Apparatus and systems for enhanced light trapping in solar cells

Abstract

An apparatus for coupling light onto a semiconductor substrate in a solar cell includes a cover layer having a first surface configured to receive solar light, having a first index of refraction with respect to the solar light, and having a second surface disposed in contact with the semiconductor substrate; and the apparatus also includes a cavity disposed inside the cover layer, wherein: the cavity has a second index of refraction being smaller than the first index of refraction, a transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and a top length of the top end is smaller than a base length of the bottom end. Another apparatus for coupling light in a solar cell includes a cover layer including a textured top surface comprising an array of parallel saw-tooth-shaped grooves.

Description

APPARATUS AND SYSTEMS FOR ENHANCED LIGHT TRAPPINGIN SOLAR CELLSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application No. 63 / 612,445 filed December 20, 2023, the content of which is incorporated herein by reference it its entirety.TECHNICAL FIELD

[0002] The general area of technology of the present disclosure is related to solar cells, particularly related to apparatus and systems for enhanced light trapping in photovoltaic solar cells.BACKGROUND INFORMATION

[0003] Over past years, the efficiency of solar cell has been continuously improving. One factor affecting the conversion efficiency of the solar cells is due to reflections: not all of the solar light that reaches a top surface of the solar cell is absorbed by the solar cell, and a certain amount of solar light is reflected away from the solar cell. Therefore, a solar cell’s efficiency can be increased by minimizing the amount of light reflected away from the solar cell.

[0004] The present disclosure describes various embodiments including apparatus and / or systems for enhanced light trapping in solar cells, thus, improving efficiency of solar cells.BRIEF SUMMARY

[0005] This summary is a brief description of certain aspects of this disclosure. It is not intended to limit the scope of this disclosure.

[0006] The present disclosure is directed to an apparatus for coupling light onto a semiconductor substrate in a solar cell. The apparatus includes a cover layer disposed on a semiconductor substrate, wherein: a first surface of the cover layer is configured to receive solar light, the cover layer has a first index of refraction with respect to the solar light, a second surface of the cover layer is disposed in contact with the semiconductor substrate andtransmitting the received solar light towards the semiconductor substrate, and the first surface and the second surface are opposite surfaces with respect to the cover layer, a first direction being from the second surface to the first surface, and a second direction being perpendicularly to the first direction; and a cavity disposed inside the cover layer, wherein: the cavity has a second index of refraction with respect to the solar light, the second index of refraction is smaller than the first index of refraction, the cavity has a longitudinal axis along the second direction, a transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and a top length of the top end is smaller than a base length of the bottom end.

[0007] The present disclosure describes a cover layer for coupling light onto a semiconductor substrate in a solar cell. The cover layer includes a first surface being a textured surface comprising an array of parallel saw-tooth-shaped grooves, the first surface configured to receive solar light; and a second surface disposed on and in contact with the semiconductor substrate, the second surface configured to transmit the received solar light towards the semiconductor substrate, wherein the first surface and the second surface are opposite surfaces with respect to the cover layer.

[0008] The present disclosure also describes a system for coupling light onto a semiconductor substrate in a solar cell. The system includes a cover layer disposed on a semiconductor substrate, wherein: a first surface of the cover layer is a textured surface comprising an array of parallel saw-tooth-shaped grooves, the first surface is configured to receive solar light, the cover layer has a first index of refraction with respect to the solar light, a second surface of the cover layer is disposed in contact with the semiconductor substrate and transmitting the received solar light towards the semiconductor substrate, and the first surface and the second surface are opposite surfaces with respect to the cover layer, a first direction being from the second surface to the first surface, and a second direction being perpendicularly to the first direction; a cavity disposed inside the cover layer, wherein: the cavity has a second index of refraction with respect to the solar light, the second index of refraction is smaller than the first index of refraction, the cavity has a longitudinal axis along the second direction, a transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and a top length of the top end is smaller than a base length of the bottom end; and wherein a longitudinal axis of the textured surface for the first surface of the cover layer is perpendicular to the longitudinal axis of the cavity.

[0009] The present disclosure discloses various embodiments directed to method for fabricating any portion or any combinations of apparatus, devices, and / or systems in the present disclosure.

[0010] The present disclosure discloses various embodiments directed to an electronic apparatus, including a memory storing one or more programs and a processor electrically coupled to the memory and configured to execute the one or more programs to perform any method or step or their combination in this disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Various exemplary embodiments of the present disclosure are described in detail below with reference to the following drawings. The drawings are provided for purposes of illustration only and merely depict exemplary embodiments of the present disclosure to facilitate the understanding of the present disclosure. Therefore, the drawings should not be considered as limiting of the breadth, scope, or applicability of the present disclosure. It should be noted that for clarity and ease of illustration these drawings are not necessarily drawn to scale.

[0012] FIG. 1A shows a schematic diagram of various embodiments in the present disclosure.

[0013] FIG. IB shows another schematic diagram of various embodiments in the present disclosure.

[0014] FIG. 1C shows another schematic diagram of various embodiments in the present disclosure.

[0015] FIG. ID shows another schematic diagram of various embodiments in the present disclosure.

