Abstract:
A method of forming a multijunction solar cell comprising an upper subcell, a middle subcell, and a lower subcell by providing a first substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on said substrate having a first band gap; forming a second solar subcell over said first subcell having a second band gap smaller than said first band gap; forming a grading interlayer over said second subcell having a third band gap larger than said second band gap; forming a third solar subcell having a fourth band gap smaller than said second band gap such that said third subcell is lattice mismatched with respect to said second subcell; and etching a via from the top of the third subcell to the substrate to enable both anode and cathode contacts to be placed on the backside of the solar cell.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is also related to co-pending U.S. patent application Ser. No. 11/109,016 filed Apr. 19, 2005. 
         [0002]    This application is also related to co-pending U.S. patent application Ser. No. 11/445,793 filed Jun. 2, 2006. 
         [0003]    This application is also related to co-pending U.S. patent application Ser. No. 11/500,053 filed Aug. 7, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present invention relates to the field of solar cell semiconductor devices, and particularly to integrated semiconductor structures including a multijunction solar cell and a conducting via that allows both anode and cathode terminals to be placed on the back side of the cell. 
         [0006]    2. Description of the Related Art 
         [0007]    Photovoltaic cells, also called solar cells, are one of the most important new energy sources that have become available in the past several years. Considerable effort has gone into solar cell development. As a result, solar cells are currently being used in a number of commercial and consumer-oriented applications. While significant progress has been made in this area, the requirement for solar cells to meet the needs of more sophisticated applications has not kept pace with demand. Applications such as satellites used in data communications have dramatically increased the demand for solar cells with improved power and energy conversion characteristics. 
         [0008]    In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as the payloads become more sophisticated, the design efficiency of solar cells, which act as the power conversion devices for the on-board power systems, become increasingly more important. 
         [0009]    Solar cells are often fabricated in vertical, multijunction structures, and disposed in horizontal arrays, with the individual solar cell connected together in a 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. 
         [0010]    Inverted metamorphic solar cell structures such as described in U.S. Pat. No. 6,951,819, the paper of 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 co-pending U.S. patent application Ser. No. 11/445,793 filed Jun. 2, 2006, of the present assignee, present an important development in future commercial solar cell products. 
         [0011]    Since a solar cell is fabricated as a vertical, multijunction structure, one electrical contact is usually placed on the top surface of the cell, and the other contact on the bottom of the cell, to avoid internal interconnections which may affect reliability and cost. A variety of designs are also known in which both contacts are placed on one side of the cell, including as represented in U.S. patent application Ser. No. 11/109,016 of the instant assignee. 
         [0012]    Prior to the present invention, there has not been a inverted metamorphic solar cell with both anode and cathode contacts on the same side of the cell. 
       SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
       [0013]    It is an object of the present invention to provide an improved multijunction solar cell with both anode and cathode contacts on the backside of the cell. 
         [0014]    It is an object of the invention to provide an improved inverted metamorphic solar cell. 
         [0015]    It is another object of the invention to provide an electrical interconnection via in a multi-solar cell structure that is fabricated on a substrate which is removed during processing. 
         [0016]    It is still another object of the invention to provide a method of manufacturing an inverted metamorphic solar cell as a thin, flexible film with contacts on one side of the cell. 
         [0017]    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. 
       2. Features of the Invention 
       [0018]    Briefly, and in general terms, the present invention provides a method of manufacturing a solar cell by providing a first substrate; depositing on the substrate a sequence of layers of semiconductor material that forms at least one cell of a multifunction solar cell; etching a via from the top surface of the sequence of layers to the first substrate; providing a second substrate over the sequence of layers, and removing the first substrate. 
         [0019]    In another aspect, the present invention provides a method of manufacturing a solar cell having a front side and back side by providing a first substrate; depositing on the substrate a sequence of layers of semiconductor material that forms at least one cell of a multijunction solar cell; providing a second substrate over the sequence of layers; and removing the first substrate. A first electrode is then formed on the back side of the solar cell, and an electrical connection is formed between the top cell of the multijunction solar cell and a second electrode on the back side of the solar cell. 
         [0020]    In another aspect, the present invention provides a solar cell including a semiconductor body having a sequence of layers forming a multijunction solar cell including; a first solar subcell 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; a grading interlayer disposed over the second subcell having a third band gap larger than the second band gap, and a third subcell disposed over the interlayer such that the third solar subcell is lattice mismatched with respect to the second subcell and has a fourth band gap smaller than the third band gap, with anode and cathode contacts on the backside of the solar cell. 
