INSTRUCTIONS FOR PREPARATION OF PAPERS - Q-Cells

Presented at the 25th European PV Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain
LASER MARKING OF
SILICON SOLAR CELLS IN MASS PRODUCTION
Uli vom Bauer, Jörg Müller, Toralf Patzlaff 1 , Daniel Binder, Steffen Geissler, Marco Spallek, Philip Kappe,
Bettina Brammer, Daniel Felsch
Q-Cells SE, Sonnenallee 17-21, D-06766 Bitterfeld-Wolfen, Germany
1 Calyxo GmbH Sonnenallee 1a, D-06766 Bitterfeld-Wolfen, Germany
ABSTRACT: Solar Systems are capital intensive goods that will operate over a long period of time. In order to
positively impact lifetime quality requirements, intensified process- and quality control is desirable. Laser marking is
providing a meaningful contribution to these requirements. Applied to raw wafer at the beginning of the
manufacturing process, each solar cell becomes traceable along the entire value chain and over the whole lifetime –
since laser marking is a hard physical coding. As usual it takes transition work from patented innovation at laboratory
stage till mass production proof. At Q-Cells we have gained vast experience by marking great number of solar cells
under mass production conditions. The paper highlights some of the key factors to be considered for high yielding inline
laser marking of silicon solar cells, since interest of customers and suppliers is rising for this topic in the
industry.
Keywords: laser marking, data matrix code, traceability, quality, process enhancement, value chain
1 INTRODUCTION
A major challenge for the PV industry in the next
years is to reduce the production costs to reach grid
parity.
Therefore not only capacity must be increased in order to
benefit from scaling effects, but also the production
effectiveness must be further enhanced, as well as the
consumables and equipment costs must be reduced to
save capital cost and last not least technological progress
must be accelerated.
At Q Cells, we do not only benefit from enlarged
production capacity but also have significantly expanded
our research & development activities in search of
technological answers to these challenges.
Laser marking has been proven to be an effective
measure for quality enhancements and process stability.
As such it has been successfully integrated in the cell
production lines at Q-Cells.
Laser marking therefore will not only foster products
of superior quality but also underline technology
leadership of advanced cell supplier in the PV industry.
Securing Investment is an essential aspect of PV-systems
strong quality requirements. Solar Panels are capital
intensive goods that require significant investment and
will generate return of invest (ROI) only after a rather
long period of hassle free operation, at conditions as
specified by their datasheets and certificates. Hence
investors take great care to understand the reliability and
durability as well as maximum power generation over
lifetime. Operators and owner of PV-power plants as well
as residential roof top-panel owner pay outmost attention
to the yield of their installations.
Laser marking provides further support of another
concern: it will help to avoid product forgery, since it is a
hard codes unique number on each wafer or solar cell.
Once written into the silicon it will stay with the product
over its entire life.
A summary of the overall top level benefits provided
by laser marking is shown in figure 1.
Figure 1: Summary of benefits as shown at PVSEC
poster session
2 NO IMPACT ON CELL PERFORMANCE
Since the usability of laser marking is significantly
determined by its location at the solar cells front side, it is
more than evident that no impact on performance of the
solar cell is a more than mandatory requirement. Impact
on cell in this context is electrical performance (laser
marking should not reduce power outcome of a cell by
any means) and also optical performance of solar cellssince
the optical homogeneity of cells used in one solar
panel need to satisfy the customers strong visual
expectations.
The marking on the front side however ensures that
cells can be traced by and large also after installation in a
module after the lamination process. The use of certain
coated or structured glass may challenge the readability
of a laser marking in a final module, however our own
tests and improvements in reader technology suggest
already today, that laser marking is a helpful tool to
identify and trace back solar cells also after their final
instalment at solar systems in power plants and solar
parks. Furthermore considering an environmental
friendly product circle (provided through industry
initiatives like PV-Cycle), laser marking information can
be used for providing helpful information for recycling of
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Presented at the 25th European PV Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain
solar panels. Recycling bodies may be grateful for any
hint on materials used some decades later, when solar
products reach their recycling plant, long after they have
been made.
happen if the laser marks are “burned” to deeply into the
silicon wafer, rather then being written in a specified
range of allowed depths at surface level.
