WO2012173487A1

WO2012173487A1 - Photovoltaic system and connector for a photovoltaic cell with interdigitated contacts

Info

Publication number: WO2012173487A1
Application number: PCT/NL2012/050429
Authority: WO
Inventors: Ilkay CESAR; Markus Johan Jansen
Original assignee: Stichting Onderzoek Centrum Nederland
Priority date: 2011-06-17
Filing date: 2012-06-18
Publication date: 2012-12-20
Also published as: DE112012002511T5; NL2006966C2; CN103843154A

Abstract

A photo-voltaic cell system has a semi-conductor substrate having a surface with interdigitated first and second contact areas thereon, providing contact to emitter and base contact areas in or on the semi-conductor. A connection string comprises a conductor line and electrically isolating material in rings around the conductor line. Parts of the conductor line between the rings are joined to a plurality of the first contact areas, in electrical contact with the first contact areas, with electrically isolating material from the rings between the conductor line and the second contact areas.

Description

Title: Photovoltaic system and connector for a photovoltaic cell with interdigitated contacts

The invention relates to a photovoltaic system having interdigitated back contact (IBC) design, to a connector suitable for use as an electrically conductive connector for electrical contact areas in a photovoltaic system, to a connector assembly and to a method for manufacturing a connector.

Photovoltaic cells are semi-conductor devices that are designed to convert photons into electrical energy. A well-known use is their use as solar panels, see US 4,234,352 for example. In general, the photovoltaic cell comprises a semiconductive material, which acts as a photo-active material wherein light energy (photons) is converted into excited mobile charge carriers. The semiconductive material generally has a p-n junction defined therein. In a planar cell the p-n junction is usually formed near a surface of the

semiconductive material. Conductors on p and n regions on the surface of the semiconductive material form contacts (also contact areas) for outputting current from the cell: the contacts form the electrical output poles of the cell.

IBC cells have interdigitated contacts of opposite polarity (emitter and base contact) on the back surface of the cell (the surface facing away from the light source during use) are generally known. Such cells are e.g. described in US 2008/0210301 and in WO 2009/092426. IBC cells have numerous advantages over conventional photovoltaic cells with front side metal grids and blanket or grid metalized backside contacts, including improved photo- generation due to elimination of front grid shading, much reduced grid series resistance, and improved "blue" photo-response since heavy front surface doping is not required to minimize front contact resistance and since there are no front contacts. In addition to the performance advantages, the back/contact cell structure allows simplified module assembly due to coplanar contacts.

A commonly used method to connect contact areas of a photovoltaic cell is to provide busbars (wider conductor regions) at the ends of the contact areas. Interconnection of a plurality of photovoltaic cells is accomplished by providing busbars extending along several cells. A first busbar of the cell connects a first plurality of contact areas (digits), forming a comb-like structure wherein the digits form the teeth of the comb, which structure serves as the base contact during use. A mirror-image shaped emitter base contact is formed by a second plurality of contact areas connected by a second busbar, provided on the same surface of the cell as base contact. The digits of the base respectively the emitter contact are provided on the surface in an alternating manner, whereby the individual base digits of a cell are generally adjacent to two emitter digits and vice versa. Hence such design is called an interdigitated back contact.

A drawback of such design is significant efficiency loss with increasing cell size, in particular if cells are provided with diameter of about 15 cm or more.

Further, generally a dielectric layer is provided on the photoactive material. Localised openings are provided in this dielectric layer (serving as an insulator). Metal is deposited and metal in the openings makes electrical contact between the contacts and the photoactive material. As a result a large part of the backside is covered with metal. This is disadvantageous in that it results in increased costs and imposes extra requirements to the insulating properties of the dielectric layer, because essentially the whole cell is exposed to an electrical potential over the dielectric layer.

WO 2006/123938 discloses a photo-voltaic cell with connections by means of external conductors that are joined to contact areas on the semiconductor body. A conductor with an isolating layer is disclosed for this purpose. The document discloses that interruptions or openings may be provided in the isolating layer to allow for contact with the semi-conductor. However, when the connection strings are fed to the photo-voltaic cell in a way that does not prevent torsion of the strings, for example from a spindle, it cannot be guaranteed that the openings will face the photo-voltaic cell. US 2010/0024881 discloses a photo-voltaic cell with various connections by means of external conductors that are joined to contact areas on the semi-conductor body. This includes foils on which the conductors are applied with isolating material over them at locations where there is a risk of short circuit to the "wrong" contact areas.

US2009/0025788 disclosed a photo-voltaic cell with external conductive wires that are joined to contact areas on the semi-conductor body. Isolating material is provided on the semi-conductor body to isolate the wires from the "wrong" contacts.

