OPTIMIZING THE FRONT SIDE METALLIZATION PROCESS USING THE CORESCAN
A.S.H. van der Heide, J.H. Bultman, J. Hoornstra, A. Schönecker, G.P. Wyers, W.C. Sinke
ECN Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands
ABSTRACT
In solar cell manufacturing it is difficult to optimize
screen printed front side metallization, in particular to
obtain good contact over the entire cell. The Corescan
enables mapping of contact resistance of full cell
surfaces. Using these mappings, it is possible to
diagnose and exclude reasons for poor contact formation.
In this study, several process conditions have been
studied. Cross-belt temperature non-uniformity in the
firing furnace turns out to be a reason for large contact
resistance differences. This shows that the acceptable
firing temperature range can be considerably increased if
a constant temperature profile across the belt width is
realized. Further, it is shown that plasma etching for
isolation gives cause for an increase of the contact
resistance of the fingers at the cell edges. Finally, contact
resistance differences related to emitter non-uniformity
are found. To summarize, the Corescan is a powerful tool
that enables easy optimization of front side metallization.
far, only the Transmission Line Method (TLM) was
available. This technique, however, is slow and it is not
able to give information as a function of position. The
technique is even based on the assumption of a positionindependent contact resistance.
In contrast to the TLM method, the Corescan does
not have these limitations. With this instrument (see Fig.
1), the contact resistance is locally measured, and a
complete scan for a 10 cm x 10 cm cell takes only 6
minutes. The Corescan is commercially available from
ECN's daughter company SunLab [4].
INTRODUCTION
To reach optimum solar cell efficiencies, it is
necessary to reduce losses to a minimum. One type of
loss is the resistance loss, which is subdivided in series
resistance and parallel resistance losses. This
contribution focuses on the localization and reduction of
the most problematic series resistance contributor in
industrial solar cells: the contact resistance of the front
side metallization. The localization is done with the
recently introduced Corescan [1,2,3], derived from
COntact REsistance scan.
For approximately 90% of solar cells produced in
industry, metallization is applied by screen printing of
metal paste, followed by electrical and mechanical
contact formation performed by sintering at high
temperature in a belt furnace. Important parameters
determining the quality of the contact formation are e.g.
screen print parameters like print pressure, paste
rheology and screen quality, belt furnace parameters like
temperature profile, uniformity and stability, but also
emitter sheet resistance and anti-reflection coating
thickness. Although fill factors around 78% are achieved
in the laboratory, fill factors in industry are regularly found
to be 5-10% lower due to the process sensitivity of the
front side contact formation.
To reduce the contact resistance, it is important to be
able to measure it, preferably as a function of position. So
Fig. 1. Picture of Corescan
By varying cell processing conditions and measuring
the resulting contact resistance distributions, it is now
easy to determine the reason for not optimally contacted
cells, and realize process optimization. Parameters
investigated in this paper are firing furnace temperature
settings and uniformity, the influence of plasma etching
for edge isolation and emitter non-uniformity. Screen print
influences have been reported in [2].
THE CORESCAN
In general, the Corescan is an instrument to
determine where resistance loss in a solar cell occurs, for
both parallel and series resistance. The patented
methods [5] used by the Corescan are based on the
analysis of the potential distribution on a cell while a
current flow is induced in the cell.
For parallel resistance (shunt locating) the cell is
slightly forward biased in the dark. For contact resistance
the cell is short-circuited and a local illumination is applied
around the potential probe. The shunt method is
described in detail in [1] and is compared with lock-in
thermography in [6]. As contact resistance is the main
subject of this paper, shunt resistance will not further be
discussed in this paper.
To compare the conventional TLM method with the
Corescan, the TLM method is explained in more detail
here. The TLM method was first published in the sixties
and is still used today. For more information see e.g. [7].
The contact resistance is proportional to the potential
difference Vce between the edge of the finger and the
silicon adjacent to the finger. To be more precise, the
contact resistance for fingers is best expressed in terms
of the line contact resistance Rcl, which is defined as
R cl ≡
Fig. 2. The Transmission Line Method (TLM)
In the method (see Fig. 2) the resistance R is
measured between one finger and the next ones, and a
graph of R against distance is constructed. The data
points will be located on a straight line, assuming that the
emitter sheet resistance is constant and that the contact
resistance is the same for each finger. The interception of
this line with the R axis is proportional to the contact
resistance. The busbar must be disconnected from the
fingers to avoid parallel conduction. So, in practice small
samples have to be cut from a full cell. Main
disadvantages of the TLM method are that it only works
when the contact resistance is the same for all fingers
(which is often not the case), that it only reveals
information on a limited cell surface and, that the method
is rather time consuming.
