Abstract:
A system for generating electrical power from solar radiation utilizing a III-V compound multijunction semiconductor solar cell; a concentrator for focusing sunlight on the solar cell, including a concave trough-shaped reflector; and a heat spreader connected to the solar cell for cooling the cell.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to co-pending U.S. patent application Ser. No. 11/109,016 filed Apr. 19, 2005, and Ser. No. 11/280,379 filed Nov. 16, 2005. 
         [0002]    This application is also related to co-pending U.S. patent application Ser. No. 11/45,793 filed Jun. 2, 2006 and assigned to the common assignee. 
     
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention. 
         [0004]    The present invention relates generally to terrestrial solar power systems for the conversion of sunlight into electrical energy, and, more particularly, to the use of III-V compound semiconductor solar cells in conjunction with reflector concentrators which are connected in an array for unitary movement to track the sun. 
         [0005]    2. Description of the Related Art. 
         [0006]    Commercially available silicon solar cells for terrestrial solar power application have efficiencies ranging from 8% to 15%. Compound semiconductor solar cells, based on III-V compounds, have 28% efficiency in normal operating conditions and 32.6% efficiency under concentration. Moreover, it is well known that concentrating solar energy onto the photovoltaic cell increases the cell&#39;s efficiency. 
         [0007]    Terrestrial solar power systems currently use silicon solar cells in view of their low cost and widespread availability. Although compound semiconductor solar cells have been widely used in satellite applications, in which their power-to-weight efficiencies are more important than cost-per-watt considerations in selecting such devices, such solar cells have not yet been designed and configured for terrestrial systems, nor have terrestrial solar power systems been configured and optimized to utilize compound semiconductor solar cells. 
         [0008]    In conventional solar cells constructed with silicon (Si) substrates, one electrical contact is typically placed on a light absorbing or front side of the solar cell and a second contact is placed on the back side of the cell. A photoactive semiconductor is disposed on a light-absorbing side of the substrate and includes one or more p-n junctions, which creates electron flow as light is absorbed within the cell. 
         [0009]    The contact on the face of the cell where light enters is generally expanded in the form of a grid pattern over the surface of the front side and is generally composed of a good conductor such as a metal. The grid pattern does not cover the entire face of the cell since grid materials, though good electrical conductors, are generally not transparent to light. 
         [0010]    The grid pattern on the face of the cell is generally widely spaced to allow light to enter the solar cell but not to the extent that the electrical contact layer will have difficulty collecting the current produced by the electron flow in the cell. The back electrical contact has not such diametrically opposing restrictions. The back contact simply functions as an electrical contact and thus typically covers the entire back surface of the cell. Because the back contact must be a very good electrical conductor, it is almost always made of metal layer. 
         [0011]    The placement of both anode and cathode contacts on the back side of the cell simplifies the interconnection of individual solar cells in a horizontal array, in which the cells are electrically connected in series. Such back contact designs are known from PCT Patent Publication WO 2005/076960 AZ of Gee et al. for silicon cells, and U.S. patent application Ser. No. 11/109/016 filed Apr. 19, 2005, herein incorporated by reference, of the present assignee, for compound semiconductor solar cells. 
         [0012]    Another aspect of terrestrial solar power system is the use of concentrators (such as lenses and mirrors) to focus the incoming sun rays onto the solar cell or solar cell array. The geometric design of such systems also requires a solar tracking mechanism, which allows the plane of the solar cell to continuously face the sun as the sun traverses the sky during the day, thereby optimizing the amount of sunlight impinging upon the cell. 
         [0013]    Still another aspect of concentrator-based solar power cell configuration design is the design of heat dissipating structures or coolant techniques for dissipating the associated heat generated by the intense light impinging on the surface of the semiconductor body. Prior art designs, such as described in PCT Patent Application No. 02/080286 A1, published Oct. 10, 2002, utilize a complex coolant flow path in thermal contact with the (silicon) photovoltaic cells. 
