WO2009090172A2

WO2009090172A2 - Apparatus and method for manufacturing fast and low cost electrical contacts to solar modules

Info

Publication number: WO2009090172A2
Application number: PCT/EP2009/050327
Authority: WO
Inventors: Matthias Krieger
Original assignee: Oerlikon Trading Ag, Trübbach
Priority date: 2008-01-14
Filing date: 2009-01-13
Publication date: 2009-07-23
Also published as: WO2009090172A3; TW200934012A

Abstract

The present invention relates to a method for manufacturing fast and low cost electrical contacts to solar modules. It further relates to a corresponding apparatus and to the use of a special adhesive. To shorten processing time and to save energy reducing stress to a solar module during fabrication this invention suggests to establish an electrical contact with the back electrode (5) of the cell (1) using a highly dedicated fast curing conductive adhesive (13) in combination with a specially designed locally active induction heating coil (11) parallel to said electrical contact (7) on the back electrode (5).

Description

Apparatus and method for manufacturing fast and low cost electrical contacts to solar modules

Field of the invention

This invention relates to the manufacturing of photovoltaic cells and more precisely to an apparatus and a method for manufacturing fast and low cost electrical contacts to thin film solar modules, where this invention addresses especially to the steps of establishing an electrical contact with the so called back electrode of the cell, which is usually comprised of a transparent conductive oxide layer, a so called TCO layer.

Background of the invention

A thin film solar cell 1 according to Prior Art is basically construed as shown in Figure Ia. A substrate (e. g. glass, plastic) 2, transparent for a relevant part of the spectrum of solar radiation (indicated by arrows 6) is being used as carrier for a photovoltaic layer stack. A first electrode 3 composed of a transparent conductive oxide (e. g. ITO indium tin oxide, ZnO or alike) is applied on said substrate 2. A photoactive layer 4 comprising a p-i-n layer stack (p-doped, intrinsic, n-doped semiconductor material, e. g. silicon) is deposited on said front electrode 3 and provides for the photoelectric effect. A back electrode 5, again a suitable TCO, completes the main structural elements of a PV (photovoltaic) cell 1. Back reflector layers, encapsulation and so forth have been omitted. In order to close the electric circuit and to allow the use of the PV cell, (at least) two contact points or stripes have to be arranged on the before mentioned electrode layers. They are indicated by 7 and 8 in Figure Ia respectively.

Figure Ib shows a known type of thin film solar module 1 in a top view to give an example for the dimensions for a cell of this type. A thin film solar module 20 exhibits individual cells 23 that are electrically operatively connected in series. This module 20 exhibits contact stripes 21, 22 arranged adjacent to and electrically connected to a respective contact area of one of said cells 23, preferably the first and/or last of said serially connected cells. Length L and width W of said solar modules may vary, common sizes (W x L) range from 0.3m x 0.5m via 1.1m x 1.3m to 2.2m x 2.6m. The length of the contact stripes 21, 22 is in the order of L or W.

The process establishing reliable electric contact 7, 8 to a conducting thin film electrode 3, 5 of a thin film solar cell/module 1 is an important and crucial step in connecting the photoactive solar cell with the outside world. The electrical conduction and mechanical properties of this contact are vital for long lifetime and reliable electrical performance of a solar module.

Traditionally, such contacts are either soldered with rather hot process steps or glued with silver-filled conductive adhesives. However, soldering causes thermal stress to the substrate 2. Further, silver-filled paste can be applied on the respective electrode layer by a screen printing process and later on hardened by a thermal treatment step. WO 2005/077062 A2 contains an overview of important parts of the state of the art and discloses techniques and electroplated items comprising electrically conductive polymers introducing application of directly electro-palatable resins DER in a number of compositions.

Although the advantages of adhesive based contacts over soldering are established, there is room for improvement when focus is put on high speed in-line manufacturing technology. Most commonly known conductive adhesives need to be cured for reliable mechanical adhesion and electrical conductivity. However, the required curing times at moderate temperatures below 160⁰C often exceed several minutes, up to half an hour. These curing times make it impossible to match this process step with typical tact times of in-line manufacturing flows often found in encapsulation manufacturing steps in solar module production. To accommodate for such long times, large footprint hot air circulation curing ovens need to be installed with integrated buffer capacity. The facility requirements for electricity and air convection are typically quite substantial and offer room for cost reduction.

