NL2015844B1

NL2015844B1 - Enhanced metallization of silicon solar cells.

Info

Publication number: NL2015844B1
Application number: NL2015844A
Authority: NL
Inventors: Cesar Ilkay; Olivier Lenzmann Frank; Eugène Bende Evert
Original assignee: Stichting Energieonderzoek Centrum Nederland
Priority date: 2015-11-23
Filing date: 2015-11-23
Publication date: 2017-06-07
Also published as: TW201725742A; WO2017091068A1

Abstract

A solar cell includes a silicon substrate provided on at least one surface of the substrate with a covering dielectric layer and with a metallic conductive pattern. The metallic conductive pattern includes a patterned aluminium based paste layer. The patterned aluminium based paste layer is bordered at the edges by a bounding element extending along each of said edges. The dielectric layer has an opening in between the edges of two bounding elements facing each other. In the opening a layered structure is arranged with an alloy including Al and Si in contact with the patterned aluminium based paste layer and a p+ doped layer bordered on one side by the said alloy layer and on the other side by the matrix of said silicon substrate.

Description

Enhanced metallization of silicon solar cells

Field of the invention

The present invention relates to a solar cell comprising a silicon substrate provided on at least one surface of the substrate with a covering dielectric layer and with a metallic conductive pattern.

Also, the invention relates to a method for manufacturing such a solar cell.

Background

In J. Krause et al., Solar Energy Materials & Solar Cells 95 (2011), pp. 2151-2160, is disclosed that rear surface contacts and metallization in a silicon solar cells can be formed by printing aluminium paste on the rear surface of the solar cell substrate and subsequently, annealing the solar cell substrate. Aluminium paste is printed on the rear surface of the silicon substrate. Typically, the rear surface is provided with a dielectric layer. During the annealing process an etching component, e.g. glass-frit, from the Al paste opens the dielectric layer. Thereafter Al melts and subsequently Al dissolves Si, after which silicon diffuses into Al and Al into Silicon resulting in an Al-Si liquid, and during cool down silicon is segregated from the melt thereby forming a doped p+ layer by epitaxial growth and below the eutectic temperature a layer of an Al-Si alloy. The formation process can be understood from the Al-Si phase diagram.

The Al-Si alloy layer is between the p-type Al doped silicon layer and the Al paste on the rear surface of the substrate. Al-paste may also contain Boron. Then the p+ layer can also contain Boron atoms in addition to Al-atoms.

In the solar cell the p-type doped silicon layer acts as a back surface field (BSF) layer.

It has been observed that such a formation of rear surface contacts and metallization may have issues that can cause contact recombination effects at the edges of the layered structure. See the publication of I. Cesar et al., “Efficiency Gain For Bi-Facial Multi-Crystalline Solar Cell With Uncapped AI2O3 And Local Firing-Through Al-BSF”, 39th IEEE Photovoltaic Specialists Conference, 16-21 June, 2013, Tampa, Florida, USA. One contact recombination effect occurs when the aluminium based paste is not shielded from the substrate by the layered structure but extends laterally over the dielectric layer on the silicon substrate. The aluminium paste can degrade the passivating properties of the dielectric layer.

Another contact recombination effect occurs at the edges of the aluminium paste where the Al-Si alloy layer or the Al-paste is not or insufficiently shielded by p+ dopant layer, such that the Al-Si alloy layer or the Al-paste is in direct contact with the matrix of the silicon substrate.

Krause shows that the Al-paste mass per unit of contact area is an important parameter for determining the thickness of the p+ dopant layer and thereby the shielding properties of the BSF layer. In general, a thicker BSF layer leads to lower contact recombination. Also, a specific BSF thickness can be obtained at a lower maximum anneal temperature when the Al-paste mass per unit of contact area is higher.

It is an object of the present invention to provide a method for manufacturing a solar cell and a solar cell that overcome the disadvantages of the prior art.

It is also an object of the invention to provide a structure that allows to deposit a relatively high Al-paste mass per unit of contact area in comparison with the prior art.

It is further an object of the invention to provide a method that creates a metallisation structure that is relatively less susceptible to misalignment.

