EP2022086A1

EP2022086A1 - Method for producing photovoltaic cells and modules from silicon wafers

Info

Publication number: EP2022086A1
Application number: EP07748136A
Authority: EP
Inventors: Göran Fajersson
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-05-24
Filing date: 2007-05-16
Publication date: 2009-02-11
Also published as: SE0601150L; EP2022086A4; WO2007136318A1; SE529315C2

Abstract

Method for the production of photovoltaic modules from mono- or multi-crystalline silicon wafers and from wafers produced by ribbon silicon methods, whereby prior to said production each silicon wafer is affixed to a carrier by the use of vacuum, an adhesive or a frame. The wafers remain affixed to their respective carriers during the necessary processing steps, where-after the wafers are loosened from their carriers and placed onto glass sheets upon finalization of the last production step.

Description

Method for producing photovoltaic cells and modules from silicon wafers

Field of the invention

The present application relates to a method for producing photovoltaic cells and modules from mono- and multi-crystalline silicon wafers and wafers produced by ribbon silicon methods. In the established methods for such production a wafer with a thickness of from 0.15 to 0.3 mm is processed. The wafer is transported through a number of processing steps, which comprise one or more of cleaning, surface etching, phosforisation at a temperature of 700-1000 ⁰C, edge isolation, etching to get rid of phosphorous silicates, nitration and screen printing. The wafer is handled in different ways depending on the degree of automisation. As the wafer hitherto is handled as a plate in stands and on transport bands it must absorb all stress without being damaged and still remain form stable. In the present invention the wafer is not handled as a stand-alone plate, but affixed to a carrier.

Background art

EP 106305 OA discloses a holder for thin semiconductor wafers with uneven sur- face. The wafers are kept in place by use of vacuum.

US2003196682 discloses a holder for semiconductor wafers where the wafers are kept in place by use of adhesive.

Legend of figures

Figure 1 shows schematically a carrier 2 for a single wafer 1. Figure 2 shows schematically a carrier 2 with one wafer 1 on each side.

Figure 3 shows schematically a number of carriers 2 fitted next to each other on a common rail 3 thereby creating a carrier rack 4.

Figure 4 shows schematically a carrier rack 4 made in one piece.

Figure 5 shows schematically a carrier rack 4 in the form of a tray 11. Figure 6 shows schematically a carrier rack 4 being placed horizontally, vertically and in an inclined position respectively.

Figure 7 shows schematically a carrier rack 4 to which the wafers 1 are affixed using vacuum.

Figure 8 shows schematically a carrier rack 4 to which the wafers 1 are affixed using adhesive.

Figure 9 shows schematically an alternative way to affix the wafers 1 on a carrier 2 using adhesive by placing the latter in strings 8 on the carrier.

Figure 10 shows schematically how the carrier rack 4 is drawn aside after rows of wafers 1 are fixed to a glass sheet. Figure 11 shows schematically a carrier rack 4 with two rulers serving as a frame for the wafers 1.

Figure 12 shows schematically a carrier rack 4 together with a U-shaped frame 11. Figure 13 shows schematically a frame that keeps the wafers 1 hi place and shades 3-5 mm of the wafers 1.

Figure 14 shows schematically a frame that keeps the wafers 1 in place and shades very little.

Figure 15 shows schematically how a number of carrier racks 4 are put in a rack module. Figure 16 shows schematically how a number of wafers 1 are electrically connected in series.

Description of the invention

Solar grade silicon is sawn to wafers 1 with desired thickness and adequate tolerances. Wafers 1 may also be produced by the so called ribbon silicon methods. The wafers 1 can be made of mono- as well as of multi-crystalline silicon. A silicon wafer 1 with a thickness of from 0.03 to 0.25 mm, preferably from 0.05 to 0.15 mm, is affixed to a carrier 2. The carrier 2 is made of a material with good stability even at the temperature necessary for the subsequent phosforisation process, meaning 700-1000 ⁰C. One example of a suitable such material is a stainless steel that can withstand all the chemicals, which are used in the processes. Other examples are ceramic materials or composites of ceramics, other metals and other inorganic materials. The carrier 2 may also be made of one or more of said materials in combination. In order not to contaminate the wafer 1 during the phosforisation process a layer of very clean silicon dioxide or ultimately a sheet of solar grade silicon can be provided on the carrier surface. In Figures 1- 4 are shown different ways to affix a wafer 1 to a carrier 2. A carrier

