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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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Nor

mal

ized

out

put

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

FFIC

IEN

CY

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