SOLAR CELL HAVING A TRANSPARENT CONDUCTIVE OXIDE CONTACT LAYER WITH AN OXYGEN GRADIENT
A solar cell includes a first electrode located over a substrate, at least one first conductivity type semiconductor layer located over the first electrode, at least one second conductivity type semiconductor layer located over the first conductivity semiconductor layer, and a transparent conductive oxide contact layer located over the second conductivity semiconductor layer. The first surface of the transparent conductive oxide contact layer may be located closer to the second conductivity type semiconductor layer than the second surface of the transparent conductive oxide contact layer, and the transparent conductive oxide contact layer may have 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.
1 . A solar cell, comprising:
a first electrode located over a substrate;
at least one first conductivity type semiconductor layer located over the first electrode;
at least one second conductivity type semiconductor layer located over the first conductivity semiconductor layer; and
a transparent conductive oxide contact layer located 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.
2 . The solar cell of claim 1 , wherein the transparent conductive oxide contact layer has an oxygen concentration that decreases continuously as a function of thickness for at least the first portion of the contact layer thickness.
3 . The solar cell of claim 2 , wherein the transparent conductive oxide contact layer has a substantially continuous profile of refractive index as a function of thickness.
4 . The solar cell of claim 1 , wherein the transparent conductive oxide contact layer has an oxygen concentration that decreases in at least two discrete steps as a function of thickness for at least the first portion of the contact layer thickness.
5 . The solar cell of claim 1 , wherein the transparent conductive oxide contact layer comprises material selected from the group consisting of ITO, AZO, Cd 2 SnO 4 , Zn 2 In 2 O 5 , In 2-x-y Sn x Zn y O 3 , CdO, or Ga 2 O 3 .
6 . The solar cell of claim 1 , wherein:
the at least one first conductivity type semiconductor layer comprises a p-type copper indium selenide (CIS) based alloy material absorber layer; and
the at least one second conductivity type semiconductor layer comprises a n-type cadmium zinc sulfide, cadmium telluride, cadmium sulfide, zinc sulfide, or zinc magnesium oxide semiconductor layer.
7 . The solar cell of claim 1 , wherein:
the at least one first conductivity type semiconductor layer comprises a p-type copper indium gallium selenide absorber layer;
the at least one second conductivity type semiconductor layer comprises a n-type cadmium sulfide semiconductor layer; and
the transparent conductive oxide contact layer comprises an AZO layer which has a higher resistivity at the first surface than at the second surface.
8 . The solar cell of claim 7 , wherein:
the AZO layer has a constant aluminum content in a range of 1.5 to 4 weight percent as a function of thickness;
the oxygen concentration at the first surface of the AZO layer is from 36 atomic percent to 40 atomic percent; and
the oxygen concentration at the second surface of the AZO layer is from 28 atomic percent to 35 atomic percent.
9 . The solar cell of claim 8 , further comprising an ITO layer located on the second surface of the AZO layer, the ITO layer having a lower resistivity than the second surface of the AZO layer.
10 . The solar cell of claim 1 , wherein the transparent conductive oxide contact layer has a substantially constant concentration of the same one or more metals as a function of thickness.
11 . 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.
12 . The method of claim 11 , wherein the step of forming the transparent conductive oxide contact layer comprises sputtering at least one metal oxide target in a process atmosphere having a variable oxygen content.
13 . The method of claim 12 , wherein the transparent conductive oxide contact layer comprises material selected from the group consisting of ITO, AZO, Cd 2 SnO 4 , Zn 2 In 2 O 5 , In 2-x-y Sn x Zn y O 3 , CdO, or Ga 2 O 3 .
14 . The method of claim 12 , wherein:
forming the at least one first conductivity type semiconductor layer comprises forming a p-type copper indium selenide (CIS) based alloy material absorber layer; and
forming the at least one second conductivity type semiconductor layer comprises forming a n-type cadmium zinc sulfide, cadmium telluride, cadmium sulfide, zinc sulfide, or zinc magnesium oxide semiconductor layer.
15 . The method of claim 12 , wherein:
forming the at least one first conductivity type semiconductor layer comprises forming a p-type copper indium gallium selenide absorber layer;
forming the at least one second conductivity type semiconductor layer comprises forming a n-type cadmium sulfide semiconductor layer; and
forming the transparent conductive oxide contact layer comprises forming an AZO layer which has a higher resistivity at the first surface than at the second surface and which has a constant aluminum content in a range of 1.5 to 4 weight percent as a function of thickness.
16 . The method of claim 15 , further comprising forming an ITO layer on the second surface of the AZO layer, the ITO layer having a lower resistivity than the second surface of the AZO layer.
17 . The method of claim 12 , wherein the oxygen content in the process atmosphere is decreased from between about 5% and about 20% during sputtering of an lower portion of the transparent conductive oxide contact layer to between about 0% and about 10% during sputtering of an upper portion of the transparent conductive oxide contact layer which is formed over the lower portion.
18 . The method of claim 17 , wherein the oxygen content in the process atmosphere is decreased continuously for at least the first portion of the sputtering of the transparent conductive oxide contact layer such that the transparent conductive oxide contact layer has an oxygen concentration that decreases continuously as a function of thickness for at least a portion of the contact layer thickness.
19 . The method of claim 18 , wherein the transparent conductive oxide contact layer has a substantially continuous profile of refractive index as a function of thickness.
20 . The method of claim 17 , wherein the oxygen content in the process atmosphere is decreased in at least two discrete steps for at least the first portion of the sputtering of the transparent conductive oxide contact layer such that the transparent conductive oxide contact layer has an oxygen concentration that decreases in at least two discrete steps as a function of thickness for at least a portion of the contact layer thickness.
21 . The method of claim 17 , wherein the oxygen content in the process atmosphere is provided by gas selected from the group consisting of O 2 , N 2 O, O 3 , or H 2 O and wherein the process atmosphere further comprises an inert sputtering gas.
22 . The method of claim 17 , wherein the step of sputtering comprises sputtering the transparent conductive oxide contact layer in a batch sputtering process in which the oxygen content in the process atmosphere is decreased as a function of time.
23 . The method of claim 17 , wherein:
the step of sputtering comprises sputtering the transparent conductive oxide contact layer in a continuous sputtering process over a continuously moving web substrate using at least a first and a second metal oxide sputtering targets;
the first metal oxide sputtering target is located upstream relative to the second metal oxide sputtering target with respect to a movement direction of the web substrate; and
the oxygen content in the process atmosphere adjacent to the first metal oxide sputtering target is higher than the oxygen content in the process atmosphere adjacent to the second metal oxide sputtering target.
24 . The method of claim 23 , wherein:
the first metal oxide sputtering target is located in a first sputtering chamber and the second metal oxide sputtering target is located in a second sputtering chamber which is isolated from the first sputtering chamber;
the oxygen content in the process atmosphere in the first sputtering chamber is higher than the oxygen content in the process atmosphere in the second sputtering chamber; and
the web substrate continuously extends and moves through the first and the second chambers during the sputtering of the transparent conductive oxide contact layer.
25 . The method of claim 11 , wherein the transparent conductive oxide contact layer has a substantially constant concentration of the same one or more metals as a function of thickness.
26 . The method of claim 11 , wherein the transparent conductive oxide contact layer has a constant oxygen concentration as a function of thickness for at least a second portion of the contact layer thickness adjacent to at least one of the first surface or the second surface.
27 . 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.