Summary

Among others, it is an object to provide for a photovoltaic cell and a method of manufacturing such a cell wherein external conductor lines are used wherein the risk of short circuit is reduced.

A method of manufacturing a photo-voltaic cell system according to claim 1 is provided.

By usingrings of electrically isolating material on the conductor line manufacturing is simplified and short circuit, can be prevented even if the orientation of the conductor line varies due to torsion (rotation around the length of the conductor line), for example when the conductor has a circular or substantially circular cross-section.

In an embodiment, the connection string is fed from a spindle directly or indirectly to the semi-conductor substrate. Although this provides for a simple way of feeding the string, it may lead to torsion. The use of rings makes it possible to avoid detrimental effects of torsion. Another example where torsion may occur is in a process wherein one or more strings are stretched prior to attachment to the substrate, whether they are fed from a spindle or not. In an embodiment a plurality of connection strings may be used. With a large number of strings the risk of short circuits due to torsion could increase, but this is avoided by means of the rings.

In an embodiment solder material is provided on the conductor line. This facilitates joining. In an embodiment the solder material is applied after applying the rings of isolating material. This saves solder material and prevents that solder material is present on the conductor line where it could cause a short circuit. In an embodiment the solder material is provided on the conductor line of the connection string or each connection string in further rings around the conductor line. Thus, torsion cannot cause areas of the conductor line without solder material to face the contact areas to which it should be joined.

In other embodiments, a meltable metal alloy may be used as solder material. Alternatively other materials such as a medium like a paste comprising conductive grains may be used. In an embodiment the grains may be sintered during soldering. In another embodiment the solder material or medium may be provided on the substrate instead of in addition to on the connection string.

In an embodiment the photo-voltaic cell is temporarily kept in a bent state when the conductor line is joined to the first contacts of the photo-voltaic cell, the surface of the photo-voltaic cell on which the conductor line is joined having a convex curvature in said bent state. This reduces stress during later use.

A photo-voltaic cell system of is provided with

- a semi-conductor substrate having a surface with interdigitated first and second contact areas thereon, providing contact to emitter and base contact areas in or on the semi-conductor respectively, or to base and emitter contact areas in or on the semi-conductor respectively,

- a connection string comprising a conductor line and electrically isolating material in rings around the conductor line, with joining parts of the conductor line between the rings joined to a plurality of the first contact areas, in electrical contact with the first contact areas, with electrically isolating material from the rings between the conductor line and the second contact areas, the connection string extending beyond the semi-conductor substrate.

Brief description of the drawing

These and other objects and advantageous aspects will become apparent from a description of exemplary embodiments illustrated in the following figures

Figure 1, la shows a connection string

Figure 2a, b show a photovoltaic cell with a connector Figure 3 shows an assembly of photovoltaic cells

Figure 4 shows a photovoltaic cell with a connector Figure 5 shows a string manufacturing arrangement

Figure 6 shows an assembly manufacturing arrangement

Figure 7 shows a woven connector

Detailed description of exemplary embodiments

Figure 1 shows a cross-section of a connection string for connecting to contact areas of a photovoltaic cell. The connection string (not to scale) comprises an elongated metal core 10, isolating windings in the form of rings 12 of electrically isolating material on core 10 at successive positions in the axial direction 16 of core 10, mutually separated by solder rings 14 of solder material. In the cross-section, each isolating ring 12 and each solder ring 14 shows as two regions, but it should be understood that each isolating ring 12 and each solder ring 14 may extend around core 10 continuously in a circumferential path. Figure la shows a cross-section of an alternative connection string for connecting to contact areas of a photovoltaic cell. In this embodiment the windings are part of string 18a-f of electrically isolating material, running helically around metal core 10. (As is known per se, "helical" means in a curve corresponding to a helix, or spiralling around the core, but as used herein "helically around" refers to the relative positions of metal core 10 and the string, i.e. the string is said to run helically around the core also when the core runs run helically around the string). The figure shows successive cross- sections 18a-f through this string: the isolating string runs from cross-section 18a to cross-section 18b and from there to cross-section 18c and so on. Metal core 10 is provided with a layer 19 of solder material. Although an embodiment is shown wherein metal core 10 is straight, and the string of isolating material revolves around it, it should be appreciated that metal core 10 and the string of isolating material way be a twisted pair of wires, so that each could be said to run helically around the other. At the other extreme the string of isolating material could be straight, with the metal core 10 revolving around it. As used herein, the string of isolating material is said to run helically around metal core 10 also in this case.