2 for a single wafer 1 may according to Figures 1 and 2 have a wafer 1 placed on one side or wafers 1 placed on both sides. In Figure 3 is shown how a number of carriers 2 can be affixed next to each other on a common rail thereby creating a carrier rack. A first carrier rack 4 for a number of wafers 1 can be made hi one piece as shown in Figure 4. A first carrier rack 4 according to Figures 3 and 4 may in the same way as in Figures 1 and 2 have a wafer 1 on one side or on both sides. A carrier rack can also have the form of a tray. This type of second carrier rack 11 is handled as a separate unit in all the processes - see Figure 5. The below description of the present invention pertains to a first carrier rack 4 according to Figure 4 and to a second carrier rack 11 according to Figure 5. The present description may though be understood mutatis mutandis to cover also the other embodiments described above. The wafers 1 are mounted on the carrier rack 4, 11 so that they are stably affixed during all the processes. There are different methods to affix the wafers 1 on the carrier rack, such as using vacuum, adhesive, welding, framing and electrostatic force. For the present invention the use of vacuum, adhesive and framing is of particular interest. Electrostatic force can be used as a complement in some of the process steps. The wafer 1 can be placed on the carrier rack 4, 11 by hand. At this moment the carrier rack 4, 11 as shown in Figure 6 can be vertical, horizontal or inclined. It is important that the wafer 1 is placed correctly on the carrier rack 4, 11. A ruler 3 may be built into the carrier rack 4, 11. It is advantageous if this ruler 3 can be pushed into the carrier 2 when a pressure is applied to it. To achieve this feature the ruler may e g be mounted on springs, which force the ruler outwardly when not under pressure and which allow the ruler 3 to be pushed into the carrier when under pressure. It is possible to fully or partly automize the placing of the wafers 1 on the carrier rack 4, 11.

If vacuum is used to affix the wafers 1 to the carrier rack 4, 11, the vacuum is applied through a number of holes or slits 5 in the carrier rack 4, 11 as shown in Figure 7. In one embodiment with five wafers 1 the carrier rack 4, 11 is placed in 45 degrees angle versus the horizontal plane and all five wafers 1 are standing on the ruler 3. The distance between the wafers 1 is adjusted exactly to the distance needed in the subsequent processing steps. Now the vacuum is applied so that the wafers 1 are firmly affixed on the carrier rack 4, 11. If a carrier rack 4, 11 is used for having wafers 1 on its both sides there is a wall in the interior of the carrier rack 4, 11 , which makes it possible to apply vacuum on both sides separately. In order to have wafers 1 on both sides the carrier rack 4, 11 is turned once wafers 1 have been placed on the first side whereupon the same procedure carried out on the other side. The carrier rack 4, 11 is now ready for further processing.

As an alternative to using vacuum the wafers 1 may be affixed on the carrier rack 4, 11 with adhesive 6. The adhesive 6 used is preferably a metal or an inorganic material. The adhesive 6 should not contain metals that harm the wafers 1 performance after the phosforisation process. Affixing the wafer 1 onto the carrier rack 4, 11 can be done in different ways, e g as follows: - The adhesive 6 is fixed by using pressure. This pressure must be less than fhe pressure that may damage the wafer 1.

- The adhesive 6 is fixed by the carrier rack 4, 11 being heated in a rather small area. - The adhesive 6 is fixed with small quantities of organic glue. The glue is destroyed and evaporated during the phosforisation process, meaning that the glue is only a temporary adhesive. The permanent adhesion is established between the carrier 2, the permanent adhesive 6 and the wafer 1.

