Passivated Emitter Rear Locally Patterned Epitaxial Solar Cell

Abstract

Passivated emitter rear local epitaxy (PERL-e) thin Si solar cells may be formed with a heavily doped epitaxial back surface field (BSF) layer, which is patterned to form well spaced point contacts to the silicon base on the rear of the solar cell. The back side of the cell may be finished with a dielectric passivation layer and a metallization layer for making electrical contact to the cell. PERL-e thick Si solar cells may be formed with heavily doped epitaxial films as the back point contacts, where the point contacts are defined by the provision of a selectively patterned thermal oxide on the rear wafer surface. Furthermore, absorption of longer wavelength, infrared (IR), light in thin silicon solar cells may be improved by the addition of a dielectric stack on the rear surface of the solar cell (a back reflector), the stack acting to reflect the longer wavelength light back through the active layers of the solar cell.

What is claimed is: 
     
         1 . A silicon solar cell, said solar cell having a front surface and a back surface, comprising:
 a base layer;   a back surface field structure contacting said base layer at a multiplicity of contact areas on the back surface of said base layer, said back surface field structure being epitaxial to said base layer at said contact areas, and   a passivation layer covering the portion of the back surface of said base layer which is not said multiplicity of contact areas.   
     
     
         2 . The solar cell as in  claim 1 , wherein said base layer is single crystal silicon. 
     
     
         3 . The solar cell as in  claim 1 , wherein said base layer is multicrystalline silicon. 
     
     
         4 . The solar cell as in  claim 1 , wherein said base layer is polycrystalline silicon. 
     
     
         5 . The solar cell as in  claim 1 , wherein said back surface field structure includes a multiplicity of silicon mesas corresponding to said multiplicity of contact areas. 
     
     
         6 . The solar cell as in  claim 1 , wherein said back surface field structure is a continuous layer of silicon. 
     
     
         7 . The solar cell as in  claim 6 , wherein said continuous layer of silicon is polycrystalline. 
     
     
         8 . The solar cell as in  claim 1 , further comprising a dielectric layer covering said passivation layer, wherein said dielectric layer and said passivation layer are formed of different materials, said different materials and the thicknesses of said dielectric layer and said passivation layer being chosen to improve the reflection of infrared light back into the active layers of said solar cell. 
     
     
         9 . The solar cell as in  claim 1 , wherein said contact areas are less than 1% of the area of the back surface of said base. 
     
     
         10 . The solar cell as in  claim 1 , wherein said passivation layer is a layer of thermal silicon oxide formed on said base layer. 
     
     
         11 . The solar cell as in  claim 1 , wherein said multiplicity of contact areas are in an evenly spaced array. 
     
     
         12 . A method of fabricating a silicon solar cell, said solar cell having front and back surfaces, said solar cell including a base layer, said method comprising:
 forming a passivation layer on the back surface of said base layer;   patterning said passivation layer to form a multiplicity of apertures in said passivation layer corresponding to contact areas on the back surface of said base layer; and   epitaxially depositing a back surface field structure on said contact areas on the back surface of said base layer.   
     
     
         13 . The method as in  claim 12 , further comprising depositing a metal layer on the back surface of said back surface field structure. 
     
     
         14 . The method as in  claim 12 , wherein said passivation layer is a layer of thermal silicon oxide. 
     
     
         15 . The method as in  claim 12 , further comprising, after said forming and before said patterning, forming a dielectric layer over said passivation layer, wherein said patterning includes patterning said passivation layer and said dielectric layer to form a multiplicity of apertures in said passivation layer and said dielectric layer corresponding to contact areas on the back surface of said base layer. 
     
     
         16 . The method as in  claim 12 , wherein said back surface field structure is continuous and polycrystalline. 
     
     
         17 . The method as in  claim 12 , wherein said base layer is a silicon wafer. 
     
     
         18 . A method of fabricating a silicon solar cell, said solar cell having front and back surfaces, said solar cell including a base layer, said method comprising:
 providing a back surface field layer and a base layer, said back surface field layer being on the back surface of said base layer, the interface between said back surface field layer and said base layer being an epitaxial crystalline interface;   patterning said back surface field layer to form a multiplicity of mesas on the back surface of said base layer;   depositing a passivation layer over said multiplicity of mesas and the back surface of said base layer; and   patterning said passivation layer to form a multiplicity of apertures corresponding to said multiplicity of mesas.   
     
     
         19 . The method as in  claim 18 , further comprising depositing a metal layer over said passivation layer and the surfaces of said multiplicity of mesas exposed by said multiplicity of apertures. 
     
     
         20 . The method as in  claim 18 , further comprising, after said depositing and before said patterning said passivation layer, forming a dielectric layer over said passivation layer, wherein said patterning includes patterning said passivation layer and said dielectric layer to form a multiplicity of apertures in said passivation layer and said dielectric layer corresponding to said multiplicity of mesas. 
     
     
         21 . The method as in  claim 18 , wherein said providing includes:
 providing a silicon substrate with a porous silicon separation layer on the surface thereof;   epitaxially depositing said back surface field layer on said porous silicon separation layer;   epitaxially depositing said base layer on said back surface field layer; and   separating said base layer on said back surface field layer from said silicon substrate.