Summary of the invention

The object is achieved by a solar cell comprising a silicon substrate provided on at least one surface of the substrate with a covering dielectric layer and with a metallic conductive pattern; the metallic conductive pattern comprising a patterned aluminium based paste layer; the patterned aluminium based paste layer being bordered at the edges by a bounding element extending along each of said edges; the dielectric layer having an opening in between the edges of two bounding elements facing each other; in the opening a layered structure is arranged with an alloy comprising A1 and Si in contact with the patterned aluminium based paste layer and a p+ doped layer bordered on one side by the said alloy layer and on the other side by the matrix of said silicon substrate. Advantageously, the use of bounding elements along the edges of the aluminium metallisation pattern provides a relatively small aperture for the creation of the p+ doped silicon sub layer and the Al-Si alloy sub layer in the silicon substrate. The aperture controls the shape of these sub layers as created during the anneal step as described above.

Since the reaction of A1 with the silicon substrate takes place through the small aperture the formed aluminium containing sub layers are rounded, i.e. arcuate, between the bounding elements such that the p+ doped silicon (or BSF) layer is between the Al-Si alloy layer and the matrix of the silicon substrate. Thus contact recombination between the Al-Si alloy and the silicon matrix is reduced due to the presence of the doped p+ layer in between. In addition, the bounding elements also prevent contact of the aluminium based paste material with the surface of the dielectric layer. This avoids degradation of the passivation of the dielectric layer and thus prevents contact recombination. This is in particular important if the Al-paste is a firing-through paste with an etchant particles, e.g. glass frit.

Since the material of the bounding elements is inert, i.e. does not react with the dielectric layer or the silicon substrate at the annealing temperature, the bounding elements act as an inert template during the anneal step. Typically, the anneal step is done between about 660°C and about 900 °C.

In addition, as the aperture (i.e. the width between the bounding elements) determines the surface area where A1 reacts with Si, the bounding elements do not require a high aspect ratio and may have a rounded cross-section or sloped side walls like a mound or embankment. The bounding elements can be printed in a similar manner as the aluminium based paste, e.g. by screen printing.

Moreover, the bounding elements provide some tolerance in the printing of the aluminium based paste material by masking the edges of the A1 paste metallisation pattern. In screen printing the size of the screen and position of the screen pattern may slightly vary due to variation in forces exerting on the screen. The bounding elements can assist to prevent misprint of the aluminium based paste material.

According to an aspect, the invention provides a solar cell as described above, wherein said layered structure having a rounded shape protruding from the surface into the silicon substrate while at least lateral ends of the p+ dopant silicon layer border at and extend at least partially under the bounding elements.

According to an aspect, the invention provides a solar cell as described above, wherein the bounding elements consist of non-contacting, non-firing-through paste. The nonfiring-through property provides that the paste material will not dissolve the underlying dielectric layer. The paste is also non-contacting, i.e. not providing electrical contact with the underlying layer.

According to an aspect, the invention provides a solar cell as described above, wherein the material of the bounding elements is characterized as an inert material with respect to the dielectric layer material and to aluminium.

According to an aspect, the invention provides a solar cell as described above, wherein the material of the bounding elements comprises aluminium-oxide particles or aluminium particles coated with an aluminium oxide.

According to an aspect, the invention provides a solar cell as described above, wherein the bounding elements comprise an aluminium oxide or aluminium nitride based material.

According to an aspect, the invention provides a solar cell as described above, wherein the aluminium based paste material is a firing-though aluminium paste.

According to an aspect, the invention provides a solar cell as described above, wherein the aluminium based paste material additionally comprises boron.

According to an aspect, the invention provides a solar cell as described above, wherein the bounding elements have sloped side walls.

According to an aspect, the invention provides a solar cell as described above, wherein a side wall of the bounding element extends partially under the patterned layer of aluminium based paste material.

According to an aspect, the invention provides a solar cell as described above, wherein the material of the bounding elements is at least inert up to an alloying temperature for formation of the alloy comprising A1 and Si from an A1 based paste material with silicon.

