3 Oerlikon Christopher Con Tan Tine

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Thin-Film Silicon Cell Technology:Current and Near Future

Dr. Johannes Meier, Head of R&D Thin FilmDr. Chris Constantine, Director-New Technologies


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Page 2 01/10/2008 Oerlikon Solar - Constantine, 24 Sept 08

Agenda

IntroductionCrystalline silicon thin-film silicon

Material issues

Amorphous siliconMicrocrystalline silicon

Solar cell devices

p-i-n solar cellsMicromorph tandem cells


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Si Wafer PV Thin-Film Si PV

Bulk absorber material:

High quality silicon safe wafer

developed by microelectronicsemiconductor industry over50 yrs (not by PV!)

PV cell technology: Front andback side passivation of wafer for

internal electrical field and light-trapping & contacting of wafer

ZnO

glass

c-Si:H

a-Si:H

ZnO

Substrate is a part of the device

Absorber material needs to be

deposited:- Electronic properties- Interfaces & structure- Light-trapping- Uniformity of layers

- Monolithic interconnection- TCO (T, Haze, Rsq)

- High rate & good quality overlarge areas (m2)


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Comparison of Atomic Structure of c-Si and a-Si:H

Single crystal silicon

Hydrogenated amorphous silicon

Courtesy IMT-NE


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(electrons)

Ec

Ef

Ev

phi

n

*

(holes)

E

a-Si:H

Substrate(glass)

back contact (1-2m)

n-layer (~20nm)

i-layer (~250 nm )

p-layer (~20nm)front contact

(1-2m)

h

The Amorphous p-i-n Solar Cell: ~0.3 m Thickness!

p/n doped layers create electric fieldi-layer = active layer, where:

photons are absorbed

electron-hole pairs are generated,separated and transported to the contactsCourtesy IMT-NE


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Deposition of Amorphous Silicon

Amorphous silicon is deposited by a

plasma process from a mixture ofsilane and hydrogen

Plasma deposition parametersinfluence the quality of the layers

High Frequency RF Increase Plasma

Energy and Dep Rate

Gas Inlets

SubstrateElectrode(Ground)

PoweredElectrode

Frequency

Generator

Impedance

MatchingUnit

Vacuum System

Substrate

H2

SiH

B H62

PumpingUnit

4

Plasma

Heater

Heater

200 - 250C

PH3

/

PECVD (Plasma EnhancedChemical Vapor Deposition)

Courtesy IMT-NE


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Transition to Microcrystalline Silicon by Hydrogen Dilution

X-Rays diffraction spectra of c-Si:H layers on glass as a function

of SC= SiH4/SiH4+H2

5

10

15

20

25

5 5.5 6 6.5 7 7.5 8nanocrystalsize

[nm]

SiH4 /(SiH4+H2) [%]

(220)

(111)

(311)

Nanocrystallite size :

E. Vallat-Sauvain et al., Advances in Microcrystalline Silicon Solar Cell Technologies (2006)


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Microstructure of Intrinsic c-Si:H

i-layer microstructure depends on preparation conditions (SC) and on substrate(underlying layer type)!

Typical AFM topography top view of a 2m

c-Si:H layer on glass: rough! (rms 16 nm),depends on layer thickness

3 m

Corresponding TEM projected view



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Microstructure as a Function of Silane Concentration


SC=SiH4/SiH4+H2

crystallinity

High crystallinity,

Porous material

Typical material

microstructure forbest devices

Protocrystalline

material

Amorphous

material

30-100%10 %


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Microcrystalline Silicon p-i-n Solar Cell

Institut de Microtechnique Neuchtel in 1994World-wide first thin film c-Si cell deposited at 200 C

By H2-dilution in the same reactor

Substrate

TCO / BC

n

i-layer

pTCO

h

Subst rate

TCO / BC

ni-layer

pTCO

h

a-Si:H

c-Si:Ha-Si:H

c-Si:H


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c-Si:H Solar Cells: Spectral Response and Jsc

Typical QE depends on Collection Cell thickness Light-trapping

Short-circuit current obtained:

JSC

20 to 30 mA/cm2

for i-layer thicknesses of 1-3 m400 600 800 1000

Spektralre

sponse[a.u.]

