Compound semiconductor solar cell

1. A compound semiconductor solar cell, comprising:

a light absorbing layer comprising a first compound semiconductor epitaxial layer that has a first conductive type and a second compound semiconductor epitaxial layer that has a second conductive type and is positioned on an inside surface of the first compound semiconductor epitaxial layer;

a window layer disposed at a front surface of the second compound semiconductor epitaxial layer;

a front epitaxial contact layer that that has the second conductive type and is disposed at a front side of the window layer;

an anti-reflection film that is disposed at the front side of the window layer;

a first electrode that is positioned on the front epitaxial contact layer;

a back epitaxial contact layer at least partially positioned on an outside surface of the first compound semiconductor epitaxial layer facing opposite to the inside surface, the back epitaxial contact layer having the first conductive type and an impurity concentration that is greater than an impurity concentration of impurities contained in the first compound semiconductor epitaxial layer; and

a second electrode that is positioned on the back epitaxial contact layer, the second electrode including (i) a plurality of first portions that directly contact the back epitaxial contact layer and form ohmic contact with the back epitaxial contact layer and (ii) a plurality of second portions that directly contact the outside surface of the first compound semiconductor epitaxial layer,

wherein the plurality of first portions and the plurality of second portions are made of a same material.

2. The compound semiconductor solar cell of claim 1 , wherein each of the plurality of second portions is 100 nm to 300 nm thicker than each of the plurality of first portions.

3. The compound semiconductor solar cell of claim 2 , wherein a thickness of the back epitaxial contact layer is from 100 nm to 300 nm

wherein a first distance between each of the plurality of first portions and a back surface of the second electrode is less than a second distance between each of the plurality of second portions and the back surface of the second electrode, and

wherein a thickness of each of the plurality of second portions is the same or substantially similar to a sum of the first distance and the thickness of the back epitaxial contact layer.

4. The compound semiconductor solar cell of claim 2 , wherein the back epitaxial contact layer includes a plurality of dots that are spaced apart from each other.

5. The compound semiconductor solar cell of claim 4 , wherein a sum of areas of the plurality of dots is from 10% to 50% of an area of the light absorbing layer or an area of the second electrode.

6. The compound semiconductor solar cell of claim 5 , wherein the plurality of dots have a circular, an elliptical, or a quadrangular planar shape and have one or more sizes that are within a first range from 5 μm to 100 μm.

7. The compound semiconductor solar cell of claim 6 , wherein the plurality of dots have a first size that is within the first range.

8. The compound semiconductor solar cell of claim 6 , wherein the plurality of dots have at least two different sizes that are within the first range.

9. The compound semiconductor solar cell of claim 5 , wherein the plurality of dots are uniformly arranged on the back epitaxial contact layer.

10. The compound semiconductor solar cell of claim 9 , wherein the plurality of dots are arranged in a lattice shape, and

wherein a distance between two adjacent first dots of the plurality of dots is from 10 μm to 200 μm, the two adjacent first dots being arranged in a first direction.

11. The compound semiconductor solar cell of claim 10 , wherein a distance between two adjacent second dots of the plurality of dots is from 10 μm to 200 μm, the two adjacent second dots being arranged in a second direction that is different from the first direction.

12. The compound semiconductor solar cell of claim 9 , wherein the plurality of dots are arranged in a rhomboid shape, and

wherein a distance between two adjacent dots of the plurality dots is from 15 μm to 350 μm.

13. The compound semiconductor solar cell of claim 5 , wherein the back epitaxial contact layer comprises: a linear portion that is located below the first electrode.

14. The compound semiconductor solar cell of claim 1 , wherein the back epitaxial contact layer comprises:

a plurality of first linear portions spaced apart from each other.

15. The compound semiconductor solar cell of claim 14 , wherein a sum of areas of the plurality of first linear portions is from 10% to 50% of an area of the light absorbing layer or an area of the second electrode.

16. The compound semiconductor solar cell of claim 15 , wherein the plurality of first linear portions have a lattice pattern.

17. The compound semiconductor solar cell of claim 16 , wherein the plurality of first linear portions have a first width in a third direction.

18. The compound semiconductor solar cell of claim 14 , wherein a first distance between two adjacent first linear portions, of the plurality of first linear portions, that are arranged in a first direction is from 30 μm to 200 μm, and wherein a second distance between two adjacent first linear portions, of the plurality of first linear portions that are arranged in a second direction that is orthogonal to the first direction is from 30 μm to 200 μm.

19. The compound semiconductor solar cell of claim 1 , wherein the plurality of first portions are spaced apart from one another in a first direction,

wherein each of the plurality of first portions is disposed between two of the plurality of second portions, and

wherein each of the plurality of second portions is disposed between two of the plurality of first portions.

20. The compound semiconductor solar cell of claim 19 , wherein a first width of each of the plurality of first portions in the first direction is greater than a second width of each of the plurality of second portions.