According to an aspect, the invention provides a solar cell as described above, wherein the alloy comprising A1 and Si additionally comprises Boron.

According to an aspect, the invention provides a solar cell as described above, wherein the alloying temperature is between about 660°C and about 900°C.

Moreover, the present invention relates to a method for manufacturing a solar cell based on a silicon substrate, comprising — Creating one or more doped semiconductor structures on the silicon substrate; — Creating a dielectric on a surface of the silicon substrate; — On the dielectric layer creating a metallic conductive pattern comprising printing a patterned aluminium based paste layer, wherein the creation of the metallisation pattern comprises: — preceding the printing of the patterned aluminium based paste layer, creating bounding elements that extends in parallel along the edges of the patterned aluminium based paste layer to be created; — then printing the aluminium based paste material between the bounding elements to form the patterned aluminium based paste layer, such that the aluminium based paste material is in contact with the dielectric layer between the bounding elements.

After creating the solar cell structure on the silicon substrate, including for example the creation of emitter layer(s), anti-reflection layers and/or front side metallisation patterns, the method of the invention provides the step of creating a metallisation pattern based on aluminium based paste material. It will be understood that the method of the invention provides the step of creating a metallisation pattern based on aluminium based paste material as a back-end step.

Advantageously, the method provides that the metallisation pattern is created by a printing process, without the need of a dedicated patterned masking layer.

In an embodiment, the printing process is a screen printing process.

The skilled in the art will appreciate that as the method of the invention is a “back-end” process, the method can be used in the manufacturing of various types of solar cell such as p-type solar cell with front surface emitter, a p-type interdigitated back contact solar cell with a n+ emitter on the rear side and a p+ BSF layer bordering the Al-Si alloy and Al-paste metallisation structure.

Advantageously, the method also provides the formation of an alternating p+ and n+ dopant layer structure without the need to selectively remove parts of the n+ region first, e.g. by selective etching or by locally preventing the formation n+ layers, e.g. by using diffusion barriers. The alloying process comprises the dissolution of silicon into aluminium such that also the dopant atoms of the n+ layer are dissolved. Since these atoms migrate from a small, typically sub micrometer layer, to the thick, typically more than ten micrometer, layer of the Al-Si melt, the concentration of the n+ dopant is reduced. During cool down in the anneal process when Silicon is segregated from the melt, a new p+ doped silicon layer is epitaxially grown. In this layer the A1 doping atoms have a higher concentration than the doping atoms that before the anneal constituted the n+ layer, resulting in a net p+ doped layer. The same holds for Aluminium based paste that also has Boron compounds.

According to an aspect, the invention provides a method as described above , further comprising: annealing the silicon substrate with the created metallisation pattern, such that during heat up the aluminium based paste material, optionally comprising Boron, opens the contacted dielectric layer, with Aluminium dissolving silicon from the silicon substrate, forming an Al-Si melt, optionally an Al-Si-B melt, and A1 diffusing into the said melt, and silicon diffusing into said melt, optionally B diffusing into said melt, and after cool down results in a layered structure comprising a lower sublayer of an p+ doped silicon layer, comprising A1 and optionally Boron as dopants where silicon is segregated from the melt, and an upper sublayer of an Al-Si alloy, optionally Al-Si-B alloy, and the patterned layer of aluminium based paste material borders on the upper sublayer, wherein the bounding elements consist of an inert material with respect to the dielectric layer material and to aluminium.

According to an aspect, the invention provides a method as described above, wherein the aluminium based paste material is printed with at least 10 mg per contact area in cm2; the contact area being defined by a distance between the bounding elements at the edges of the patterned aluminium based paste layer per length of patterned layer. According to an aspect, the invention provides a method as described above, wherein the annealing is carried out at a temperature between about 660°C and about 900°C. According to an aspect, the invention provides a method as described above, having a p-type solar cell with an n+ emitter on at least one surface where the p+ contact is formed by an anneal of an aluminium based paste material where A1 melts and A1 dissolves the silicon including the n+ emitter forming an Al-Si melt, where A1 diffuses into Si and Si diffuses into Al, and during cool down silicon is segregated from the Al-Si melt and is leaving behind the layered structure of the p+ doped silicon layer and the Al-Si alloy layer, resulting in a p+/n+junction at the surface.

