WO2006137746A1

WO2006137746A1 - Stress relieving ribbons

Info

Publication number: WO2006137746A1
Application number: PCT/NO2006/000240
Authority: WO
Inventors: Erik Sauar; Daniele Margadonna; Tim Lommasson; Helge Cato Aamodt; Ingmar ÅSBERG; Andreas Bentzen
Original assignee: Renewable Energy Corp ASA
Current assignee: Renewable Energy Corp ASA
Priority date: 2005-06-24
Filing date: 2006-06-23
Publication date: 2006-12-28
Anticipated expiration: 2007-12-24

Abstract

The present invention relates to a method for electrical connection of solar cells into modules and solar cells equipped with the electrical connection according to the invention, where at least one electrical conductor/ribbon is soldered to at least the back side of the solar cells such that the at least one electrical conductor/ribbon is only soldered onto the at least back side of the solar cell in two or more interspaced soldering areas (B), forming free space section(s) of each of the at least one electrical conductor/ribbon extending between two adjacent soldering areas (B) which constitutes stress relieving deformability zones.

Description

STRESS RELIEVING RIBBONS

The present invention relates to a method for electrical connection of solar cells and solar cells equipped with the electrical connection according to the invention.

Background

The current practice to assembly solar cells in photovoltaic modules makes use of copper ribbons with various coatings (tin, silver, lead etc) for the electrical connection of each solar cell to the next. The ribbon is frequently first soldered on the front of each cell along the soldering track (2 or more tracks on each cell). The ribbon is then soldered on the back of the next cell in order to build a string of series connected cells. In some arrangements, ribbons are soldered simultaneously on the front and on the back. In other arrangements, ribbons are soldered only at the back. An example of typical present connection and assembly of solar panels according to prior art is schematically illustrated in Figures 1 to 4. Fig 1 and 2 illustrate the front side and the rear side of a solar cell 10 according to prior art, and will be described briefly herein. The solar cell 10 is for example of the crystalline silicon type and comprises two soldering tracks 12 on each side. The soldering tracks 12 for example comprise a thin layer of silver printed onto the solar cell surface.

Fig. 3 shows how solar cells 10 of the abovementioned type are typically interconnected according to prior art. It shall be noted that the interconnection described herein is primarily referring to the electrical connection between the solar cells, and to a lesser extent the physical connection. The physical assembling of the solar cells to each other and to other parts of the solar panel device is provided by solar cell fastening means (not shown). Such solar cell fastening means are considered known per se for a man skilled in the art and will not be described here. The solar cells are interconnected by means of copper ribbons 14, usually with a coating to improve solderability. In the shown example, a first end of the ribbon 14 is soldered to the front side of a first solar cell and a second end of the ribbon 14 is soldered to the rear side of the adjacent solar cell, as illustrated in fig. 4.

These soldering methods are today applied to solar cells made on silicon having thickness in the range of 200 - 350 micron and area in the range of 150-250 cm².

In order to save material and process costs, solar cell manufacturers are developing new fabrication processes to improve efficiency and/or to reduce material consumption, and the development aims at: reduced wafer thickness, down towards 100 micron and possibly even below increased cell area up towards 500 cm² new design of the solar cell device so that both plus and minus contact are placed on the back of the cell, since all equipment located on the front side of the cell will occupy energy producing area. there is also a trend towards higher soldering temperatures in order to avoid lead containing products, such as the silver-based pastes on the cell and the thin solder coating on the copper ribbon. Lead-free soldering normally takes place at higher temperatures due to higher melting points.

In order to connect the above new cells together, the conventional mentioned methods cannot be easily applied. When a continuous ribbon is soldered along the soldering track in the complete length, the ribbon (typically copper) and silicon wafer are mechanically coupled at the soldering temperature (typically 200 - 400 ⁰C). When the cell cools down from the solidification temperature of the solder to room temperature, the silicon is consequently subjected to mechanical stress since the ribbon (typically copper) is shrinking more than the silicon. The problem becomes more important when the wafer thickness is reduced, and/or when the cell area is increased or when the soldering temperature is increased due to the use of (lead-free) higher temperature solder coatings. The mechanical stress is resulting in an increased occurrence of broken cells during soldering and cooling, regardless of whether the soldering is made manually or automatically by a machine, and such breakage is very costly.