         [0021]    In another aspect of the present invention provides a multijunction solar cell having a front side surface and a back side surface including a first solar subcell adjacent the front side surface having a first band gap; a second solar subcell disposed over the first subcell and having a second band gap smaller than said first band gap; a grading interlayer disposed over the second subcell and having a third band gap greater than the second band gap; and a third solar subcell adjacent the back side surface and disposed over the interlayer, the third subcell being lattice mismatched with respect to said second subcell and having a fourth band gap smaller than the third band gap. A via is formed in the first, second, and third solar cells with an electrical conductor extending through the via. An insulated contact pad is provided on the back side surface and electrically connected to the conductor to form a first terminal of the solar cell on the back side surface. A second terminal is formed on the back side surface by a metal layer making contact with a contact layer on the back side. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    These and other features and advantages of this invention will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
           [0023]      FIG. 1  is an enlarged cross-sectional view of the solar cell structure according to the present invention at the end of the process steps of forming a multijunction solar cell on a first substrate; 
           [0024]      FIG. 2  is a cross-sectional view of the structure of  FIG. 1  with a via etched to the first substrate; 
           [0025]      FIG. 3  is a cross-sectional view of the solar cell structure of  FIG. 2  after the next process step according to the present invention including depositing a dielectric layer and a conductive layer in the via; 
           [0026]      FIG. 4  is a cross-sectional view of the solar cell of  FIG. 3  after the next process step according to the present invention in which a wafer carrier or surrogate second substrate is adhered to the “top” side of the solar cell structure; 
           [0027]      FIG. 5  is a cross-sectional view of the solar cell of  FIG. 4  after the next process step according to the present invention in which the first substrate is removed; 
           [0028]      FIG. 6  is a cross-sectional view of the solar cell of  FIG. 5  after the next process step according to the present invention in which a cap layer and metal contact layer is deposited on the structure; 
           [0029]      FIG. 7  is a cross-sectional view of the solar cell of  FIG. 6  after the next process step according to the present invention in which a cover glass is adhered to the solar cell structure on one side, and the surrogate second substrate removed on the other side; and 
           [0030]      FIGS. 8A and 8B  are top and bottom plan views, respectively, of a wafer including the solar cell of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]    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. 
         [0032]      FIG. 1  depicts the multijunction solar cell according to the present invention after formation of the three subcells A, B and C on a substrate. More particularly, there is shown a first substrate  101 , which may be either gallium arsenide (GaAs), germanium (Ge), or other suitable material. In the case of a Ge substrate, a nucleation layer  102  such as InGaP 2 , is deposited on the substrate. On the substrate, or over the nucleation layer  102  in the case of a Ge substrate, a buffer layer  103  of InGaAs, and an etch stop layer  104  of InAlP 2  are further deposited. A contact layer  105  of InGaAs is then deposited on layer  104 , and a window layer  106  of InAlP 2  is deposited on the contact layer. The subcell A, consisting of an n+ emitter layer  107  of InGaP 2  and a p-type base layer  108  of InGaP 2 , is then deposited on the window layer  106 . 
         [0033]    Although the preferred embodiment utilizes the III-V semiconductor materials described above, the embodiment is only illustrative, and it should be noted that the multijunction solar cell structure could be formed by any suitable combination of group III to V elements listed in the periodic table subject to lattice constant and band gap requirements, wherein the group III includes boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (T). The group IV includes carbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi). 
         [0034]    In the preferred embodiment, the substrate  101  is gallium arsenide, the emitter layer  107  is composed of InGa(Al)P 2 , and the base layer is composed of InGa(Al)P 2 . The use of parenthesis in the formula is standard nomenclature to indicate that the amount of aluminum may vary from 0 to 30%. 
         [0035]    On top of the base layer  108  is deposited a p+ type back surface field (“BSF”) layer  109  of InGaAlP which is used to reduce recombination loss. 
         [0036]    The BSF layer  109  drives minority carriers from the region near the base/BSF interface surface to minimize the effect of recombination loss. In other words, a BSF layer  109  reduces recombination loss at the backside of the solar subcell A and thereby reduces the recombination in the base. 
         [0037]    On top of the BSF layer  109  is deposited a sequence of heavily doped p-type (such as AlGaAs) and n-type layers  110  (such as InGaP 2 ) which forms a tunnel diode which is a circuit element to connect cell A to cell B. 
         [0038]    On top of the tunnel diode layers  110  a window layer  111  of n++ InAlP 2  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. 
