WO2009039666A1

WO2009039666A1 - Flexible solar module and method of making same

Info

Publication number: WO2009039666A1
Application number: PCT/CA2008/001729
Authority: WO
Inventors: Adrian Howard Kitai; Wei Zhang; Yanxia Zhu
Original assignee: Mcmaster University
Priority date: 2007-09-28
Filing date: 2008-09-29
Publication date: 2009-04-02

Abstract

A flexible solar module is disclosed comprising a plurality of electrically interconnected solar cell chips arranged in an array and a plurality of optical elements adhered directly to each of the plurality of solar cell chips which direct light to the plurality of solar cell chips. The optical elements are adhered to the solar cell chips such that there are no air gaps between the plurality of solar cell chips and the plurality of optical elements. The area of the light collecting area of each of the plurality of optical elements may be equal to or greater than the area of the top each of the solar cell chips.

Description

TITLE: FLEXIBLE SOLAR MODULE AND METHOD OF MAKING SAME

[0001] The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described in any way.

CROSS REFERENCES TO RELATED APPLICATIONS

[0002] This application claims the benefit of U. S. Provisional

Application No. 60/976,122 filed September 28, 2007, and the entire contents of which are hereby incorporated by reference.

FIELD

[0003] Applicants' teachings relate to a method and apparatus to provide a flexible sheet having solar cells. Moreover, applicants' teachings are directed towards a method and apparatus to produce a flexible solar module consisting of many small cells that are interconnected to one another.

[0004] These and other features of the applicants' teachings are set forth herein.

SUMMARY

[0005] The invention provides, in one aspect, a flexible solar module comprising: a plurality of interconnected solar cell chips arranged in an array, each solar cell chip having a light receiving surface and two or more electrodes; and a plurality of optical elements, each having a light receiving area and a light emitting area, the light emitting area of each optical element being adhered to the light receiving surface of the plurality of solar cell chips with no air gaps between the light emitting area of each optical element and the light receiving surface to which it is adhered, each light collecting area receiving light incident upon the light collecting area and directing the light to the light receiving surface of the plurality of solar cell chips, so that the light collecting area can receive light over a wide range of angles.

i [0006] In another aspect, the invention provides that the light receiving surface of each of the plurality of solar cell chips has a first surface area, the light collecting area of each of the plurality of optical elements has a second surface area and the second surface area is greater than the first surface area.

[0007] In another aspect, the invention provides that the plurality of optical elements are inverted truncated pyramids.

[0008] In another aspect, the invention provides that the plurality of optical elements are transparent plates.

[0009] In another aspect, the invention provides that the flexible solar module further comprises a light reflecting layer deposited on surfaces of the optical element other than the light collecting area and the light emitting area.

[0010] In another aspect, the invention provides that the plurality of solar cell chips are silicon.

[0011] In another aspect, the invention provides that the electrodes of the plurality of solar cell chips are electrically connected to two or more wires.

[0012] In another aspect, the invention provides that the two or more wires are any one of: silver wires and copper wires.

[0013] In another aspect, the invention provides that the solar module further comprises one or more flexible electrode substrates having conductive stripes, wherein each of the plurality of solar cell chips has an electrode pattern on a first side and the plurality of solar cell chips are interconnected by adhering the solar cell chips to the first side of the one or more flexible electrode substrates such that the electrode pattern of each of the plurality of solar cell chips aligns with the conductive stripes.

[0014] In another aspect, the invention provides that the conductive stripes are copper.

[0015] In another aspect, the invention provides a method of fabricating a flexible solar module, the method comprising: aligning a plurality of interconnected solar cell chips in an array, each solar cell chip having a light receiving surface and two or more electrodes; and adhering one of a plurality of optical elements to the light receiving surface of each of the plurality of solar cell chips with no air gaps between each optical element and the light receiving surface to which it is adhered, each optical element having a light collecting area which receives light incident upon the light collecting area and directs the light to the light receiving surface of the plurality of solar cell chips, so that the light collecting area can receive light over a range of angles.

[0016] In another aspect, the invention provides that the light receiving surface of each of the plurality of solar cell chips has a first surface area, the light collecting area of each of the plurality of optical elements has a second surface area and the second surface area is greater than the first surface area.

