The Multi-Busbar Design: an Overview
Stefan Braun1, Giso Hahn1
Robin Nissler2, Christoph Pönisch2,
Dirk Habermann2
Universität Konstanz1
www.uni-konstanz.de/photovoltaics
Gebr. Schmid GmbH2
4th Metallization Workshop, May 8th 2013, Konstanz
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Motivation
Optimized Solar Cell Design for Module Integration*
Ag consumption
Solar cell optimization
Module
=
+
Multi-Busbar Solar Cell
*S.
Braun et al., “Solar Cell Improvement by Using a Multi-Busbar Design as Front Electrode”, Energy Procedia 21, 227-233 (2012)
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Outline
• Motivation
• Cell Concepts
- Differences between 3-busbar and multi-busbar solar cell
• Simulation
- Advantages of multi-busbar design on cell- and module-level
• Results of Experiment
- Solar cells & one cell modules
• Summary
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Cell Concepts
SE Solar Cell (6 inch Cz)
Alkaline textured
SiNx:H layer
Screen printed fingers
Full area Al BSF
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Cell Concepts
SE Solar Cell (6 inch Cz)
Alkaline textured
3-Busbar
SiNx:H layer
Multi-Busbar
Screen printed fingers
Full area Al BSF
3 Busbars
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Round Sn coated Cu wires
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Extraction IV Parameters
Starting Point
2-diode model simulation
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Extraction IV Parameters
Starting Point
2-diode model simulation
Selective Emitter
reference cell*
Extracted
parameters
jsc [mA/cm2] Voc [mV] FF [%]
37.63
638.7
79.78
η [%]
19.17
j01 [fA/cm2]
j02 [nA/cm2]
rp [cm2]
rs [cm2]
jph [mA/cm2]
538
17
10000
0.433
40.5**
**without
shading
*B. Tjahjono et al.,”Optimizing selective emitter technology in one year of full scale production”, Proc. 26th EUPVSEC Hamburg, p. 901-905
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Simulation
How many busbars do we need?
19.4
Adding busbars to cell structure
Busbar width 1.4 mm
19.2
19.0
 [%]
IV setup
IV setup
18.8
18.6
18.4
Cell
Stringed
Stringed opt.
18.2
18.0
3
4
5
6
7
Busbars
8
9
10
Shading dominates
  drop
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Simulation
How many busbars do we need?
Stringed front side
19.4
Busbar width 1.4 mm
Height of rectangular tabbing 200 µm
19.2
 [%]
19.0
18.8
18.6
18.4
Cell
Stringed
Stringed opt.
18.2
18.0
•  drop due to stringing
• Additional Rs by tabbing
• IV measurement does not represent module
..performance
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3
4
5
6
7
Busbars
8
9
10
Shading dominates
  drop
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Simulation
How many busbars do we need?
Stringed front side
19.4
Busbar width variable
Height of rectangular tabbing 200 µm
19.2
 [%]
19.0
18.8
18.6
18.4
Cell
Stringed
Stringed opt.
18.2
18.0
3
4
5
6
7
Busbars
8
9
10
Rs reduction dominates
 η gain
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Simulation
How many busbars do we need?
Stringed front side
 [%]
1.4
19.2
1.2
19.0
1.0
18.8
0.8
18.6
0.6
18.4
18.2
18.0
0.4
Cell
Stringed
Stringed opt.
3
4
5
6
7
Busbars
0.2
8
9
Busbar width [mm]
19.4
Busbar width variable
Height of rectangular tabbing 200 µm
0.0
10
Rs reduction dominates
 η gain
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String Geometry
Tabbing
Wire
Reflection on string
Reflection on wire
Effectively shaded area: ~100%
Effectively shaded area: ~70%
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String Geometry
Tabbing
Wire
Reflection on string
Total reflection on glass
Effectively shaded area: ~100%
Effectively shaded area: ~36%
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String Geometry
*A.W.
Tabbing
Wire
Reflection on string
Partial reflection on glass
Effectively shaded area: ~100%
Effectively shaded area: 30%*
Blakers, "Shading losses of solar-cell metal grids“, Journal of Applied Physics, vol. 71, p. 5237, 1992
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Finger Design
Multi-Busbar (15 wires), 6 inch
3-Busbar, 6 inch
L = 25 mm
L = 5 mm
Series resistance rs of
finger grid:
rs ~ L2
 rs 3BB 25 times higher
with same grid structure
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Finger Design
Multi-Busbar (15 wires), 6 inch
3-Busbar, 6 inch
L = 25 mm
L = 5 mm
1.0
Finger spacing constant (2 mm)
0.8
2
rs [cm ]
Aspect ratio height/width = 0.5
Gaussian shaped fingers
IV setup
3 Busbars
15 Wires
0.6
0.4
0.2
0.0
10 20 30 40 50 60 70 80 90
Finger width [µm]
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Finger Design
Multi-Busbar (15 wires), 6 inch
3-Busbar, 6 inch
L = 5 mm
0.6
Finger spacing variable
2
rs [cm ]
Aspect ratio height/width = 0.5
Gaussian shaped fingers
IV setup
2.4
0.5
2.0
0.4
1.6
0.3
3 Busbars
15 Wires
1.2
0.2
0.8
0.1
0.4
Finger spacing [mm]
L = 25 mm
0.0
0.0
10 20 30 40 50 60 70 80 90
Finger width [µm]
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Ag Consumption & Efficiency
0.6
IV setup
3 Busbars
15 Wires
0.5
19.8
0.4
19.6
0.3
19.4
0.2
19.2
0.1
19.0
0.0
18.8
10 20 30 40 50 60 70 80 90
Finger width [µm]
[%]
2
rs [cm ]
20.0
Finger width
η
3 Busbars
50 µm
19.3%
15 Wires
17 µm
19.7%
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Ag Consumption & Efficiency
0.6
19.8
0.5
0.4
19.6
0.4
120
0.3
19.4
0.3
90
0.2
19.2
0.2
60
0.1
19.0
0.1
30
0.0
18.8
10 20 30 40 50 60 70 80 90
Finger width [µm]
[%]
2
2
3 Busbars
15 Wires
0.5
rs [cm ]
IV setup
20.0
3 Busbars
15 Wires
150
0.0
0
10 20 30 40 50 60 70 80 90
Finger width [µm]
Finger width
η
3 Busbars
50 µm
19.3%
108 mg
15 Wires
17 µm
19.7%
6.7 mg
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Ag paste [mg]
IV setup
rs [cm ]
0.6
15
Ag consumption
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Aluminium Rear Side
How many Ag Pads on Al Rear Side?