The adhesive 6 can be applied in different ways. In Figure 8 is shown how the adhesive 6 is applied in a small area. The adhesive 6 can be put on the surface of the carrier rack 4, 11. It may also be put in circular cavities in the carrier rack 4, 11. The number of cavities may vary depending on the size of the wafer. By smaller wafers five cavities is a sufficient number. If the cavities in the carrier rack 4, 11 are slightly conical 7, as shown in Figure 8, the affixing of the wafer 1 to the carrier rack 4, 11 can be stronger. One possibility to secure that the wafer 1 can be loosened from the carrier rack 4, 11 in a later process is that in the middle of each cavity there is a small hole, in which an overpressure may be generated. The conical cavities 7 must be just so large that the adhesive easily gets loose without the wafer 1 being damaged. When the wafer 1 is subsequently affixed to a glass sheet in a later step the adhesive may be ground away. If it is not detrimental to the further steps it may stay as it is. In Figure 9 is shown an alternative way to utilise the adhesive 6. Here the adhesive is applied in the form of at least two strings 8, each with rectangular or circular cross section. In the carrier rack 4, 11 there is also in this case a cavity that fits to the strings 8 so that it to a large extent is fitted into the surface of the carrier rack 4, 11. Also in this alternative an overpressure can be utilised to loosen the wafer 1. The adhesive 6 in this alternative can also be utilised as a conductor for electricity. The adhesive strings 8 may also comprise electrically conducting means 28, such as metal wire - see Figure 10. In this case the strings 8 are first placed into rails 2 cut out or holders 18 in the carrier rack 4, 11. The holders 18 are fixed at intervals cut out of the carrier rack 4, 11. The wafers 1 are then mounted on the strings with adhesive 8. After all the processes are done and the wafers 1 fixed to the glass sheet 12 the carrier rack 4, 11 is pulled aside so it gets free from the strings 8. The carrier rack 4, 11 goes back and gets new strings 8 and wafers 1 affixed to it.

When the wafers 1 are not too thin over about 0,1 mm they can be carried by gravity plus a two-sided frame. Two rulers 3 of the same type as in Figure 6 and put so that the wafer 1 fits exactly between them can be a sufficient support for the wafers 1 - see Figure 11. In Figure 12 is shown another method. There are two pieces made of for example ceramics. One piece forms a tray 11 and another U-shaped piece supports the tray 11 creating a frame 13. After that the tray 11 and the U-shaped frame 13 have been mounted together, the wafers 1 are put in a well-defined position. The tray 11 with the U- shaped frame 13 is then handled through all the process steps described below. When it is ready to be put on the glass sheet 12 first the U-shaped frame 13 is taken away, then the tray 11 is mounted from beneath on to the glass sheet 12, which has resin on its underneath side. In Figure 13 a method is described where the wafers 1 are put on carrier rack 4, 11 in well-defined positions. A frame 14 of a suitable material is placed above the wafers 1 so that it touches all the sides of the wafers 1. Where the wafer 1 has an adjacent wafer 1 the frame 14 touches both of these. The frame 14 in this way shades 2-4 mm of all the sides of the wafer 1. The frame 14 is then pressed on to the carrier rack 4, 11 , so that they together create a package and the wafers 1 are firmly fit between the carrier rack 4, 11 and the frame 14. In the wet processes the package is either dipped into the liquid or alternatively the liquid is put into the frame 14 for sufficient time to give desired result. The frame 14 in Figure 13 can also have a very sharp triangular or needle-shaped cross-section 15 - see Figure 14. In this way the frame 14 can keep the wafers 1 in place in different ways without shading any significant area of the wafers 1. In Figure 15 is shown how a number of carrier racks 4, 11 can form a rack module 16. A bar 17 can be used to press the frames against the wafers 1. This gives a possibility to keep an exact force on the frames and in this way make a good package for all processes where the frames can be used. During edge isolation and screen printing the frames need to be taken away and another method used to affix the wafers 1.