Figure 2: Digital Tracing by laser marking along the
full value chain of the solar industry: from raw wafer
till PC-Cycle.
3 WRITING OF MARKS
Laser marking starts with the writing of marks, e.g.
the data matrix code. The well defined writing process
actually is the essential step of laser marking.
optimized
non-optimized
Figure 4: Writing Laser marks with different laser
parameter: variation in size, shape and pulse duration
in order to optimize readability.
Therefore one hand parameters such as laser power,
wavelength and duration of writing need to be defined
and confirmed by practical test.
On the other hand, despite preferably rather shallow laser
writing, the marks shall be deep enough, to avoid getting
etched away at the wet-bench processes during solar cell
production.
Best results have been achieved with Nd:YAG –
Laser, operating at wavelength of e.g. 1064 nm with
pulse durations of 20 ~ 50 ns and power of 5 ~ 30 W,
creating dots of 100 µm diameter and 10~30 μm depth –
before etching.
Last not least the shape and depth will have major
impact on the readability. Test like shown in figure 4 did
confirm that without optimized writing process the
readability of laser marking will fall behind requirements
of mass production which are significantly higher than
90%.
Figure 3: Example of an optimized and a non
optimized code, where solar cell fingers are printed
across the data matrix code, whereas the optimized
code fit well between solar cell fingers. See also
Chapter 6 Data-Matrix-Code for more info on code
redundancy.
Writing Code has great impact on readability and
specifying the right placement of the laser marking code
can improve greatly the readability. Quite obvious is the
importance of the placement of the code in respect to the
finger layout, as shown in Figure 3. But moreover also
the active area of operation of the laser writing the code
and the field of view in which the data-matrix reader can
safely detect the code, need to be considered. Best results
have been achieved in placing the code close to the centre
of solar cells considering bus-bar and finger layout.
Great care need to be taken that laser marks are
designed, which do not influence the electrical parameter
of a cell, e.g. create shunts. This failure mode could
Figure 5: Impact on readability through specifying the
right process for WRITING laser marking code.
4 READING OF MARKS
As already pointed out in the previous section of
writing the laser marks, it is essential to create optimized
laser marks in order to achieve the highest possible
reading rates.
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Presented at the 25th European PV Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain
Figure 6: Example of an industrial type data-matrixreader
from IOSS GmbH. This reader with adjustable
light and camera parameters in order to optimize
readability was used for results shown in this paper.
In our experiments we covered inspection systems and
light sources of different supplier. As a prime example
this paper is focussing on results achieved with IOSS
data-matrix-reader (DMR), in reference to the
demonstration shown at the exhibition.
Throughout our production line the surface of solar
cells changes as a result of various etching, oxidation or
heating processes. Hence an optimization of reading
equipment had to be undertaken.
While the light source in regular DMR was provided
by blue LED, we found improved reading rate with red
LED, especially after wet bench processes. Further
enhancement was done by optimized shutter speed in
order to regulate the exposure time.
Figures 5 and 7 provide insight on the very vast
variation of readability as a function of write-process
optimization of the laser marks (figure 5) and as a
function of read-process optimization (figure 7).
Figure 7: Impact on readability through specifying the
right process settings for READING laser marking
code.
For best results in industrial mass production process
it is obvious that both measures need to be taken. The
optimization is done through learning in iterative steps
and learning is enhanced with greater volume produced.
Figure 8: Sequence for the checking of change in micro-crack behaviour through laser marking did confirm no difference
compared to the peer group of un-laser-marked wafer.
