WO2003012883A2

WO2003012883A2 - Method for contacting thin-film electrodes

Info

Publication number: WO2003012883A2
Application number: PCT/EP2002/008638
Authority: WO
Inventors: Franz Karg; Heinz-Ingo Schneider; Helmut Vogt
Original assignee: Shell Solar Gmbh
Priority date: 2001-08-02
Filing date: 2002-08-02
Publication date: 2003-02-13
Also published as: DE10137778A1; WO2003012883A3

Abstract

With thin-film components, in particular thin-film solar modules, conductive layers (4) are contacted with a metal strip (5) by ultrasonic welding according to the invention. For that purpose, a sonotrode (6) is set onto the metal strip (5) laid upon the conductive layer (4) with a contact force. Subsequently, the sonotrode (6) is oscillated and the conductive layer (4) is bonded with the metal strip (5).

Description

METHOD FOR CONTACTING THIN-FILM ELECTRODES

The invention relates to a method for providing an electric contact between a metallic or transparent thin- film electrode and a metal strip, and to the use of a device for carrying out this method. In many applications of thin-film components made on glass, synthetic, metal or ceramic substrates an electric contact has to be made to the outside world for the purpose of control and/or to feed and carry away current. In thin-film solar modules, for example, the cells lying on the ends of a serial connection have to be contacted in order to carry away the current generated by the solar module. To this end, a metal strip is applied onto a conductive layer. The conductive layer is either a metallic back electrode, consisting of for example molybdenum, or a transparent thin-film electrode consisting of doped zinc-, tin- or indium-tin oxide. Molybdenum electrodes are used mainly with so-called CIS solar cells. Transparent electrodes are widely used not only in various solar cells but also in flat screens, electrochromic or heatable windows, used for example in the automotive field.

An example of a thin-film solar module is a module that comprises a transparent substrate, on which have been deposited a transparent thin-film first electrode, a semi-conductor layer (or light-absorbing layer) and a thin-film second electrode. The conductive layer that forms the first electrode can also be a set of interconnected conductive strips, and the same applies to the second electrode. The transparent thin-film first electrode, the semi-conductor layer and the thin-film second electrode form a layered structure. In case an electric contact has to be made between the first electrode and the outside world, a location is selected ■ on the conductive layer that forms the first electrode (which in this case includes removing the part of the semi-conductor layer and second electrode that eventually covers the location) , and a metallic strip is connected to the first electrode at the selected location. In case an electric contact has to be -made between the second electrode and the outside world, a location is selected on the conductive layer forming the second electrode, and a metallic strip is connected to the second electrode at the selected location. When the connection or connections have been made, the layered structure is encapsulated by an embedding material that encloses the layered structure, and on top of the embedding material a plate is secured that forms the back of the module. This configuration is sometimes called a superstrate configuration; because the module is build on a transparent substrate that is the front side (or light- receiving side) of the module.

An alternative to the superstrate configuration is the substrate configuration of a thin-film solar module. The thin-film solar module of the substrate configuration comprises a substrate, on which have been deposited a (not necessarily transparent) back .electrode, a semiconductor layer (or light-absorbing layer) and a thin- film front electrode. The conductive layer that forms the back electrode can also be a set of interconnected conductive strips, and the same applies to the front electrode. The thin-film back electrode, the semiconductor layer and the thin-film front electrode form a layered structure. In case an electric contact has to be made between the back electrode and the outside world, a location is selected on the -conductive layer that forms the back electrode (which in this case includes removing the part of the semi-conductor layer that covers the location) , and a metallic strip is connected to the back electrode at the selected location. In case an electric contact has to be made between the front electrode and the outside world, a location is selected on the conductive layer forming the front electrode, and a metallic strip is connected to the front electrode at the selected location. When the connection or connections have been made, the layered structure is encapsulated by an embedding material that encloses the layered structure, and on top of the embedding material a transparent cover plate is secured that forms the light- receiving side of the module.

An example of the thin-film module of the substrate configuration is a CIS solar cell, in which the substrate is for example a glass or ceramic plate, or a metal or synthetic foil.

A further example for thin-film contacting originates from the field of electrochromic windows. Such a window is a layered structure consisting of two parallel glass substrates that are each provided with a thin-film transparent electrode on the surfaces facing each other. The transparent electrode can, for example consist of indium-tin oxide (ITO) . In between the glass substrates there is a so-called active zone that is filled for example with a layer of a metal oxide, a layer of a transparent solid electrolyte and an ionic reservoir layer. The optical transmission of the active zone can be influenced by applying a potential difference between the thin-film transparent electrodes. The change of transmission of the active zone is based upon the electrochromic effect of the metal oxide, which is for example M0O3 or WO3. In order to apply a potential difference to the thin- film electrodes, a metal strip has to be connected to the thin-film electrodes.

