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Chen, Yu

Lauzurica Santiago, Sara

Molpeceres Álvarez, Carlos Luis

Morales Furió, Miguel

Muñoz Martín, David

The main objective of this work is to adapt Laser Induced Forward Transfer (LIFT), a well-known laser direct writing technique for material transfer, to define metallic contacts (fingers and busbars) onto c-Si cells. A layer of a commercial silver paste (viscosity around 30-50 kcPs), with thickness in the order of tens of microns, is applied over a glass substrate using a coater.. The glass with the silver paste is set at a controlled gap over the c-Si cell. A solid state pulsed laser (532 nm) is focused on the glass/silver interface producing a droplet of silver that it is transferred to the acceptor substra.te The process parameters (silver paste thickness, gap and laser parameters -spot size, pulse energy and overlapping of pulses) are modified and the morphology of the voxels is studied using confocal microscopy. Long lines are printed with a scanner and their uniformity, width, and height are studied. Examples of metallization of large areas (up to 10 cm x 10 cm) over c-Si cells are presented

Archivo Digital UPM

High volume transfer of high viscosity silver pastes using Laser Direct-Write Processing for metallization of c-Si cells

Lart, R.A.

OpenGrey Repository

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Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

25/5/16!
1!
Centro Láser
UPM !
High volume transfer of high viscosity silver 
pastes using Laser Direct-Write Processing 
for metallization of c-Si cells!
M. Morales, Y. Chen,  D. Munoz-Martin, !
S. Lauzurica, C. Molpeceres !
Centro Láser
UPM !
1.  Introduction: metallization of solar cells !
2.  Laser-Induced Forward Transfer (LIFT)!
3.  Experimental setup!
4.  Parameterization of the LIFT process!
5.  LIFT printing of long lines!
6.  Large area metallization!
7.  Summary!
8.  Acknowledgements!
Outlook !
Centro Láser
UPM ! Metallization of Solar Cells !
!
Front-side Metallization: key process for  enhancing efficiency 
in a cost effective way !
•  Screen-printing!
Luque &  
Hegedus, 
Handbook of 
Photovoltaic 
Science and 
Engeeniering 
(2003)!
Centro Láser
UPM ! Metallization of Solar Cells !
!
Front-side Metallization: key process for  enhancing efficiency 
in a cost effective way !
•  Screen-printing!
•  Ink–jet printing!
Teng & Vest, IEEE 
Trans. Compon., 
Hybrids, Manuf. 
Technol (1988)!
25/5/16!
2!
Centro Láser
UPM ! Metallization of Solar Cells !
!
Front-side Metallization: key process for  enhancing efficiency 
in a cost effective way !
•  Screen-printing!
•  Ink–jet printing!
•  High-effiency approaches: photolitography / evaporation / plating!
Zhao et al, Sol. Energ. Mat. Energ. C. (2001)!
Centro Láser
UPM ! Metallization of Solar Cells !
!
Front-side Metallization: key process for  enhancing efficiency 
in a cost effective way !
•  Screen-printing!
•  Ink–jet printing!
•  High-effiency approaches: photolitography / evaporation / plating!
•  Laser Printing based on Laser-Induced Forward Transfer (LIFT)!
Centro Láser
UPM ! LIFT !
LIFT: Laser-Induced Forward Transfer!
Brown et al, Microfluid Nanofluid  (2011)!
Some materials deposited using LIFT:!
-  Metals, Oxides, Nanopowders!
-  Organic polymers, Biomaterials & living cells!
-  Conductive inks, Ag nanoparticles pastes!
Centro Láser
UPM ! Experimental setup !
!