         [0014]    Still another aspect of a solar cell system is the physical structure of the semiconductor material constituting the solar cell. Solar cells are often fabricated in vertical, multijunction structures, and disposed in horizontal arrays, with the individual solar cells connected together in an electrical series. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current. One type of multijunction structure useful in the design according to the present invention is the inverted metamorphic solar cell structures, such as described in U.S. Pat. No. 6,951,819, M. W. Wanless et al, Lattice Mismatched Approaches for High Performance, III-V Photovoltaic Energy Converters (Conference Proceedings of the 31 st  IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press, 2005) and U.S. patent application Ser. No. 11/445,793 of the present assignee, filed Jun. 2, 2006, and herein incorporated by reference 
         [0015]    Although a variety of design features described above have been known for use in solar cell arrays and solar energy systems, they have not been utilized together or adapted in an integrated manner in a terrestrial solar energy system prior to the present invention. 
       SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
       [0016]    It is an object of the present invention to provide an improved multijunction solar cell for terrestrial power application 
         [0017]    It is another object of the invention to provide an inverted metamorphic solar cell for terrestrial power applications. 
         [0018]    It is still another object of the invention to provide an inverted metamorphic solar cell as a thin, flexible film that conforms to the non-planar support of a solar concentrator. 
         [0019]    It is still another object of the invention to provide a solar cell as a thin, flexible film that conforms to the non-planar support of a heat spreader. 
         [0020]    It is still another object of the invention to provide a solar cell as a thin, flexible film that conforms to the non-planar image plane of a solar concentrator. 
         [0021]    It is still another object of the invention to provide a III-V semiconductor solar cell with a reflective or refractive solar concentrator for terrestrial power applications. 
         [0022]    It is still another object of the invention to provide a III-V semiconductor solar cell with a solar tracker for terrestrial power applications. 
         2 . Features of the Invention 
       [0023]    Briefly, and in general terms, the invention provides a system for generating electrical power form solar radiation utilizing a III-V compound semiconductor solar cell, a concentrator for focusing sunlight on the solar cell, including a concave trough-shaped reflector, a solar tracker coupled to said concentrator so as to align the concentrator with the rays of the sun as the sun traverses the sky so that the sunlight is focused on the solar cell, a heat spreader connected to said solar cell for cooling said cell, and an electrical circuit connected to the solar cell for transferring electrical energy from the cell. 
         [0024]    In another aspect, the present invention provides a thin, flexible solar cell including a semiconductor body having an upper surface; a multijunction solar cell disposed on the upper surfaces; a first solar subcell on the substrate having a first band gap; a second solar subcell disposed over the first subcell and having a second band gap smaller than the first band gap; and a grading interlayer disposed over the second subcell interlayer having a third band gap larger than the second band gap, and a third solar subcell over the second solar subcell such that the third solar subcell is lattice mismatched with respect to the second subcell and the third subcell has a fourth band gap smaller than the third band gap, and a support for mounting the solar cell in a non-planar configuration so as to capture the sunlight in a concentrator. 
         [0025]    In one aspect, the present invention provides a solar cell including a semiconductor structure that includes a first III-V semiconductor region forming a first surface of the semiconductor structure and having a first polarity and a second III-V semiconductor region forming a second surface of the semiconductor structure and having a second polarity. The structure further includes at least one insulating via formed in the semiconductor structure from the first surface to the second surface, an electrical connection extending through the via and an insulated contact pad on the first surface of the semiconductor structure, the electrical connection extending from the second semiconductor region to the insulated contact pad so as to form a terminal of the second semiconductor region on the first surface, and a heat dissipating support on which the solar cell is mounted. 
