Abstract:
Solar cell receiver subassemblies for use in a concentrating solar system that concentrates the solar energy onto a solar cell for converting solar energy to electricity. The subassemblies may include an optical element defining an optical channel and forming an optical path. The subassemblies may also include a solar cell receiver comprising a support and a solar cell mounted on the support adjacent to the optical element and in the optical path of the optical channel. The solar cell may include one or more III-V compound semiconductor layers and may be capable of generating in excess of 20 watts of peak DC power. The subassemblies may also include a heat shield mounted over and peripherally adjacent to exterior sides of the optical element to cover and block concentrated light from reaching a surface of the support adjacent to the solar cell.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 12/553,813, filed on Sep. 3, 2009, the contents of which are incorporated herein by reference in its entirety. 
     The disclosure of this application is related to U.S. application Ser. No. 12/485,684, filed on Jun. 16, 2009; U.S. application Ser. No. 12/246,295, filed on Oct. 6, 2008; U.S. application Ser. No. 12/069,642 filed on Feb. 11, 2008; U.S. application Ser. No. 12/264,369, filed on Nov. 4, 2008 which is a divisional of Ser. No. 12/069,642; U.S. application Ser. No. 11/849,033, filed on Aug. 31, 2007; U.S. application Ser. No. 11/830,576, filed on Jul. 30, 2007; and U.S. application Ser. No. 11/500,053, filed on Aug. 7, 2006; the contents of each are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present application is directed to a solar cell receiver subassembly for use in a concentrating solar system, more particularly, to a solar cell receiver subassembly with a heat shield to protect elements of the subassembly from concentrated solar energy. 
     BACKGROUND 
     Historically, solar power (both in space and terrestrially) has been predominantly provided by silicon solar cells. In the past several years, however, high-volume manufacturing of high-efficiency III-V compound semiconductor multijunction solar cells for space applications has enabled the consideration of this alternative technology for terrestrial power generation. Compared to silicon, III-V compound semiconductor multijunction cells are generally more radiation resistant and have greater energy conversion efficiencies, but they tend to cost more to manufacture. Some current III-V compound semiconductor multijunction cells have energy efficiencies that exceed 27%, whereas silicon technologies generally reach only about 17% efficiency. Under concentration, some current III-V compound semiconductor multijunction cells have energy efficiencies that exceed 37%. 
     Generally speaking, the multijunction cells are of n-on-p polarity and are composed of a vertical stack of InGaP/(In)GaAs/Ge semiconductor structures. The III-V compound semiconductor multijunction solar cell layers are typically grown via metal-organic chemical vapor deposition (MOCVD) on germanium (Ge) substrates. The use of the Ge substrate permits a junction to be formed between n- and p-type Ge, thereby utilizing the substrate for forming the bottom or low band gap subcell. The solar cell structures are typically grown on 100-mm diameter Ge wafers with an average mass density of about 86 mg/cm 2 . In some processes, the epitaxial layer uniformity across a platter that holds 12 or 13 Ge substrates during the MOCVD growth process is better than 99.5%. The epitaxial wafers can subsequently be processed into finished solar cell devices through automated robotic photolithography, metallization, chemical cleaning and etching, antireflection (AR) coating, dicing, and testing processes. The n- and p-contact metallization is typically comprised of predominately Ag with a thin Au cap layer to protect the Ag from oxidation. The AR coating is a dual-layer TiO x /Al 2 O 3  dielectric stack, whose spectral reflectivity characteristics are designed to minimize reflection at the coverglass-interconnect-cell (CIC) or solar cell assembly (SCA) level, as well as, maximizing the end-of-life (EOL) performance of the cells. 
     In some compound semiconductor multijunction cells, the middle cell is an InGaAs cell as opposed to a GaAs cell. The indium concentration may be in the range of about 1.5% for the InGaAs middle cell. In some implementations, such an arrangement exhibits increased efficiency. The advantage in using InGaAs layers is that such layers are substantially better lattice matched to the Ge substrate. 