•    Low temperature screen-printing Ag pastes.!
•  Deposition onto donor substrate using a commercial paint 
coater.!
!
•  Basic LIFT configuration (no intermediate absorbing layer or 
assisting liquid matrix).!
 !
•  Diode Pumped Solid State Laser (Spectra Physics Explorer)!
•  Nd:YVO4 emiting 532 nm!
•  Pulse duration 14 ns!
!
•    Optical Scanner (ScanLab HurryScan)!
•  F-Theta Lens, focal 250 mm!
•  Focused beam diameter 22-25 µm!
!
!
!
25/5/16!
3!
Centro Láser
UPM ! Silver paste !
Dupont PV17F
Viscosity 28-40 kcps
Solid Content @750 ºC 91.0 %
Non-newtonian, 
pseudoplastic, thixotropic 
fluids !
Centro Láser
UPM ! Deposition onto donor substrate !
Commercial paint coater (RK PrintCoat Instruments). !
Incorporates spreading blade adjustable using micrometers!
Thickness, roughness (~ 2 um) and waviness (~ 5-8 um) are meassured for 
every experiment by means of confocal microscopy!
0 200 400 600 800 1000 1200
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
Aplicador micrométrico 20 µm, velocidad 1,5 , 1 línea de plata
z 
(µ
m
)
x (nm)
Rugosidad : rms = 1,14 µm, Vp=6,7 µm 
Centro Láser
UPM ! LIFT configuration 
Experimental scheme!
Laser system!
Centro Láser
UPM !
Laser! Explorer@532nm !
Laser frequency! 20 kHz!
Pulse duration! 14 ns!
Pulse energy! 0.1 – 35 uJ!
Paste thickness ! 30 um, 50 um, 80 um!
GAP! 50 um!
Receiving substrate! Monocrystalline Silicon wafer (520 um)!
LIFT parameterization !
25/5/16!
4!
Centro Láser
UPM !
Acceptor substrate! Donor substrate!
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 30 um!
GAP! 50 um!
Pulse energy! 1.6 uJ!
•  Transfer threshold!
•  Explosive transfer!
•  All paste is removed from the 
donor substrate!
•  Transferred paste height is 
similar to silver powder size!
W: 117 um!
H: 5 um!
W: 91 um!
H: 33 um!
LIFT parameterization ! Centro LáserUPM !
Acceptor substrate! Donor substrate!
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 30 um!
GAP! 50 um!
Pulse energy! 14.4 uJ!
•  Explosive transfer!
•  All paste is removed from the 
donor substrate!
•  Transferred paste height is 
similar to silver powder size!
W: 205 um!
H: 4 um!
W: 243 um!
H: 32 um!
LIFT parameterization !
Centro Láser
UPM !
Acceptor substrate! Donor substrate!
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 50 um!
GAP! 50 um!
Pulse energy! 3.2 uJ!
•  Transfer threshold!
•  Compact transfer is obtained!
•  All the paste is removed from the 
donor substrate!
W: 88 um!
H: 12 um!
W: 85 um!
H: 55 um!
LIFT parameterization ! Centro LáserUPM !
Acceptor substrate! Donor substrate!
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 50 um!
GAP! 50 um!
Pulse energy! 14.4 uJ!
•  Compact transfer is obtained!
•  Spray with silver particle and 
thinner is present. !
•  All the paste is removed from the 
donor substrate!
W: 236 um!
H: 13 um!
W: 132 um!
H: 57 um!
LIFT parameterization !
25/5/16!
5!
Centro Láser
UPM !
Acceptor substrate! Donor substrate!
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 50 um!
GAP! 50 um!
Pulse energy! 34.7 uJ!
•  Explosive transfer but with 
heights higer that particle size!
•  All the paste is removed from the 
donor substrate!
W: 211 um!
H: 9 um!
W: 136 um!
H: 56 um!
LIFT parameterization ! Centro LáserUPM ! LIFT printing of long lines !
Laser! Explorer@532nm !
Laser frequency! 20 kHz, 50 kHz!
Pulse duration! 14 ns!
Pulse energy! 0.1 – 35 uJ!
Paste thickness ! 80 um!
GAP! 50 um!