[0016] FIG. IE shows another schematic diagram of various embodiments in the present disclosure.

[0017] FIG. 2A shows a schematic diagram of various embodiments of a textured cover layer in the present disclosure.

[0018] FIG. 2B shows another schematic diagram of various embodiments of the textured cover layer in the present disclosure.

[0019] FIG. 2C shows another schematic diagram of various embodiments of the textured cover layer in the present disclosure.

[0020] FIG. 3 shows a schematic diagram of an exemplary embodiment in the present disclosure.

[0021] FIG. 4A show a geometric view of a baseline solar cell with grooved air / glass interface and pyramidal textured silicon, illustrating a unit solar cell with a schematic diagram of Ist-order scattered photons.

[0022] FIG. 4B show another geometric view of the baseline solar cell with grooved air / glass interface and pyramidal textured silicon, illustrating the unit solar cell with the schematic diagram of Ist-order scattered photons.

[0023] FIG. 5A shows a geometric view of an exemplary embodiment of a solar cell containing an air-filled, triangular cavity, illustrating a unit solar cell with a schematic diagram.

[0024] FIG. 5B shows another geometric view of the exemplary embodiment of the solar cell containing the air-filled, triangular cavity, illustrating a unit solar cell with a schematic diagram.

[0025] FIG. 5C shows a three-dimensional (3D) geometric view of an exemplary embodiment of a solar cell containing an air-filled, triangular cavity.

[0026] FIG. 6 shows a geometry view of a solar cell containing a textured air / glass interface in the present disclosure.

[0027] FIG. 7 shows a cross-section diagram of a saw-tooth textured cover glass with some light rays of normally incident illumination.

[0028] FIG. 8 shows a chart of angular dependence of the absorption to sunlight of various glass textures in the present disclosure.

[0029] FIG. 9 shows a chart of decrease in solar irradiance with increasing angular distance from the zenith.DETAILED DESCRIPTION OF THE INVENTION

[0030] The invention will now be described in detail hereinafter with reference to the accompanied drawings, which form a part of the present invention, and which show, by way of illustration, specific examples of embodiments. Please note that the invention may, however,be embodied in a variety of different forms and, therefore, the covered or claimed subject matter is intended to be construed as not being limited to any of the embodiments to be set forth below. Please also note that the invention may be embodied as methods, devices, components, or systems. Accordingly, embodiments of the invention may, for example, take the form of hardware, software, firmware or any combination thereof.

[0031] Throughout the specification and claims, terms may have nuanced meanings suggested or implied in context beyond an explicitly stated meaning. Likewise, the phrase “in one embodiment or implementation” (or “in some implementations”) as used herein does not necessarily refer to the same embodiment(s) / implementation(s) and the phrase “in another embodiment or implementation” (or “in other implementations”) as used herein does not necessarily refer to different embodiment(s) / implementation(s). It is intended, for example, that claimed subject matter includes combinations of exemplary embodiments in whole or in part.

[0032] In general, terminology may be understood at least in part from usage in context. For example, terms, such as “and”, “or”, or “and / or,” as used herein may include a variety of meanings that may depend at least in part upon the context in which such terms are used. Typically, “or” if used to associate a list, such as A, B or C, is intended to mean A, B, and C, here used in the inclusive sense, as well as A, B or C, here used in the exclusive sense. In addition, the term “one or more” or “at least one” as used herein, depending at least in part upon context, may be used to describe any feature, structure, or characteristic in a singular sense or may be used to describe combinations of features, structures or characteristics in a plural sense. Similarly, terms, such as “a”, “an”, or “the”, again, may be understood to convey a singular usage or to convey a plural usage, depending at least in part upon context. In addition, the term “based on” or “determined by” may be understood as not necessarily intended to convey an exclusive set of factors and may, instead, allow for existence of additional factors not necessarily expressly described, again, depending at least in part on context.

[0033] Over past years, the efficiency of solar cell has been continuously improving. One factor affecting the conversion efficiency of the solar cells is due to reflections: not all of the solar light that reaches a top surface of the solar cell is absorbed by the solar cell, and a certain amount of solar light is reflected away from the solar cell. Therefore, a solar cell’s efficiency can be increased by minimizing the amount of light reflected away from the solar cell.

[0034] In some implementations, one improvement to solar cells may use a textured cover glass enabling better coupling of the incident light to the cell. In some implementations, another enhancement may use textured silicon to prevent much of the reflection from this component. In some implementations, improvement may use a photon downshifting layer in which UV light (with low quantum efficiency) is absorbed and re-emitted at longer wavelengths (with high quantum efficiency) thereby improving cell performance.

[0035] The present disclosure describes various embodiments including apparatus and / or systems for coupling light onto a semiconductor substrate in a solar cell. In various embodiments, referring to FIGs. 1A-1E, a cover layer 110 on a semiconductor substrate 170 may include a cavity 150, which may be used to enable better coupling of incident solar light 180 to the semiconductor substrate 170 (e.g., silicon containing photovoltaic layer). The index of refraction of the cavity is smaller than the index of refraction of the cover layer, and / or a base end 154 of the cavity may be highly reflective with respect to the solar light. The cover layer may receive the solar light 180 and transmit the received solar light 182 towards the semiconductor substrate.

