fundamental reactions during the formation of fired silver ... · fundamental reactions during the...
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Fundamental Reactions During the Formation of Fired Silver Contacts and Solar Cell Results
M. Hörteis, S. W. Glunz
2nd workshop on Metallization for Crystalline Silicon Solar Cells
Konstanz, 15.04.2010
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Materials used for the front side metallizationIngredients
Glas frit (Lead oxide, Bismuthoxide…)
Front side silver paste Silver
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Development of ink/pastes Preparation
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Paste deposition on the front sideUsed technologies
Printing techniques
Screen printing
Stencil printing
Pad printing
Direct write
Inkjet
Aerosol jet
Extrusion/Dispensing
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Formation of the contactSchematic
Printed layer
Fired at T= 800°C
optionally enhanced by a plating step
n-emitter
p-base
d=18 µm
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Reactions during contact formationInk – Solar cell
n-emitterDeposited ink
Opening of the ARC (SiNx) Ink reacts with Silicon Nitride
Contact formation Ink reacts with Silicon
n-emitter
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Investigated materialsRelated to the contact formation
Silicon
Silicon nitride (ARC)SilverGlas frit
(Lead oxide, Bismuth oxide…)
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Basic reactions TG-DTA measurement – Si + PbO
M. Hörteis, Adv. Funct. Mater. 2010, 20, 476–484
10 20 30 40 50 60 70 80 90 10011012013099.0
99.2
99.4
99.6
99.8
100.0
mas
s [%
]
time [min]
Si + PbO
0
100
200
300
400
500
600
700
800
900
temperature
tem
pera
ture
[°C
]
mass-signal
Balance
Furnace
Sample
TC1TC2
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Basic reactions TG-DTA measurement – Si + PbO
-1.5
-1.0
-0.5
0.0
0.5
DTA
[μV
/mg]DTA-Signal
tempe
rature
656°C
10 20 30 40 50 60 70 80 90 1001101201300
100
200
300
400
500
600
700
800
900Si + PbO
time [min]
tem
pera
ture
[°C
]Balance
Furnace
Sample
TC1TC2
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1
2
3
1: Silicon
2: Lead
3: Glass
Si
Pb
Glas
Si + 2 PbO SiO/PbOglas + 2 Pb>600°C
Inte
nsity
[a.u
.]
Energy [keV]
Si
Inte
nsity
[a.u
.]
Energy [keV]
Pb
Pb
Inte
nsity
[a.u
.]
Energy [keV]
PbSi
O
Basic reactions SEM + EDX analysis – Si + PbO
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-0.50
-0.25
0.00
DTA
-sig
nal [μV
/mg]
20 40 60 80 100 120 140 160 1800
100
200
300
400
500
600
700
800
900
Tem
pera
tur [
°C]
Si + PbO + Ag
Zeit [min]
Reaction under the presence of silver
Re-crystallization during cooling
Basic reactions TG-DTA measurements – Si + PbO + Ag
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Basic reactions - Si + PbO + Ag Phase diagram Ag-Pb
800 700 600 500 400 300 200 100
600°C
300°C
DTA
-sig
nal [
a.u.
]
temperature [°C] Ag [at. %]
Eutectic atca. 304°C
600°C
304°C
962°C
tem
pera
ture
[°C
]
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1
2
Basic reactions - Si + PbO + Ag SEM + EDX analysis
0 2 4 6 8 10 12 14 16 18 20
Ag Pb In
tens
ity [a
.u.]
Energy [keV]
Pb
0 2 4 6 8 10 12 14 16 18 20
Inte
nsity
[a.u
.]