According to an aspect, the invention provides a method as described above, wherein the aluminium based paste material, the p+ doped silicon layer and the Al-Si alloy layer contain Boron in addition.

Advantageous embodiments are further defined by the dependent claims.

Brief description of drawings

The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are shown. The drawings are intended exclusively for illustrative purposes and not to restrict the inventive concept. The scope of the invention is only limited by the definitions presented in the appended claims.

In the drawings

Figure 1 shows a cross-section of a p-type front emitter solar cell according to an embodiment of the invention;

Figure 2 shows a cross-section of a p-type interdigitated back-contact solar cell with a gap between p+ region and n+ region according to an embodiment of the invention; Figure 3 shows a cross-section of a p-type interdigitated back-contact solar cell without a gap between p+ region and n+ region according to an embodiment of the invention; Figure 4 shows a cross-section of an n-type interdigitated back-contact solar cell according to an embodiment of the invention;

Figure 5 shows a cross-section of an n-type interdigitated back-contact solar cell according to an embodiment of the invention, and

Figures 6 and 7 schematically show a top view of a metallisation structure in accordance with a respective embodiment of the present invention.

Detailed description of embodiments

Figure 1 shows a cross-section of a p-type front emitter solar cell 10 according to an embodiment of the invention.

The solar cell of figure 1 comprises a p-type silicon substrate 100, i.e. the silicon substrate has a base p conductivity type. On a first surface A of the substrate 100, an n+ type layer 112 is arranged. On surface B opposite surface A the surface of the substrate is covered by a dielectric layer 102. On surface B a pair of bounding elements 104 delimit an opening C in the dielectric layer 102. In an embodiment, the bounding elements 104 are line-shaped as they extend along the surface B. In between the bounding elements 104 a linear body 106 of aluminium based paste material is present which forms a metallisation line on surface B.

The dielectric layer may be a single layer or a stack of sub layers, selected from for example AI2O3 and SiNx.

In the silicon substrate 100 between the bounding elements 104 a layered structure 108, 110 or double layer is present that extends from the surface B into the silicon substrate. The double layer comprises close to the surface B a upper layer 108 of an Al-Si alloy (an alloy comprising A1 and Si) and deeper in the silicon substrate, a lower layer 110 of p+ -type aluminium doped silicon.

Both the upper layer 108 and the lower layer 110 have a rounded cross-section extending into the silicon substrate in a manner that the lower layer 110 separates the upper layer from the silicon substrate matrix which prevents contact recombination between the upper layer and the silicon substrate matrix. The lateral ends of the p+ doped silicon layer 110 border on the interface between the bounding elements 104 and the silicon substrate.

In an embodiment the p+ doped silicon layer extends partially under the bounding elements 104.

The aluminium based paste material body 106 is in contact with the upper layer of Al-Si alloy, and can act as metallisation line.

The bounding elements may have sloped side walls and extend partially under the aluminium based paste body 106. The edge of the aluminium based paste material body 106 is then under a right or oblique angle (< 90°) with the surface B.

In an embodiment, the bounding elements have a height of at least one pm.

The layered structure is formed as follows:

After creating the semiconductor structure in the silicon substrate , i.e. the front side n+ layer 112, and the rear side dielectric layer, the metallisation process comprises the formation of a metallisation line pattern 106 from aluminium based paste. As a first step a pattern of line-shaped bounding elements 104 is created in which the line-shaped elements 104 are positioned on the dielectric layer 102 as an outline in which subsequently the metallisation line pattern is to be created. In an embodiment, the lineshaped bounding elements 104 are created by a paste printing process.

In a next step, in between the line-shaped bounding elements 104, a body of aluminium based paste material is applied on the dielectric layer 102 by a paste printing process. Subsequently, the substrate provided with the body of aluminium based paste 106 bordered by the line-shaped bounding elements 104 is subjected to an annealing process. The annealing takes place at elevated temperature, above the eutectic temperature of Al-Si, typically at a temperature between about 660°C and about 900°C.

