Method and a Tape for Interconnecting Single Solar Cells into Solar Cell Modules

Abstract

The invention relates to a method of interconnecting photovoltaic cells into a module by using a tape carrying tabbing wires (wires). The tape with wires, according to the invention, makes it possible to build modules having a reduced ohmic loss and thus an increased power output from the module. The reduction of ohmic losses is obtained by having wires on the backside of the cells which have a larger cross section area for the electrical current generated in the cells than the cross section area of the wires used on the topside of the cells.

Description

The present invention relates to interconnecting individual solar cells into solar cell modules by use of a specially designed tape in order to reduce Ohmic losses.Background to the InventionThe invention originates from problems with ohmic losses when interconnecting individual solar cells into solar cells modules. The solar cells are in literature also called Photovoltaic cells (PV cells) and the solar cell modules are called PV modules. In the following we will use the denominations “PV cells” and “PV modules” or just “cells” and “modules”. Modules are produced at an increasing rate and the market has yearly growth of about 20 percent. Still, the competition is fierce among the manufactures of modules. Manufactures of modules are striving to reduce the cost of modules both by reducing the cost of manufacture and increasing the efficiency of modules in the conversion of solar radiation into electrical power. This increase in efficiency is obtained at cell level and module level. At cell level the increase is obtained by increasing the efficiency of individual cell. At module level the “increase” in efficiency is obtained by reducing the loss caused by the interconnection technic used when interconnecting individual cells into modules. When interconnecting cells into modules the interconnection gives a reduction of the nominal power output with af about 15 percent. The cause behind this reduction will be explained later. Interconnection can also be called “bonding”. The wires used when interconnecting cells is be called “bonding wires” or “tabbing wires” or just “wires”. By modifying the wires used on the topside and backside of the cells the ohmic losses can be reduced. To be able to do this a tape carrying wires for or bonding can be used. The tape with wires and how it is used will be described in the ensuing text. In the next sections the traditional technic for bonding and the bonding technic with a tape are described. The use of tape for bonding is described in patent DE19652810A1.The present invention concerns of using a tape carrying wires in order to reduce the ohmic losses in bonding of cells into modules by having different the wires on topside and backside of cells when building a module.More details are given in the following. The layout of the description is:- Description of a PV cell- Description of a PV module- Traditional method of building modules- Ohmic losses in a module- Description of tape a and method for bonding of PV cells into modules- Tape used according to the invention in order to reduce ohmic losses.Description of a PV cellThe PV cell comprises a number of thin layers on a supporting plate. The layers have different functions which together results in a functional PV cell. There are a number of different types of PV cells, but the two main types are: thin film PV cells and silicon PV cells. In the following we will describe a silicon cell. The incoming radiation from the sun creates a voltage across the active layer in the cell and an electric current can be taken from the cell via electrical connections on the top- and backside of the cell. The topside and backside have different polarities. The topside is the side which receives the incoming solar radiation. The electrical connection on the topside is called topcontact and the contact on the backside backcontact. The connection on the topside is made on a thin transparent layer which is electrically conductive. The transparent conduction layer is called TCO (Transparent Conduction Oxide). The TCO layer is thin, due to low transparency and can thus only handle small current densities. Therefore, a matrix (a grid) of highly conducting material is imposed on top of the TCO layer to collect the current. The grid consists of a pattern with fingers and busbars. The fingers are narrow (small width) metallic strips with a width of 0, 05 to 0,2 mm. The distance between individual fingers is 2 to 10 mm. Busbars are running across and mostly perpendicular to the fingers. The busbars contact the fingers, and it is the busbars that collect the current from the fingers. As described later the ability of the busbar to carry current is increased by the wires used to interconnect cells. Her we stop to clarify that the wires that interconnect cells are placed on and contacts the busbars. The number of busbars can vary, the trend is in an increasing number of busbars. The busbars have a width between 1, 5 to 2, 5 mm but other widths may be used. As noted above the width of the fingers are in the order 0, 05 to 0,2 mm. The grid with fingers and busbars will however reduce the efficiency of the PV cell because the nontransparent material in the grid shades the surface of the cell from the solar radiation. In the making of the cell, it is thus important to have as small as possible shading area from the grid. But the fingers and busbars and wires also cause an ohmic loss.The backside of an ordinary PV cell does not need to be transparent to light and the contact on the backside