GB2493219A - Back Surface Field Silicon Solar Cell - Google Patents

Abstract

A silicon solar cell and a method for preparing a rear contact bus bar region of a solar cell, the solar cell comprising a silicon substrate 3, an aluminium-containing layer 5 on a rear side surface 7 of the silicon substrate 3 and an additional metallic bus bar contact layer 15 on the rear side surface 7 of the silicon substrate 3; wherein the aluminium-containing layer 5 comprises an exposure area 9 at which silicon of the substrate 3 is exposed and the additional metallic bus bar contact layer 15 is deposited onto the exposure area 9 by spray coating techniques. The metallic bus bar contact layer preferably comprises less than 50 vol% silver and more preferably comprises no silver. Such spray coating techniques allow using metals other than silver thereby enabling substantial cost-savings. Furthermore, as the bus bar contact layer 15 directly contacts the exposed area 9 of the silicon substrate 3, good ohmic and mechanically adhesive contact may be established and peeling-off may be prevented.

Description

Silicon solar cell and method for preparing rear contact busbar region for same with spray coating

FIELD OF THE INVENTION

The present invention relates to a silicon solar cell having a solderable rear contact busbar region for interconnecting neighboring solar cells. Furthermore, the present invention relates a method for preparing a rear contact busbar region for such solar cell.

TECHNICAL BACKGROUND

Solar cells are used to convert sunlight into electricity using a photovoltaic effect. A general object is to achieve high conversion efficiency and high reliability balanced by a need for low production costs. ens * . * ** S

Most of the industrially manufactured solar cells currently are based on silicon * * wafer technology and use screen printing techniques for preparing metal contacts to a wafer substrate. Therein, typically a rear side of the solar cell is covered with an aluminium containing layer which generates a back surface field (BSF).

In order to be able to connect neighboring solar cells in a modul, at least part of the metal contacts are typically provided with a solderable material such that solder strips may be soldered to each of neighboring solar cells in order to electrically connect them. Conventionally, a screen printing paste containing silver particles is used for generating the solderable metal contacts.

However, as the prize of silver continuously grew over the last years and as silver containing screen printing pastes typically contain between 60% and 90% of silver content, a metallisation using such pastes significantly contributes to overall costs of a solar cell.

Alternative metallisation techniques have been proposed. For example, US 4,320,251 proposes to use arc plasma spraying in order to apply ohmic contacts to a semiconductor for solar cell manufacture.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a silicon solar cell having a rear contact structure enabling both high efficiency potential and easy manufacture at low cost as well as high long-term reliability. Furthermore, it is an object of the present invention to provide a method for preparing such rear contact structure.

Such objects may be met with the subject-matter of the independent claims.

Advantageous embodiments are defined in the dependent claims.

According to a first aspect of the present invention, a solar cell is proposed comprising a silicon substrate, an aluminium-containing layer on a rear side surface of the silicon substrate and an additional metallic bus bar contact layer on the rear side surface of the silicon substrate. Therein, the aluminium-*t.

containing layer comprises an exposure area at which silicon of the substrate is exposed. The additional metallic bus bar contact layer is deposited onto the exposure area by spray coating techniques.

A gist of the proposed silicon solar cell may be seen as based on the following ideas and recognitions: While it was known that it is beneficial for an efficiency of a solar cell to provide an aluminium-containing layer on a rear side surface of the solar cell substrate in order to generate a back surface field, and while spray coating techniques as such are known for a long time, problems have been observed when attempting to apply metal contacts to an aluminium-containing layer using spray coating techniques. For example, it has been observed that such metal contacts may show insufficient adhesion and may therefore, upon load, peel-off from the substrate surface. However, it has been observed by the inventors of the present application that adhesion properties of a metallic layer applied by spray coating techniques to a rear side surface of a solar cell substrate may be significantly better in case the applied metallic layer directly contacts an exposed silicon surface instead of contacting a surface of an aluminium-containing layer. Based on such recognition, it is proposed to provide a silicon solar cell the rear side surface of which is not completely covered with an aluminium-containing BSF layer but comprises non-covered exposure areas at which the silicon of the substrate is exposed and may therefore be directly contacted by an additional metallic layer.

In the following, possible features and advantages of embodiments of the proposed solar cell are explained in detail.

The silicon substrate used for the solar cell may be any type of substrate. For example, silicon wafers or silicon thin films may be used. The silicon substrate may be mono-crystalline, multi-crystalline or amorphous.