The carrier racks 4, 11 can also be fitted to a conveyer belt 29 or conveyer chain 30. The conveyer belt 29 or conveyer chain 30 is provided with a device fixing each carrier rack 4, 11 to the conveyer belt 29 or conveyer chain 30 during all the processes. The carrier racks 4, 11 may be placed on the conveyer belt 29 or conveyer chain 30 in a number of parallel lines. In the last step, when each wafer 1 is put on the glass sheet 12, the carrier racks 4, 11 can with some force be freed from the conveyer belt 29 or conveyer chain 30. The conveyer belt 29 or conveyer chain 30 can have support devices for frames 14, 15 if not vacuum or adhesive is used to fix the wafer 1 to the carrier rack 4, 11. The carrier rack 4, 11 with the wafers 1 affixed to it according to the vacuum method, the adhesive method or the frame method is now ready to be processed in different steps known in the art and further steps to be envisaged for treating the cells to obtain increased lifetime and higher efficiencies. The carrier rack 4, 11 is handled with suitable transport facilities and if convenient with robots. One of the last steps hi the process is often screen printing, during which a silver and/or aluniinium paste is printed on the wafer 1.

To connect the wafers 1 electrically in parallel small pieces of a conductor of an established type can be applied hi the established way. The processes with the wafers 1 on the carriers 2 are now finalized and the wafers 1 have been electrically connected in parallel.

The invention is especially useful for producing a number of wafers 1 electrically connected hi parallel. If there is a wish to instead produce the wafers 1 electrically connected in series e g the method schematically shown in Figure 16 may be used. Small pieces of electrically conducting wires are applied on the wafers 1 before the latter are placed on the carrier rack 4, 11. Part of these connecting wires is made to extend more or less straight out from the wafer 1 surface during all the processes. This means that the wires must withstand the phosforisation process temperature. The screen printing must be modified so that it may handle these wires.

For the invention it is very attractive to have long wafers. That is a wafer that has a normal wafers width and a length corresponding to 3, 4 or more wafers. The length of the wafer will create the width of the module. Such a long wafer can e g be produced by sawing an ingot along its long side or by the ribbon silicon methods. The long wafer is processed in all the needed processes up to screen printing. Screen printing is modified to give good conductivity over the long wafers whole length. A supplementary conductor can be fitted over the whole length.

In all the methods above now electrical conductors are fitted to get the electricity out from the first and or the last wafers 1 in the row.

Before the wafers 1 are affixed to the glass sheet 12 it is preferable to conduct a test in order to establish whether there is a good electrical contact between the wafers 1. This test should indicate between which wafers 1 there is a defect, which may be subsequently repaired.

The wafers 1 are now placed on a glass sheet 12, which will become the front side of the final product, being a solar module. Said sheet 12 may alternatively be made of another durable and transparent material, e g carbonate plastic. A suitable resin for ex- ample melamine resin, which has little influence on the efficiency of the solar module, is applied in a thin layer on the glass sheet 12 or on the front side of the wafers 1. The carrier rack 4, 11 is placed so that the row of finalized wafers 1 is located in a well defined position above or under the horizontally placed glass sheet 12. The carrier rack 4, 11 is put with a precise power against the glass sheet 12 where the resin affixes the carrier rack 4, 11 to the sheet 12. Heat can be used to reduce the time for the resin to harden. If vacuum is used it is now released. Alternatively may be used an in the art known laminate form resin for fixing the finalized wafers 1 to the glass sheet 12. When an adhesive is used an overpressure is utilised to loosen the wafers 1 from the carrier rack 4, 11 or the method shown in Figure 10.

Alternatively the wafers 1 may be taken away from the carrier racks 4, 11 and be stored separately once finalized. Then the wafers 1 may be affixed to the sheets 12 in a later and separate process.

The carrier rack 4, also when formed as a tray 11 is cleaned and is reutilized so that new wafers 1 may be placed thereon. Each row of wafers 1 on a carrier rack 4, 11 normally becomes a finalized row in the solar module. On the glass sheet 12 new carrier racks 4, 11 bring new rows located beside the former ones so that the solar module will ultimately get the desired amount of rows of wafers 1. Now the glass sheet 12 with the rows of wafers 1 is further processed on its back side. Examples of such processes are: - Plasma etching or etching with KOH to isolate the emitter layer from the p- layer. This can alternatively be done when the wafers 1 are affixed on the carrier rack 4, 11 that is on the front side.

