Post on 01-Feb-2018
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