Aerosol-Printed Silicon Solar Cell Exceeding
20% Efficieny
Crystal-Clear-Workshop
M. Hörteis
Fraunhofer-Institut für
Solare Energiesysteme ISE
Utrecht, 01.10.2008
Crystal-Clear-Workshop, Metallization 2008
Motivation, Screen print contact - “new”-contact
ƒ Reduced contact area
ƒ Good electrical contacts on lowly
doped emitters
Æreduced shading losses
Æimproved blue response
Æreduced recombination
Æreduced recombination
ƒ Aspect ratio
Æhigher conductivity
Æreduced series resistance
15 µm
100 µm
Crystal-Clear-Workshop, Metallization 2008
Content
ƒ
Seed layer concepts
ƒ
Aerosol print technique
ƒ
Ink preparation at Fraunhofer ISE
ƒ
Contact structure and formation
ƒ
Application: High efficiency solar cell
Crystal-Clear-Workshop, Metallization 2008
Two layer concept: Seed-layer and conductive-layer
Seed-layer
ƒ
ƒ
ƒ
ƒ
ƒ
Evaporation of contact metals trough a mask
Direct laser scribing
Electroless plating of contact metals
Fine line screen print
Aerosol-jet print / Ink-jet print
Conductive-layer
ƒ Plating (light induced or electroless)
ƒ Multiple printing of highly conductive materials
Crystal-Clear-Workshop, Metallization 2008
Light-induced plating
ƒ
Backside connected
via external power supply
to Silver anode
ƒ
Illumination induces
negative potential on front
side
ƒ
Positively charged Ag+-ions
are attracted by the
illuminated front side and
deposited along the seed
layer
Crystal-Clear-Workshop, Metallization 2008
Light-induced-plating – industrial realization
ƒ Inline LIP at Fraunhofer ISE
ƒ Two machines are available,
for Ag and Cu
Crystal-Clear-Workshop, Metallization 2008
Schematic of an aerosol printer
Atomization
gas
Atomizer
Virtual
impactor
Sheath gas
Print head
Nozzle
Substrate
XY-Table
Crystal-Clear-Workshop, Metallization 2008
Aerosol print head
Metal Aerosol
Sheath gas
d=18 µm
Nozzle
Aerosol
Jet
Ø=100 µm (nozzle opening)
XY-Table
Crystal-Clear-Workshop, Metallization 2008
Printing systems
© Optomec
Crystal-Clear-Workshop, Metallization 2008
Simulation: optimized contact width
el. losses
ƒ Influence of electrical
losses
ƒ Dependent on contact
resistivity
12
2
ρc> 10 mΩ cm
11
Total loss p [%rel]
ƒ Influence of optical losses
opt.losses
10
10
minimum
5
9
3
8
0.1
7
6
1
0.5
1
2
ρc= 0.01 mΩ cm
10
20
Rsh = 55 Ω/sq
30
40
Contact width wc [μm]
Mette A., New Concepts for Front Side Metallization of Industrial Silicon Solar Cells, Universität Freiburg
50
Crystal-Clear-Workshop, Metallization 2008
Functional ink materials
ƒ Metal powder - Silver
⇒ Conductivity and contact
ƒ Glass frit (Metal Oxides)
⇒ Contact formation and adhesion
ƒ Organic vehicle system (Solvents, binder, dispersant
agents, rheological additives…)
⇒ Adjusting the ink on the used printing system
(Screen print, Aerosol print, Inkjet print…)
Crystal-Clear-Workshop, Metallization 2008
Ink - Preparation
Tools:
ƒ Crushing
ÆMortar
ÆBall mill
ƒ Mixing
ÆAgitatiors
Æspatula and beaker
ƒ Dispersing
ÆUltra sonic finger/bath
Æ3 mill chair
www.d-firma.de/bilder/mischen.jpg
Crystal-Clear-Workshop, Metallization 2008
Aerosol contact after printing
