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太阳级硅中杂质对电池效率的影响(英文)


Specification of solar grade silicon: How impurities affect efficiency
Bart Geerligs

20th EPVSEC Barcelona 2005, 2AO1.3

Outline
Objective and introduction I

ngots and cells from artificially contaminated silicon feedstock: – Titanium – Aluminum Analysis of results Implications for feedstock specification

20th EPVSEC Barcelona 2005, 2AO1.3
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Objective
Determine allowable concentrations of impurities in silicon feedstock for mc-Si solar cells. Reasons: specific Silicon produced for Photovoltaics possibility of Low-cost and abundant Silicon feedstock from carbothermic reduction of quartz. SiO2 + 2C → Si + 2CO Fe, Ti, Al, and C are major impurities in silicon from carbothermic reduction. What are the target levels for these impurities?
20th EPVSEC Barcelona 2005, 2AO1.3
3

Which specs are available?
Si wafer manufacturers: “we like to be on the safe side, the SEMI poly-Si spec [<0.1 ppmw total metals] works for us…” For PV, there exist more specific earlier studies:

J.R. Davis, et al., IEEE Trans El. Dev. ED-27, 677 (1980) Cz-growth

also, Fally et al., Revue Phys. Appl. 22, 529 (1987) mc-Si, but no info on Ti

20th EPVSEC Barcelona 2005, 2AO1.3
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Experimental procedure

poly-Si feedstock with added impurity

directional solidification furnace

ingot & wafers

20th EPVSEC Barcelona 2005, 2AO1.3
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Experimental procedure

poly-Si feedstock with added impurity

directional solidification furnace industrial in-line cell process SiNx:H coating 14.5 – 15% cell efficiency

ingot & wafers

20th EPVSEC Barcelona 2005, 2AO1.3
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Titanium
10 ppmw (parts-per-million by weight) of Ti were added to the feedstock
20

JscVoc (W/cm )

2

18

15%rel

25%rel

16 reference S10: Ti 0 20 40 60 80 100

14

position in ingot towards top (%)

20th EPVSEC Barcelona 2005, 2AO1.3
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Titanium
10 ppmw (parts-per-million by weight) of Ti were added to the feedstock
20 0
IQE (%)

18

16 reference S10: Ti 0 20 40 60 80 100

-50

14

-100

ingot S10, position in ingot: 13% 19% 24% 54% 65% 86% 400 600 800 1000 1200 wavelength (nm)

JscVoc (W/cm )

2

position in ingot towards top (%)

Reduction of Jsc due to strongly reduced red-response in IQE
20th EPVSEC Barcelona 2005, 2AO1.3
8

Aluminum
5 ppmw of Al were added to the feedstock
20

JscVoc (W/cm )

2

16

15%rel

25%rel

12

reference S6: Al 0 20 40 60 80 100

position in ingot towards top (%)

20th EPVSEC Barcelona 2005, 2AO1.3
9

Aluminum
5 ppmw of Al were added to the feedstock
20 0 JscVoc (W/cm )
2

16

IQE (%)

-50

12

reference S6: Al 0 20 40 60 80 100 -100

ingot S6, position in ingot: 14% 30% 53% 69% 400 600 800 1000 1200

position in ingot towards top (%)

wavelength (nm)

Reduction of Jsc again due to reduced red-response in IQE

20th EPVSEC Barcelona 2005, 2AO1.3
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Analysis of results
15-25% reduction of JscVoc due to 5 ppmw Al or 10 ppmw of Ti is too much to be acceptable. How can we determine the maximum allowable concentration? Can the cell efficiency for other concentrations be modeled and predicted? Is there experimental data to verify such a model?

20th EPVSEC Barcelona 2005, 2AO1.3
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Model for analysis
5

segregation during ingot growth

4 Cs (a.u.) 3 2 1 0.0 0

1 impurity concentration Cs ∝ 1 x

0.2 0.4 0.6 0.8 1.0 20 40 60 80 100 position in ingot x position in ingot towards top (%)

20th EPVSEC Barcelona 2005, 2AO1.3
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Model for analysis
1 5 Leff in solar cell (a.u.) Cs (a.u.)

segregation during ingot growth

4 3 2
top 1 0 0.0 0 bottom

1 impurity concentration Cs ∝ 1 x
If impurity dominates recombination

1 1 ∝ ∝ Cs 2 Leff τ eff
Leff ∝ 1 x

0.4 (1-x) 0.6 0.8 1.0 1 √ (square root ofposition in ingot xtowards bottom) position in ingot