By using traditional cells (negative contact on the front and positive contact on the back) the problem is to some extent self-compensating, because nearly the same length of ribbon is soldered on both front and back. Hence, the silicon wafer will experience compression forces, but not bend. In case of cells having both contacts on the back, the cell will be subjected to a non -compensated stress and show significant bending. Significant bending typically leads to higher breakage rates in the further production process and during the later outdoor life-time of the completed solar module.

Prior art Suzuki (JP 2005142282) has tried to solve the problem of stress between the ribbon and wafer by making partial cuts into the ribbons in various configurations. Suzukis invention however, will imply higher electrical losses due to the reduced cross- section of the ribbon, will imply a slightly more costly ribbon and it is not clear whether or not the invention will be able to release sufficient stress. It is thus a need for novel methods for soldering wafers without inducing unacceptable levels of mechanical stress due to different thermal expansion of wafer and ribbon material. Objective of the invention

Consequently, the main objective of the present invention is to provide a method for electrical connection of solar cells that solves the abovementioned problems.

A further objective of the invention is to reduce the stress effect in the cell caused by the electrical connection, and in this way make it possible to use solar cells with increased surface area, decreased thickness and/or lead-free soldering without increasing the electrical losses and/or the cost of the ribbon.

The objectives of the invention may be obtained by the features set forth in the following description of the invention and/or in the appended patent claims. Description of the invention

The present invention is based on the realisation that mechanical stress due to use of ribbon(s) with different thermal expansion than the solar wafer, may be solved by equipping the ribbon(s) with one or more deformable stress relieving sections formed by only soldering the ribbon(s) onto the solar cell in two or more interspaced sections on the solar cell surface, and thus forming one or more intersections where the ribbon(s) is/are not fastened to the solar cell surface. These one or more intersections will then be free to bend and thus relive mechanical stress induced by differences in thermal expansion of solar cell and ribbon(s). A further deformability and thus stress relieving capacity will be obtained by allowing the length of the ribbon(s) in the intersection zones to be larger than the distance along the solar cell surface between the soldering sections such that the ribbon(s) will be more or less bent above these intersections. The intersections may be applied on one or both sides of the solar cell. The characteristic length of the soldering section may vary from a point-like soldering spot to elongated soldering areas extending almost from edge-to-edge of the solar cell.

The use of soldering sections for fastening the ribbon(s) will not only solve the problem with induced mechanical stress due to difference in thermal contraction after the soldering process which operates at 200 — 400 ⁰C, but will also prolong the service time of solar panels since bending forces induced by the thermal variations will be more or less eliminated. The difference in day and night temperatures in solar panels might be in the order of 50 - 60 °C, which will induce periodic stresses in conventional cells which may lead to breakages, unacceptable bending inside panels etc.

The term "ribbon" as used herein includes any conceivable electric conducting structure connected to the contacts of solar cells in order to interconnect solar cells into modules. Ribbons are often made of copper coated with tin, lead, silver etc., but the invention may employ any electric conducting material enabling charge carriers to enter an external circuit. List of figures

Fig. 1 shows a front view of a solar cell with soldering tracks according to prior art;

Fig. 2 shows a rear view of a solar cell with soldering tracks according to prior art; Fig. 3 illustrates the interconnection of the solar cells in fig. 1 and 2; Fig. 4 shows a cross sectional view of some of the interconnected solar cells in fig. 3;

Fig. 5 and 6 shows a front view and a rear view of a solar cell according to an example embodiment of the invention, respectively;

Fig. 7 illustrates a moulding device for the corrugated ribbon according to an example embodiment of the invention;

Fig 8 illustrates the use of the corrugated ribbon as made by the example device illustrated in Figure 7;

Fig. 9 illustrates how a corrugated ribbon can be fastened to a solar cell; Fig. 10 illustrates a ribbon which is corrugated in the same plane as the cell itself; Fig. 11 a - e illustrates a second embodiment of the invention; and Fig. 12 a - b illustrates a third embodiment of the invention.

Preferred embodiments of the invention.

The invention will now be described in further detail by way of preferred embodiments of the invention. These embodiments should not be considered a limitation of the general nature of the inventive idea of using deformable zones in the ribbons to relieve build up of mechanical stress due to differences in thermal expansion between ribbons and cell.