Problems known in the art

Essentially two aspects of traditional curing processes for silver- conductive adhesives are encountered. First, the equipment required to match the long curing time with encapsulation manufacturing tact times must contain substrate buffer space for up to e. g. 20 substrates. Such substrates have, especially in large-area thin- film-PV cell production a size of several square meters. Thus the equipment requires enormous footprint and electricity consumption to heat up a respective large volume is substantial. Secondly, to save the expensive curing equipment at this process stage, the potential fixing of contact strips on wet adhesive by tape is possible, but eliminates the possibility to perform reliable I/V measurement for quality check before the following white paint screen printing process. Said white paint layer serves as a back reflector and protection layer. A rework of the module after screen printing is no longer possible. Thus one faces the unsatisfactory choice of either investing too much in curing equipment, eventually even two identical equipment sets for thermal treatment of both contact curing and white paint curing, with cooling of the modules in between process steps. Alternatively one would have to eliminate the possibility of adding a true quality checkpoint before screen printing. The risk of introducing yield loss in the crucial contacting process step without room for improvement is substantial. A solution for an optimized contacting process at lower cost, while offering a true I/V quality checkpoint before screen printing is thus highly desirable.

Brief description of the drawings

Figure Ia shows the basic elements of a pin-structure plus front- and back electrode of a PV cell in a side view. Figure Ib shows a know type of thin film solar module in a top view. Figure 2 shows an embodiment of the invention with an induction coil and a substrate to be treated and prepared with a structure to form a contact stripe. Figure 3 shows a first embodiment of the invention suitable for mass production processes. Figure 4 shows a second embodiment of the invention suitable for mass production processes preferably of large scale thin film solar modules.

Solution according to the invention

The invention replaces the traditional hot air circulation curing process for established silver-filled adhesives in a batch type oven with buffer capacity by a true in-line curing solution at the speed of the encapsulation manufacturing line of less than one minute. In addition, the heat applied according to the state of the art may reduce the electrical efficiency and/or life span of a solar cell, too .

To obtain such short cycle times during production at a better quality of the solar cells produced, a highly dedicated fast curing conductive adhesive is used in combination with a specially designed locally active induction heating coil parallel to the Cu-Sn contact strip. Applicable adhesives usually contain silver particles (>50%) plus acrylates and cure by the effect of heat. An example is the experimental glue adhesive XCA 80229 or XCA80239 manufactured by Emerson & Cuming, said adhesive exhibiting a specific weight of 3,8- 4,0 g/cm³ and a viscosity of 30- 40 Pa . s .

For adhesive bonding of a metal member to a non metallic support magnetic induction heating has been introduced by e.g. GB 2 064 506 A in the automobile sector for the application for fixing rear mirror to the windscreen. Induction heating of a conducting metal strip can be tuned and closed loop temperature controlled with a pyrometer very effectively and rapidly, without substantial heating of the float glass substrates to avoid thermal stress to a major extend. It is the unique combination of said very rapidly curing adhesive at moderate temperature of only 150⁰C in less than 15 seconds and the highly effective local induction heating process that allows the fabrication of a reliable electrical contact in such short time that heating of the entire float glass substrate area is no longer necessary, thus saving time and electrical energy. This opens the door for true in-line manufacturing processing at very high tact rates of less than one minute. The requirement for a buffer capacity to accommodate for production flow is completely eliminated. Thus, a production process according to the present invention combines the advantages of contact-free production known from autoclaves with short application time steps of soldering processes .

A sketch of the induction coil and substrate geometry for smaller substrate size is given in Figure 2: The drawing is not to scale and presented to make the underlying principle clear. An expert will easily adopt and scale up said principle to the respective application, irrespective of substrate size. A substrate 10, e. g. a finished PV cell on float glass including front- and back-electrode, is being positioned adjacent to an induction coil 11. Due to the nature of the application this induction coil is preferably designed in an elongated, oval form for improved treatment of elongated contact strips. The distance between induction coil 11 and substrate 10 can be adjusted to be a few mm, the substrate may be placed on a distance piece made from an insulating material, e. g. a temperature resistant plastic. Reference 14 marks the position of the coil below the substrate 10. A metal contact strip 12 to be affixed (contacted) to the substrate 10 is being placed on a layer of adhesive 13 of the afore mentioned kind. The contact strip 12 preferably comprises Cu- Sn. The extent of adhesive applied to substrate 10 may vary due to technical needs, the amount shown in figure 2 is just exemplary. 15 denote the contacts of the induction coil 11 to a respective power supply including further control units not shown in here. An apparatus according to the invention therefore comprises an induction coil 11 in preferably elongated form (flattened or oblate oval form, the coil having preferably 1-3 windings essentially framing said oval), a mount for placing a substrate 10 adjacent to said substrate 10 in close relationship. During operation said mount carries a substrate 10, on which an adhesive 13 is being applied plus a contact strip 12.