0.2

Leff can be determined from the red-response of the IQE
20th EPVSEC Barcelona 2005, 2AO1.3
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Comparing Leff from IQE with model
Leff follows expected decrease to top of ingot. (exception: bottom of S6)
Leff (m) from IQE 150 S10 (Ti) S6 (Al)

100

50
top 0 0.0 bottom

0.6 0.8 1.0 √(1-x) (square root of position towards bottom of ingot)

0.2

0.4

20th EPVSEC Barcelona 2005, 2AO1.3
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Comparing Leff from IQE with model
Leff follows expected decrease to top of ingot. (exception: bottom of S6) Conclusion: Relation between feedstock contamination and recombination is linear (no non-linear effects from precipitation, etc.).
Leff (m) from IQE 150 S10 (Ti) S6 (Al)

100

50
top 0 0.0 bottom

0.6 0.8 1.0 √(1-x) (square root of position towards bottom of ingot)

0.2

0.4

20th EPVSEC Barcelona 2005, 2AO1.3
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Comparing Leff from IQE with model
Leff follows expected decrease to top of ingot. (exception: bottom of S6) Conclusion: Relation between feedstock contamination and recombination is linear (no non-linear effects from precipitation, etc.). (at least for Al, Ti, for the used concentrations and probably lower)
20th EPVSEC Barcelona 2005, 2AO1.3
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Leff (m) from IQE

150

S10 (Ti) S6 (Al)

100

50
top 0 0.0 bottom

0.6 0.8 1.0 √(1-x) (square root of position towards bottom of ingot)

0.2

0.4

Do-It-Yourself specification of solar grade silicon
1. Construct PC1D model for your cell process, and calculate cell efficiency versus Leff 2. Use 1/Leff2 ∝ CL (CL is impurity concentration in the feedstock) “generic” plot of cell efficiency versus CL (CL in a.u.). 3. One data point (impurity concentration and cell efficiency) to calibrate CL-scale. 4. Choose acceptance level of cell efficiency (cost analysis! e.g. 97%rel efficiency if feedstock 25% lower cost). 5. Read required impurity concentration from plot.

20th EPVSEC Barcelona 2005, 2AO1.3
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Graphical presentation of D-I-Y feedstock specification
relative to high-purity feedstock
14.5% cell techn. 17% cell techn.

1.0

cell efficiency (%rel)

0.9

0.8

our results: 5 ppmw Al or 10 ppmw Ti

0.7

1 10 100 impurity concentration (a.u.)
20th EPVSEC Barcelona 2005, 2AO1.3

18

Graphical presentation of D-I-Y feedstock specification
relative to high-purity feedstock
14.5% cell techn. 17% cell techn.

1.0

cell efficiency (%rel)

0.9

0.8

our results: 5 ppmw Al or 10 ppmw Ti 60x reduction

0.7

1 10 100 impurity concentration (a.u.)
20th EPVSEC Barcelona 2005, 2AO1.3

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Graphical presentation of D-I-Y feedstock specification
relative to high-purity feedstock

1.0

spec: 0.1 ppmw Al or 0.2 ppmw Ti

14.5% cell techn. 17% cell techn.

cell efficiency (%rel)

0.9

0.8

our results: 5 ppmw Al or 10 ppmw Ti 60x reduction

0.7

1 10 100 impurity concentration (a.u.)
20th EPVSEC Barcelona 2005, 2AO1.3

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Conclusions
Clear impact of Ti and Al at ppm level. Dependence of impact on position in ingot modeled according to segregation and linear relation between Leff-2 and Cfeedstock. Extrapolated feedstock specification based on 3%rel cell efficiency reduction: Al: 0.1 ppmw Ti: 0.2 ppmw See the paper for more details, also on carbon, mix of impurities, Fe, and modelling of economics!
20th EPVSEC Barcelona 2005, 2AO1.3
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Thank you for your attention
Acknowledgements Oyvind Mjs, NTNU Trondheim ScanArc, Scanwafer, HCT EC for contracts SOLSILC, SPURT, and SISI Coauthors: Petra Manshanden, Paul Wyers (ECN Solar Energy), Eivind vrelid, Ola Raaness, Aud Waernes (Sintef), Benno Wiersma (Sunergy)

22

22

20th EPVSEC Barcelona 2005, 2AO1.3 20th EPVSEC Barcelona 2005, 2AO1.3


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