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Presented at the 25th European PV Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain
5 MECHANICAL STRESS TEST
Another important factor for implementing laser
marking technology into an industrial mass-production
process is the stability of the process during volume
production. But also is the proof that there is no impact
on the product itself. Great care has been taken in
confirming that the new feature will have no impact on
cell handling, will cause no micro cracks or breakage and
has no affect on the yield of production.
Micro crack investigations have been carried out with
wafer stacks from 100 to 300 wafer each stack, for both
manual handling and automated handling. Wafers haven
been carried and shifted around in real mass production
environment simultaneously with their non –laser marked
counterparts as shown in figure 8.
After initial micro-crack investigation, transportation
and handling of wafer took place to apply same stresslevel
as wafer would experience during regular
production. By additional twist test any potentially
available micro crack would see further amplification.
The twist test was carried out with about 70% of the force
of a prior determined breakage force.
Red: no marks Green: flat marks Blue: deep marks
Figure 10: Investigation of solar cells stability: the
force required to create breakage shows no correlation
with laser marking.
6 DATA MATRIX CODE
The chosen code is a 2-dimensional industry standard
data matrix code called DataMatrixECC200. It is a defacto
industry standard with a significant robustness. It
contains about 25% redundancy achieved through a Reed
Solomon Error Correction.
Figure 9: Investigation of solar cells stability: the ballon-ring
test did provide evidence that breakage cracks
have no correlation with the laser marks.
Additional Investigation carried out by the
independent Fraunhofer CSP institute performing the
Ball-On-Ring-Test-Method on 160 μm thick solar cells
samples, with a variation of different depth of the marks.
The cracks do evidently not have their root cause in any
laser mark, and ongoing cracks are running rather in
between the individual marks than across them. This
confirmed that there is no significant influence on
breakage strength in the vicinity of the laser marks.
Figure 10 shows the distribution of the force required to
create breakage of a solar cell, in relation to the depth of
the laser marks.
correctable
non correctable
Figure 11: Top picture shows how the data is
distributed across the data-matrix for best data
reliability and recovery. The lower two pictures are
examples of correctable and non-correctable code.
In a 14x14 data matrix code there are 12x12 dots left
over for data. The outer lines of the code are required for
better code identification by the reader. The real data are
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Presented at the 25th European PV Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain
scrambled and strategically distributed over the whole
area in order to reach highest robustness against missing
dots, disturbance or corruption of the code as shown in
Figure 11.
Especially for purposes in the solar industry (for example
the occasional possibility that solar cell fingers could be
printed over the code) the capability and structure of error
correction is of importance.
ACKNOWLEDGMENTS
We gratefully acknowledge fruitful discussions and
contributions by equipment provider, external institutes
namely Fraunhofer and last but not least customers
supporting our development.
REFERENCES
[1] Patent: WO/2007/099138 Solar cell marking method,
and solar cell, Jörg Müller et al.
[2] Fraunhofer CSP “Strength Characterization of Laser
Marked Solar Cells“, V 63/2008 – “Strength
Characterization of Laser Marked Solar Cells”
[3] ASTM 1239-95 “Reporting Uniaxial Strength Data
and Estimating Weibull Distribution Parameters for
Advanced Ceramics”, 1995
[4] IOSS GmbH, H.Richter, J Gässler, Information
about Data Matrix Code (2010)
[5] St. Geissler, G Simons, Microcrack-Investigations
(2008).
Figure 12: readability of a data-matrix code.
Figure 12 shows a real-life photo from the chosen
IOSS reader and its analysing software, where rot dots
indicating the readability status of the code on the screen
Since the code is available in various sizes, e.g. 14x14,
12x26 or 8x32 it need to be chosen in consideration of
various parameter including finger pitch of a solar cell,
capability to store different amount of data or the time
required to write the code, as well as possible space
constrains. Q-Cells did experiments to optimize code size
and placement on the solar cell surface for better massproduction
results.
An example of an actual implementation of a laser
marking ID Code is given in figure 13. The encoded
information can contain information such as: producer
identifier, check digit, year/ week, marker & running
number.
Figure 13: example of data-matrix code content
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