Abstract:
A method of forming a multijunction solar cell comprising an upper subcell, a middle subcell, and a lower subcell comprising providing 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; and forming a grading interlayer over said second sub cell 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 mis-matched with respect to said second subcell.

Description:
GOVERNMENT RIGHTS STATEMENT 
       [0001]    The U.S. Government has certain rights pursuant to the provisions of DFAR 227-12 (January 1997) to practice or have practiced the subject invention for or on behalf of the United States throughout the world. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to the field of solar cell semiconductor devices, and particularly to integrated semiconductor structures including a multijunction solar cell including a metamorphic layer. 
         [0004]    2. Description of the Related Art 
         [0005]    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. 
         [0006]    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, solar cells, which act as the power conversion devices for the on-board power systems, become increasingly more important. 
         [0007]    Solar cells are often fabricated in vertical, multijunction structures, and disposed in horizontal arrays, with the individual solar cells 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. 
         [0008]    Inverted metamorphic solar cell structures such as described in U.S. Pat. No. 6,951,819 and 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) present an important starting point for the development of future commercial products. The structures described in such prior art present a number of practical difficulties relating to the appropriate choice of materials and fabrication steps, in particular associated with the lattice mismatched layers between the “lower” subcell (the subcell with the lowest bandgap) and the adjacent subcell. 
         [0009]    Prior to the present invention, the materials and fabrication steps disclosed in the prior art have not been adequate to produce a commercial viable, manufacturable, and energy efficient solar cell. 
       SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
       [0010]    It is an object of the present invention to provide an improved multijunction solar cell. 
         [0011]    It is an object of the invention to provide an improved inverted metamorphic solar cell. 
         [0012]    It is another object of the invention to provide in a multi-cell structure, an interlayer between a second subcell and a third lattice-mis-matched subcell that maximizes the energy efficiency of the solar cell. 
         [0013]    It is still another object of the invention to provide a method of manufacturing an inverted metamorphic solar cell as a thin, flexible film. 
         [0014]    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 
       [0015]    Briefly, and in general terms, the present invention provides a solar cell including a semiconductor body having an upper surface; a multi junction 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. 
         [0016]    In another aspect, the present invention provides 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; and forming a grading interlayer over said second subcell having a third band gap larger than said second band gap forming said at least one lower subcell over said middle subcell such that said at least one lower subcell is lattice mis-matched with respect to said middle subcell and said third subcell has a fourth band gap smaller than said second band gap. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    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: 
           [0018]      FIG. 1  is an enlarged cross-sectional view of the solar cell according to the present invention at the end of the process steps of forming the layers of the solar cell; 
           [0019]      FIG. 2  is a cross-sectional view of the solar cell of  FIG. 1  after the next process step according to the present invention; 
           [0020]      FIG. 3  is a cross-sectional view of the solar cell of  FIG. 2  after the next process step according to the present invention; 
           [0021]      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 surrogate substrate is attached; 
           [0022]      FIG. 5A  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 original substrate is removed; 
           [0023]      FIG. 5B  is another cross-sectional view of the solar cell of  FIG. 4  after the next process step according to the present invention in which the original substrate is removed; 
           [0024]      FIG. 6A  is a top plan view of a wafer in which the solar cells according to the present invention are fabricated; 
           [0025]      FIG. 6B  is a bottom plan view of a wafer in which the solar cells according to the present invention are fabricated; 
           [0026]      FIG. 7  is a top plan view of the wafer of  FIG. 6B  after the next process step according to the present invention; 
           [0027]      FIG. 8  is a cross-sectional view of the solar cell of  FIG. 5B  after the next process step according to the present invention; 
           [0028]      FIG. 9  is a cross-sectional view of the solar cell of  FIG. 8  after the next process step according to the present invention; 
           [0029]      FIG. 10  is a cross-sectional view of the solar cell of  FIG. 9  after the next process step according to the present invention; 
           [0030]      FIG. 11  is a cross-sectional view of the solar cell of  FIG. 10  after the next process step according to the present invention; 
           [0031]      FIG. 12  is a cross-sectional view of the solar cell of  FIG. 11  after the next process step according to the present invention; 
           [0032]      FIG. 13  is a cross-sectional view of the solar cell of  FIG. 12  after the next process step according to the present invention; 
           [0033]      FIG. 14  is a cross-sectional view of the solar cell of  FIG. 13  after the next process step according to the present invention; 
           [0034]      FIG. 15  is a cross-sectional view of the solar cell of  FIG. 14  after the next process step according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    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. 
         [0036]      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 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  is deposited on the substrate. On the substrate, or over the nucleation layer  102 , a buffer layer  103 , and an etch stop layer  104  are further deposited. A contact layer  105  is then deposited on layer  104 , and a window layer  106  is deposited on the contact layer. The subcell A, consisting of an n+ emitter layer  107  and a p-type base layer  108 , is then deposited on the window layer  106 . 
         [0037]    It should be noted that the multi junction 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). 
