WO2011158118A2

WO2011158118A2 - Method and device for forming an electrical contact pattern on a solar cell

Info

Publication number: WO2011158118A2
Application number: PCT/IB2011/001736
Authority: WO
Inventors: Chetan Singh Solanki
Original assignee: Indian Institute Of Technology
Priority date: 2010-06-14
Filing date: 2011-06-14
Publication date: 2011-12-22
Also published as: WO2011158118A3

Abstract

A method of forming an electrical contact pattern on a solar cell (100) comprising the steps of forming a pre-form of the electrical contact pattern on a top surface (144) of the solar cell (100) thermosetting the pre-form contact pattern having a MERCK IS ISHAPE etching paste (142) disposed thereon to a temperature of about 390°C, and subjecting the solar cell (100) to metallization process for deposition of metal on the etched contact pattern.

Description

TITLE OF THE INVENTION

Method and device for forming an electrical contact pattern on a solar cell

FIELD OF TH E INVENTION

[0001] The present invention relates to solar cells and more particularly, to formation of current carrying electrical contact patterns on the solar cells. DESCRIPTION OF THE BACKGROUND ART

[0002] Commercially available solar cells have l imited efficiencies in terms of an electric contact patterning required on the front and back surfaces of the solar cells for carrying electric charge from the solar cell to an external load. Typically, such patterning is formed out by well-known screen printing technology that primarily involves etching of the pattern and deposition of metal composition both on the front as well as on the back surfaces of the solar cells. An example of the design of the pattern includes a finger-bus bar pattern.

[0003] Screen printing technology generally includes the steps of disposing a mesh designed in a shape of a finger-bus bar pattern on a front side of the solar cell. Thereafter, a metallic paste that includes a si lver powder, glass frits, and organics is applied over the mesh. Usually, a lead borosilicate glass with high lead oxide content is used as glass frits whereas the organics comprises of a binder, solvent, and certain additives that ensures proper printing properties when applied on the solar cells.

[0004] Once the metallic paste is settled on the mesh, the mesh is removed from the solar cell and the solar cell leaving behind the metallic paste in the form of a finger-bus bar pattern on an antireflective coating (usually SiNx, Silicon Nitride) provided on the top surface of the solar cell. The next step in screen printing technology involves, placing the solar cell with the metallic paste deposited thereon within an infra-red (IR) furnace operating at high temperatures. Thereafter, subjecting the solar cell to firing within the furnace for pattern formation and metal deposition on the formed pattern. The firing is generally carried out at high temperatures to the tune of 800°C- 900°C and for sufficient time to ensure proper etching and metal deposition.

[0005] During firing, the glass frit plays the most important role as it etches the antireflective coating on the solar cell. Due to this etching, the silver powder and the organics, which are dried and burnt off, are deposited on the etched portions to enable formation of electrical contact for the solar cell. However, during metal deposition, some amount of glass frits are also accumulated along with the silver powder and organics on the solar cell within the etched portions. It is observed that these traces of glass frits affect the current transport from the semiconductor into the silver film to a great extent. Additionally, the glass also dissolves several percent of silver and enhances the sintering process of the fine silver powder during firing. Such unwanted deposition of the glass frits leads to porosity within the conductive metal contact formed on the solar cell that hinders flow of electric charge therein. Thus, current collecting capacity of the solar cells is compromised resulting in the overall efficiency to be on the lower side.

[00061 However, the major problem lies in the complete solar cell unit being exposed to heating to higher temperatures range of about 800-900°C. Such high temperatures, under any condition, are very detrimental to the performance and efficiency of the solar cell used in the photovoltaic and related industries. It is well known that increase in temperature enhances recombination of the electric charge carriers within the solar cell. This recombination also sufficiently affects the overall performance and efficiency of the solar cell. Then, finger widths of the finger-busbar pattern are also large that results in occupying larger surface area on the solar cell. This increases resistance to the incident solar rays on the solar cell. Moreover, the problems like, time and power consumption in screen printed process, are also the associated drawbacks with the screen printing processes that need to be addressed. [0007] Thus there is a need for a process for making electric contact pattern on the solar cell that at least addresses some of the above mentioned drawbacks, operates within lesser operating temperatures, and yet maintains good conductivity for carrying electric charges.

SUMMARY OF THE INVENTION

[0008] Disclosed herein in a method of forming an electrical contact pattern on a solar cell, the method includes the steps of forming a pre-form of the electrical contact pattern on a top surface of the solar cell, thermosetting the pre-form contact pattern having a MERCK ISISHAPE etching paste disposed thereon to a temperature of about 390°C, and subjecting the solar cell to metallization process for deposition of metal on the etched contact pattern.