The Corescan method for contact resistance is
based on the measurement of the potential jump between
a finger and the adjacent silicon while current flows
through the cell (see Fig. 3). This current is generated
locally with a light beam, while the cell is short-circuited
externally. A probe, centered in the light beam detects the
potential at the front surface and is displaced together
with the beam over the cell. The probe is pressed against
the surface by the probe holder weight and scratches
through the anti-reflection coating to enable electrical
contact with the silicon surface.
Fig. 3. Corescan in contact resistance mode
Vce
,
J sc d
(1)
where Jsc is the local short circuit current density
generated by the beam and d is the distance between the
fingers. There are several reasons to prefer Rcl above the
conventionally used specific contact resistance ρc in case
of screen printed fingers. In the first place, the contact
interface is very inhomogeneous, secondly the sheet
resistance of the silicon below the fingers is not known.
Therefore, the model to calculate the current pattern
below a contact collecting lateral currents does not apply
and it is impossible to calculate ρc correctly (in fact, we
cannot even speak of a single ρc value). It is better to
incorporate the entire situation below the contact in a
single value, as is accomplished by using the Rcl
definition given above. This value can be directly
calculated from the Corescan data: Vce can be
determined from the measured potential data, while the
light intensity is adjusted before each scan in such way
that Jsc has a predetermined value.
Fig. 4. Corescan (part of one scan line)
An example scan performed at Jsc = 30 mA/cm2 (see
Fig. 4) shows the following typical features of a Corescan:
1. Fingers are at the lowest potential because they are
in contact with the grounded busbars,
2. Between the fingers the potential has a parabolic
shape due to emitter sheet resistance,
3. Potential jumps proportional to the contact resistance
occur at the finger edges where the probe moves
from metal to the adjacent silicon,
4. A finger interruption causes a 4 times higher
parabola due to effectively doubled finger spacing.
Although line scans are instructive, complete cell
scans must be presented in 2D/3D graphs; examples of
2D graphs will be shown in the next section.
Typical advantages of the Corescan method are its
speed (typically 6 min for a 10 cm x 10 cm cell), the full
cell surface scanning, and the automation of the
measurement.
METALLIZATION PROCESS OPTIMIZATION
To demonstrate the application of the Corescan for
contact formation optimization, example scans on cells
with SiN anti-reflection coating and screen printed
contacts are given in this section.
In the first example, cells were IR fired at the same
belt speed but with different furnace temperature settings.
The temperature setting ranged from T-45 °C to T+30 °C.
The Corescan results are given in Fig. 6. The measured
Fill Factor is indicated, and also a picture of the aluminum
backside at T+45 °C is shown.
FF 72 %, T-45 °C
FF 74 %, T-30 °C
FF 74%, T-15 °C
FF 75 %, T °C
In this example, maximum FF is reached at
temperature T. The degradation in FF for the other
temperature settings is caused by increased contact
resistance losses.
The first observation is that it is possible to deduce
from the contact resistance distribution whether a cell is
over-fired or under-fired. This enables easy optimization
of the temperature setting.
The second observation is the brownish color (or
darker gray in B&W) on the left and right sides of the cell.
This reveals that the aluminum backside reached a much
higher temperature on the sides than in the center. The
similarity between the color pattern of the aluminum
backside and the contact resistance figure for the same
temperature is striking. The higher temperature
apparently results in a strongly increased contact
resistance.
Comparing the color of the aluminum backside of
different cells, the temperature difference over one cell
was estimated to be 45±10 °C. Work is underway to
investigate the cross-belt temperature non-uniformity in
the furnace. A large improvement could be gained, if the
temperature difference could be reduced to a few
degrees, and especially for wide belts. This would
increase the firing window by tens of degrees.
Another example of temperature non-uniformity is
shown in Fig. 5.
Fig. 5. Corescan on a cell supported at 4 points
during firing.