[0036] In some implementations, the cover layer 110 may include a cover glass of a solar cell, and the cavity may be regarded as an object or a structure contained / embedded within the cover glass, resulting in increased device efficiency over prior technology. The purpose of this object is first to redirect the light incoming to the solar cell thereby creating a region or regions devoid of any light. This redirection can be done carefully so as to not introduce enough angular content (when combined with the angular content of the incoming light field) to cause a portion of the light field to be redirected out of the cell before reaching the silicon. The second purpose of this object is to provide a highly reflective structure so that any light that is reflected from the silicon (or traveling skyward from a photon downshifting emitter) is recycled back to the silicon for additional chances at absorption.

[0037] In some implementations, referring to FIG. 1 A, the cover layer 110 is disposed on the semiconductor substrate 170. The cover layer 110 has a first surface 112 and a second surface 114. The first surface 112 is a “top” surface of the cover layer in FIG. 1A, and the second surface 114 is a “bottom” (or “base”) surface of the cover layer in FIG. 1 A, and thus, the first surface and the second surface are opposite surfaces with respect to the cover layer.

[0038] The first surface 112 of the cover layer is configured to receive solar light 180, the cover layer has a first index of refraction with respect to the solar light. The second surface 114of the cover layer is disposed in contact with the semiconductor substrate 170 and transmitting the received solar light 182 towards the semiconductor substrate.

[0039] A first direction 191 is a direction from the second surface to the first surface, a second direction 192 is perpendicular to the first direction, and a third direction 193 is perpendicular to both of the first direction and the second direction. In some implementations, the second direction may be regarded as “x” direction, the third direction may be regarded as “y” direction, and the first direction may be regarded as “z” direction. Thus, the three direction forms a x-y-z three-dimensional (3D) direction system. In some implementations, when any textured pattern (if there is any) on cover layer surface is not considered, the first direction is perpendicular to the first / second surface of the cover layer, the second direction is parallel to the first / second surface of the cover layer, and the third direction is also parallel to the first / second surface of the cover layer.

[0040] In some implementations as shown in FIG. 1A, the cavity 150 may have a longitudinal axis 159 substantially parallel to (also referred as “along”) the second direction.

[0041] The cavity 150 that is disposed inside the cover layer may have a transverse crosssection in a plane defined by the first direction 191 and the third direction 193, as shown in FIG. IB. Referring to its transverse cross-section, the cavity has a top end 152 and a bottom end 154, and has a height 157 along the first direction In some implementations, the top end 152 is closer than the bottom end 154 to the first surface 112, and / or the bottom end 154 is closer than the top end 152 to the second surface 114.

[0042] Referring to the transverse cross-section, the top end 152 has a top length 153 along the third direction 193; and the bottom end 154 has a base length 155 along the third direction 193. The top length 153 is smaller than the base length 155. For a non-limiting example, the top length 153 may be a fraction of the base length 155, and the fraction may be a value smaller than 1 / 2. In some implementations as shown in FIG. 1C, the top length of the top end 152 may be zero, resulting in the fraction to be zero, and thus, the transverse cross-section of the cavity may have a triangular shape.

[0043] The base length 155 may be between 50% and 90% of a width 111 of the solar cell. For a non-limiting example, the base length 155 may be about 80% of the width 111 of the solar cell.

[0044] The cavity may have an aspect ratio, which is defined by its height 157 divided by its base length 155; and the aspect ratio may be a value between 1 and 5, inclusive. For a non-limiting example, the aspect ratio is 3.25, which means the height is longer than (i.e., to be 3.25 times as) the base length.

[0045] For non-limiting examples as shown in FIGs. ID and IE, the transverse crosssection of the cavity may have various shapes. The two sides (158a and 158b) of the cavity may not be straight line (i.e., may not be a flat surface in three-dimensional view), and may be curved (i.e., a curved surface in three-dimensional view) for example as convex surfaces in FIG. ID or concave surface in FIG. IE. In some implementations, for the examples in FIGs. ID and IE, the top length of the top end may be zero, similar to FIG. 1C.

[0046] In some implementations, the base end is reflective to the solar light, so as to reflect towards the semiconductor substrate any solar light reflected from the semiconductor substrate. The high reflectivity of the base end may be achieved by a reflective coating, for example, a metallic coating (e.g., silver) or dielectric coating suitable for solar lights.

[0047] In some implementations, two sides of the cavity (158a and 158b in FIG. IB) are high reflective to the solar light. The high reflectivity may be achieved by effects of total internal reflection because the index of refraction of the cavity is smaller than the index of refraction of the cover layer. In some implementations, the high reflectivity of the two sides may be achieved by a reflective coating, for example, a metallic coating (e.g., silver) or dielectric coating suitable for solar lights.

[0048] In some implementations, the cavity is sealed and isolated from outer environment so that dirt or humidity does not enter into the cavity. The cavity may contain a medium which has different index of refraction from the cover layer, the medium may include at least one of the following: a type of pure gas (e.g., pure Nitrogen gas) or a mixture of various types of gases (e.g., a dry air), or any other transparent material (e.g., in its solid or liquid form) that has a smaller index of refraction than the cover layer (e.g., glass).