On the rear side of the silicon substrate, an aluminium-containing layer may be generated using various techniques resulting in various structures. For example, in atypical industrial application, an aluminium-containing paste is screenprinted onto the rear side surface and is subsequently fired at elevated temperatures. In the firing process, the aluminium forms an eutectic phase with the silicon which, upon solidification, then may form the aluminium-containing * layer creating the BSF. Alternatively, other deposition techniques such as evaporation, sputtering, printing techniques other than screenprinting, etc. may be used for generating the aluminium-containing layer.

The aluminium-containing layer is applied such that it does not cover the entire rear side surface of the silicon substrate but such that a specific exposure area remains uncovered. Accordingly, in this exposure area, the silicon of the substrate is exposed and may accessed from outside.

The exposure area may form a continuous area extending for example substantially from one edge of the rear side surface to an opposite edge of the rear side surface. Alternatively, the exposure area may form a discontinuous area extending e.g. substantially from one edge to the opposite edge of the rear side surface wherein the discontinuous area comprises a plurality of exposure dots at which silicon of the substrate is exposed and intermediate areas of the aluminium-containing layer laterally separate neighbouring exposure dots.

In other words, the exposure area may be formed by a continuous line on the rear side surface of the silicon substrate which is not covered by the aluminium-containing layer or, alternatively, by a plurality of dot-like non-covered regions arranged along such line. The line may reach from one edge or close to one edge to an opposite edge or close to an opposite edge of the solar cell substrate.

Typically, the line is linear but other contours are possible.

The entire exposure area may be between 0.1 and 10%, preferably between 2 and 6%, of the entire area of the rear side surface of the substrate. Accordingly, as only a minor fraction of the rear side surface is exposed and not covered by the aluminium-containing BSF layer, a major part of the rear side surface may be well passivated by the BSF. On the other hand, the exposure area is sufficiently * :" large in order to provide sufficient adhesive contact area between the substrate rear side and the additional metallic layer directly contacting the silicon at the exposure area. * * * ** *

Onto the exposure area, an additional metallic layer is deposited subsequently * ** serving as a bus bar contact layer. This metallic bus bar contact layer may extend along lines such that linear solar strips may be subsequently soldered to the metallic bus bar contact layer. Typically, a bus bar is a central metalized contact which may collect charge carriers from a larger surrounding area which area may be e.g. covered by a contact grid or a full-area contact having a lower conductivity than the bus bar contact. The bus bar may then be electrically and mechanically connected to conductive strips which may be used to interconnect neighbouring solar cells for e.g. generating strings in a module.

While, in principle, both the exposure area and the additional metallic bus bar contact layer may have any suitable geometry or pattern and while, generally, the geometry or pattern of the bus bar contact layer may be chosen independently of the geometry and pattern of the exposure area, it may be preferred to provide the metallic bus bar contact layer such that it covers substantially only the exposure area. In other words, the deposition of metal for the bus bar contact layer may be restricted to the exposure area. Such restriction of the area to be covered by the additional metallic layer may reduce efforts and costs for depositing such layer.

One specific feature of the proposed solar cell may be seen in that the additional metallic bus bar contact layer is deposited by spray coating techniques. Such spray coating techniques imply methodical as well as structural differences compared to other metal deposition techniques and particularly compared to screen printing techniques..

Thermal spray coating is a relatively mature technology for generating metallic layers. It may be reliably used in industrial manufacturing processes. The technology uses hot jet methods to accelerate and melt or soften powder particles that are fed into the jet. Cold jet methods are also available. High-speed particles or droplets may be deposited on a surface and form the desired coated layer.

Different types of spray coating techniques are known such as plasma spraying, flame spraying or cold spraying. In order to generate small area metal layers, spray direct write technologies may be used in which down-scaled nozzle geometry and jet dimensions allow for creating deposition patterns having dimensions as small as 0.1 mm. Other spray coating techniques use patterned masks for restricting the deposited layer to a specific area.

Unlike other metal deposition techniques, spray coating may allow for directly and locally apply metals to a silicon surface and generating both goods mechanical adhesive contacts and good electrically conducting contacts between the applied metal layer and the substrate's surface. Furthermore, spray coating techniques allow depositing substantially pure metal layers directly mechanically and electrically contacting an underlying substrate's surface. Typically, no additional components are needed for enabling or increasing adhesion to a substrate's surface.

In contrast hereto, for example screenprinted metal layers comprise additional components such as meltable metal oxide particles (frequently referred to as "glass frites") intermixed between metal particles, these metal oxide particles melting and possibly etching a substrate's surface during a subsequent firing procedure at elevated temperatures thereby generating the desired mechanical adhesive contact.