Wavelength

a-Si:H

c-Si:H


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Stability with Respect to Light-Soaking (Staebler-Wronski)

a-Si:H

0

0.2

0.4

0.6

0.8

1

100 102 104 106

Exposure time [sec]

Norm

alizedeffi

ciency[%]

cell temperatureT = 485C

c-Si:H

1h 10 h

(final) (init)intensity(suns)

thicknesscell

7.7 %7.7 %103.6 mc-Si:H5.1 %10.0 %60.6 ma-Si:H

100 h

Well-designed c-Si:H cellsare stable!


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MicromorphTandem Cell Concept (IMT Neuchtel 1994)

Gap

1.1 eV

1.7 eV

c-Si:H

a-Si:H(top cell)

ZnO

1.5 - 3 m

0.2 - 0.3 m

back contact

light

glass

Optimal Bandgap Combination for tandem cells: 1.1 eV & 1.7 eVT.J. Coutts et al., PVSEC-12

ZnO

c-Si:H

a-Si:H

ZnO

glass

Micro- morph

SEM micrograph

Courtesy of IMT-NE


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Light Trapping by Rough TCO

Reduction of fabrication time Cost reduction (/Wp) Improvement of stability in case of amorphous cells

Reduction of thickness in silicon thin film solar cells:

s u n l i g h t

a-Si:Hoptical path

silver or Aluminum

TCO TCO

top contact(transparent

infraredmirror


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Light-trapping by Front TCOs Surface Morphology

SnO2 (Best commercial TCO) Oerlikon Solar LPCVD ZnO

as-grown


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1.4 m2 a-Si:H p-i-n Modules on

LPCVD ZnO and Commercial SnO2

ZnO

SnO2

LPCVD ZnO SnO2


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Concept of TCO Intermediate Reflector: a-Si:H/TCO/c-Si:H)

standardtandem cell

c-Si:Ha-Si:H

tandem cell

with intermediate

ZnO reflector

back reflector

Introduced by IMT Neuchtel 1996


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Effect of Intermediate Reflector on Spectral Response

400 500 600 700 800 900 1000

QE[a.u.]

Wavelength [nm]

a-Si:H/ /c-Si:HZnOJsc-increase in the a-Si:H top cell

Improvement of stability and

efficiency


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Japan: Intermediate Reflector by PECVD (in-situ)

K. Yamamoto et al. WCPEC-3 (2003, Osaka)


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Future Trends to Improve Efficiencies

More and more complex

stacked structures (> 15 layers)for enhanced efficiency

Challenge for industrialisation:Build universal PECVDreactors for deposition of very

thin individual functional layers

a-Si:H,a-SiO:H

a-SiC:H,..Int. Reflector

Top cell

Middle cell a-Si:H, c-Si:H,a-SiGe:H

Int. Reflector

Bottom cellc-Si:H,

c-SiGe:H,..

Back reflector

Subst rate

TCO

n

pTCO

ARC

p

n

p

n

h

i

i

i

ARC


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Summary: Why Thin-Film Silicon Solar Cells?

Much less material involved* which is non-toxic and abundant

Lower temperature processes (a-Si:H ~180-250C) allows low costsubstrates, like glass, st.-steel, etc. (encapsulation only from one side)

Composition of multi-junction cells (tandems, triples)

Monolithic series connection of modules

Large-area deposition process

Better temperature coefficient (higher kWh/kWp)

Low energy payback time (1-2 years) Still further room for improvement (advanced light-trapping)

Attractive advantages compared to Si wafer-based PV

(a-Si:H 0.2g SiH4/Wp vers. 10 g/Wp for c-Si)*

*P. Lechner et al., Proc. 21 EPVSEC(2006)


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Thank You !

3 Oerlikon Christopher Con Tan Tine

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