[0049] In some implementations, the transverse cross-section of the cavity has a triangular shape. In some implementations, the cover layer comprises a planar cover glass and an etyleen vinyl acetate (EVA) layer. In some implementations, the first surface of the cover layer comprises at least one of the following: a flat surface, a grooved-textured surface, an upright pyramid-textured surface, an inverted pyramid-textured surface, a saw tooth-textured surface. In some implementations, the textured surface for the first surface of the cover layer has a longitudinal axis perpendicular to the second direction. In some implementations, the grooved- textured surface has grooves with a groove angle of about 45 degree. In some implementations,the semiconductor substrate comprises a Silicon poly-crystalline substrate. In some implementations, the second surface of the cover layer comprises at least one of the following: a flat surface, a groove-textured surface, or a pyramidal-textured surface. In some implementations, the solar light comprises at least one of the following: a sun light, or a sky light.

[0050] In some implementations, there may be an emitter layer disposed between the cover layer and the semiconductor substrate, the emitter layer configured to downshift ultraviolet (UV) light to light with longer wavelengths. When a portion of the downshifted light with longer wavelengths transmits towards the cavity (away from the semiconductor substrate), the portion may be reflected by the base end towards the semiconductor substrate, thus improving the percentage / efficiency of solar lights coupling to the solar cells.

[0051] The present disclosure describes various embodiments including a cover layer with a textured surface for coupling light onto a semiconductor substrate in a solar cell. In various embodiments, referring to FIGs. 2A and 2B, a cover layer 110 on a semiconductor substrate 170 may have a first (textured) surface 112, which may be used to enable better coupling of incident solar light 180 to the semiconductor substrate 170 (e.g., silicon containing photovoltaic layer). The cover layer may receive the solar light 180 and transmit the received solar light 182 towards the semiconductor substrate.

[0052] The various embodiments with a cover layer with a textured surface may improve the performance (efficiency) of crystalline silicon solar cells. To enhance the coupling of light to the solar cell’s cover layer (e.g., cover glass), either an anti-reflection (AR) coating or a surface texture may be applied to the air / glass interface thereby reducing reflection losses and improving device performance. In some implementations, there may be some disadvantage of AR coatings, for example, AR coatings may last a relatively short amount of time (e.g., on the order of months) in comparison with lifetimes expected for solar cells (e.g., several decades).

[0053] Texturing the surface of the cover layer of the solar cell may be a much more robust approach in comparison to AR coatings, and various textures on the cover glass surface may improve light trapping to the cover glass as compared with an uncoated flat glass surface. The various embodiments in the present disclosure may further improve light trapping efficiency, indicating that such a novel surface texture is needed to further improve solar cell efficiency.

[0054] Referring to FIG. 2A, a cover layer 110 (e.g., a cover glass) has a first surface 112 with a surface texture consisting of a series of grooves with an asymmetric cross-section (e.g.,saw tooth cross-section). As described in details in later paragraphs of the present disclosure, the saw-tooth textured cover layer (cover glass) has the following advantages as compared with uncoated flat glass and other types of textured glasses containing symmetric cross-sections, for example, grooves, upright pyramids, and inverted pyramids: higher coupling of normally incident light to the cover glass, higher coupling of Lambertian light (i.e., skylight) to the cover glass, higher performance of solar cells during sunny days for fixed panels and for panels with sun tracking, and / or higher performance of solar cells during overcast days.

[0055] The first surface 112, being a textured surface, includes an array of parallel sawtooth-shaped grooves, and the parallel saw-tooth shaped grooves may have a longitudinal axis 297. The first surface configured to receive solar light 180. The cover layer 110 has a second surface 114 disposed on and in contact with the semiconductor substrate 170, and the second surface is configured to transmit the received solar light 182 towards the semiconductor substrate. The first surface and the second surface are opposite surfaces with respect to the cover layer. A first direction 295 may be defined as pointing from the second surface towards the first surface of the cover layer.

[0056] In some implementations referring to FIGs. 2B and 2C, each saw-tooth-shaped groove 210 in the array of parallel saw-tooth-shaped grooves comprises a first subunit 212 and a second subunit 214, wherein the first subunit comprises a vertical side 212; and the second subunit comprises a sloping side 214.

[0057] In some implementations, the vertical side 212 is substantially parallel to the first direction 295 that is a direction from the second surface to the first surface.

[0058] In some implementations, the sloping side has a sloping angle 215 between 35 and 55 degrees. For a non-limiting example, the sloping angle 215 may be about 45 degree. Here, and in all other portions of the present disclosure unless otherwise specified, “about” a value may refer to a range from 97% of the value to 103% of the value, inclusive.

[0059] In some implementations, a height 213 of the vertical side may be between 1 micrometer and 50 millimeters, inclusive. For a non-limiting example, the height 213 may be about 1 to 2 millimeters, which would result in a cover layer with an economically reasonable and practical thickness and / or, in a manufacturing point of view, is relatively easy to fabricate.

[0060] In some implementations, the cover layer comprises a planar cover glass, or a combination of cover glass and an etyleen vinyl acetate (EVA) layer.

