Patent Application
20150263183
The present invention is directed to solar cell interconnectors with at least two layers and to a solar cell array of at least two solar cells electrically interconnected with said interconnectors. The invention is further directed to a method of interconnecting solar cells of such a solar cell array with said interconnectors.
Solar cells are the primary source of electrical power generation in space. For realizing the required operational voltage, numerous solar cells are typically connected in series, called a “string”.
Furthermore, in space applications, the strings are usually bonded with a silicon adhesive 6 to a sandwich panel 5 with carbon fiber facesheet and an aluminum honeycomb core with a polymer frontside insulation. For radiation protection, a cover glass 9, 9′ is bonded to the solar cell 1, 1′ with transparent silicone adhesive 8. These substrates have a large thermal expansion mismatch relative to the solar cell 1, 1′. During eclipse phases with temperature fluctuations up to 200° C., the gap 7 between two solar cells 1, 1′, inside one string, varies by several 10 μm, which leads to significant thermo-mechanical stress in the interconnector material. To cope with this, it is state of the art to use 10 to 30 μm thick metal foils like Ag or Au, both of which might have some additives for strengthening. The advantage of these two materials is, next to the good electrical conductivity, that they can be welded directly onto the Ag cell contacts. However, for such high conductive materials, the actual thickness for interconnector application is higher than required from an electrical point of view. Depending on interconnector geometry and welding area, only a few micrometers would be sufficient. This higher thickness eases the handling and production processes, however decreases the overall lifetime, as the thickness contributes linearly to the resulting stress, which is largest in the highest, loop-like portion of the interconnector.
Other materials often used as interconnector materials are Mo, a nickel-cobalt ferrous alloy (e.g. known under the trade names KOVAR) or a nickel-iron alloy (e.g. known under the trade name INVAR). The low coefficient of thermal expansion of these materials reduces the contribution of the thermal expansion of the interconnector to the overall gap variation making them good candidates for missions with several 10.000 eclipse phases, i.e. in a low earth orbit. As these materials cannot be welded or soldered directly to the solar cell contacts, however, they are additionally plated with several μm of Ag or Au, which makes the manufacturing process sensitive and expensive.
The second main requirement that an ideal interconnector has to fulfil is a high resistance against erosion by energetic Xe ions. These ions, with energies in the range of some 100 eV, are emitted by electric thrusters used for station keeping and increasingly also for the orbit rising of satellites. In this energy range all metallic materials exhibit a non-negligible sputtering yield, defined as the number of sputtered atoms per incident ion. It is known from the non-patent literature “M. Tartz, T. Heyn, C. Bundesmann, C. Zimmermann, and H. Neumann, Sputter yields of Mo, Ti, W, Al, Ag under xenon ion incidence, Eur. Phys. J. D 61, 587-592 (2011)” that typical values are ranging from 0.12 for 200 eV Xe impinging on Ti, to 0.3 for the combination Al/Xe to 1.3 for Ag/Xe. This sputtering yield further increases at non-normal angles of incidence.
The geometric shape of the interconnector is designed to minimize internal stresses during cycling and therefore the interconnectors 2 are usually S-shaped, which makes them significantly protruding above the cell cover glasses 9, 9′. Especially this protruding part is exposed to the Xe ions and presents itself at a wide range of incidence angles. This will result in a non-homogeneous erosion profile and detrimental stress concentration. Of course, it is possible to select interconnector materials like Al or Ti, which have a lower sputter yield than Ag, for example. But even in this case, significant material is sputtered away and re-deposition of material at solar cell edges can present a reliability risk by providing an electric shunt path.
For all materials an increase in interconnector thickness is thus no viable option and in addition would increase the thermo-mechanical stresses and reduce fatigue life, as mentioned before. In conclusion, all possible metallic interconnector materials only have a limited tolerance to Xe erosion before the reliability of the solar array is negatively affected.
Accordingly, there is still a demand for highly electrical conductive foil materials having an improved thermo-mechanical cycling stability combined with an improved Xe ion erosion resistance.