[0009] In some embodiments, the step of forming pre-form of the electrical contact pattern on the top surface of the solar cell further includes applying the etching paste on the insulating layer, providing a heating device having a heating surface protruding outwardly from the heating device, the heating surface formed to have a shape similar to that of the electrical contact pattern, and contactably positioning the heating surface on the etching paste so that the heating surface forms a pre-form of the electrical contact pattern on the insulating layer. [0010] In some embodiments, the step of forming pre-form of the electrical contact pattern on the top surface of the solar cell further includes providing a heating device having a heating surface protruding outwardly from the heating device, the heating surface formed to have a shape similar to that of the electrical contact pattern, applying a layer of etching paste on the heating surface, and contactably positioning the heating surface having the etching paste applied thereto on the insulating layer of the solar cell.

[0011] In another embodiment of the present invention, the step of subjecting the solar cell to metallization process further includes deposition of Ni by electroless Ni plating technique a composition of NiCl₂, NaP02H2-H20 and other chemicals on the etched electrical contact pattern.

[0012] In another aspect to the present invention, a heating device connected to a temperature control unit and capable of forming a contact pattern on a solar cell when contacted and operated therewith includes a conductive base portion having a plurality of holes formed therein, a first heating member connected to one of the plurality of holes for providing necessary heating effect to the heating device, a sensor disposed within other of the plurality of holes for sensing temperature of the heating device and based on the sensed temperature sending signal to the temperature control unit, and a heating surface protruding outwardly from the base portion and having a shape representative of the electrical contact pattern. [0013] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0014] It is to be understood that both the foregoing general description and the following detailed description of the present embodiments of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.

[0015] A BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above-mentioned and other features and advantages of the various embodiments of the invention, and the manner of attaining them, will become more apparent will be better understood by reference to the accompanying drawings, wherein:

[0017] FIG. 1 is a cross-sectional view of a solar cell used in various embodiments of the present invention;

[0018] FIG. 2 is a perspective view of a heating device according to an embodiment of the present invention;

[0019] FIG. 3 shows an electrical connection of the heating device of FIG. 2 with a temperature control unit;

[0020] FIG. 4 shows application of an etching paste on the heating device of FIG. 2 according to one embodiment of the present invention;

(0021 ] FIG. 5 shows the step where the heating device of FIG. 4 is about to be positioned on the solar cell of FIG. 1 ;

[0022] FIG. 6 shows the step where the heating device of FIG. 4 connected with the temperature control unit is contactably positioned over the solar cell of FIG. 1 ;

[0023| FIG. 7 shows an etched finger-bus bar pattern formed on the solar cell after completion of the etching process; [0024] FIG. 8 shows application of an etching paste on the solar cell of FIG. 1 according to one embodiment of the present invention;

[0025] FIG. 9 shows an etching step carried on the solar cell of FIG. 8; and

[0026] FIG. 10 shows an electro-less nickel plating technique for deposition of metal on the formed finger-bus bar pattern on the solar cell of FIG. 7, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] FIG. 1 shows a cross-sectional view of a solar cell 100 used in the various embodiments of the present invention described in the foregoing specification. As a person skilled in the art would know, solar cells used in photovoltaic technologies are generally made from crystalline -silicon composite material and includes a P-type semiconductor, an N-type semiconductor, and an insulating layer. The solar cell 100, as shown in FIG. 1 , has an N-type semiconductor 102 formed over a P-type semiconductor 1 04, and an insulating layer 106 disposed on the top of the N-type semiconductor 102. The insulating layer 106 acts as an anti-reflective coating to the solar cell 100 that causes total internal reflection of the solar rays incident on the solar cell 100. Preferably, the insulting layer 1 06 is made from SiN_x (Silicon Nitride).

[0028] As detailed in the foregoing description, selective portions of the insulating layer 106 are subjected to chemical treatment/etching process for formation of the electrical contact pattern thereon. Preferably, the electric contact pattern may be formed to represent a shape of a finger-bus bar pattern well-known in the art, for carrying and distributing electric current from the solar cell 100 to an external load (not shown). However, other shapes of the electric contact pattern are also considered within the scope of the present invention. Additionally, a bottom surface 108 of the P-type semiconductor 104 may also have an insulating layer 106 that may be subjected to chemical treatment/etching process simi lar to that provided on top of the N-type semiconductor 102 for forming similar or other designed electric contact patterns. However, for the sake of bring clarity to the various embodiments of the present invention in the foregoing description, electric contact pattern formed only on the top of the solar cell 100 will be explained.