can be made on a metalized surface that partially or completely covers the backside of the PV cell. However, bifacial cells have been developed in which the backside of the cell receives some reflected light. This reflected light is of about 5 to 10 percent of the light hitting the frontside. Bifacial cell has a backside which have similar photo voltaic properties as the frontside. For later use it is noted that the backside of a bifacial cell needs to be transparent and cannot be metalized.Description of a PV moduleA number of individual PV cells are electrical connected to each other to form a PV module. The cells are interconnected in series in which the positive electrode (usually the backside of the cell) is connected to the negative electrode of the cell (usually the topside of the cell) of a neighboring cell. The cells are placed in a string and the cells in the string are as said above interconnected in series. The interconnection is made with earlier described wires. The number of cells in the string may vary but a typical number is 10. When half cells are used the number is 20. The denomination “half-cell” means that one cell is divided into two separate cells. The cell voltage of an individual cell is of about 0, 5 volts. Ten cells interconnected in series thus have a voltage of 5 volts. Six of the strings described above are placed close and parallel to each other to form the module. The strings are then interconnected in series and 6 strings with 10 cells in each string give a module having a voltage of 30 volts and a surface area of 1, 6 m2 The description above describes a typical PV module but other numbers of PV cells in the module than that described above can be used. The string of cells are strengthened and mechanically supported by different sheets in the process of production which will be described in the next section.Traditional method of building modulesIn the above a description of the layout of a module is described. In this section it is explained how the module is built by the traditional method. The use of tape will be described later. The above described strings of cells are interconnected in a production machine called stringer. In the stringer electrical conduction wires are drawn from the topside of one cell to the backside of a neighboring cell. In the following text we will use “wire” when referring to the “interconnecting electrical conduction wire”In the stringer, cells are progressively laid out and separate pieces of wires are drawn from the topside of one cell to the backside of a neighboring cell. The wire has a length which is about the length of two cells. The wires is during layout soldered to the topside busbar and the backside of the cells. This soldering is necessary to keep the cells in a string that can be handled in the further building of the module. A number of cells are bonded together into a string comprising a number of cells. The usual number is ten, but other numbers can be used. A number of these strings, usually 6, are placed side by side to form a module.The 6 strings of interconnected cells are now moved to and placed topside down on a sealing sheet which in turn rests on a topglass. As before the topside is the side that receives the incoming light. The sealing sheet is of a polymer material. The most common material in the sealing sheet is a material called EVA (Ethylene Vinyl Acetate) but other materials can be used. The strings are connected in series in a soldering process and outgoing contacts for the complete module are soldered in place. The module with the cells is covered with a second sealing sheet. The second sealing sheet is usually of the same type as the above mentioned first sealing sheet. The second sealing sheet is thereafter covered with a backsheet of a suitable material, usually a polymer. This backsheet faces the outer environment and is the backside is of the module. Instead of a polymer backsheet, the backside can be covered with a sheet of glass. Here it is noted, if a bifacial cell is used a transparent cover on the backsheet must be used. The complete module with topglass, first sealing sheet, PV cells with interconnecting wires, second sealing sheet and backsheet or backglass is heated (baked) in vacuum at a temperature of 150 °C. In the baking the sealing sheets softens, melts and fix the module into a robust and sealed unit that can withstand the outer environment. The baking process is also called “lamination”.A more detailed description of the lamination is given in the following.The complete module with topglass, first sealing sheet, PV cells with interconnecting wires, second sealing sheet and backsheet is placed on a heated (150 °C) plate in a laminator chamber. The chamber is closed and evacuated to vacuum. The vacuum chamber has a flexible membrane in the lid at the inside of the vacuum chamber. At start of the lamination process the membrane is sucked up against the lid by the vacuum. When the chamber is evacuated the space between the lid and the membrane is opened to the ambient and the membrane will, due to the vacuum in the chamber, push the module against the heated lower part of the chamber. This “push" will be exerted on the module for about 10 minutes. The push and the heat will soften and melt the sealing sheets and fix the module to a robust unit. The vacuum in the chamber is vented, the chamber is opened and the module is removed from the chamber.Ohmic losses in a moduleWhen interconnecting cells into modules losses from the interconnection is incurred. The losses are caused by the following- ohmic losses in fingers, wires and TCO layer, shadow losses from fingers