EP 0 758 145 A2 describes a connection process for manufacturing a circuit module by performing connection between an external electrode of a component and a conductor of a transparent substrate by applying a laser beam through the substrate heating the connection spot to create phase transition and/or diffusion.
EP 1 305 988 A1 discloses a method for producing an electrical connection between a flexible circuit board with single sided access and a metallic contact partner comprising the steps of:
U.S. Pat. No. 6,983,539 B2 discloses a method of forming an electrical connection between a TAB (tape automated bonding) circuit and electrical contact bumps, the TAB circuit including a polymeric base and electrical leads formed on the polymeric base, said method comprising the steps of:
Thus, it is a first object of the present invention to provide a solar cell interconnector having an improved thermo-mechanical cycling stability with high electrical conductivity and being Xe ion erosion resistant as well as handling friendly. It is a further object of the invention to provide a high reliable solar cell array with said solar cell interconnectors. It is still another object of the invention to provide a method of interconnecting solar cells with said interconnectors.
The first object of the present invention is achieved by the solar cell connector with the features of claim 1.
The solar cell interconnector according to the invention comprises at least two layers, a first, substrate layer and a second, electrically conductive layer. The first, substrate layer consists of a polymeric material and the second, electrically conductive layer consists of a metal material deposited on the substrate layer.
The present invention comprises a new bi-layer sandwich material for the use as solar cell interconnector particularly for space applications but not necessarily limited to space. This sandwich material is a combination of a substrate layer, e.g. a flexible thin polymer foil, and an electrically conductive layer placed on said substrate layer, e.g. a conductive metal film.
The solar cell interconnector according to claim 1 provides superior resistance to erosion via sputtering, for example by Xe ions, and thermo-mechanical fatigue, while at the same time lending itself to flexible, laser based interconnection and structuring methods. The basic idea of the invention is thus to combine the elasticity features of the polymeric material of the substrate layer with the electrical conductivity features of the conductive layer in one single device.
It is advantageous when the polymeric material of the substrate layer comprises high yield strength properties, in particular a yield strength >100 MPa.
In a preferred embodiment the polymeric material of the substrate layer comprises a low Young's modulus, in particular a Young's modulus <10 MPa.
In yet another preferred embodiment the material, preferably a metal material, of the electrically conductive layer comprises a low electrical resistivity, in particular a resistivity less than 5×10−6 Ohm. In other words, the electrically conductive layer has sufficient conductivity.
It is furthermore advantageous when the substrate layer is resistant against particle radiation and/or ultraviolet light radiation and/or vacuum exposure. This will increase reliability as well as durability of the interconnector. The resistance against particle radiation includes resistance against impinging ions, e.g. Xe ions emitted from electrical propulsion thrusters.
Preferably, the material of the electrically conductive layer is selected such that the thermal expansion coefficient of the electrically conductive layer is substantially the same as the thermal expansion coefficient of the substrate.
In a preferred embodiment of the inventive interconnector the material of the substrate layer is polyimide or ETFE. At the application date of this patent application suitable polyimides were, for example, known under the trade names “KAPTON” from DuPont or “UPILEX” from UBE Industries. A preferred polyimide is 1.1 poly(4,4′-oxydiphenylene-pyromellitimide) [trade names “KAPTON HN” or “KAPTON VN”]. Another preferred polyimide is 3,3′,4,4′-biphenyl tetracarboxylic dianhydride/1,4-phenylenediamine (BPDA-PDA) [trade name “UPILEX-S”]. Another suitable polymer material for the solar cell interconnectors of the present application is ethylene tetrafluoroethylene (ETFE) [e.g. known under the trade name “TEFZEL” from DuPont].
The material of the electrically conductive layer is preferably Ag or Au or Au88/Ge12 or Al.
The substrate layer thickness is, in an advantageous embodiment of the invention, within a range of 5 to 50 μm, preferably within a range of 10 to 25 μm, most preferably 12.5 μm.
