Patent Application
20200335648
The disclosure is related generally to a single toe interconnect for solar cells.
In order to make use of electrical power generated by solar cells from sunlight, the current must be transported from the solar cell to the external circuit. This is typically done using metal gridlines (fine features collecting current from all areas of the device), one or more busbars (larger feature aggregating current from many gridlines), contact pads (locations to attach tabs/interconnects to connect to other devices), and interconnects to span between the contact pads and other devices. These grids, busbars, contact pads, tabs and interconnects all result in parasitic losses.
The metal used for grids, busbars, contact pads and interconnects also imposes power losses on the solar cell's conversion efficiency, by preventing light from reaching the semiconductor layers of the solar cell, from parasitic losses, and with series resistance. For state-of-the-art space solar cells, metal coverage of the solar cell is the primary loss factor. Optimization of gridlines and busbar designs is standard practice to reduce obscuration, but the design and location of contacts and interconnects has received little attention.
Space solar cells typically have two or three interconnects spaced as widely apart as possible. This common practice is intended to reduce the risk of an open circuit if a solar cell cracks, but also results in current flowing relatively long distances along the busbars. Removal of an interconnect for large-format solar cells is known to cause efficiency loss. For example, the most recent evolution has been to reduce interconnects from three to two for all solar cells with an area less than 65 cm2, with the remaining two interconnects being placed as widely apart as possible.
However, widely spaced interconnects increase resistance losses when conducting current to the interconnects and/or result in increased dimensions of the busbars that reduces cell current. Moreover, widely spaced interconnects increase the risk that cell cracks could isolate all interconnects, creating on open circuit. In addition, interconnects only being available in multiples of two leads to over- or under-designed solutions.
Thus, there is a need in the art for improved solar cell interconnects. The present disclosure satisfies this need.
To overcome the limitations described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present disclosure describes an interconnect for making electrical connections for a solar cell, wherein the interconnect consists of a single toe, without a second toe connected to the single toe, and with no connected crossbars. The present disclosure also describes a method for fabricating the interconnect and a method for making electrical connections using the interconnect.
The interconnect is comprised of one or more layers of conductive, bondable material, selected from aluminum (Al), copper (Cu), silver (Ag), gold (Au), molybdenum (Mo), Kovar® (an iron-nickel-cobalt alloy), Invar (64FeNi), or an alloy or combination thereof. The interconnect is about 0.25 to 1 inch in length, 40 to 100 mils in width, and 1 to 2 mils in thickness.
A plurality of the interconnects may be placed uniformly across an edge of the solar cell. In one example, three interconnects are placed uniformly across an edge of the solar cell, when the solar cell has an area less than 60 cm2. In another example, four or more interconnects are placed uniformly across an edge of the solar cell, when the solar cell has an area greater than 60 cm2.
Referring now to the drawings in which like reference numbers represent corresponding parts throughout:
In the following description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific example in which the disclosure may be practiced. It is to be understood that other examples may be utilized and structural changes may be made without departing from the scope of the present disclosure.
Overview
This disclosure improves electrical connections between solar cells by replacing relatively fewer, large interconnects with relatively more, smaller interconnects. Currently, space-grade interconnects are comprised of two parallel flat metal “toes” or “legs” connected with crossbars. In this disclosure, an interconnect consists of a single flat metal toe, without a second toe, and with no connected crossbars.
Making interconnects with only a single toe allows the number of interconnects to be properly sized to the cell and program, including odd numbers of interconnects. Removing the crossbars allows interconnects to be placed more uniformly across the solar cell's edges, increasing solar cell efficiency, reducing material cost, improving reliability and enabling advanced manufacturing, such as real-time laser cutting of interconnects.
Technical Description
As shown in
The limitations of this configuration include:
As noted above, metal coverage of the solar cell 10 is the primary loss factor. Gridlines 14 and busbars 16 have been optimized to reduce obscuration, but interconnects 12 have received little attention.
This disclosure overcomes these limitations by providing a new design for an improved interconnect with a single toe or leg, and distributing the interconnects relatively uniformly across an edge of the solar cell.