[0017] In another aspect, the invention provides that the plurality of optical elements are inverted truncated pyramids.

[0018] In another aspect, the invention provides that the plurality of optical elements are transparent plates.

[0019] In another aspect, the invention provides that the method further comprises depositing a light reflecting layer onto surfaces of the optical element other than the light collecting surface and the light emitting surface.

[0020] In another aspect, the invention provides that the solar cell chips are silicon.

[0021] In another aspect, the invention provides that the electrodes of the plurality of solar cell chips are electrically connected to two or more wires.

[0022] In another aspect, the invention provides that the two or more wires are any one of: silver wires and copper wires.

[0023] In another aspect, the invention provides that the plurality of solar cell chips have an electrode pattern on a first side and the method further comprises adhering two or more flexible electrode substrates having conductive stripes to the first side of the plurality of solar cell chips where the electrode pattern aligns with the conductive stripes.

[0024] In another aspect, the invention provides that the conductive stripes are copper. DRAWINGS

[0025] The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the applicants' teachings in any way.

[0026] Figure 1a is a cross-sectional view representative of some embodiments of a flexible solar module according to applicants' teachings;

[0027] Figure 1b is a top and side view of an optical element of the flexible solar module of Figure 1 ;

[0028] Figure 2a is a cross-sectional view representative of some further embodiments of the flexible solar module according to applicants' teachings;

[0029] Figure 2b is a top and side view of an optical element of the flexible solar module of Figure 2a;

[0030] Figures 3 to 8b are top and bottom views showing a method of making the flexible solar module of Figure 1 according to some embodiments of applicants' teachings;

[0031] Figures 9 to 12 are top and bottom views showing a second method of making a flexible solar module according to some embodiments of the applicants' teachings;

[0032] Figures 13 and 14 are cross-sectional views of the solar module fabricated according to the second method; and

[0033] Figure 15 is a graph showing the output power of a solar cell chip versus angle of incident light for two examples of optical elements mounted on solar cell chips according to some embodiments of the applicants' teachings.

DESCRIPTION OF VARIOUS EMBODIMENTS

[0034] Applicants' teachings relate to a method and apparatus to provide a flexible sheet having solar cell chips. Moreover, applicants' teachings are directed towards a method and apparatus to produce a flexible solar module consisting of many small cells that are interconnected to one another. The flexible solar module can have a high conversion efficiency (of the order of about 20%) and output power. Further, the flexibility allows for greater portability and ease of mounting of the solar module compared to conventional solar power devices. For example, but not limited to, the flexible solar modules of applicants' teachings may be bent or rolled, and may be bonded to a variety of curved surfaces including, for example, but not limited to, roofs and vehicle panels.

[0035] The optical elements, explained below, in some embodiments of the applicants' teaching, increase the amount of light incident on each solar cell chip and decrease the required size of each chip.

[0036] The resulting flexible solar module does not require tracking the sun over the course of the day. Tracking mechanisms can add to the cost and size of solar cell installations. Examples of systems using optical elements or mounts that do require tracking are given in U. S. Publication No. 2008/0123313 to Home et al., U. S. Publication No. 2006/0274439 to Gordon et al, and U. S. Publication No. 2007/0089778 to Home et al. In these examples, as well as many other concentrator designs, incoming light is focused to a small spot. Designs of this nature work only over a small range of incident light angles, and do not generally perform well in cloudy conditions.

[0037] The applicants' teaching specifies optical element designs that may achieve a degree of concentration while simultaneously allowing incident light to be varied over a wide range of angles. This removes the requirement for tracking and, on cloudy days, permits effective operation even when light is scattered over a wide range of incident angles.

[0038] Figure 1a is an illustration of some embodiments of applicants' teachings showing a flexible solar module 10 constructed in accordance with some methods of applicants' teachings, as will hereinafter be described. In various embodiments, flexible solar module 10 utilizes proven materials and processes for its production. In some embodiments of applicants' teachings, flexible solar module 10 comprises interconnected solar cell chips 12. In some embodiments of applicants' teachings, these solar cell chips 12 can be interconnected by top wires 17 and bottom wires 19, as will hereinafter be described in Example A. Alternatively, in other embodiments of applicants' teachings, the solar cell chips may be interconnected using a flexible electrode substrate, as will hereinafter be described in Example B. Each solar cell chip 12 has a light receiving surface 14.