Receiving substrate! Monocrystalline Silicon wafer (520 um)!
Process speed! 20 – 2000 mm/s!
Centro Láser
UPM ! LIFT printing of long lines !
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 80 um!
GAP! 50 um!
Pulse energy! 14.5 uJ!
Process speed! 2 m/s!
Laser frequency! 20 kHz!
Centro Láser
UPM ! LIFT printing of long lines !
Heigth: 55 µm!
Width: 90 µm – 150 µm!
Aspect Ratio: 0.36 – 0.61 !
25/5/16!
6!
Centro Láser
UPM !
•  The LIFT process 
generates a column of 
paste that connects both 
substrates.!
•  When the glass substrate is 
removed the paste is 
streched until the final 
shape is obtained.!
•  This could explain the high 
aspect ratio and high 
transferred volume.!
•  The paste thickness and 
the gap are key variables in 
the transfer of high viscosity 
pastes!
LIFT printing of long lines ! Centro LáserUPM !
Aspect Ratio ≈ 0.25 - 0.40 !
Voxel ≈ 300 pL!
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 80 um!
GAP! 50 um!
Pulse energy! 14.5 uJ !
Laser frequency! 20 kHz!
Process speed! 2 m/s!
Line length! 3 cm!
LIFT printing of long lines !
Centro Láser
UPM ! Large area metallization !
Laser! Explorer@532nm !
Spot diameter! 25 um!
Paste thickness ! 80 um!
GAP! 50 um!
Pulse energy! 14.5 uJ!
Process speed! 2 m/s!
Line length! 3 cm!
•  O p t i c a l s c a n n e r s a l l o w s f a s t 
processing and flexible design to print 
large areas.!
•  Good predeposit conditions are 
needed.!
Centro Láser
UPM ! Summary !
1)  The minimum energy required to transfer the paste increases 
with the thickness of the paste.!
2)  Viscosity of the paste plays a fundamental role. Thinner additives 
are needed for controlling the paste viscosity.!
3)  The paste thickness and the gap are identified as key variables 
in the transfer of high viscosity pastes. !
4)  Lines with higher height than the gap suggest that the paste 
forms a union with the acceptor substrate and the final shape is 
reached once the donor substrate is removed.!
!
5)  Lines deposited using best parameters have good shape and 
large aspect ratios (~ 0.3). The volume transferred per pulse 
(voxel) is quite large (~ 300 pL).!
6)  Optical scanners allows fast processing and flexible design to 
print large areas.!
7)  LIFT is a promising technique for the metallization of PV devices 
using commercial screen-printing pastes. !
25/5/16!
7!
Centro Láser
UPM ! Acknowledgments ! Centro LáserUPM ! Acknowledgments !
This work has been supported by the Spanish MINECO projects SIMLASPV 
(ENE2011-23359),  HELLO (ENE2013-48629-C4-3-R) and EUROPEAN COMISSION 
APPOLO  FP7-2013-NMP-ICT-FOF. 609355!
Prof. M. Morales       D. Canteli     J.M. López      Prof. C. Molpeceres
      PhD. D. Muñoz                C. Yu             PhD. S. Lauzurica  J.J. García-Ballesteros
Centro Láser
UPM ! Laser sintering !
Experimental setup:!
!
•   DPSS Laser (Spectra Physics Millenia)!
•  Nd:YVO4 emiting 532 nm!
•  CW, power up to 20W!
•  Optical Scanner (ScanLab HurryScan)!
•  F-Theta Lens, focal 250 mm!
•  Focused beam diameter 20 µm!
!
Metallization comprise different steps:!
1.  Pre-metallization processes!
2.  Metallization!
3.  Curing, sintering and firing!
All-laser based process: !
Laser heating using a CW source.!
25/5/16!
8!
Centro Láser
UPM !
Lines deposited using screen-printing (Heraeus)!
Laser sintering ! Centro LáserUPM !
Lines deposited using LIFT (Heraeus)!
Laser sintering !
Centro Láser
UPM !


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