The use of spray coating techniques for depositing the bus bar contact layer allows for this layer comprising less than 50 vol-%, preferably less than 10 vol- %, of silver. Preferably, the metallic bus bar contact layer comprises substantially no silver. Therein, "substantially" may mean that no silver is specifically added but silver is avoided as far as technologically possible or as far as economically helpful.

For example, spray coating techniques allow using cheaper conductive metal powder comprising for example tin (Sn) particles or particle of other compounds or alloys such as tin-silver (SnAg). Powders comprising other metals such as nickel (Ni), aluminium (Al), molybdenum (Mo), nickel-aluminium (Ni-Al), nickel-phosphorus (Ni-P) and nickel-boron-silicon (Ni-B-Si) alloys may also be : used. Accordingly, the excessive costs for silver occurring when conventional silver paste screenprinting is used for contact formation may be prevented. * ..

Preferably, the metallic bus bar contact layer comprises a solderable material.

For example, particles comprising pure solderable metals such as tin or silver or comprising a compound or alloy with a solderable metal may be used. Due to the solderable material, conductor strips may subsequently be soldered to the metallic bus bar contact layer for interconnecting with neighbouring solar cells.

According to a second aspect of the present invention, a method for preparing a rear contact bus bar region of a solar cell is proposed, the method comprising the following steps preferably in the indicated order: providing a silicon substrate; generating an aluminium-containing layer on a rear side surface of the silicon substrate and depositing by spray coating an additional metal contact layer on the rear side surface of the silicon substrate. Therein, the aluminium-containing layer comprises local openings forming an exposure area at which silicon of the substrate is exposed and the additional metal contact layer is deposited onto the exposure area.

The proposed method may be used for preparing the rear contact bus bar region of the silicon solar cell described further above.

Preferably in the proposed method, no silver-containing paste is printed onto the rear side surface of the silicon substrate. In other words, while printing techniques such as screenprinting may be used for generating the aluminium- containing layer, the proposed method should dispense with using silver-containing pastes for the formation of a rear contact bus bar region of the solar cell in order to reduce costs. Such dispensing of silver-containing paste is possible as, instead of silver pads screenprinted onto the rear side of conventional solar cells for soldering interconnecting strips to these pads, a metal contact layer is deposited using spray coating which allows preparing metal contacts of other metals than silver. As these metal contacts are deposited directly onto the exposed silicon substrate, good mechanical adhesion may be obtained such that strips soldered to such bus bar contact layer will not peel-off : easily.

Preferably, the aluminium-containing layer is generated by applying an aluminium-containing paste onto the tear side surface of the silicon substrate and subsequently firing the aluminium-containing paste and then, after such firing, the additional metal contact layer is spray-coated onto the rear side surface of the silicon substrate. In other words, the aluminium-containing BSF layer may be generated in accordance with conventional screenprinting technology but solder pads are not generated in screenprinting technology but by using spray coating technology applied after the formation of the aluminium-containing layer.

It may be noted that possible features and advantages of embodiments of the present invention are described herein with respect to the proposed silicon solar cell or with respect to the proposed method for preparing a rear contact structure for such solar cell. One skilled in the art will recognize that the different features may be arbitrarily combined and features of the solar cell may be realized in a corresponding manner in the preparation method and vice versa in order to implement further advantageous embodiments and realize synergetic effects.

Furthermore, one skilled in the art will realize that the silicon solar cell may have more features than described herein as is typical for such solar cells. For example, the solar cell may comprise differently doped regions, dielectric layers at surfaces thereof as anti-reflection coating, surface passivation, etc. and additional electrical contact structures on a front and/or rear side of the solar cell substrate, to mention only a few examples. Furthermore, the proposed method may be part of a method for preparing an entire solar cell, such method comprising various additional method steps such as diffusion steps, passivation steps, metallization steps, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, features and advantages of embodiments of the present invention are described with respect to the enclosed drawings. Therein, neither the description nor the drawings shall be interpreted as limiting the invention. * * * ** .

Fig. I shows a top view onto a rear side surface of a silicon solar cell according to an embodiment of the present invention.

Fig. 2 shows a top view onto a rear side surface of a silicon solar cell according to an alternative embodiment of the present invention.

Fig. 3 shows a cross-sectional view along the portion A-A of Fig. 1 or 2 of a solar cell substrate prior to applying an additional metallic bus bar contact layer.

Fig. 4 shows a cross-sectional view along the portion A-A of Fig I or 2 of a solar cell substrate after applying an additional metallic bus bar contact layer.

Fig. 5 shows a test arrangement for testing the electrical performance of a silicon solar cell according to an embodiment of the present invention.

The figures are only schematical and not to scale. Same or similar features are designed with same reference signs throughout the figures.