Although interruptions in the form of helical windings provide a simple way of making contact, it should be noted that this makes the manufacturing process susceptible to errors in the case of torsion of the connection strings. Torsion could result in isolation with the intended contact or electrical contact with a contact that should be kept isolated from the connection string.

Figure 2a shows a view of part of a back surface of a photo-voltaic cell (not to scale) comprising a set of base contact areas 22 in the form of metal fingers that extend parallel to each other, and a set of emitter contact areas 24 in the form of metal fingers that extend parallel to each other and to the metal fingers of the base contact areas 22. Base contact areas 22 and emitter contact areas 24 alternate along a direction transverse to the fingers, an emitter contact area 24 being provided between each pair of successive parallel base contact areas 22.

A first and second connection string 26, 28 are provided on the back surface 20, extending in the direction transverse to the fingers. First connection string 26 is positioned so that parts of its isolating windings between the metal core and the substrate are located over the emitter contact areas 24. When rings of isolating material are used, first connection string 26 may be positioned so that its isolating rings (indicated by hatching) are located over the emitter contact areas 24 and its solder contact rings are located over the base contact areas 22. Second connection string 26 is positioned so that parts of its isolating windings between the metal core and the substrate are located over the base contact areas 24. When rings of solating material are used, second connection string 28 may be positioned so that its isolating rings are located over the base contact areas 22 and its solder contact rings are located over the emitter contact areas 24.

Figure 2b shows a cross-section of part of the photo-voltaic cell through a virtual plane through the axis of first connection string 26 for the embodiment with rings. The virtual plane cross-sects base contact areas 22 and emitter contact areas 24 perpendicularly to their axial direction. Part of the isolating rings 12 lie between metal core 10 of first connection string 26 and emitter contact areas 24. Solder rings 14 join metal core 10 of first connection string 26 to base contact areas 22.

As is known per se, the photo-voltaic cell may comprise a semiconductor substrate of a first conductivity type (n-type for example), with areas of a second conductivity type (p-type for example) on its surface (as used herein "on" refers both to a layer of the second conductivity deposited on the semiconductor substrate and to a layer created by adding doping in the semiconductor substrate adjacent its surface. Each area of the second conductivity type forms an emitter area, which is coupled to the semiconductor body via a semi-conductor junction. In an interdigitated back contact cell the areas of the second conductivity type are provided in a patterned way on the semiconductor, alternating with areas of the first conductivity type (preferably with enhanced doping relative to the semi-conductor substrate). Each area of the first conductivity type forms a base area coupled to the semi-conductor substrate without intervening semi-conductor junction. Base and emitter contact areas 22, 24 are formed by a conductor, (comprising sintered metal grains, or be plated or evaporated material) in contact with the base and emitter areas respectively. Other layers may be present, such as an isolation layer on the back surface, a reflective coating etc. The contact between the contact areas and the base and emitter areas may be provided through such layers.

For the sake of clarity, only the contact areas 22, 24 are shown in the figures. The semiconductor body, the base area and the emitter areas are and other layers are not shown in the figures. First and second connection string 26, 28 are used to connect such conductors of base and emitter contact areas 22, 24 respectively.

During manufacture a connection string and the semi-conductor substrate are processed separately, after which the connection string is joined to the semi-conductor substrate. That is, on one hand the connection string with its elongated metal core 10, isolating rings 12 solder rings 14 is produced and the other hand the semi-conductor substrate with emitter and base areas and first and second contact areas 22, 28 is produced. A connection string of any length may be produced, which is subsequently cut into sections that are joined to the semi-conductor substrate (before or after cutting). The connection string may be joined to the semi-conductor substrate by heating, at a temperature that is sufficiently high to make the solder material of solder rings 14 flowable.

Although an embodiment has been shown wherein the solder material is part of the connection string, provided on metal core 10 prior to joining the connection string to the semi-conductor substrate, it should be appreciated that this is not strictly necessary. Alternatively the solder material may be applied to the semi-conductor substrate, a metal core with or without solder material being joined to the semi-conductor substrate with the aid of the solder material on the semi-conductor substrate. As another alternative the solder material may be applied to the metal core at assembly The solder material need not be applied in a ring around the entire

circumference of metal core 10: it may suffice to provide solder material on a patch on part of the circumference, for example on half or a quarter of the circumference. Similarly, the isolation material need not be provided in rings: it could cover a patch on part of the circumference and/or it could extend to part of the circumference at the axial position where the solder material is provided. However, this may make it necessary to control torsion of the connection string during assembly. Metal core 10 may have a circular cross- section. However, alternatively metal core 10 may be strip shaped. Metal core 10 may have any cross-section, for example an elliptical cross-section with a major axis that is longer than its minor axis. A flattened cross-section may make it easier to control torsion during assembly.