         [0026]    In another aspect, the present invention provides a solar cell module including a thin film semiconductor body including a multijunction solar cell and having first and second electrical contacts on the back surface thereof, a support for mounting the solar cell and making electrical contact with the first and second contacts, and a heat spreader attached to the support for dissipating heat from the semiconductor body. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  shows a highly simplified block diagram of a terrestrial solar cell system under an illustrated embodiment of the invention; 
           [0028]      FIG. 2  shows a cross-sectional view of an inverted metamorphic solar cell that may be used in the present invention; 
           [0029]      FIG. 3  shows an enlarged cross-sectional view of a first embodiment of the collection optics used in the present invention; and 
           [0030]      FIG. 4  shows a cross-sectional view of a second embodiment of the collection optics used in the present invention. 
       
    
    
       [0031]    Additional objects, advantages, and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility. 
       DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    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. 
         [0033]    The present invention relates generally to terrestrial solar power systems for the conversion of sunlight into electrical energy, and to the use of Ill-V compound semiconductor solar cells in conjunction with optical components such as reflectors or concentrators which are connected in an array for unitary movement to track the sun. 
         [0034]    In one aspect, the invention relates to the design of a solar power system as depicted in  FIG. 1 .  FIG. 1  depicts the sun  100  traversing the sky along a path  101  which varies with latitude and day of the year. Solar collectors  102  are pointed at the sun so as to maximize the exposure of the solar cells (not shown) directly to the sun&#39;s parallel incoming rays. The collectors  102  may be organized as an array which is mounted on a rotatable platform  103  to allow the array to track the sun  100  as the sun moves during the day. The platform  103  is in turn mounted on a fixed support  104  which may be mounted on a building or other terrestrial structure. The support  104  may include electrical circuitry to transfer the electrical current supplied by the array  102  to a battery, power distribution system, or grid. 
         [0035]    A solar tracking arrangement  106  is provided which may either store solar angle data in a database, or utilize photodetectors or other optical components  107  to detect the position of the sun  100 . The tracking arrangement processes the stored or detected sun angle information, and transmits appropriate control signals to the support  104  to cause the platform  103  and collector optics array  102  to be continuously positioned both in azimuth and altitude angles by means of a drive  105 , schematically shown in the Figure. A variety of solar tracking arrangements are known to those skilled in the art, and therefore need not be described in detail here. 
         [0036]      FIG. 2  depicts the multijunction inverted metamorphic solar cell that may be used in one embodiment of the present invention, including three subcells A, B and C. More particularly, the solar cell is formed using the process in U.S. patent application Ser. No. 11/445,793 filed Jun. 2, 2006. As shown in the Figure, the top surface of the solar cell includes grid lines  501  which are directly deposited over the contact layer  105 . An antireflective (ARC) dielectric layer is deposited over the entire surface of the solar cell. An adhesive is deposited over the ARC layer to secure a cover glass. The solar cell structure includes a window layer  106  adjacent to the contact layer  105 . The subcell A, consisting of an n+ emitter layer  107  and a p-type base layer  108 , is then formed on the window layer  106 . 
         [0037]    In the preferred embodiment, the n+ type emitter layer  107  is composed of InGA(Al)P, and the base layer  108  is composed of InGa(Al)P. 
         [0038]    Adjacent to the base layer  108  is deposited a back surface field (“BSF”) layer  109  used to reduce recombination loss. The BSF layer  109  drives minority carriers from the region near the base/BSF interface surface to minimize the effect of recombination loss. 
         [0039]    On the BSF layer  109  is deposited a sequence of heavily doped p-type and n-type layers  10  which forms a tunnel diode, a circuit element that functions to electrically connect cell A to cell B. 
         [0040]    On the tunnel diode layers  110  a window layer  111  is deposited. The window layer  111  used in the subcell B also operates to reduce the recombination loss. The window layer  111  also improves the passivation of the cell surface of the underlying junctions. It should be apparent to one skilled in the art, that additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present invention. 