- The wafer's back side is treated to achieve the desired electrical conductivity. This can be done with different methods. Examples of useful methods are the Al-BSF Aluminum Back Surface Field method and the LFC Laser Fired Contact method. Some of the methods utilise short heating, which should be done so that the glass sheet 12 is not hurt. One possibility is to heat the glass sheet 12 to a higher temperature not to generate too much stress in it.

- If the rows are connected in parallel on the front side then the back side is now also connected in parallel, with established types of conductors.

- The final step is to connect the rows electrically in parallel or hi series. This connection is preferably done on one of the sides of the glass sheets 12, where a free space is created. This can be done manually or automatically. The solar module is now active and electrically connected, so it can be tested to secure its function. After testing the solar module is laminated with established polymer sheets using conventional methods. After the mounting of electrical contacts and final testing the solar module is ready for shipment. The present invention has the following main advantages in comparison with methods used hitherto:

- The silicon wafers 1, which have a thickness of from 0.03 mm to 0.25 mm and which due to their small thickness may not be handled using conventional methods, are conveniently mounted on a carrier rack 4, 11 according to the captioned vacuum, adhesive or frame method.

- The silicon wafers 1 are processed to activate the front side into solar cells when they are affixed to the carrier rack 4, 11.

- Electrical contact is created between silicon wafers 1 in a row being affixed to the carrier rack 4, 11, whereby the wafers 1 are not subject to risk of breakage. - The row of silicon wafers 1 that are electrically connected on the front side is conveniently attached to a glass sheet 12, which will become the front side of the final solar module.

- The glass sheet 12 with several rows of silicon wafers is processed so the back side of each silicon wafer 1 is isolated from its front side and gets a conductive layer on its back side.

- The rows' back sides are electrically connected when they are attached on the glass sheet.

- All rows are electrically connected on one side or both sides of the glass sheet 12 when they are attached to the glass sheet 12. - In using the carrier rack 4, 11 the very thin wafers can be processed without breakage.

- All the process steps are possible to automize.

Claims

1. Method for the production of photovoltaic modules from mono- or multi-crystalline silicon wafers 1 comprising one or more processing steps, characterizedin that each silicon wafer (1) is affixed to a carrier (2) by the use of vacuum, an adhesive or a frame, that the silicon wafers (1) remain affixed to their respective carriers (2) during all production steps, that each carrier (2) carries a silicon wafer (1) on one of its sides only or carries a silicon wafer (1) on each of its sides, that a number of carriers (2) are fitted on a common rail (3) thereby creating a carrier rack (4), that the silicon wafers (1) on a carrier rack (4) are electrically connected in parallel or in series.

2. Method according to claim l,characterizedin that the silicon wafers

(1) are loosened from their carriers (2) and are placed onto transparent and durable sheets (12) , preferable glass sheets, upon finalization of the last production step, these sheets (12) becoming the front side of a solar module.

3. Method according to claim Ior2, characterizedin that the carrier rack (4) is formed as a tray (11), which may cooperate with a frame (13) for keeping the silicon wafer (1).

4. Method according to any preceding claim, characterizedin that the carrier racks (4, 11) are fitted to a conveyer belt (29) or to a conveyer chain (30), whereby the carrier racks (4, 11) are placed in one or more lines.

5. Method according to any preceding claim, characterizedin that the carriers (2) are made of a material withstanding up to 1000⁰C and having a good chemical stability.

6. Method according to claim 5, characterizedin that the material is selected from one or more of stainless steel, other metals, ceramic material and composite material.

7. Method according to any preceding claim, characterizedin that the silicon wafers (1) have a thickness of from 0.03 mm to 0.25 mm, preferably from 0.05 mm to 0.15 mm.

