Multijunction solar cells with electrically conductive polyimide adhesive

1. A solar cell comprising:

a permanent supporting substrate composed of a carbon fiber reinforced polymer utilizing a conductive polyimide binding resin;

a metal contact layer bonded to the permanent supporting substrate with a polyimide adhesive; and

a sequence of layers of semiconductor material forming a multijunction solar cell directly adjacent the metal contact layer, wherein the sequence of layers of semiconductor material includes a first or top solar subcell having a first band gap;

a grading interlayer disposed under the first solar subcell having a third band gap larger than said second band gap;

a second solar subcell disposed under the grading interlayer and having a second band gap smaller than said first band gap; and

a third solar subcell having a fourth band gap smaller than said second band gap such that said third subcell is lattice mismatched with respect to said second subcell, wherein the third solar subcell is directly adjacent to the metal contact layer.

2. A solar cell as defined in claim 1 , wherein said first solar subcell is composed of an InGa(Al)P emitter region and an InGa(Al)P base region.

3. A solar cell as defined in claim 1 , wherein said grading interlayer is composed of (In x Ga 1-x ) y Al 1-y As, wherein 0<x<1 and 0<y<1 with x and y selected such that the band gap of each interlayer remains constant throughout its thickness.

4. A solar cell as defined in claim 1 , wherein said first subcell is composed of an GaInP, GaAs, GalnAs, GaAsSb, or GaInAsN emitter region and an GaAs, GalnAs, GaAsSb, or GaInAsN base region, and the second subcell is composed of an InGaP emitter layer and a GaAs or GalnAs base layer.

5. The solar cell as defined in claim 1 , wherein the lower solar subcell is composed of an InGaAs base and emitter layer, or an InGaAs base layer and an InGaP emitter layer.

6. The solar cell as defined in claim 1 , wherein the graded interlayer is compositionally graded to lattice match the middle subcell on one side and the lower subcell on the other side, and is composed of (In x Ga 1-x ) y Al 1-y As, wherein 0<x<1 and 0<y<1 with x and y selected such that the band gap of the interlayer remains constant throughout its thickness and greater than said second band gap.

7. A solar cell as defined in claim 1 , further comprising an adhesive polyimide bonded to the permanent supporting substrate at a curing temperature above 350 degrees C.; and wherein the cured adhesive polyimide layer surface together with the permanent supporting substrate is bonded to the surface of the sequence of layers of semiconductor material at a temperature of about 300 degrees C.

8. A solar cell as defined in claim 1 , wherein the supporting substrate is a flexible polyimide film.

9. A solar cell as defined in claim 1 , wherein the supporting substrate includes a plurality of layers of composite sheets embedded in a matrix of cyanate ester adhesive.

10. A solar cell as defined in claim 9 , wherein the stack of layers of composite sheets are co-cured.

11. A solar cell as defined in claim 1 , wherein the permanent adhesive is a conductive polyimide adhesive.

12. A solar cell as defined in claim 1 , wherein the conductive polyimide adhesive includes a polyimide resin and carbon black particles or silver flakes.

13. A solar cell as defined in claim 1 , wherein the conductive polyimide adhesive includes carbon black particles having a size less than or equal to 5 micrometers.

14. A solar cell assembly comprising:

a sequence of layers of semiconductor material forming a multijunction solar cell including an upper or top subcell, and a bottom subcell;

a metal contact layer disposed adjacent to and making electrical contact with the bottom subcell;

a supporting substrate disposed adjacent to the metal contact layer, wherein the supporting substrate is a composite of a polyimide film, a metallic foil and a polyimide film; and

a permanent conductive polyimide adhesive bonding the supporting substrate to the metal contact layer.

15. A solar cell assembly as defined in claim 14 , wherein the supporting substrate includes a plurality of layers of composite sheets embedded in a matrix of cyanate ester adhesive.

16. A solar cell assembly as defined in claim 15 , wherein the stack of layers of composite sheets are co-cured.

17. A solar cell assembly as defined in claim 14 , wherein the permanent supporting substrate is an aluminum honeycomb substrate with a carbon composite face sheet.

18. A solar cell assembly as defined in claim 14 , wherein the conductive polyimide adhesive includes a polyimide resin and carbon black particles or silver flakes.