An apparatus, method of fabricating and method of making electrical connections using this new design are illustrated in
In one example, the interconnect 24 is comprised of one or more layers of conductive, bondable material, for example, in the form of a metal foil, selected from aluminum (Al), copper (Cu), silver (Ag), gold (Au), molybdenum (Mo), Kovar® (an iron-nickel-cobalt alloy), Invar (64FeNi), or an alloy, or combination thereof. The interconnect 24 may be about 0.25 to 1 inch in length, 40 to 100 mils in width, and 1 to 2 mils in thickness.
In one example, the plurality of the interconnects comprise three interconnects 24 placed uniformly, i.e., with equal space between each interconnect 24, across an edge of the solar cell 10, when the solar cell 10 has an area less than 60 cm2. In another example, the plurality of the interconnects 24 comprise four or more interconnects 24 placed uniformly across an edge of the solar cell 10, when the solar cell 10 has an area greater than 60 cm2.
This interconnect 24 reduces both current and voltage losses in the busbars 16, while reducing assembly attrition and maintaining risk mitigation against open circuits. The result is improved efficiency, reduced parasitic loss, and improved manufacturability by eliminating the double toe interconnects 12 with connected crossbars 22.
From an operational efficiency perspective, there are reasons one might not look to this solution of a single toe interconnect 24. For example, the reduced metal used for the interconnect 24 may increase the potential for damage.
However, use of the single toe interconnects 24 between solar cells 10 provides a number of benefits, including increased redundancy, reduced assembly attrition, more flexible design options, increased reliability, improved efficiency, reduced material cost, and advanced manufacturing.
For example, the number of interconnections can be increased using the single toe interconnects 24 to increase redundancy. Alternatively, the number of interconnections using the single toe interconnects 24 can be decreased to reduce assembly attrition. This disclosure provides more flexible design options, in that the solar cell 10 can have more points of interconnection (more redundancy) or fewer points of interconnection (reduced assembly attrition) using the new interconnects 24.
Specifically, by removing the crossbar 22 between double toes 20, this disclosure provides more design freedom to select the number of interconnections and the location of those interconnections using the new interconnects 24. For example, odd numbers of interconnections for the solar cell 10 are possible using the interconnects 24, whereas the interconnects 12 always result in even numbers of interconnections due to the double toes 20.
This disclosure can provide equal, greater, or less reliability than currently required as determined by the number, spacing and/or locations of interconnects 24. For example, the number, spacing and locations of the interconnections can be selected to improve solar cell 10 efficiency (evenly spaced) or positioned based on some other constraints. Specifically, the location of interconnections using the interconnects 24 are not constrained by the crossbars 22.
Removing the crossbars 22 allows the interconnects 24 to be placed more uniformly across the edges of the solar cell 10, which simultaneously increases solar cell 10 efficiency, reduces material cost, and improves reliability. For example, even spacing of interconnects 24 leads to improved reliability by reducing the probability that solar cell 10 cracks can electrically isolate the interconnects 24 to create an open circuit. Even spacing of interconnects 24 also leads to higher solar cell 10 efficiency due to reduced busbar 16 obscuration and resistance losses.
Single toe interconnects 24 inherently reduce the amount of material consumed in fabrication and therefore the cost of the interconnect 24. The crossbars 22 increase material consumption and cost of interconnects 12, and increases obscuration for contact pads 18 on a solar cell 10. Single toe interconnects 24 are more compatible with advanced manufacturing concepts, such as real-time laser cutting of the interconnects 24 from raw stock material. Single-toe interconnects 24 also provide mechanical stress relief for the solar cell 10.
Fabrication
Examples of the disclosure may be described in the context of a method 28 of fabricating one or more interconnects 24, a solar cell 10 using the interconnects 24, a solar cell panel comprised of the solar cells 10, and/or a space vehicle such as a satellite including the solar cell panel, comprising steps 30-42, as shown in
As illustrated in
Each of the processes of method 28 may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of manufacturers and major-system subcontractors; a third party may include without limitation any number of venders, subcontractors, and suppliers; and an operator may be a satellite company, military entity, service organization, and so on.
As shown in
Functional Block Diagram
At least one of the solar cells 10 includes at least the metal gridlines 14 collecting current from all areas of the solar cell 10, the busbars 16 aggregating current from the gridlines 14, and contact pads 18 used to attach the interconnects 24 to span between the contact pads 18 and other devices (not shown) for making electrical connections.
