Compositionally-graded band gap heterojunction solar cell

Abstract

A photovoltaic device includes a composition modulated semiconductor structure including a p-doped first semiconductor material layer, a first intrinsic compositionally-graded semiconductor material layer, an intrinsic semiconductor material layer, a second intrinsic compositionally-graded semiconductor layer, and an n-doped first semiconductor material layer. The first and second intrinsic compositionally-graded semiconductor material layers include an alloy of a first semiconductor material having a greater band gap width and a second semiconductor material having a smaller band gap with, and the concentration of the second semiconductor material increases toward the intrinsic semiconductor material layer in the first and second compositionally-graded semiconductor material layers. The photovoltaic device provides an open circuit voltage comparable to that of the first semiconductor material, and a short circuit current comparable to that of the second semiconductor material, thereby increasing the efficiency of the photovoltaic device.

What is claimed is: 
     
         1 . A photovoltaic device comprising a stack, from top to bottom, of a first intrinsic compositionally-graded semiconductor layer including a first compositionally-graded alloy of a first semiconductor material and a second semiconductor material, an intrinsic semiconductor material layer including at least said second semiconductor material, and a second intrinsic compositionally-graded semiconductor layer including a second compositionally-graded alloy of said first semiconductor material and said second semiconductor material, wherein an atomic concentration of said second semiconductor material monotonically decreases with distance from an interface with said intrinsic semiconductor layer in said first compositionally-graded alloy and said second compositionally-graded alloy. 
     
     
         2 . The photovoltaic device of  claim 1 , wherein said first semiconductor material has a greater band gap width than said second semiconductor material. 
     
     
         3 . The photovoltaic device of  claim 2 , wherein said first semiconductor material is silicon and said second semiconductor material is germanium. 
     
     
         4 . The photovoltaic device of  claim 2 , wherein said first compositionally-graded alloy, said intrinsic semiconductor material layer, and said second compositionally-graded alloy are amorphous. 
     
     
         5 . The photovoltaic device of  claim 4 , wherein said first compositionally-graded alloy, said intrinsic semiconductor material layer, and said second compositionally-graded alloy include hydrogenated amorphous semiconductor materials. 
     
     
         6 . The photovoltaic device of  claim 2 , wherein said first compositionally-graded alloy, said intrinsic semiconductor material layer, and said second compositionally-graded alloy are polycrystalline. 
     
     
         7 . The photovoltaic device of  claim 2 , wherein said first compositionally-graded alloy, said intrinsic semiconductor material layer, and said second compositionally-graded alloy are single crystalline and epitaxially aligned to one another. 
     
     
         8 . The photovoltaic device of  claim 1 , wherein said atomic concentration of said second semiconductor material strictly decreases with distance from an interface with said intrinsic semiconductor layer in said first compositionally-graded alloy and said second compositionally-graded alloy. 
     
     
         9 . The photovoltaic device of  claim 1 , wherein said atomic concentration of said second semiconductor material is 100% at a first interface between said first intrinsic compositionally-graded semiconductor layer and said intrinsic semiconductor layer, and said atomic concentration of said second semiconductor material is 100% at a second interface between said second intrinsic compositionally-graded semiconductor layer and said intrinsic semiconductor layer. 
     
     
         10 . The photovoltaic device of  claim 9 , wherein said atomic concentration of said second semiconductor material is 0% at a first end surface of said first intrinsic compositionally-graded semiconductor layer, and said atomic concentration of said second semiconductor material is 0% at a second end surface of said second intrinsic compositionally-graded semiconductor layer. 
     
     
         11 . The photovoltaic device of  claim 10 , further comprising:
 a p-doped first semiconductor material layer consisting of a p-doped first semiconductor material and located on said first end surface, and   an n-doped first semiconductor material layer consisting of an n-doped first semiconductor material and located on said first end surface.   
     
     
         12 . The photovoltaic device of  claim 11 , wherein said first semiconductor material is silicon and said second semiconductor material is germanium. 
     
     
         13 . The photovoltaic device of  claim 12 , wherein said first semiconductor material is amorphous hydrogenated silicon and said second semiconductor material is amorphous hydrogenated germanium. 
     
     
         14 . The photovoltaic device of  claim 11 , wherein a p-i junction is coincident with a material junction between said p-doped first semiconductor material layer and said first intrinsic compositionally-graded semiconductor layer. 
     
     
         15 . The photovoltaic device of  claim 14 , wherein an i-n junction is coincident with a material junction between said n-doped first semiconductor material layer and said second intrinsic compositionally-graded semiconductor layer. 
     
     
         16 . The photovoltaic device of  claim 1 , wherein said first intrinsic compositionally-graded semiconductor layer has a thickness from 10 nm to 200 nm, said an intrinsic semiconductor material layer has a thickness from 10 nm to 400 nm, and said second intrinsic compositionally-graded semiconductor layer has a thickness from 10 nm to 200 nm. 
     
     
         17 . The photovoltaic device of  claim 1 , further comprising;
 a transparent conductive material layer electrically connected to said first intrinsic compositionally-graded semiconductor layer; and   at least one back reflector layer electrically connected to said second intrinsic compositionally-graded semiconductor layer.   
     
     
         18 . The photovoltaic device of  claim 1 , wherein said intrinsic semiconductor material layer consists of said second semiconductor material. 
     
     
         19 . A method of forming a photovoltaic device comprising:
 forming a first intrinsic compositionally-graded semiconductor layer on a substrate;   forming an intrinsic semiconductor material layer on said first intrinsic compositionally-graded semiconductor layer, said intrinsic semiconductor material layer including at least said second semiconductor material; and   forming a second intrinsic compositionally-graded semiconductor layer on said intrinsic semiconductor material layer, said second intrinsic compositionally-graded semiconductor layer including a second compositionally-graded alloy of said first semiconductor material and said second semiconductor material, wherein an atomic concentration of said second semiconductor material monotonically decreases with distance from an interface with said intrinsic semiconductor layer in said first compositionally-graded alloy and said second compositionally-graded alloy, and said second conductivity type is the opposite of said first conductivity type.   
     
     
         20 . The method of  claim 19 , wherein said first semiconductor material has a greater band gap width than said second semiconductor material. 
     
     
         21 . The method of  claim 20 , wherein said first compositionally-graded alloy, said intrinsic semiconductor material layer, and said second compositionally-graded alloy are amorphous. 
     
     
         22 . The method of  claim 19 , wherein said atomic concentration of said second semiconductor material strictly decreases with distance from an interface with said intrinsic semiconductor layer in said first compositionally-graded alloy and said second compositionally-graded alloy. 
     
     
         23 . The method of  claim 19 , wherein said atomic concentration of said second semiconductor material is 100% at a first interface between said first intrinsic compositionally-graded semiconductor layer and said intrinsic semiconductor layer, and said atomic concentration of said second semiconductor material is 100% at a second interface between said second intrinsic compositionally-graded semiconductor layer and said intrinsic semiconductor layer. 
     
     
         24 . The method of  claim 19 , further comprising:
 forming a doped first semiconductor material layer consisting of said first semiconductor material and dopants of said first conductivity type on said substrate, and   forming another doped first semiconductor material layer consisting of said first semiconductor material and dopants of said second conductivity type on said second intrinsic compositionally-graded semiconductor layer.   
     
     
         25 . The method of  claim 1 , wherein said substrate is optically transparent, and said method further comprises;
 forming a transparent conductive material layer on said substrate; and   forming at least one back reflector layer on said second intrinsic compositionally-graded semiconductor layer.