Al-Si alloy formation in narrow p-Si contact areas

Transcription

Al-Si alloy formation in narrow p-Si contact areas
Al-Si alloy formation in narrow
p-Si contact areas
Elias URREJOLA1, Kristian PETER1, Heiko PLAGWITZ2, Gunnar SCHUBERT2
1
International Solar Energy Research Center - ISC - Konstanz, Germany
2 Sunways AG, Konstanz, Germany
2nd Workshop on Metallization for Crystalline Silicon Solar Cells
Session II: Contact Formation II
14.04.2010
Outline
• Motivation: local contact rear side PERC
• Result:
contact resistivity (TLM)
• Analysis:
micrographs SEM
• Solar cell results and summary
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Motivation – PERC cell
Local Al contact analysis for the next generation solar cells
laser opening,
alloying process:
Pyramids 60 µm
Extended analysis of the Al-Si alloying formation
Ref: G. Beaucarne, 1st Workshop on Metallization for Crystalline Silicon Solar Cells, (2008).
Uruena, et al. 24th EUPSEC (2009), p. 1483.
Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
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ISCISC-Konstanz
Motivation – PERC cell
i-PERC (2D)
Ag contact
passivated emitter
dielectric stack
alloyed junction p- Si
Al contact
• high efficiency
• reduced Si thickness
• dielectric passivation at the rear
• local contact formation (LBSF)
high internal reflection
lower recombination
p-Si
d1
dielectric
barrier
d2
screen printed Al fingers
on dielectric openings
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Motivation – PERC cell
Model of the local contact formation PERC (rear)
Analysis: Contact resistance and contact formation
p-Si passivation
p-Si
etching lines (d1)
d1
dielectric
barrier
d2
d1: dielectric barrier opening width [µm]
d2: screen printed Al finger width [µm]
screen printed
Al-finger (d2)
firing (alloying)
TLM, SEM
d2 >> d1
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Contact resistivity result (TLM)
Screen printed Al finger, d2 = 700 µm
p-Si
d1
dielectric
barrier
d2
10% covered area
FF loss 1%
Contact resistivity
variation:
not expected?
ρc doping, T
contact area?
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
The Al-Si alloying process
RT: After drying
RT: After firing
heating
cooling
down
Alloying by rapid
thermal annealing
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Reduction of the contact area
from Reference
Screen printed 700 µm Al finger width on p-Si surface
to PERC
d1 < 100µm, d2 > 500 µm h = 50µm
Screen printed 700 µm Al finger width on 100 µm dielectric opening width
lower contact resistivity (less FF loss)
more passivated area (lower recombination)
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Al-Si alloy junction
d2
dielectric
barrier
d1
p-Si
(SEM/EDX)
Al matrix (Al-17%Si)
Al-12.6%Si
20µm
BSF (Al-1%Si)
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Analysis: Al-Si alloy formation
d2
Screen printed Al finger, d2 = 700 µm
dielectric
barrier
d1
p-Si
d1 = 80µm
20µm
40µm
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Analysis: Al-Si alloy formation
d2
Screen printed Al finger, d2 = 700 µm
d1 = 80µm
dielectric
barrier
d1
p-Si
d1 = 175µm
40µm
20µm
50µm
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Analysis: Al-Si alloy formation
d2
Screen printed Al finger, d2 = 700 µm
d1 = 80µm
d1 = 175µm
a
b
20µm
40µm
dielectric
barrier
d1
p-Si
d1 = 250µm
50µm
b
same Al mass,
material,
firing, printing
conditions >
Contact resistivity
dependence Al-Si alloy
geometry
a
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Analysis: Al-Si alloy formation
d2
Screen printed Al finger, d2 = 700 µm
50µm
dielectric
barrier
d1
p-Si
High
overlapping
of Al
strong alloy
HCl 13
Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Discussion: contact resistivity
d2
a
dielectric
barrier
d1
p-Si
d1a
d1 = 2 d1a = 2 d1b
20µm
∆d1c
a
d1
d1 + d1c
d1 + d1c‘
d1 + d1c‘‘
High contact resistivity d1c planar surface
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
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ISCISC-Konstanz
Summary of experiments
d2
dielectric
barrier
d1
d1
p-Si
p-Si
High overlapping of paste amount
deep Al-Si alloy at the edge.
p-Si
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
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ISCISC-Konstanz
At the solar cell level
Gruppe
N°
i-PERC Reference Eta
[%]
VOC
ISC
FF
[mV] [mA/cm²] [%]
Rs
[Ωcm2]
best
PERC
16,0 619,7
33,7
76,8
1,1
mean
PERC
15,8 616,9
33,4
76,9
1,1
Reference 15,6 612,3
32,9
77,6
0,9
mean Reference 15,6 611,1
32,8
77,6
0,9
best
mc-Si wafers
gain: ƞ: 0.4 %, Jsc: 1 mA/cm2, Voc: 7 mV
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Summary
• Increasing Ratio: Al finger [µm] / opening [µm]
deep Al-Si alloy formation
minimization of contact resistivity ρc
• Increasing opening width
identical alloying forms at the edges
planar surface increasing with Ratio
• Shallow dielectric barrier
lowest ρc of 8 mΩcm2
lower FF losses
• Solar cell level d1<100µm (opening), d2>600µm (Al mass)
less metallization
more passivated area (less recombination)
gain: ƞ: 0.4 %, Jsc: 1 mA/cm2, Voc: 7 mV
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
Urrejola
ISCISC-Konstanz
Local rear contacts
PERC solar cell:
narrow openings
but high material overlapping
c
b
p-Si
d1
dielectric
barrier
a
d2
Thank you!
elias.urrejola@isc-konstanz.de
Acknowledgements:
German Federal Ministry of Education and Research (contract no. 03SSF0335I)
Merck for the kind supply of the etching paste
Appendix 1
200 µm
600 µm
900µm
100 µm
100µm
200 µm
200µm
c
b
p-Si
d1
dielectric
barrier
a
d2
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
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ISCISC-Konstanz
Appendix 2
• BSF strongly dependence on the temperature
• Homogeneous eutectic at high temperatures
1. T: 620°C, Al: 800µm
BSF: 4µm, opening: 150µm,
thickness: 20µm
2. T: 710°C, Al: 800µm
BSF: 7µm, opening: 150µm,
thickness: 20µm
better Rc.
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
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ISCISC-Konstanz
Appendix 3
Transmission line model (TLM):
Two probes are used to send a current through the strip while the voltage across two other probes is
measured. A plot of this voltage as a function of distance yields a transfer length Lt. The contact resistivity is
related to Lt and the sheet resistivity Rs of the diffused layer
Ref: H. Plagwitz, Thesis, (2007).
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Al-Si alloy formation in narrow p-Si contact areas - Elias Urrejola.
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