Energy [keV]
Ag
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Basic reactions Summing-up
Reaction between Lead oxide and Silicon
Lead oxide is reduced to lead, and simultaneously Si is oxidized
Silver is liquefied far below its melting point
Silver – lead melt recrystallizes during cooling
During the reaction glass is produced
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Basic reactions – Si3N4 + PbOOpening of the passivation-layer
Mass loss at T = 685°C
10 20 30 40 50 60 70 80 90 100 110 120 13098.0
98.2
98.4
98.6
98.8
99.0
99.2
99.4
99.6
99.8
100.0
mas
s [%
]
time [min]
mass
685°C
SiNx - PbO
0
100
200
300
400
500
600
700
800
900
tem
pera
ture
[°C
]
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Basic reactions – Si3N4 + PbOOpening of the passivation-layer
10 20 30 40 50 60 70 80 90 100 110 120 13098.0
98.2
98.4
98.6
98.8
99.0
99.2
99.4
99.6
99.8
100.0
mas
s [%
]
time [min]
mass
685°C
-17.5
-15.0
-12.5
-10.0
-7.5
-5.0
-2.5
0.0
2.5
5.0
PbO - SiNx
SiNx - PbO
DTA
-Sig
nal [
µV/m
g]
Mass loss at T = 685°C
Exothermic reaction
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Basic reactions – Si3N4 + PbOOpening of the passivation-layer
10 20 30 40 50 60 70 80 90 100 110 120 13098.0
98.2
98.4
98.6
98.8
99.0
99.2
99.4
99.6
99.8
100.0
6
8
10
12
mas
s [%
]
time [min]
mass
685°C
PbO - SiNx
SiNx - PbO
MS
-Sig
nal [
1x10
-9A]
N2 (28)
12 PbO + 2 Si3N4 6 SiO2 + 12 Pb + 4 N2 (↑)
Mass loss at T = 685°C
Exothermic reaction
Evolution of nitrogen
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Basic reactions Summing-up
Reaction between Lead oxide and Silicon Lead oxide is reduced to lead, and simultaneously Si/SiNx are oxidizedSilver is liquefied far below its melting pointSilver – lead melt recrystallizes during cooling, forming an el. contactDuring contact formation additional glass is produced
Opening of the passivation Layer (SiNx)Similar reaction as with pure SiliconSiNx is oxidized and glass is formed
Contact formation Formation of an isolating glass layer
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Ink Optimization Variation of the glass content
Density of contact-crystallites is increased with increasing glass content
For more than 10% of glass, the contact resistance is increased again, due to an increased glass layer
0 10 20 300.0
0.5
1.0
1.5
2.0
2.5
3.0
cont
act r
esis
tanc
e R
cxW [Ω
cm]
glass content [%]
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Optimized ink, applied on high efficiency solar cellsPrinted and fired contacts vs. evaporated contacts
Is it possible to close the gap between screen printed contacts and high efficiency contacts?
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Application on High efficiency solar cellsContact geometries
industrial-type contact vs. high-efficiency contact
37 µm
55 µm
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Application on High efficiency solar cellsPrinted and fired contacts vs. evaporated contacts
Contact on shallow emitter
fine line printing
contact firing through ARC
Light induced plating
3 process steps for the front side metallization
Contact on a deep emitter
photolithographically opening of passivation layer
evaporation of seed contact TiPdAg
Light induced plating
8 process steps for the front side metallization
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Application on High efficiency solar cellsCell structure
LIP-SilverPrinted and fired contact / evaporated contacts
SiNx-PECVD Antireflexioncoating / thermal grown SiO2
Shallow 110 Ω/sq. POCl3 –emitter / deep 120 Ω/sq. emitter
LFC point contacts
ALD-Al2O3 / PECVD – SiO2
PVD - Aluminum
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Application on High efficiency solar cellsSolar cell results
Main difference in open circuit voltage
For higher solar cell efficiencies a passivated front side is necessary
Gap between high efficiency solar cells and “industrial-type” solar cells is going to be closed
21.779.839.86844deep120high efficiency type
21.180.140.26564shallow110industrial type
[%][%] [mA/cm²][mV][cm²][Ω/sq.]ηFFJSCVOCARshContacts
best cell results
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Matthias Hörteis
Fraunhofer-Institut für Solare Energiesysteme ISE