The aluminium based paste material has a composition that at the elevated temperature allows the paste material to open the dielectric layer 102, in such a way that the aluminium based paste contacts the silicon matrix of the silicon substrate. Advantageously, using a firing-through Al-based paste, i.e. Al-based paste with an etchant component, e.g. glass frit, can open the dielectric layer, e.g. Aluminium-oxide or a stack layer of Aluminium-oxide and Silicon-nitride, in the anneal process. This prevents the need of opening the dielectric layer by other means, like local etching or by laser drilling.

Next, A1 from the aluminium based paste reacts with silicon from the silicon substrate matrix, which results after cooling in the layered structure of a lower layer 110 of p+ type A1 doped silicon (BSF layer) and an upper layer 108 of Al-Si alloy embedded in the silicon substrate. The lower layer 110 of p+ type A1 doped silicon extends under the bounding elements 104. If the Al-based paste also contains Boron, the p+ doped silicon layer 110 also contains Boron atoms.

As the bounding elements consist of a relatively inert material that does not react with the dielectric layer 102 or the silicon substrate 100 during the annealing process, the bounding elements 104 acts as mask or template for the aluminium based paste and allows to form the layered structure 108, 110 in a well-defined manner as the reaction of A1 with the silicon is controlled by the aperture or width between the rounded elements 104.

Also, the bounding elements provide that the creation of the metallisation structure becomes less susceptible to a low viscosity of the aluminium based paste material and undesirable flow of the paste material is avoided.

After the metallisation step of the rear surface B, a front side contact 114 can be created on the n+ type front side emitter layer 112. Or the front side contact is created simultaneously with the anneal step of rear side metallisation (‘co-firing’).

The above described metallisation structure comprising the lower layer 110 of p+ type A1 and/or Boron doped silicon, an upper layer 108 of Al-Si alloy, a body 106 of aluminium based paste material and bounding elements 104 of an relatively inert material, in which the body 106 of aluminium based paste material is in contact with the upper layer of Al-Si alloy and is bordered by the bounding elements 104, can also be applied in other types of silicon based solar cells.

The invention provides a method in which the metallisation pattern can be created by a printing process without the need of a patterning the dielectric layer, such as an AI2O3-SiNx layer or by patterning a stack of an AbCh-diclcctric layer and a syloxane capping layer that is patterned by etching or laser drilling. In an embodiment, the printing process of the paste(s) is a screen printing process.

Below, examples of such solar cells in accordance with the present invention are described with reference to Figures 2-7.

It is noted that the following examples relate to some embodiments of the invention. The skilled in the art will appreciate that the invention is not limited to the examples presented here.

In Figures 2-7 entities with the same reference number as shown in Figure 1 refer to identical or corresponding entities.

Figure 2 shows a cross-section of a p-type interdigitated back-contact, IBC, solar cell according to an embodiment of the invention.

The metallisation structure 104, 106, 108, 110 is created in a p-type IBC solar cell 20. The IBC solar cell 20 comprises a p-type silicon substrate 100 which is covered at a surface B with a dielectric layer 102. During use surface B will act as back surface for contacting to some external terminal structure such as a conductive back-sheet of a solar panel (not shown).

The metallisation structure 104, 106, 108, 110 is positioned at an opening C in the dielectric layer and is similar as described above with reference to Figure 1.

In the surface B of the substrate adjacent to the metallisation structure 104, 106, 108, 110 an n+ doped layer 116, covered by dielectric layer 102, is present that can act as counter electrode for the metallisation structure.

Between the n+ doped layer 116 and the lower layer 110 of p+ A1 doped silicon a gap 120 of base p conductivity type silicon is arranged.

Further the n+ doped silicon layer 116 is provided with a contact 118 that connects to the n+ doped silicon layer 116 by an opening D in the dielectric layer 102.

The surface A opposite surface B, may be covered by other layers such as an n+ doped layer or a p+ doped layer or alternating n+/p+ doped layers and/or a dielectric layer, e.g. S£Nx or AI2O3, optionally comprising electrical surface charge, causing an inversion or an accumulation layer of the charge carriers in the silicon matrix and/or an anti-reflection coating layer, as will be appreciated by the person skilled in the art.