[0029] FIG. 2 shows a perspective view of a heating device 1 10 according to an embodiment of the present invention. The heating device 1 10 has a flat base portion 1 12 made of a conductive material such various metallic materials known in the art. Further, the heating device 1 10 is micro- machined to have a heating surface 1 14 that extends upwardly from the base portion 1 12. The heating surface 1 14 serves as an interface for heat transfer between the solar cell 100 and the heating device 1 10 when the heating device 1 10 contacts the solar cell 100. Preferably, edges 1 16 of the heating surface 1 14 are micro-machined to have a shape similar to that of the electrical contact pattern that is to be formed on the solar cell 1 00. According to an embodiment of the present invention, the heating surface I 14 of the heating device 1 10 is micro-machined to have a shape of a finger-bus bar pattern 1 18.

[0030] As shown in FIG. 2, the base portion 1 12 has a plurality of holes 1 20 formed therein that extends from a first side 122 of the base portion 1 12 to an opposite second side 1 24 or, may stop along a certain distance before the opposite second side 124. In a first hole 126 a first heating member 126, having a power rating of 150W, is disposed which when connected with a temperature control unit 130 (FIG. 3) provides the necessary heating effect to the base portion 1 1 2 as well as the heating surface 1 14. According to another embodiment of the present invention, there may be a second heating element 130 disposed within a second hole 134. Preferably, a sensor 1 36 that senses temperature of the heating device 1 10 is also connected to a third hole 138 that is disposed in between the two heating elements. Benefit of disposing the two heating elements with the sensor 136 positioned therebetween is two folds. First, the heating elements by virtue of their positioning within the base portion 1 12 uniformly heat the heating surface 1 14 to a pre-requisite temperature. Second, as the sensor 1 36 is symmetrically positioned in between the two heating elements, the sensor 136 accurately measures the temperature of the heating surface 1 14.

(0031 J FIG. 3 shows an electrical connection 140 between the heating device 1 10 and the temperature control unit 130. As noted above, due to arrangement of the sensor 136 in between the two heating elements, nearly accurate measurement of the temperature of the heating device 1 10 is possibly achieved. Once the sensor 136 senses the temperature of the heating device 1 10, it gives signal to the temperature control unit 130 and accordingly temperature of the heating element is regulated. Typically, during chemical treatment/etching the heating surface 1 14 contacts an etching paste and transfers heat energy thereto so that the etching paste performs etching function. The etching pastes are highly temperature sensitive and if not operated at their standard operating temperature, then the whole etching process may not function properly. Therefore, accurate measurement of the heating surface 1 14, provided by the above arrangement, becomes a must for such etching processes.

[0032] Formation of electrical contact pattern on the solar cel l 100 by using the heating device 1 10 will be described below. According to one embodiment of the present invention as shown in FIG. 4, a chemical semi-molten etching paste 142, preferably that of M ERCK ISISHPE™ is uniformly applied over the heating surface 1 14 of the heating device. Preferably, the etching paste 142 is uniformly applied over the edges 1 16 of finger-bus bar pattern 1 18 shape of the heating surface 1 14 so that the etching paste 142 also takes shape of the finger-bus bar pattern 1 18. One method of applying the etching paste 142 over the edges 1 16 is to form this pattern on a thin sheet of plastic and then tightly positioning the plastic sheet on the edges 1 16 of the heating surface 1 14 so that the pattern of the etching paste 142 matches the finger-bus bar pattern 1 18 of the heating surface 1 14. In this manner, edges 1 16 of the heating surface 1 14 have the etching paste 142 adhered thereon. However, in various other embodiments, other ways may be used to coat the etching paste 142 on the edges 1 16 of the heating surface 1 14 and be considered within the scope of the present invention. It will also be appreciated by a person skilled in the art that several other etching pastes operating at lower temperatures may be used in the various embodiments of the present invention and be considered within the scope of the present invention.

[0033] FIG. 5 shows the step where the heating device 1 10 electrically connected with the temperature control unit 130 (not shown) positioned adjacent to the insulating layer 106 of the solar cell 100. Fully positioned heating device 1 10 over the solar cell 100 with the heating device 1 10 electrically connected 140 with the temperature control unit 130 is illustrated in FIG. 6. In this position, the edges 1 16 of the heating surface 1 14 having the etching paste 142 coated thereon only contacts selective portions of a top surface 144 of the insulating layer 106. Quite clearly, these selective portions have an imprint of the shape of the finger-bus bar pattern 1 1 8 (not shown) of the heating surface 1 14. This leads to formation of a pre-form of the electrical contact pattern on the insulating layer 106. This pre-form pattern acts as a guide pattern along which formation of 'final' electrical contact pattern on the solar cell 1 00 takes place when subjected to chemical treatment/etching process. [0034] Once the pre-form of the electrical contact pattern is formed on the top surface 144 of the insulating layer 106, the temperature of the heating member is slowly raised and thermosetted to a temperature of about 390°C. In this way, the pre-form of the electrical contact pattern is thermosetted at a temperature of 390°C. This allows the heating surface 1 14 to transfer heat to the etching paste 142 attached thereto and as a result of that the etching paste 142 is heated up. Further, the MERCK IS1SHPE ^{1 M} etching paste 142 by virtue of its etching properties begins to perform its etching function at this temperature of 390°C. This marks the beginning of the etching step on the insulating layer 106 as described in the foregoing paragraphs.

