Method of polyimide diffusion bonding for multijunction solar cells

1. A method of manufacturing a solar cell comprising:

providing a first semiconductor substrate with an epitaxial sequence of layers of semiconductor material forming a solar cell deposited over the first semiconductor substrate using an MOCVD reactor;

depositing a metal layer on top of the sequence of layers of semiconductor material, the metal layer including a top surface layer composed of gold or silver;

providing a polymer film;

depositing a first metallic adhesion layer on one surface of the polymer film, the first metallic adhesion layer having a coefficient of thermal expansion substantially different from that of the top surface layer;

depositing a second metal adhesion layer on the first metallic adhesion layer, the second metal adhesion layer having a different composition from the first metallic adhesion layer and having no chemical elements in common with the first metallic adhesion layer; and

adjoining the second metal adhesion layer of the polymer film to the metal layer on top of the sequence of layers and bonding the second metal adhesion layer permanently to the metal layer on top of the sequence of layers by diffusion of a chemical element of one layer into the other layer under thermocompression of the semiconductor substrate and the polymer film.

2. A method as defined in claim 1 , further comprising removing the first semiconductor substrate.

3. A method as defined in claim 1 , wherein the first metallic adhesion layer comprises Cr, Ir, Ni, Mo, or W.

4. A method as defined in claim 1 , wherein the second metal adhesion layer comprises gold.

5. A method as defined in claim 1 , wherein the polymer film comprises a poly (4,4′-oxydiphenylene-pyromellitimide) material.

6. A method as defined in claim 1 , wherein the epitaxial sequence of layers of semiconductor material include an upper first solar subcell having a first band gap adjacent to the first semiconductor substrate; a second solar subcell adjacent to said first solar subcell and having a second band gap smaller than said first band gap; a third solar subcell adjacent to said second solar subcell and having a third band gap smaller than said second band gap; a graded interlayer adjacent to said third solar subcell, said second graded interlayer having a fourth band gap greater than said third band gap; and a lower fourth solar subcell adjacent to said graded interlayer, said lower subcell having a fifth band gap smaller than said third band gap such that said fourth subcell is lattice mismatched with respect to said third subcell.

7. A method as defined in claim 1 , further comprising a separation layer deposited over the first semiconductor substrate and between the first semiconductor substrate and the epitaxial sequence of layers of semiconductor material forming a solar cell.

8. A method as defined in claim 2 , wherein the removing step includes immersing the solar cell in hydrofluoric acid.

9. A method as defined in claim 1 , wherein the thermocompressive diffusion bonding includes using pressure in the range of 0.5 to 2.0 MPa at a temperature in the range of 150 to 300 degrees Centigrade for a time period between five and thirty minutes.

10. A method as defined in claim 1 , wherein the polymer film is corona or plasma treated to improve metal adhesion.

11. A method as defined in claim 1 , further comprising bonding the second surface of the polymer film to a supporting substrate utilizing a polymeric bond or a diffusion bond.

12. A method of manufacturing a solar cell comprising:

providing a first semiconductor substrate and a separation layer on the first semiconductor substrate;

using an MOCVD reactor, depositing an epitaxial sequence of layers of semiconductor material over the separation layer to form a solar cell, wherein the epitaxial sequence of layers include:

an upper first solar subcell having a first band gap adjacent to the first semiconductor substrate;

a second solar subcell adjacent to the first solar subcell and having a second band gap smaller than the first band gap;

a third solar subcell adjacent to the second solar subcell and having a third band gap smaller than the second band gap;

a graded interlayer adjacent to the third solar subcell, the graded interlayer having a fourth band gap greater than the third band gap; and

a lower fourth solar subcell adjacent to the graded interlayer, the lower subcell having a fifth band gap smaller than the third band gap such that the fourth subcell is lattice mismatched with respect to the third subcell;

depositing a metal layer on top of the sequence of layers of semiconductor material, the metal layer including a top surface layer comprising gold or silver;

providing a polymer film comprising a poly (4,4′-oxydiphenylene-pyromellitimide) material;

depositing a first metallic adhesion layer on one surface of the polymer film, the first metallic adhesion layer comprising Cr, Ir, Ni, Mo, or W and having a coefficient of thermal expansion substantially different from that of the top surface layer;

depositing a second metal adhesion layer comprising gold on the first metallic adhesion layer, the second metal adhesion layer having a different composition from the first metallic adhesion layer and having no chemical elements in common with the first metallic adhesion layer;

adjoining the second metal adhesion layer of the polymer film to the metal layer of the sequence of layers;

permanently bonding the second metal adhesion layer to the metal layer on top of the sequence of layers by diffusion boding using a thermocompressive technique using pressure in the range of 0.5 to 2.0 MPa at a temperature in the range of 150 to 300 degrees Centigrade for a time period between five and thirty minutes; and

removing the first semiconductor substrate by immersing the assembled body in hydrofluoric acid.