Back Metal Layers in Inverted Metamorphic Multijunction Solar Cells
A multijunction solar cell comprising an upper first solar subcell having a first band gap; a middle second solar subcell adjacent to the first solar subcell and having a second band gap smaller than the first band gap, and having a base layer and an emitter layer; a graded interlayer adjacent to said second solar subcell, having a third band gap greater than the second band gap; a lower solar subcell adjacent to the grading interlayer, having a fourth band gap smaller than said second band gap such that the third subcell is lattice mismatched with respect to said second subcell; and a metal electrode layer deposited on said lower subcell and having a coefficient of thermal expansion substantially similar to that of the subcells.
1 . A multijunction semiconductor solar cell comprising:
an upper first solar subcell having a first band gap;
a middle second solar subcell adjacent to said first solar subcell and having a second band gap smaller than said first band gap, and having a base layer and an emitter layer,
a graded interlayer adjacent to said second solar subcell; said graded interlayer having a third band gap greater than said second band gap;
a lower third solar subcell adjacent to said graded interlayer, said lower third subcell having a fourth band gap smaller than said second band gap such that the third subcell is lattice mismatched with respect to the second subcell; and
a metal electrode layer deposited on the lower subcell and having a coefficient of thermal expansion within a range of 0 to 10 ppm per degree Kelvin different from that of the adjacent semiconductor material.
2 . A multijunction solar cell as defined in claim 1 , wherein the coefficient of thermal expansion of the metal electrode layer is in the range of 5 to 7 ppm per degree Kelvin.
3 . The multijunction solar cell of claim 1 , wherein the metal electrode layer has a coefficient of thermal expansion that has a value less than 15 ppm per degree Kelvin.
4 . The multijunction solar cell of claim 1 , wherein the metal electrode layer has a coefficient of thermal expansion that has a value within 50% of the coefficient of thermal expansion of the adjacent semiconductor material.
5 . The multijunction solar cell of claim 1 , wherein the metal electrode layer has a coefficient of thermal expansion that has a value within 10% of the coefficient of thermal expansion of the adjacent semiconductor material.
6 . The multijunction solar cell of claim 1 , wherein the metal electrode layer comprises molybdenum.
7 . The multijunction solar cell of claim 1 , wherein the metal electrode layer includes a Ti/Au/Mo sequence of layers.
8 . The multijunction solar cell of claim 1 , wherein the metal electrode layer includes a Mo/Ag/Au sequence of layers.
9 . The multijunction solar cell of claim 1 , further comprising a supporting substrate adjacent to the metal electrode layer.
10 . A multijunction solar cell as defined in claim 9 , wherein the supporting substrate is composed of a silicon aluminum alloy having approximately 80% silicon and 20% aluminum.
11 . The multijunction solar cell of claim 9 , further comprising an adhesive layer bonding the supporting substrate to the metal electrode layer.
12 . The multijunction solar cell of claim 8 , wherein the adhesive layer has a thickness of less than 5 microns.
13 . The multijunction solar cell of claim 1 , wherein the graded interlayer is compositionally graded to lattice match the middle subcell on one side and the bottom subcell on the other side.
14 . The multijunction solar cell of claim 1 , wherein said 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 middle second solar subcell and less than or equal to that of the lower third subcell.
15 . The multijunction solar cell as defined in claim 1 , wherein the graded interlayer is composed of (In x Ga 1-x ) y Al 1-y As, with x and y selected such that the band gap of the graded interlayer remains constant throughout its thickness.
16 . The multijunction solar cell as defined in claim 1 , wherein the upper first solar subcell is composed of InGa(Al)P.
17 . A multijunction solar cell as defined in claim 1 , wherein the middle second subcell is composed of an InGaP emitter layer and a GaAs or In 0.015 GaAs base layer.
18 . A multijunction solar cell as defined in claim 1 , wherein the lower third solar subcell is composed of an InGaAs base layer and an InGaP emitter layer that is lattice matched to the base layer.
19 . A semiconductor device comprising:
a semiconductor body having a thickness of less than 50 microns and having a front surface and a back surface, wherein the semiconductor body has a predetermined coefficient of thermal expansion; and
a metal electrode layer having a coefficient of thermal expansion arranged to closely match the coefficient of thermal expansion of the semiconductor body and deposited on at least one of the front and back surfaces.
20 . A method of manufacturing a solar cell comprising:
providing a first substrate;
depositing on a first substrate a sequence of layers of semiconductor material forming a solar cell;
depositing a metal electrode layer having a coefficient of thermal expansion substantially similar to that of the semiconductor material on top of the sequence of layers; and
removing the first substrate.