The electrically conductive layer thickness is preferably within a range of 1 to 10 μm, more preferably within a range of 1 to 5 μm.
It is very advantageous when the electrically conductive layer thickness is selected in order to minimize stresses in the electrically conductive layer for a given bending radius. This will enhance the durability and reliability of the interconnector.
One of a lot of advantageous practical examples is designed such that the electrically conductive layer thickness is 1.9 μm and the substrate layer thickness is 12.5 μm.
The object of the present invention directed to a solar cell array is achieved by a solar cell array with the features of claim 13.
This solar cell array according to the present invention is formed of at least two solar cells electrically interconnected with solar cell interconnectors according to the present invention, the solar cells each having a first surface exposed to an ion source, wherein the solar cell interconnectors are mounted to the solar cells in such a way that the respective substrate layer of each interconnector is directed to the ion source and forms thus an outer exposed surface area shielding the respective electrically conductive layer underneath from erosion due to sputtering. Such ions can, for example, be Xe ions from electric propulsion thrusters.
In another embodiment, it is advantageous when the solar cell interconnectors are embedded in silicone.
A method of interconnecting solar cells of a solar cell array according to the invention is defined in claim 15.
This inventive method comprises the steps of
The present invention provides a mechanical cycling stable, handling friendly and particularly Xe ion erosion resistant solar cell interconnector, which can be welded directly to the solar cell contacts.
The invention is hereinafter described by example with reference to the drawings. In these drawings
Thus, the layer structure of a practical example consists of a polyimide film layer 20 coated with a conductive metal layer 21. It is, for example, a 7 μm to 25 μm thick “KAPTON HN” or “KAPTON VN” foil coated with 1 μm to 5 μm Ag, Au, Au88/Ge12 or Al, but is not limited neither to the stated thicknesses, nor to the stated materials and can also include a dedicated undercoating for the chosen conductive material as shown exemplarily in
Contrary to most fatigue test setups, the variation of the inter solar cell gap on a solar array due to temperature represents a strain controlled setup. For a given strain ε the resulting stress σ is given by Eε, where E denotes the materials Young's modulus. The Young's modulus of “KAPTON HN”, as an example of polyimide, and different metallic materials that are commonly used as solar cell interconnectors are compared in Table 1. It can be seen that the Young's modulus of “KAPTON” is a factor 28 times lower than the next lowest value for a metallic material, in this case Ag. The stresses in the interconnector are reduced by the same amount. The yield strength, on the other hand, of “KAPTON” is 75 MPa and thus only approximately a factor 4 lower than the yield strength of most metals with low Young's modulus. In summary while the stresses in a metal interconnector can result in plastic deformation and consequently in a fatigue behaviour in the low cycle fatigue range, the ratio yield strength/Young's modulus of polyimide is one order of magnitude lower, and fatigue during typical space missions is of no concern.
1) Dupont Kapton HN Data Sheet
2) ASM International Metals Handbook, Volume 2, 10th edition
By creating a bilayer system according to
In order to illustrate the reduction in stress levels, a beam with rectangular cross section is chosen as a model system, composed of this bilayer system. It is fixed at one end and deflected at the other end by a fixed amount such that bending radius of the beam remains the same, independent of its stiffness.