• Pad size 4 mm x 0.5 mm
continuous
6
• 15 round Cu wires
• Rsheet Al = 10 m/sq. ~ 18 µm
5
4
3
2
Pads on
unit cell
Al rear side
?
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Aluminium Rear Side
How many Ag Pads on Al Rear Side?
• Pad size 4 mm x 0.5 mm
0.50
• 15 round Cu wires
0.45
• Rsheet Al = 10 m/sq. ~ 18 µm
2
rs [cm ]
0.40
rs <0.3
cm2
is sufficient
2 Pads
3 Pads
4 Pads
5 Pads
6 Pads
Continuous
0.30
0.25
0.20
Number of pads
depends on wire
diameter
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0.35
Stringed rear side
0.15
220
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240
260
280
Wire diameter [µm]
300
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Aluminium Rear Side
How many Ag Pads on Al Rear Side?
TinPad*
0.50
0.45
2
rs [cm ]
0.40
0.35
Stringed rear side
2 Pads
3 Pads
4 Pads
5 Pads
6 Pads
Continuous
0.30
0.25
0.20
0.15
220
Sn pads instead of Ag pads
 Full area Al BSF
*TinPad
240
260
280
Wire diameter [µm]
300
by Schmid Group
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Aluminium Rear Side
How many Ag Pads on Al Rear Side?
TinPad*
0.50
0.45
2
rs [cm ]
0.40
0.35
Stringed rear side
2 Pads
3 Pads
4 Pads
5 Pads
6 Pads
Continuous
0.30
0.25
0.20
0.15
220
Sn pads instead of Ag pads
 Full area Al BSF
*TinPad
240
260
280
Wire diameter [µm]
300
by Schmid Group
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Experiment
Process Flow Solar Cell Level
Cz-Si, 6 inch, 2 Ohmcm
Alkaline texture
POCl3 emitter diffusion 55 Ohm/sq.
3 busbars + fingers
Inkjet masking
Front pads + fingers
Emitter etch back 110 Ohm/sq.
Chemical edge isolation
SiNx:H deposition
Screen printing full area Al BSF
3 busbars + fingers
Screen printing front side
Front pads + fingers
Finger width 70 µm
Spacing 2.1 mm
Co-firing
Finger width 50 µm
Spacing 1.8 mm
Pads 500 x 700 µm2
IV measurement
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Experiment
Process Flow Module Level
TinPad
3 ribbons
Stringing front- / rear side
15 wires
Collector ribbons
Lamination process
IV measurement
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Experiment
Process Flow Module Level
TinPad
3 ribbons
Stringing front- / rear side
15 wires
Collector ribbons
V,I
Lamination process
V,I
Aperture area - IV measurement
Independently measured @
European Solar Test Installation
in Ispra, Italy
V,I
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V,I
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Experiment: IV Results
Average IV Parameters of 3x Solar Cells
Design
Ag [mg]
Voc [mV]
jsc [mA/cm²]
FF [%]
rs [Ωcm2]
η [%]
3BB
140
640
38.1
80.0
0.41
19.45
MBB
68
640
37.7
80.2
0.32
19.34
Delta
-72
0
-0.4
+0.2
-0.09
-0.11
• Ag paste reduction >50% for multi-busbar cells (72 mg)
• jsc loss -0.4 mA/cm²
• Efficiency loss -0.11%
Similar η on cell level
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Experiment: IV Results
Average IV Parameters of 3x Solar Cells and 3x 1-Cell Modules
Design
Ag [mg]
Voc [mV]
jsc [mA/cm²]
FF [%]
rs [Ωcm2]
η [%]
3BB
140
640
38.1
80.0
0.41
19.45
MBB
68
640
37.7
80.2
0.32
19.34
Delta
-72
0
-0.4
+0.2
-0.09
-0.11
Design
Ag [mg]
Voc [mV]
jsc [mA/cm²]
FF [%]
rs [Ωcm2]
η [%]
3BB
140
638
37.1
76.7
1.19
18.16*
MBB
68
637
37.3
77.9
0.96
18.49*
Delta
-72
-1
+0.2
+1.2
-0.23
+0.33
*aperture area
High module efficiencies with
multi-busbar design
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Conclusion
Advantages of Multi-Busbar Solar Cell Design
• Multiple busbars can boost module efficiency
• Technology has high potential for metal reduction
• Ag reduction via fine line printing 72 mg  50%abs
• Module efficiencies 18.16% 3BB
18.49% MBB
• Best multi-busbar module efficiency 18.57%
• Outlook:
- Optimized front pad design  increased jsc
 η 18.7% possible (Simulation)
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Thank you for your attention!
Special thanks to: Schmid Group for funding
Harald Müllejans, Diego Pavanello, Elena Salis, Roberto Galleano for IV measurements
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