WO2013104066A1

WO2013104066A1 - Method and apparatus for increasing the efficiency of solar cells

Info

Publication number: WO2013104066A1
Application number: PCT/CA2013/000033
Authority: WO
Inventors: William A. G. GOUGH
Original assignee: Astral Automation Inc.; Whitehead, Lorne A.
Priority date: 2012-01-13
Filing date: 2013-01-11
Publication date: 2013-07-18

Abstract

A solar cell assembly is provided which receives incident radiant energy from a source. An optical layer is provided between the energy source and the solar cell. The optical layer does not significantly reflect incident light but substantially reflects light emitted from the solar cell back into the solar cell using a combination of refraction and total internal reflection. The optical layer is unbonded to the surface of the solar cell but rather a gap is provided between the optical layer and the solar cell, which gap has a refractive index substantially less than that of the solar cell and preferably close to 1.0.

Description

Method and Apparatus for Increasing the Efficiency of Solar Cells

Cross Reference To Related Application

[0001] The present application claims the benefits, under 35 U.S.C. § 119(e), of U.S.

Provisional Application Serial No. 61/586,572 filed January 13, 2012 entitled "Method and Apparatus for Increasing the Efficiency of Solar Cells" which is incorporated herein by this reference.

Technical Field

[0002] The invention relates to the field of solar cells and photovoltaic devices, and more particularly to films and coatings for solar cells and photovoltaic devices to improve their efficiency.

Background

[0003] Recent advances in the field of photovoltaic cell technology have achieved new records for conversion efficiency of light to electrical energy. These new generation solar cells operate under the principle of "Photon Recycling" where the energy from the incident light source tends to create re-emitted photons in the cell. These re-emitted photons are absorbed by the cell to produce electrical energy and heat, as well as additional re-emitted photons. Some of the re- emitted photons leave the cell as light, which represents a loss of energy and a reduction in the cell efficiency. It has been proposed by others that the cell efficiency can be improved by incorporating a high reflectance mirror at the back of the cell (the side of the cell facing away from the incident light source) so that re-emitted photons would be reflected back to the cell to improve cell efficiency. However, photons that are re-emitted from the side of the cell that receives the light from the source are lost, as a simple reflector could not be used to return these re-emitted photons without interfering with the incident light source. Cell efficiency could be improved if the re-emitted photons on the incident side of the cell could also be reflected back to the cell in a manner that does not significantly interfere with the transmission of light to the cell from the light source.

[0004] Solar cells are also being produced using thin film techniques. In these cells, some of the incident light can pass through the solar cell film without being absorbed. A reflective surface is applied to the back of the cell so that any light that was not absorbed can be returned to the solar cell film. Similar to the problem described above, any un-absorbed light that is emitted from the incident side of the cell is lost back to the light source. Cell efficiency could be improved if the re-emitted light on the incident side of the cell could also be reflected back to the cell in a manner that does not significantly interfere with the light source.

[0005] In addition to the light which is lost due to re-emitted or un-absorbed light from the solar cell as described above, there is also light which is lost on the incident side of the cell simply due to reflection. The Silicon and Gallium Arsenide compounds used in the manufacture of the solar cells have a high Refraction Index (3.4 to 4.0 or higher) which results in a high reflectance (30% or higher) for incident light. Anti-reflection techniques that use quarter wavelength interference layers with matched Refraction Index values are typically applied to the surface of the solar cell to reduce the amount of reflected light significantly. Cell efficiency could be further improved if any reflected light on the incident side of the cell could be returned to the cell in a manner that does not significantly interfere with the transmission of light into the cell from the source.

[0006] Another approach to improving the performance of solar cells is disclosed in Raymond et al. US Patent no. 8,338,693 issued December 25, 2012, which discloses the use of a PV enhancement film applied to the surface of the solar cell. The PV enhancement film includes a plurality of total internal reflection (TIR) elements on the substrate opposite the light-receiving surface which traps some of the reflected light to provide additional chances for absorption. In this design the TIR layer is bonded directly to the surface of the solar cell. Bonding the TIR layer directly to the solar cell surface increases the amount of light that can escape from inside the solar cell, which represents a loss in efficiency. There remains a need therefore for films and coatings for solar cells and photovoltaic devices which further improve their efficiency.

[0007] The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.

Summary

[0008] The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.

