The present invention relates to a photovoltaic module with a number of solar cells which have a front side contact structure that includes a number of linear contact fingers disposed in parallel and at least one bus bar extending perpendicular thereto.
Solar cells are used for the purpose of converting the energy of electromagnetic radiations, particularly sunlight into electrical energy. The energy conversion is based on the fact that the radiation in a solar cell is subjected to an absorption, whereby positive and negative charge carriers (“Electron-hole pairs”) are generated. The generated free charges are then isolated from each other, in order to be dissipated to isolated contacts.
Generally, solar cells have a square silicon substrate, in which two regions are configured with different conductivity or doping. There is a p-n junction between both the regions, which is also referred to as “Base” and “Emitter”. This p-n-junction generates an inner electrical field, which causes the above described isolation of the charge carriers generated by radiation.
Generally, the front side emitter-contact structure of the solar cell includes a grid-like arrangement made of linear metallic contact elements, which are also referred to as contact fingers. In addition, metallic bus bars extending transverse to the contact fingers and having a bigger width are provided. Generally, the rear side base-contact structure has a metallic layer configured flat, on which the metallic rear side contact elements are disposed. Cell connectors are connected to the front and the rear side contact elements.
A number of solar cells are always interconnected in a photovoltaic (PV) or solar module. Generally, the solar cells are interconnected in series via the cell connectors in so-called strings, which in turn are connected in the form of a series connection. The solar cells interconnected in this manner are located in a transparent embedded layer, which is disposed between a front side glass cover and a rear side film cover.
Generally, the cell connectors are tin-plated copper strips, which are soldered on the front side bus bars and the rear side contact elements. While attaching the cell connector to the front side bus bars, often there is an inaccurate positioning. Then, the cell connector is supported at least on one side on the contact fingers and exerts a force effect on the contact finger via the solder, which can lead to finger breakages and cell breakages in the bus bar region. The forces are developed on the contact fingers by different coefficients of thermal expansion of silicon substrate, metallizing paste, solder, copper strip, encapsulation material, rear side film or glass cover. The narrow sensitive contact fingers are often subjected to mechanical stress during cooling of the solder and during the lamination process on the solar module. Further mechanical stress develops by temperature fluctuations of day and night, summer and winter, as well as by snow and wind loads on the solar module. The finger breakages and cell breakages caused have a negative effect on the module output and increase the electrical degradation of the module. However, frequently the bus bars are also configured narrower than the cell connector in order to save silver paste. In these cases, there is mechanical stress on the fingers even without inaccurate positioning of the cell connector, because the cell connector exerts a force effect on the sensitive contact finger via the solder.
The object of the present invention consists of providing a photovoltaic module with a number of solar cells, which have an improved front side contact structure.
This object is achieved by a photovoltaic module according to claim 1. Further advantageous embodiments of the invention are claimed in the dependent claims.
According to the invention, the photovoltaic module has a number of solar cells, which respectively include a pre-processed silicon wafer with a contact structure that has a number of linear contact fingers disposed in parallel in a first direction and at least one bus bar disposed perpendicular to the first direction. The bus bar extends over the contact finger in a second direction and includes contact surfaces in the region of the respective contact fingers, which protrude over the contact finger in the first and in the second direction and are electrically connected to the contact fingers. Furthermore, at least one strip-like cell connector is provided; which extends over the bus bar and is electrically connected to the contact surfaces of the bus bars. The width of the cell connector is smaller in the first direction than the length of the contact surface in the first direction.
In accordance with the invention, the layout of the contact structure in the bus bars, which is used for combining the emitter-charge carriers detected over the contact fingers is configured in the form of broadened contact surfaces, which ensures that the cell connectors narrower in comparison to these contact surfaces do not come in mechanical contact with the contact fingers even in an inaccurate position by the attachment of the cell connector. Thus, by reliably preventing the plating of the cell connector on the contact fingers, the number of finger breakages and cell breakages can be reduced and thereby the risk of an electrical degradation of the solar cell and of the photovoltaic module made therefrom is reduced.
According to a preferred embodiment, the bus bar has a sequence of short and long contact surfaces in the direction of cell connector. The short contact surfaces serve for material-saving and thereby for a cost-reduction, since the contact surfaces are generally made of silver. At the same time, the shadow area of the solar cell front side, over which the incident light falls, is also reduced. The wider contact surfaces are used for an excellent electrical and mechanical contact between the cell connectors and the contact surfaces generally made of copper and reliably prevent the peeling-off of the cell connector from the contact surfaces at mechanical stress to which the solar cell or the photovoltaic module is subjected during manufacture or during use.