First aspect of the invention

The first aspect of the invention is related to the use of a corrugated ribbon 24 for electrical connection of the solar cell. Preferred embodiments according to the first aspect of the invention are illustrated in figures 5 — 10. The method according to the first aspect of the invention obtains the stress relieve by applying soldering to the solar cell (20) or onto a soldering area (Br) of a corrugated ribbon (24), and soldering the soldering area (Br) of the corrugated ribbon (24) to the solar cell (20). Further, the solar cell according to the first aspect of the invention is characterised in that the device comprises a corrugated ribbon (24) having at least two interspaced soldering areas (B_r) which are soldered to the solar cell (20), either by applying solder to each soldering area (B_r) of the corrugated ribbon (24) or by applying soldering to corresponding soldering areas (B) of the solar cell (20). The first aspect of the invention is considered to be the best mode of the invention.

The front side and the rear side of a solar cell 20 according to a first preferred embodiment of the invention is schematically shown in Figure 5 and 6, respectively. There are shown two tracks 22 on each side. In this embodiment, the soldering tracks 22 comprises areas B with length denoted b and an area C with length denoted c. Areas B, C may be adapted to the ribbon according to the invention, which will be described below. On the rear side (Fig. 6), soldering occurs only on areas B. On the front side, soldering may be continuously or only on areas B. In the first preferred embodiment, the soldering tracks 22 comprise a thin layer of primarily silver printed onto the solar cell surface. In the embodiment shown on the figures, the soldering track 22 is continuously applied to the solar cell on the front side, and on defined regions on the back side.

The corrugated ribbon 24 for interconnecting the preferred embodiments of the solar cells 20, is shown in Fig. 7. Preferably the corrugated ribbon comprises a coated copper conductor having a thickness in the range 0.1 — 0.3 mm and a width from 1 to 3 mm. The corrugated ribbon 24 is corrugated with a height denoted a_r. The corrugated ribbon further comprises periodical bends or angles along its length, where one period or pitch length p_r is equal to the length b_r of one soldering area B_r plus the length of the free path denoted c_r, see Figure 7. The soldering areas B_r is equal to the length b_r times the width of the corrugated ribbon 24. The areas B of the soldering track 22 may be adapted to soldering areas B_r of the corrugated ribbon 24, or as shown in Figure 5, may be wider than the soldering areas B_r of the ribbon(s). The length c of area C of the soldering track 22 should preferably be adapted to the free length c_r of the corrugated ribbon 24.

The frequent bends or angles will provide flexibility, since the free length c_r of the ribbon is not fastened to any parts of the solar cell or the solar panel device. This flexibility will reduce the stress effect to the cell, since there is some elasticity provided by the bent part of the ribbon. Consequently, the stress accumulated along the soldering track 22 can be substantially reduced if the following is considered.

First, the ratio b_r/c_r will influence on the stress. Supposing that b_r/c_r = 0, e.g. b_r =0, the ribbon has no contact with the cell, and there is no stress on the cell. Supposing b_r/c_r = oo; e.g. c_r =0, the corrugated ribbon is soldered all along the soldering track, and the stress is at its maximum. In a preferred embodiment, the ratio b_r/c_r is between 1/10 and ¹A, in a more preferred embodiment the ratio b_r/c_r is between 1/6 and 1/3. Furthermore, in a preferred embodiment the total length of the ribbon is so much longer than the soldering track that after cooling down from the soldering temperature, both the ribbon and the soldering track are equally long. For a 156 mm long silicon based solar cell and a soldering temperature in the range of 225 ⁰C, the ideal additional length of the copper based ribbon at the high temperature is approximately 0.42 millimetres, which is equivalent to 0.27 % of the ribbon's length.

In fig. 8, a method for production of the corrugated ribbon is illustrated. Here, the corrugation of the ribbon 24 is obtained by pressing a long and relatively flat ribbon between a first moulding plate 40 comprising spaced teeth 41 and a second moulding plate 42 comprising spaced apertures 43. The length b_r and length c_r are also indicated in the drawing.

The choice of ratio b_r/c_r and the number of soldering points on the printed soldering track will among other factors depend on the electric conductivity of the soldering tracks on the cell. Furthermore, the pitch length p_r will depend on the soldering method, where four methods are frequently used today:

- hot tip soldering infrared heating - heating by other electromagnetic frequencies (e.g. Laser)

- hot air heating

The chosen design of the corrugated ribbon will vary in different applications, and a person skilled in the art with knowledge of the present disclosure will perform evaluations and computations to achieve the wanted performance for his or her application. For instance, when using the hot tip soldering method, the pitch length p_r cannot be too short. A hot tip 50 must be placed in contact with the soldering length b_r, as illustrated in fig. 9. Here, a lower corrugated ribbon 24 is soldered to the rear side of the solar cell 20 at the same time as an upper corrugated ribbon 24 is soldered to the front side of the solar cell 20 using the hot tip 50. The lower corrugated ribbon is supported by a preheated plate 51 having a temperature below the soldering temperature. The hot tip 50 is having a temperature of ca 200 - 350 °C and both the lower and the upper corrugated ribbon are soldered to the solar cell simultaneously by the hot tip 50.