[0035J During etching, the selective portions of the insulating layer 106 that are in physical contact with the etching paste 142 as well as available in the vicinity of the etching paste 142 are etched. As noted above, this etching is carried along the pre-form of the contact pattern acting as a guide pattern towards the top surface 146 (See FIG. 9) of the N-type semiconductor 102. Thermosetting of the pre-form with the etching paste 142 adhered to the heating surface 1 14 is allowed for approximately 90 seconds. Throughout this time period, all of the selective portions of the insulating layer 106 across its width (W) are etched thereby exposing a top surface 146 of the N- type semiconductor 102. It will be understood by a skilled person that the exposed portions of the N- type semiconductor 102 corresponds to the final electrical contact pattern formed thereon. Thus, at the end of the 90 second time period, the insulating layer 106 has developed cavities 148 (shown in FIG. 9) that have a shape similar to that of the pre-form of the electrical contact pattern. After the etching step, the solar cel l 100 is thoroughly washed with known washing solutions for removing the untreated etching paste 142 and debris of the insulating layer 106. A thoroughly washed solar cell 100 having the finger-bus bar electrical contact pattern is shown in FIG. 7. [0036] Generally, the width of the finger that is obtained at the end of the above described etching process is 250μι^η spacing. This finger width of this dimension is achieved because of the fact that the thickness of the edges 1 16 of the heating surface 1 14 has a similar dimension. It will be understood by skilled person that a benefit of using the above explained heating device 1 1 0 is that width of the finger-bus bar pattern 1 1 8 is dependent on the width of the edges 1 1 6 of the heating surface 1 14 of the heating device 1 10. Thus, by controlling the width of the edges 1 16 one can very easily control the width of the finger-bus bar pattern 1 1 8 to be formed on the solar cell 100. Thus, finger widths of even lesser dimensions than that of 250μηι may be easily achieved by varying the thickness of the edges 1 16 of the heating surface 1 14. Accordingly, the metal deposited on the etched electric contact pattern also has a smaller width and this provides some benefits over the known processes. For example, requirement of larger surface areas on the top of the solar cell 100 is reduced considerably resulting in overall efficiency of the solar cell 100 being increased. Another benefit is that the problem of recombination for charge carriers moving within the solar cell 100 is also reduced.

[0037| FIG. 8 illustrates another embodiment of the present invention. In this embodiment the

MERCK ISISUPE™ etching paste 142 is applied on the top surface 144 of the insulating layer 106. The etching paste 142, which is generally in sub-molten form, is uniformly applied over the insulating layer 106 by any device that is capable of uniformly applying such solutions over an even solid surface. The heating device 1 10, as described above in various embodiments, having the heating surface 1 14 that has a shape of the electrical contact pattern is positioned on the solar cell 100.

[0038) As shown in FIG. 9, the heating device 1 10 is positioned on top of the etching paste

142 in such a manner that the heating surface 1 14 contacts the etching paste 142 to form a pre-form of the electrical contact pattern. Actually, when the heating surface 1 14 makes contact with the etching paste 142, the electrical contact pattern shape of the heating surface 1 14 is imprinted on the etching paste 142 thereby forming the pre-form of the pattern. Thereafter, as shown in FIG. 9, similar steps of thermosetting and etching at a temperature of about 390°C and for duration of 90 seconds, as described in the embodiment where the etching paste 142 is applied on the heating device 1 10, follows. Thereafter, as noted above, the etched pattern is thoroughly washed to obtain a final electrical contact pattern on the N-type semiconductor 102 of the solar cell 100.