and wires, contact losses, light transmission losses in TCO layer. In the present invention we are concerned with the ohmic losses in the wires. On the topside the choice of wire dimensions is a compromise between ohmic loss in the wire and the wire shadowing active cell area. This compromise, optimization, results in a wire having a ohmic loss in the topside which reduces the power output of about 2 to 4 percent. Now to the backside. If the wire on the backside has the same dimension as the topside wire, the same ohmic loss as the front side loss is also incurred from the backside wire. In the traditional method of bonding a continuous wire with the same dimension along the wire has to be used on topside and backside.Description of a tape and method for bonding of PV cells into modules A tape that carries wires can be used to interconnect cells. The tape has an adhesive that fixes the tape and wire to cells. Also, the tape fixes the cells together into string that can be handled without the above needed soldering during further building of a module. The wires carried by the tape are interrupted at certain intervals in a pattern that enables an interconnection of the PV cells in series. The final interconnection of all of the PV cells in the module and closing of the electrical circuit is obtained in the laminating step. The procedure to use the tape is -The process starts with a first tape (tape 1) carrying wires is laid out on a carrier belt with the wires on top of tape. The tape on the carrier belt is preferably held in place on the belt by a vacuum system. The first tape with the wires which is placed on the carrier belt has an adhesive on the side facing upward. That is, a side not facing the carrier belt. The laying out of the first tape with wires is now followed by the laying out of cells on the belt and on the first tape with wires. The desired number of cells (usually 10, 20 if half cells are used) are now placed on the belt with the topside down and facing the first tape with wires. The cells in the now laid out string are separated with a small distance between the cells. The wires carried by the tape have, as described above, been cut in such a manner that a portion of the wire extends in the said gap between two cells. The building of the module continues and a second set of tape with wires is placed on the backside of the cells that just have been placed in a string. The tape on the backside has an adhesive on the side facing the backside of the cells. The tape with the adhesive further facilitates the fixing of cells into a sturdy string that can be handled. The wires in the second set are now resting on and contact the backcontact on the backsides of the cells. The second tape with wires that is placed on the backside of the PV cells have a part of the wire that extends across of the free space between two PV cells. We now have wires on the topside and on the backside of the PV cells and a part of the wire on the two sides of a cell overlaps in the free space between two cells. It is these overlapping wires that contact each other in the process of lamination thus forming the contact in series between the top- and backside of two adjacent cells. The process above is now repeated until the wanted number of strings in the module is obtained. When the desired number of strings have being built the process continuous with the building of the module as described when building modules with the traditional method The strings are placed on sealing sheet which is placed on a topglass. This is followed by a second sealing sheet and finally a cover sheet. The cover sheet can be a glass sheet.The module is now ready for lamination. The lamination is done at a temperature of about 150 °C, as described earlier. At the temperature of 150 °C the sealing sheets soften and melt. The earlier described push from the outside and the vacuum on the inside will bring the free wires in the space between the PV cells in contact with each outer. During the pushing and heating the wires also contact the fingers on the topside and the contacts on the backside. The wires are furnished with a solder that melts and solders at the lamination temperature and thus the wires are soldered to each other, to the fingers and to the backcontactTape used according to the invention in order to reduce ohmic losses,In the above it is described how cells are interconnected in series in a string in the building of PV modules by tapes laid out on the topside and backside of the cells. The tapes carry wires that electrically connect the cells in series. In the invention it is realized that the tapes on topside of backside can be of different identities and the tape carrying wires on the topside and backside can differ from each other. According to the invention can the wire carried by the backside tape have a larger cross section than the wire carried by the topside tape in order to reduce the ohmic loss and thus increasing the power output from the module. The mechanism behind losses when interconnecting cells into modules have been explained earlier. Now, the background to the inventionOn the topside the choice of wire dimensions is, as explained before, a compromise between ohmic loss in the wire and the wire shadowing active cell area. This compromise, optimization, results in a wire having a ohmic loss in the topside which reduces the power output of about 2 to 4 %. Now to the backside. The same ohmic loss is also incurred from the backside wire if the wire on the backside has the same dimension as the topside wire. In the traditional method of bonding, the same wire has to be used on topside and