[0061] The various embodiments / implementations described in the present disclosure may be combined in part or in all. For a non-limiting example as shown in FIG. 3, a system embodiment may include a portion or all of the embodiment in FIG. 1 A and a portion or all of the embodiment in FIG. 2A. A cover layer 110 disposed on a semiconductor substrate 170. The first surface 112 of the cover layer is a textured surface comprising an array of parallel sawtooth-shaped grooves, and the first surface is configured to receive solar light 180. The cover layer has a first index of refraction with respect to the solar light. A second surface 114 of the cover layer is disposed in contact with the semiconductor substrate and transmitting the received solar light 182 towards the semiconductor substrate. The first surface and the second surface are opposite surfaces with respect to the cover layer. A first direction 191 is defined as a direction from the second surface to the first surface, and a second direction 192 is perpendicularly to the first direction;

[0062] A cavity 150 is disposed inside the cover layer. The cavity has a second index of refraction with respect to the solar light, and the second index of refraction is smaller than the first index of refraction. The cavity has a longitudinal axis 159 along the second direction. A transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and a top length of the top end is smaller than a base length of the bottom end.

[0063] A longitudinal axis 297 of the textured surface for the first surface of the cover layer is perpendicular to the longitudinal axis 159 of the cavity. The longitudinal axis 159 of the cavity may be parallel to the second direction 192, and the longitudinal axis 297 of the sawtooth grooves of the first surface 112 may be parallel to the third direction 193.

[0064] The present disclosure also includes various embodiment for fabricating a portion or all of any single apparatus, devices, and systems, or a combinations of any portions of more than two apparatus, devices, and systems described in the present disclosure.

[0065] In some implementations, one method for fabricating an apparatus for coupling light onto a semiconductor substrate in a solar cell may include a portion or all of the following steps. In one step, the method includes disposing a cover layer on a semiconductor substrate, wherein: a first surface of the cover layer is configured to receive solar light, the cover layer has a first index of refraction with respect to the solar light, a second surface of the cover layer is disposed in contact with the semiconductor substrate and transmitting the received solar light towards the semiconductor substrate, and the first surface and the second surface are oppositesurfaces with respect to the cover layer, a first direction being from the second surface to the first surface, and / or a second direction being perpendicularly to the first direction. In another step, the method includes disposing a cavity inside the cover layer, wherein: the cavity has a second index of refraction with respect to the solar light, the second index of refraction is smaller than the first index of refraction, the cavity has a longitudinal axis along the second direction, a transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and / or a top length of the top end is smaller than a base length of the bottom end.

[0066] In some implementations, another method for fabricating a cover layer for coupling light onto a semiconductor substrate in a solar cell includes a portion or all of the following steps. In one step, the method includes using a textured surface comprising an array of parallel saw-tooth-shaped grooves as a first surface of the cover layer, the first surface configured to receive solar light. In another step, the method includes disposing a second surface on and in contact with the semiconductor substrate, the second surface configured to transmit the received solar light towards the semiconductor substrate. The first surface and the second surface are opposite surfaces with respect to the cover layer.

[0067] In some implementations, another method for fabricating a system for coupling light onto a semiconductor substrate in a solar cell includes a portion or all of the following steps. In one step, the method includes disposing a cover layer on a semiconductor substrate, wherein: a first surface of the cover layer is a textured surface comprising an array of parallel saw-tooth-shaped grooves, the first surface is configured to receive solar light, the cover layer has a first index of refraction with respect to the solar light, a second surface of the cover layer is disposed in contact with the semiconductor substrate and transmitting the received solar light towards the semiconductor substrate, and / or the first surface and the second surface are opposite surfaces with respect to the cover layer, a first direction being from the second surface to the first surface, and a second direction being perpendicularly to the first direction. In another step, the method includes disposing a cavity inside the cover layer, wherein: the cavity has a second index of refraction with respect to the solar light, the second index of refraction is smaller than the first index of refraction, the cavity has a longitudinal axis along the second direction, a transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and / or a top length of the top end is smaller than a base length of the bottom end. A longitudinal axis of the textured surface for the first surface of the cover layer is perpendicular to the longitudinal axis of the cavity.

[0068] The present disclosure describes various exemplary embodiments in the following paragraphs, which merely serve as examples not limitations to the application of the present disclosure.Embodiment Set I

[0069] The present disclosure describes various embodiments for an “object” disposed within a cover glass of a solar cell, resulting in increased device efficiency.

[0070] In some implementations, a simplified silicon poly-crystalline solar cell (including only the relevant optical components) consists of a device with a planar cover glass, followed by an EVA layer (with the same or similar index of refraction as the glass) in contact with a thick (~ 100 - 200 micrometer) silicon poly-crystalline substrate. This type of device may suffer from reflective losses both from the air / glass interface and as well as from the EVA silicon interface. In order to decrease these losses, silicon solar cells are provided with a textured cover glass and with textured silicon. Once such approach uses a textured cover glass with parallel grooves (at the air / glass interface) and a pyramidal textured silicon substrate. This type of structure, while a significant improvement over the planar case, still suffers from some unwanted reflection losses. The invention disclosed here is applied to this grooved-glass, pyramidal-silicon type of device (and is applicable to other types of devices as well). In addition to the remaining reflective losses of incoming sunlight, this type of device does little to mitigate the radiation losses from solar cells incorporating photon shifting layers (that is, layers in which UV light is absorbed and re-radiated at longer wavelengths). The invention here not only reduces the reflective losses from incident sun light but also the radiative losses from photon shifting.