In a method for manufacturing a contact area on a substrate 10 the contact strip 12 is being heated by said induction coil 11 and the contact strip 12 is being contacted to the electrode layer via the electrically conductive adhesive.

High frequency currents connected to the coil 11 induce eddy currents through a layer of substrate 2, 10 in the contact strip 12 which results in a locally restricted heating of said contact strip 12. The heating of the contact strip 12 again initializes the curing or linking of the adhesive via polymerization. This way the thermal load to the substrate 2, 10 can be focused and delimited exactly to the area where it is needed reducing thermal and mechanical stress to the substrate 2, 10, too.

Further ohmic contact stripes and additional contact fingers 24 consisting of electrically conductive epoxy adhesive may be located of the thin film solar stack in screen printing of metallic thick film pastes or ink in thick film printing or even a spraying process. Even without metal stripes or wires the contents of metal powder or metal flakes within a electrically conductive epoxy adhesive are sufficient for curing it in a single step of magnetic induction heating. Thus, on one substrate 2 a number of individual cells 23 can be e.g. connected in series easily using the cured conductive epoxy adhesive now forming additional contact fingers 24 at a predefined Ohmic resistivity, see Figure 2 for an example to be processed using a novel method according to this invention. Figure 3 shows a first embodiment of the invention suitable for mass production processes using a continuous kiln 17 adapted to the present invention. This curing apparatus 17 is no longer a known type of electrically heated conveyor furnace. It comprises at least one coil 11 as shown in Figure 2 that radiates a high frequency electro magnetic field causing eddy currents in the assembly adjacent to the coil 11 to cure the adhesive 13. In the meantime the whole assembly on the substrate 2, 10 is conveyed at a constant speed v to bring a further region 16 covered by uncured adhesive 13 next under the coil 11. The substrate 10 used may be a large strip processed as some kind of a multiple printed panel or there may be conveyed a number of separated substrates 2 cut to an appropriate size already.

The speed v and the width w of the coil 11 are determined by a person skilled in the art in that way to assure sufficient heating of the adhesive 13 over a predetermined time. In a further embodiment a detector (not shown in here) is used for detecting a region 16 to electrically switch on and off the coil 11. In an alternative embodiment the pattern of the regions 16 on the substrate 2 is known. After adjusting once a metered way length causes the switching on and off of the coil 11.

Figure 4 shows a second embodiment of the invention suitable for mass production processes preferably of large scale thin film solar modules in a more stationary way, that is a production step causing a minimum of motion to the substrate 10. The appliance 18 shown contains means 25 for positioning the substrate 10 in sufficient accuracy on a kind of a desk 26 for applying the adhesive 13 and subsequent a metal stripe 12 to form an electrical conductive connection or even the electrical contacts after curing. Not further shown in detail in Figure 4, a flap gate 27 is provided that covers said assembly, where the flap gate 27 in this embodiment comprises two coils 11, 11' of the type shown in Figure 2. With the exact positioning of the adhesive 13 and the metal contact stripes 12 on the substrate 10, the closing of the flap gate 27 locates the coils 11, 11' into a position designed for short term induction curing the adhesive 13 creating an electrical conductive connection of the electrode to an electrical contact stripe 12 respectively. As the assembly in this step takes place without further moving the solar module this embodiment is designed for producing large scale solar panels of W x L up to 2.2m x 2.6m to ease accurate application on the prepared substrate 10 and the handling of all items concerned in this production step as well.

Further advantages of the invention

The method for manufacturing a contact area according to the invention can be employed in a much wider range than only thin film PV module encapsulation process. The method is applicable for many delicate optical elements that require high reliability electrical contacts to the outside world can be realized with the same manufacturing technology. Such manufacturing steps can open the door for miniaturization of thermally sensitive packages combining glass, and possibly also injection molded temperature sensitive polymers.