By using for example infrared and/or hot air heating, the corrugated ribbon is placed over the soldering tracks and held in contact by pins. The soldering occurs only in the soldering area B, where the metal is in contact with the cell.

The abovementioned detailed description is especially provided to illustrate and to describe the preferred embodiments of the invention. However, the description is by no means limiting the invention to the specific embodiments. The soldering track 22 can be discontinuously applied to the solar cell, so that only the areas B on fig. 5 and 6 are coated. Alternatively, the soldering is applied along the soldering areas B_r of the corrugated ribbon(s) before fastening the ribbon(s) to the solar cell. The ribbon(s) is/are then connected to the next solar cell or to other parts of the electrical circuit arrangement of the solar panel device. Moreover, it is of course possible to apply more than one soldering track 22 for each polarity on each solar cell.

The corrugation described so far is a corrugation created so that the ribbon section c_r is not soldered onto the cell, but standing some minimum distance (between zero and a_r due to the shrinking during cool-down) away from the cell surface. Another embodiment of the invention can be accommodated by bending the ribbon in the other plane so that when you see the ribbon soldered onto the cell it no longer forms a straight line as can be seen in Fig. 10. This embodiment has the advantage that when mechanical pressure is applied onto the cell surface (as for example in the lamination process of module manufacturing) there will be no additional stress applied to the cell. This arrangement is particularly effective when both negative and positive contacts are placed on the back of the cell. For this specific embodiment of the first aspect of the invention, the dimensions and the pattern of the bending are less important. The critical factor is simply to provide at least one bend between each soldering point.

In further embodiments of the first aspect of the invention, both positive and negative contacts are brought to soldering areas on the back side, and there are more soldering tracks or areas on the back side and none on the front. The back side contacting can be achieved in several ways well defined in the literature, for example emitter wrap through or metal wrap-through. In this case, soldering stress induced on the back is not compensated by equivalent stress on the front, and special soldering technique must be used to avoid the occurrence of bended, and possibly thereafter, broken cells.

Second aspect of the invention.

The second aspect of the invention relates to use of a "comb" for creating the deformable interconnection zones of the ribbons. This embodiment of the invention will now be described with reference to Figure 11 a-e.

A comb 119 according to the invention comprises a holding structure 120 periodically applied with comb teeth 122 of an inert (not easily solderable) material such as TEFLON^®. The comb 119 is brought in position on each side of the cell 110 (Fig. l ib) by means of a positioning device (not shown), so that the end 124 of the comb teeth 122 are covering the soldering track 112 of the cell (Fig. 1 Ic).

Thereafter, a copper ribbon 130 is placed on the soldering track 112 and over the comb teeth 122. The copper ribbon 130 is soldered onto the soldering track 112 by one of the above-mentioned methods. Consequently, the comb teeth 122 of inert material are increasing the "travelling distance" of the ribbon while the silicon distance is non-affected. Finally, the remaining comb teeth 122 of inert material are removed from the cell after the soldering has started and before the cool-down is completed. Since the comb teeth in this way are increasing the length of the ribbons at the highest temperature, this additional length can be "consumed" during the cooling and shrinking of the ribbon.

Third aspect of the invention

A third aspect of the invention will now be described with reference to

Figure 12 a-b. A combined ribbon 219 according to the invention is comprising a copper ribbon 230 periodically applied with tiny spacer ribbons 220 of a soft, slow- or non- reacting, transparent material such as for example TEFLON^® or EVA^®. The combined ribbon 219 is brought in position onto the soldering track 212 of the cell 210 by means of positioning device (not shown). The spacer ribbons 220 can be tape applied to the copper ribbon 230. The combined ribbon 219 can be pre-cut to desired length, or can be kept as a continuous roll of ribbon that is cut in situ before it is positioned.

The combined ribbon 219 is soldered onto the soldering track 112 by one of the above-mentioned methods. Consequently, the spacer ribbons 220 of inert material are providing additional travelling distance for the copper ribbon relative to the soldering track 212 on the silicon. During subsequent cooling, the ribbons will hence shrink more than the silicon, and since the applied ribbons on the tape are soft, the ribbons will be allowed to shrink and release stress.