DESCRIPTION OF PREFERRED EMBODIMENTS

Fig I shows a top view onto a rear side surface of a silicon solar celL I according to an embodiment of the present invention. Fig. 3 shows a cross-sectional view of a portion of the solar cell of Fig. 1 along the lines A-A indicated in Fig. 1 prior to applying an additional metallic bus bar contact layer.

Fig. 4 shows a cross-sectional view of the same portion after applying the additional metallic bus bar contact layer.

*fl.fl A silicon wafer serves as a substrate 3. An aluminium-containing layer 5 is applied on top of the rear side surface 7 of this silicon substrate 3. The * aluminium-containing layer may be formed by screenprinting an aluminium- :.: containing paste onto respective portions of the rear side surface of the silicon substrate and subsequently firing the paste in order to generate an aluminium-containing BSF layer. The aluminium-containing layer 5 does not cover the entire rear side surface 7 of the silicon substrate 3 but leaves an exposure area 9 uncovered. At this exposure area 9, the rear side surface 7 of the silicon substrate 3 is not covered but exposed to the environment. The exposure area 9 extends from one edge of the silicon substrate to an opposite edge and corresponds to the location where a linear solder strip shall subsequently be soldered to the rear side contact of the solar cell for interconnection with neighbouring solar cells, Thus, a length of the exposure area may correspond to a length of the solar cell substrate 3. A width of the exposure area may be in a range of 0.3-10mm, preferably in a range of 2 to 5mm.

Fig. 2 shows an alternative embodiment of a silicon solar cell 1', In contrast to the embodiment of Fig. 1, an exposure area 9' is discontinuous and comprises a plurality of dots 11 at which the silicon of the substrate 3 is exposed.

Intermediate areas 13 of the aluminiumcontaining layer 5 laterally separate neighbouring exposure dots II. While a width of the exposure dots 11 may be similar to the width of the linear exposure area of Fig. 1, a length of such exposure dots may be any within a large range of for example 0.2-80 mm. The dots may have any shape such as for example rectangular, round or elliptical.

As shown in Fig. 4, an additionaL metallic bus bar contact layer 15 is deposited onto the exposure area 9. This additional metallic bus bar contact layer 15 is deposited using spray coating techniques and therefore has structuraL and functional features typically resulting from such spray coating deposition. The spray coated bus bar contact layer 15 may be deposited with any suitable thickness and may be thicker or thinner than the aluminium-containing layer 5.

The spray coated bus bar contact layer 15 may get good adhesion with any silicon layer, silicon nitride layer or other layer at the surface of the silicon o substrate 3. Furthermore, it may be used to establish a good contact with any test probe pin as explained below with respect to Fig. 5. * 0 * S * 0S S

The bus bar contact layer 15 may be locally deposited such as to substantially only cover the exposure area 9 and not to extend beyond this area 9 into an area covered by the aluminium-containing layer 5. As, at the exposure area 9, the bus bar contact layer 15 directly contacts the exposed silicon rear side surface 7, good ohmic and adhesive contact may be established between the bus bar contact layer 15 and the silicon substrate 3. As the bus bar contact layer 15 does not extend significantly beyond such exposure area 9, no peeling-off of such layer occurs easily.

Furthermore, as a silver-less solderable material may be used for the bus bar contact layer 15, this layer 15 may enable a cheap and reliable option for soldering an intercormection strip to the rear side contact of the solar cell 1.

Fig. 5 shows an advantageous option to test electrical properties of a silicon solar cell substrate as shown in Fig. 3 before applying the additional metallic bus bar contact layer 15. With an embodiment of a solar cell 1' as shown in Fig. 2, intermediate regions 13 between neighbouring exposure dots 11 are covered with the aluminium-containing layer 5. Accordingly, an electrical test arrangement 17 having two longitudinal test arms 19 may be arranged along a line B-B as indicated in Fig. 2. The test arms 19 comprise a plurality of test needles 21. The arrangement and spacing of these test needles 21 is adapted such that any of the test needles 21 may come in contact with a rear side of the solar cell only in regions covered by the aluminium-containing layer 5, i.e. at the intermediate areas 13.

Accordingly, electrical properties of the substrate of the solar cell 1 may be e**so * easily tested even before depositing the bus bar contact layer 15 and thus also * before soldering any contact strip to the solar cell I. Therein, the arms 19 of the test arrangement 17 may be located at positions actually corresponding to the position of the bus bar to be applied later with subsequent processing steps * Accordingly, the electric properties measured by a measuring device 23 are in good correspondence to electrical properties of the finalized solar cell.