Contact areas 22 and emitter contact areas 24 may be provided in a periodically repeating pattern, as seen in a direction transverse to the direction of the fingers. Similarly isolating rings 12 and solder rings 14 on the connection strings may be provided in a periodically repeating pattern in this direction. The period lengths of the patterns of the contact areas and the connection string may be at least substantially the same, i.e. not so different that there may no contact to, or isolation from, a contact are at one end of the connection of the connection string on a photo-voltaic cell if there is contact, or isolation at the opposite end of the connection string on the photo-voltaic cell. The connection wire may extend in a direction perpendicular to the direction of the fingers, in which case the period length of the contacts along that direction may be substantially the same as the period length of the connection wire. But the connection wire may extend in another direction transverse to the fingers e.g. at an angle that deviates from ninety degrees, in a direction perpendicular to the direction of the fingers. In this case the period length of the contacts along that transverse direction may be substantially the same as the period length of the connection wire. However, any pattern of distances between the contacts may be used, not necessary that a periodic pattern, and/or the connection wire need not be provided along a straight line. In this case the pattern of distances between the rings on connection wire need not be periodic. However, a periodic pattern with straight connection wires simplifies manufacture and perpendicular angles minimize material cost.

Figure 3 shows an assembly of photovoltaic cells, comprising a first, second and third photovoltaic cell 30a-d connected in series by connection string 32a,b, 34a,b. In the assembly first, second and third photovoltaic cell 30a-c are fixed in a row to a support structure (not shown), with the base and emitter contact areas of first, second and third photovoltaic cell 30a-c in parallel with each other and transverse to the row direction in which first, second and third photovoltaic cell 30a-c succeed each other along the row.

A first connection string 32a extends over first and second photovoltaic cell 30a, b, but not to third photovoltaic cell 30c, on the back surface of first and second photovoltaic cell 30a, b. Extending in the direction transverse to the fingers. First connection string 32a is positioned so that its isolating rings are located over the emitter contact areas of first photovoltaic cell 30a and its solder contact rings are located over the base contact areas of first photovoltaic cell 30a. First connection string 32a is positioned so that its isolating rings are located over the base contact areas of second photovoltaic cell 30b and its solder contact rings are located over the emitter contact areas of second photovoltaic cell 30b. Thus, first connection string 32a connects first and second photovoltaic cell 30a, b in series.

A second connection string 32b extends over second and third photovoltaic cell 30b, c, but not to first photovoltaic cell 30a, on the back surface of second and third photovoltaic cells 30b, c. Second connection string 32b is connected similarly to second and third photovoltaic cell 30b, c as first connection string 32a is connected to first and second photovoltaic cell 30a,b. Thus first and connection string 32a,b together connect first, second and third photovoltaic cell 30a-c in series.

In the example of figure 3, third and fourth connection strings 34a, b,

34a,b extend only over first and third photovoltaic cell 30a, c respectively, electrically connected to the emitter contact areas of the first photovoltaic cell 30a and the base contact areas of the third photovoltaic cell 30c respectively. But it should be understood that more than three photovoltaic cells may be placed and connected in series and in that case, third and/or fourth connection strings, may be connected like first and second connection strings 32a, b, connecting first and third photovoltaic cell 30a, c in series with adjacent photovoltaic cells (not shown), if present.

Preferably, connection strings 32a, b have a slackened part at the transitions between the photovoltaic cell 30a-c, for example in the form of a loop, or partial loop. This allows prevents that stress arises due relative motion of photovoltaic cell 30a-c. By applying a curvature during joining that is opposite to the curvature after shrinking the cell will remain flat.

Although examples have been shown with two connection strings connected to each photovoltaic cell, it should be appreciated that only one connection string may be used, to contact one of the base and/or emitter contact areas 22, 24, the other being connected by a conventional bus bar.

Although examples have been shown with only two connection strings connected to each photovoltaic cell, it is preferred that a larger number connection strings, for example at least ten is used for a photovoltaic cell, connected in parallel and spaced from each other in the direction of the fingers.

Figure 4 shows such an arrangement with a plurality of first connection strings 40, evenly spaced from each other and a plurality of second connection strings 42, evenly spaced from each other. Each first connection string 40 is connected as described for the first connection string of figure 2a,b. Each second connection string 42 is connected as described for the second connection string of figure 2a,b. Use of parallel strings has the advantage that the base and/or emitter contact areas 22, 24 can be made narrower or less thick without severe performance loss, which makes it possible to save material used in the base and/or emitter contact areas 22, 24. As will appreciated this is partly achieved if a plurality of connection strings is used for contacting only one of the base and/or emitter contact areas 22, 24.

