Abstract:
A solar cell including a semiconductor body with a multijunction solar cell and an integral bypass diode, and a pair of vias extending between the upper and lower surfaces, forming determined on the lower surface and electrically coupling the anode of the bypass diode with the conductive grid on the upper surface.

Description:
GOVERNMENT RIGHTS STATEMENT 
     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. Portions of the material in this specification arose as a result of U.S. Government support under a subcontract to a U.S. Government Agency. The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on resonable terms as provided for by the terms of the contract. 
    
    
     REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 11/109,016 filed Apr. 19, 2005. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Description of the Related Art 
     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. 
     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. 
     Solar cells are often fabricated in vertical, multijunction structures, and 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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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, and as represented in U.S. patent application Ser. No. 11/109,616 of the instant assignee. The presence of a bypass diode in an integrated solar cell presents another complication, since a connection must be made from both the multijunction solar cell and from the bypass diode on the top surface of a first cell to the bottom surface of the adjoining cell in the array. 
     Prior art interconnection arrangements for solar cell arrays have utilized an electrical contact to the top layer of the wafer to contact the anode of the bypass diode, and to the bottom surface of the adjoining cell. Although such an arrangement is generally satisfactory for most applications, there are certain applications in which present more stringent design requirement with both terminals of the solar cell and the bypass diode on the same side of the cell are required. Prior to the present invention, solar cell solar celldesigns have not been able to meet such design requirements. 
     SUMMARY OF THE INVENTION 
     1. Objects of the Invention 
     It is an object of the present invention to provide an improved solar cell array. 
     It is another object of the invention to provide a via in a solar cell with an integral bypass diode so that both anode and cathode terminals are on one side. 
     It is another object of the present invention to provide a multiple via configuration in a solar cell semiconductor device for both the surface grids and the anode of the bypass diode for improved reliability and higher manufacturing yields. 
     It is still another object of the invention to provide an improved method for manufacturing a solar cell array by utilizing multi-contact via interconnects between the upper surface and the backside of the wafer including a bypass diode. 
     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, with 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 
     Briefly, and in general terms, the present invention provides a solar cell including a semiconductor body having an upper surface and an opposing lower surface; a multijunction solar cell disposed between said upper and lower surfaces; a bypass diode disposed on the semiconductor body and electrically connected across the multijunction solar cells for passing current when the solar cell is shaded; and an electrically conducting via making contact with a terminal of the bypass diode and extending from the upper surface to a contact on the lower surface of the semiconductor body. 
     The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, bets will be understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1A  is a top plan view of a semiconductor wafer with solar cell with a bypass diode; 
         FIG. 1B  is an enlarged top view of the solar cell of  FIG. 1A  showing the bypass diode; 
         FIG. 2  is an enlarged cross-sectional view of the solar cell with a bypass diode through the A-A plane of  FIG. 1B ; 
         FIG. 3  is an enlarged top view of the solar cell with a bypass diode through the B-B plane of  FIG. 1B ; 
         FIG. 4A  is a top plan view of the solar cell with a bypass diode as cut from the wafer depicted in  FIG. 1A ; 
         FIG. 4B  is a bottom plan view of the solar cell with a bypass diode shown in  FIG. 4A ; 
         FIG. 5  is a circuit diagram of the solar cell with a bypass diode of  FIG. 4A ; 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     The present invention relates to the interconnection of the anode and cathode contacts of the III-V multi-junction solar cells in an array by use of metallic clips or jumpers which are bonded or welded to the adjoining cells. Solar cell semiconductor devices, such as those depicted in U.S. Pat. No. 6,680,432, often include bypass diodes epitaxially grown on the substrate but separated from the solar cell structure by a trench that provides electrical isolation of the solar cell and the bypass diode. The surface of the trench is covered by a dielectric material of any suitable material that is nonconductive and reduces the possibility of an electrical chart or shunt path being created along the edge of the cell. 
       FIG. 1A  is a top plan view of a semiconductor wafer with solar cell with a bypass diode according to the present invention. A typical wafer  500  can accommodate two cells  501  and  502  which are scribed or cut out from the wafer  500  at the end of the fabrication process. The peripheral edge material  503  of the semiconductor wafer is discarded. Unlike configurations of the prior art, there are no electrical contacts on the top surface or cathode of the cells  501  or  502 . The bypass diode  503  of cell  501 , and the bypass diode  503  of cell  502  are also depicted, and are shown in greater detail in the enlarged portion of the wafer as shown in  FIG. 1B . 
