Method for manufacturing inverted metamorphic multijunction solar cells
A method of fabricating both a multijunction solar cell and an inverted metamorphic multijunction solar cell in a single process using a MOCVD reactor by forming a first multijunction solar cell on a semiconductor substrate; forming a release layer over the first solar cell; forming an inverted metamorphic second solar cell over the release layer; and etching the release layer so as to separate the multijunction first solar cell and the inverted metamorphic second solar cell.
1. A method of fabricating both a multijunction solar cell and an inverted metamorphic multijunction solar cell in a single process using a MOCVD reactor comprising:
providing a semiconductor substrate;
forming a first multijunction solar cell on said semiconductor substrate;
forming a release layer over the first solar cell;
forming an inverted metamorphic second solar cell including: (i) growing a first solar subcell having a first band gap on said release layer; (ii) growing a second solar subcell over said first subcell having a second band gap smaller than said first band gap; growing a first grading interlayer over said second solar subcell; (iii) growing a third solar subcell over said grading interlayer having a fourth band gap smaller than said second band gap such that said third solar subcell is lattice mismatched with respect to said second solar subcell; and
etching the release layer so as to separate the multijunction first solar cell and the inverted metamorphic second solar cell.
2. A method as defined in claim 1 , wherein the substrate is a germanium substrate, and forming a first multijunction solar cell further comprises forming a first photoactive junction in said substrate to form a bottom solar subcell;
forming a gallium arsenide middle cell disposed on said substrate; and
forming an indium gallium phosphide top cell disposed over said middle cell.
3. A method as defined in claim 1 , further comprising:
forming a second graded interlayer adjacent to said third solar subcell; said second graded interlayer having a fifth band gap greater than said fourth band gap;
forming a lower fourth solar subcell adjacent to said second graded interlayer, said lower subcell having a sixth band gap smaller than said fourth band gap such that said fourth subcell is lattice mismatched with respect to said third subcell; and
mounting a surrogate substrate on top of fourth solar subcell.
4. A method as defined in claim 3 , wherein the lower fourth subcell has a band gap in the range of 0.6 to 0.8 eV; the third subcell has a band gap in the range of 0.9 to 1.1 eV, the second subcell has a band gap in the range of 1.35 to 1.45 eV, and the first subcell has a band gap in the range of 1.8 to 2.1 eV.
5. A method as defined in claim 1 , wherein the surrogate substrate is composed of sapphire, GaAs, glass, Ge or Si.
6. A method as defined in claim 3 , wherein the first graded interlayer is compositionally graded to lattice match the second subcell on one side and the third subcell on the other side, and the second graded interlayer is compositionally graded to lattice match the third subcell on one side and the bottom fourth subcell on the other side.
7. A method as defined in claim 1 , wherein said first graded interlayer is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the second subcell and less than or equal to that of the third subcell, and having a band gap energy greater than that of the second subcell and of the third subcell.
8. A method as defined in claim 3 , wherein said second graded interlayer is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the third subcell and less than or equal to that of the bottom fourth subcell, and having a band gap energy greater than that of the third subcell and of the fourth subcell.
9. A method as defined in claim 3 , wherein the first and second graded interlayers are composed of (In x Ga 1-x ) y Al 1-y As with 0<x<1, 0<y<1, and x and y selected such that the band gap of each interlayer remains constant throughout its thickness.
10. A method as defined in claim 3 , wherein the band gap of the first graded interlayer remains constant at 1.51 eV, and the band gap of the second graded interlayer remains constant at 1.1 eV.
11. A method as defined in claim 3 , wherein the first subcell is composed of an InGaP emitter layer and an InGaP base layer, the second subcell is composed of InGaP emitter layer and a InGaAs base layer, the third subcell is composed of an InGaP emitter layer and an InGaAs base layer, and the bottom fourth subcell is composed of an InGaAs base layer and an InGaAs emitter layer lattice matched to the base layer.
12. A method as defined in claim 1 , wherein the release layer is composed of AlAs or AlGaAs.
13. A method as defined in claim 1 , wherein etching the release layers utilized a hydrofluoric wet etch.
14. A method as defined in claim 1 , further comprising forming a semiconductor contact layer disposed over the lower subcell of the second solar cell, and depositing a metal layer disposed over the semiconductor contact layer.
15. A method as defined in claim 14 , wherein the metal layer has a strain that is opposite in sign from the layers comprising the second solar cell.
16. A method as defined in claim 14 , wherein the metal layer includes a first layer adjacent to the contact layer composed of titanium or nickel, and a second layer adjacent to the first layer composed of silver.
17. A method as defined in claim 14 , wherein the metal layer is from 5 to 15 microns in thickness.
18. A method as defined in claim 15 , wherein the deposition of the metal layer is controlled so that the deposited metal layer bows the adjacent semiconductor structure so the epitaxial layer deposited over the release layer are pulled away from the release layer.
19. A method as defined in claim 14 , wherein the metal layer is deposited by e-beam evaporation and adjusting deposition parameters including deposition rate, substrate temperature, and layer thickness so as to introduce tensile rather than compressive stress.
20. A method as defined in claim 14 , wherein the metal layer is composed of a sequence of layers including Ti/Pt/Ag, Ti/Ni/Ag, Ni/Pt/Ag, Cr/Ni/Ag, Ni/Ag, Cr/Ag, or Ti/Ag.