Although the preceding examples have been described for the embodiment with rings, it should be appreciated similar connections may be realized with a windings that form part of a string that runs helically around the metal core.

Figure 5 shows an exemplary arrangement for manufacturing a connection string, comprising a first and second spindle 50, 52, a series of baths 54a-c and a UV light source 56. The manufacture of a single connection string from a single metal wire 58 that forms the core of the connection string will be described, but it should be appreciated that a plurality of connection strings may be manufactured in parallel, from a plurality of parallel metal wires. In operation metal wire 58 (e.g. a Cu or Al wire) is provided on first spindle 50. First bath 52a contains a liquid containing UV polymerizable monomers. From first spindle 50 metal wire 58 is led through a first bath 52a via one or more guide wheels. In first bath 52a a liquid film is formed on metal wire 58. From first bath 52a metal wire 58 is led along UV light source 56.

UV light source 56 lights the film on metal wire 58 in an axially periodic pattern (here the axial direction is taken to be the length direction of metal wire 58). In an embodiment, the pitch (period length) of this pattern equals the pitch of base contact areas 22 and emitter contact areas 24 of the photo-voltaic cell for which the connection string will be used. A shutter may be used for example, or a movable screen that is moved along with metal wire 58 over part of the path of metal wire 58 where metal wire 58 is led along UV light source 56. In an embodiment UV light source 56 is configured to light the film around the entire circumference metal wire 58 at the axial positions where the film is lighted. Thus, the film is made to polymerize in rings around metal wire 58 at selected axial positions. However, if rings are not used, UV light source 56 may light only part of the circumference. For example, if the metal core of the connection string is strip formed, only one side may be exposed in a periodic pattern.

From UV light source 56 metal wire 58 is led through a second bath 54b. Second bath 54b contains a solvent for removing unpolymerized monomers of the film on metal wire 58. From second bath 54b metal wire 58 is led through third bath 54c. Third bath 54c may a galvanic bath, a voltage being applied between metal wire 58 and an electrode in the bath to plate exposed rings of metal wire 58 with solder material. As a result a connection string is formed, comprising metal wire 58 as a metal core, rings of polymer as rings of isolating material and rings of solder material. From third bath 54c the connection string is wound on second spindle 52, for later use to connect base and/or emitter contacts.

Although one way of manufacturing the connection string has been described by way of example, it should be appreciated that many alternatives are possible. For example, instead of using axially periodic polymerization, a metal wire with a continuous isolation layer may be used, which is selectively removed in successive rings along the metal wire, for example by local heating of machining. As another example, a bath of liquid solder material may be used to deposit the solder material on the metal wire in rings where no isolator material is present. As a further example, solder material may be applied to the metal wire first and the isolating material may be added later, the isolating material being removed in successive rings, or deposited selectively only in successive rings.

The embodiment with windings that form part of a string that runs helically around the metal core may be manufactured by providing a separate string and metal core (with a solder layer on it) and winding string and the metal core against each other. Alternatively, this embodiment may be manufactured in a similar way, for example by rotating UV light source 56 around the metal wire 58 during illumination, instead of using a shutter, with a period of revolution that corresponds to the period of the shutter. Instead of a revolving light source, a plurality of light sources may be used that are activated in a rvolving pattern, or a revolving mirror may be used to direct light to the metal wire in a revolving pattern.

Figure 6 shows an exemplary manufacturing arrangement for applying a connection string 60 to photovoltaic cells 62. The manufacturing arrangement for applying the connection string comprises a conveyor belt 64 with convexly curved suction cushions 65, a spindle 66, a roller 67 and a cutter 68. In operation photo-voltaic cells 62 are transported by conveyor belt 64 clamped to suction cushions 65 by applying suction through suction cushions 65. Although the operation will be described for a row of photo-voltaic cells 62 with one photo-voltaic cell 62 on each successive suction cushion 65 on conveyor belt 64, it should be appreciated that a plurality of photo-voltaic cells 62 may be provided in parallel on each suction cushion 65, so that a plurality of rows of photo-voltaic cells 62 may be processed in parallel.