[0039] Once the formation of the electrical contact pattern on the solar cell 100 is complete the solar cell 100 and more particularly the formed electric contact pattern thereof is subjected to metallization process for deposition of metallic material thereon. Preferably, the metallization process is done through various well known metal deposition techniques. FIG. 10 shows one embodiment of metal deposition technique according to the present invention in which metal is deposited by electroless nickel plating process. In the electroless nickel plating process the etched electric contact pattern is submerged into an electrolytic solution that had composition of a composition of NiCl₂ , NaP02H2 H2() and other chemicals. The Electroless nickel (EN) plating is an auto-catalytic reaction used to deposit a coating of nickel chloride on a substrate. Experimentally it is observed that deposition of metal is carried out at a temperature of 80°C and generally takes 5 minutes in the electroless process. According to another embodiment, the metallization on the etched electric contact pattern may also be done by well known thermal evaporation technique that requires similar lesser operating temperatures.

[0040| Preferably, Al (Aluminum) metal is deposited on the bottom surface 108, i.e., bottom of the P-type semiconductor 104, in the finger-bus bar contact pattern for carrying electric current to an external load. Preferably, deposition of Al on the bottom surface 108 of the solar cell 100 in the above pattern may be carried by the above explained etching and metal deposition processes. Alternatively, A I metal deposition on the bottom surface 108 of the solar cell 100 may also be carried by evaporation process. The reason for using Al as metal is because of the fact that Al makes good ohmic contact with the silicon material of the P-type solar cell 100.

[0041 ] Thus, a skilled person in the art would easily recognize from the above described above embodiments is that the temperature to which the solar cell 100 is selectively subjected for etching of the electrical contact pattern and deposition of the metal composition is much less. To the contrary, temperature requirements in the prior art processes that required heating of the whole solar cell unit to higher temperatures to the tune of 800^HC - 900°C. Thus, above noted drawbacks associated with the solar cells exposed to higher temperature is reduced significantly. Additionally, replacing the etching agent of glass frits with MERCK ISlSHPE ^rM etching paste 142 drawback of porosity of metal deposited is also avoided. Moreover, the number of steps and process time required etching and deposit metal on the solar cell is also reduced. [0042] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. A method of forming an electrical contact pattern on a solar cell, the method comprising the steps of:

forming a pre-form of the electrical contact pattern on a top surface of the solar cell:

thermosetting the pre-form contact pattern having a MKRCK ISISHAPE! etching paste disposed thereon to a temperature of about 3 0°C; and

subieciing the solar cell to metallization process for deposition of metal on the etched contact pattern.

2. The method according to claim 1, the solar cell comprising a P-type semiconductor, an N-type semiconductor and an insulating layer positioned on the top of the N-type semiconductor, wherein the insulating layer forms the top surface of the solar cell.

3. The method according to claim 2, wherein the step of forming pre-form of the electrical contact pattern on the top surface of the solar cell further includes:

applying the etching paste on the insulating layer;

providing a heating device having a heating surface protruding outwardly from the heating device, the heating surface formed to have a shape similar to that of the electrical contact pattern; and conlaetably positioning the heating surface on the etching paste so that the heating surface forms a pre-form of the electrical contact pattern on the insulating layer.

4. The method according to claim 2, wherein the step of forming pre-form of the electrical contact pattern on the lop surface of ihe soiarcell further includes: provid ing a heating device having a heating surface protruding outwardly from the heating device, the heating surface formed to have a shape sim i lar to that of the electrical contact pattern; applying a layer of etching paste on the heating surface; and

contactably positioning the heating surface hav ing the etching paste applied thereto on the insulating layer of the solar cell.

5. The method according to claim 2, wherein the step of thermosetting further includes etching of the insulating layer available in the vicinity of the etching paste and along corresponding to the pre-form of the electrical contact pattern.

6. The method according to claim 1 , wherein the step o f subjecting the solar cell to metal l ization process further includes an electrolcssplating Ni plating technique a composition of Nidi .

NaP02 l 12 1 120 and other chemicals.

8. A heating device connected to a temperature control unit and capable of forming a contact pattern on a solar cel l when contacted and operated therewith comprising:

a conductive base portion having a plurality o f holes formed therein;

a first heating member connected to one of the plurality of holes for providing necessary heating effect to the heating device;

a sensor disposed within other of the plurality of holes for sensing temperature of the heating dev ice and based on th sensed temperature sending signal to the temperature control unit: and

a !ieating surface protruding outward ly from the base portion and having a shape

representative of the electrical contact pattern.

9. The heal ing device according to claim 8, wherein the heating surface has a M ERCK I S I SHAPE etching paste appl ied thereon, the heating surface being contactably d isposed on a top surface of the solar cel l and heated to a temperature of about 390°C so as to etch the top surface to form a pattern represent ing the contact pattern.

10. The heat ing device according to claim 8, further including a second heating member connected with another of the plurality of ho les, wherein the sensor is positioned in between the first and the second heati ng elements.