Abstract:
An integral semiconductor device having a sequence of layers of semiconductor material. The semiconductor device may include a first region in which the sequence of layers of semiconductor material forms at least one cell of a multijunction solar cell including a metamorphic layer with a graded lattice constant. The semiconductor device may also include a second region, spaced apart from the first region, in which the sequence of layers in the second region forms a support for a bypass diode that functions to pass current when the solar cell is shaded.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a division of U.S. patent application Ser. No. 11/614,332, filed Dec. 21, 2006. This application also is related to co-pending U.S. patent application Ser. No. 11/445,793 filed Jun. 2, 2006. 
     
    
     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 and an integral bypass diode. 
         [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]    When solar cells in an array are all receiving sunlight, or are illuminated, each cell in the array will be forward biased and will be carrying current. However, if any of the cells are not illuminated, because of shadowing or damage, those shadowed cells are still in the array circuit and may be forced to become reversed biased in order to carry the current generated by the illuminated cells. This reverse biasing can degrade the cells and can ultimately render the cells inoperable. In order to prevent reverse biasing, a diode structure in parallel with the solar cells in a single multijunction cell is often implemented. 
         [0009]    However, when the solar cell is not receiving sunlight, whether because of shading by a movement of the satellite, or as a result of damage to the cell, then resistance exists along the cell path. As solar cells exist in an array, current from illuminated cells must pass through shaded cells. If there were no diode, the current would force its way through the cell layers, reversing the bias of such cells and permanently degrading, if not destroying the electrical characteristics of such cells. 
         [0010]    If the cell contains a diode, however, the current can be offered an alternative, parallel path, and the shaded cells will be preserved. The problem with this concept has been the difficulty in creating a diode that is relatively easy to manufacture and which uses a very low level of voltage to turn on and operate. 
         [0011]    The purpose of the bypass diode is to draw the current away from the shadowed or damaged cell. The bypass diode becomes forward biased when the shadowed cell becomes reverse biased. Since the solar cell and the bypass diode are in parallel, rather than forcing current through the shadowed cell, the diode draws the current away from the shadowed cell and completes the electrical current to maintain the connection in the next cell. 
         [0012]    If a cell is shaded or otherwise not receiving sunlight, in order for the current to choose the diode path, the turn on voltage for the diode path must be less than the breakdown voltage along the cell path. The breakdown voltage along the cell path will typically be at least five volts, if not more. In an implementation utilizing a Schottky bypass diode. The Schottky contact requires a relatively small amount of voltage to “turn on”, about 600 millivolts. However, in a multijunction solar cell with a germanium substrate, to pass through the Ge junction the bias of the Ge junction must be reversed, requiring a large voltage. Reversing the bias of the Ge junction requires approximately 9.4 volts, so nearly ten volts are needed for the current to follow the diode path. Ten volts used to reverse the bias of the Ge junction is ten volts less than otherwise would be available for other applications. 
         [0013]    The use of bypass diodes in connection with solar cells is known from U.S. Pat. Nos. 6,103,970; 6,359,210; 6,600,100; 6,617,508; 6,680,432; and 7,115,811. 
         [0014]    Inverted metamorphic solar cell structures such as described in U.S. Pat. No. 6,951,819 and M. W. Wanlass et al., Lattice Mismatched Approaches for High Performance, III-V Photovoltaic Energy Converters (Conference Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press, 2005) and copending U.S. patent application Ser. No. 11/445,793 filed Jun. 2, 2006, of the present assignee, present an important starting point for the development of future commercial products with high energy conversion of efficiency. 
         [0015]    Prior to the present invention, the materials and fabrication steps disclosed in the prior art have not been described on energy efficient solar cell based on an inverted metamorphic structure with an integral bypass diode. 
       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 with an integral bypass diode. 
         [0017]    It is an object of the invention to provide an improved inverted metamorphic solar cell. 
         [0018]    It is another object of the invention to provide an integral bypass diode in a multi-solar cell structure, with at least two adjacent lattice mismatched subcells that maximizes the energy efficiency of the solar cell. 
         [0019]    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 an integral bypass diode. 
         [0020]    Some implementations or embodiments of the invention may achieve fewer than all of the foregoing objects. 
         [0021]    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. 