         [0039]    On top of the window layer  111  the layers of cell B are deposited: the emitter layer  112 , and the p-type base layer  113 . These layers are preferably composed of InGaP 2  for the emitter and either GaAs or In 0.015 GaAs for the base, respectively, although any other suitable materials consistent with lattice constant and band gap requirements may be used as well. 
         [0040]    On top of the cell B is deposited a BSF layer  114  of p+ type AlGaAs 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 to connect cell B to cell C. A buffer layer  115   a , preferably InGaAs, is deposited over the tunnel diode  115 , with a thickness of about 1.0 micron. A metamorphic buffer layer  116  is then deposited over the buffer layer  115   a . The layer  116  is preferably a compositionally step-graded composition of InGaAlAs deposited as a series of layers with monotonically changing lattice constant that provides 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. 
         [0041]    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. 
         [0042]    On top of the metamorphic buffer layer  116  another n+ window layer  117  is deposited. The window layer  117  improves the passivation of the cell surface of the underlying junctions. Additional layers may be provided without departing from the scope of the present invention. 
         [0043]    On top of the window layer  117  the layers of subcell C are deposited; the n-type emitter layer  118  and the p type base layer  119 . In the preferred embodiment, the emitter layer is composed of GaInAs and the base layer is composed of GaInAs with about a 1.0 ev bandgap, although any other semiconductor materials with suitable lattice constant and band gap requirements may be used as well. 
         [0044]    On top of the base layer  119  of subcell C a back surface field (BSF) layer  120 , preferably composed of GaInAsP, is deposited. 
         [0045]    Over or on top of the BSF layer  120  is deposited a p+ contact layer  121 , preferably of p+ type InGaAs. 
         [0046]      FIG. 2  is a cross-sectional view of the structure of  FIG. 1  after the process step of a via  150  being etched from the top surface of the deposited layers  102  through  121  by dry or wet chemical processes to the substrate  101 . 
         [0047]      FIG. 3  is a cross-sectional view of the solar cell structure of  FIG. 2  after the next sequence of process step according to the present invention including depositing a back metal layer over the p+ contact layer  121 , and depositing a dielectric layer  161  in the interior of the via  150  and over a portion of the back metal contact layer. A conductive layer  162  is then deposited in the via  150  and over the dielectric layer  161 . The layer  162  serves as a wrap through front contact for the solar cell. 
         [0048]      FIG. 4  is a cross-sectional view of the solar cell of  FIG. 3  (how oriented with the substrate  101  at the top of the Figure) after the next process step according to the present invention. A wafer carrier or surrogate second substrate is adhered to the “top” side of the solar cell structure, which is now at the bottom of the Figure. In the preferred embodiment, the surrogate substrate is sapphire about 1000 microns in thickness, and is perforated with holes about 1 mm in diameter, spaced 4 mm apart, to aid in subsequent removal of the substrate. 
         [0049]      FIG. 5  is a cross-sectional view of the solar cell of  FIG. 4  after the next process step according to the present invention in which the first substrate  101  is removed by a lapping or grinding process. 
         [0050]      FIG. 6  is a cross-sectional view of the solar cell of  FIG. 5  after the next process step according to the present invention in which a cap layer is deposited over a portion of the nucleation layer in the region of the via  150  and metal contact layer is deposited over the cap layer, making electrical contact with the metal layer  161  inside the via  150 . An antireflective coating (ARC) layer is then applied over the surface of the nucleation layer. 
         [0051]      FIG. 7  is a cross-sectional view of the solar cell of  FIG. 6  after the next process step according to the present invention in which an adhesive is applied over the front metal layer and the ARC layer, and a cover glass is adhered to the solar cell structure. On the other side, the surrogate second substrate is then removed by dissolving the adhesive attaching it, or any other suitable technique. 
         [0052]      FIGS. 8A and 8B  are top and bottom plan views, respectively of a wafer including the solar cell of the present invention. In  FIG. 8A , Cell  1  of each wafer is illustrated in greater detail with grid lines  501 , a bus  502 , and circular regions  503  in which a via  150  extends through the wafer such as shown in previous cross-sectional views. 
         [0053]      FIG. 8B  depicts the back side contact region  505  and a wrap through front contact region  504  with vias  503  corresponding to those shown in  FIG. 8A . 
         [0054]    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 of constructions differing from the types described above. 
         [0055]    While the invention has been illustrated and described as embodied in a multijunction inverted metamorphic solar cell, 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. 
         [0056]    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.