MULTIJUNCTION SOLAR CELLS ON BULK GeSi SUBSTRATE

Abstract

A solar cell comprising a bulk germanium silicon growth substrate; a diffused photoactive junction in the germanium silicon substrate; and a sequence of subcells grown over the substrate, with the first grown subcell either being lattice matched or lattice mis-matched to the growth substrate.

1 . A method of manufacturing a multijunction solar cell comprising:
 providing a growth substrate;   forming a first solar subcell in the growth substrate;   growing a sequence of layers of semiconductor material using a disposition process to form a solar cell comprising a plurality of subcells including a first middle subcell disposed over the growth substrate and having a band gap in the range of 0.9 to 1.6 eV, at least a second middle subcell disposed over the first middle subcell and having a band gap in the range of approximately 1.55 to 1.8 eV and an upper subcell disposed over the last middle subcell and a band gap in the range of 2.0 to 2.20 eV;   wherein the growth substrate is composed of GeSi with the Ge content in the GeSi substrate in the range of 85% to 87%.   
     
     
         2 . A method as defined in  claim 1 , wherein the first solar subcell has a band gap of less than 2.15 eV, the second middle solar subcell has a band gap of less than 1.73 eV; and the first middle solar subcell has a band gap in the range of 1.15 to 1.2 eV. 
     
     
         3 . A method as defined in  claim 1 , wherein the first solar subcell has a band gap of 2.05 eV, and the first solar subcell has an indirect band gap of 0.7 to 1.1 eV, or 0.85 to 1.65 eV. 
     
     
         4 . A method as defined in  claim 1 , wherein the band gap of the first middle solar subcell is less than 1.41 eV, and greater than that of the first solar subcell. 
     
     
         5 . A method as defined in  claim 1 , further comprising:
 providing a distributed Bragg reflector (DBR) layer adjacent to and disposed between the first middle and the first solar subcells and arranged so that light can enter and pass through the first middle solar subcell and at least a portion of which can be reflected back into the first middle solar subcell by the DBR layer, and is composed of a plurality of alternating sublayers of lattice matched materials with discontinuities in their respective indices of refraction; and   wherein the difference in refractive indices between alternating sublayers 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.   
     
     
         6 . A method as defined in  claim 5 , wherein the DBR layer includes a first DBR layer composed of a plurality of p type In z Al x Ga 1-x-z As sublayers, and a second DBR layer disposed over and adjacent to the first DBR layer and composed of a plurality of p type In w Al y Ga 1-y-w As sublayers, where 0<w<1, 0<y<1, 0<z<1 and y is greater than x, thereby increasing the reflection bandwidth of the DBR layer. 
     
     
         7 . A method as defined in  claim 1 , wherein the growth substrate is lattice mismatched with respect to the first middle subcell, and has a band gap between 0.83 and 0.88 eV as measured at 300 degrees Kelvin, corresponding to a percentage of Si in the GeSi substrate ranging between 13.0 and 15.0 percent by mole fraction. 
     
     
         8 . A method as defined in  claim 1 , wherein the first subcell is composed of a base layer of (In x Ga 1-x ) 1-y Al y P where x is 0.505, and y is 0.142, corresponding to a band gap of 2.10 eV, and an emitter layer of (In x Ga 1-x ) 1-y Al y P where x is 0.505, and y is 0.107, corresponding to a band gap of 2.05 eV. 
     
     
         9 . A method as defined in  claim 1 , further comprising a tunnel diode disposed over the growth substrate, and an intermediate layer disposed between the first middle subcell and the tunnel diode, wherein the intermediate layer is compositionally graded to lattice match the first middle solar subcell on one side and the tunnel diode on the other side and 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 than or equal to that of the first middle solar subcell and different from that of the tunnel diode, and having a band gap energy greater than that of the growth substrate. 
     
     
         10 . A method as defined in  claim 1 , further comprising an intermediate layer disposed between the first middle subcell and the growth substrate wherein the intermediate layer is compositionally step-graded with between one and four steps to lattice match the growth substrate on one side and composed of In x Ga 1-x As or (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 is in the range of 1.15 to 1.41 eV throughout its thickness. 
     
     
         11 . A method as defined in  claim 10 , wherein the intermediate layer has a graded band gap in the range of 1.15 to 1.41 eV, or 1.2 to 1.35 eV, or 1.25 to 1.30 eV. 
     
     
         12 . A method as defined in  claim 1 , wherein either (i) the emitter layer; or (ii) the base layer and emitter layer, or the upper subcell have different lattice constants from the lattice constant of the second middle subcell. 
     
     
         13 . A method as defined in  claim 1 , further comprising:
 providing a distributed Bragg reflector (DBR) layer adjacent to and beneath the first middle solar subcell and arranged so that light can enter and pass through the first middle solar subcell and at least a portion of which can be reflected back into the first middle solar subcell by the DBR layer, 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, 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, and wherein the DBR layer includes a first DBR layer composed of a plurality of p type In z Al x Ga 1-x-z As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type In w Al y Ga 1-y-w As layers, where 0<w<1, 0<x<1, 0<y<1, 0<z<1 and y is greater than x; and   providing an intermediate layer disposed between the DBR layer and the first solar subcell, wherein the intermediate layer is compositionally step-graded to lattice match the DBR layer on one side and the first solar subcell on the other side, and 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 than or equal to that of the DBR layer and less than or equal to that of the first solar subcell, and having a band gap energy greater than that of the first solar subcell.   
     
     
         14 . A method as defined in  claim 1 , wherein each subcell includes an emitter region and a base region, and one or more of the subcells have a base region having a gradation in doping that increases exponentially from 1×10 15  atoms per cubic centimeter adjacent the p-n junction to 4×10 18  atoms per cubic centimeter adjacent to the adjoining layer at the rear of the base, and an emitter region having a gradation in doping that decreases from approximately 5×10 18  atoms per cubic centimeter in the region immediately adjacent the adjoining layer to 5×10 17  atoms per cubic centimeter in the region adjacent to the p-n junction. 
     
     
         15 . A method as defined in  claim 9 , wherein at least one of the upper sublayers of the intermediate layer has a larger lattice constant than the adjacent layers of the upper sublayer disposed directly above the intermediate layer. 
     
     
         16 . A method as defined in  claim 1 , wherein the difference in lattice constant between the adjacent first middle subcell and the first subcell is in the range of 0.1 to 0.2 Angstroms. 
     
     
         17 . A method as defined in  claim 1 , further comprising an inactive majority carrier layer (i.e., a window, BSF, or tunnel diode layer) disposed over the first middle subcell or second middle solar subcell, and having a lattice constant that is greater than that of the first middle subcell and the first subcell so that the tunnel diode layers are strained in tension. 
     
     
         18 . A method as defined in  claim 1 , further comprising a first threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 microns disposed over said second middle solar subcell. 
     
     
         19 . A method as defined in  claim 18 , further comprising a second threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 micron and composed of InGa(Al)P, the second threading dislocation inhibition layer being disposed over and directly adjacent to said grading interlayer for reducing the propagation of threading dislocations, said second threading dislocation inhibition layer having a composition different from a composition of the first threading dislocation inhibition layer. 
     
     
         20 . A method as defined in  claim 1 , including providing an intermediate layer disposed between the first middle solar subcell and the first solar subcell so as to provide a gradual transition in lattice constant in semiconductor structure from the first middle solar subcell to the first solar subcell, wherein the intermediate layer has a band gap that in constant throughout its thickness, and wherein the multijunction solar cell is a four junction solar cell in which the numerical sum of the band gaps of the four solar subcells, divided by four, is equal to 1.35 eV.