[0071] In some implementations as shown in FIGs. 4A and 4B, a solar cell device without a cavity inside may be used as a starting (or baseline) solar cell device, containing a grooved, air / glass interface and pyramidal -textured silicon. To simplify description, since the EVA layer can be assumed to have the same index as the glass, the device can be successfully modeled with a single, glass layer. In this device, the air / glass interface consists of a series of parallel grooves with a groove angle of 45°. The textured silicon consists of a rectangular array of pyramids with a base angle of 54°, which is typical of textured silicon. The red rays may represent sun light illuminating the cell at normal incidence. Just above the silicon layer is an absorber / emitter layer capable of downshifting UV light to longer wavelengths. This layer, modeled as a Lambertian emitter, is used for evaluating the amount of downshifted light thatis trapped by the device. The blue (downward component) and green (upward component) rays may represent light emitted by this source. In the model, the four sides of the unit cell may be assumed as 100% reflective so as to represent a solar cell with infinite lateral extent. Once light enters the silicon layer below the pyramidal texture, it is considered to be completely absorbed by this thick silicon layer.

[0072] This starting device may be modeled using a ray tracing program (for example, ZEMAX). It shows that 96.9% of the incident sunlight is captured by the device with the remaining reflected 3.1% leaving the device at the air / glass interface. For the downshifting emitter, 75.0% of this light is captured; and 25% is lost out the top (air / glass) interface. These capture values serve as the benchmark for comparison to the embodiments with cavity inside the cover glass.

[0073] In some implementations as shown in FIGs. 5A-5C , a cover glass includes an airfilled cavity with a triangular cross-section. The cavity (or referred as object) may be disposed within the cover glass, that is, between an upper, textured air / glass interface and a lower, glass / EVA interface. The purpose of this cavity / object may include: i) to redirect the incoming light to generate a light-free region in the glass while still ensuring that this incoming light will reach the silicon surface, and / or ii) provide a reflective surface for recycling light reflected from the silicon.

[0074] In some implementations, a base of the triangular cavity (side closest to the silicon) is 80% of the unit cell with and the length is 3.25X the length of this base. A long axis of the cavity is perpendicular to the grooves on the upper air / glass interface. This orientation, between the long axis of the cavity and the direction of the grooves at the air / glass interface allows the incoming light to reach the silicon. If the orientation between these two structures are parallel, some of the incoming light would be redirected out of the device without encountering the silicon. This is because the grooves impart a large angular content to some of the incoming light which would add to the angular deviation introduced by the triangular cavity resulting in some of the light redirected out of the device. By having these structures perpendicular, the total extent of angular content of the incoming beam may be minimized and all of the light entering the device reaches the silicon layer. FIG. 5C shows a three-dimensional view of the unit cell of the solar cell, illustrating this perpendicular orientation. All three sides of the triangular cavity have high reflectivity. The high reflectivity of the two sloping sides of the cavity are a result of total internal reflection between the glass / air interface of the cavityand the bottom face has a high reflective coating (e.g. thin film reflector with ~ 100% reflectivity).

[0075] In some implementations, the device with a cavity inside may be modeled using a ray tracing program (e.g., ZEMAX). The modeling results show that 98.3% of the incident sunlight is captured by the device with the remaining reflected 1.7% leaving the device at the air / glass interface. For the downshifting emitter, 90.9% of this light is captured; 9.1% is lost out the top (air / glass) interface. Therefore, the capture of the incoming light is enhanced by 1.44% and the capture of the downshifted light is increased by 21.2%. Assuming a typical silicon solar cell efficiency of 22% and an additional cell efficiency of 1% from downshifting, these gains represent a significant increase in (absolute) cell efficiency of 0.53% (= 0.32% from the light entering the cell plus 0.21% from the enhanced capture of the down shifted light).Embodiment Set II

[0076] The present disclosure describes various embodiments for a top surface of a solar cell being textured with an asymmetric cross-section (e.g., saw-tooth cross-section), resulting in increased device efficiency.

[0077] In some implementations, referring to FIG. 6, various surface textures may be compared for their performance in trapping light in solar cells: flat, grooved, upright pyramids, inverted pyramids, and saw-tooth cross-section. The performance of each of these surfaces may be evaluated in terms of the fraction of light that is captured by the glass and also by the resulting absorption by the silicon in the solar cell. These results were obtained by a ray tracing program (e.g., ZEMAX). This model of the solar cell consists of a thick glass / EVA layer in which the air / glass interface is either flat or textured. Since the EVA and glass typically have a similar index of refraction, this combination of materials is modeled as a single layer with an index of 1.5. This layer is in contact with a silicon layer textured with an array of pyramids with a slope angle of 54 degrees. This textured silicon is coated with a thin, conformal layer of SisN4. The index of refraction of the silicon is 4.0 and that of the SislS is 2.05. The silicon is sufficiently thick so that no light exits the bottom interface.

