Abstract:
A concentrator photovoltaic solar cell array for terrestrial use for generating electrical power from solar radiation including a multifunction III-V compound semiconductor solar cell with material composition and bandgaps to maximize absorption in the AM1.5 spectral region, and a thickness of one micron or greater so as to be able to produce in excess of 15 watts of DC power with conversion efficiency in excess of 37%. The aggregate surface area of the grid pattern covers approximately 2 to 5% of the top cell.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to co-pending U.S. patent application Ser. No. 11/500,053 filed Aug. 7, 2006, and U.S. patent application Ser. Nos. 11/830,576 and 11/830,636 filed Jul. 30, 2007 by the common assignee. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to the design of solar cells for concentrator terrestrial solar power systems for the conversion of sunlight into electrical energy, and, more particularly to a solar cell structure using III-V compound semiconductors and the optimization of various parameters in order for the cell to operate at optimum efficiency. 
         [0004]    2. Description of the Related Art 
         [0005]    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. Moreover, it is well known that concentrating solar energy onto a III-V compound semiconductor photovoltaic cell increases the cell&#39;s efficiency to over 37% efficiency under concentration. 
         [0006]    Terrestrial solar power systems currently use silicon solar cells in view of their low cost and widespread availability. Although III-V 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 III-V semiconductor solar cells have not yet been designed for optimum coverage of the solar spectrum present at the earth&#39;s surface (known as air mass 1.5 or AM1.5). Such cells have not been configured or optimized for use in solar tracking terrestrial systems, nor have existing commercial terrestrial solar power systems been configured and optimized to utilize compound semiconductor solar cells. 
         [0007]    In the design of both silicon and III-V compound semiconductor solar cells, 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. Grid lines extend over the top surface of the cell to capture this electron flow which then connect into the front contact or bonding pad. 
         [0008]    An important aspect of specifying the design of a solar cell is the physical structure (composition, bandgaps, and layer thicknesses) of the semiconductor material layers constituting the solar cell. Solar cells are often fabricated in vertical, multijunction structures to utilize materials with different bandgaps and convert as much of the solar spectrum as possible. One type of multijunction structure useful in the design according to the present invention is the triple junction solar cell structure consisting of a germanium bottom cell, a gallium arsenide (GaAs) middle cell, and an indium gallium phosphide (InGaP) top cell. 
         [0009]    Prior to the present invention, there has not been an optimal combination of semiconductor structural features in a triple junction III-V compound semiconductor solar cell suitable for terrestrial applications under high (over 500×) concentration, or a determination of the optimum design parameters to maximize efficiency of the cell. 
       SUMMARY OF THE INVENTION 
       [0010]    1. Objects of the Invention 
         [0011]    It is an object of the present invention to provide an improved III-V compound semiconductor multijunction solar cell for terrestrial power applications producing in excess of 15 watts of peak DC power per solar cell. 
         [0012]    It is still another object of the invention to provide a grid structure on the front surface of a III-V semiconductor solar cell to accommodate high current for concentrator photovoltaic terrestrial power applications. 
         [0013]    It is still another object of the invention to provide a III-V semiconductor solar cell with a relatively thick front or top subcell semiconductor layers having a composition optimized for high concentration AM1.5 solar radiation for terrestrial power applications. 
         [0014]    It is still another object of the invention to provide a terrestrial solar power system constituted by a plurality of solar cell arrays with concentration optics adapted to permit the solar cells to operate at optimum efficiency. 
         [0015]    2. Features of the Invention 
         [0016]    Briefly, and in general terms, the present invention provides a concentrator photovoltaic solar cell for producing energy from the sun including a germanium substrate including a first photoactive junction and forming a bottom solar subcell; a gallium arsenide middle cell disposed on said substrate; an indium gallium phosphide top cell disposed over the middle cell and having a bandgap to maximize absorption in the AM1.5 spectral region; and a surface grid disposed over the top cell and having a grid pattern which covers from 2 to 5% of the top cell surface area and configured for conduction of the relatively high current created by the solar cell. 
         [0017]    In another aspect, the present invention provides a concentrator photovoltaic solar cell for producing energy from the sun including a bottom subcell including a first photoactive junction, a middle cell disposed on said bottom cell and including a second photoactive junction; and a top cell disposed over said middle cell and having a photoactive junction and bandgap to maximize absorption in the AM1.5 spectral region with a top layer sheet resistance of less than 500 ohms/square and adapted operate at an concentration level of greater than twenty suns. 