[0039] In accordance with various embodiments of applicants' teachings, solar cell chips 12 are, for example, but not limited to, silicon cells. Single crystal and multi-crystalline silicon, for example, are suitable solar cell materials having both proven stability and performance in the field. Silicon technology is also relatively efficient, currently yielding about 14% to about 20% conversion efficiency. Other suitable materials can include, for example, but not limited to, cadmium telluride (CdTe) and gallium arsenide (GaAs) or the solar cells may be triple junction solar cells having germanium substrates. The solar cell chips 12 may be diced from larger silicon cells.

[0040] While the solar cell chips used in the following examples are square, it is to be understood that the solar cell chips could be any shape such as, but not limited to, round or hexagonal.

[0041] An optical element 16, as shown in Figure 1b, can be mounted on each solar cell chip 12 to direct and concentrate light onto the light receiving surfaces 14 of the solar cell chips 12 and thereby increase the power generated. Each optical element 16 has a light collecting area 22 and a light emitting area 24.

[0042] In various embodiments of the applicants' teachings, the surface area of the light collecting area 22 of each optical element 16 is greater than the surface area of the light receiving surface 14 of the solar cell chips 12. For example, the optical element depicted in Figure 1b is in the form of an inverted truncated pyramid such that the area of the end of the optical element attached to the solar cell chip is smaller than the area of the opposite side from which light is collected, see for example, Figure 1a. The truncated pyramid directs light onto the solar cell chip and increases the power generated from a given chip, and moreover is able to accept sunlight over a wide range of incident angles. The surfaces of the sloping faces 20 of the optical element 16 in Figure 1b may be curved or flat.

[0043] It is to be understood that the shape of the optical element 16 may vary according to the shape of the solar cell chip 12. For example, if the solar cells chips 12 were round, the optical element 16 may be in the form of a truncated cone

[0044] Alternatively, for the flexible solar module 10 as shown in Figure

2a, the optical elements may be transparent support plates 18, as shown in Figure 2b, mounted on the cells 12 to provide additional mechanical strength while allowing light incident over a wide range of angles to reach the said cells 12

[0045] The optical elements 16,18 are preferably adhered directly to the solar cell chips 12 such that there are no air gaps between the light receiving surface 14 of the solar cell chips 12 and the light emitting area 24 of the optical element 16,18

[0046] Aspects of the applicants' teachings may be further understood in light of the following examples of the method of making the flexible solar module 10, which should not be construed as limiting the scope of the present teachings in any way.

EXAMPLE A

[0047] Referring now to Figures 3 and 4, a method of producing the flexible solar module 10 in accordance with a first method of the applicants' teachings is described. In this example, the solar cell chips 12 have both front and rear electrodes. In this example, each chip has dimensions of 8.2 mm by 8.2 mm, yielding a light receiving surface 14 of 67.2 mm².

[0048] According to this exemplary method, the top wires 17 are first soldered onto temporary copper tapes 11 to hold them straight and ready for attachment onto the solar cell chips 12 as shown in Figure 3. This should preferably be done above a smooth flat working surface. The copper tapes 11 are, for example, but not limited to, SPI #05012 AB Copper tape. The wires 17 may be thin silver wires, such as, but not limited to, Alfa Aesar 00303. The solar cell chips 12 are then placed underneath the top wires 17 facing upwards, as shown in Figure 4.

[0049] The top wires 17 are soldered onto areas of the cells 12 where the wires 17 intersect with the front electrodes of the cells 12. This can be accomplished by, for example, applying solder paste (for example, but not limited to, Indium 3.2 as made by Indium Corporation), baking the unit until the solder paste melts and then cooling the unit to room temperature.