The stresses in a bilayer beam of such composition can be estimated by an equivalent width technique as schematically illustrated in
The location y of the stress free neutral fiber N.A. is calculated as
The maximum stress in the Ag layer for a given bending radius occurs at its outermost surface and depends linearly on its distance from the neutral fiber N.A. For any given “KAPTON” thickness DKapton, there is therefore an optimum Ag thickness x. For very small silver thicknesses, N.A. stays essentially in the middle of the “KAPTON” layer. Increasing x pushes N.A. closer to the silver layer. A certain point, however, if N.A. is already close to the silver layer increasing the silver layer thickness further only starts to increase the distance from the neutral fibre again. The optimum thickness x* is derived as
and the position of N.A. is then:
For the example of a 12.5 μm thick “KAPTON” foil and the EAg/EKapton ratio R of 28, the optimum Ag thickness is 1.9 μm and the neutral fibre is located 12.1 μm away from the bottom surface. The maximum stress is the same as in a 2*(1.9+0.4) μm=4.6 μm pure Ag beam and thus more than 2.5 times lower than in a 12.5 μm pure Ag beam. Since N.A. is pushed almost to the surface of the “KAPTON” layer, the maximum stresses in the “KAPTON” layer are essentially doubled, but based on the argument above, that is still easily tolerable by the polyimide. In its application as a solar cell interconnector, the reduced stiffness of the bilayer interconnector compared to a pure metal one, in addition results in a larger portion of the interconnector being able to take up the stress due to inter cell gap variation.
In terms of electrical resistivity this layer thickness is more than sufficient. For an interconnector of a width w=5 mm, and a length l=5 mm, featuring x=2 μm of Ag with a specific resistance ρ of 2×10−8 Ωm a resistance R, R=ρl/wx, of 10 mΩ results. For a cell equipped with three interconnectors of this type and delivering 0.5 A of current, a voltage drop of less than 2 mV results.
The bilayer interconnector according to the present invention has not only superior fatigue resistance, but can also tolerate higher Xe ion fluences. The sputtering yield for “KAPTON” is a factor 4 lower than for Aluminum. According to the non-patent literature: “A. P. Yalin, B. Rubin, S. R. Domingue, Z. Glueckert, and J. D. Williams, Differential Sputter Yields Of Boron Nitride, Quartz, and Kapton Due to Low Energy Xe+ Bombardment, 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit (doi: 10.2514/6.2007-5314)” the sputter yield for Kapton at 250 eV and perpendicular incidence is 0.008 mm3/C. In contrast the sputter yield in Al of 0.3 atoms/ion translates into 0.03 mm3/C. Therefore the “KAPTON” substrate layer can provide efficient protection to the metalized layer from Xe ion erosion, provided the interconnector is oriented appropriately.
The configuration in the example of
In the example of
On most solar cells, a top contact design can be provided easily according to state of the art methods, as for example described in EP 1 693 899 A2. Contacts 122, 122′ of both polarities are then on the front surface of the cell. In this case the interconnection is straightforward as shown in
But also solar cells with contacts on top and bottom can be safely interconnected with the solar cell interconnector according to the invention. Merely the interconnector has to be turned upside down at contact spot 223 in the protected area underneath the cell, as shown in
The upside down transfer of the solar cell interconnector 202 is done by folding the interconnector under 45°, which brings the interconnector metal layer 221 in contact with the cell metallisation layer 204, and the “KAPTON” substrate layer 220 to the front. The same connecting technology, i.e. placing welding spots 235 onto the interconnector 202, can be used as on the cell front side contacts.
The 45° folding can be performed in two ways. By folding the metal layer 221 side onto the metal layer side (lower example in
Due to the superior fatigue resistance of the solar cell interconnector according to the present invention, also an encapsulated configuration becomes possible, as illustrated in
In these examples the entire gap 325, 425 between two solar cells 301, 301′; 401, 401′ is filled with a filling material, for example with a lay down silicone adhesive with which the solar cells are mounted to a substructure 305, 405 or with a cover glass adhesive with which the cover glasses are mounted to the solar cells. In these examples the whole solar cell interconnectors 302, 402 are embedded in the filling material including the interconnector 302, 402. In this example the electric connection of the metal layer 321, 421 of the interconnector 302, 402 is carried out as described with respect to
In order to aid in stress relaxation, the solar cell interconnector can also be patterned in the gap along an axis parallel to the welding pad, which increases its effective length. The encapsulated configuration has the advantage that the silicone adhesive provides an effective protection against atomic oxygen found in low earth orbit applications.