backside.However, according to the invention, wires carried by the tape on the backside can have a larger cross section are for the electrical current than the wire used on the topside. As an example, by doubling the area for the electrical current on the backside wire , the ohmic loss caused by wires on the backside is reduced by 50 percent. In the example above, this gives an increase in power output from the module with 1 to 2 percent. Preferably the wire on the backside can have a larger width. For a bifacial cell this has some consequences as this results in increased shadowing. But this is of minor importance as the light hitting the backside is much less than the light hitting the topside.Summary of the InventionThe objective of the invention is to use a tape carrying wires to interconnect cells in series and by this obtain a reduction of the ohmic losses. The reduction is achieved by the use of different wire cross section areas of the tapes carried by the tapes on the topside and backside when interconnecting cells in series. According to the presented invention the tapes set forth by description is characterized of features of claim 1 in which the tape used on the on backside carries wires for the conduction of electrical current and said wires have a lower ohmic loss for the electrical current generated in the cells than the ohmic loss caused by the wires used on the topside of the cells.The invention also relates to a method according to claim 2 in which the interconnected cells are bifacial cells.The invention also relates to a method according to claim 3 in which the wires used on the topside and backside on cells are of different material.The invention also relates to a method according to claim 4 in which the wires on the topside and backside have circular cross sections.The invention also relates to a method according to claim 5 in which the wires on the topside and backside have a rectangular cross section.The invention also relates to a method according to claim 6 in which the wires on the topside have a circular cross section and the wires on backside have a rectangular cross section.The invention also relates to a method according to claim 7 in which the material in the tape that carries the wires have different optical properties on the topside and backside.The invention also relates to a method according to claim 8 in which the tapes that carries the wires have different width on topside and backside.The invention also relates to a method according to claim 9 in which the wires used on the backside of a cell in a module has a larger cross section area for the conduction of the electrical current than the cross section area of the wires used on the topside of the cell.Brief Description of the DrawingsFigure 1 shows a Photovoltaic Cell (PV cell) with an active layer, a topcontact, and a backcontact.Figure 2 shows the contact pattern (the grid) used to transport current from the topside of a PV cell.Figure 3 shows the interconnection in series between the topcontact and backcontact of PV cells.Figure 4 shows a string with wires drawn between the PV cells.Figure 5 shows a cross section in a module with laid out string of cells with also laid out tape with wires on topside and backside of the cells. The figure shows the module before lamination.Figure 6 shows a cross section in the string shown in figure 5. Figure 6 shows the tape with wires according to the invention.Figure 7 shows a string with the tape with wires in a module after lamination.Detailed Description of Preferred EmbodimentsFigure 1 shows a PV cell 1 having an active layer 2, a topcontact 3 and a backcontact 4. Topside 6 receives incoming solar radiation 5. The solar radiation is schematically shown with arrows. Figure 1 also defines a backside 7 of the PV cell. The incoming solar radiation 5 generates a voltage in the active layer. The voltage sets up a current which is led from the PV cell via the topcontact 3 and the backcontact 4.Figure 2 shows a PV cell 20 on which a grid has been applied on the PV cell. The figure shows the topside. The grid comprises fingers 21 and busbars 22. Figure also has a pointer 23 which indicates the part of the topside which is not covered (shaded) by the grid). Figure 2 shows a common pattern of a grid on a PV cell, but other types of patterns may occur.Figure 3 shows how PV cells 1 , 34 are interconnected in series via wires 31 which extends from one side 33 having positive to side 32 having negative polarity.Figure 4 shows a module built with the traditional method in which a continuous wire 43 is drawn from a topside 6 of one PV cell to the backside 7 of a neighboring PV cell. PV cells 1 are assembled into a module 40. Figure 4 only shows a side view of a section of a string with a number of the PV cells 1. The module has a topglass 41, a first transparent sealing sheet 42. The sealing sheet 42 is usually made a polymer material called EVA (Ethylene Vinyl Acetate). Solar radiation passes through the topglass 41 and the sealing sheet 42. On the cells there is a second sealing sheet 46. The assembly of the module is finished with a backsheet 47. In figure 4 the module is shown after lamination. The sealing sheet 42, 46 has melted and merged in a space 48 between the cells. Figure 4 only shows a part of a string in a complete module. A complete module usually has 6 strings with ten PV cells in each string. For clarity it shall be pointed out that figure 4 shows the module with distorted scales. The actual PV cell 1 has in reality a thickness of about 0, 2 mm and the length of the cell is of about 200 mm. The distance between two adjacent cells is of about 2 mm.Figure 5 