[0009] The invention therefore provides a solar cell which is provided, between the light source and the surface of the solar cell which receives incident light from the source, with an optical layer which does not significantly reflect incident light but which substantially reflects light emitted from the solar cell back into the solar cell using a combination of refraction and Total Internal Reflection. The optical layer is separated from the surface of the solar cell by a gap having a refractive index substantially less than that of the solar cell, and preferably close to 1.0.

In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.

Brief Description of Drawings

[00011] Exemplary embodiments are illustrated in referenced figures of the

drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

[00012] Figure 1 is a schematic diagram illustrating a cross-section of a solar cell according to an embodiment of the invention incorporating un-bonded mounting and showing trajectory examples for incident light.

[00013] Figure 2 is a schematic diagram illustrating a cross-section of a solar cell according to an embodiment of the invention incorporating un-bonded mounting with base layer and showing trajectory examples for incident light.

[00014] Figure 3 is a schematic diagram illustrating a cross-section of a solar cell according to an embodiment of the invention incorporating unbonded mounting and showing re-emitted light returned to the cell by the optical layer.

[00015] Figure 4 is a schematic diagram illustrating a cross-section of a solar cell according to an embodiment of the invention incorporating unbonded mounting with base layer and showing re-emitted light returned to the cell by the optical layer. [00016] Figure 5 is a schematic diagram illustrating in perspective view an application of a second optical layer to increase the amount of light returned to the solar cell.

[00017] Figure 6 is a schematic diagram illustrating a cross-section of an

assembled solar cell according to an embodiment of the invention incorporating unbonded mounting.

[00018] Figure 7 is a schematic diagram illustrating a cross-section of an

assembled solar cell according to an embodiment of the invention incorporating an optical layer having a base layer.

Description

[00019] Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

[00020] For purposes of this application the term "solar cell" includes any

photovoltaic element capable of converting radiant energy into electricity or other device for collecting radiant energy. "Radiant energy" is the energy of electromagnetic waves including light and other forms of solar energy. [00021] With reference to Fig. 1-4, 6 and 7, the invention is comprised of a layer 10 of transparent optical material with a prismatic cross section (hereafter referred to as the "Optical Layer") that is located above the surface 11 of the solar cell 12 on the side that is incident to the light source, between the light source and the solar cell 12. The prismatic shape of the Optical Layer 10 permits transmission of light to the solar cell 12, but has highly reflective properties for light that is re-emitted from the surface of the solar cell 12. This Optical Layer 10 is designed to permit substantially all of the source light energy within a specified range of incident angles to reach the solar cell 12 while reflecting a significant portion of any re-emitted light back to the solar cell 12 for conversion into electricity. The re-emitted light is returned to the solar cell 12 using a combination of refraction and Total Internal Reflection, which occurs as a result of the prismatic shape and Refractive Index of the Optical Layer 10. The Optical Layer 10 can be constructed of glass or optical plastic material, which typically provides a Refractive Index in the range of 1.4 to 1.6 or higher. The application of this Optical Layer 10 to a solar cell 12 will cause a significant portion of any re-emitted light from the solar cell 12 to be returned to the solar cell 12, resulting in an increase in efficiency of electricity production for a given amount of incident light energy. Re-emitted light from all other sides of the solar cell 12 is reflected back to the cell using simple mirrors 40 applied to these other cell surfaces.

[00022] The Optical Layer 10 is separated from the solar cell surface 11 by a gap

13. Bonding the Optical Layer 10 directly to the solar cell surface 11 would increase the amount of light that can escape from inside the solar cell 12, which represents a loss in efficiency. This increased loss occurs because bonding the Optical Layer 10 to the solar cell surface 11 increases the value of the critical angle for Total Internal Reflection of light that is traveling inside the solar cell 12 itself, which allows more light energy to escape from the solar cell 12 before it is converted into electricity. By maximizing the relative difference in the refractive index at the solar cell surface 11 , between the refractive index of the solar cell and that of the adjoining medium, the smallest possible critical angle for total internal reflection to occur inside the silicon photocell is achieved. The index of refraction of silicon is about 3.5 and that of a vacuum is 1.0. The optimum gap 13 is a vacuum, but for practical purposes since air has a refractive index only slightly greater than a vacuum, air will generally be preferred as the medium in gap 13 for structural and manufacturing reasons. Almost all gases have a refractive index near 1.0, so any transparent gas with a refractive index near 1.0 would be an acceptable alternative to vacuum or air. Most solids or liquids have a refractive index of about 1.3 or greater. These materials may also be an aceptable means to fill the gap, but the aim however is to have a gap between the Optical Layer 10 and the solar cell 12 which has a refractive index as close as possible to 1. For structural and manufacturing reasons the width of gap 13 is preferably as small as possible, but there is no required width of gap 13.

