TRANSLATION OF PCT/EP2011/002963
METHOD FOR PRODUCING A METALLIC CONTACT STRUCTURE OF A
PHOTOVOLTAIC SOLAR CELL
Description
The invention relates to a method for producing a metallic contact structure of a
photovoltaic solar cell according to the preamble of claim 1.
A photovoltaic solar cell is formed of a planar semiconductor element in which, by
means of incident electromagnetic radiation, generation of electron-hole pairs is
obtained and charge carrier separation takes place at at least one pn junction, such
that an electrical potential difference arises between at least two electrical contacts
of the solar cell and electrical power can be tapped off from the solar cell via an
external electric circuit connected to said contacts.
In this case, the charge carriers are collected via metallic contact structures, such
that, by making contact with said contact structures at one or more contact points,
the charge carriers can be fed into the external electric circuit.
Competing requirements arise with regard to the design of the metallic contact
structure: On the one hand, the average path length of a charge carrier in the
semiconductor substrate to the closest contact point with a metallic contact
structure is intended to be small so that ohmic losses on account of conduction
resistances within the semiconductor are kept small. On the other hand, the contact
area between metallic contact structure and semiconductor substrate is intended to
be small since a high recombination rate is present at the contact area, compared
with electrically passivated surfaces of the semiconductor substrate.
Particularly in the case of solar cells in which an emitter, and thus also the pn
junction separating the charge carrier pairs generated, is formed at or in the region
of the front side of the solar cell, said front side being designed for light incidence,
electrical contact is made with the base typically by means of a metallic

contact-making layer which is arranged on the rear side ad which is electrically
conductively connected to the semiconductor substrate. In order to obtain high
efficiencies, an efficient rear-side passivation, i.e. the obtaining of a low surface
recombination rate for minority charge carriers, in particular in the region of the
rear-side surface of the semiconductor substrate, and a formation of the electrical
contact with a low contact resistance are essential in this case.
Thus, solar cell structures are known in which the rear side of the semiconductor
substrate is covered substantially over the whole area with a passivation layer or
insulating layer embodied as a silicon nitride layer, silicon dioxide layer or
aluminum oxide layer or layer system, such that low surface recombination rates
are obtained. It is only at point contacts that the passivation layer is opened in a
linear fashion over a small area or in a point-like fashion and there is an electrically
conductive connection to a metallic contact-making layer arranged on the
passivation layer. In this case, the total area of the distributed contacts is
significantly smaller than the area of the rear side of the solar cell. One such solar
cell structure is, for example, the PERL structure (passivated emitter, rear locally
defused) as described in J. Benick, B. Hoex, G. Dingemans, A. Richter, M. Hermle,
and S.W. Glunz "High-efficiency n-type silicon solar cells with front side boron
emitter," in Proceedings of the 24th European Photovoltaic Solar Energy Conference
(Hamburg, Germany), 2009 and in Zhao et al, Proc. of the 21st IEEE PVSC, 333
(1990). This structure, which makes possible rear-side contact-making in order to
achieve high efficiencies, requires additional photolithography steps during
production, such that an industrial implementation of said solar cell structure is not
practicable or is at least highly cost-intensive.
The present invention is therefore based on the object of providing a method for
producing a metallic contact structure of a photovoltaic solar cell which can be
implemented industrially in a cost-effective manner and at the same time makes
possible high efficiencies through the combination of regions passivated over a large
area with local contact-making regions to form a metallic contact structure.
This object is achieved by means of a method as claimed in claim 1. Advantageous
configurations of the method according to the invention are found in claims 2 to 18.

The method according to the invention for producing a metallic contact structure of
a photovoltaic solar cell comprises the following method steps:
In a method step A, an electrically non-conductive insulating layer is applied to a
surface of a semiconductor substrate, if appropriate to one or more intermediate
layers covering said surface.
In a method step B, a metallic contact layer is applied to the insulating layer, if
appropriate to one or more intermediate layers covering the insulating layer.
Furthermore, one or more local electrically conductive connections between
semiconductor substrate and contact layer are produced through the insulating
layer and, if appropriate, further intermediate layers situated between contact layer
and semiconductor substrate.
The result of method steps A and B is, therefore, that the surface of the
semiconductor substrate, largely by means of the insulating layer, on the one hand
is electrically insulated from the metallic contact layer and on the other hand is
passivated with regard to the recombination activities and only at a plurality of
local regions is there an electrically conductive connection between the
semiconductor substrate and contact layer through the insulating layer.
What is essential is that in method step B the metallic contact layer is formed in a
manner comprising the following method steps:
In a method step B1 a first paste containing metal particles is applied at a plurality
of local regions. These are those local regions at which an electrically conductive
connection to the semiconductor substrate is intended to be produced.
In a method step B2, a second paste containing metal particles is applied areally, in
a manner at least partly covering at least the regions covered with the first paste
and also the regions situated therebetween- The layer formed by means of the
second paste thus covers at least the regions of the first paste which are applied in
method step B1 and also at least partly the regions situated therebetween.