shows a cross section of a module with a tape according to the invention with wires 52a and 52b carried by tapes 51a and 51b. The module is shown before lamination. The figure shows a topglass 41 facing down. Again, it is the topside which receives the incoming solar radiation. Sealing sheet 42 is placed on the topglass 41. On top of the sealing sheet 42 there is tape 51a with the wires 52a facing the cells. Cells 1 are placed on the tape 51a carrying wires 52a. The second tape 51b with the wires 52b is placed on the PV cells 1 with the wire 52b facing the cell. Thereafter, a second sealing sheet 46 is placed on tape 51b and cells. The second sealing sheet 46 is covered with a backsheet 47. In figure 5 the topside 6 and the backside 7 of the cells are defined. Figure 5 shows the module before lamination. The module after lamination is shown in figure 7. We can already here note that it is the free part of the wires 52a and 52b that contacts each other in gap 57 during lamination. Until now nothing has been said about the geometrical form of the wires. Figure 5 only shows the wires in general. The wires can have different geometries such as round, flat (rectangular) or some other form. Also, the wires 52a, 52b can have geometries that differ between wire 52a and 52b an example, wire 52a can be circular and wire 52b can be flat or the other way around. Figure 6 depicts a circula wire 62a and a flat wire 62b.Here it is pointed out that for clarity, topglass, sealing sheet, tape and wire are drawn with a distance between each other. In reality they are close to each other.Figure 6 shows a cross section defined in figure 5. Figure 6 shows the topglass 41, the first sealing sheet 42, the tape 51a, wire 62a. Here the wires 52a in figure 5 are shown as a circular wire 62a in figure 6. Its recalled that wires 52a, 52b in figure 5 are shown as a wire in general and that the wires 52a, 52b can have different geometrical shapes. The tape 51a carrying the wire 62a faces the topside 6 of cell 1. Said tape 51a has an adhesive on the side that faces cell 1. Wire 62b contacts the backside 7 of cell. In figure 6 wire 62b is shown as a rectangular flat wire. As before the tape 51b has an adhesive on the side facing cell 1. The second sealing sheet 46 is seen on the cells. The module is finished with cover sheet 47. In figure 5 extra space was used between topglass, sealing sheet tape and wire in order to depict the different entities. This extra space is, however, not needed in figure 6.Figure 7 shows module 70 after lamination with PV cells 1 interconnected by wires 52a, 52b which are carried by the tapes 51a, 51b.The wire 52a contacts the topside 6 and the wire 52b contacts the backside 7. Figure 7 shows the module after lamination and the wires 52a, 52b are now brought in contact with each other in gap 57 and a contact in series between individual PV cells has been obtained. The wires have been furnished with a solder that solders the wires 51a and 51b to each other during lamination at position 58. Also, the wires 52a and 52b are soldered to the topside and backside during lamination. The sealing sheets 42, 46 have melted, and filled the gap 57 between the PV cells. For clarification consult figure 5 in which the sealing sheets, tape and extra tape are shown before lamination. In figure 7 the tapes 51a, 51b have merged with the melted sealing sheet and thus not seen in figure 7.

Claims

Method for electrically connecting individual photovoltaic solar cells, solar cells (1), which are used in the manufacture of a solar cell module, said solar cell module comprising a cover glass (41), sealing layer (42, 46), a number of solar cells (1) and a back layer (47), said solar cells having a top side (6) and a back side (7) having an active layer (2) in which an electric current is generated, a top contact (3) on the top side (6) and a back contact (4) on the back side (7), and electric current is conducted from the solar cells via the back contact and the top contact, said solar cells being arranged in strings with a gap (57) between individual solar cells in the string, said solar cells in the string being interconnected by means of wires (52a, 52b) of an electrically conductive material and said wires being carried by tapes (51a, 51b), and said wires (52a, 52b) contacts the top (6) and back (7) of the solar cells as well as each other in the gap (57) during lamination, characterized in that the wires (52b) on the back (7) of the cells have a larger cross-sectional area for the electric current generated in the cells than the cross-sectional area of the wires (52a) used on the top (6) of the cells.2Method according to claim 1, characterized in that the cells (1) used are bifacial cells.3Method according to any one of claims 1 or 2, characterized in that the material of the wires (52a, 52b) has different electrical properties for conducting electric current.4Method according to any one of claims 1, 2 or 3, characterized in that the wires (52a, 52b) have circular cross-sections.5Method according to any one of claims 1, 2 or 3, characterized in that the wires (52a, 52b) have a rectangular shape.Method according to any one of claims 1, 2 or 3, characterized in that the threads (52a) carried by the tape (51a) have circular cross-sections and that the threads (52b) carried by the tape (51b) have rectangular cross-sections.7Method according to any one of claims 1, 2, 3, 4, 5 or 6, characterized in that the tape (51a) has different optical properties than the tape (51b).8Method according to any one of claims 1, 2, 3, 4, 5, 6 or 7, characterized in that the tape (51a) has a different width than the tape (51b).