List of reference nummerals :

1 thin film solar cell

2 substrate

3 first electrode

4 photovoltaic layer stack having a p-i-n-structure

5 second electrode / back electrode

6 solar radiation / light

7 electrical contact point or stripe arranged on the before mentioned electrode layer

8 electrical contact point or stripe arranged on the before mentioned electrode layer

9

10 substrate

11 induction coil

12 contact strip

13 adhesive

14 Reference marking the position of the coil 11 below the substrate 10

15 contact of the induction coil 11

16 region covered with uncured adhesive 13

17 continuous kiln

18 stationary appliance 19

20 thin film solar module

21 contact stripe

22 contact stripe

23 individual cell

24 contact finger

25 means for positioning of the substrate 10 on the desk 26

26 Desk

27 flap gate

L length of a solar module

W width of a solar module v constant speed

Claims

1. Method for manufacturing fast and low cost electrical contacts to solar modules, characterized in establishing an electrical contact with the back electrode (5) of the cell (1) using a highly dedicated fast curing conductive adhesive (13) in combination with a specially designed locally active induction heating coil (11) parallel to said electrical contact (7) on the back electrode (5) .

2. Method according to claim 1, characterized in that a Cu-Sn contact strip (12) is used as electrical contact.

3. Method according to claim 1, characterized in that the adhesive (13) contains silver particles at a range of more than 50 % plus acrylates and cures by the effect of heat.

4. Method according to claim 1, characterized in that induction heating of a conducting metal strip (12) is tuned and closed loop temperature controlled using a pyrometer.

5. Method according to claim 1, characterized in that rapidly curing the adhesive is carried out at moderate temperature of about 150⁰C in less than 10 - 20 seconds, preferably in 15 seconds .

6. Method according to claim 1, characterized in that on one substrate (2) at least two individual cells (23) are electrically connected by applying a region (16) covered by uncured adhesive (13) and curing the conductive adhesive (13) to form a contact finger (24) .

7. Method according to claim 1, characterized in that an assembly of a substrate (2) and at least one region (16) covered by uncured adhesive (13) is conveyed at a constant speed (v) to bring the region (16) next under the coil (11) for curing.

8. Method according to the preceding claim, characterized in that a detector is used for detecting a region (16) to electrically switch on and off the coil (11) .

9. Method according to claim 7, characterized in that the pattern of the regions (16) on the substrate (2, 10) is known and after adjusting once a metered way length causes the switching on and off of the coil (11) .

10. Method according to claim 1, characterized in that the substrate (10) is positioned in sufficient accuracy, the adhesive (13) is applied and a flap gate 27 covers said assembly, where the flap gate 27 in this embodiment comprises two coils (11, 11') emitting high frequency electro magnetic radiation for curing the adhesive (13) .

11. Apparatus for manufacturing fast and low cost electrical contacts to solar modules, where a solar module (1) comprises a first electrode (3) applied on a substrate (2), a photoactive layer (4) comprising silicon and a back electrode (5) , where at least two contact points or stripes are arranged on said electrode layers (3, 5) characterized in comprising at least one induction coil (11) for short term induction curing an adhesive (13), that is disposed on at least one of the electrode layers (3, 5) creating an electrical conductive connection to an electrical contact stripe (12) .

12. Apparatus according to claim 11, characterized in that it is a continuous kiln (17) comprising at least one coil (11) radiating a high frequency electro magnetic field causing eddy currents adjacent to the coil (11) to cure the adhesive (13) while the substrate (2) is conveyed at a speed (v) .

13. Apparatus according to claim 11, further comprising means (25) for positioning the substrate (2, 10) in sufficient accuracy on a desk (26) for applying the adhesive (13) to form a electrical conductive connection after curing, where a flap gate (27) is provided that covers said assembly, and the flap gate (27) comprises at least one coil (11, 11') for short term induction curing.

14. Use of a fast curing conductive adhesive in combination with a specially designed locally active induction heating coil (11) for curing by the effect of heat caused by induced eddy currents to achieve an ohmic contact stripe or an additional contact or a contact finger (24) or an electrical conductive connection to an metal contact stripe (12) .