       FIG. 1B  is an enlarged top plan view of the bypass diode  502  implemented in the solar cell of  FIG. 1A . The Figure also shows some of the vertical conductors  806  which extend in parallel over the top surface of the cell  501  and function to make electrical contact with the top layer of the cell  501  and collect charge when the surface is illuminated. There is also depicted an electrically conducting bus  807  which extends around the periphery of the cell  501  and functions to electrically connect each of the conductors  806 . 
     The top plan view of the bypass diode  503  shows that it is semicircular in shape, and as will be seen in the cross-section views of  FIGS. 2 and 3 , is preferably implemented as a mesa structure fabricated over the top layer of the cell  501 . Two spaced apart metal layers  630  and  632  extend over portions of the three sides of the mesa and function to make electrical contact between the top layer of the bypass diode  503  and the substrate located at the bottom of the trench  650 . Another metal layer which functions as the electrical contact to the top terminal of the diode, which in the preferred embodiment is the anode of the diode. 
       FIG. 2  illustrates a detailed cross-sectional view of an integral semiconductor structure with a triple junction solar cell structure  640  and a bypass diode  503  through the A-A plane of  FIG. 1B . The structure includes a substrate  602 , a triple junction solar cell  640 , a bypass diode  620 , a trench or well  650 , and an electrical shunt layer  630 . The triple junction solar cell structure  640  further includes a bottom, middle and top subcells  604 ,  606 , and  608  respectively. One of the conductive grid lines  806  is deposited over a lateral conduction layer  610 . The contact pad  804  is also depicted as deposited over the bypass diode  620 . 
     A layer of non-conduction or dielectric material causes the edges of the cell  501  to prevent the possibility of an electrical short or shunt path being created along the edge of the cell as will be seen in  FIG. 3 . In the preferred structure according to the present invention, the same material that is normally used as an antireflection coating (ARC) over the surface of the cell  501  is an ideal choice for this dielectric material. The ARC layer is normally applied to the top of the solar cell in current commercial devices, to reduce front surface reflection. The preferred embodiment uses a fabrication sequence of steps that allows the ARC layer to extend not only over the surface of the solar cell, but into the mesa trench to provide the protection of the vertical edge of the solar cell mesa. 
     In greater particularity, in one embodiment, the substrate is a p-type germanium (“Ge”) substrate  602  which is entirely covered with a metallic layer on the backside of the semiconductor wafer  500  to form a lower metal contact pad  802  (as shown in  FIG. 43 ). The bottom cell  604  contains a p-type Ge base layer  810 , an n-type Ge emitter layer  812 , and an n-type GaAs layer  814 . The base layer  810  is deposited over the substrate  602 . The layer  810  in one embodiment can be formed through diffusion of atoms into the Ge substrate from a deposited emitter layer  812 . After the bottom cell  604  is fabricated, a sequence of p-type and n-type tunneling junction layers  815 , are deposited to form a structure sometimes referred to as a tunneling diode that provides a circuit element to connect the bottom cell  604  with the next subcell  606 . 
     The middle cell  606  further includes a back surface field (“BSF”) layer  820 , a p-type GaAs base layer  822 , and n-type GaAs emitter layer  824 , and an n-type gallium indium phosphide.sub.2 (GalnP.sub.2) window layer  826 . The base layer  822  is deposited over the BSF layer  820  once the BSF layer  820  is deposited over the tunneling junction layers  816 . The window layer  826  is subsequently deposited on the emitter layer  824  after the emitter layer  824  is deposited on the base layer  822 . The BSF layer  820  is used to reduce the recombination loss in the middle cell  606 . The BSF layer  820  drives minority carriers from a highly doped region near the back surface to minimize the effect of recombination loss. In other words, a BSF layer  820  reduces recombination loss at the backside of the solar cell and thereby reduces the recombination at the emitter region. 
     The window layer  826  used in the middle cell  606  also operates to reduce the recombination loss. The window layer  826  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. Before depositing the layers of the top cell  608 , p-type and n-type tunneling junction layers  830  are deposited over the middle cell  606  to form a tunnel diode connecting the middle cell  606  to the top cell  608 . 
     The top cell  608 , according to this embodiment, includes layer sequence of a p-type indium gallium aluminum phosphide.sub.2 (“InGaAlP”) BSF layer  840 , a p-type GalnAP.sub.2 base layer  842 , and n-type.sub.2 emitter layer  844 , and an n-type aluminum indium phosphide.sub.2 (“AllnP.sub.2”) window layer  846 . The base layer  842  of the top cell  608  is deposited on the BSF layer  840  once the BSF layer  840  is deposited over the tunneling junction layers  730 . The window layer  846  is subsequently deposited on the emitter layer  844  after the layer  844  is deposited on the base layer  842 . 