FF 74 %, T+15 °C
FF 69 %, T+30 °C
FF 62 %, T+45 °C
T+45 °C, backside
Fig. 6. Corescan potential mappings on cells fired at
different set temperatures and a picture of the Al
backside for the last cell. Lighter areas have higher
contact resistance.
Four regular oriented spots of poor contact are seen
in the center of a specially prepared test cell. Also the
area between the four spots shows enhanced contact
resistance as compared to the outside areas of the cell.
Due to the presence of supports placed underneath the
cell during firing the temperature distribution was
influenced, and as a consequence non-uniformly
distributed. The four supports are clearly revealed in the
scan by a higher contact resistance.
In Fig. 7 the Corescan shows a narrow band of high
contact resistance along the edges of the cell (the fact
that there are no high potentials visible at the lower edge
is due to that the probe scanned exactly over the outer
metallization line). The plasma etching of the edges as
used for cell isolation causes the high contact resistance.
Fig. 7. Increased contact resistance of the fingers at the
cell edges caused by plasma etching used for edge
isolation.
The last example shows contact resistance nonuniformity caused by a non-uniform emitter doping. One
of the methods to apply e.g. phosphorous dopant on the
solar cell is by spinning. In this example, a cell on both
sides doped by spinning has higher contact resistance at
the rim of a circle with the same diameter as the spinning
head (see Fig. 8.).
accomplish, because the TLM method is time-consuming
and assumes a uniform contact resistance. The latter is
often an invalid assumption as shown by the examples in
this paper.
From the given examples, the influence of the
individual process steps on the contact resistance is
clearly shown.
The firing temperature and non-uniformity of the
temperature distribution in the furnace appear to have a
large impact on the contact resistance. This leads to the
important conclusion that the range of acceptable firing
temperature settings increases by tens of degrees, if a
uniform cross-belt temperature is realized.
The other examples demonstrate that also other
process steps, such as plasma etching of cell edges for
isolation, and non-uniform emitter doping, can locally
increase the contact resistance.
The optimization of the front side metallization
process is much easier when using the Corescan. It is no
longer necessary to optimize without knowledge of the
contact resistance distribution, which is often the key to
find the reason for poor contact formation.
REFERENCES
[1] A.S.H. van der Heide, A. Schönecker, G.P. Wyers and
th
W.C. Sinke, Proceedings 16 European Photovoltaic
Solar Energy Conference, Glasgow (United Kingdom),
1438 (2000)
[2] A.S.H. van der Heide, J.H. Bultman, J. Hoornstra and
th
A. Schönecker, 11 NREL workshop on crystalline
silicon solar cell materials and processes, Estes Park
(Colorado, USA), 293 (2001)
Fig. 8. Increased contact resistance in a circular pattern
caused by the spinning head used for spinning of dopant
fluid.
Apparently, some of the dopant material has been
removed from the front side when placing the cell upside
down to spin the cell backside, resulting in lower doping
and a higher contact resistance. In addition to the circle,
there is a high contact resistance region in the center.
This is also caused by the furnace temperature nonuniformity discussed earlier. At the upper part of the
figure the higher potential is not caused by increased
contact resistance, but by finger interruptions.
Other methods of emitter doping can also lead to
non-uniform sheet resistance, for example the doping
level on cells made in tube furnaces usually decreases to
the cell center due to a decreasing gas concentration.
Whether these doping differences are so large that the
contact resistance is influenced can be easily verified by
performing a Corescan.
CONCLUSIONS
With the Corescan, that enables mapping of contact
resistance on full cell surfaces, a new tool is available to
optimize the front side metallization. With the traditional
TLM method, optimization is difficult or even impossible to
[3] A.S.H. van der Heide, J.H. Bultman, J. Hoornstra and
th
A. Schönecker, Proceedings 17 European
Photovoltaic Solar Energy Conference, Munich
(Germany), in press
[4] http://www.sunlab.nl
[5] A.S.H. van der Heide, Dutch patent 1013204, 5 April
2001, worldwide patent pending
[6] O. Breitenstein, J.P. Rakotoniaina and A.S.H. van der
th
Heide, 11 NREL workshop on crystalline silicon solar
cell materials and processes, Estes Park (Colorado,
USA), 253 (2001)
[7] G.K. Reeves, H.B. Harrison, IEEE Electron Device
Letters, 3, 111 (1982)