Understanding and avoiding the formation of voids for rear

Understanding and avoiding the
formation of voids for rear passivated
silicon solar cells
Elias URREJOLAnow with 2, Kristian PETER 1, Heiko PLAGWITZ 2, Gunnar SCHUBERT 2
1
International Solar Energy Research Center - ISC - Konstanz, Germany
2
SUNWAYS AG Photovoltaic Technology, Konstanz, Germany
3rd Workshop on
Metallization for Crystalline Silicon Solar Cells
Wednesday, 26 October 2011
Part I.
Motivation
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Solar Cell Structures: Total vs. Local
Total: Al-BSF Solar Cell
Local: i-PERC [1]
p-Si
L-BSF
BSF
Al rear side
L-BSF
BSF
[1] G.
Agostinelli et al. 20th EUPVSEC, Barcelona pp.647-650 (2005).
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Motivation: Void instead of Eutectic
El-Analysis
FF ~ 78.6%
?
L-BSF
void formation
Rear side completely passivated
uncertain origin?[1-[1-4]
[1]
F. Huster, 20th EUPVSEC, Barcelona, p. 1466-1469 (2005)
F.S. Grasso et al, 2nd Workshop on Metallization, Konstanz, pp. 15-21 (2010)
[3] T. Lauermann et al, 35th IEEE PVSC, pp. 28-33 (2010)
[4] S. Gatz et al., Energy Procedia, vol. 8, 318 (2011)
[2]
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Part II.
Understanding Void Formation
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Contact Formation: Total vs. Local
Standard Process [1]
Local Al-Si Contact [2, 5, 6]
Al-Si phase diagram (equilibrium) [3]
Al
Si
Cooling
liquid
down
Peak
temperature
heating
Solid
850°C
Si diffusion in Al
Tpeak
[4]
(a) (b)
(a,b)
(c)
(c)
(a) AlAl-rich solidus in lamellar structure
(b) Eutectic (Al - 12.6 % Si)
(c) BSF: SiSi-rich solidus
[1]
DSi
DAl
F. Huster, 20th EUPVSEC, Barcelona, p. 1466-1469 (2005).
F.S. Grasso et al. 2nd Workshop on Metallization, Konstanz, pp. 15-21 (2010).
[3] J.L. Murray and A.J. McAlister, Bulletin of Alloy Phase Diagrams 5, 74-84 (1984).
[4] E. Urrejola et al. Applied Physics Letters 98, 153508 (2011).
[5] V.A. Popovich et al. 24th EUPVSEC, Hamburg, pp. 555-558 (2009)
[6] M. Rauer et al. Energy Procedia vol 8, 200 (2011).
[2]
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Silicon Diffusion in Aluminum [1]
[1]
E. Urrejola, et al. Applied Physics Letters,
Letters, vol. 98, 153508 (2011)
d2: Si diffusion in Al
d2
Al
d1
d1
3 [µm/°C]
p-Si
d1: local contact opening
Spread of Si in the Al matrix d2 increases by 3 [µm/°C]
within Tpeak : {750 - 950}°C
does not depend on the contact area
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Impact of Al Amount on Mass Transport [1]
[1]
E. Urrejola, et al. Journal of Applied Physics vol. 107, 124516 (2010)
•Al responsible for mass transport
• Si diffuses faster than Al in Al-Si alloys
(DSi > DAl)[2]
saturation
[2] J.O.
McCaldin and H. Sankur, Applied Physics Letters, vol. 19, 524 (1971).
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Impact of ∆ Pitch on Void Formation
d2
Al
d1
p-Si
Lp
Lp: contact spacing
∆Lp
Lp = 2 mm
Lp < 0.25 mm
influence on void formation?
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Impact of ∆ Pitch on Void Formations
Same Al paste, same firing Difference?
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Profile of Si Distribution in Al (EDX Line)
Small contact spacing
Si
Si almost
homogeneously
distributed in the Al
matrix
Si
Si
Si
Large contact spacing
~ no Si in middle part!
Si
Void: Si directly obove the opening < Si overlying the dielectric barrier
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Comparable with Literature
C. J. Kircher,
Kircher, Journal of Applied Physics,
vol. 47, 5394 (1976).