According to another preferred embodiment, the lengths of the contact surfaces in the region of the edges of the pre-processed silicon wafer with reference to the further contact surfaces of the bus bars is maximum. In particular, extremely high forces act on the cell connector in the region of the transition towards the solar cell mainly during the manufacture of the module, so that here there is an increased risk of a peeling off of the cell connector from the contact surfaces.
According to another preferred embodiment, the length of the contact surfaces is varied from the edge of the pre-processed silicon wafer towards the center of the pre-processed silicon wafer, such that the length of the contact surfaces in the direction of cell connector reduces and has a minimum preferably in the region of the center of the solar cell. By such a layout, an optimal compromise with reference to the material use, shadowing of the solar cell front side and sufficient adhesion is achieved on the contact surfaces at mechanical stress on the cell connectors.
According to another preferred embodiment, the corners of the contact surfaces are configured rounded-off or chamfered. This applies preferably also for the transitions of the contact surfaces towards the contact fingers. Thus, the stress peaks developed on the corners or in the transition region are avoided, which can lead to cell breakages. The same also applies to the rear side contact structure, the contact surfaces of which are likewise configured preferably rounded-off or chamfered, in order to avoid such stress peaks.
According to another preferred embodiment, the bus bars additionally have webs, which are narrower than the contact surfaces and interconnect these, wherein the transitions of the contact surfaces towards the webs are configured rounded-off or chamfered. By the additional webs under the cell connectors between the contact surfaces, there is a possibility to make an improved electrical and mechanical contact with the cell connectors, without requiring increasing peel-off of the solar cells front side by additional non-transparent contact layers, because the webs disappear under the always available cell connectors. Furthermore, the webs simplify the capacity of electrical contact of the solar cell by means of needle plates in the cell tester.
The invention is explained in more details in the following with the help of the Figures. They show:
With the help of the figures, a solar cell and a photovoltaic module are described, in which an improved front side contact structure of the solar cell ensures a reduction of finger breakages and cell breakages.
As
Therefore, the front side contact structure includes a plurality of metallic contact elements, which are subsequently also referred to as contact fingers. These are configured—as shown in the FIG. 4—relatively thin and linear and extend over the solar cell in the shape of a parallel grid. The contact finger structure leads to just a slight shadowing of the solar cells front side, over which light radiation falls. The contact fingers 132 are preferably embedded in an anti-reflection layer 120, whereby the light reflection is suppressed on the surface, which minimizes the luminous efficacy. In addition to the contact fingers 132 extending in parallel, the front side contact structure of the solar cell preferably includes a number of metallic bus bars 135, which are also referred to as busbars in the following. The bus bars 135 are disposed perpendicular to the linear contact fingers 132 and extend out over the contact fingers. The bus bars 135 combine the charge carriers detected over the contact fingers from the emitter region 112 and forward them via the cell connector to the adjacent solar cells. The contact fingers 132 and the bus bars 135 are preferably composed of silver and are normally applied by means of a printing process, in which silver paste is used.
The rear side contact structure of the solar cell includes, as
While attaching the strip-like cell connector—which are generally copper strips—on the front side busbars, the mass production processes of the solar cells often result in inaccurate positioning of the cell connector, whereby then the cell connector moved against the busbars rests on the contact fingers. Through this contact, then forces in the contact fingers are coupled, e.g. by the different coefficients of thermal expansion of silicon substrate, silver paste, solder, for attaching the copper strips, copper, encapsulation material, rear side film or glass cover on the front side. This mechanical stress on the contact finger, which develops during the manufacturing processes, e.g. during cooling of the solder and the subsequent lamination process, or even by temperature fluctuation during solar cells operation, can then lead to damages to the contact fingers, particularly lead to breakages, which has a negative effect on the photovoltaic module output.
Such finger breakages and cell breakages are prevented by the improved front side contact structures in accordance with the invention, in which the continuous busbars schematically shown in
Configurations possible in accordance with the invention, of the front side contact structures in accordance with the invention are schematically represented in the
In the embodiment shown in
Another embodiment is shown in
This also applies for the embodiment shown in
In principle, most diverse sequences of short and long contact surfaces can be conceived. For example, rather than as shown in
Alternatively, as shown in the
Number | Date | Country | Kind |
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10 2013 212 845.2 | Jul 2013 | DE | national |