[0078] Referring to FIG. 7, a saw-tooth texture may include an array of parallel grooves whose cross-section is shown in the figure. Each “tooth” of this texture includes a vertical side and a sloping face at an angle of 45° with the average surface plane. To properly compare theperformance of this textured surface with grooved, upright pyramidal and inverted pyramidal surfaces, the slope angle of all the faces of these surfaces were also set to 45°.

[0079] For a first comparison, the amount of light captured in the cover glass with various textures may be modeled. Obviously, any light that is reflected from a given surface texture cannot be recovered and is lost. Two illumination conditions were considered: direct, normally incident light (such as that from sunlight incident on a solar panel suitably oriented), and Lambertian light (or skylight, such as that emitted by the entire sky excluding the direct sunlight). These results are summarized in the middle two columns of Table 1. The capture for the normally incident illumination on the saw tooth texture is nearly 100%; -0.4% higher than the other textures and 4% higher than the (uncoated) flat case. For the Lambertian (skylight) case the capture is 1.6 - 1.8% higher for the saw tooth as compared with the other textures and 7.4% higher as compared with a flat cover glass. It should be noted that the Lambertian column, in addition to the skylight contribution can be interpreted as the total contribution to the cell on overcast days. On a sunny day, roughly 65% of the total light incident on a solar cell (directed at the sun) comes from the sun while the remaining 35% comes from the sky (modeled here with a Lambertian source.) Using these weighting factors the total light captured by the cover glass is listed in the last column of Table 1 (labeled “Combined”). The saw tooth textured glass captures about 99.3% of the total incident light; -0.8% more than the other textured glasses and -5.2% more light than for flat glass. The values in Table 1 could be used to calculate the improvement in cell efficiency if all the light captured by the cover glass is absorbed by the silicon. (For example, if this were true, an 0.8% increase in light capture would translate to an increase in cell efficiency of 0.16% assuming a typical nominal cell efficiency of 20%. ) However, the silicon interface in all cells reflects some light (and to varying degrees). Therefore, to get a better estimate of the impact on cell efficiency it may be necessary to determine how much of this reflected light is lost by the cell.Table 1 : Light capture for flat and several textured cover glasses

[0080] In some implementations, the above analysis may be repeated except this time the light is allowed to travel throughout the cell, undergoing reflection and absorption at the silicon layer as well as recycling within and escaping from the glass layer. In this manner, an estimate of the impact that the glass surface texture has on cell performance can be made (as opposed to just the capture efficiency of the cover glass). The results of this analysis are displayed in Table 2. Since the results in Table 2 consider only normal incidence light (from the sun), these results correspond to solar panels that have the capability to track the movement of the sun across the sky. As before, the cover glasses with texture trap significantly more light than the flat case. For the saw tooth texture, the enhancement as compared with the other textures ranges from 0.8 - 1.2% on overcast days (“Lambertian column”) and from 0.18 - 0.30% on sunny days (“Combined column”).Table 2: Performance of solar cells

[0081] In some implementations, a comparison may be made among the various glass textures for solar panels that are stationary and do not track the sun across the sky. To accomplish this, the sensitivity of each textured cell may be evaluated as function of illumination angle. FIG. 8 graphically shows the results of angular dependence of the absorption to sunlight of various glass textures. For the saw tooth and grooved textures, the plane defined by the angle of incident light and the surface normal is parallel to the surface grooves, the saw tooth structure, overall, has the highest absorption as the illumination angle is varied from 0° (along the surface normal) to 75°. As expected, all the textures have superior performance as compared with the flat cover glass case.

[0082] To properly calculate cell performance as the sun moves across the sky, the angular dependence of the solar illumination is required. Two effects impact the power on a stationary solar cell as the sun travels from the zenith to the horizon. The first effect is simply the change in the projected area of the cell. This is given by cos 0, where 0 is the angle between the sun and the surface normal of the cell. The second effect is known as atmospheric extinction and is the dimming of the sun as a result of the sunlight traversing a thicker amount of the atmosphere as the sun approaches the horizon. FIG. 9 shows a decrease in solar irradiance with increasing angular distance from the zenith. The total effect (black curve) results from the product of the cosine (red curve) and the extinction (blue curve). In some implementations, the total curve in this figure was used to weight the angular dependencies in FIG. 8.

[0083] When the sun traversing the zenith is considered, the results are listed in Table 3. In this table, the Lambertian (overcast condition) is the same as in Table 2. For a sunny day (“Combined” column) the saw tooth texture gives an improvement over the other textures ranging from 0.7 - 1.4%. The “Lambertian” column is the same as that column in Table 2.Table 3 : Cell performance for fixed panels (no sun tracking).

[0084] The present disclosure describes various embodiments with a textured solar cell cover glass with a saw tooth cross-section, which is evaluated and shows superior light trapping as compared with other textures. In addition, calculations show superior cell performance for solar panels with and without sun tracking. In addition, performance is improved under overcast conditions.