Summarized and expressed in different wording, possible advantages of embodiments of the present inventive silicon solar cell and the method for preparing a rear contact bus bar region of a solar cell as proposed herein are as follows: Cost reduction: Significant cost savings may be achieved due to the possible replacement of silver-containing screenprinting paste for back bus bar formation with other cheaper conductive metal powder such as e.g. tin or tin-silver powder.

Thereby, the proposed silicon solar cell may be immune to ever increasing silver prices for backside printing. Overall costs for manufacturing the back bus bar are estimated to decrease by about 0.015.

Increase in aluminium coverage: The back aluminium coverage may be increased to enhance the backside passivation which is expected to improve the performance of the solar cell.

Decrease of contact resistance: A decrease in contact resistance and increase in fill factor at a module level is expected.

More stable solderability at module level: Experiments show that the spray-coated bus bar contact coukl get stable solderability and strong peel force between a ribbon strip and a back bus bar.

*.ø. 0 * Finally, it should be noted that the term "comprising" does not exclude other elements * .. * 0 * or steps and the "a" or "an" does not exclude a plurality. Also elements described in *:::: association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims. * ** * * * *.* *

LIST OF REFERENCE SIGNS

I solar cell 3 substrate aluminium-containing layer 7 rear side surface 9,9' exposure area 11 exposure dots 13 intermediate region metallic busbar contact layer 17 test arrangement 19 arms 2 1 test needle 23 measurement device * *

I

*4*Ie* * I *.fl * ** ** I * 44 * I * 444 *

Claims (1)

1. <claim-text>CLAIMS1. Silicon solar cell (1), comprising: a silicon substrate (3); an aluminium-containing layer (5) on a rear side surface (7) of the silicon substrate (3); and an additional metallic busbar contact layer (15) on the rear side surface (7) of the silicon substrate (3); characterized in that the aluminium-containing layer (5) comprises an exposure area (9, 9') at which silicon of the substrate (3) is exposed; and the additional metallic busbar contact layer (15) is deposited onto the exposure area (9. 9') by spray coating techniques.</claim-text> <claim-text>2. Silicon solar cell according to claim 1, wherein the metallic busbar contact layer (15) comprises less than 50 vol-% of silver.</claim-text> <claim-text>3. Silicon solar cell according to claim 2, wherein the metallic busbar contact layer (15) compriscs no silver.</claim-text> <claim-text>4. Silicon solar cell according to one of claims Ito 3, wherein the metallic busbar contact layer (15) comprises a solderable material. *0***</claim-text> <claim-text>5. Silicon solar cell according to one of claims Ito 4, wherein the metallic busbar contact layer (15) extends along lines such that linear solder strips may be soldered to the metallic busbar contact layer (15).* * 6. Silicon solar cell according to one of claims ito 5, wherein an exposure " area (9) forms a continuous area extending substantially from one edge to an opposite edge of the rear side surface (7), 7. Silicon solar cell according to one of claims Ito 5, wherein an exposure area (9') forms a discontinuous area extending substantially from one edge to an opposite edge of the rear side surface (7) wherein the discontinuous exposure area (9') comprises a plurality of exposure dots (II) at which silicon of the substrate (3) is exposed and intermediate areas (13) of the aluminium-containing layer (5) laterally separating neighboring exposure dots (11).8. Silicon solar cell according to one of claims ito 7, wherein the metallic busbar contact layer (15) covers substantially only the exposure area (9, 9').9. Silicon solar cell according to one of claims Ito 8, wherein the exposure area (9, 9') is between 0.1% and 10% of the entire area of the rear side surface (7).10. Method for preparing a rear contact busbar region of a solar cell (1), the method comprising: providing a silicon substrate (3); generating an aluminium-containing layer (5) on a rear side surface (7) of the silicon substrate (3); and depositing by spray coating an additional metallic busbar contact layer (15) on the rear side surface (7) of the silicon substrate (3); characterized in that the aluminium-containing layer (5) comprises an exposure area (9, 9') at which silicon of the substrate (3) is exposed; and the additional metallic busbar contact layer (15) is deposited onto the exposure area (9, 9'). *fl*"* 11. The method according to claim 10, wherein no silver containing paste is 4* printed onto the rear side surface (7) of the silicon substrate (3).* ** 12 The method according to claim 10 or 11, wherein the aluminium-containing layer (5) is generated by applying an aluminium containing paste OlltO the rear side surface (7) of the silicon substrate (3) and subsequently firing the aluminium-containing paste; and wherein the additional metal contact layer (15) is spray coated on the rear side surface (7) of the silicon substrate (3) after the firing of the aluminiumScontaining pasteS * SS..... * S * ** S. P * .5 * fl* *</claim-text>