For thin-film components of the type described above long-time stability is required; especially the stability of all electric contacts at conditions of severe thermal cycling and/or humidity has to be ensured. For that purpose there exist a number of international quality tests which are described for example in the IEC 1646 standard for solar modules. Previous experience has shown that especially the damp heat test in accordance with IEC 1646 is critical and has to be executed prior to any new process step. In this damp heat test the electric resistance of the contact is measured before and after subjecting the thin-film component to a temperature of 85 °C and a relative humidity of 85% for 1 000 hours. In previous methods electrode layers in thin-film modules have been contacted in a two-step process after uncovering the conductive layer, by first pretinning the uncovered surface of the electrode layer and subsequent soldering a metal strip to the soldering joint in a conventional way. This method has the disadvantage that subsequent process steps, for example the encapsulation of the component, are limited to values of about 200 °C, which is the temperature at which the solder starts to melt. In addition, a two-step process is expensive, and automation of the process is difficult. Further, it has been found that the surfaces of the contacts become oxidised in presence of humidity, which results in an increased contact resistance. Still further, solders with high pull strengths contain heavy metals.

Alternatively, contacting methods are. used that employ conductive adhesives to fix the metal strips. However, long-time stability at the climate test conditions mentioned above is poor. It is known from the field of crystalline silicon solar cells to apply a metal strip to a printed electrode and to fix it by means of ultrasonic welding, see, for example USA patent specifications No. 3 620 847 and No. 4 957 133. However, the design and the manufacturing process for silicon solar cells and solar modules provide other preconditions for the use of the ultrasonic welding technology, especially because the contacting takes place with printed electrodes that consist of silver and aluminium flakes in a porous glass frit and have a structure and surface quality that are generally more easily to deal with and more readily to prepare for ultrasonic welding.

It is an object of the present invention to provide an advantageous alternative to the conventional soldering methods for providing a contact between a conductive layer of a thin-film component, in particular of a thin- film solar module, and a metal strip.

To this end the method for providing a contact between a conductive layer of a thin-film component, in particular of a thin-film solar module, and a metal strip according to the present invention comprises the steps of:

- selecting a location on the surface of the conductive layer of the thin-film component; placing the metal strip onto the selected location of the conductive layer; and

- ultrasonic welding the metal strip to the conductive layer, wherein the ultrasonic welding step comprises: placing a sonotrode with a predetermined contact force on the metal strip; oscillating the sonotrode with a predetermined oscillation amplitude and a predetermined oscillation frequency; and - _ -

lifting off the sonotrode from the thin-film component .

The invention further relates^' to a thin-film component, particularly thin-film solar module with at least one conductive layer and a metal strip which is at least spot-welded ultrasonically to the conductive layer.

The invention also relates to using an ultrasonic welding device for ultrasonic welding of a conductive layer of a thin-film component, in particular a thin-film solar module and a metal strip.

For thin-film solar modules, particularly CIS solar modules, these possibilities of producing and forming the electrodes and surfaces to be contacted do not prevail. The thin film layers to be contacted such as metals and metal oxides are high-density films deposited by sputtering or chemical vapour deposition. They have a much harder and smoother surface than screen-printed contacts used in crystalline silicon technology. For that reason, ultrasonic welding technology has never been taken into consideration or discussed for ultrasonic welding of thin-film solar modules.

With the invention, this approach is taken for the first time, and the conditions are demonstrated at which contacting of thin-film electron layers by means of ultrasonic welding can be successfully applied while avoiding the disadvantages in the prior art described above.

In the following the invention will be described in more detail on the basis of an exemplary embodiment with reference to the Figures, in which

Figure 1 shows schematically and not to scale the structure of a device for carrying out the method according to the invention; and

Figure 2 shows the chronological sequence of a contacting process according to the invention. Reference is now made to Figure 1. Figure 1 shows a holding device 1 for thin-film modules, for example a vacuum clamping table. On the holding device 1 a thin- film component 2 is positioned. The thin-film component is for example a thin-film solar cell of the substrate configuration, comprising a substrate 3, onto which the layered structure of the thin-film module are applied directly or indirectly. For the sake of clarity, the layered structure is not shown in detail in Figure 1, only the conductive layer 4 with a thickness of typically 50 to 5 000 nanometer, representing an electrode layer, is shown.

