Abstract:
A solar cell receiver subassembly for use in a concentrating solar system which concentrates the solar energy onto a solar cell by a factor of 1000 or more for converting solar energy to electricity, including an optical element defining an optical channel, a solar cell receiver having a support; a solar cell mounted on the support adjacent to the optical element and in the optical path of the optical channel, the solar cell comprising one or more III-V compound semiconductor layers and capable of generating in excess of 20 watts of peak DC power; a diode mounted on the support and coupled in parallel with the solar cell; and first and second electrical contacts mounted on the support and coupled in parallel with the solar cell and the diode; and an encapsulant covering the support, the solar cell, the diode, and at least a portion of the exterior sides of the optical element.

Description:
RELATED APPLICATIONS 
       [0001]    The disclosure of this application is related to co-pending U.S. application Ser. No. 12/485,684, filed on Jun. 16, 2009; co-pending 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. 12/254,369, 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 which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present application is directed to a solar cell subassembly for use in a concentrator photovoltaic system, more particularly, to an encapsulated solar cell receiver including a solar cell, metallized ceramic substrate and a concentrator optical element. 
       BACKGROUND 
       [0003]    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 multifunction 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%. 
         [0004]    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. 
         [0005]    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 
       [0006]    According to an embodiment, a solar cell subassembly for converting solar energy to electricity includes an optical element defining an optical channel, a solar cell receiver comprising: 
         [0000]    a support; a solar cell mounted on the support adjacent to the optical element and in the optical path of the optical channel, the solar cell comprising one or more III-V compound semiconductor layers and capable of generating in excess of 20 watts of peak DC power; and an encapsulant covering the support, the solar cell, and at least a portion of the exterior sides of the optical element. 
         [0007]    In another aspect, the present invention provides a method of manufacturing a solar cell receiver, comprising: providing a support; mounting a solar cell comprising one or more III-V compound semiconductor layers and capable of generating in excess of 20 watts of peak DC power on the support; mounting an optical element defining an optical channel over the solar cell so that the solar cell is disposed in the optical path of the optical channel; and encapsulating the support, the solar cell, and at least a portion of the exterior sides of the optical element. 
         [0008]    Of course, 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 
         [0009]      FIG. 1  is a partially exploded perspective view of an embodiment of a solar cell receiver including a solar cell, a metallized ceramic substrate and a heat sink. 
           [0010]      FIG. 2  shows the solar cell and the metallized ceramic substrate of  FIG. 1  in more detail. 
           [0011]      FIG. 3  is a cross-sectional view of the solar cell, the metallized ceramic substrate and the heat sink shown in  FIG. 1 . 
           [0012]      FIG. 4  is a cross-sectional view of the solar cell, the metallized ceramic substrate and the heat sink shown in  FIG. 3  after attaching the concentrator optical element and the encapsulant. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    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. 
         [0014]    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. 
         [0015]    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.” 
         [0016]      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 multifunction solar cell having n-on-p polarity and is composed of InGaP/(In)GaAs III-V compounds on a Ge substrate. In each case, the solar cell  102  is positioned to receive focused solar energy from a secondary optical element  104 . 
         [0017]    The secondary optical element  104  is positioned between the solar cell  102  and a primary focusing element (not shown) such as a lens. The secondary optical element  104  is generally designed to collect solar energy concentrated by the corresponding lens toward the upper surface of the solar cell  102 . The secondary optical element  104  includes an entry aperture  105  that receives light beams from the corresponding lens and an exit aperture  107  that transmits the light beams to the solar cell  102 . The secondary optical element  104  includes an intermediate region  112  between the apertures  105 ,  107 . Under ideal conditions, the lens associated with the secondary optical element  104  focuses the light directly to the solar cell  102  without the light hitting against the secondary optical element  104 . 
         [0018]    In most circumstances, the lens does not focus light directly on the solar cell  102 . This may 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 during construction, misalignment during operation due to tracker error, structural flexing, and wind load. Thus, under most conditions, the lens focuses the light such that it reflects off the secondary optical element  104 . 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°. 
         [0019]    The secondary optical element  104  therefore acts as a light spill catcher to cause more of the light to reach the solar cell  102  in circumstances when the corresponding lens does not focus light directly on the solar cell  102 . The secondary optical element  104  can include a reflective multi-layer intermediate region such as the kind disclosed in U.S. patent application Ser. No. 12/402,814 filed on Mar. 12, 2009, the content of which is incorporated herein by reference in its entirety. The reflective multi-layer intermediate region can be formed from different materials and have different optical characteristics so that the reflectivity of the light beams off secondary optical element  104  and transmitted to the solar cell  102  optimizes the aggregate irradiance on the surface of the solar cell  102  over the incident solar spectrum. For example, in some implementations, the inner surface of the body  112  of the secondary optical element  104  can be coated with silver or another material for high reflectivity. In some cases, the reflective coating is protected by a passivation coating such as SiO 2  to protect the secondary optical element  104  against oxidation, tarnish or corrosion. 
         [0020]    The body  112  of the secondary optical element  104  has one or more mounting tabs  114  for attaching the body  112  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. 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. 
         [0021]    In one embodiment as shown in  FIG. 2 , a concentrator  106  is disposed 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 receiver. 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 cell  102  converts the incoming sunlight directly into electricity by the photovoltaic effect. 
         [0022]    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). 
         [0023]    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 a 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. 
         [0024]      FIG. 2  illustrates the solar cell  102  and the ceramic substrate  126  in more detail. The ceramic substrate  126  has metallized upper and lower surfaces  128  and  130 . Both surfaces  128  and  130  of the ceramic substrate  126  are metallized 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  metallized 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 metallized 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. 