     SUMMARY 
     The present application is directed to solar cell receiver subassemblies for use in a concentrating solar system that concentrates the solar energy onto a solar cell for converting solar energy to electricity. The subassemblies may include an optical element defining an optical channel and forming an optical path. The subassemblies may also include a solar cell receiver comprising a support and a solar cell mounted on the support adjacent to the optical element and in the optical path of the optical channel. The solar cell may include one or more III-V compound semiconductor layers and may be capable of generating in excess of 20 watts of peak DC power. The subassemblies may also include a heat shield mounted over and peripherally adjacent to exterior sides of the optical element to cover and block concentrated light from reaching a surface of the support adjacent to the solar cell. 
     Some embodiments may incorporate or implement fewer of the aspects or features noted in the foregoing summaries. 
     The present invention is not limited to the above features and advantages. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially exploded perspective view of an embodiment of a solar cell receiver including a solar cell, a metalized ceramic substrate and a heat sink positioned relative to a primary focusing element. 
         FIG. 2  is a partially exploded perspective view of an embodiment of a solar cell receiver including a concentrator, a solar cell, and a ceramic substrate. 
         FIG. 3  is perspective view of an embodiment of a solar cell receiver with a concentrator, a solar cell, and a ceramic substrate. 
         FIG. 4  is a perspective view of an embodiment of a solar cell receiver with a heat shield. 
     
    
    
     DETAILED DESCRIPTION 
     Details of the present invention will now be described including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of the actual embodiment nor the relative dimensions of the depicted elements, and are not drawn to scale. 
     Solar cell receivers include a solar cell for converting solar energy into electricity. In various implementations described herein, a triple-junction III-V compound semiconductor solar cell is employed, but other types of solar cells could be used depending upon the application. Solar cell receivers often contain additional components, e.g., connectors for coupling to an output device or other solar cell receivers. 
     For some applications, a solar cell receiver may be implemented as part of a solar cell module. A solar cell module may include a solar cell receiver and a lens coupled to the solar cell receiver. The lens is used to focus received light onto the solar cell receiver. As a result of the lens, a greater concentration of solar energy can be received by the solar cell receiver. In some implementations, the lens is adapted to concentrate solar energy by a factor of 400 or more. For example, under 500-Sun concentration, 1 cm 2  of solar cell area produces the same amount of electrical power as 500 cm 2  of solar cell area would, without concentration. The use of concentration, therefore, allows substitution of cost-effective materials such as lenses and mirrors for the more costly semiconductor cell material. Two or more solar cell modules may be grouped together into an array. These arrays are sometimes referred to as “panels” or “solar panels.” 
       FIG. 1  illustrates an embodiment of a solar cell receiver  100  including a solar cell  102 . In one embodiment, the solar cell  102  is a triple-junction III-V compound semiconductor solar cell which comprises a top cell, a middle cell and a bottom cell arranged in series. In another embodiment, the solar cell  102  is a multijunction solar cell having n-on-p polarity and is composed of InGaP/(In)GaAs III-V compounds on a Ge substrate. The light received by the solar cell receiver  100  may initially pass through various components, including a lens  200 , a secondary optical element  104 , and a concentrator  106 . Various embodiments may include different combinations and arrangements of components. Some embodiments may include fewer components. In some embodiments, the light passes through a single component prior to reaching the solar cell  102 . 
     The lens  200  concentrates light towards the solar cell receiver  100 . The distance between the lens  200  and the solar cell receiver  100  may vary depending upon the specific construction of each, and/or the overall construction of the solar module. The lens  200  may be Fresnel lenses, or may be conventional spherical lenses. An advantage of Fresnel lenses is that they require less material compared to a conventional spherical lens. In one embodiment, the lens  200  has a rectangular shape. In one more specific embodiment, the lens  200  is 9 inches by 9 inches. The lens  200  may be made from various materials, including but not limited to acrylic, plastic, and glass. 
     Under ideal conditions, the lens  200  focuses the light directly to the solar cell  102 . However, in most circumstances, this does not occur due to a variety of causes, including but not limited to chromatic aberration of a refractive lens design, misalignment of the solar cell  102  relative to the lens  200  during construction, misalignment during operation due to tracker error, structural flexing, and wind load. The difference between an ideal setup and a misaligned setup may be a minor variation in the positioning of the lens of less than 1°. 