         [0022]    Some implementations or embodiments of the present invention may incorporate or implement fewer of the aspects and features noted in the foregoing summaries. 
       2. Features of the Invention 
       [0023]    In another aspect briefly, and the 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, including a first region in which at least one layer of the sequence of layers forms at least one layer of a bypass diode to pass current when the solar cell is shaded, and a second region in which the sequence of layers of semiconductor material forms at least one cell of a multijunction solar cell; providing a second substrate over the second region; and removing the first substrate. 
         [0024]    The present invention further provides a solar cell with an integral bypass diode including a semiconductor body having a sequence of layers including a first region 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, and a second region including a bypass diode. 
         [0025]    In another aspect, the present invention provides a solar cell semiconductor device having a sequence of layers of semiconductor material, including a first region in which the sequence of layers of material forms at least one cell of a multijunction solar cell, and a second region in which the corresponding sequence of layers forms a support for a bypass diode to protect said cell against reverse biasing wherein the sequence of layers in the first region forming the at least one cell and the sequence of layers in the second region forming the support to the bypass diode are identical and wherein each layer in the first region has substantially the same composition and thickness as the corresponding layer in the second region. 
         [0026]    The sequence of layers includes a discontinuous lateral conduction layer forming two electronically isolated portions, the first portion making an electrical contact to an active region of said solar cell in one region, and the second portion making electrical contact to an active region of the bypass diode. 
         [0027]    A conductive layer is deposited on the sequence of layers; and a conductor connects the second portion and the bypass diode to the conductive layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    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: 
           [0029]      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 bypass diode and solar cell on a first substrate; 
           [0030]      FIG. 2  is a cross-sectional view of the solar cell of  FIG. 1  after the next process step according to the present invention including adhering a surrogate substrate to the top of the structure; 
           [0031]      FIG. 3  is a cross-sectional view of the solar cell of  FIG. 2  after the next process step according to the present invention depicted including removing the original substrate; 
           [0032]      FIG. 4  is a cross-sectional view of the solar cell of  FIG. 3  after the next process step according to the present invention including etching a trench so that the semiconductor body is formed into two spaced apart regions; 
           [0033]      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 certain layers in the left side region are removed, and a step formed in the right side region; 
           [0034]      FIG. 6  is another cross-sectional view of the solar cell of  FIG. 5  after the next process step according to the present invention in which a dielectric layer is formed over the right side region; 
           [0035]      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 portion of the dielectric layer is removed; 
           [0036]      FIG. 8  is a cross-sectional view of the solar cell of  FIG. 7  after the next process step according to the present invention in which a conductive layer is deposited; 
           [0037]      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 contact layers are deposited; and 
           [0038]      FIG. 10  is a circuit diagram of the solar cell and bypass diode according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0039]    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. 
         [0040]      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  100 , which may be either gallium arsenide (GaAs), germanium (Ge), or other suitable material. A sequence of layers forming a diode is then deposited on the substrate. For example, a p+ GaAs diode emitter layer  101 , an intrinsic GaAs layer  102 , and a n type GaAs  103  are deposited, followed by an etch stop layer  104  of n+ type GaInP 2 . A contact layer  105  of n++ GaAs is then deposited on layer  104 , and a n+AlInP 2  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 , are then deposited on the window layer  106 . 
         [0041]    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), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). 
         [0042]    In the preferred embodiment, the substrate  100  is gallium arsenide, the emitter layer  107  is composed of GaInP 2 , and the base layer is composed of p type GaInP 2 . The use of the parenthesis in the formula is standard nomenclature to indicate that the amount of aluminum may vary from 0 to 30%. 
         [0043]    On top of the base layer  108  is deposited a back surface field (“BSF”) layer of p+ type AlGaInP  109  used to reduce recombination loss. 
         [0044]    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. 
         [0045]    On top of the BSF layer  109  is deposited a sequence of heavily doped p-type and n-type GaAs layers  110  which forms a tunnel diode which is a circuit element to connect cell A to cell B. 
         [0046]    On top of the tunnel diode layers  110  a n+ InAlP.sub.2 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. 
         [0047]    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 GaInP 2  and GaAs (or In 0.015 GaAs) respectively, although any other suitable materials consistent with lattice constant and band gap requirements may be used as well. 
