Inverted metamorphic multijunction solar cell

Abstract

A method of manufacturing a solar cell comprising: providing a growth substrate depositing on the growth substrate an epitaxial sequence of layers of semiconductor material forming at least a first and second solar subcells depositing a semiconductor contact layer on top of the second solar subcell depositing a reflective metal layer over said semiconductor contact layer such that the reflectivity of the reflective metal layer is greater than 80% in the wavelength range 850 to 2000 nm depositing a contact metal layer composed on said reflective metal layer mounting and bonding a supporting substrate on top of the contact metal layer and removing the growth substrate.

1 . A method of manufacturing a solar cell comprising:
 providing a growth substrate;   depositing on the growth substrate an epitaxial sequence of layers of semiconductor material forming at least a first and second solar subcells;   depositing a semiconductor contact layer on top of the second solar subcell;   depositing a reflective metal layer composed of any one or more of the following metals or alloys thereof: Al, Be, and Ni to a thickness between 50 nm and 5 microns over said semiconductor contact layer; such that the reflectivity of the reflective metal layer is greater than 80% in the wavelength range 850-2000 nm;   depositing a contact metal layer composed of one or more layers of Ag, Au, and Ti on said reflective metal layer;   mounting and bonding a surrogate substrate on top of the contact metal layer; and   removing the first substrate.   
     
     
         2 . A method as defined in  claim 1 , wherein the reflective metal layer comprises one or more of the following metals or alloys thereof: Ag, Al, Au, Be, Cu, Ni or Ti, and further comprising depositing a diffusion barrier layer directly on said semiconductor contact layer, wherein the reflective metal layer is deposited directly on the diffusion barrier layer. 
     
     
         3 . A method as defined in  claim 2 , wherein the diffusion barrier layer is composed of one or more layers of Cr, Pd, Pt, Si, Ti or TiN. 
     
     
         4 . A method as defined in  claim 2 , wherein the diffusion layer has a thickness of between 0.1 nm and 10 nm. 
     
     
         5 . A method as defined in  claim 1 , further comprising:
 depositing on the growth substrate a first sequence of layers of semiconductor material forming a first solar subcell, a second solar subcell, and a third solar subcell;   depositing on said third solar subcell a first grading interlayer; and   depositing on said first grading interlayer a second sequence of layers of semiconductor material forming a fourth solar subcell, the fourth solar subcell being lattice mismatched to the third solar subcell.   
     
     
         6 . A method as defined in  claim 5 , wherein the first graded interlayer is compositionally graded to lattice matched the third solar subcell on one side and the lower fourth solar subcell on the other side, and is composed of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater than or equal to that of the third solar subcell and less than or equal to that of the lower fourth solar subcell, and having a band gap energy greater than that of the third solar subcell and the fourth solar subcell. 
     
     
         7 . A method as defined in  claim 5 , wherein, the fourth solar subcell has a band gap in the range of approximately 1.05 to 1.15 eV, the third solar subcell has a band gap in the range of approximately 1.40 to 1.50 eV, the second solar subcell has a band gap in the range of approximately 1.65 to 1.78 eV and the first solar subcell has a band fap in the range of 1.92 to 2.2 eV, and wherein the graded interlayer is composed of (In x Ga 1-x )Al 1-y As with 0<x<1, 0<y<1, and x and y selected such that the band gap remains constant throughout its thickness, and the band gap of the graded interlayer remains at a constant value in the range of 1.42 to 1.60 eV throughout its thickness. 
     
     
         8 . A method as defined in  claim 1 , wherein the upper first solar subcell is composed of AlGaInP, the second solar subcell is composed of an InGaP emitter layer and a AlGaAs base layer, the third solar subcell is composed of GaAs, and the lower fourth solar subcell is composed of InGaAs. 
     
     
         9 . A method as defined in  claim 1 , further comprising:
 a distributed Bragg reflector (DBR) layer adjacent to and between the second and the third solar subcells and arranged so that light can enter and pass through the second solar subcell and at least a portion of which can be reflected back into the second subcell by the DBR layer.   
     
     
         10 . A method as defined in  claim 1 , further comprising:
 a distributed Bragg reflector (DBR) layer adjacent to and between the third solar subcell and the graded interlayer and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer.   
     
     
         11 . A method as defined in  claim 10 , wherein the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction. 
     
     
         12 . A method as defined in  claim 11 , wherein the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength. 
     
     
         13 . A method as defined in  claim 12 , wherein the DBR layer includes a first DBR layer composed of a plurality of p type Al x Ga 1-x As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al y Ga 1-y As layers, with 0<x<1, 0<y<1, and where y is greater than x. 
     
     
         14 . A method as defined in  claim 9 , wherein the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction. 
     
     
         15 . The multijunction solar cell as defined in  claim 14 , wherein the difference in refractive indices between alternating layers in maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength. 
     
     
         16 . A method as defined in  claim 15 , wherein the DBR layer includes a first DBR layer composed of a plurality of p type Al x Ga 1-x As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al y Ga 1-y As layers, with 0<x<1, 0<y<1, and where y is greater than x. 
     
     
         17 . A method as defined in  claim 4 , wherein the thickness of the diffusion barrier layer in between 1.0 and 3.0 nm. 
     
     
         18 . A method of manufacturing a solar cell comprising:
 providing a growth substrate;   depositing on the growth substrate an epitaxial sequence of layers of a III-V compound semiconductor material forming at least a first top or light-facing solar subcell and a second bottom solar subcell, the second bottom solar subcell has a top surface and a bottom surface;   depositing a diffusion barrier layer directly on the bottom surface of the bottom solar subcell;   depositing a reflective metal layer directly on the diffusion barrier layer to a thickness between 50 nm and 5 microns over said semiconductor contact layer such that the reflectivity of the reflective metal layer is greater than 80% in the wavelength range 850-2000 nm; and   depositing a contact metal layer composed of one or more layers of Ag, Au, and Ti, on said reflective metal layer.   
     
     
         19 . A method as defined in  claim 18 , wherein the diffusion barrier layer is composed of one or more of the following or alloys thereof: Cr, Pd, Pt, Si, Ti, or TiN deposited to an aggregate thickness between 0.1 and 10.0 nm. 
     
     
         20 . A solar cell comprising:
 an epitaxial sequence of layers of III-V compound semiconductor material forming at least a top light-facing solar subcell and a bottom solar subcell, the bottom solar subcell having a top surface and a bottom surface;   a reflective metal layer deposed directly on the bottom surface of the bottom solar subcell composed of one or more of the following metals or alloys thereof: Al, Be, and Ni such that the reflectivity of the reflective metal layer is greater than 80% reflectivity in the wavelength range of 850-2000 nm and having a thickness between 50 nm and 5 microns;   a contact metal layer composed of one or more layers of Ag, Au, and Ti having a top surface deposed on the reflective metal layer, and a bottom surface; and   a supporting substrate bonded to the bottom surface of the contact metal layer.