Multijunction solar cells

Abstract

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein the upper first solar subcell covers less than the entire upper surface of the second solar subcell, leaving an exposed portion of the second solar subcell that lies in the path of the incoming light beam.

1 . A multijunction solar cell comprising:
 an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam;   a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap;   wherein the upper first solar subcell covers less than the entire upper surface of the second solar subcell, leaving an exposed portion of the second solar subcell that lies in the path of the incoming light beam.   
     
     
         2 . A solar cell as defined in  claim 1 , further comprising a lateral conduction layer disposed between the upper first solar subcell and the second solar subcell. 
     
     
         3 . A solar cell as defined in  claim 1 , wherein the area of the portion constitutes between 5% and 25% of the surface area of the solar cell. 
     
     
         4 . A solar cell as defined in  claim 1 , wherein the portion comprises a plurality of parallel spaced apart strips extending across the width of the solar cell. 
     
     
         5 . A solar cell as defined in  claim 4 , further comprising a plurality of parallel grid lines disposed over the upper first solar subcell, and wherein the strips are disposed between alternating grid lines. 
     
     
         6 . A solar cell as defined in  claim 1 , wherein the portion comprises a peripheral region of the solar cell. 
     
     
         7 . A solar cell as defined in  claim 1 , wherein the portion comprises a plurality of irregularly shaped regions extending over the surface of the solar cell. 
     
     
         8 . A solar cell as defined in  claim 7 , wherein the irregularly shaped regions comprises alphanumeric characters. 
     
     
         9 . A solar cell as defined in  claim 8 , wherein the alphanumeric characters represent a unique serial number for each solar cell. 
     
     
         10 . A solar cell as defined in  claim 7 , wherein the irregularly shaped region comprises a pictorial design such as a logo. 
     
     
         11 . A solar cell as defined in  claim 1 , further comprising grid lines disposed over the upper first solar subcell, and wherein the portion comprises a plurality of discrete spaced apart regions disposed between the grid lines disposed over the surface of the upper first solar subcell. 
     
     
         12 . A multijunction solar cell as defined in  claim 1 , wherein:
 the upper first subcell is composed of indium gallium aluminum phosphide having a band gap in the range of 2.0 to 2.2 eV;   the second solar subcell includes an emitter layer composed of indium gallium phosphide or aluminum gallium arsenide, and a base layer composed of aluminum gallium arsenide and having a band gap in the range of 1.6 to 1.8 eV;   and further comprising a third solar subcell disposed below the second solar subcell; and   a fourth solar subcell disposed below the third solar subcell.   
     
     
         13 . A solar cell as defined in  claim 10 , further comprising a distributed Bragg reflector (DBR) structure disposed below the second solar subcell and above the third solar subcell, wherein the DBR structure includes a first DBR layer composed of a plurality of n type or p type Al x Ga 1-x As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of n or p type Al y Ga 1-y As layers, where 0<x<1, 0<y<1, and y is greater than x. 
     
     
         14 . A multijunction solar cell, comprising:
 a first subcell that initially receives incident light upon said solar cell, said first subcell being made of a first material system having a band gap in the range of 1.85 to 1.95 eV, a first thickness, and producing a first photogenerated current density output;   a second subcell having a first portion that receives said incident light after said first subcell receives said incident light, and a second portion that receives said incident light directly, said second subcell being disposed immediately adjacent said first subcell, being made of said first material system, having a band gap in the range of 1.3 to 1.42 eV, a second thickness that is greater than said first thickness, and producing a second photogenerated current density output that is substantially equal in amount to said first photogenerated current density output; and   a third bottom subcell that is disposed in said solar cell such that said third bottom subcell is disposed below the second subcell, said third subcell having a third photogenerated current density output that is equal to or greater than said first photogenerated current density output.   
     
     
         15 . A solar cell as defined in  claim 14 , further comprising:
 a lateral conduction layer disposed between the second and the third subcells and composed of gallium arsenide (GaAs) or gallium indium phosphide (GaInP).   
     
     
         16 . A solar cell as defined in  claim 14 , further comprising:
 a bottom subcell that is disposed in said cell such that said bottom subcell is disposed below the third subcell and is the last of the subcells to receive said incident light, said bottom subcell having a bottom photogenerated current output that is greater than said first photogenerated current output.   
     
     
         17 . A solar cell as defined in  claim 14 , wherein the thickness of the first subcell is between 600 nm and 1200 nm. 
     
     
         18 . A solar cell as defined in  claim 14 , wherein the EOL to BOL ratio of the short circuit current of the second subcell is greater than 95%. 
     
     
         19 . A solar cell as defined in  claim 14 , wherein the short circuit current density in the first subcell over the first portion is approximately equal to the short circuit density of the second subcell. 
     
     
         20 . A solar cell as defined in  claim 14 , wherein:
 the upper first subcell is composed of indium gallium aluminum phosphide; and   the second solar subcell includes an emitter layer composed of indium gallium phosphide or aluminum gallium arsenide, and a base layer composed of aluminum gallium arsenide.