Photovoltaic devices and method of making

1. A photovoltaic device, comprising:

a layer stack comprising a transparent conductive layer; and

an absorber layer disposed on the layer stack, wherein:

the absorber layer comprises cadmium, selenium, tellurium, and oxygen,

the absorber layer comprises a first region disposed proximate to the layer stack relative to a second region,

the first region of the absorber layer is disposed at a front interface of the absorber layer,

the second region of the absorber layer is disposed at a back interface of the absorber layer,

the first region of the absorber layer has a thickness between 100 nanometers to 3000 nanometers,

the second region of the absorber layer has a thickness between 100 nanometers to 3000 nanometers, and

a ratio of an average atomic concentration of selenium in the first region of the absorber layer to an average atomic concentration of selenium in the second region of the absorber layer is greater than 2.

2. The photovoltaic device of claim 1 , wherein the absorber layer comprises a plurality of grains separated by grain boundaries.

3. The photovoltaic device of claim 1 , wherein the absorber layer comprises a plurality of grains separated by grain boundaries and an atomic concentration of selenium in the grain boundaries is higher than an atomic concentration of selenium in the grains.

4. The photovoltaic device of claim 1 , wherein an atomic concentration of selenium varies non-linearly across a thickness of the absorber layer; and wherein the absorber layer includes a varying concentration of selenium such that there is a higher concentration of selenium near the front interface relative to the back interface.

5. The photovoltaic device of claim 1 , wherein the atomic concentration of selenium varies exponentially across the thickness of the absorber layer.

6. The photovoltaic device of claim 1 , wherein the absorber layer comprises a CdTe 1-x Se x alloy, wherein a Se substitution fraction, x, has a value in a range from 0.20 to 0.25 at the front interface.

7. The photovoltaic device of claim 1 , wherein the layer stack comprises:

the transparent conductive layer disposed on a support; and

a buffer layer disposed between the transparent conductive layer and the absorber layer.

8. The photovoltaic device of claim 1 , wherein the layer stack comprises:

the transparent conductive layer disposed on a support;

a buffer layer disposed over the transparent conductive layer; and

an interlayer disposed between the buffer layer and the absorber layer.

9. The photovoltaic device of claim 1 , further comprising a p+-type semiconducting layer disposed between a back contact layer and the absorber layer, wherein the p+-type semiconducting layer comprises a material selected from zinc telluride, magnesium telluride, manganese telluride, beryllium telluride, mercury telluride, arsenic telluride, antimony telluride, copper telluride, elemental tellurium, or combinations thereof.

10. The photovoltaic device of claim 1 , wherein at least a portion of the absorber layer is a ternary compound or a quaternary compound.

11. The photovoltaic device of claim 1 , wherein the absorber layer further comprises sulfur, copper, chlorine, lead, zinc, mercury, or combinations thereof.

12. The photovoltaic device of claim 1 , wherein an amount of oxygen varies across a thickness of the absorber layer.

13. The photovoltaic device of claim 1 , wherein the ratio of the average atomic concentration of selenium in the first region of the absorber layer to the average atomic concentration of selenium in the second region of the absorber layer is greater than 10.

14. A method of making a photovoltaic device, comprising:

providing an absorber layer on a layer stack, and

providing a back contact layer on the absorber layer;

wherein:

the absorber layer comprises cadmium, tellurium, selenium, and oxygen;

an atomic concentration of selenium varies non-linearly across a thickness of the absorber layer;

the absorber layer includes a varying concentration of selenium such that there is a higher concentration of selenium near the front interface relative to the back interface; and

the absorber layer comprises a first region and a second region, wherein:

the first region is disposed proximate to the layer stack relative to the second region,

the first region is disposed at a front interface of the absorber layer,

the second region is disposed at a back interface of the absorber layer,

the first region has a thickness between 100 nanometers to 3000 nanometers,

the second region has a thickness between 100 nanometers to 3000 nanometers,

a ratio of an average atomic concentration of selenium in the first region to an average atomic concentration of selenium in the second region is greater than 2, and

the average atomic concentration of selenium in the absorber layer is in a range from 0.01 atomic percent to 25 atomic percent of the absorber layer.

15. The method of claim 14 , wherein the step of providing an absorber layer comprises co-depositing a selenium source material and a semiconductor material.

16. The method of claim 14 , wherein the step of providing an absorber layer comprises:

disposing a selenium source layer on the layer stack, wherein the selenium source comprises elemental selenium, cadmium selenide, hydrogen selenide, organo-metallic selenium, or combinations thereof;

disposing a semiconductor material on the selenium source layer, wherein the semiconductor material comprises cadmium and tellurium; and

introducing selenium from the selenium source layer into at least a portion of the absorber layer.

17. The method of claim 16 , wherein the selenium source layer has an average thickness in a range from about 1 nanometer to about 1000 nanometers.