     The secondary optical element  104  is positioned along an optical path between the lens  200  and the solar cell  102  and acts as a light spill catcher to cause more of the light to reach the solar cell  102  in circumstances when the lens  200  does not focus light directly on the solar cell  102 . The secondary optical element  104  includes an entry aperture  105  that receives concentrated light from the lens  200  and an exit aperture  107  that transmits the concentrated light towards the solar cell  102 . The secondary optical element  104  may include a tapered shape with the entry aperture  105  being larger than the exit aperture  107 . As shown in this implementation, the secondary optical element  104  has four reflective walls. In other implementations, different shapes (e.g., three-sided to form a triangular cross-section) may be employed. The secondary optical element  104  can be made of metal, plastic, or glass or other materials. 
     An intermediate region  112  extends between the apertures  105 ,  107  and includes a reflective surface to direct the light towards the solar cell  102 . The reflective surface can be formed from different materials and have different optical characteristics. For example, in some implementations, the reflective surface includes a silver coating or other material for high reflectivity. In some cases, the reflective coating is protected by a passivation coating such as SiO 2  to protect against oxidation, tarnish or corrosion. An example of an optical element with a reflective surface is disclosed in U.S. patent application Ser. No. 12/402,814 filed on Mar. 12, 2009, which is incorporated herein by reference in its entirety. 
     The secondary optical element  104  has one or more mounting tabs  114  for attaching to a bracket  116  via one or more fasteners  118 . The bracket  116  is provided for mounting the secondary optical element  104  to a heat sink  120  via one or more fasteners  122 . The bracket  116  is thermally conductive so that heat energy generated by the secondary optical element  104  during operation can be transferred to the heat sink  120  and dissipated. 
     In one embodiment, a concentrator  106  is disposed along the optical path between the exit aperture  107  of the secondary optical element  104  and the solar cell  102 . The concentrator  106  is preferably glass and has an optical inlet  108  and an optical outlet  110 . In one embodiment, the concentrator  106  is solid glass. The concentrator  106  amplifies the light exiting the secondary optical element  104  and directs the amplified light toward the solar cell  102 . In some implementations, the concentrator  106  has a generally square cross section that tapers from the inlet  108  to the outlet  110 . In some implementations, the optical inlet  108  of the concentrator  106  is square-shaped and is about 2 cm×2 cm and the optical outlet  110  is about 0.9 cm×0.9 cm. The dimensions of the concentrator  106  may vary with the design of the solar cell module and the solar cell receiver  100 . For example, in some implementations the dimensions of the optical outlet  110  are approximately the same as the dimensions of the solar cell  102 . In one embodiment, the concentrator  106  is a 2× concentrator. The bottom surface of the concentrator  106  can be directly attached to the upper surface of the solar cell  102  using an adhesive such as a silicone adhesive. 
     The solar energy received by the solar cell  102  is concentrated by the lens  200  and the concentrator  106 . In one embodiment, the light is concentrated by a factor of 1000 or more. 
       FIG. 2  illustrates the solar cell  102  positioned along the optical path to receive the concentrated light and being mounted on a ceramic substrate  126 . The ceramic substrate  126  has metalized upper and lower surfaces  128  and  130 . Both surfaces  128  and  130  of the ceramic substrate  126  are metalized to increase the heat transfer capacity of the ceramic substrate  126 , enabling the solar cell receiver  100  to more adequately handle rapid temperature changes that occur due to abrupt changes in solar cell operating conditions. For example, the solar cell  102  generates heat energy when converting light to electricity. Having both the upper and lower surfaces  128  and  130  of the ceramic substrate  126  metalized provides for a faster transfer of the heat energy from the solar cell  102  to the heat sink  120  for dissipation. The opposite condition occurs when the solar cell  102  becomes suddenly shaded. That is, the solar cell  102  stops producing electricity and rapidly cools as does the secondary optical element  104 . The metalized upper and lower surfaces  128  and  130  of the ceramic substrate  126  prevent the solar cell  102  from cooling too rapidly by transferring heat energy from the heat sink  120  to the solar cell  102 , and depending on the thermal conditions, to the secondary optical element  104  as well. The increased heat transfer capacity of the solar cell receiver  100  reduces the amount of stress imparted to the interface between the solar cell  102  and the ceramic substrate  126  during rapid temperature changes, ensuring a reliable solar cell-to-substrate interface. 
