This document discusses laser processes that can be used to improve bifacial solar cells. It describes how selective laser doping can be used to dope emitter regions and bulk silicon to improve contacts and reduce shadowing on both sides of the solar cell. Laser transferred contacts are also discussed as a way to create fine line metallization patterns with reduced shadowing and contact resistance. Experimental results show that these laser processes can increase cell efficiency compared to standard diffusion processes. The laser techniques allow high selectivity and resolution and are well-suited for bifacial solar cell fabrication.

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Add-on laser processes for bifacial solar
cells
P.C. Lill, M. Dahlinger, E. Hoffmann, T.C. Röder,
S.J. Eisele, J.R. Köhler, and J.H. Werner
Institut für Photovoltaik (ipv), Stuttgart, Germany
1st Workshop on bifacial solar cells,
April 23rd, 2012

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Advantages
high selectivity
low temperature
ambient air processes
versatile

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Laser processes for bifacial solar cells
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ARC
emitter
screen printed
metallization
ARC screen printed
metallization
selectively laser doped
bulk
BSF
locally increased
doping
improved contact
improved contact
reduced shadowing
front and rear side
laser transferred contacts

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Laser doping process
silicon substrate
precursor
layer
laser doped
region
X Y
4
silicon substrate
doping precursor
liquid silicondoped region
laser pulse

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Benefits of laser doping
highly flexible process
applicable for front and
rear side
variety of doping precursors
5
industrially relevant
only one additional process
step
commercially available
tools
selectively laser doped

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Full area laser doped emitters
6*confirmed by ISE CalLab
S. J. Eisele, T. C. Röder, J. R. Köhler, and J. H. Werner, Appl. Phys. Lett. 95, 133501 (2009).
SiO2 - Al point contacts
SiO2/ZnS/MgF2
Ti/Pd/Ag contacts
p-type
n+ laser doped emitter
Cell structure
4 cm2 on polished Fz
Voc
[mV]
Jsc
[mA/cm2]
FF
[%]
η
[%]
677 35.2 79.1 18.9*

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Cell structure
4 cm2 on polished Fz
Voc
[mV]
Jsc
[mA/cm2]
FF
[%]
η
[%]
677 35.2 79.1 18.9*
641 34.6 73.6 16.3
Full area laser doped emitters
6*confirmed by ISE CalLab
S. J. Eisele, T. C. Röder, J. R. Köhler, and J. H. Werner, Appl. Phys. Lett. 95, 133501 (2009).
SiO2 - Al point contacts
SiO2/ZnS/MgF2
Ti/Pd/Ag contacts
p-type
n+ laser doped emitter
SiO2 - Ti/Pd/Ag point contacts
SiO2/SiNx
Al contacts
p+ laser doped emitter
n-type
furnace
n+-BSF

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80 100 120 140 160
0
50
100
150
200
250
300
350
emittersaturation
currentdensityJ0e
[fA/cm²]
emitter sheet resistance sh
[/sq]
High quality emitters
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2.8 Ωcm, p-type Fz
Phosphorus
n+-emitter
SiNx
SiNx
3 Ωcm, n-type Fz
Boron
p+-emitter
SiO2
SiO2

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TexturingTexturing
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Industrial concept – selective emitter
Diffusion
PSG removal
Metallization
Laser irradiation
SiNx deposition
Diffusion
Laser irradiation
PSG removal
SiNx deposition
Metallization
Δη = +0.8% abs

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Cell
structure
Rsh,emitter
[Ω/sq]
Voc
[mV]
Jsc
[mA/cm2]
FF
[%]
η
[%]
60 623 36.5 76.9 17.5
80 625 37.1 76.1 17.7
Cell results
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standard emitter

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Cell
structure
Rsh,emitter
[Ω/sq]
Voc
[mV]
Jsc
[mA/cm2]
FF
[%]
η
[%]
60 623 36.5 76.9 17.5
80 625 37.1 76.1 17.7
100 635 37.1 77.8 18.3
Cell results
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selective emitter
standard emitter

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Laser processes for bifacial solar cells
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ARC
emitter
screen printed
metallization
ARC screen printed
metallization
selectively laser doped
bulk
BSF
locally increased
doping
improved contact
improved contact
reduced shadowing
w < 30 µm
laser transferred contacts

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metal
glass
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Laser transferred contacts
SiNx
laser
wlaser

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Benefits of LTC fine line metallization
laser transferred contacts
seed layer deposition
contact emitter through ARC
Ni, Al, Sb, Ti etc.
low contact resistance
Rc < 1 mcm2 @ 100 /sq
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plating
improve conductivity
material: Ni/Cu
good aspect ratio A  1:3
reduced shadowing
w < 30 µm

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Ni/Cu plating
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LTC fine line solar cell
LTC metallization
Ni/Cu plating
Texturing
Diffusion
P-glass removal
SiNx deposition
LTC metallization
Rear contact
SE laser doping
η = 17.1%

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LTC solar cells
14*confirmed by ISE CalLab
LTC
metal
ACell
[cm2]
Voc
[mV]
Jsc
[mA/cm2]
FF
[%]
η
[%]
Ni 4 615 35.6 78.1 17.1*
Ni with selective
emitter
50 µm
contact
finger
laser
doped
area

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50 µm
contact
finger
LTC solar cells
14*confirmed by ISE CalLab
LTC
metal
ACell
[cm2]
Voc
[mV]
Jsc
[mA/cm2]
FF
[%]
η
[%]
Ni 4 615 35.6 78.1 17.1*
Sb 4 612 37.3 76.4 17.4
Ni with selective
emitter
Sb self doped
contact
50 µm
contact
finger
laser
doped
area

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Conclusions
laser processes
high selectivity and spatial resolution
low temperature processes
laser doping
n and p-type doping
variety of precursors
LTC fine line metallization
reduce shadowing
reduce contact resistance
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selectively laser doped
laser transferred contacts
perfectly suited for bifacial solar cells!

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Thank you for your attention!