[0085] While the particular invention has been described with reference to illustrative embodiments, this description is not meant to be limiting. Various modifications of the illustrative embodiments and additional embodiments of the invention will be apparent to one of ordinary skill in the art from this description. Those skilled in the art will readily recognizethat these and various other modifications can be made to the exemplary embodiments, illustrated and described herein, without departing from the spirit and scope of the present invention. It is therefore contemplated that the appended claims will cover any such modifications and alternate embodiments. Certain proportions within the illustrations may be exaggerated, while other proportions may be minimized. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive.

Claims

CLAIMSWhat is claimed is:

1. An apparatus for coupling light onto a semiconductor substrate in a solar cell, the apparatus comprising: a cover layer disposed on a semiconductor substrate, wherein: a first surface of the cover layer is configured to receive solar light, the cover layer has a first index of refraction with respect to the solar light, a second surface of the cover layer is disposed in contact with the semiconductor substrate and transmitting the received solar light towards the semiconductor substrate, and the first surface and the second surface are opposite surfaces with respect to the cover layer, a first direction being from the second surface to the first surface, and a second direction being perpendicularly to the first direction; and a cavity disposed inside the cover layer, wherein: the cavity has a second index of refraction with respect to the solar light, the second index of refraction is smaller than the first index of refraction, the cavity has a longitudinal axis along the second direction, a transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and a top length of the top end is smaller than a base length of the bottom end.

2. The apparatus according to claim 1, wherein a height along the first direction in the transverse cross-section of the cavity is larger than the base length of the bottom end.

3. The apparatus according to claim 1, wherein the base end is reflective to the solar light.

4. The apparatus according to claim 1, wherein the cavity contains a medium different from material of the cover layer, the medium may include at least one of the following: a type of pure gas, a dry air, or a nitrogen gas.

5. The apparatus according to claim 1, wherein the transverse cross-section of the cavity has a triangular shape.

6. The apparatus according to claim 1, wherein the cover layer comprises a planar cover glass and an etyleen vinyl acetate (EVA) layer.

7. The apparatus according to claim 1, wherein the first surface of the cover layer comprises at least one of the following: a flat surface, a grooved-textured surface, a upright pyramid-textured surface, an inverted pyramid-textured surface, a saw tooth-textured surface.

8. The apparatus according to claim 7, wherein a textured surface for the first surface of the cover layer has a longitudinal axis perpendicular to the second direction.

9. The apparatus according to claim 7, wherein the grooved-textured surface has grooves with a groove angle of about 45 degree.

10. The apparatus according to claim 1, wherein the semiconductor substrate comprises a Silicon poly-crystalline substrate.

11. The apparatus according to claim 1, wherein the second surface of the cover layer comprises at least one of the following: a flat surface, a groove-textured surface, or a pyramidal -textured surface.

12. The apparatus according to claim 1, wherein the solar light comprises at least one of the following: a sun light, or a sky light.

13. The apparatus according to claim 1, further comprising: an emitter layer disposed between the cover layer and the semiconductor substrate, the emitter layer configured to downshift ultraviolet (UV) light to light with longer wavelengths.

14. A cover layer for coupling light onto a semiconductor substrate in a solar cell, the cover layer comprising: a first surface being a textured surface comprising an array of parallel saw-toothshaped grooves, the first surface configured to receive solar light; and a second surface disposed on and in contact with the semiconductor substrate, the second surface configured to transmit the received solar light towards the semiconductor substrate, wherein the first surface and the second surface are opposite surfaces with respect to the cover layer.

15. The cover layer according to claim 14, wherein: each saw-tooth-shaped groove in the array of parallel saw-tooth-shaped grooves comprises a first subunit and a second subunit; the first subunit comprises a vertical side; and the second subunit comprises a sloping side.

16. The cover layer according to claim 15, wherein the vertical side is substantially parallel to a first direction that is a direction from the second surface to the first surface.

17. The cover layer according to claim 15, wherein the sloping side has a sloping angle of 45 degree.

18. The cover layer according to claim 15, wherein a height of the vertical side is between 1 micrometer and 50 millimeters, inclusive.

19. The cover layer according to claim 14, wherein the cover layer comprises a planar cover glass and an etyleen vinyl acetate (EVA) layer.

20. A system for coupling light onto a semiconductor substrate in a solar cell, the system comprising: a cover layer disposed on a semiconductor substrate, wherein: a first surface of the cover layer is a textured surface comprising an array of parallel saw-tooth-shaped grooves, the first surface is configured to receive solar light, the cover layer has a first index of refraction with respect to the solar light, a second surface of the cover layer is disposed in contact with the semiconductor substrate and transmitting the received solar light towards the semiconductor substrate, and the first surface and the second surface are opposite surfaces with respect to the cover layer, a first direction being from the second surface to the first surface, and a second direction being perpendicularly to the first direction; a cavity disposed inside the cover layer, wherein: the cavity has a second index of refraction with respect to the solar light, the second index of refraction is smaller than the first index of refraction, the cavity has a longitudinal axis along the second direction, a transverse cross-section of the cavity has a top end and a bottom end, and the top end is closer than the bottom end to the first surface, and a top length of the top end is smaller than a base length of the bottom end; and wherein a longitudinal axis of the textured surface for the first surface of the cover layer is perpendicular to the longitudinal axis of the cavity.

Images