8. Method according to any preceding claim, characterizedin that the silicon wafers (1) are affixed to the carrier (2) by the use of an adhesive being selected from adhesives withstanding up to 1000⁰C, such as ceramic adhesives.

9. Method according to claim 8, characterizedin that the adhesive is placed in small cavities on the carrier (2), said cavities being essentially placed in a circle.

10. Method according to claim 8,characterizedin that the adhesive is placed in strings (8) on the carrier, the strings (8) preferably having each a rectangular cross-section area.

11. Method according to claim 10, characterizedin that the strings (8) comprise electrically conducting means (28).

12. Method according to any preceding claim, characterizedin that the carriers (2) are fitted with a hole under each silicon wafer (1) to be used for inlet of gas under pressure for loosening the silicon wafers (1) from the carriers (2) once the final production step has been carried out.

13. Method according to any preceding claim, characterizedin that said production comprises one or more of the steps of cleaning, surface etching, phosforisation, edge isolation, etching to get rid of phosphorous silicates, nitration and screen printing.

14. Method for the production of photovoltaic modules from mono- or multi- crystalline silicon wafers (1) comprising one or more processing steps, characte- r i z e d in that each silicon wafer (1) is affixed to a carrier (2) by the use of vacuum, an adhesive or a frame, that the silicon wafers (1) remain affixed to their respective carriers (2) during all production steps, that said production comprises one or more of the steps of cleaning, surface etching, phosforisation, edge isolation, etching to get rid of phosphorous silicates, nitration and screen printing.

15. Method according to claim 14,characterizedin that that the silicon wafers (1) are loosened from their carriers (2) and are placed onto transparent and durable sheets (12), preferable glass sheets, upon finalization of the last production step, these sheets (12) becoming the front side of a solar module,

16. Method according to claim 14 or 15, characterizedin that each carrier (2) carries a silicon wafer (1) on one of its sides only or carries a silicon wafer (1) on each of its sides.

17. Method according to claim 14, 15 or 16, c h a r a c t e r i z e d in that a number of carriers (2) are fitted on a common rail (3) thereby creating a carrier rack (4).

18. Method according to any one of claims 14-17, characterizedin that the carrier rack (4) is formed as a tray (11), which may cooperate with a frame (13) for keeping the silicon wafers (1).

19. Method according to any one of claims 14-18, characterizedin that the carriers (2) are fitted to a conveyer belt (29) or to a conveyer chain (30), whereby the carrier racks (4, 11) are placed in one or more lines.

20. Method according to any one of claims 14-19, characterizedin that the carriers (2) are made of a material withstanding up to 1000⁰C and having a good chemical stability.

21. Method according to claim 20, characterizedin that the material is selected from one or more of stainless steel, other metals, ceramic material and composite material.

22. Method according to any one of claims 14-21,characterizedin that the silicon wafers (1) have a thickness of from 0.03 mm to 0.25 mm, preferably from 0.05 mm to 0.15 mm.

23. Method according to any one of claims 14-22, characterizedin that the silicon wafers (1) are affixed to the carrier (2) by the use of an adhesive being selected from adhesives withstanding up to 1000⁰C, such as ceramic adhesives.

24. Method according to claim 23, characterizedin that the adhesive is placed in small cavities on the carrier (2), said cavities being essentially placed in a circle.

25. Method according to claim 23,characterizedin that the adhesive is placed in strings (8) on the carrier, the strings (8) preferably having each a rectangular cross-section area.

26. Method according to claim 25, characterizedin that the strings (8) comprise electrically conducting means (28).

27. Method according to any one of claims 14-26, characterizedin that the carriers (2) are fitted with a hole under each silicon wafer (1) to be used for inlet of gas under pressure for loosening the silicon wafers (1) from the carriers (2) once the final production step has been carried out.

28. Method according to any one of claims 14-27, characterizedin that the silicon wafers (1) on a carrier rack (4) are electrically connected in parallel or in series, that the silicon wafers (1) are loosened from their carriers (2) and are placed onto transparent and durable sheets (12) , preferable glass sheets, upon finalization of the last production step, these sheets (12) becoming the front side of a solar module.