[00023] Figure 1 shows an example of the transmission path for incident light to the solar cell at different angles to the Optical Layer 10 made from a material with a Refraction Index of 1.59. Preferably the sides 17 of prismatic elements 19 of the Optical Layer 10 form angles A to the horizontal of 45 degrees. In order to maximize the amount of light returned to the solar cell surface 11 from all directions equally, the prismatic elements 19 on the Optical Layer 10 should be symmetric with an internal apex angle A of 90 degrees at the top and equal internal angles B of 45 degrees at the base. Other prism geometries are possible, but they will result in light transmission and reflection performance that is efficient only at a particular range of incident angles. As shown in the Fig. 1 , all of the incident light 15 with an angle of approximately 35 degrees or less from the vertical is directed to the surface of the solar cell by the Optical Layer 10. Incident light with angles greater than approximately 35 degrees from vertical is either lost or must make several reflections in the Optical Layer 10 to reach the solar cell surface, which represents a decrease in efficiency.

For maximum efficiency, the solar cell 12 must be maintained in good alignment with the light source, such as in the case of automatic sun- tracking type installations or use solar concentrators. Solar

concentrators may or may not track the sun but in either case will provide collimation of the light presented to the solar cell to meet the incident requirements for the prism layer. Where the incident light is concentrated, the benefit of the prism layer is even more significant because the proportion of energy saved by the prism layer results in a greater value of net energy produced by the solar cell.

The Optical Layer 10 may require a base layer 14 for structural purposes depending on the method used to construct the prismatic surface. This base layer 14 is not required to achieve the desired optical properties. The base layer 14 should be as thin as possible to reduce transmission losses as the light passes through the Optical Layer 10. Figure 2 shows the transmission path for incident light to the solar cell at different angles to the Optical Layer 10 when a base layer is present in the Optical Layer 10.

Figure 3 shows the transmission path for re-emitted light from the solar cell 12 15 to the solar cell at different angles to the Optical Layer 10, as indicated in the figure, for the embodiment shown in Fig. 1 when the Optical Layer 10 is not bonded to the solar cell 12 and there is a region 13 of air between the Optical Layer 10 and the surface 11 of the solar cell 12. The Optical Layer 10 returns light 20 that is reflected, un- absorbed, or re-emitted from the solar cell 12 on the incident side of the solar cell as shown in Figure 3. Simple mirrors 40 are used to return light to the solar cell on the bottom and sides of the solar cell, as indicated by reflector 40 in the figures. Figure 4 shows examples of the return path for light 20 that is reflected, un-absorbed, or re-emitted from the solar cell 12 on the incident side of the solar cell 12 at different angles to the Optical Layer when a base layer 14 is present.

The light is returned to the solar cell 12 either by Total Internal

Reflection within a single prism in the Optical Layer 10, indicated at 26, or by a series of refractions through adjacent prisms in the Optical Layer 10 in combination with total internal reflection, indicated at 24. Again, in order to maximize the amount of light returned to the solar cell surface 11 from all directions equally, the prisms on the Optical Layer 10 should be symmetric with an internal apex angle of 90 degrees at the top and equal internal angles of 45 degrees at the base. Other prism geometries are possible, but they will result in light transmission and reflection performance that is efficient only at a particular range of incident angles.

The amount of light that can be returned to the solar cell increases as the Refractive Index of the Optical Layer 10 material increases, so it is desirable to use material with a high Refractive Index to maximize solar cell efficiency.

Some light 28 does escape from the Optical Layer 10 for a certain range of trajectories of light emitted from the solar cell 12. It is possible to reduce the amount of light that escapes from the solar cell 12 (and is not returned to the solar cell 12) by applying another Optical Layer 30 oriented with the prisms aligned perpendicularly to the first Optical Layer 10. Figure 5 shows the physical arrangement of the Optical Layers 10, 30 for this configuration. Additional Optical Layers could also be applied at equally spaced relative intersection angles, but the reduction in transmission of incident light due to the additional layers of material must be considered in contrast to the amount of benefit that can be realized through increased recovery of the light emitted from the solar cell 12.