A plurality of connection strings 60 (only one shown) are provided wound in parallel on spindle 66. From spindle 66 connection strings 60 are fed between a photo-voltaic cell 62 on one of the suction cushions 65 and roller 67. Base and emitter connection strings, i.e. connection strings 60 for making electrical contact to base contacts and emitter contacts respectively, are fed with an offset between the positions of the rings of solder material. Roller 67 is pressed against the photo-voltaic cell 62 with the connection strings in between. Supply of connection strings 60 from spindle 66 is synchronized to the position of photo-voltaic cell 62 that presses against roller 67, so that solder material from the base and emitter connection strings makes electrical contact to base contacts and emitter contacts of photo-voltaic cell 62

respectively and isolation material on the base and emitter connection strings is interposed between the string's core and the emitter and base contacts and of photo-voltaic cell 62 respectively.

Roller 67 may be heated so as to make the solder material on connection strings 60 flow. The solder material is allowed to cool after passing under roller 67. Subsequently, cutter 68 removes a part of the connection strings 60. For a first part of the connection strings 60, which make electrical contact to the base contacts, cutter 68 does so between successive pairs of photo-voltaic cells 62, so that the connection string runs on between the photovoltaic cells 62 within each pair. For a second part of the connection strings 60, which make electrical contact to the base contacts, cutter 68 does so between successive photo-voltaic cells 62 within a pair but not between the pairs. Thus, each time a continuous section of connection string 60 is attached to two adjacent photovoltaic cells 62. At the end of a series connection of photo-voltaic cells 62, connection strings 60 may be attached to only one photo-voltaic cell 62.

The use of convexly curved suction cushions 65 during attachment of connection strings 60 has the advantage that some slack is created when photo-voltaic cells 62 are released from suction cushions 65. This reduces stress in the photo-voltaic cells 62.

Although one way of applying the connection string has been described by way of example, it should be appreciated that many alternatives are possible. For example, in one embodiment parallel connection strings are first attached to a flexible foil (e.g. by means of adhesive or heating), after which the foil with the parallel connection strings if attached to the photo- voltaic cells. This facilitates alignment.

Figure 7 shows a woven connector mat comprising electrically conductive connection strings 70a,b and electrically isolating isolator strings 72a,b as warp and woof. Electrically conductive connection strings 70a,b run parallel to each other. Each electrically conductive connection string 70a, b comprises a metal core covered by a solder layer. Alternating connection strings 70a, 70b for contacting base and emitter contacts may distinguished. Isolator strings 72a,b run parallel to each other. Alternating isolator strings 72a, 72b for isolating base and emitter contacts may distinguished. Each isolator string 72a,b runs transverse to electrically conductive connection strings 70a,b, alternately passing above and below successive electrically conductive connection strings 70a,b. Isolator strings 72b for isolating emitter contacts pass above first ones of the connection strings 70a (for contacting base contacts) and below first ones of the connection strings 70b (for contacting emitter contacts). The next isolator strings 72a for contacting emitter contacts conversely passes below and above the first and second ones of the connection strings 70a,b respectively.

In a photo-voltaic cell, the connector mat may be attached on the back surface, with isolator strings 72a, 72b for isolating base and emitter contacts over the base and emitter fingers respectively and connection strings 70a, 70b for contacting base and emitter contacts joined to base and emitter contacts respectively. The conductor mat may be manufactured by weaving. The photo-voltaic cell may be assembled by applying the woven mat to the back surface, with isolator strings 72a, 72b aligned with the base and emitter fingers, followed by heating to connect connection strings 70a, 70b. The woven mat may be applied using a manufacturing arrangement similar to that of figure 6, supplying the mat from a spindle, and pressing the mat against the photo-voltaic cell, for example when the cell is held in a curved surface. As in the case of a connection string with rings or patched, strings of circular or flattened cross-section may be used. Flattened isolator strings 72a, 72b have the advantage that they can be more easily kept in alignment with contact areas. Flattened connection strings 70a, 70b have the advantage that they provide for broader joints to the contact areas. As in the case of a connection string with rings or patches of isolating material, the solder material on connection strings 70a, 70b could be omitted, for example is solder material is provided on the contacts of the cell. Connection strings 70a for the base contacts may be replaced in the mat by other strings if the connections to the base contacts are otherwise provided on the semi-conductor substrate.

Conversely, connection strings 70b for the emitter contacts may be replaced in the mat by other strings if the connections to the emitter contacts are otherwise provided on the semi-conductor substrate. Thus in some

embodiments only one set of connection strings may be used.

As will be appreciated, both the woven mat and the strings with isolator rings or patches provided for application of one or more pre- manufactured string against the back surface of a photovoltaic cell, to provide connections to at least one set of a plurality of contact areas on a photo-voltaic cell. This string provides for a common connection to the plurality of contact areas on the same semi-conductor substrate, for example to at least three distinct contact areas, even when no deposited electrical conductors on the semi-conductor substrate are available that connect these contact areas.

