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Innovative Fine-Line Screen Printing Metallization Reduces Silver Consumption



Together with their project partners, scientists at the
Photovoltaic Technology Evaluation Center PV-TEC at the Fraunhofer Institute
for Solar Energy Systems ISEin Freiburg have succeeded in improving the
traditional screen printing process for the fine-line metallization of silicon
solar cells. Using specially developed fine-line screens, the project team was
able to create contact fingers with a width of merely 19 µm and a height of 18
µm in a single printing step. This means that up to 30 percent less silver is
needed, which in turn leads to a significant reduction in manufacturing costs.




Inside of a screen printing machine © Fraunhofer ISE /
Dirk Mahler

Silicon solar cells rely on metal electrodes on their front
and rear sides to carry the electrical energy generated in the semiconductor
material from light irradiation. To this end, a flatbed screen printing process
is typically used to print a fine contact grid onto the front side of the cell.
This grid should block as little as possible of the active cell surface from
exposure to light and must be sufficiently conductive to keep the solar cells'
series resistance low. The technological challenge in the screen printing
process lies in creating the narrowest possible continuous contact fingers with
a sufficient height for good lateral conductivity. Printing extremely fine
contact fingers requires the use of highly engineered specialized screens and
metallization pastes in addition to complete mastery of the screen printing
metallization process.

“Working together with industry partners in fine-line screen
printing metallization, in particular with screen manufacturers Koenen GmbH and
Murakami Co. Ltd. as well as screen chemical supplier Kissel + Wolf GmbH, we
have managed to reduce the contact fingers' width to less than 20 micrometers —
a reduction of 30 to 40 percent compared with the current industry standard,”
explains Dr.-Ing. Andreas Lorenz, project manager in the Printing Technology
group at Fraunhofer ISE. Innovative fine-mesh screens were used in the
metallization of passivated emitter and rear contact (PERC) solar cells in two
independent test series. Using such a screen made it possible to create contact
fingers with a width of merely 19 µm and a height of 18 µm in a single printing
step. Not only are the contact fingers extremely narrow, their electrical
properties are also outstanding. When integrated into modules — particularly
with newer technologies such as multi-busbar interconnection with 8 to 15
busbars — they enable a notable reduction of the power loss in the contact
fingers. These newly developed screen printing processes require up to 30
percent less silver compared with the current industry standard with a contact
finger width of approximately 30 - 40 percent compared with the current industry standard,” explains Dr.-Ing.
Andreas Lorenz, project manager in the Printing Technology group at
Fraunhofer ISE. Innovative fine-mesh screens were used in the
metallization of passivated emitter and rear contact (PERC) solar cells
in two independent test series. Using such a screen made it possible to
create contact fingers with a width of merely 19 µm and a height of
18 µm in a single printing step. Not only are the contact fingers
extremely narrow, their electrical properties are also outstanding. When
integrated into modules — particularly with newer technologies such as
multi-busbar interconnection with 8 to 15 busbars — they enable a
notable reduction of the power loss in the contact fingers. These newly
developed screen printing processes require up to 30 percent less silver
compared with the current industry standard with a contact finger width
of approximately 30 µm.

Scanning electron microscope image of the cross-sectional surface of a fine-line screen-printed contact finger. © Fraunhofer ISE

As part of the experiment, PERC solar cells were metallized
using the optimal screen parameters; a nominal finger width of 24 µm was
selected due to the limitation with regard to the available number of busbars
on the solar cell (in this case 5). The best PERC solar cell in this test
series achieved an efficiency of h = 22.1%.

“Using highly engineered screen and paste systems for
fine-line metallization, it could be possible to start manufacturing solar
cells with nearly invisible contact fingers on an industrial scale in the near
future. This would represent a great advantage for applications in integrated
photovoltaics, where aesthetic, homogeneous module surfaces are in demand,”
says Dr. Florian Clement, Head of the Production Technology — Structuring and
Metallization Department at Fraunhofer ISE.


© Fraunhofer ISE, Koenen GmbH Scanning electron microscope image of a finger opening in a fine-mesh screen.


The results of these test series will be presented at two
upcoming PV conferences — the 36th EU PVSEC in Marseille, France, and the 29th
PVSEC in Xi'an, China.

The collaborative project “FINALE”, which focusses the
development of fine-line screen printing processes and in which the researchers
partly achieved these results, was funded by the German Federal Ministry for
Economic Affairs and Energy BMWi and conducted in collaboration with industry
partners Koenen GmbH, Kissel + Wolf GmbH and Wickon Hightech GmbH.

Further fine-line screen printing results which are
presented here were realized within a cooperation between Murakami Co. Ltd. and
Fraunhofer ISE.


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