Exponentially Doped Layers In Inverted Metamorphic Multijunction Solar Cells

Abstract

A method of forming a multifunction solar cell including an upper subcell, a middle subcell, and a lower subcell, including providing first substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on the substrate having a first band gap; forming a second solar subcell over the first solar subcell having a second band gap smaller than the first band gap; forming a grading interlayer over the second subcell, the grading interlayer having a third band gap greater than the second band gap; and forming a third solar subcell over the grading interlayer having a fourth band gap smaller than the second band gap such that the third subcell is lattice mismatched with respect to the second subcell, wherein at least one of the bases of a solar subcell has an exponentially doped profile.

1 - 20 . (canceled) 
     
     
         21 . A multifunction solar cell comprising an upper subcell, a middle subcell, and a lower or bottom subcell, comprising:
 a first solar subcell having a base and an emitter having a first band gap;   a second solar subcell having a base and an emitter disposed over said first solar subcell having a second band gap smaller than said first band gap;   a grading interlayer disposed over said second solar subcell, said grading interlayer having a third band gap greater than said second band gap; and   a third solar subcell having a base and an emitter disposed over said grading interlayer having a fourth band gap smaller than said second band gap such that said third subcell is lattice mismatched with respect to said second subcell,   wherein at least one of the bases has an exponentially doped profile.   
     
     
         22 . A solar cell as defined in  claim 21 , wherein the base in said first solar cell has an exponential gradation in doping from 1×10 16  per cubic centimeter adjacent the base-emitter junction to 1×10 18  per cubic centimeter adjacent the adjoining layer. 
     
     
         23 . A solar cell as defined in  claim 21 , wherein the base in said second solar cell has an exponential gradation in doping from 1×10 16  per cubic centimeter adjacent the base-emitter junction to 1×10 18  per cubic centimeter adjacent the adjoining layer. 
     
     
         24 . A solar cell as defined in  claim 21 , wherein the base in said third solar cell has an exponential gradation in doping from 1×10 16  per cubic centimeter adjacent the base-emitter junction to 1×10 18  per cubic centimeter adjacent the adjoining layer. 
     
     
         25 . A solar cell as defined in  claim 21 , wherein the emitter layer in at least one of the solar cells has an increasing gradation in doping from 5×10 17  per cubic centimeter adjacent the base-emitter junction to 5×10 18  per cubic centimeter adjacent the adjoining layer. 
     
     
         26 . A solar cell as defined in  claim 24 , wherein said third solar cell is the bottom subcell and the exponential gradation in doping results in an increase in short circuit current to a level approximately equal to the short circuit current in a higher subcell. 
     
     
         27 . A solar cell as defined in  claim 21 , wherein the first solar cell is formed on a first substrate composed of GaAs. 
     
     
         28 . A solar cell as defined in  claim 21 , wherein said first solar subcell is composed of an InGa(Al)P emitter region and an InGa(Al)P base region. 
     
     
         29 . A solar cell as defined in  claim 21 , wherein said second solar subcell is composed of an InGaP emitter region and an GaAs base region. 
     
     
         30 . A solar cell as defined in  claim 21 , wherein said grading interlayer is composed of InGaAlAs. 
     
     
         31 . A solar cell as defined in  claim 30 , wherein said grading interlayer is composed of nine steps of layers with monotonically changing lattice constant. 
     
     
         32 . A solar cell as defined in  claim 21 , wherein said third solar subcell is composed of InGaAs. 
     
     
         33 . A solar cell as defined in  claim 21 , further comprising a barrier layer about one micron in thickness disposed adjacent said grading interlayer for preventing threading dislocations from propagating. 
     
     
         34 . A semiconductor structure for use in manufacturing a solar cell comprising:
 a first substrate;   a sequence of layers of semiconductor material forming a solar cell including at least one base layer with exponential doping;   mounting a surrogate substrate on top of the sequence of layers disposed on said first substrate; and   a surrogate substrate mounted on top of the sequence of layers.   
     
     
         35 . A structure as defined in  claim 34 , wherein the sequence of layers of semiconductor material forms a triple junction solar cell, including first, second and third solar subcells. 
     
     
         36 . A solar cell comprising:
 a top cell including base and emitter layers composed of InGaP semiconductor material;   a middle cell emitter layer of GaAs semiconductor material and a base layer of InGaP semiconductor material; and   a bottom cell including an emitter and base layer of InGaAs semiconductor material,   wherein at least one of the base layers has an exponentially doped profile.   
     
     
         37 . A solar cell as defined in  claim 36 , further comprising a first substrate for the growth of semiconductor material, wherein said top cell is directly grown over said first substrate and is lattice matched thereto, and said bottom cell is lattice mismatched with respect to said substrate. 
     
     
         38 . A solar cell as defined in  claim 37 , further comprising a second substrate for mounting the solar cell on a rigid support.