A method step B3 involves globally heating the semiconductor substrate in a
temperature step.
In this case, the first paste and second paste and also the temperature step or the
temperature steps following method step B1 are designed in such a way that the
first paste penetrates through the insulating layer and forms an electrically
conductive contact directly with the semiconductor substrate, whereas the second
paste does not penetrate through the insulating layer. Consequently, the metal
layer that forms from the second paste is electrically conductively connected to the
semiconductor substrate only indirectly via the first paste or the metal structures
that form therefrom.
The method according to the invention thus makes it possible for the first time to
obtain, in a simple manner which can be implemented industrially in a
cost-effective way, an electrical contact-making, in particular of the rear side of a
solar cell, which has regions passivated over a large area, on the one hand, and local
contact-making regions, on the other hand. The use of two different pastes, only the
first of which penetrates through the insulating layer in the temperature step or the
temperatures step following paste application, makes it possible to obtain, by
cost-effective methods, the abovementioned advantageous contact-making structure
having a plurality of local contact-making regions and regions passivated over a
large area on the surface of the semiconductor substrate.
In particular, for applying the pastes, it is possible to have recourse to methods
which are known per se and already implemented industrially, such as, for
example, screen printing or inkjet printing methods, which are already established
for high throughput rates in the inline method and thus cost-effective production
processes.
Various methods can be used for applying the pastes: screen printing, stencil
printing, dispensing, inkjetting, laser transfer printing, pad printing, intaglio
printing and other printing and deposition methods. Preferably, paste application is
followed by a drying step in order to solidify the pastes.

Furthermore, for producing the metallic contact structure by means of the method
according to the invention, it is possible to dispense entirely with cost-intensive
photolithography. Only the first paste has to be applied in a targeted manner in the
regions provided for the local contact-making. However, this is already possible
nowadays without any problems for example using the already known methods
mentioned above, in particular screen printing, stencil printing or inkjet printing
methods or dispensing. The insulating layer and the second paste can be applied
over a large area, and in particular over the whole area, and so here as well no
masking steps whatsoever are necessary.
With the method according to the invention, therefore, the contact-making structure
realized hitherto in cost-intensive complex processes, for producing high-efficiency
solar cells, becomes implementable in a cost-effective industrial production method.
The global heating in method step B3 is preferably effected in a fast firing furnace
as a firing step in which the wafers are conveyed through the hot region of the
furnace in a continuous method. In this case, preferably temperatures of up to
1000°C are attained, preferably maximum temperatures of less than 900°C being
used. The firing process will preferably be made very short, such that the wafers
stay in the furnace only for a few minutes and are heated to the temperature only
for a few seconds, as a result of which this process differs distinctly from pure
drying processes.
Preferably, the second paste containing metal particles is applied areally between
the regions covered with the first paste in such a way that the second paste
connects the regions covered with the first paste, in such a way that after method
step B3 there is an electrically conductive connection between the structures
resulting from the first paste by means of the structure resulting from the second
paste. In this case, preferably at least two adjacent structures resulting from the
first paste are electrically conductively connected by the structure resulting from
the second paste. With further preference, each structure resulting from the first
paste is electrically conductively connected to at least one further structure