[0050] Referring now to Figure 5, optical elements 16 can be adhered directly to the solar cell chips 12 using, for example, UV curable glue such as, but not limited to Type J-91 Summers Optical Lens bond. This substantially strengthens the solar cell chip so as to prevent chip breakage. The optical elements can be cut from, for example, but not limited to, 0.2 mm thick Thermo Scientific microscope cover glass or 1.1 mm thick Eagle XG Corning Glass or 4 mm thick Solite 2000 AGC flat glass. The optical elements 16 used in this example, shown in Figure 1b, could be 4 mm in thickness and be cut from 4 mm thick Solite 2000 flat glass. The light collecting area 22 of each optical element 16 in this example could measure 12.8 mm by 12.8 mm, yielding a light collecting area 22 of 163.8 mm². The light emitting area 24 of each optical element in this example could measure 8.2 mm by 8.2 mm, yielding a light emitting area 24 of 67.2 mm². The relative performance of the optical elements is illustrated in Figure 15, referred to below.

[0051] Alternatively, flat transparent plates 18, as shown in Figure 2b, may be mounted on the front of the solar cell chips 12, as shown in Figure 2a. For the purposes of this example, the whole unit can then be cured under a UV lamp for, in this example, approximately ten minutes. Once the glue is cured, the wires 17 can be cut from the copper tapes 11.

[0052] A Gel Pack, such as Gel-Film PF-20X120-X4, can be placed above the solar cell chips 12. The jointed solar cell chips 12 can then be removed from the working surface and flipped over to expose the back side of the solar cell chips 12. [0053] Referring now to Figure 6, a single wire 19 is adhered to the back side of the solar cell chips 12 using, for example, a silver epoxy such as, but not limited to, Epoxy Technology H20S. The wire 19 may be a thin silver wire, such as, but not limited to, Alfa Aesar 00303.

[0054] Referring now to Figures 7 and 8, a flexible solar array according to this exemplary method is shown. The preceding steps may be repeated for multiple rows of solar cell chips 12. These rows of solar cell chips 12 can be assembled into an array, as shown in Figure 7.

[0055] In this example, a sheet of Ethylene-vinyl acetate (EVA) is placed onto the backside of the array of solar cell chips 12 and a piece of Mylar is placed onto the EVA sheet. The EVA is, for example, but not limited to, Encapsolar™ PV- 135D (as made by Stevens Urthane). The whole unit is then laminated under heat and pressure.

[0056] According to this example, the top wires 17 and bottom wires 19 are soldered to two copper tapes 21 placed on either end of the array of solar cell chips 12. The copper tapes 21 are then folded around to the back side of the array of solar cell chips 12. Figures 8a and 8b show back views of a representative flexible solar cell chip arrays according to this exemplary method. Figure 8a shows a parallel connection example in which the current from each row of solar cells chips is added together to produce a low voltage, high current output. In the case of silicon solar chips, the total output voltage is approximately 0.5 volts. Figure 8b shows a series connection example, in which the voltage from each row of solar cell chips is added together. In the case of silicon solar cell chips, the total voltage available between the two electrodes 40, 42 will be approximately 3.5 volts.

[0057] The flexible solar cell chip array may optionally be provided with encapsulation. This encapsulation may be provided by a DuPont Tefzel front sheet bonded to the front of the array with EVA in a vacuum lamination process, for example. Encapsulation materials must be carefully selected to avoid negatively affecting the optical performance of optical elements. [0058] In addition, a light reflecting layer may optionally be added to those surfaces of the optical element other than the light collecting areas 22 and the light emitting areas 24.

[0059] The flexible solar module 10, as illustrated in Figures 1a and 2a, can then have a suitable load applied through leads connected to the copper tapes 21.

