Method and device for cadmium-free solar cells

Abstract

A method for fabricating a thin film photovoltaic device is provided. The method includes providing a substrate comprising a surface region made of a thin-film photovoltaic absorber including copper, indium, gallium, selenium, and sulfur species. Additionally, the method includes applying a dip-in chemical bath deposition process for forming a buffer layer containing at least zinc-oxygen-sulfide material but substantially free of cadmium species. Furthermore, the method includes producing a chemical bath including steps of heating a bath of water to about 75° C., adding aqueous ammonia to mix with the bath of water, adding a solution of sodium hydroxide , adding zinc salt solution, and adding a solution of thiourea. The dip-in chemical bath deposition process includes immersing a plurality of substrates formed with the thin-film photovoltaic absorber substantially vertically in the chemical bath for 30 minutes to form the zinc-oxygen-sulfide buffer layer followed by a cleaning and drying process.

What is claimed is: 
     
         1 . A method for fabricating a thin film photovoltaic device, the method comprising:
 immersing a substrate in a chemical bath, wherein the substrate comprises a photovoltaic absorber material, and wherein the chemical bath includes a zinc species, an ammonia species, an organosulfur species, and a sodium hydroxide species;   while the substrate is immersed, forming a buffer layer including at least a zinc-oxygen-sulfide compound; and   removing the substrate having at least the photovoltaic absorber material and the buffer material from the chemical bath.   
     
     
         2 . The method of  claim 1  wherein the photovoltaic absorber material comprises a thin film of CuInGaSe 2  or CuInGa(SeS) 2  compound material. 
     
     
         3 . The method of  claim 1  wherein the organosulfur species comprises thiourea. 
     
     
         4 . The method of  claim 1  wherein the ammonia species is derived from an aqueous ammonia. 
     
     
         5 . The method of  claim 1  wherein the zinc species is derived from a hydrated zinc sulfate, a hydrated zinc chloride, or hydrated zinc nitrate. 
     
     
         6 . The method of  claim 3  wherein the thiourea has a concentration at least 25 times greater than that of the zinc salt species in the aqueous solution. 
     
     
         7 . The method of  claim 1  further comprising:
 producing the chemical bath by the steps of:
 heating a bath of water to a first temperature; 
 adding aqueous ammonia to mix with the bath of water to form an ammonia solution; 
 adding a solution of sodium hydroxide into the ammonia solution; 
 adding a solution of zinc salt; 
 adding a solution of thiourea. 
 
 
     
     
         8 . The method of  claim 7  wherein the chemical bath comprises a concentration of [OH − ] of about 4 mMole or more. 
     
     
         9 . The method of  claim 7  wherein the chemical bath comprises a [Zn] concentration of more than 1 mMole. 
     
     
         10 . The method of  claim 7  wherein the chemical bath comprises a concentration of thiourea about 60 mMole. 
     
     
         11 . The method of  claim 7  wherein the chemical bath comprises has a concentration of [NH 4 OH] about 1.6 Mole. 
     
     
         12 . The method of  claim 1  wherein the substrate comprises soda lime glass. 
     
     
         13 . The method of  claim 1  further comprising subjecting the substrate having the buffer material to a cleaning solution. 
     
     
         14 . The method of  claim 13  wherein the cleaning solution comprises rinse water. 
     
     
         15 . The method of  claim 1  wherein immersing the substrate comprises dipping the substrate substantially vertically in the chemical bath for about a first period of time. 
     
     
         16 . A thin film photovoltaic device produced according to the process of  claim 1 .