resulting from the first paste by means of a structure resulting from the second
paste.
Advantageously, the second paste containing metal particles is applied areally in
such a way that at least 30%, preferably at least 50%, of the area of the
intermediate regions, which intermediate regions are situated between the regions
covered with the first paste, is covered by the second paste.
Preferably, in method step B2 the second paste is applied in such a way that it
completely covers the regions covered with the first paste and also the regions
situated therebetween. This ensures, in particular, a low electrical transverse
conduction resistance. It likewise lies within the scope of the invention for the
regions situated therebetween to be covered only partly, for example in a linear
fashion, with the second paste. This ensures an electrically conductive connection by
the second paste, or the metal structure arising therefrom, paste being saved on
account of the partial coverage in comparison with the whole-area coverage of the
regions situated therebetween. It likewise lies within the scope of the invention to
use this contact-making structure for the formation of solar cells with which contact
is made on the rear side, such as MWT, EWT or back junction solar cells.
For interconnecting a solar cell with an external electric circuit or further solar cells
in module interconnection, typically metallic contact structures of the solar cell are
soldered to a metallic cell connector by means of a soldering method. On account of
their composition, however, the metallic contact structures are typically not suitable
or only suitable to a limited extent for a soldering connection, for which reason
usually so-called "soldering pads", that is to say local metallic contact structures
which, firstly, form an electrically conductive connection to the metallic
contact-making structure of the solar cell and, secondly, have good solderable
properties, are additionally applied to the solar cell.
In one preferred embodiment of the method according to the invention, therefore,
method step B additionally comprises a method step Ba, wherein in method step Ba
a third paste containing metal particles is applied on at least one local partial
region, said third paste being designed in such a way that the third paste forms

solderable metallic structures after the action of heat. In this way, a soldering pad
is additionally formed in a cost-effective procedure which can be implemented in
particular in the inline method.
Particularly advantageously, the third paste is actually arranged prior to the second
paste on the substrate, such that the second paste can overlap the third paste in
regions in order that a particularly good electrical and mechanical connection of
these two pastes is formed in the subsequent firing step.
In one particularly simple embodiment, the third paste and the first paste are
identical and are arranged in one step, for example by means of a screen printing
process. The second paste is then arranged in such a way that the previously
produced soldering pads are not covered or are only covered in regions and are thus
accessible for subsequently making contact with the solar cell for example by means
of soldering or adhesive bonding.
The third paste is preferably applied in such a way that the metallic structure
resulting therefrom is electrically conductively connected to the metallic structure
resulting from the second paste and/or the metallic structure resulting from the
first paste.
Preferably, the third paste and also the temperature step or the temperature steps
following method step B1 are designed in such a way that the third paste does not
penetrate through the insulating layer. In this preferred embodiment of the method
according to the invention, furthermore in method step Bl the first paste is applied
to at least one first group and one second group of local regions, wherein in method
step B2 the second paste is applied in a manner covering the regions of the first
group which are covered with the first paste and also the regions situated
therebetween, and in method step Ba the third paste is applied in a manner
covering the regions of the second group which are covered with the first paste and
also the regions situated therebetween.
In this advantageous embodiment, therefore, exclusively the first paste, or the
metallic structures resulting therefrom, penetrates through the insulating layer.

The metallic structure resulting from the third paste is, firstly, electrically
conductively connected to the semiconductor substrate via the local regions of the
second group that are covered with the first paste, and it is furthermore electrically
conductively connected to the second paste or the metallic structure resulting
therefrom.
In an alternative advantageous embodiment, the third paste is designed in such a
way that it penetrates through the insulating layer in the temperature step or the
temperature steps following method step B1. In this embodiment, therefore, an
electrical contact between the metallic contact structure resulting from the third
paste and the semiconductor substrate is effected directly, wherein, in this
embodiment, too, the metal structure resulting from the third paste is electrically
conductively connected to the metal structure resulting from the second paste. In
particular it is advantageous in this case that the first and third pastes are
identical and are applied to the semiconductor substrate preferably simultaneously,
preferably in a common process step.
Preferably, the second and third pastes are applied in a manner electrically
conductively connected to one another. In this case, it lies within the scope of the
invention to apply the second and third pastes such that they do not overlap, but
directly adjoin one another. This affords the advantage that, firstly, defined regions
are formed with regard to the soldering pads on the contact-making side of the solar
cell in the case of small height differences and, secondly, an electrically conductive
connection between second paste and third paste or the metal structures resulting
therefrom, said connection exhibiting low ohmic conduction losses, is ensured by the
pastes adjoining one another.
In order to ensure a secure electrically conductive connection between first paste
and third paste or the structures resulting therefrom, it is advantageous to apply to
the second and third pastes in an overlapping fashion, preferably in such a way that
the third paste overlaps the second paste. Preferably, the pastes are applied in an
overlapping fashion such that they overlap by a length in the range of between
0.1 mm and 2 mm.