     The metalized upper surface  128  of the ceramic substrate  126  is in contact with the solar cell  102  and has separated conductive regions  132  and  134  for providing isolated electrically conductive paths to the solar cell  102 . The first conductive region  132  provides an anode electrical contact point for the solar cell  102  and the second conductive region  134  provides a cathode electrical contact point for the solar cell  102 . 
     In one embodiment, an upper conductive contact area  140  is disposed at the upper surface  138  of the solar cell  102  and occupies the perimeter of the solar cell  102 . In some implementations, the upper conductive contact area  140  can be smaller or larger to accommodate the desired connection type. For example, the upper conductive contact area  140  may touch only one, two or three sides (or portions thereof) of the solar cell  102 . In some implementations, the upper conductive contact area  140  is made as small as possible to maximize the area that converts solar energy into electricity, while still allowing electrical connection. While the particular dimensions of the solar cell  102  will vary depending on the application, standard dimensions are about a 1 cm 2 . For example, a standard set of dimensions can be about 12.58 mm×12.58 mm overall, about 0.160 mm thick, and a total active area of about 108 mm 2 . For example, in a solar cell  102  that is approximately 12.58 mm×12.58 mm, the upper conductive contact area  140  can be about 0.98 mm wide and the active area can be about 10 mm×10 mm. 
     The upper conductive contact area  140  of the solar cell  102  may be formed of a variety of conductive materials, e.g., copper, silver, and/or gold-coated silver. In this implementation, it is the n-conductivity cathode (i.e. emitter) side of the solar cell  102  that receives light, and accordingly, the upper conductive contact area  140  is disposed on the cathode side of the solar cell  102 . In one embodiment, the upper conductive contact area  140  of the solar cell  102  is wire bonded to the second conductive region  134  of the metalized upper surface  128  of the ceramic substrate  126  via one or more bonding wires  142 . 
     A bypass diode  124  is connected in parallel with the solar cell  102 . In some implementations, the diode  124  is a semiconductor device such as a Schottky bypass diode or an epitaxially grown p-n junction. For purposes of illustration, the bypass diode  124  is a Schottky bypass diode. External connection terminals  125  and  127  are provided for connecting the solar cell  102  and the diode  124  to other devices, e.g., adjacent solar cell receivers (not shown). 
     The functionality of the bypass diode  124  can be appreciated by considering multiple solar cells  102  connected in series. Each solar cell  102  can be envisioned as a battery, with the cathode of each of the diodes  124  being connected to the positive terminal of the associated “battery” and the anode of each of the diodes  124  being connected to the negative terminal of the associated “battery.” When one of the serially-connected solar cell receivers  100  becomes damaged or shadowed, its voltage output is reduced or eliminated (e.g., to below a threshold voltage associated with the diode  124 ). Therefore, the associated diode  124  becomes forward-biased, and a bypass current flows only through that diode  124  (and not the solar cell  102 ). In this manner, the non-damaged or non-shadowed solar cell receivers  100  continue to generate electricity from the solar energy received by those solar cells. If not for the bypass diode  124 , substantially all of the electricity produced by the other solar cell receivers would pass through the shadowed or damaged solar cell receiver, destroying it, and creating an open circuit within, e.g., the panel or array. The solar cell receiver  100  also includes the ceramic substrate  126  such as an alumina substrate for mounting of the solar cell  102  and the heat sink  120  for dissipating heat generated by the solar cell  102  during operation. 
     The bypass diode  124  couples the first conductive region  132  of the metalized upper surface  128  to the second conductive region  134 . In one embodiment, a cathode terminal of the bypass diode  124  is connected to the anode terminal of the solar cell  102  via the first conductive region  132  and an anode terminal of the bypass diode  124  is electrically connected to the cathode terminal of the solar cell  102  via the second conductive region  134 . The anode terminal of the solar cell  102  is formed by a lower conductive surface. The cathode terminal of the solar cell  102  is formed by the upper conductive contact area  140  of the solar cell  102  as described above. The external connection terminals  125  and  127  disposed on the metalized upper surface  128  of the ceramic substrate  126  provide for electrical coupling of a device to the solar cell  102  and the bypass diode  124 . In some implementations, the connector terminals  125  and  127  correspond to anode and cathode terminals, and are designed to accept receptacle plugs (not shown) for connection to adjacent solar cell receivers. 
