.1eV «High» absorption in the red-near I.R.
Enhanced absorption: double junction/tandem
Wavelength (nm)
Ligh
t int
ensi
ty (
kW/m
2 µm
) “spectrum splitting.”
Micromorph cell efficiency 11-14% Micromorph module efficiency 8.5-10.8%
glass
TCOa-Si:H – Top cell
µc-Si:H – Top cell
Back reflector
~3mm
~2µm
Light
Amorphous and Microcrystalline silicon
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Two materials with the same process: PECVD
a-Si:H Eg=1.8eV µc-Si:H Eg=1.1 eV
Typical process temperature: -180C a-Si:H -150C µc-Si:H Plasma conditions: 13.56 MHz 10-15kW Deposition rate ~ 0.5nm/s
Cathode
Anode
Plasma Substrate
From Single to Multiple junctions • Single Junction
–aSi:H cell with enhanced light trapping – TCO and Texturing Efficiency: 6 to 8% on module
• Double Junction / Tandem cell –highest theoretical efficiency: combination of absorber materials having band gap 1.8 eV (a-Si:H) for the top and 1.1 eV (µc-Si:H) for the bottom cell. Efficiency 12.5% on cell 10% on large module
• Triple junction / Quadruple Junction –a-SiGe:H middle absorber –a-Si:H/a-SiGe:H/ µc-Si:H Efficiency: 14-15% on cell, 12% on large module
Possible drawbacks of triple junction:
• Reduced throughput:~ 25% lower with respect to Tandem
• Power stabilization weakness (light induced degradation) of a-SiGe:H (15% to 18% LID degradation factor)
• Quadruple Junction Approach: Higher Voc and improved LID degradation
glass
textured TCO
a-Si:H top absorber
a-SiGe:Hmiddle absorber
µc-Si:H bottom absorber
ZnOAg
Eg: 1.752eV
Eg: 1.45eV
Eg: 1.1eV
Light trapping
p-i-n a-Si:H
p-i-n uc-Si:H
TCO
glass light
TCO Back reflector
~700nm
~250nm
~1.6µm
~50nm
Asahi VU APCVD (SnO2:F)
Asahi W
ZnO:B -MOCVD W text ZnO
• Texturing causes light scattering, increasing the optical path of photons in silicon • Natural texturing can be achieved during the CVD deposition process • Double feature texture (possible both for SnO2 and ZnO): higher and smaller
texturing shapes can be reached (but not ready for production!)
Impact of texturing and different TCO material
• ZnO has better transmittance at long wavelengths
• Higher Haze can be achieved with MOCVD
• Texturing (increases optical path) improves the currents generated in the top and bottom cells
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EXTE
RN
AL Q
UA
NTU
M E
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Wavelength (nm)
ZnO - H=20%ZnO - H=20%ZnO - H=20%SnO2-H=10%SnO2-H=10%SnO2-H=10%
a-Si:H
µc-Si:H
SnO2:F
ZnO:B
low haze TCO high haze TCO
TCO a-Si:H
µc-Si:H
BR
Haze (%)
Transmittance(%)
Thin Film Si Roadmap Multi Junction Full spectrum Triple J Double J
9-10% 10-12% 12-14%
SnO2:F U-Valley ZnO Double TCO Plasmonics
14-16%
Improving absorber layers and cell structure
Improved light trapping
Efficiency:
a-Si:H / µc-Si:H a-Si:H / a-SiGe:H/µc-Si:H
TCO
Back contact
Eg1 Eg2
Eg3
Eg4
Multiple gap solar cell
Long term research: beyond 16% efficiency
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Ultra-thin c-Si 3D Structures Ag Nanowires in TCO
Si - nanowires solar cells Plasmonic resonators
EU Research Programs