Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/40—Optical elements or arrangements
  • H10F77/42—Optical elements or arrangements directly associated or integrated with photovoltaic cells, e.g. light-reflecting means or light-concentrating means
  • H10F77/488—Reflecting light-concentrating means, e.g. parabolic mirrors or concentrators using total internal reflection
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F19/00—Integrated devices, or assemblies of multiple devices, comprising at least one photovoltaic cell covered by group H10F10/00, e.g. photovoltaic modules
  • H10F19/80—Encapsulations or containers for integrated devices, or assemblies of multiple devices, having photovoltaic cells
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/40—Optical elements or arrangements
  • H10F77/42—Optical elements or arrangements directly associated or integrated with photovoltaic cells, e.g. light-reflecting means or light-concentrating means
  • H10F77/484—Refractive light-concentrating means, e.g. lenses
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/52—PV systems with concentrators

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• TIR total internal reflection
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Figure 6 is a schematic diagram illustrating a cross-section of an
• Figure 7 is a schematic diagram illustrating a cross-section of an
• 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.
• 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.
• the Optical Layer 10 is separated from the solar cell surface 11 by a gap
• 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.
• 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.
• 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.
• the sides 17 of prismatic elements 19 of the Optical Layer 10 form angles A to the horizontal of 45 degrees.
• 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.
• 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.
• 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
• 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.
• 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 VikuitiTM product family of optical films.
• TIR Total-Internal-Reflection
• 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.
• a transparent optical film 32 is applied to the Optical Layer as part of the assembled solar cell as shown in Figure 6.
• 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.
• the Refraction Index for the coating material should be about 1.26.
• Magnesium Fluoride 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.
• the 3M VikuitiTM 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.
• 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.

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Citations (4)

• 2013
  • 2013-01-11 WO PCT/CA2013/000033 patent/WO2013104066A1/fr not_active Ceased

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