The first step of the method according to the present invention is selecting a location on the surface of the conductive layer 4 of the thin-film component where the contact with a metal strip has to be made. This step may include uncovering conductive layer 4 in the neighbourhood of the selected location.

Then a metal strip 5, for example an Al strip, is positioned on the conductive layer 4 at the selected location. Above the metal strip 5 a sonotrode 6 is positioned.

The sonotrode 6 can be a massive steel sonotrode or bond wedge sonotrode, and it has a rough working surface 7. The sonotrode 6 is mechanically connected to an ultrasonic generator (not shown) , which generator causes during normal operation the sonotrode 6 to oscillate in the direction of arrow A. During normal operation, the oscillating sonotrode applies vibratory energy at ultrasonic frequencies in a plane parallel to the welding.

To provide an ultrasonic weld, the sonotrode 6 is placed onto the metallic strip 5 that lays on the conductive layer 4 with a predetermined contact force. Then the sonotrode is oscillated along its longitudinal axis in the direction of arrow A. The oscillation of the sonotrode 6 has a predetermined oscillation frequency and predetermined oscillation amplitude. Through the roughness of the working surface 7 of the sonotrode 6, . the transmission of the oscillation into the metal strip 5 is possible in a way so that a relative motion occurs between the fixed thin-film module 2 and the metal strip 5, more precisely between the conductive layer 4 and the metal strip 5. The relative oscillating movement causes the removal of oxide layers on the facing surfaces of objects to be welded, and finally the both parts 4 and 5 are spot-welded together.

After completion of the weld, the sonotrode 6 is lifted away from the thin-film component 2. Applicant had investigated thoroughly the parameters that influence the quality of the ultrasonic weld, and it was found that the method of the present invention can be performed economically and can result in contacts having a very high quality, in particular in CIS ' solar modules, if the parameters, contact force, oscillation frequency and oscillation amplitude are selected in the following ranges .

The contact force is suitably between 30 and 600 N, and preferably between 200 and 400 N, the oscillation frequency is suitably between 10 and 80 kHz and preferably between 30 and 40 kHz, and the oscillation amplitude is suitably between 3 and 50 micrometer and preferably between 5 and 10 micrometer.

These values apply for ultrasonic generators having a power of between about 300 watts and 1 000 watts. In this context it has to be considered that ultrasonic generators with a higher power are not to be preferred unconditionally, because only about 10 to 20% of the energy is effective on the welding point and because it has been found that particularly the transient oscillation build-up at the beginning of the welding process in connection with the exertion of the contact force plays an important role in the execution of the method according to the invention. Therefore, the ultrasonic welding according to the invention is advantageously performed by first placing the sonotrode 6 onto the contact area, and then waiting until the predetermined contact force F_SOJ_J_ is reached, as shown in the time period T__ of the diagram in Figure 2.

After the time period T]_ the sonotrode is then oscillated, and it oscillates during the time period T2.

The time periods T_ and T2 can be predetermined, from past experience. Alternatively, the time periods can be ■ set during the ultrasonic welding process. In this case, Ti is set as the time when F_soj_]_ is reached, and T2 is set when a predetermined amount of vibratory energy has been introduced into the weld.

Suitably, T2 is between 0.02 and 2 s, and more suitably T2 is between 0.10 and 0.30 s.

Applicant had further found that for the contacting electrode layers of thin-film modules a sonotrode with a working surface having a regular surface structure is advantageously used. In particular, the use of a sonotrode with a working surface having regular and irregular pyramidal structure with the tips of the pyramids being spaced about 0.05 to 0.15 mm (tip to tip) has proven to be particularly suitable.

Examples for the electrode layers to be contacted are thin films from molybdenum, tin-, zinc-- or indium-tin oxide which are used in solar cells having absorbers consisting of chalcopyrite semiconductors (for example CuInSe2), amorphous silicon, CdTe, but also in dye sensitised solar cells, in LCD displays, electrochromic windows and heatable window panes. It has been proven by this invention that, in contrary to expectations, the surface quality of the areas 4 to be contacted does not have to meet highest requirements. And that the film deletion of coatings covering the electrode areas 4 can be achieved with comparatively low and therefore economically justifiable expense, particularly when using Al or Al/Cu metal strips, does not only provide satisfactory but unexpectedly good contacting results. The metal strips to be applied according to the invention are preferably Al metal strips which yield good results particularly in the contacting of ZnO and molybdenum electrode layers. The width of the strips is suitably between 1 and 5 mm and preferably between 2 and 3 mm, and the thickness is suitably between 20 and 400 micrometer and more suitably between 80 and 300 micrometer.

