Back contact solar cell assemblies

Abstract

A back contact solar cell assembly and methods for its manufacture and assembly onto a panel for use in space vehicles are described. The solar cell assembly includes a compound semiconductor multijunction solar cell having a contact at the top surface of the solar cell, a conductive semiconductor element extending from the contact on the top surface to the back surface of the assembly where it forms a first back contact of a first polarity type, and a second back contact of a second polarity at the back surface of the assembly electrically coupled to the back surface of the solar cell.

1 . A method of fabricating a back contact multijunction solar cell assembly comprising:
 providing a semiconductor substrate;   depositing epitaxial layers of semiconductor material over the substrate to form a plurality of solar subcells; including a top or light facing solar subcell;   depositing a plurality of grid lines extending over the top or light receiving surface of the multifunction solar cell;   forming an electrical contact on a bottom or back surface of the multi junction solar cell, opposite to the top or light receiving surface;   forming a first bus bar disposed on the top or light receiving surface of the multijunction solar cell and conductively connected to a first set of said grid lines and having a first portion extending substantially parallel to and proximate a second edge of the solar cell;   forming a second bus bar disposed on the top or light receiving surface of the multijunction solar cell spaced apart from and electrically isolated from the first bus bar, the second bus bar being conductively connected to a second set of grid lines and having a first portion extending substantially parallel to and proximate another edge of the solar cell;   forming a first discrete conductive stand-off component composed of semiconductor material and disposed and spaced apart from the multijunction solar cell and proximate to the first bus bar and electrically coupled thereto, the first stand-off component extending from the top or light receiving surface of the multijunction solar cell to the bottom or back surface of the multijunction solar cell to form a first electrical contact of a second polarity type on the bottom of the assembly; and   wherein a second discrete conductive stand-off component is composed of semiconductor material and spaced apart from the multijunction solar cell and proximate to the second bus bar and electrically coupled thereto, the second stand-off component extending from the top or light receiving surface of the multijunction solar cell to the bottom or back surface of the multijunction solar cell to form a second electrical contact of a second polarity type on the bottom of the assembly.   
     
     
         2 . A method as defined in  claim 1 , further comprising:
 coupling a first element to the first bus bar to form a first electrical contact of a first polarity type to the solar cell; and   coupling a second interconnect element to the second bus bar to form a second electrical contact of a first polarity type to the solar cell.   
     
     
         3 . A method as defined in  claim 1 , wherein the solar cell includes a first edge; a second edge parallel to and opposite the first edge; a third edge orthogonal to the first edge; and a fourth edge parallel to and opposite the third edge and orthogonal to the first edge, and wherein a bounding rectangle is defined by lines extending along the first edge, the second edge, the third edge, and the fourth edge. 
     
     
         4 . A method as defined in  claim 3 , wherein:
 (a) the first discrete conductive stand-off component is disposed within the bounding rectangle on one side of the solar cell adjacent the first edge of the solar cell; and   (b) the second discrete conductive stand-off component is disposed within the bounding rectangle on an opposite second side of the solar cell adjacent the second edge of the solar cell.   
     
     
         5 . A method as defined in  claim 4 , wherein the first and second stand-off components each have a polygonal cross-section and each extends from the top surface of the solar cell to the bottom of the solar cell and forms a first and second respective electrical contacts of a first polarity type on the bottom of the solar cell. 
     
     
         6 . A method as defined in  claim 1 , wherein the stand-off components are composed of a highly doped semiconductor material. 
     
     
         7 . A method as defined in  claim 1 , wherein the stand-off components are composed of gallium arsenide. 
     
     
         8 . A method as defined in  claim 1 , wherein the first interconnect element is electrically coupled to the top surface of the first stand-off component, and the second interconnect element is electrically coupled to the top surface of the second stand-off component. 
     
     
         9 . A method as defined in  claim 3 , further comprising:
 a third stand-off component disposed within the bounding rectangle on said one side of the solar cell, and disposed between the first edge and the fourth edge of the solar cell.   
     
     
         10 . A method as defined in  claim 3 , further comprising a bypass diode disposed within the bounding rectangle and disposed between the second edge and the third edge of the solar cell. 
     
     
         11 . A method as defined in  claim 3 ,
 wherein the first bus bar is conductively connected to a first end position of said grid lines and having a first portion extending substantially parallel to and proximate to the third edge of the solar cell; and   wherein the first interconnect couples the first bus bar with the top surface of the first discrete conductive stand-off component.   
     
     
         12 . A method as defined in  claim 4 ,
 wherein the second interconnect element couples the second bus bar with the top surface of the second discrete conductive stand-off component.   
     
     
         13 . A method as defined in  claim 1 , wherein the grid lines are arranged parallel to one another and substantially orthogonal to the first and second bus bars. 
     
     
         14 . A method as defined in  claim 3 , wherein there is no bus bar along the first and second edges of the solar cell. 
     
     
         15 . A method as defined in  claim 1 , wherein each discrete conductive stand-off component is a discrete semiconductor element shaped as a triangular prism having a side length from 2 to 25 mm and a height from 120 to 150 microns. 
     
     
         16 . A method as defined in  claim 1 , wherein each of the discrete conductive stand-off components are disposed in opposite corners of the solar cell. 
     
     
         17 . A method as defined in  claim 10 , wherein the bypass diode is triangular in shape having a first external edge that is collinear with one of the four long edges of the solar cell and a second external edge that is collinear with the edge of one of the cropped corners of the cell. 
     
     
         18 . A method assembly as defined in  claim 15 , wherein each of the discrete semiconductor elements has first and second end surfaces which are metallized with a metal to a thickness of approximately 5 microns to form a contact or bonding pad. 
     
     
         19 . A method of producing a solar cell assembly comprising;
 providing a flexible substrate;   providing a plurality of conductive traces on the substrate, the plurality of conductive traces including a first conductive trace and a second conductive trace, each of the conductive traces being electrically isolated from one another and at least partly adhered to the substrate, each of the conductive traces comprising a first end portion and a second end portion;   providing a plurality of compound semiconductor solar cells including a first solar cell and a second solar cell, each solar cell comprising a top surface with a top contact of a first polarity and a back surface with a back contact of a second polarity;   bonding the back contact of the first solar cell to the first end portion of the first conductive trace;   bonding the back contact of the second solar cell to the first end portion of the second conductive trace; and   bonding the second end portion of the first conductive trace to the top contact of the second solar cell for connecting the first solar cell and the second solar cell in electrical series.   
     
     
         20 . A solar cell assembly comprising;
 a flexible substrate;   a plurality of conductive traces disposed on the substrate, the plurality of conductive traces including a first conductive trace and a second conductive trace, each of the conductive traces being electrically isolated from one another and at least partly adhered to the substrate, each of the conductive traces comprising a first end portion and a second end portion;   a plurality of III-IV compound semiconductor solar cells including a first solar cell and a second solar cell, each solar cell comprising a top surface with a top contact of a first polarity and a back surface with a back contact of a second polarity;   the back contact of the first solar cell being bonded to the first end portion of the first conductive trace;   the back contact of the second solar cell being bonded to the first end portion of the second conductive trace; and   the second end portion of the first conductive trace being bonded to the top contact of the second solar cell for connecting the first solar cell and the second solar cell in electrical series.