         [0025]    The metallized 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 . The solar cell  102  has a conductive lower surface  136  out-of-view in  FIG. 2 , but visible in the cross-section of  FIG. 3  that is positioned on and connected to the first conductive region  132  of the metallized upper surface  128  of the ceramic substrate  126 . The opposing upper surface  138  of the solar cell  102  has a conductive contact area  140  connected to the second conductive region  134  of the ceramic substrate  126 . 
         [0026]    In one embodiment, the conductive lower surface  136  of the solar cell  102  forms an anode terminal of the solar cell  102  and the conductive contact area  140  disposed at the upper surface  138  of the solar cell  102  forms a cathode terminal. According to this embodiment, the conductive lower surface  136  of the solar cell  102  is positioned on the first conductive region  132  of the ceramic substrate  126  and electrically isolated from the second conductive region  134  to ensure proper operation of the solar cell  102 . In one embodiment, the first conductive region  132  of the ceramic substrate  126  is at least partly surrounded on three sides by the second conductive region  134  about a periphery region of the ceramic substrate  126 . 
         [0027]    In one embodiment, the conductive contact area  140  disposed at the upper surface  138  of the solar cell  102  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. 
         [0028]    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 metallized upper surface  128  of the ceramic substrate  126  via one or more bonding wires  142 . 
         [0029]    The bypass diode couples the first conductive region  132  of the metallized upper surface  128  of the ceramic substrate  126  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  of the ceramic substrate  126  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  of the ceramic substrate  126 . The anode terminal of the solar cell  102  is formed by the lower conductive surface  136  of the solar cell  102  as described above and is out-of-view in  FIG. 2 , but visible in the cross-section of  FIG. 3 . The cathode terminal of the solar cell  102  is formed by the upper conductive contact area  140  of the solar cell  102  also as described above. The external connection terminals  125  and  127  disposed on the metallized 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. 
         [0030]    The upper surface  128  of the ceramic substrate  126  can be metallized by attaching 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 lower metallized surface is out of view in  FIG. 2 , but visible in the cross-section of  FIG. 3 ). 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. The lower surface  130  of the ceramic substrate  126  can be similarly metallized and attached to the heat sink  120 . 
         [0031]      FIG. 3  illustrates a cross-sectional view of the solar cell  102 , ceramic substrate  126  and heat sink  120  of the solar cell receiver  100  along the line labeled X-X′ in  FIG. 1 . The secondary optical element  104 , light concentrator  106  and terminals  125 ,  127  are not shown in  FIG. 3  for ease of illustration. The upper and lower surfaces  128  and  130  of the ceramic substrate  126  are metallized. The upper metallized surface  128  of the substrate  126  has separated conductive regions  132  and  134  for providing electrically isolated anode and cathode connections to the solar cell  102  as described above. 
         [0032]    The solar cell  102  has a conductive lower surface  136  connected to the conductive region  132  of the metallized upper surface  128  of the ceramic substrate  126 . In one embodiment, the conductive lower surface  136  of the solar cell  102  forms the anode terminal of the solar cell  102  and the conductive contact area  140  disposed at the upper surface  138  of the solar cell  102  forms the cathode terminal of the solar cell  102 . The conductive lower surface  136  of the solar cell  102  is positioned on the first conductive region  132  of the metallized upper surface  128  of the ceramic substrate  126  and electrically isolated from the second conductive region  134  to ensure proper operation of the solar cell  102 . 
         [0033]    The lower surface  130  of the ceramic substrate  126  also has a metallization layer  148  that is bonded to the heat sink  120  with a highly thermally conductive attach media  150 , such as a metal-filled epoxy adhesive or solder. Filling an epoxy adhesive with metal increases the thermal conductivity of the interface between the ceramic substrate  126  and the heat sink  120 , further improving the heat transfer characteristics of the solar cell receiver  100 . In one embodiment, the highly thermally conductive attach media  150  is a metal-filled epoxy adhesive having a thickness t epoxy  of approximately 1 to 3 mils. The metal-filled epoxy adhesive can be applied to the lower metallized surface  130  of the ceramic substrate  126 , the heat sink  120  or both and then cured to bond the heat sink  120  to the substrate  126 . In one embodiment, the heat sink  120  is a single-piece extruded aluminum heat sink as shown in  FIG. 1 . 
         [0034]    The solar cell receiver  100  can be manufactured by providing the metallized ceramic substrate  126  and connecting the conductive lower surface  136  of the solar cell  102  to the first conductive region  132  of the metallized upper surface  128  of the substrate  126 . The conductive contact area  140  disposed at the upper surface  138  of the solar cell  102  is connected to the second conductive region  134  of the metallized upper surface  128  of the ceramic substrate  126 , e.g. via one or more bond wires  142 . The heat sink  120  is bonded to the lower metallized surface  130  of the ceramic substrate  126  with the metal-filled epoxy adhesive  150 . 
         [0035]      FIG. 4  illustrates a cross-sectional view of the solar cell  102 , ceramic substrate  126  and heat sink  120  of the solar cell receiver  100  along the line labeled X-X′ in  FIG. 1  after the bonding of the light concentrator  106  to the upper surface  138  of the solar cell  102  by means of a suitable light transparent adhesive  151 . After attachment of the light concentrator  106 , the solar cell  102  is surrounded by an encapsulant  152 , one embodiment of which may be silicone based. The encapsulant is applied over the entire portion of the ceramic substrate  126  surrounding the solar cell  102 , including over the region between the heat sink  120  and the metallized lower surface  130  of the ceramic substrate  126 , as well as optionally over the diode  124 , and then the encapsulant is subsequently cured by heat or other suitable process. 
         [0036]    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. 
         [0037]    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. 
         [0038]    The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.