Back contact to film silicon on metal for photovoltaic cells

Abstract

A crystal oriented metal back contact for solar cells is disclosed herein. In one embodiment, a photovoltaic device and methods for making the photovoltaic device are disclosed. The photovoltaic device includes a metal substrate with a crystalline orientation and a heteroepitaxial crystal silicon layer having the same crystal orientation of the metal substrate. A heteroepitaxial buffer layer having the crystal orientation of the metal substrate is positioned between the substrate and the crystal silicon layer to reduce diffusion of metal from the metal foil into the crystal silicon layer and provide chemical compatibility with the heteroepitaxial crystal silicon layer. Additionally, the buffer layer includes one or more electrically conductive pathways to electrically couple the crystal silicon layer and the metal substrate.

What is claimed is: 
     
         1 . A method for creating a conductive pathway in a photovoltaic cell comprising:
 obtaining a layered photovoltaic device comprising:
 a metal substrate with a crystal orientation; 
 a crystal semiconductor layer with the crystal orientation; and 
 a heteroepitaxially grown buffer layer positioned between the substrate and the crystal semiconductor layer; and 
   forming one or more conductive pathways between the crystal semiconductor layer and the metal substrate, the pathways being through at least a portion of the buffer layer.   
     
     
         2 . The method of  claim 1  further comprising directing a laser beam on the crystal semiconductor layer, the laser in an operating spectrum such that the laser does not interfere with atomic placements crystal orientation of the crystal semiconductor layer, the laser being absorbed at the metal substrate to form the conductive pathway between the crystal semiconductor layer and the metal substrate, the pathway including material from the metal substrate. 
     
     
         3 . The method  claim 2  wherein the operating spectrum of the laser comprises wavelengths within the infra red portion of the electromagnetic spectrum. 
     
     
         4 . The method of  claim 1  further comprising directing a laser on the metal substrate, the laser drilling holes through the metal substrate and causing metal from the metal substrate to diffuse through the buffer layer to electrically couple the metal substrate and the crystal silicon layer. 
     
     
         5 . The method of  claim 1  further comprising:
 applying an etchant to a backside of the metal substrate, the etchant creating apertures through the metal substrate and the buffer layer; and 
 filling the apertures with an electrically conductive material to electrically couple the metal substrate and the crystal silicon layer. 
 
     
     
         6 . A method for creating a conductive pathway in a photovoltaic cell comprising:
 forming apertures in a buffer layer grown over a metal substrate, the metal substrate and the buffer layer having a common crystalline structure; and   heteroepitaxially growing a crystal semiconductor layer after forming the apertures in the buffer layer, the apertures being filled with crystal semiconductor to form a conductive pathway in the buffer layer between the crystal semiconductor layer and the metal substrate.   
     
     
         7 . The method of  claim 6  further comprising:
 creating structures on the metal substrate prior to heteroepitaxially growing the buffer layer; and 
 removing the structures to form the apertures in the buffer layer. 
 
     
     
         8 . The method of  claim 6  further comprising:
 applying nanoparticles to the metal substrate prior to heteroepitaxially growing the buffer layer over the nanoparticles; and 
 mechanically or chemically removing the nanoparticles to form the apertures, wherein heteroepitaxially growing the crystal semiconductor fills the apertures to create vias electrically coupling the metal substrate and the crystal semiconductor layer. 
 
     
     
         9 . The method of  claim 6  further comprising:
 providing a mask over an edge of the metal substrate prior to heteroepitaxially growing the buffer layer; and 
 removing the mask after growing the buffer layer to form the apertures, wherein heteroepitaxially growing the crystal semiconductor fills the apertures to create vias electrically coupling the metal substrate and the crystal silicon layer. 
 
     
     
         10 . The method of  claim 6  further comprising:
 applying etchant to the buffer layer; and
 allowing the etchant to form the apertures in the buffer layer, wherein heteroepitaxially growing the crystal semiconductor fills the apertures to create vias electrically coupling the metal substrate and the crystal silicon layer. 
 
 
     
     
         11 . The method of  claim 10  wherein the etchant is applied in the form of droplets by an inkjet printer to form a grid array pattern, wherein the droplets are spaced on the buffer layer. 
     
     
         12 . The method of  claim 10  wherein the etchant is applied by spraying a fine mist of etchant onto the buffer layer to form a random pattern. 
     
     
         13 . The method of  claim 6  further comprising directing a laser at the buffer layer to form the apertures in the buffer layer, wherein heteroepitaxially growing the crystal semiconductor fills the apertures to create vias electrically coupling the metal substrate and the crystal semiconductor layer. 
     
     
         14 . A crystal silicon product comprising:
 a metal foil substrate having a crystalline structure with a first orientation;   a heteroepitaxial crystal silicon semiconductor layer having a crystalline structure with a second orientation that matches the first orientation; and   a heteroepitaxial oxide buffer layer having a crystalline structure with a third orientation that matches the first orientation, the buffer layer positioned between the metal foil substrate and the crystal silicon semiconductor layer.   
     
     
         15 . The crystal silicon product of  claim 14 , wherein the heteroepitaxial oxide buffer layer is positioned to insulate the crystal silicon semiconductor layer from diffusion of metal from the metal foil substrate. 
     
     
         16 . The crystal silicon product of  claim 14 , wherein grains in the crystal silicon semiconductor layer substantially match orientations of the metal foil substrate. 
     
     
         17 . A method for fabricating a crystal silicon photovoltaic device, the method comprising:
 growing at least one oxide buffer layer heteroepitaxially on a metal substrate having a crystalline structure with a first orientation; and   growing a silicon layer heteroepitaxially on the at least one oxide buffer layer, the silicon layer having a grain size substantially the same as the metal substrate.   
     
     
         18 . The method of  claim 17 , further comprising:
 fabricating the metal substrate to have the crystalline structure with the first orientation using a Rolling Assisted Biaxially Textured Substrate (RABiTS) process.   
     
     
         19 . The method of  claim 17 , wherein electron beam evaporation is used to grow the buffer layers. 
     
     
         20 . The method of  claim 17 , wherein the silicon layer is heteroepitaxially grown using hot wire chemical vapor deposition (HWCVD). 
     
     
         21 . The method of  claim 20 , wherein HWCVD is performed with the following conditions:
 about 20 sccm of SiH 4  flow;   a single, coiled tungsten filament about 12 inches long, heated with about a 16 A current;   about 10 mTorr pressure in the chamber during growth; substrate temperatures between about 600° C. to 800° C.; and   a base pressure of about 5×10 −7  Torr.   
     
     
         22 . The method of  claim 17 , wherein the metal substrate comprises vacuum cast Ni-5W. 
     
     
         23 . The method of  claim 17 , wherein the metal substrate comprises non-vacuum cast Ni-3W. 
     
     
         24 . The method of  claim 17 , wherein the metal substrate comprises NiW foil with oriented grains of about 50 μm. 
     
     
         25 . The method of  claim 17 , wherein a first oxide buffer layer of the at least one oxide buffer layer includes about 60 nm of MgO and 120 nm of γ-Al 2 O 3 .