The metallisation structure 104, 106, 108, 110 can be manufactured after formation of the semiconductor structure (the n+ doped layer 116, the dielectric layer 102 and any layer on surface A) by the method as described above with reference to Figure 1.

Figure 3 shows a cross-section of a p-type interdigitated back-contact solar cell 30 according to an embodiment of the invention.

The metallisation structure 104, 106, 108, 110 is created in a p-type IBC solar cell 30. The IBC solar cell 30 comprises a p-type silicon substrate 100 which is covered at a surface B with a dielectric layer 102.

In the surface B of the substrate adjacent to the metallisation structure 104, 106, 108, 110 an n+ doped layer 116, covered by dielectric layer 102, is present.

In this embodiment, the n+ doped layer 116 borders directly on the lower p+ A1 doped silicon layer 110 without a gap. At the border between the n+ doped layer 116 and the A1 doped silicon layer 110 a sharp p/n transition may be present.

It is noted that the structure of the IBC solar cell 30 can be formed in a relatively simple manner by first creating the n+ doped layer 116 in surface B of the substrate and covering surface B with the dielectric layer 102. After these steps, parallel line-shaped bounding elements 104 are printed according to a desired metallisation pattern. Next a body of aluminium based paste material 106 is printed on the dielectric layer 102 in between the parallel line shaped bounding elements 104.

By using an aluminium based paste material that is configured as a firing-through paste (e.g. containing a glass-frit), in a subsequent anneal step the metallisation structure 104, 106, 108, 110 is formed. The material of the line-shaped bounding elements 104 is configured as a non-firing-through paste.

In this embodiment, no removal step of parts of the emitter (layer 116) is required. Figure 4 shows a cross-section of an n-type interdigitated back-contact solar cell 40 according to an embodiment of the invention.

The metallisation structure 104, 106, 108, 110 is created in a p-type IBC solar cell 40. The IBC solar cell 40 comprises an n-type silicon substrate 101 which is covered at a surface B with a dielectric layer 102. During use surface B will act as back surface for contacting to for example, a conductive back-sheet of a solar panel (not shown).

In the surface B of the substrate adjacent to the metallisation structure 104, 106, 108, 110 an n+ doped layer 116, covered by dielectric layer 102, is present that can act as counter electrode for the metallisation structure. A gap 124 is present between the n+ doped layer 116 and the lower layer 110 of P+ A1 doped silicon.

In this embodiment, an interruption of the n+ doped layer 116 at the location of the metallisation structure 104, 106, 108, 110, is required e.g. by an additional selective etching step or by a masked diffusion step. After this step, parallel line-shaped bounding elements 104 are printed according to a desired metallisation pattern. Next a body of aluminium based paste material 106 is printed in between the parallel line shaped rounded elements 104.

By using an aluminium based paste material that is configured as a firing-through paste, in a subsequent anneal step the metallisation structure 104, 106, 108, 110 is formed. The material of the line-shaped bounding elements 104 is configured as a nonfiring-through paste.

Figure 5 shows a cross-section of an n-type interdigitated back-contact solar 50 cell according to an embodiment of the invention.

The n-type IBC solar cell 50 is similar to the n-type IBC solar cell 40 as described above with reference to Figure 4, except that in the embodiment shown in Figure 5, the n+ doped layer 116 borders directly on the lower layer 110 of p+ A1 doped silicon.

In this embodiment, the layered structure 108, 110 of the p+ doped silicon layer and the Al-Si alloy layer can be formed by using a contacting firing-through aluminium based paste material. Patterning of the n+ doped layer 116, e.g. by selective etching or by masked diffusion, can be omitted.

It is noted that the metallisation structure and the method of manufacturing allow to print relatively more aluminium per unit contact area in comparison to the methods from the prior art thanks to the creation of the line-shaped bounding elements 104 between which the body of aluminium based paste material is printed on the dielectric layer or if the dielectric layer is removed before, on the surface of the substrate. This has the benefit of creating a thicker BSF at a specific temperature or that the same BSF thickness can be obtained at a lower maximum anneal temperature. A lower maximum anneal temperature can have benefits for maintaining the passivating properties of the dielectric layer, since the passivating properties can deteriorate with increasing anneal temperatures.