In a further preferred embodiment of the method according to the invention, the
third paste is applied to the second paste. As a result, although this gives rise to
height differences on the contact-making side, in return the accuracy of the printing
methods can be provided with a higher tolerance, since the regions printed with
second and third pastes no longer adjoin one another.
In a further advantageous embodiment, the second paste is designed in such a way
that it does not penetrate through the insulating layer, and that, after the action of
heat or as a result of a further aftertreatment, said second paste forms a metallic
structure that is solderable or electrically interconnectable in some other way, or
such a structure is formed on said second paste. The separate production of
soldering pads is thereby obviated. In this preferred embodiment, therefore, the
entire second paste applied areally, or the metal structure resulting therefrom, is
suitable for a solderable connection or an alternative interconnection technology. In
particular, the second paste, or the metal structure resulting therefrom, is suitable
for electrical contact-making by means of conductive adhesives, as described in
"Fast and easy single step module assembly for back-contacted C-Si solar cells with
conductive adhesives", 2003, Bultman, Osaka.
The pastes containing metal particles known to the person skilled in the art from
previously known screen printing or inkjet printing methods or other methods for
producing metallic contact structures can be used for implementing the method
according to the invention. In particular, the person skilled in the art knows, for a
predefined insulating layer in predefined temperature steps, which pastes achieve
penetration through the insulation layer and which do not.
The insulating layer is preferably embodied as a dielectric layer, in particular
preferably as a silicon nitride layer, silicon oxide layer, aluminum oxide layer,
silicon carbide layer, titanium dioxide layer, or as a layer containing mixtures of the
materials mentioned. The embodiment of the insulating layer as a layer system
composed of a plurality of layers likewise lies within the scope of the invention.
Advantageously, the insulating layer or the layer system used as an insulating
layer has a thickness in the range of between 5 nm and 500 nm, in particular
between 20 nm and 250 nm, with further preference between 50 nm and 250 nm. In

particular, a layer system composed of a layer of aluminum oxide and a layer of
silicon nitride preferably having a total thickness of 100 nm or a layer system
composed of a layer of silicon oxide and a layer of silicon nitride, preferably having a
total thickness of 200 nm, has proved to be advantageous in experiments conducted
by the applicant, in particular with regard to the insulation effect.
Preferably, the first and second pastes have the following specification: the first
paste preferably contains substances that penetrate through the insulating layer in
a high-temperature step. In particular, the first paste preferably contains glass frit,
preferably up to 10% lead and/or bismuth-boron glass, or pure lead and/or bismuth
oxides. In a further preferred embodiment, the first paste contains one or more
oxides, preferably from the group: GeO2, P2O5, Na2O, K2O, CaO, A12O3, MgO,
TiO2, ZnO and B203.
In order to support the formation of a better local back surface field, the use of a
first paste enriched with phosphorus or other elements corresponding to the base
doping type is advantageous. If the base doping is a p-type doping, the first paste
contains corresponding dopants that allow a p-type doping. In particular,
aluminum, boron and/or gallium can be contained.
The second paste preferably contains above mentioned substances which do not
attack the passivation or only attack it in small amounts (preferably less than 2%).
In particular, the second paste preferably contains no glass frit, or only glass frit
with a proportion of less than 1%. In particular, an aluminum paste or a copper
paste is preferably used as second paste. The aluminum paste is distinguished by a
large proportion of aluminum-containing metal particles (preferably greater than
50%).
The third paste preferably has the following specifications:
Preferably, the third paste has a silver proportion of at least 70%, in order to ensure
a good solderability. The use of a third paste enriched with other solderable metals
is likewise within the scope of the invention, preferably tin, gold, copper, iron or

nickel or combinations thereof. The total proportion of the solderable metals in the
third paste is preferably at least 70%.
It is advantageous to use a third paste which additionally comprises additives as
mentioned with regard to the first paste, if the intention is for the third paste to
penetrate through the insulating layer. In particular, in this case it is advantageous
to use a third paste which additionally comprises additives for local high doping
which correspond to the base doping type, preferably phosphorus or other elements
in the case of an n-type doping and preferably aluminum or boron or other elements
in the case of a p-type base doping.
In a further preferred embodiment, after method step B1 and before method step B2
the first paste is dried. The drying process preferably comprises the following
method steps: heating to a temperature at which the paste carrier materials used
evaporate at least partly, preferably completely. Heating to a temperature in the
temperature range of between 150°C and 300°C is preferably effected in the drying
process.
The separate drying of the first paste prior to applying the second paste has the
advantage that the first paste is dry and thus maintains its form when the second
paste is applied. Moreover, no intermixing of the paste constituents takes place,
which prevents the through-firing effect from being weakened.
In a further preferred embodiment of the method according to the invention, no
high-temperature step i.e. in particular no heating to more than 500°C, takes place
between method step Bl and method step B2.
In particular, it is advantageous to treat the first and second pastes and
particularly preferably also the third paste in a common temperature step, such
that all applied pastes are converted into metallic contact structures in the common
temperature step in method step B3. Particularly simple and cost-effective
processing is made possible as a result.

