Polysilicon

Wafers

Cells Modules

RenewableEnergy

CorporationHIT solar cells

L. CarnelScanwafer

HIT solar cells

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HIT = Heterojunction with Intrinsic Thin-layer

HIT solar cells: first used by Sanyo in 1992 and now used forhigh-efficiency solar cells (250 MWp in 2007)

Heterojunction emitter: two different semiconductormaterials (a-Si:H and c-Si) create the pn-junction

diffused homojunction emitter

Intrinsic: between the p and n type material there is anundoped (“intrinsic”) amorphous Si layers

Thin layer: total a-Si:H layer is typically less than 20 nm

HIT solar cells

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The cross-section is different than ”traditional” structures

”SANYO” cell ”TRADITIONAL” cell

HIT solar cells

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a-Si:H/c-Si heterojunction gives a band offset

p+i a-Si:H n c-Si n+ c-Si p c-Si

EfEg,c-Si

ΔEv

ΔEc

EfEg,c-Si

Eg,a-Si:H

HIT solar cells

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Why Heterojunction ?

a-Si:H passivates the surface very well (Seff below 10 cm/spossible) and creates pn-junction

Surface passivation by reducing surface dangling bonddensity and by field effect passivation of doped a-Si:H

Low-temperature approach (< 200 °C)

HIT solar cells

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Sanyo is still far ahead of laboratories …

16,07762833,21 cm2n CZAIST

16,58064432,21 cm2p FZIMEC

18,27766735,41 cm2p FZNREL

16,3746900,3 cm2p FZ

18,57963436,81 cm2p FZ

19,1826820,3 cm2n FZIMT

22,37972538,9100 cm2n CZSanyo

n FZ

FZ / CZ

1 cm2

Area Eff(%)

FF(%)

Voc(mV)

Jsc(mA/cm2)

19,87963939,3HMI

HIT solar cells

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OUTLINE

Monocrystalline silicon Interface between a-Si and c-Si

Transparency of a-Si:H and ITO

Texturing of c-Si

Polycrystalline silicon Introduction

Heterojunction vs. homojunction emitter

HIT solar cells

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1. Importance of interface properties

The lifetime of the passivated structure is excellent precursorto study the passivation of the HIT emitter

S. Taira, et al.

HIT solar cells

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1. The influence of the intrinsic amorphous Si layer

Provides extremely good surface passivation (3 orders ofmagnitude less defects than doped a-Si:H)

N. Jensen

HIT solar cells

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1. Deposition temperature and annealing influences thesurface passivation

S. Dauwe, et al.

HIT solar cells

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1. The heterojunction needs an abrupt interface

High deposition temperature leads to epitaxial Si and bad Voc

T.H. Wang, et al.

HIT solar cells

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2. Thickness of a-Si:H is compromise between high Vocand high Jsc

a-Si:H has direct bandgap high absorption coefficient

Minimise thickness to reduce the absorption in the a-Si:H(lost for collection)

N. Jensen, et al.

HIT solar cells

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2. ITO provides high transparency and low resistivity

Both requirements lead to a compromise since they dependon the dopant density of the ITO

S. Taira, et al.

HIT solar cells

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3. Texturing of c-Si needs to be smooth

The texturing of the c-Si needs to be rounded so thatcontinuous a-Si film is deposited

N. Takuo, et al.

HIT solar cells

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OUTLINE

Monocrystalline silicon Interface between a-Si and c-Si

Transparency of a-Si:H and ITO

Texturing of c-Si

Polycrystalline silicon Introduction

Heterojunction vs. homojunction emitter

HIT solar cells

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1. The cross-section of a thin-film polysilicon solar cell

HIT solar cells

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1. The grain size of polycrystalline silicon << mc-Si

5 µm5 µm

Fine-grained polySi

Coarse-grained polySi

HIT solar cells

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1. Homojunction emitter is a barrier for H atoms

Without emitter

With emitter

Depth (µm)

HIT solar cells

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1. Different processing due to different temperatures

HOMOJUNCTION HETEROJUNCTION

ARC deposition

homojunction

passivation

substrate

growth of p+ p layer

passivation

heterojunction

HIT solar cells

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2. Heterojunction increases Voc by 100 mV, before andafter hydrogenation

Homojunction

Heterojunction

Current density

(mA/cm2)

Voltage

(V)

HIT solar cells

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2. The heterojunction leads to base limited solar cell

Illumination

(Suns) Voltage(V)

HeterojunctionHomojunction

HIT solar cells

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2. Reason for the higher Voc is the absence of preferentialdoping spikes

3 µm

3 µm

n+ emitter

p-type layer + n+

peaks

p+ backside

2 µm

ITO layer

p-type layer

HIT solar cells

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Conclusion

HIT solar cells result in efficiencies > 22 %

Large discrepancy between results at Sanyo and researchlaboratories

Reduction of interface recombination is key to success

Also for small-grained polysilicon layers the HIT emitter isbeneficial

HIT solar cells

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Acknowledgements

IMEC-SOLO; Scanwafer T&U; Dr. A. Ulyashin