     According to this embodiment, an n-type GaAs window or cap layer  846  is employed for enhancing better contact with metal materials. The cap layer  846  is deposited over the top cell  608 . The lateral conduction layer  610 , formed of n-type GaAs, is deposited over the cap layer  846 . An n-type GaInP 2  stop etch layer is deposited over the lateral conduction layer  610 . After the stop etch layer is deposited, the layers that will form the bypass diode on one portion of the cell  501  are epitaxially deposited over the entire wafer. 
     The bypass diode layers include in one embodiment, a sequence of an n-type GaAs layer  860 , an i-type GaAs layer  862 , and a p-type GaAs layer  864 . The n-type layer  860  is deposited over the stop etch layer  612 . The i-type layer  862  is deposited over the n-type layer  860 . The p-type layer  864  is deposited over the i-type layer  862 . After layer  864  is deposited, a contact pad  804  is deposited over the bypass diode  620 . Once the anode contact pad  804  is formed, the polarity is formed, the result is that an integral p-i-n bypass diode with p-on-n polarity is formed over the solar cell. In another embodiment, an n-i-p bypass diode with n-on-p polarity can be formed over solar cell structure using similar process described above. It should be apparent to one skilled in the art that additional layer(s) may be added or deleted in the bypass diode  620  without departing from the scope of the present invention. 
     In one embodiment, a metal shunt layer  630  is deposited into a portion of the well  650 . One end of the shunt  630  makes electrical contact with the substrate  602  and the other end of the shunt  630  makes electrical contact with the lateral conduction layer  610 , and thereby to an active layer of the triple junction cell  644 . An anti-reflection (AR) coating  808  may be deposited over certain parts of the solar cell to enhance solar cell performance. 
     A via  900  is etched from the bottom surface of the substrate though to the top surface of the wafer making a small circular opening on the top surface of the wafer. A dielectric layer is deposited over the interior surface of the via to insulate the layers, and a metal layer deposited over the dielectric layer to form an electrical path  902  between the top and bottom surfaces of the wafer. In particular, the path  902  makes electrical contact with the contact layer  804  of the bypass diode and forms a contact surface on the bottom of the wafer. 
     In addition to via  900 , vias  920 ,  930 ,  940  and  950  are etched from the bottom surface of the substrate to make electrical contact with four of the grid lines  806  on the top surface of the wafer, at contact points respectively, as shown in  FIGS. 4A and 4B . Connections  910  extend between the vias  900 ,  920 ,  930 ,  940 ,  950 . 
     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, 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). 
       FIG. 5  is a circuit diagram  700  illustrating a triple junction solar cell structure and a bypass diode  620 . The device  700  includes a top cell  608 , a middle cell  606 , a bottom cell  604 , a bypass diode  620 , a resistance block  702 , and four paths  710 - 716 . In one embodiment, the resistance block  702  includes resistance from the shortened portion of the multijunction solar cell structure that is situated underneath of the bypass diode  620  and the resistance from the shunt  630  shown in  FIG. 2 . 
     During normal operation (e.g., when solar cells  604 - 608  are exposed to sunlight, solar light, light, radiation, and/or photons), the solar cells  604 - 608  are forward biased. They convert solar energy to electrical energy and the generated electric current between the neighboring solar cells connected in series. It should be noted that the terms sunlight, solar light, light, radiation, and/or photons may be used interchangeably herein. In this embodiment, solar cells are organized in a series. While solar cells  604 - 608  are in forward biased, bypass diode  620  is reverse biased because bypass diode  620  has an opposite polarity from solar cells. Thus, when bypass diode  620  is in reverse bias mode, no electric current passes through the bypass diode  620 . 
     Then electrical current generated from the neighboring solar cells arrives at solar cells  604 - 608  via path  710 , solar cells  604 - 608  pass total electrical current, which includes the current converted by solar cells  604 - 608  and the current arriving from neighboring solar cells through path  710 , to path  716  via path  712 . Path  716  may be connected to another solar cell (such as solar cell  521  depicted in  FIG. 6 ) and/or other electrical devices. 
     However, during the situation in which the solar cells  601 - 608  are in reverse bias mode when, for example, solar cells  604 - 608  are shadowed, the bypass diode  620  becomes forward biased. In this situation, bypass diode  620  become active and passes the current from neighboring solar cells via path  710  to path  716  through path  714 . In other words, when the solar cells  604 - 608  are in reverse bias mode, the bypass diode  620  becomes forward biased and uses path  714  to pass the current from path  710  to path  716 . Thus, in this circuit configuration, bypass diode protects the cells  604 - 608 .