Preferably, at least one of the pastes, preferably the paste which penetrates through
the insulation layer, is enriched with silicon or a silicon compound, preferably with
silicon. It lies within the scope of the invention for the enrichment to consist in a
concentration of between 0.1% and 12%, in particular in a concentration of between
4% and 8%. However, the enrichment preferably consists in a concentration of
between 0.1% and 99%, with further preference in a concentration of between 8%
and 80%, particularly preferably in a concentration of 12% to 60%.
The enrichment with silicon or a silicon compound has the advantage that the
alloying process between the metal contained in the paste and the silicon
semiconductor substrate, which process is crucial for the electrical conductivity,
inter alia, proceeds with significantly less damage and the recombination near the
contact point is thus reduced.
It likewise lies within the scope of the invention to admix aluminum-silicon alloys
with the paste. In this case, it is advantageous to use a paste having a concentration
of aluminum-silicon alloy of at least 1%, preferably at least 10%, with further
preference at least 25%. In this case, it is particularly advantageous for the paste to
have a silicon proportion in the range of 5% to 90%, preferably in the range of 12%
to 80%. In particular, it is advantageous to use a eutectic mixture having a
proportion of approximately 12% silicon.
An admixture of aluminum-silicon alloys has the advantage that the melting point
of the paste decreases and the alloying process proceeds more homogenously.
In a further preferred embodiment, the second paste and/or the third paste
contain(s) silver, preferably in a concentration of at least 70%, particularly in the
range of 80% to 100%. This ensures a solderability of the metal structure produced
of the second paste and/or the third paste.
In a further preferred embodiment, the first paste and/or the third paste contain(s)
a dopant of the base doping type of the solar cell to be produced. This results in a
local high doping in the regions in which the first paste or the metal structure
resulting therefrom is directly in electrical contact with the semiconductor

structure. As a result of such a local high doping on account of the dopant of the
first paste, firstly the contact resistance and secondly the recombination rate are in
each reduced at the contact area metal/semiconductor surface and a further
increase in the efficiency of the solar cell is thus obtained.
It is furthermore advantageous to use an aluminum-containing paste as first paste,
preferably having an aluminum content of 70% to 100%. As a result, a rear-side
p+-type emitter can be formed on a semiconductor substrate having an n-type
doping during the high-temperature step. On p-type semiconductors, it is possible to
produce a local high doping and thus a p+type back surface field that reduces the
recombination rate.
In a further preferred embodiment, an A1 paste enriched with silicon can be used as
first paste for cell concepts in which a p-type emitter is at least partly required on
the rear side. Said emitter arises during the high-temperature process at the
locations of the rear side which are covered with the aluminum-containing paste. In
this case, the A1 paste can be applied either over the whole area or in a structured
fashion.
In one preferred embodiment, therefore, at least the first paste and/or the third
paste contain(s) a dopant of the emitter doping type, i.e. having a doping type
opposite to the base doping type of the solar cell to be produced. As a result, a local
emitter is formed on the rear side.
Preferably, method step B3 involves a global heating of the semiconductor structure
to at least 700°C for a time duration of at least 5 s. Preferably, a temperature
maximum of at least 730°C is achieved during the aforementioned time duration.
A particularly cost-effective industrially implementable configuration of the method
according to the invention results from the fact that at least one of the pastes,
preferably all of the pastes, is/are applied by means of inkjet printing methods or
aerosol printing methods. The application of pastes containing metal particles by
means of inkjet printing methods is known per se and described for example in