DURING COOLING:
The reduced Si concentration in the
Al adjacent to contact area is due to
Si diffusing back and regrowing on
the Si surface
J. O. McCaldin,
McCaldin, Applied Physics Letters, vol.
20, 171 (1972).
DURING COOLING:
1. dissolved Si is unable to go the
distance to the substrate and
remains as precipitates
2. Si at the AlAl-Si interface can easily
diffuse to the substrate
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Concentration of Si in Al (EDX/EDS)
L-BSF
Al - 84%
Si - 16%
Large V
Al - 69%
Si - 12%
O - 19%
Small V
Al - 62%
Si - 18%
O - 20%
Large V
Al - 72%
Si - 10%
O - 18%
Small V
Al - 65%
Si - 12%
O - 23%
Volume contained in void distributed in the Al matrix
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Hypothesis of Void Formation
Non equilibrium process!
void complete formed
At the Tpeak:
Al/Si melt transferred into Al matrix
void is formed
During cooling:
Si tries to diffuse back and solidifies in Al
(high concentration of Si at the interface)
no BSF was found (?)
DSi > DAl
void with BSF
During cooling down:
Si recrystallizes epitaxially + Al Si (BSF)
If cooling is faster than recrystallization and DSi
Si does not reach the interface and void is
formed. Rest of Si solidifies in Al.
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Understanding the Formation of Voids
void no BSF eutectic below Al
void + BSF + eutectic formations
L-BSF
eutectic below Al
Al - 87%
Si - 13%
voids appear at Tpeak
and / or
during cooling down
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Part III.
How to avoid the voids?
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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1. Effect of gravity on the Al-Si morphology [1]
[1]
E. Urrejola, et al. Journal of Applied Physics, vol. 110, 056104
056104 (2011)
Due to Al2O3 the Al paste does not move during the alloying process [2],
thus this phenomenon must be due to gravity [1]
Case B
Case A
[2] 1
L. Sardi et al. Solar Cells, vol 11, 51 (1984).
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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2. Optimization of Contact Spacing [1]
[1]
E. Urrejola, et al. Energy Procedia,
Procedia, vol. 8, 331 (2011)
Lp
[µm]
Eutectic or void
depth [µ
[µm]
BSF
[µm]
Voids/eutectic
[%]
100
14±
14±2
5.7 – 7
8
250
23±
23±2
4.5 – 4.7
25
700
23±
23±2
4.1 – 4.5
87
1000
27±
27±2
-
100
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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3. Others from Literature
Reducing DSi
SiAl
(e.g. Si in the Al paste [1, 2]
well formed L-BSF
reduced recombination [3])
Paste development to optimize Si – Al mass transfer [4]
Firing optimization [5]
[1]
1 M. Rauer et al. IEEE Electron Device Letters vol. 32, 916 (2011)
A. Urueña et al. 24th EUPVSEC, pp. 1483-1486 (2009).
[3] J. Müller et al. IEEE Transactions on Electron Devices vol. 58, 3239 (2011).
[4] V. Meemongkolkiat, 4th WCPEC, Hawaii pp. 1138-1141 (2006).
[5] T. Lauermann et al. 26th EUPVSEC, Hamburg pp. 1137-1143 (2011)
[2]
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Summary
• Si diffusion into Al must be controlled
• Voids formation depends on:
–
–
–
–
Diffusion of Si into Al and temperature: DSi > DAl
Concentration and distribution of Si in the Al layer
Amount of Al and contact spacing
Others: paste, firing parameters, surface ablation
• Avoiding voids ☺:
–
–
–
–
Al on top during firing
Reducing DSi
SiAl (e.g. Si in Al before firing)
Contact spacing optimization
Paste development to optimize Si – Al mass transfer
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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Acknowledgements
The authors gratefully acknowledge the financial support by the German
Federal Ministry of Education and Research within the framework of the
Leading-Edge Cluster Competition and the research cluster Solarvalley
Central Germany under contract No. 03SSF0335I,
and Merck KGaA for the kind supply of the etching paste.
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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THANK YOU FOR YOUR ATTENTION
[email protected]
Understanding and avoiding the formation of voids for rear passivated silicon solar cells - Elias Urrejola
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