Example of an Embodiment of the Invention in an Assembled Solar Cell

The Optical Layer 10 is designed to allow transmission of the incident light to the solar cell 12 while simultaneously exhibiting a high reflectance to the re-emitted light from the cell. The Optical Layer 10 has a prismatic shape as shown in Fig. 1-4. One example of an Optical Layer 10 that exhibits these properties is the Total-Internal-Reflection (TIR) film originally patented by Lome Whitehead et al. This film has been applied in computer and television display technology to increase the brightness of LCD panels. It is commercially produced by the 3M Company under the Vikuiti™ product family of optical films. The film is embossed with a prismatic cross sectional pattern and can be used as a single layer film or arranged in two layers where the axes of the prismatic patterns are perpendicular. This material may be used as the Optical Layer 10 in an embodiment of this invention.

The Optical Layer 10 uses a symmetric prism shape when viewed in cross-section with an internal angle A of 90 degrees at the apex and internal angles B of 45 degrees at the base. This prism geometry provides good performance from a wide range of light trajectory angles for both incoming light and reflected light from the cell.

The Optical Layer 10 must be protected from the environment to preserve the optical properties of the prismatic surface. The prisms can only properly refract and internally reflect the light if the surfaces are clean and free from moisture. In order to provide environmental protection of the prism surfaces, a transparent optical film 32 is applied to the Optical Layer as part of the assembled solar cell as shown in Figure 6.

[00033] The Optical Layer 10 may require a base layer 14 for structural purposes depending on the method used to construct the prismatic surface as described previously. Figure 7 shows an assembled solar cell that uses an Optical Layer that includes a Base Layer 14 which may be secured to vertical side walls 41. The top and bottom surfaces 34, 36 of the Optical Layer 10 are preferably coated with an anti-reflective material with a Refraction Index equal to the square root of the Refraction Index of the material used for the Optical Layer 10. For an Optical Layer 10 with a Refraction Index of 1.59, the Refraction Index for the coating material should be about 1.26. Materials with such a low Refraction Index are not common, so Magnesium Fluoride (MgF) may be used as it has a low Refraction Index of 1.3. Magnesium Fluoride is commonly used to reduce reflections on windows and eye glasses. Although the Refraction Index is higher than desired for this example, it will still provide improved transmission of the incident light through the Optical Layer 10 to the solar cell 12. The thickness of the coating should be equal to one- quarter wavelength of the light frequency that represents the greatest proportion of incident energy to the solar cell that will result in maximum energy conversion. For visible light, this range is typically around 600 nm, so the thickness would be about 150 nm. Both surfaces of the Transparent Protective Layer 32 should also have this anti- reflection coating applied.

The Optical Layer 10 should be constructed as thin as possible to reduce light transmission losses and the amount of material used to manufacture the solar cell 12. As an example, the 3M Vikuiti™ prismatic film has a thickness of about 150 to 160 microns. The Optical Layer 10 should be constructed using an optically transparent material with as high a Refractive Index as possible. The higher the Refractive Index of the prismatic Optical Layer 10, the greater the range of incident angles of light that will be returned to the solar cell by refraction and total internal reflection, resulting in higher cell efficiency. Most available optical films are either glass or plastic with Refractive Index values of 1.4 to 1.6 or higher. All the light trajectory examples shown are based on an Optical Layer with a Refractive Index of 1.59.

Anti-reflection coatings that use quarter wavelength interference layers with matched Refraction Index values are preferably applied to the surface of the solar cell to reduce the amount of reflected light at the surface of the solar cell. [00037] Mirrors or reflectors 40 are applied to the bottom and sides of the solar cell 12 to return any emitted, reflected, or un-absorbed light back to the solar cell in order to provide another opportunity to convert the light into electricity, as indicated by reflector 40 in Fig. 6, 7. These mirrors can be constructed from metallized glass or plastic film with a highly polished aluminum or silver surface.