"Spray and inkjet printing of hybrid nanoparticle-metal-organic inks for Ag and Cu
metallizations", Curtis, 2002, Mat. Res. Soc. Symp. Proc.
The method according to the invention is preferably used for producing a solar cell
which is essentially formed by a silicon semiconductor substrate. In particular, the
method according to the invention is advantageous for forming the contact structure
on the rear side of the solar cell, opposite the light incidence side.
In the case where the base doping and the doping achieved by the first paste have
the same doping type (for example p-type), it is advantageous if the proportion of
coverage of the solar cell side by the first paste, or the structure emerging
therefrom, is less than 50%, particularly advantageously less than 12%, and
preferably less than 7%. It is likewise advantageous in this case if at least one
dimension of partial regions of first contact structures is less than 500 μm,
preferably less than 200 μm, furthermore less than 100 μm, in particular less than
50 μm. These partial regions are preferably embodied in a punctiform or linear
fashion, wherein at least one dimension of the structures is preferably less than
500 μm, preferably less than 200 μm, furthermore less than 100 μm, in particular
less than 50 μm. Preferably, the first paste is applied in such a way that
punctiform, linear and/or comb-like contact-making areas known per se are formed
between metallization and semiconductor.
If the base doping is opposite to the doping which can be achieved by the first paste,
a coverage of greater than 7%, preferably greater than 12%, and in particular
greater than 50%, of the solar cell side by the first paste, or the structure emerging
therefrom, is advantageous. It is likewise advantageous in this case if at least one
dimension of partial regions of first contact structures is greater than 50 μm,
preferably greater than 100 μm, furthermore greater than 200 μm, in particular
greater than 500 μm. These partial regions are preferably formed in a punctiform or
linear fashion, wherein at least one dimension of the structures is greater than
50 urn, preferably greater than 100 μm, furthermore greater than 200 μm, in
particular greater than 500 μm.

The semiconductor substrate used is preferably a silicon wafer, in particular having
a base doping corresponding to base resistances in the range of 0.1 ohm cm to
10 ohm cm, preferably in the range of 1 ohm cm to 5 ohm cm. Furthermore, the use
of semiconductor substrates having a thickness of less than 250 μm, preferably less
than 170 μm, preferably less than 100 μm, is advantageous.
Further preferred embodiments and advantageous features are explained below
with reference to the figures and the description of the figures, in which:
Figure 1 shows a partial section from a solar cell whose rear-side metallic
contact structure was produced by means of a first exemplary
embodiment of the method according to the invention and has
soldering pads which do not penetrate through the insulating layer,
Figure 2 shows a partial section from a solar cell whose rear-side metallic
contact structure was produced by means of a second exemplary
embodiment of the method according to the invention, in which the
soldering pad penetrates through the insulating layer, and
Figure 3 shows a partial section from a solar cell in which the rear-side metallic
contact structure was formed by means of a third exemplary
embodiment of the method according to the invention, in which the
entire metal layer produced areally on the rear side is formed as a
soldering pad.
Figures 1 to 3 in each case illustrate a partial section from a solar cell produced
from a semiconductor substrate 1 embodied as a silicon wafer. The solar cell
continues toward the right and left. In particular, the solar cell has a multiplicity of
local electrical contact-making regions, only some regions being illustrated for the
sake of better clarity in Figures 1 to 3. Furthermore, the structures and dopings at
the front side of the solar cell illustrated have not been illustrated; customary solar
cell structures, in particular customary arrangements of emitter and metallic
emitter contact structures making contact with the emitter and also passivation

layers and antireflection layers and textures optimizing light incidence are
conceivable here in Figures 1 to 3.
Identical reference signs designate identical elements in Figures 1 to 3.
The semiconductor substrate 1 is a p-doped silicon wafer having a base doping of
approximately 2.7 ohm cm.
The metallic contact structure illustrated in figure 1 was produced by means of a
first exemplary embodiment of the method according to the invention, comprising
the following method steps:
In a method step A, a silicon dioxide layer 2 having a thickness of 250 nm was
applied and etched back to a thickness of 100 nm in further process steps. The
following parameters were used for the oxidation: thermal oxidation at 900°C for
150 min in a water vapor atmosphere (partial pressure approximately 90-100%).
Afterward, in a method step Bl, a multiplicity of local regions of a first paste
containing metal particles were applied, two regions 3a and 3b being identified by
way of example in figure 1.
In this case, the local region 3a belongs to a first group of local regions and the local
region 3b belongs to a second group of local regions. The first paste was applied to
the insulating layer at the local regions 3a, 3b. Afterward, in a method step B2, a
second paste was applied in the regions 4a and 4b, in such a way that the second
paste covers the first group the regions of the first group which are covered with the
first paste 3a, and also the regions situated therebetween.
Furthermore, in a method step Ba, a third paste containing metal particles was
applied in a region 5, such that second paste and third paste adjoin one another and
have an electrical connection. The pastes can also overlap for a better electrical and
mechanical connection.