         [0018]    In another aspect, the present invention provides a concentrator photovoltaic solar cell for producing energy from the sun including a germanium substrate including a first photoactive junction a gallium arsenide middle cell disposed on said substrate; and an indium gallium phosphide top cell disposed over said middle cell and having a bandgap to maximize absorption in the AM1.5 spectral region and a thickness greater than 8000 Angstroms in order to carry the increased current associated with concentrated sunlight on the surface of said top cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a highly enlarged cross-sectional view of a terrestrial solar cell constructed in accordance with the present invention; 
           [0020]      FIG. 2  is a top plan view of the solar cell of  FIG. 1  showing the grid lines in a first embodiment; 
           [0021]      FIG. 3  is a top plan view of the solar cell of  FIG. 1  showing the grid lines in a second embodiment. 
           [0022]      FIG. 4  is a graph showing the efficiency of a solar cell having a structure according to the present invention as a function of the surface coverage of the grid lines. 
       
    
    
       [0023]    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 a preferred embodiment, 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 
       [0024]    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. 
         [0025]    The related U.S. patent application Ser. No. 11/830,636 of assignee, herein incorporated by reference, relates generally to a terrestrial solar power system for the conversion of sunlight into electrical energy utilizing a plurality of mounted arrays spaced in a grid over the ground, to the optimum size and aspect ratio of the solar cell array mounted for unitary movement on a cross-arm of a vertical support that tracks the sun, and to the design of the subarrays, modules or panels that constitute the array. 
         [0026]    The design of a typical semiconductor structure of a triple junction III-V compound semiconductor solar cell is more particularly described in U.S. Pat. No. 6,680,432, herein incorporated by reference. Since such cells are described as optimized for space (AMO) solar radiation, one aspect of the present invention is the modification or adaptation of such cell designs for concentrator photovoltaic applications with terrestrial (AM1.5) solar spectrum radiation according to the present invention. 
         [0027]    As shown in the illustrated example of  FIG. 1 , the bottom subcell  10  includes a substrate  11 ,  12  formed of p-type germanium (“Ge”), the bottom portion which also serves as a base layer of the subcell  10 . A metal contact layer or pad  14  is formed on the bottom of base layer  11  to provide an electrical contact to the multijunction solar cell. The bottom subcell  10  farther includes, for example, an n-type Ge emitter region  12 , and an n-type nucleation layer  13 . The nucleation layer  13  is deposited over the substrate  11 ,  12 , and the emitter layer  12  is formed in the Ge substrate by diffusion of dopants from upper layers into the Ge substrate, thereby changing upper portion  12  of the p-type germanium substrate to an n-type region  12 . A heavily doped n-type gallium arsenide layer  14  is deposited over the nucleation layer  13 , and is a source of arsenic dopants into the emitter region  12 . 
         [0028]    Although the growth substrate and base layer  11  is preferably a p-type Ge growth substrate and base layer, other semiconductor materials may be also be used as the growth substrate and base layer, or only as a growth substrate. Examples of such substrates include, but not limited to, GaAs, InP, GaSb, InAs, InSb, GaP, Si, SiGe, SiC, Al 2 O 3 , Mo, stainless steel, soda-lime glass, and SiO 2    
         [0029]    Heavily doped p-type aluminum gallium arsenide (“AlGaAs”) and (“GaAs”) tunneling junction layers  14 ,  15  may be deposited over the nucleation layer  13  to form a tunnel diode and provide a low resistance pathway between the bottom subcell and the middle subcell  20 . 
         [0030]    The middle subcell  20  includes a highly doped p-type aluminum gallium arsenide (“AlGaAs”) back surface field (“BSF”) layer  16 , a p-type InGaAs base layer  17 , a highly doped n-type indium gallium phosphide (“InGaP 2 ”) emitter layer  18  and a highly doped n-type indium aluminum phosphide (“AlInP 2 ”) window layer  19 . 
         [0031]    The window layer typically has the same doping type as the emitter, but with a higher doping concentration than the emitter. Moreover, it is often desirable for the window layer to have a higher band gap than the emitter, in order to suppress minority-carrier photogeneration and injection in the window, thereby reducing the recombination that would otherwise occur in the window layer. Note that a variety of different semiconductor materials may be used for the window, emitter, base and/or BSF layers of the photovoltaic cell, including AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and other materials and still fall within the spirit of the present invention. 
         [0032]    The InGaAs base layer  17  of the middle subcell  307  can include, for example, approximately 1.5% Indium. Other compositions may be used as well. The base layer  17  is formed over the BSF layer  16  after the BSF layer is deposited over the tunneling junction layers  14 ,  15  of the bottom subcell  10 . 
         [0033]    The BSF layer  16  is provided to reduce the recombination loss in the middle subcell  20 . The BSF layer  16  drives minority carriers from a highly doped region near the back surface to minimize the effect of recombination loss. Thus, the BSF layer  16  reduces recombination loss at the backside of the solar cell and thereby reduces recombination at the base layer/BSF layer interface. The window layer  19  is deposited on the emitter layer  18  of the middle subcell  20  after the emitter layer is deposited. The window layer  19  in the middle subcell  20  also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. 