[00038] For fixed-mounted solar panels it will be desirable to have some means to concentrate light from wide incidence angles. While various means can be used to concentrate the incident light, one means to accomplish the concentration may be by means of a planar convex lens to improve the acceptance angle for incident light. In this way incident light with an angle approaching 90 degrees from vertical can be captured. [00039] While a number of exemplary aspects and embodiments have been

discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the invention be interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope. While the preferred embodiment has been described in relation to photovoltaic cells, the invention also has application to solar thermal panels or other applications for trapping light energy.

Claims

WHAT IS CLAIMED IS:

1. A solar cell assembly for collecting incident radiant energy from a source of radiant energy, said solar cell assembly comprising:

i) a solar cell comprising an upper surface which receives incident light from said source of radiant energy; and

ii) an optical layer mounted between said source of radiant energy and said solar cell, wherein said optical layer comprises an array of total internal reflection elements which substantially reflect light re-emitted from the solar cell back into the solar cell by a combination of refraction and total internal reflection and which do not significantly reflect incident light;

wherein said optical layer is spaced from said solar cell, the refractive index of said space being substantially less than the refractive index of said solar cell.

2. The solar cell assembly of claim 1 wherein said space has a refractive index close to 1.0.

3. The solar cell assembly of claims 1 or 2 wherein said total internal reflection elements each comprise a prismatic cross section.

4. The solar cell assembly of claims 1 or 2 wherein said space is a vacuum.

5. The solar cell assembly of claim 1 or 2 wherein said space is an air-filled gap.

6. The solar cell assembly of claims 1 to 5 wherein said optical layer is

constructed of a transparent material having a refractive index greater than approximately 1.4.

7. The solar cell assembly of claim 1 to 6 wherein said solar cell further comprises a lower surface which is mirrored to reflect light which has passed through said solar cell back into said solar cell.

8. The solar cell assembly of claims 1 to 7 wherein said solar cell further

comprises opposed sides joining said upper and lower surfaces, wherein said opposed sides are mirrored to reflect light which has passed through said solar cell back into said solar cell.

9. The solar cell assembly of claims 1 to 8 wherein each said total internal reflection element is symmetric with an internal apex angle of 90 degrees at the top and equal internal angles of 45 degrees at the base. 10. The solar cell assembly of claims 1 to 9 wherein said optical layer comprises a total internal reflective film.

The solar cell assembly of claim 10 wherein said total internal reflective film is Vikuiti™ prismatic film.

The solar cell assembly of claim 10 wherein said total internal reflective film has a thickness of about 150 to 160 microns.

The solar cell assembly of claims 1 to 12 wherein said optical layer comprises a transparent base layer for mounting said optical layer in relation to said solar cell in said solar cell assembly.

The solar cell assembly of claims 1 to 13 wherein a transparent optical film is applied to the optical layer as a protective layer.

A method of collecting radiant energy from a source of radiant energy, said method comprising providing a solar cell assembly comprising:

The method of claim 15 wherein said space has a refractive index close to 1.0.

The method of claim 15 wherein said total internal reflection elements each comprise a prismatic cross section.

18. The method of claim 15 or 16 wherein said space is a vacuum.

19. The method of claim 15 or 16 wherein said space is an air-filled gap. 20. The method of claims 15 to 19 wherein said optical layer is constructed of a transparent material having a refractive index greater than approximately 1.4.

21. The method of claims 15 to 20 wherein said solar cell further comprises a lower surface which is mirrored to reflect light which has passed through said solar cell back into said solar cell.

22. The method of claims 15 to 21 wherein said solar cell further comprises

opposed sides joining said upper and lower surfaces, wherein said opposed sides are mirrored to reflect light which has passed through said solar cell back into said solar cell.

23. The method of claims 15 to 22 wherein each said total internal reflection

element is symmetric with an internal apex angle of 90 degrees at the top and equal internal angles of 45 degrees at the base.

24. The method of claims 15 to 23 wherein said optical layer comprises a total internal reflective film.

25. The method of claim 24 wherein said total internal reflective film is Vikuiti™ prismatic film.

26. The method of claim 24 wherein said total internal reflective film has a

thickness of about 150 to 160 microns. 27. The method of claims 15 to 26 wherein said optical layer comprises a

transparent base layer for mounting said optical layer in relation to said solar cell.

The method of claims 15 to 27 wherein a transparent optical film is applied to the optical layer as a protective layer.