In a common temperature step in a method step B3, global heating was effected in
one temperature step. The temperature step was performed in a continuous furnace
with the following parameters: heating within 10 s to 500°C, maintaining this
temperature for 10 s. Further heating to 800°C within 5 s, cooling to room
temperature within 15-25 s.
In this temperature step, firstly metallic structures were formed from first, second
and third pastes, wherein only the first paste is designed to penetrate through the
insulating layer 2. This was achieved by virtue of the fact that the first paste
contains lead glass frit in a concentration of between 1% and 5%, whereas the
second and third pastes contain no glass frit. It is likewise possible to use a second
and third paste which contain an inadequate concentration or unsuitable type of
glass frit for penetrating through said layer.
As a result, after carrying out method step B3, as illustrated in figure 1, there is
therefore a direct electrically conductive connection between first paste, or the
metallic structure resulting therefrom, in the local regions 3a, 3b, whereas second
and third pastes, or the metallic structures resulting therefrom, are electrically
conductively connected to the semiconductor substrate only indirectly via the first
paste or the metal structure resulting therefrom. In this exemplary embodiment,
therefore, local structures of the first paste were also arranged below the regions in
which the third paste is arranged.
The third paste has a silver proportion of approximately 90%, such that a good
solderability is provided and the third paste or the metallic structure resulting
therefrom thus functions as a soldering pad.
In a second exemplary embodiment of the method according to the invention, the
third paste is designed in such a way that it likewise penetrates through the
insulating layer 2 in the temperature step carried out in method step B3. Therefore,
in this exemplary embodiment, it is not necessary for local regions in the region 5 in
which the third paste is applied to be printed with the first paste. The remaining
method steps are identical to the method steps described for Figure 1.

As illustrated in Figure 2, therefore, this exemplary embodiment has the result that
the soldering pad formed by means of the third paste in the region 5 is directly
electrically conductively connected to the semiconductor substrate 1 over the whole
area, whereas the second paste or the metal structure resulting therefrom is
electrically conductively connected to the semiconductor substrate only indirectly
via the first paste or the metal structure resulting therefrom.
In this exemplary embodiment, therefore, some regions in which the first paste has
to be printed are omitted in comparison with the exemplary embodiment in
accordance with Figure 1, thus resulting in a saving of paste and a better electrical
contact between soldering pad and semiconductor substrate.
In contrast to the first exemplary embodiment, the third paste has the following
parameters: it contains glass frit (bismuth/lead glass frit) in order to enable
through-firing of the insulation layer. Furthermore, the third paste contains dopant
of the base doping type in order to achieve a better contact and a lower
recombination rate.
In a third exemplary embodiment of the method according to the invention, only a
first and a second paste are used. In this case, the second paste is applied, over the
whole area, to the insulating layer or the previously applied first paste. The process
parameters correspond to those described in the first exemplary embodiment.
The result is illustrated in Figure 3: the second paste 6, or the metallic structure
resulting therefrom, covers the whole area of the rear side of the solar cell to be
produced, wherein there is an electrically conductive contact with the
semiconductor substrate 1 only in the local regions in which the first paste was
printed.
In this exemplary embodiment, therefore, only two pastes have to be printed, with
the result that a further simplification of the production process is obtained.
For a good solderability, the second paste in this case has a silver proportion of 90%.

All of the abovementioned percentages with regard to the constituents of the pastes
relate to percent by mass.

CLAIMS
1. A method for processing a metallic contact structure of a photovoltaic solar
cell,
comprising the following method steps:
A applying an electrically non-conductive insulating layer (2) to a surface of a
semiconductor substrate, if appropriate to one or more intermediate layers
covering said surface,
B applying a metallic contact layer to the insulating layer (2), if appropriate to
one or more intermediate layers covering the insulating layer (2), and
producing one or more local electrically conductive connections between
semiconductor substrate (1) and contact layer through the insulating layer (2)
and, if appropriate, further intermediate layers,
characterized
in that in method step B the metallic contact layer is formed in a manner
comprising the following method steps:
B1 a first paste containing metal particles is applied at a plurality of local
regions,
B2 a second paste containing metal particles is applied areally, in a manner
at least partly covering at least the regions covered with the first paste
and also the regions situated therebetween, and
B3 globally heating the semiconductor substrate and at least the first and
second pastes in a temperature step,
wherein the first paste and the second paste (4a, 4b) and also the
temperature step or further temperature steps following method step Bl are
designed in such a way that the first paste (3a, 3b) penetrates through the
insulating layer (2) and forms an electrically conductive contact directly with
the semiconductor substrate (1), whereas the second paste (4a, 4b) does not
penetrate through the insulating layer (2) and is electrically conductively
connected to the semiconductor substrate (1) only indirectly via the first
paste (3a, 3b).