         [0034]    Before depositing the layers of the top cell  30 , heavily doped n-type InAlP 2  and p-type InGaP 2  tunneling junction layers  21 ,  22  respectively may be deposited over the middle subcell  20 , forming a tunnel diode. 
         [0035]    The tunnel diode layers disposed between subcells have a thickness adapted to support a current density through the tunnel diodes of greater than 50 amps/square centimeter. 
         [0036]    In the illustrated example, the top subcell  30  includes a highly doped p-type indium gallium aluminum phosphide (“InGaAlP”) BSF layer  23 , a p-type InGaP 2  base layer  24 , a highly doped n-type InGaP 2  emitter layer  25  and a highly doped n-type InAlP 2  window layer  26 . The base layer  24  of the top subcell  30  is deposited over the BSF layer  23  after the BSF layer  23  is formed over the tunneling junction layers  21 ,  22  of the middle subcell  20 . The window layer  26  is deposited over the emitter layer  25  of the top subcell after the emitter layer  25  is formed over the base layer  24 . A cap layer  27  may be deposited and patterned into separate contact regions over the window layer  26  of the top subcell  30 . 
         [0037]    The cap layer  27  serves as an electrical contact from the top subcell  309  to metal grid layer  40 . The sheet resistance of the top cell is preferably about 250 ohms/square centimeters, and in any event less than 500 ohms/square. The doped cap layer  27  can be a semiconductor layer such as, for example, a GaAs or InGaAs layer. An anti-reflection coating  28  can also be provided on the surface of window layer  26  in between the contact regions of cap layer  27 . 
         [0038]    The resulting solar cell has band gaps of 1.9 eV, and 0.7 eV for the top, middle, and bottom subcells. The solar cell has an open circuit voltage (V oc ) of at least 3.0 volts, a responsivity at short circuit at least 0.13 amps per watt, a fill factor (FF) of at least 0.70, and an efficiency at least 35% under air mass 1.5 (AM1.5) or similar terrestrial spectrum at 25 degrees Centigrade, when illuminated by concentrated sunlight by a factor in excess of 500×, so as to produce in excess of 15 watts of DC power. 
         [0039]      FIG. 2  is a top plan view of the solar cell of  FIG. 1  showing the grid lines  40  in a first embodiment. In particular,  FIG. 1  depicts the cross-section through the A-A plane of  FIG. 2 , including two typical grid lines  40 . The grid lines  40  are arranged into four identical quadrants Q 1 , Q 2 , Q 3  and Q 4  over the active area of the solar cell. It is noted that in this embodiment the cell is four-fold rotationally symmetric, i.e. the cell can be rotated 90° and each configuration of the grid lines in the cell after rotation is identical to the previous configuration of the grid lines prior to rotation. 
         [0040]      FIG. 3  is a top plan view of the solar cell of  FIG. 1  showing the grid lines in a second embodiment. In particular, the grid lines extend between two bus bars on opposite sides of the cell. Either the first or the second embodiments, have a thickness or height of 4 microns or more, a width of less than 5 microns, and a pitch (i.e., distance between centers of adjacent grid lines) of greater than 100 micron but less than 200 microns. 
         [0041]    The aggregate surface area of the grid pattern covers approximately 2.0% to 5.0% of the surface area of the top cell. The grid pattern and line dimensions are selected to carry the relatively high current produced by the solar cell. 
         [0042]      FIG. 4  is a graph showing the efficiency of a solar cell having a structure according to the present invention as a function of the surface coverage of the grid lines as a percent of the total surface area of the solar cell. The graph peaks in the range of 2 to 5% of the surface area, and thus according to the present invention the surface coverage of the grid lines is selected in that range. 
         [0043]    Although the invention has been described in certain specific embodiments of semiconductor structures, and grid designs, many additional modifications and variations would be apparent to those skilled in the art. 
         [0044]    For example, the present invention may utilize one or more homojunction cells or subcells, i.e., a cell or subcell in which the p-n junction is formed between a p-type semiconductor and an n-type semiconductor both of which have the same chemical composition and the same band gap, differing only in the dopant species and types. Alternatively, the present invention may utilize one or more heterojunction cells or subcells, i.e., a cell or subcell in which the p-n junction is formed between a p-type semiconductor and an n-type semiconductor having different chemical compositions of the semiconductor material in the n-type and n-type regions, and/or different band gap energies in the p-type regions, in addition to utilizing different dopant species and type in the p-type and n-type regions that form the p-n junction. This aspect of the present invention is, therefore, considered in all respects to be illustrative and not restrictive. The scope of this aspect of the invention is indicated by the relevant appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein. 
         [0045]    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 terrestrial solar cell systems and constructions differing from the types described above. 
         [0046]    While the aspect of the invention has been illustrated and described as embodied in a solar cell semiconductor structure 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. 
         [0047]    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.