Method for forming photovoltaic cells employing multinary semiconductor films

DETAILED DESCRIPTION What is claimed is: 1.
A method for use in the manufacture of photovoltaic cells having a film of a Cu-ternary semiconductor compound made up of a plurality of constituent elements, comprising the steps of: magnetron sputtering at least two constituent elements of the Cu-ternary compound simultaneously onto a substrate in a first preselected proportion; and subsequently magnetron sputtering said at least two constituent elements simultaneously onto the substrate in at least one other preselected proportion; whereby a composite sputtered film of the Cu-ternary compound having a preselected concentration gradient of at least one of the constituent elements is produced on the substrate.
2.
The method of claim 1 wherein the sputtering steps are performed at room temperature and the method further comprises the step of subsequently heat treating the composite sputtered film to produce a macroscopically homogeneous film.
3.
The method of claim 2 wherein the heat treatment is carried out at a temperature of at least approximately 350 degrees Celsius.
4.
The method of claim 1 wherein said Cu-ternary semiconductor compound is a chalcopyrite containing at least one Group III element, and the preselected concentration gradient comprises a gradient in the ratio of the concentration of Cu to the concentration of the Group III element.
5.
The method of claim 4 wherein the Cu-ternary semiconductor compound includes a Group VI element and the constituent elements are sputtered from stable alloys containing the Group VI element.
6.
The method of claim 5 which further comprises the step of subsequently heat treating the film in a reactive atmosphere containing the Group VI element.
7.
The method of claim 4 wherein the semiconductor compound is CuInSe.
sub.
2 and the constituent elements are sputtered from stable alloy targets containing Se.
8.
The method of claim 7 wherein the alloy targets comprise Cu.
sub.
2 Se and In.
sub.
2 Se.
sub.
3, respectively.
9.
The method of claim 1 wherein the step of sputtering the constituent elements in at least one other preselected proportion comprises varying the relative sputtering rate of at least one of the constituent elements during the sputtering process