         [0038]    In the preferred embodiment, the substrate  101  is gallium arsenide, the emitter layer  107  is composed of InGa(Al)P, and the base layer is composed of InGa(Al)P. 
         [0039]    On top of the base layer  108  is deposited a back surface field (“BSF”) layer  109  used to reduce recombination loss. 
         [0040]    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. 
         [0041]    On top of the BSF layer  109  is deposited a sequence of heavily doped p-type and n-type layers  110  which forms a tunnel diode which is a circuit element to connect cell A to cell B. 
         [0042]    On top of 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. 
         [0043]    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 and In 0.015 GaAs respectively, although any other suitable materials consistent with lattice constant and band gap requirements may be used as well. 
         [0044]    On top of the 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 to connect cell B to cell C. A buffer layer  115   a , preferably InGaAs, is deposited over the tunnel diode  115 , to 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. 
         [0045]    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. 
         [0046]      FIG. 2  is a cross-sectional view of the solar cell of  FIG. 1  after the next process step according to the present invention in which a metal contact layer  122  is deposited over the p+ semiconductor contact layer  121 . The metal is preferably a sequence of Ti/Au/Ag/Au layers. 
         [0047]      FIG. 3  is a cross-sectional view of the solar cell of  FIG. 2  after the next process step according to the present invention in which an adhesive layer  123  is deposited over the metal layer  122 . The adhesive is preferably GenTak  330  (distributed by General Chemical Corp.). 
         [0048]      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 surrogate substrate, preferably sapphire, is attached. In the preferred embodiment, the surrogate substrate is about 40 mils 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. 5A  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 original substrate is removed by a sequence of lapping and/or etching steps in which the substrate  101 , the buffer layer  103 , and the etch stop layer  104 , are removed. The etchant is growth substrate dependent. 
         [0050]      FIG. 5B  is a cross-sectional view of the solar cell of  FIG. 5A  from the solar cell of  FIG. 5A  from the orientation with the surrogate substrate  124  being at the bottom of the Figure. 
         [0051]      FIG. 6A  is a top plan view of a wafer in which the solar cells according to the present invention are implemented. 
         [0052]      FIG. 6B  is a bottom plan view of the wafer with four solar cells shown in  FIG. 6A . In each cell there are grid lines  501  (more particularly shown in  FIG. 10 ), an interconnecting bus line  502 , and a contact pad  503 . 
         [0053]      FIG. 7  is a bottom plan view of the wafer of  FIG. 6B  after the next process step in which a mesa  510  is etched around the periphery of each cell using phosphide and arsenide etchants. 
         [0054]      FIG. 8  is a cross-sectional view of the solar cell of  FIG. 5B  after the next process step according to the present invention in which the sacrificial buffer layer has been removed with 4 citric 1 H 2 O 2  solution. 
         [0055]      FIG. 9  is a cross-sectional view of the solar cell of  FIG. 8  after the next process step according to the present invention in which the etch stop layer  104  is removed by HCl/H 2 O solution. 
         [0056]      FIG. 10  is a cross-sectional view of the solar cell of  FIG. 9  after the next process step according to the present invention in which a photoresist mask (not shown) is placed over the contact layer  105  as the first step in forming the grid lines  501 . The mask  200  is lifted off to form the grid lines  501 . 
         [0057]      FIG. 11  is a cross-sectional view of the solar cell of  FIG. 10  after the next process step according to the present invention in which grid lines  501  are deposited via evaporation and lithographically patterned and deposited over the contact layer  105 . The grid lines are used as a mask to etch down the surface to the window layer  106  using a citric acid/peroxide etching mixture. 
         [0058]      FIG. 12  is a cross-sectional view of the solar cell of  FIG. 11  after the next process step according to the present invention in which an antireflective (ARC) dielectric coating layer  130  is applied over the entire surface of the “bottom” side of the wafer with the grid lines  501 . 
         [0059]      FIG. 13  is a cross-sectional view of the solar cell of  FIG. 12  after the next process step according to the present invention in which the mesa  501  is etched down to the metal layer  122  using phosphide and arsenide etchants. The cross-section in the figure is depicted as seen from the A-A plane shown in  FIG. 7 . 
         [0060]    One or more silver electrodes are welded to the respective contact pads. 
         [0061]      FIG. 14  is a cross-sectional view of the solar cell of  FIG. 13  after the next process step according to the present invention after the surrogate substrate  124  and adhesive  123  are removed by EKC 922. Perforations are made over the surface, each with a diameter is 0.033 inches and separated by 0.152 inches. 
         [0062]    The perforations allow the flow of etchant through the surrogate substrate  124  to permit its lift off. 
         [0063]      FIG. 15  is a cross-sectional view of the solar cell of  FIG. 14  after the next process step according to the present invention in which an adhesive is applied over the ARC layer  130  and a coverglass attached thereto. 
         [0064]    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. 
         [0065]    While the invention has been illustrated and described as embodied in a multijunction 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. 
         [0066]    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.