     The upper surface  128  of the ceramic substrate  126  can be metalized by attaching the metallization layers  132  and  134  to the substrate. In one embodiment, holes  144  are formed in the metallization layers  132 ,  134 .  FIG. 2  shows the ceramic substrate  126  having two metallization layers  132  and  134  attached to the upper substrate surface  128 . The metallization layers  132  and  134  are attached to the upper surface  128  of the ceramic substrate  126  by high temperature reactive bonding or other type of bonding process. A lower surface of the ceramic substrate  126  can be similarly metalized and attached to the heat sink  120 . 
     In some instances, the lens  200  is misaligned relative to the solar cell receiver  100  and concentrated light from the lens  200  strikes the substrate  126  and the various components mounted on the substrate  126 . This concentrated light may burn the substrate  126  and components. To address this issue, a heat shield  150  is positioned to block the concentrated light from striking the substrate  126  and/or components as illustrated in  FIG. 4 . The heat shield  150  is constructed of materials to block the deleterious aspects of the concentrated light from reaching the various components. In one embodiment, the heat shield  150  is constructed of ceramic. Heat shields  150  may also be constructed from various other materials, including but not limited to metals, high-temperature plastics, and silvered glass. 
     In one embodiment, the heat shield  150  includes a fixed shape prior to the attachment to the solar cell receiver  100 . As illustrated in  FIG. 4 , the heat shield  150  may include a first side that faces towards the substrate  126  and a second side that faces away from the substrate  126 . The heat shield  150  may be constructed from a single piece, or may include multiple different pieces.  FIG. 4  includes the heat shield  150  formed from four separate sections that are each attached to the solar cell receiver  100 . Another embodiment includes the heat shield  150  formed as a single unit and sized and shaped to extend around the entire periphery of the solar cell  102 . In heat shields  150  with different sections, the sections may be made from the same or different materials, and may have the same or different shapes or sizes. 
     The heat shield  150  may extend around an entire periphery of the solar cell  102  as illustrated in  FIG. 4 , or a limited section of the periphery. In one embodiment, the heat shield  150  is positioned along just a single side of the solar cell  102 . In another embodiment, the heat shield  150  is positioned over just the components on the substrate  126 . In one embodiment, the heat shield  150  is positioned just over the diode  124 . 
     The heat shield  150  may be attached in various manners. In one embodiment, an adhesive  160  is used to attach the heat shield  150 . In one specific embodiment, the adhesive  160  is a silicon-based adhesive. Other adhesives  160  may also be used to attach the heat shield  150 , including epoxies and urethanes. The heat shield  150  may also be attached with mechanical fasteners, such as pins, rivets, and screws. In heat shields  150  constructed from multiple sections, the sections may be attached by different methods. 
     The heat shield  150  may be shaped to conform to the various components. The heat shield  150  may include a cut-out  152  on an underside to accommodate leads  300  that extend from the terminals  125 ,  127 . Similar cut-outs may also be included to accommodate other components, such as the diode  124 . The heat shield  150  may include a sloping side  151  as illustrated in  FIG. 4  that roughly corresponds to the tapering shape of the concentrator  106 . 
     The heat shield  150  may be laterally spaced away from the periphery of the solar cell  102 . This ensures that the heat shield  150  does not block the concentrated light from reaching the solar cell  102 . In one embodiment as illustrated in  FIG. 4 , the heat shield  150  is positioned laterally away from the solar cell  102  and adjacent to the wires  142  that connect with the solar cell  102 . In the embodiment of  FIG. 4 , the wires  142  are exposed because the misaligned concentrated light is prevented from striking the wires  142  by the tapering shape of the concentrator  106 . 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc and are also not intended to be limiting. Like terms refer to like elements throughout the description. 
     As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
     Without further analysis, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.