In order for the material combination of the interconnector according to the present invention to become suitable for use as a solar cell interconnector, an appropriate connection method of the metallic electrically conductive layer to the metallised contact pads on the solar cell is required. Laser transmission micro joining, which makes use of the optical characteristics of polyimide, is a suitable state of the art method to achieve this.
Oscillations that can be found from 800 nm to higher wavelengths are due to film thickness oscillations. The step inside the curves that can be found directly at 800 nm is a measurement artefact from the setup which was used. From the measurement it can be concluded, that for infrared wavelengths >700 nm nearly no absorption takes place inside the measured “KAPTON” foil (below 5%). About 85% of incident infrared light is transmitted through the foil.
If the foil is coated with a suitable metal like it is the case with the interconnector according to this invention as shown in
These measurements demonstrate that the solar cell interconnector according to the present invention is compatible with laser transmission welding. The suitability of this method for joining a metallised foil-substrate-combination to another metal is described in the prior art as mentioned earlier.
A necessary precondition that the solar cell interconnector according to the present invention can be used according to an intended purpose in space is its compatibility with the space environment. “KAPTON” and other polyimide films, like e.g. “UPILEX-S”, have been already widely used in space and are therefore well characterized, mainly in applications like thermal shields or electrically insulating foils. From this experience the compatibility of “KAPTON” with most space environmental effects can be concluded. The most degrading effect in space is charged particle radiation (protons, electrons). In the interconnector system “KAPTON” takes the role of mechanically stressed supporting film. Therefore it is vital to ensure that even after being exposed to the cumulated radiation dose over mission lifetime, its mechanical properties still remain sufficient.
For polymers the amount of material damage, either cross linking or chain scission, depends on the total ionizing dose deposited in the material. For a typical 15 year mission in geostationary orbit, a dose in the 108 Gy range is expected at the material surface, which decreases to 107 Gy at a “KAPTON” depth of 1 μm due to internal shielding and then further by a factor 2 to 3 in the next 10 μm. For material testing on earth, usually completely penetrating radiation, in the form of gamma radiation or electrons are used, which deposit a constant dose across the entire depth of the material. This provides decidedly worst case values.
Data for the mechanical characteristics of “KAPTON” after radiation exposure are available up to a total dose of 107 Gy as summarized in Table 2. The measurement was performed with a Co60 source. For a radiation dose of 107 Gy the tensile strength is reduced by approximately 27% and the elongation by about 48%, whereas the Young's modulus remains almost constant.
In order to cover a dose of 108 Gy not included in these literature values a “KAPTON VN” foil, 100 μm thick; irradiated with 1 MeV electrons up to a cumulated dose of 7.9×107 Gy as exit window of the van de Graaff accelerator at TU Delft, was submitted to mechanical characterization together with a control film of the same material. Four samples were investigated per test. The results are shown as average values in Table 3.
While the tensile strength Rm decreases, the yield strength Rp at 3% elongation increases by almost the same percentage value. More importantly, since the Young's modulus increases by the same amount, the ratio of yield strength to Young's modulus remains constant and therefore the mechanical properties of “KAPTON” even after 108 Gy exposure can be considered sufficient for use as an interconnector.
Finally in space the solar cell interconnectors and the solar cell array according to the invention can be exposed to temperatures enveloping −200° C. to +200° C. For both layer components of the interconnector, these temperatures are easily tolerable. “KAPTON” is used in a wide range from −269° C. to 400° C. and the metal film typically can tolerate even higher temperatures. For the assembly of both materials it is desirable not to introduce additional stresses in the interconnector by a large difference in thermal expansion coefficient. Suitable material combinations, however, are readily available. A 25 μm “KAPTON HN” film, for example, with a thermal expansion coefficient of 17×10−6/K is matched closely to Ag with a thermal expansion coefficient of 19.7×10−6/K.
Reference numerals in the claims, in the description and in the drawings are provided only for a better understanding of the invention and shall not delimit the scope of protection which is defined by the wording and meaning of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14 000 909.3 | Mar 2014 | EP | regional |