[0060] Referring now to Figure 15, a graph is shown of the maximum output power of a solar cell chip 12 versus the angle of incident light using two examples of optical elements 16,18. The horizontal axis of the graph refers to the angle in degrees formed between the incident light beam relative to the plane comprised by the light collecting area 22 of the optical element 16,18. Optical element A comprises an optical element 18 made of flat 0.2 mm thick Thermo Scientific microscope cover glass, as detailed in Figure 2b, having a length and width of 8.2 mm. Optical element B comprises an optical element 16 that is 4 mm in thickness and is cut from 4 mm thick Solite 2000 flat glass, as detailed in Figure 1b. Optical element B has a light collecting area measuring 12.8 mm by 12.8 mm, yielding a light collecting area of 163.8 mm², and a light emitting area measuring 8.2 mm by 8.2 mm, yielding a light emitting area of 67.2 mm². It can be observed from the graph of Figure 15 that the power available from the solar cell chip increases when optical element B is used. It can also be seen that, for either optical element A or optical element B, output power may be obtained over a wide range of incident angles of light. For example, the optical elements 16,18 in these examples can utilize light from about 20 degrees to about 160 degrees (the graph in Figure 15 shows the angle of incident light from about 20 degrees to about 90 degrees, the incident light from about 90 degrees to about 160 degrees, which is not shown, would be a mirror image thereof).

Example B

[0061] Another method of producing the flexible solar modules 10 is now described with reference to Figures 9 to 14. In this example, conductive paths are created by patterning the copper layer of a flexible electrode substrate 30. The solar cells required for this embodiment differ from the cells used in the previous example in that they have electrodes on the back side only. There are no electrodes on the front side of the solar cells. The solar cell chips 12, diced from the original solar cells as described previously, are therefore suitable for mounting with solder onto the flexible electrode substrate 30.

[0062] According to this exemplary method, a solar cell is diced along the electrode pattern into squares, as shown in Figure 9. The solar cell is, for example, but not limited to, a SunPower A300.

[0063] A flexible electrode substrate 30 having an electrode pattern which matches the back side of the chips is then prepared, as shown in Figure 10. This flexible electrode substrate 30 is, for example, but not limited to, a copper-clad Kapton sheet (as made by Strataflex Corp.). Solder paste is then applied to the flexible electrode substrate 30 by a screen printing method (such as service from Laytec Design and Consulting Inc.) The back sides of the solar cell chips 12 can then be placed onto the solder locations of the flexible electrode substrate 30 with the electrodes 25 of the solar cell chips 12 in alignment with the electrodes 32 on the flexible electrode substrate 30. Figures 11 and 12 show top views of the flexible electrode substrate 30 with the mounted solar cell chips 12.

[0064] Referring now to Figures 13 and 14, two alternate embodiments of the flexible solar array 10 according to the second exemplary method are shown.

[0065] The solder paste can be melted in, for example, a belt furnace, to secure the positions of the solar cell chips 12 on the flexible electrode substrate 30, producing a solder layer 36 between the solar cell chips 12 and the flexible electrode substrate 30. As shown in Figure 13, optical elements, such as flat transparent glass plates 34, can then be adhered to the solar cell chips 12 using, for example, UV curable glue (such as Type J-91 Lens Bond). Alternatively, as shown in Figure 14, other optical elements, such as lenses 16, may be adhered to the solar cell chips 12 to improve light collection. [0066] The flexible solar cell chip array may optionally be provided with encapsulation. This encapsulation may be provided by a Tefzel front sheet bonded to the front of the array with EVA in a vacuum lamination process, for example. Encapsulation materials must be carefully selected to avoid negatively affecting the optical performance of optical elements.

[0067] Making the flexible solar module using smaller solar cell chips arranged in an array, but connected together, allows the outputs of the various solar cell chips to be added together to produce the flexible solar module 10 having high conversion efficiency (and about 14-20% conversion efficiency where silicon chips are used for the solar cell chips in the examples provided) and long term stability (over 25 years continuous service, again where silicon chips are used for the solar cell chips). However, the solar module is flexible, lightweight and thin. The flexible solar module of applicants' teachings can be useful for, for example, but not limited to, portable battery chargers for marine, RV and recreational use, flexible laminates for integration into size-limited applications such as bus shelters, automotive applications, and building integrated photovoltaics (BIPV).

[0068] Moreover, a flexible solar module constructed in accordance with applicants' teachings, and as provided in the above examples, does not include a continuous glass plate. Without a glass plate, there is no need for a frame to prevent damage to the glass (for example, an aluminum frame, which can be 1.5 to 2 inches deep). Without a frame, such as aluminum, the module is thinner and flexible allowing for greater portability and ease of mounting of the solar module compared to conventional rigid panel solar power devices. For example, but not limited to, the flexible solar modules of applicants' teachings may be bent or rolled, and may be bonded to a variety of curved surfaces including, for example, but not limited to, roofs and vehicle panels.

