Solar cell having a transparent conductive oxide contact layer with an oxygen gradient

A method for making a solar cell, comprising:

• forming a first electrode located over a substrate;
• forming at least one first conductivity type semiconductor layer over the first electrode;
• forming at least one second conductivity type semiconductor layer over the first conductivity semiconductor layer;
• and
• forming a transparent conductive oxide contact layer over the second conductivity semiconductor layer,
• wherein:
• a first surface of the transparent conductive oxide contact layer is located closer to the second conductivity type semiconductor layer than a second surface of the transparent conductive oxide contact layer;
• and
• the transparent conductive oxide contact layer has an oxygen concentration that decreases continuously or in at least two discrete steps as a function of thickness for at least a first portion of the contact layer thickness in a direction from the first surface to the second surface.

A method for making a solar cell, comprising:

• forming a first electrode located over a substrate;
• forming at least one first conductivity type semiconductor layer over the first electrode;
• forming at least one second conductivity type semiconductor layer over the first conductivity semiconductor layer;
• and
• forming a transparent conductive oxide contact layer over the second conductivity semiconductor layer by sputtering at least one metal oxide target in a process atmosphere having a variable oxygen content;
• wherein:
• a first surface of the transparent conductive oxide contact layer is located closer to the second conductivity type semiconductor layer than a second surface of the transparent conductive oxide contact layer;
• the transparent conductive oxide contact layer has an oxygen concentration that decreases continuously or in at least two discrete steps as a function of thickness for at least a first portion of the contact layer thickness in a direction from the first surface to the second surface;
• the substrate is a metallic web substrate;
• and
• the steps of forming the first electrode over the substrate, forming the at least one p-type semiconductor absorber layer, forming the n-type semiconductor layer, and forming the transparent conductive oxide contact layer are conducted in corresponding process modules of a plurality of independently isolated, connected process modules without breaking vacuum, while passing the metallic web substrate from an input module to an output module through the plurality of independently isolated, connected process modules.