1,6
after printing
contact height [µm]
1,4
1,2
1,0
0,8
0,6
0,4
38 µm
0,2
0,0
20
ƒ
ƒ
ƒ
ƒ
40
60
contact width [µm]
80
Single printed, dense line
Line width <40 µm using a
200 µm nozzle
Line height about 1-2 µm
Height:width < 1/20
Crystal-Clear-Workshop, Metallization 2008
Aerosol contact after firing
1,6
after printing
after firing
contact height [µm]
1,4
1,2
1,0
0,8
0,6
0,4
35 µm
0,2
0,0
20
40
60
80
contact width [µm]
ƒ
Evaporation of solvent and
binder
ƒ Sintering of silver particles
ƒ Reduced line conductivity
⇒ Light induced plating
Crystal-Clear-Workshop, Metallization 2008
Aerosol contact after LIP
35
after printing
after firing
after plating
contact height [µm]
30
25
20
15
10
35 µm
75 µm
5
0
0
ƒ
ƒ
ƒ
20
40
60
80
contact width [µm]
100
Aspect ratio increases 1:4
improved conductivity
ρf~2×10-8 Ωm
Higher reflection (optical
width about 70% of real
contact width)
20 μm height
40 µm seed layer
80 µm contact after light induced plating
Crystal-Clear-Workshop, Metallization 2008
Reverse contact formation
LIP-Silber
plated silver
n-emitter
p-base
Crystal-Clear-Workshop, Metallization 2008
Reverse contact formation
n-emitter
Seed layer
n-emitter
p-base
ƒ LIP silver is removed by nitric acid
p-base
Crystal-Clear-Workshop, Metallization 2008
Reverse contact formation
Glass layer
n-emitter
p-base
ƒ LIP silver is removed by nitric acid
ƒ Printed silver is removed
n-emitter
Crystal-Clear-Workshop, Metallization 2008
Reverse contact formation
Silver crystallites
n-emitter
p-base
ƒ LIP silver is removed by nitric acid
ƒ Printed silver is removed
ƒ HF dip to remove the glass layer
Crystal-Clear-Workshop, Metallization 2008
Reverse contact formation
Imprints of
silver crystallites
n-emitter
p-base
ƒ
ƒ
ƒ
ƒ
LIP silver is removed by nitric acid
Printed silver is removed
HF dip to remove the
glass layer
n-emitter
Nitric acid to remove the silver
crystallites
Crystal-Clear-Workshop, Metallization 2008
Current model of the contact formation
ƒ Glass melts at about 500°C
and wets the interface inksolar cell
ƒ PbO, dissolved in the glass
reacts with the SiNx-layer
and opens the ARC
T<500°C
700<T<830°C
700<T<830°C
ƒ Both, silver and oxidized
silicon is dissolved in the
glass melt at about 800°C
ƒ Dissolved silver crystallizes
in form of small silvercrystallites during cooling
T<830°C
Schubert, G., Thick film metallisation of crystalline silicon solar cells 2006, Universität Konstanz
RT
Crystal-Clear-Workshop, Metallization 2008
Mögliche Mechanismen des Stromflusses ohne LIP
Precipitate
Glass
SiNx layer
plated silver
5
n-Si
plated silver
6
screen-printed
contact
n-emitter
p-base
4
7
SiNx layer plated silver crystallite
1
glass
2
3
precipitate screen-printed
contact
Mette A., New Concepts for Front Side Metallization of Industrial Silicon Solar Cells, Universität Freiburg
glass
Crystal-Clear-Workshop, Metallization 2008
Solar cells on Fz-Wafers
ƒ Two different inks
ƒ Diluted screen print
paste (ink A)
ƒ Designed at ISE (ink B)
ƒ On three different emitters
ƒ 50 Ω/sq.
ƒ 70 Ω/sq.
ƒ 110 Ω/sq.