[0069] While the applicants' teachings are described in conjunction with various embodiments, it is not intended that the applicants' teachings be limited to such embodiments. On the contrary, the applicants' teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art.

Claims

CLAIMS:

1. A flexible solar module comprising: a plurality of interconnected solar cell chips arranged in an array, each solar cell chip having a light receiving surface and two or more electrodes; and a plurality of optical elements, each having a light receiving area and a light emitting area, the light emitting area of each optical element being adhered to the light receiving surface of the plurality of solar cell chips with no air gaps between the light emitting area of each optical element and the light receiving surface to which it is adhered, each light collecting area receiving light incident upon the light collecting area and directing the light to the light receiving surface of the plurality of solar cell chips, so that the light collecting area can receive light over a wide range of angles.

2. The flexible solar module of claim 1, wherein the light receiving surface of each of the plurality of solar cell chips has a first surface area, the light collecting area of each of the plurality of optical elements has a second surface area and the second surface area is greater than the first surface area.

3. The flexible solar module of claims 1 or 2, wherein the plurality of optical elements are inverted truncated pyramids.

4. The flexible solar module of claim 1 , wherein the plurality of optical elements are transparent plates.

5. The flexible solar module of any one of claims 1 to 4, further comprising a light reflecting layer deposited on surfaces of the optical element other than the light collecting area and the light emitting area.

6. The flexible solar module of any one of claims 1 to 5, wherein the plurality of solar cell chips are silicon.

7. The flexible solar module of any one of claims 1 to 6, wherein the electrodes of the plurality of solar cell chips are electrically connected to two or more wires.

8. The flexible solar module of claim 7, wherein the two or more wires are any one of: silver wires and copper wires.

9. The flexible solar module of any one of claims 1 to 6 further comprising one or more flexible electrode substrates having conductive stripes, wherein each of the plurality of solar cell chips has an electrode pattern on a first side and the plurality of solar cell chips are interconnected by adhering the solar cell chips to the first side of the one or more flexible electrode substrates such that the electrode pattern of each of the plurality of solar cell chips aligns with the conductive stripes.

10. The flexible solar module of claim 9, wherein the conductive stripes are copper.

11. A method of fabricating a flexible solar module, the method comprising: a) aligning a plurality of interconnected solar cell chips in an array, each solar cell chip having a light receiving surface and two or more electrodes; and b) adhering one of a plurality of optical elements to the light receiving surface of each of the plurality of solar cell chips with no air gaps between each optical element and the light receiving surface to which it is adhered, each optical element having a light collecting area which receives light incident upon the light collecting area and directs the light to the light receiving surface of the plurality of solar cell chips, so that the light collecting area can receive light over a range of angles.

12. The method of claim 11, wherein the light receiving surface of each of the plurality of solar cell chips has a first surface area, the light collecting area of each of the plurality of optical elements has a second surface area and the second surface area is greater than the first surface area.

13. The method of claims 11 or 12, wherein the plurality of optical elements are inverted truncated pyramids.

14. The method of claim 11 , wherein the plurality of optical elements are transparent plates.

15. The method of any one of claims 11 to 14, the method further comprising depositing a light reflecting layer onto surfaces of the optical element other than the light collecting surface and the light emitting surface.

16. The method of any one of claims 11 to 15, wherein the solar cell chips are silicon.

17. The method of any one of claims 11 to 16, wherein the electrodes of the plurality of solar cell chips are electrically connected to two or more wires.

18. The method of claim 17, wherein the two or more wires are any one of: silver wires and copper wires.

19. The method of any one of claims 11 to 18, the plurality of solar cell chips having an electrode pattern on a first side and the method further comprising adhering two or more flexible electrode substrates having conductive stripes to the first side of the plurality of solar cell chips where the electrode pattern aligns with the conductive stripes.

20. The method of claim 19, wherein the conductive stripes are copper.