The surface of the metal strips may be provided with an anti-adhesive coating, which prevents that the metal strip adheres permanently to the surface of the sonotrode. With Al metal strips, the anti-adhesive coating may be provided by an aluminium oxide layer. The use of an Al/Cu metal strip, where an Al metal strip and a Cu metal strip are superposed and connected to one another (rolled), is also suitable. The sonotrode then is in contact with the Cu side of the Al/Cu strip so that the adhesion to the sonotrode surface is clearly reduced, while the Al side of the strip faces the molybdenum electrode and provides good contacting accordingly. The method according to the invention is characterized first of all by the fact that a one-step process may be realised with it, which leads to short cycle times during the contacting of electrode layers for thin-film modules. The method according to the invention provides a metallic bond between the electrode layer and - li ¬

the metal strip without any additional material. The bond is insensitive to temperature, so that no limitations to following process steps occur. Even in warm and humid environments no increase of the contact resistances can be detected. Finally, a bond with very high pull-off forces can be achieved with the method according to the invention.

Claims

C L A I M S

1. A method for providing a contact between a conductive layer. (4) of a thin-film component (2), in particular of a thin-film solar module, and a metal strip (5) comprising the steps of: - selecting a location on the surface of the conductive layer (4) of the thin-film component (2); placing the metal strip (5) onto the selected location of the conductive layer (4); and ultrasonic welding the metal strip (5) to the conductive layer (4), wherein the ultrasonic welding step comprises: placing a sonotrode (6) with a predetermined contact force on the metal strip (5) ; oscillating the sonotrode (6) with a predetermined oscillation amplitude and a predetermined oscillation frequency; and

- ^' lifting off the sonotrode (6) from the thin-film component (2) .

2. The method according to claim 1, characterized in that the contact force is between 30 and 600 N, and preferably between 200 and 400 N.

3. The method according to claim 1 or 2, characterized in that the oscillation frequency is between 10 and

80 kHz, and preferably between 30 and 40 kHz.

4. The method according to any one of the claims 1-3, characterized in that the oscillation amplitude is between 3 and 50 micrometer, and preferably between 5 and

10 micrometer.

5. The method according to any one of the claims 1-4, characterized in that the step of oscillating the sonotrode (6) is executed for a predetermined time period (T2) •

6. The method according to claim 5, characterized in that the time period (T2) of the oscillation of the sonotrode (6) is between 0.02 and 2 s, and preferably between 0.10 and 0.30 s.

7. The method according to any one of the claims 1-6, characterized in that the metal strip is an Al strip or an Al/Cu strip.

8. The method according to any one of the claims 1-7, characterized in that the conductive layer (4) is a molybdenum layer or a tin-, zinc- or indium-tin oxide layer.

9. A thin-film component, particularly thin-film solar module with at least one conductive layer (4) and a metal strip (5) which is at least spot-welded ultrasonically to the conductive layer (4) .

10. The thin-film component according to claim 9, characterized in that the conductive layer is an electrode (4) of the thin-film component^".

11. The thin-film component according to claim 9 or 10, characterized in that the conductive layer (4) consists of molybdenum, of doped zinc-, tin- or indium-tin oxide.

12. The thin-film component according to any one of the claims 9-11, characterized in that the metal strip (5) is an Al strip or an Al/Cu strip.

13. Use of an ultrasonic welding device (1, 6) for ultrasonic welding of a conductive layer (4) of a thin- film component (2), in particular a thin-film solar module and a metal strip (5) .

14. Use according to claim 13, characterized in that the conductive layer (4) consists of molybdenum,' of doped zinc-, tin- or indium-tin oxide.

15. Use according to claim 13 or 14, characterized in that the metal strip (5) is an Al strip or an Al/Cu strip.

16. Use according to any one of the claims 13-15, characterized in that the sonotrode (6) of the ultrasonic welding device has a working surface (7) with a pyramidal structure.

17. Use according to claim 16, characterized in that the pyramids of the pyramidal structure of the working surface (7) are spaced from one another in a distance between 0.35 and 0.15 mm (tip to tip).