         [0048]    On top of the cell B a p+ GaInP 2  BSF layer  114  is deposited which performs the same function as the BSF layer  109 . A p++/n++ GaAs 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  116 , preferably GaInP, is deposited over the tunnel diode  115 , to a thickness of about 1.0 micron. A metamorphic buffer layer  117  is deposited over the buffer layer  116  and is preferably a compositionally step-graded GaInP series of layers with monotonically changing lattice constant to achieve a transition in lattice constant from cell B to subcell C. In other examples the metamorphic buffer layer  117  is composed of InGaAlAs. The bandgap of layer  117  is 1.5 ev constant with a value slightly greater than the bandgap of the middle cell B. 
         [0049]    In one embodiment, as suggested in the Wanlass et al. paper, the step grade contains nine compositionally graded steps with each step layer having a thickness of 0.25 micron. 
         [0050]    On top of the metamorphic buffer layer  117  another n+GaInAs window  118  is deposited. The window layer  118  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. 
         [0051]    On top of the window layer  118  the layers of subcell C are deposited; then n+ type emitter layer  119  and the p type base layer  120 . In the preferred embodiment, the emitter layer is composed of GaInAs and the base layer is composed of p type GaInAs with about a 1.0 ev bandgap requirements although any other semiconductor material with suitable lattice constant and band gap requirements may be used as well. 
         [0052]    On top of the base layer  120  of subcell C a back surface field (BSF) layer  120 , preferably composed of GaInAsP, is deposited. 
         [0053]    Over or on top of the BSF layer  121  is deposited a p+ contact layer,  122  preferably of p+ type InGaAs. 
         [0054]      FIG. 2  is a cross-sectional view of the solar cell of  FIG. 1  after the next process steps according to the present invention in which a metal contact layer  123  is deposited over the p+ semiconductor contact layer  122 . The metal is preferably a sequence of Ti/Au/Ag/Au layers. An adhesive layer  124  is then deposited over the metal layer  123 . The adhesive is preferably GenTak  330  (distributed by General Chemical Corp.). A surrogate substrate  125 , preferably sapphire, is attached, to the structure using the adhesive layer  124 . 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. 
         [0055]      FIG. 3 , the structure of  FIG. 2  is shown with the surrogate substrate  125  at the bottom. The original substrate  100  is removed by a sequence of lapping and/or etching steps in which the substrate is removed. The choice of the etchant is dependent on the substrate used. 
         [0056]      FIG. 4  then depicts the next process steps in which trench  150  is then etched to layer  123  separating the semiconductor body into two regions,  151  and  152 . A trench  150  is then etched to layer  123  separating the semiconductor body into two regions,  151  and  152 . 
         [0057]      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 layers  101  through  104  in the left side region  151  are removed, and a step formed in the right side region  152  between layers  104  and  105 . Such processing may be implemented by known photolithography techniques. 
         [0058]      FIG. 6  is another cross-sectional view of the solar cell of  FIG. 5  after the next process step according to the present invention in which a dielectric layer  200  is formed over the right side region  152 . Such process step may be implemented by known masking, deposition, and photoresist lift off techniques; 
         [0059]      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 portion of the dielectric layer  200  is removed so that the step portion of the window layer is  106  is exposed, as well as layer  101 ; 
         [0060]      FIG. 8  is a cross-sectional view of the solar cell of  FIG. 7 , after the next process step according to the present invention in which a conductive layer  201  is deposited for electrically connecting the window layer  106  and the metal layer  123 ; 
         [0061]      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 contact layers  202  and  203  are deposited on the left side and right side regions  151  and  152  respectively. 
         [0062]      FIG. 10  is a circuit diagram of the solar cell and bypass diode according to the present invention. The cells A, B, C are arranged in the same order as shown in  FIG. 9 , with the layer  105  (in region  151 ) at the top of the semiconductor structure forming a terminal of the solar cell, and being electrically connected to layer  203 , the terminal of the bypass diode. (Such connection is not shown in  FIG. 9 ). 
         [0063]    Similarly, on the back side of the solar cell, the layer  123  forms the terminal, and is connected by conductor  201  to the terminal of the bypass diode. 
         [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, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.