Isolator material is interposed between the electrical conductor of the string and the contact areas of the other set. This isolator material may be provided in the form of patches or rings on the electrical conductor or in a woven mat.

A method of manufacturing a photo-voltaic cell system is used, the system comprising a semi-conductor substrate having a surface with interdigitated first and second contact areas thereon, providing contact to emitter and base contact areas in or on the semi-conductor respectively, or to base and emitter contact areas in or on the semi-conductor respectively, the method comprising

- providing a conductor line;

- joining the conductor line to a plurality of the first contact areas, in electrical contact with the first contact areas;

- providing electrically isolating material between the conductor line and the second contact areas.

This provides a novel photovoltaic cell system and a method of manufacturing such a system that can serve as an alternative for known photovoltaic cells, in particular a cell that overcomes one or more of the drawbacks mentioned in the background section.

Thus a conductor line is used that is pre -manufactured before it is joined to contacts on the photo-voltaic cell. In an embodiment such conductor lines are provided in electrical contact with emitter and base contacts respectively. But in another embodiment only a conductor line or lines of this type may be used for the base contacts, connecting distinct base contacts on the photo-voltaic cell, or only for the emitter contacts, connecting distinct emitter contacts on the photo-voltaic cell, the remaining contacts being connected by a conventional bus bar or bus bars.

By using a conductor line or conductor lines the need for printing or otherwise depositing conductor material to form a bus bar, with its

disadvantages, is reduced or even avoided for at least one of the base and emitter. The system may have one or more photo-voltaic cells. The photo- voltaic cell or cells of the system may have no electrical conductor on the semiconductor structure that forms an electrical connection between base contact fingers and/or between emitter contact fingers.

In an embodiment, the method comprises

- providing a plurality of conductor lines;

- joining the plurality of conductor lines, each to said plurality of the first contact areas, the conductor lines being joined in parallel electrical contact with the first contact areas ;

- providing electrically isolating material between the conductor lines and the second contact areas.

The method may comprise

- providing a further conductor line or lines;

- joining the further conductor line to a plurality of the second contact areas, or the further conductor lines each to a plurality of the second contact areas - providing further electrically isolating material between the further conductor line or lines and the first contact areas.

In this case the contacts (e.g. contact fingers) can be made smaller, e.g. less thick or less wide, which saves material.

According to one aspect the conductor line is part of a connection string comprising the electrically isolating material in patches on the conductor line, leaving parts of the conductor line electrically exposed, the electrically exposed parts alternating with the patches along a length direction of the conductor line, the method comprising

- providing a connection string;

- joining the electrically exposed parts of the connection string each to a respective one of the first contact areas, in electrical contact with the first contact areas, with the patches located between the conductor line and the second contact areas. In a further embodiment the patches form rings of electrically isolating material around the conductor line. The use of prefab patches on the conductor lines simplifies assembly.

According to one aspect the conductor line is provided in a woven mat, comprising a series of first strings mutually parallel to each other and second strings transverse to the first strings, woven through successive first strings in said series, one of the first strings comprising said conductor line, the second strings comprising said isolating material In a further embodiment each of the first strings comprises a conductor line, interdigitated first and second subsets of the first strings being joined to the plurality of first and contact areas respectively. Thus no deposition of isolation material is needed. In an embodiment, a plurality of conductor lines may be woven in parallel in the mat. This eases position control when a plurality of conductor lines is joined to a photo-voltaic cell.

Solder material may be provided on the conductor line or lines prior to joining, the conductor line or lines being joined to the first contact areas by heating the solder material. This makes it easy to join the conductor lines to the photo-voltaic cell.

In an embodiment, the system comprises a further photo-voltaic cell, wherein said conductor line is joined to a plurality of the second contact areas on the further photo-voltaic cell, electrically isolating material being provided between the conductor line and the first contact areas of the further photovoltaic cell, whereby the conductor line electrically connects emitter and base contacts of the photovoltaic cell and the further voltaic cell. In a further embodiment slack is added to the conductor line between the photo-voltaic cell and the further photo-voltaic cell. In this embodiment the system contains a plurality of photo-voltaic cells connected in series. In this case, the conductor line may run on from one photo-voltaic cell to another, contacting base contacts in one and emitter contact in the other. This the conductor lines may serve both to connect contact fingers within cells as well as between cells.

In a further embodiment the system slack is added to the conductor line between the photo-voltaic cell and the further photo-voltaic cell. This reduces the risk of damaging stress in the case of relative movement between the photo-voltaic cells.

