Distributed bragg reflector structures in multijunction solar cells

Abstract

A multijunction solar cell and its method of fabrication, having an upper first solar subcell composed of a semiconductor material including aluminum and having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; a first and second DBR structure adjacent to the third solar subcell; and a fourth solar subcell adjacent to the DBR structures and lattice matched with said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap; wherein the fourth subcell has a direct bandgap of greater than 0.75 eV.

1 . A multijunction solar cell comprising:
 a first solar subcell comprising an emitter layer and a base layer composed of aluminum gallium arsenide or indium gallium arsenide, the emitter layer and the base layer forming a photoelectric junction;   a second solar subcell disposed below the first solar subcell and comprising an emitter layer and a base layer forming a photoelectric junction; and   a combined DBR structure between the first solar subcell and the second solar subcell with no intervening solar subcells, the combined DBR structure comprising a first distributed Bragg reflector (DBR) structure and a second DBR structure,   wherein the first DBR structure is disposed beneath the base layer of the first solar subcell and composed of a plurality of alternating layers of different semiconductor materials with discontinuities in their respective indices of refraction and arranged so that light can enter and pass through the first solar subcell and at least a first portion of which is light in a first spectral wavelength range and can be reflected back into the first solar subcell by the first DBR structure, and a second portion of which is light in a second spectral wavelength range and can be transmitted through the first DBR structure to the layers disposed beneath the first DBR structure, where the second spectral wavelength range is greater in wavelength than the first spectral wavelength range, and   wherein the second DBR structure is disposed beneath and adjacent the first DBR structure, such that there are not any active subcell layers between said first DBR structure and the second DBR structure, wherein the second DBR structure is compositionally different from the first DBR structure and composed of a plurality of alternating layers of different semiconductor materials with discontinuities in their respective indices of refraction and arranged so that light can enter and pass through the first DBR structure and at least a portion of which is light in the second spectral wavelength range and can be reflected back into the first solar subcell by the second DBR structure, and a second portion of which is light in a third spectral wavelength range and can be transmitted through the second DBR structure to the second solar subcell disposed beneath the second DBR structure.   
     
     
         2 . The multijunction solar cell of  claim 1  further comprising a metamorphic layer between the combined DBR structure and the second solar subcell. 
     
     
         3 . The multijunction solar cell of  claim 2  wherein for the second solar subcell, the emitter layer is composed of germanium and the base layer is composed of germanium. 
     
     
         4 . The multijunction solar cell of  claim 1  further comprising tunnel diode layers between the combined DBR structure and the second solar subcell. 
     
     
         5 . The multijunction solar cell of  claim 1  wherein the first solar subcell further comprises a window layer, tunnel diode layers, and a back surface field (“BSF”) layer, wherein the base layer is disposed on the BSF layer, the window layer is disposed on the emitter layer, and the tunnel diode layers are disposed on the window layer. 
     
     
         6 . The multijunction solar cell of  claim 1  further comprising
 a first additional solar subcell above the first solar subcell comprising an emitter layer and a base layer composed of indium gallium aluminum phosphide, the emitter layer and the base layer forming a photoelectric junction. 
 
     
     
         7 . The multijunction solar cell of  claim 6  further comprising a first tunnel diode layer, a second tunnel diode layer, and a nucleation layer between the combined DBR structure and the second solar subcell, wherein the nucleation layer is composed of indium gallium arsenide and is disposed on the emitter layer of the first additional solar subcell and the first tunnel diode layer composed of gallium arsenide is disposed on the nucleation layer and the second tunnel diode layer composed of aluminum gallium arsenide is disposed on the first tunnel diode layer, wherein the emitter layer of the first solar subcell is composed of indium gallium phosphide or aluminum gallium arsenide. 
     
     
         8 . The multijunction solar cell of  claim 6  further comprising:
 a second additional solar subcell disposed between the first solar subcell and the first additional solar subcell, the second additional solar subcell comprising an emitter layer composed of indium gallium arsenide or aluminum gallium arsenide and a base layer composed of aluminum gallium arsenide, wherein the base layer of the first solar subcell comprises InGaAs. 
 
     
     
         9 . The multijunction solar cell according to  claim 6  further comprising a back surface field (“BSF”) layer composed of p-type aluminum gallium arsenide disposed on the combined DBR structure and a window layer composed of n-type indium gallium aluminum phosphide disposed on the emitter layer of the second solar subcell, wherein the base layer is disposed on the BSF layer. 
     
     
         10 . The multijunction solar cell of  claim 1  wherein one or more of the solar subcells have a gradation in doping in the base layer that increases approximately exponentially from approximately 1×10 15  free carriers per cubic centimeter in a region adjacent the photoelectric junction to approximately 4×10 18  free carriers per cubic centimeter in a region adjacent an adjoining layer and a gradation in doping in the emitter layer that increases from approximately 5×10 17  free carriers per cubic centimeter in a region adjacent the photoelectric junction to approximately 5×10 18  free carriers per cubic centimeter in a region immediately adjacent an adjoining layer. 
     
     
         11 . The multijunction solar cell of  claim 1  wherein the emitter layer of the first solar subcell comprises highly doped n-type indium gallium phosphide (“InGaP”). 
     
     
         12 . The multijunction solar cell of  claim 1  wherein the half width value of reflection of the first DBR structure and the second DBR structure is in a range between 250 and 350 nm. 
     
     
         13 . The multijunction solar cell of  claim 1  wherein the combined DBR structure includes alternating layers of lattice mismatched materials, the combined DBR structure includes a first DBR layer composed of a plurality of n type or 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 n or p type Al y Ga 1-y As layers, where 0<x<1, 0<y<1, and y is greater than x. 
     
     
         14 . The multijunction solar cell of  claim 1  wherein the combined DBR structure comprises a sequence of alternating first and second different semiconductor layers, and wherein an average lattice constant of the sequence of alternating first and second semiconductor layers is approximately equal to a lattice constant of a substrate. 
     
     
         15 . The multijunction solar cell of  claim 1  wherein the first solar subcell comprises a highly doped n-type indium gallium arsenide emitter layer and a highly doped n-type indium gallium aluminum phosphide window layer. 
     
     
         16 . The multijunction solar cell of  claim 1  wherein the first solar subcell has a BSF layer comprising highly doped p-type aluminum gallium arsenide (“AlGaAs”). 
     
     
         17 . The multijunction solar cell of  claim 1  wherein first spectral wavelength range—of approximately 780 to 860 nm. 
     
     
         18 . The multijunction solar cell of  claim 1  wherein the second solar subcell has a band gap in the range of approximately 1.41 eV, and the first solar subcell has a band gap in the range of 1.65 eV to 1.8 eV. 
     
     
         19 . The multijunction solar cell of  claim 1  wherein the first spectral wavelength range overlaps the second spectral wavelength range by less than 10 nm. 
     
     
         20 . The multijunction solar cell of  claim 1  wherein the first spectral wavelength range and the second spectral wavelength range correspond to the spectral absorption band of the first solar subcell.