According to an embodiment, the method provides a printing mass for the aluminium based paste material of at least 10 mg per contact area in cm2.

As a result, a relatively thicker lower layer 108 of p+ A1 doped silicon and a relatively thicker upper layer 110 of Al-Si alloy can be formed at a given annealing temperature. Alternatively, a lower layer 108 and an upper layer 110 of comparable thickness as observed in the prior art can be formed by an, in comparison with the prior art, lower annealing temperature. A further consequence of the latter effect is that since the firing temperature (of the anneal step) can be lower, a higher Voc is obtained due to a better surface passivation of the dielectric layer 102 and/or of the dielectric layers on the front surface.

The material of which the line-shaped bounding elements 104 are created can be an alumina based paste.

The aluminium based paste material can be a paste that contains aluminium globules with a diameter in the order of 1 - 10 pm in which the aluminium globules are coated with an alumina coating.

In addition, the aluminium based paste may contain additives like boron , which may enhance the p-type doping level of the lower p+ doped silicon layer 110. The boron can be in separate globules or be admixed in the aluminum.

Figure 6 schematically shows a top view of a metallisation structure in accordance with an embodiment of the present invention.

On surface B of a silicon substrate of a solar cell, a busbar and fingers type metallisation structure 200 is created on the dielectric layer 102. In the metallisation structure a number of busbars 210 extend parallel to a first direction Y (shown as vertically). A number of fingers 220 extend in a second direction X (shown as horizontally) perpendicular to first direction Y. The fingers 220 are connected to the busbars 210 to form a metallisation network. According to an embodiment, the busbars 210 consists of a Ag-based conductive paste, preferably a non-firing through paste, that is printed on the dielectric layer for soldering purposes.

The fingers 220 consist of a body 106 of aluminium based paste material that on its edges is bordered by bounding elements 104. The body of aluminium based paste material contacts the layered structure of the upper Al-Si alloy layer 108 and the lower p+ doped silicon layer 110, through an opening (not shown here) in the dielectric layer. According to an embodiment, the metallisation structure is created by first printing the Ag-based busbars 220, then printing the bounding elements 104 as outline for the body of aluminium based paste and finally printing the body of aluminium based paste inside the outline formed by the bounding elements 104. It is shown that the fingers 210 (i.e. both the bounding elements and the body of aluminium paste) overlap at least partially with the busbars 220.

Figure 7 schematically shows a top view of a metallisation structure 230 in accordance with an embodiment of the present invention.

On surface B of a silicon substrate of a solar cell, a busbar and fingers type metallisation structure 230 is created on the dielectric layer 102. The metallisation structure 230 is very similar to the metallisation structure 200 of Figure 6, except that in this embodiment the busbars 240 are now only partially formed from Ag-based paste portions 250 for soldering purposes. For the remainder the busbars 240 are formed from aluminium based paste portions 255 that enclose the Ag-based paste portions 250. The edges of the aluminium based paste in the busbars and in the fingers are bordered by bounding elements 104 as described above in more detail.

The Ag-based parts, the bounding elements and the aluminium based paste bodies and portions are printed in a same order as explained with reference to Figure 6.

The H-pattem comprising busbars and fingers is an example. The busbars might also be replaced by merely disk shaped Ag-paste based areas (‘pads’) from which A1 -paste based leads extend bordered on each side by bounding elements.

The invention has been described with reference to the preferred embodiment. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.

Claims

A solar cell comprising a silicon substrate provided on at least one surface of the substrate with a covering dielectric layer and with a metallic conductive pattern; wherein: the metallic conductive pattern comprises a patterned aluminum-based paste layer; the patterned aluminum-based paste layer is bounded at the edges by a limiting element that extends along each of the edges; the dielectric layer has an opening between the edges of two limiting elements facing each other; a layered structure is provided in the opening with a layer of an alloy comprising Al and Si, in contact with the patterned aluminum-based paste layer and a p + doped layer bounded on one side by the alloy layer and on the other side through the matrix of the silicon substrate.