Crystal-Clear-Workshop, Metallization 2008
Process flow
Front and rear passivated solar cell
Crystal-Clear-Workshop, Metallization 2008
Process flow
Aerosol seed layer print
Crystal-Clear-Workshop, Metallization 2008
Process flow
Contact firing
Crystal-Clear-Workshop, Metallization 2008
Process flow
Evaporation of Al on the rear
Crystal-Clear-Workshop, Metallization 2008
Process flow
Laser fired contacts (LFC)
Crystal-Clear-Workshop, Metallization 2008
Process flow
Light induced plating (LIP)
Crystal-Clear-Workshop, Metallization 2008
Process flow
Forming gas annealing (FGA)
Crystal-Clear-Workshop, Metallization 2008
Cell structure
ƒ
LIP-Silver
ƒ
Aerosol-printed seed layer
ƒ
Antireflexion coating and
emitter
ƒ
LFC point contacts
ƒ
Thermal oxide
ƒ
Evaporated aluminum
Crystal-Clear-Workshop, Metallization 2008
Current and voltage
39,0
670
665
38,5
660
Voc [mV]
jsc [mA]
38,0
37,5
37,0
655
650
645
36,5
640
55 (ink A)
70 (ink A) 110 (ink A) 110 (ISE ink)
Rsh [Ω/sq]
55 (ink A) 70 (ink A) 110 (ink A) 110 (ink B)
Rsh [Ω/sq]
Crystal-Clear-Workshop, Metallization 2008
IQE of the best cells of each emitter
ƒ Influence of the emitter
sheet resistance is visible
in the short wave length
region
1.0
0.9
0.8
50 Ω/sq ink A
70 Ω/sq ink A
110 Ω/sq ink A
110 Ω/sq ink B
Sim. IQE 110 Ω/sq ink B
0.7
ƒ Good reflection, due to a
passivated rear and a
aluminum mirror
0.6
R, IQE
ƒ Comparable quantum
efficiencies for all cells in
the long wave length
region
0.5
0.4
0.3
0.2
0.1
0.0
400
500
600
700 800
λ (nm)
900
1000 1100 1200
Crystal-Clear-Workshop, Metallization 2008
Fill-Factor and efficiency
0.85
21.0
20.5
20.0
0.80
19.5
η [%]
FF
19.0
0.75
18.5
18.0
17.5
0.70
17.0
16.5
16.0
0.65
55 (ink A)
70 (ink A) 110 (ink A) 110 (ink B)
Rsh (Ω/sq.)
55 (ink A)
70 (ink A) 110 (ink A) 110 (ink B)
R
sh
(Ω/sq.)
Crystal-Clear-Workshop, Metallization 2008
Efficiency over one wafer
20,3%
20,2%
20,4%
19,8%
20,3%
20,6%
(20.3%*)
20,6%
(20.3%*)
*)independently confirmed by Callab ISE
Crystal-Clear-Workshop, Metallization 2008
Contact resistance vs. emitter sheet resistance
ƒ Contact resistance
increases with increasing
emitter sheet resistance
1,4
1,2
Rc*W [Ω*cm]
ƒ The contact resistance
Rc×W for ink B (designed
at ISE) is below 0,5 Ωcm
for all emitter sheet
resistances
1,6
1,0
Ink B
Ink A
0,8
0,6
0,4
0,2
50
60
70
80
Rsh [Ω/sq.]
90
100
110
Crystal-Clear-Workshop, Metallization 2008
SEM image of Ag-crystallites on a 110 Ω/sq. Emitter
Crystal-Clear-Workshop, Metallization 2008
Conclusion
ƒ Fine line printing is possible
ƒ Low ohmic contacts can be formed on lowly doped
emitters using ISE ink
ƒ Processed solar cells achieving 20% efficiency
Crystal-Clear-Workshop, Metallization 2008
Thank you for your attention
25 µm
55 μ5m
5
More information:
M. Hörteis, S. W. Glunz PIP-850 (online available)
μm
25 μ m