In an embodiment, the photo-voltaic cell is temporarily kept in a bent state when the conductor line is joined to the first contacts of the photovoltaic cell, the surface of the photo-voltaic cell on which the conductor line is joined having a convex curvature in said bent state. The surface on which the conductor line is joined has a convex curvature in said bent state, i.e. the centre of curvature is on the other side of the photo-voltaic cell opposite this surface. This reduces the risk of damaging stress due to the conductor lines.

A photo- voltaic cell system is provided that is obtainable by any of these manufacturing methods.

Claims

1. A method of manufacturing a photo-voltaic cell system, the system comprising a semi-conductor substrate having a surface with interdigitated first and second contact areas thereon, providing contact to emitter and base contact areas in or on the semi-conductor respectively, or to base and emitter contact areas in or on the semi-conductor respectively, the method comprising

- providing a connection string comprising a conductor line and electrically isolating material in rings around the conductor line;

- joining parts of the conductor line between the rings to a plurality of the first contact areas, in electrical contact with the first contact areas, with electrically isolating material from the rings between the conductor line and the second contact areas.

2. A method according to claim 1, wherein the connection string is provided on a spindle, the connection string being fed from the spindle to the photo-voltaic cell.

3. A method according to claim 1 or 2, comprising

- providing a plurality of connection strings, each comprising a conductor line and electrically isolating material in rings around the conductor line of the connection string;

- joining parts of the conductor line of each connection string between the rings of the connection string to said plurality of the first contact areas, with the electrically isolating material from the rings between the conductor line and the second contact areas, the conductor lines of the connection strings being joined in parallel electrical contact with the first contact areas.

4. A method according to claim 3, wherein the plurality of connection strings is provided on a spindle, the connection strings being fed in parallel from the spindle to the photo-voltaic cell

5. A method according to any one of the preceding claims, comprising - providing a further connection string or strings comprising a conductor line or lines and electrically isolating material in rings around the conductor line or lines;

- joining parts of the conductor line of the further connection string or each further connection string between the rings of the further connection string to said plurality of the second contact areas, with the electrically isolating material from the rings between the conductor line and the first contact areas to a plurality of the second contact areas.

6. A method according to claim 5, wherein the further connection string or strings are provided on a spindle, the method comprising feeding the connection string and the further connection string or strings in parallel from the spindle to the photo-voltaic cell.

7. A method according to any one of the preceding claims, wherein the connection string, the plurality of connection strings, or the further connection string or strings and the connection string or plurality of connection strings are fixed on a flexible foil and fixed to said a flexible foil, prior to said joining and placed on the contact areas with the foil.

8. A method according to any one of the preceding claims, wherein the connection string, the plurality of connection strings, or the further connection string or strings and the connection string or plurality of connection strings are fed in parallel from the spindle to a flexible foil and fixed to said a flexible foil, prior to said joining and placed on the contact areas with the foil

9. A method according to any one of the preceding claims wherein solder material is provided on the conductor line of the connection string or each connection string prior to said joining, the conductor line or lines being joined to the first contact areas by heating the solder material.

10. A method according to claim 9 wherein the solder material is provided on the conductor line of the connection string or each connection string after applying the rings of isolating material.

11. A method according to claim 9 or 10 wherein the solder material is provided on the conductor line of the connection string or each connection string in further rings around the conductor line.

12. A method according to any one of the preceding claims, wherein the system comprises a further photo-voltaic cell, the method comprising joining parts of the conductor line of the connection string between the rings to a plurality of the second contact areas on the further photo-voltaic cell, with the electrically isolating material from the rings between the conductor line and the first contact areas on the further photo-voltaic cell.

13. A method according to claim 12, wherein slack is added to the conductor line between the photo-voltaic cell and the further photo-voltaic cell.

14. A method according to any one of the preceding claims, wherein the photo-voltaic cell is temporarily kept in a bent state when the conductor line is joined to the first contacts of the photo-voltaic cell, the surface of the photo- voltaic cell on which the conductor line is joined having a convex curvature in said bent state.

15. A photo- voltaic cell system obtainable by the manufacturing method of any one of the preceding claims.

16. A photo-voltaic cell system according to claim 15, comprising

- a connection string comprising a conductor line and electrically isolating material in ringss around the conductor line, with joining parts of the conductor line between the rings joined to a plurality of the first contact areas, in electrical contact with the first contact areas, with electrically isolating material from the rings between the conductor line and the second contact areas, the connection string extending beyond the semi-conductor substrate.

17. A system of photo- voltaic cells according to claim 16, comprising a further photo-voltaic cell, wherein parts of the conductor line of the connection string between the rings are joined to a plurality of the second contact areas on the further photo-voltaic cell, with the electrically isolating material from the rings between the conductor line and the first contact areas on the further photo-voltaic cell.