The solar cell of claim 1, wherein the layered structure has a rounded shape that protrudes from the surface in the silicon substrate while at least lateral ends of the p + dopant silicon layer are adjacent to and at least partially extend below the limiting elements.

Solar cell according to claim 1 or 2, wherein the limiting elements consist of non-contacting, non-burning, "non-fusing-through" paste.

Solar cell according to any of the preceding claims, wherein the material of the bounding elements is characterized as an inert material with respect to the material of the dielectric layer and aluminum.

The solar cell of claim 4, wherein the material of the limiting elements comprises aluminum oxide particles or aluminum particles coated with an aluminum oxide.

A solar cell according to claim 4 or claim 5, wherein the limiting elements comprise an aluminum oxide or material based on aluminum nitride.

The solar cell according to any of claims 1-6, wherein the material of the aluminum-based paste is a burn-through, "fining-though", aluminum paste.

A solar cell according to any of the preceding claims, wherein the aluminum-based paste material additionally comprises boron.

Solar cell according to any of the preceding claims, wherein the bounding elements have sloping side walls.

10. Solar cell according to any of claims 1-9, wherein a side wall of the limiting element partially extends below the patterned layer of aluminum-based paste material.

The solar cell of claim 5, wherein the material of the limiting elements is at least inert to an alloy temperature to form the alloy comprising Al and Si from an Al-based paste material with silicon.

The solar cell of claim 11, wherein the alloy comprising Al and Si additionally comprises boron.

The solar cell of claim 11, wherein the alloy temperature is between about 660 ° C and about 900 ° C.

A method for manufacturing a solar cell based on a silicon substrate, comprising - Forming one or more doped semiconductor structures on the silicon substrate; - Forming a dielectric on a surface of the silicon substrate; - Forming a metallic conductive pattern on the dielectric layer comprising printing a patterned aluminum-based paste layer, the formation of the metallization pattern comprising: - prior to printing the patterned aluminum-based paste layer, forming of limiting elements extending parallel along the edges of the patterned aluminum-based paste layer; - then printing the aluminum-based paste material between the limiting elements to form the patterned aluminum-based paste layer, such that the aluminum-based paste material is in contact with the dielectric layer between the limiting elements.

The method of claim 14, further comprising: heating the silicon substrate with the formed metallization pattern such that during heating, the aluminum-based paste material, which optionally comprises boron, opens the contacted dielectric layer with aluminum silicon dissolving from the silicon substrate, thereby forming an Al-Si melt, optionally an Al-Si-B melt, where Al diffuses into the melt and silicon diffuses into the melt, and optionally B diffuses into the melt, and after cooling results in a layered structure that has a lower sub layer of a p + doped silicon layer, which comprises Al and optionally B as a dopant where silicon is segregated from the melt, and a higher sub-layer of an Al-Si alloy, optionally Al-Si-B alloy, and the of a patterned layer of aluminum-based paste material is adjacent to the higher-lying sub-layer, the bounding elements consisting of an inert material with respect to the The material of the dielectric layer and aluminum.

A method according to claim 14 or 15, wherein the aluminum-based paste material is printed with at least 10 mg per contact area in cm 2; wherein the contact area is defined by a distance between the bounding elements at the edges of the patterned aluminum-based paste layer per length of the patterned layer.

A method according to any of the preceding claims 15-16, wherein the heating is performed at a temperature between about 660 ° C and about 900 ° C.

A method according to claim 14, with a p-type solar cell with an n + emitter on at least one surface where the p + contact is formed by heating an aluminum-based paste material wherein Al melts and Al dissolves silicon including the n + emitter Al-Si melt forming, Al diffusing into Si and Si diffusing into Al, and during cooling silicon segregates from the Al-Si melt and leaves the layered structure of the p + doped silicon layer and the layer of the Al-Si alloy leaving a layer of Al p + / n + junction on the surface results.

The method of claim 18, wherein the aluminum-based paste material, the p + doped silicon layer and the layer of the Al-Si alloy additionally contain boron.