In the second and third embodiments of the invention, the same periodical soldering can be achieved as in the first embodiment. Consequently, the periodic length p_r, the soldering length b_r and the free length c_r occur in the same way in all examples.

Claims

1. Method for electrical connection of solar cells, where at least one electrical conductor/ribbon is soldered to at least the back side of the solar cell, characterised in that the at least one electrical conductor/ribbon is only soldered onto the at least back side of the solar cell in two or more interspaced soldering areas (B), such that the free space section(s) (c_r) of each of the at least one electrical conductor/ribbon extending between two adjacent soldering areas (B) forms stress relieving deformability zones.

2. Method according to claim 1, characterised in that the length of the electrical conductor/ribbon between two soldering areas (B) is larger than the free space length c along the solar cell surface such that the electrical conductor/ribbon will be more or less bent in the free space zone(s).

3. Method according to claim 2, characterised in that

- the bends is obtained by using a corrugated ribbon (24) comprising periodical bends with period length p_r equal to the soldering length b_r of the soldering areas (B_r) plus the free length c_r, and which have height a_r, and

- the soldering areas (B_r) of the corrugated ribbon (24) is adapted to the soldering areas (B) on the solar cell (20).

4. Method according to claim 3, characterised in that the ratio b_r/c_r is between 1/10 and ¹A, preferably between 1/6 and 1/3.

5. Method according to any of claims 1-4, characterised in that the ribbon is between 0.1 and 0.6 % longer than the soldering track at the soldering temperature, preferably between 0.2 and 0.4 % longer.

6. Method according to any of claims 1 - 5, characterised in that the soldering is performed by hot tip soldering, infrared heating, electromagnetic heating or hot air heating.

7. Method according to any of the preceding claims characterised in that the electrical conductor/ribbon(s) is/are applied on both the front side and the back side of the solar cell.

8. Method for electrical connection of solar cells, characterised in that the method comprises:

- applying at least one soldering track for each polarity of the solar cell, - placing a series of inert bodies at regular intervals along each soldering track,

- placing an electrical conductor/ribbon on-top of each soldering track including the regularly spaced inert bodies, - soldering the electrical conductor/ribbon onto the soldering tracks between the inert bodies, and

- retrieving the inert bodies to form stress relieving deformability zones in the form of free space zone(s) (Cr).

9. Method according to claim 8, characterised in that the inert bodies are rectangularly shaped flat teeth of

Teflon^® protruding from a holding structure.

10. Method for electrical connection of solar cells, characterised in that the method comprises:

- applying at least one soldering track for each polarity of the solar cell, - placing one copper ribbon including a series of inert soft spacer bodies at regularly spaced intervals along the copper ribbon on-top and parallel to each soldering track, and

- soldering the electrical conductor/ribbon between the soft spacer bodies onto the soldering tracks.

11. Method according to claim 10, characterised in that the inert soft spacer bodies are made of Teflon^® or

EVA^®'

12. Solar cell, comprising a silicon wafer and one or more ribbons/electrical conductors 24 at least on the back side of the solar cell 20, characterised in that

- the at least one electrical conductor/ribbon 24 is soldered to the solar cell 20 in two or more interspaced soldering areas (Br) such that the section(s) of each of the at least one electrical conductor/ribbon(s) 24 extending between two adjacent soldering areas (Br) forms stress relieving deformability zones.

13. Solar cell according to claim 8, characterised in that the length of the electrical conductor/ribbon 24 between two soldering areas (Br) is larger than the free space length c along the solar cell surface such that the electrical conductor/ribbon will be more or less bent in the free space zone(s).

14. Solar cell according to claim 8 or 9, characterised in

- that the electrical conductor/ribbon 24 is a corrugated ribbon (24) of coated copper with periodical bends with period length p_r equal to the soldering length b_r of the soldering areas (B_r) plus the free length c_r, and which have height a_r, and

- that the soldering areas (B_r) of the corrugated ribbon (24) is adapted to the size of and the space between areas (B) on the solar cell (20).

15. Solar cell according to claim 14, characterised in that the ratio b_r/c_r is between 1/10 and ¹A, preferably between 1/6 and 1/3.

16. Solar cell according to claim 15, characterised in that the ribbon is between 0.1 and 0.6 % longer than the soldering track at the soldering temperature, preferably between 0.2 and

0.4 % longer.