         [0041]    On the window layer  111  of cell B are deposited: the emitter layer  112 , and the p-type base layer  113 . These layers are preferably composed of InGaP and In 0.015 GaAs respectively, although any other suitable materials consistent with lattice constant and band gap requirements may be used as well. 
         [0042]    On cell B is deposited a BSF layer  114  which performs the same function as the BSF layer  109 . A p++/n++ tunnel diode  115  is deposited over the BSF layer  114  similar to the layers  110 , again forming a circuit element that functions here to electrically connect cell B to cell C. A buffer layer  115   a,  preferably InGaAs, is deposited over the tunnel diode  115  and has a thickness of about 1.0 micron. A metamorphic buffer layer  116  is deposited over the buffer layer  115   a  which is preferably a compositionally step-graded InGaAlAs series of layers with monotonically changing lattice constant to achieve a transition in lattice constant from cell B to subcell C. The bandgap of layer  116  is 1.5 ev constant with a value slightly greater than the bandgap of the middle cell B. 
         [0043]    In one embodiment, as suggested in the Wanless et al. paper, the step grade contains nine compositionally graded steps with each step layer having a thickness of 0.25 micron. In the preferred embodiment, the interlayer is composed of InGaAlAs, with monotonically changing lattice constant, such that the bandgap remains constant at 1.50 ev. 
         [0044]    Over the metamorphic buffer layer  116  is a window layer  117  composed of In 0.78 GaP, followed by subcell C having n+ emitter layer  118  and p-type base layer  114 . These layers are preferably composed of In 0.30 GaAs. 
         [0045]    A BSF layer  120  is deposited over base layer  119 . The BSF layer  120  performs the same function with respect to cell C as BSF layers  114  and  109 . 
         [0046]    A p+ contact layer  121  is deposited over BSF layer  120  and a metal contact layer  122 , preferably a sequence of Ti/Au/Ag/Au layers is applied over layer  121 . 
         [0047]      FIG. 3  is a view of a first embodiment of the present invention using a Cassegrain reflector arrangement. In such an arrangement, the solar cell  204  may be mounted in the center of the reflector  301 , and a passive heat spreader  302 , with cooling fins  303 , may be provided. 
         [0048]    In most general terms, the solar cell module is a thin film semiconductor body including a multijunction solar cell having first and second electrical contacts on the back surface thereof. The module includes a support for mounting the solar cell and making electrical contact with the first and second contacts. A heat spreader is attached to the support of the reflector  301  for dissipating heat from the semiconductor body. 
         [0049]      FIG. 4  is an enlarged view of a parabolic trough solar collector  400  according to a second embodiment of the present invention. The trough  401  is one embodiment of the collector optics  102 , the trough  401  is positioned to face the sun so that the incoming parallel rays are focused at a focal point along a line, approximately at the center of tube element  402 . In one embodiment, the solar cell  406  (such as described in  FIG. 2 ) may be mounted and supported by the tube  402 . The tube  402  may be composed of two electrically isolated elements  403  and  404  supported by a dielectric outer support  405 . The metallic elements  403  and  404  function as a heat spreader, and may be filled with a circulating liquid to provide even greater cooling to the solar cell  406 . The tube  402  is suspended at the focal point by means of a support bracket  408 . 
         [0050]    One aspect of the present invention depicted in  FIG. 4  is that the solar cell  406  is a flexible thin film and shaped so as to conform to the surface of the tube  402 , which has a non-planar configuration, in this embodiment being cylindrical. The design of the solar cell  406  may include a metal via  407  which makes an electrical connection between the top surface of the cell  406  and element  404 . The bottom surface of the cell  406  makes electrical contact with element  403 . 
         [0051]    Although this invention has been described in certain specific embodiments, many additional modifications and variations would be apparent to those skilled in the art. The present invention is therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein. 
         [0052]    It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. 
         [0053]    While the invention has been illustrated and described as embodied in a solar power system using III-V compound semiconductors, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
         [0054]    Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.