2. The method as claimed in claim 1,
characterized
in that method step B additionally comprises a method step Ba, wherein in
method step Ba a third paste containing metal particles is applied on at least
one local partial region, said third paste being designed in such a way that
the third paste forms a solderable metallic structure after the action of heat.
3. The method as claimed in claim 2,
characterized
in that the third paste and also the temperature step or the temperature
steps following method step B1 are designed in such a way that the third
paste does not penetrate through the insulating layer (2),
in that in method step Bl the first paste (3a, 3b) is applied to at least one
first group and one second group of local regions, in method step B2 the
second paste (4a, 4b) is applied in a manner covering the regions of the first
group which are covered with the first paste and also the regions situated
therebetween, and in method step Ba the third paste (5) is applied in a
manner covering the regions of the second group which are covered with the
first paste and also the regions situated therebetween.
4. The method as claimed in claim 2,
characterized
in that the third paste (5) and also the temperature step or the temperature
steps following method step Bl are designed in such a way that the third
paste (5) penetrates through the insulating layer (2) in the temperature step
or the temperature steps following method step B1.
5. The method as claimed in any of claims 2 to 4,
characterized
in that the first paste and the third paste (5) are applied in an overlapping
fashion.
6. The method as claimed in any of claims 2 to 5,
characterized

in that first and third pastes are identical and are applied in a common
method step.
7. The method as claimed in claim 2,
characterized
in that the third paste (5) is applied to the second paste (4a, 4b).
8. The method as claimed in any of the preceding claims,
characterized
in that the first paste (3a, 3b) contains one or more substances which
penetrate through the insulating layer, preferably glass frit, and
in that the second paste (4a, 4b) contains no substances which penetrate
through the insulating layer, preferably in that the second paste contains no
glass frit.
9. The method as claimed in any of the preceding claims,
characterized
in that the insulating layer (2) is embodied as a dielectric layer, preferably as
a silicon nitride layer, silicon oxide layer, aluminum oxide layer, silicon
carbide layer, titanium dioxide layer, or as a layer containing mixtures of the
materials mentioned.
10. The method as claimed in any of the preceding claims,
characterized
in that after method step B1 and before method step B2 the first paste (3a,
3b) is dried, preferably by heating to at least 150°C.
11. The method as claimed in any of the preceding claims,
characterized
in that no high-temperature step takes place between method step B1 and
method step B2.
12. The method as claimed in any of the preceding claims,
characterized

in that at least one of the pastes is enriched with silicon, preferably in a
concentration in the range of 0.1% to 99%, preferably in the range of 8% to
80%, with further preference in the range of 12% to 60%.
13. The method as claimed in any of the preceding claims,
characterized
in that the second paste (4a, 4b) and/or the third paste (5) contain(s) silver,
preferably in a concentration in the range of 70% to 100%, preferably in the
range of 80% to 100%.
14. The method as claimed in any of the preceding claims,
characterized
in that the first paste and/or third paste (3a, 3b) contain(s) a dopant,
preferably in a concentration in the range of 0.1% to 3%, preferably in the
range of 0.5% to 2%.
15. The method as claimed in any of the preceding claims,
characterized
in that method step B3 involves heating to at least 750°C for a time duration
of at least 3 s.
16. The method as claimed in any of the preceding claims,
characterized
in that at least one of the pastes, preferably all of the pastes, is/are applied by
means of one of the following methods: screen printing method, stencil
printing method, laser transfer printing method, inkjet printing method or
dispensing.
17. The method as claimed in any of the preceding claims,
characterized
in that the first paste comprises a dopant of the base doping type, and
in that the first paste is applied in such a way that the proportion of coverage
of the solar cell side by the first paste, or the structure emerging therefrom, is

less than 50%, particularly advantageously less than 12%, and preferably
less than 7%.
18. The method as claimed in any of the preceding claims,
characterized
in that the first paste comprises a dopant of an emitter doping type, and
in that the first paste is applied in such a way that a proportion of coverage of
the solar cell side by the first paste, or the structure emerging therefrom, is
greater than 7%, particularly greater than 12%, and in particular greater
than 50%.