Distributed Bragg reflector structures in multijunction solar cells
A multijunction solar cell and its method of fabrication, including an upper and a lower solar subcell each having an emitter layer and a base layer forming a photoelectric junction; a near infrared (NIR) wideband reflector layer disposed below the upper subcell and above the lower subcell for reflecting light in the spectral range of 900 to 1050 nm which represents unused and undesired solar energy and thereby reducing the overall solar energy absorptance in the solar cell and providing thermodynamic radiative cooling of the solar cell when deployed in space outside the atmosphere.
1. A multijunction solar cell comprising:
an upper and a lower solar subcell each having an emitter layer and a base layer forming a photoelectric junction;
a near infrared (NIR) wideband reflector layer disposed below the upper subcell and above the lower subcell for reflecting light in the spectral range of 900 to 1050 nm which represents unused and undesired solar energy and thereby reducing the overall solar energy absorptance in the solar cell and providing thermodynamic radiative cooling of the solar cell when deployed in space outside the atmosphere;
the wideband reflector layer including a first distributed Bragg reflector (DBR) structure disposed between a buffer layer and a tunnel diode layer, wherein the buffer layer is a nucleation layer, and wherein the DBR structure, the tunnel layer, and the buffer layer are disposed between a same pair of solar subcells of the multijunction solar cell, the pair of solar subcells having no other solar subcell disposed between them, wherein the DBR structure is 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 upper solar subcell and at least a first portion of said light in a first spectral wavelength range can be reflected back into the upper solar subcell and out of the solar cell by the first DBR structure, and a second portion of said light in a second spectral wavelength can be transmitted through the first DBR structure to the layers disposed beneath the first DBR structure, wherein the second spectral wavelength range is greater than 1050 nm.
2. A multijunction solar cell as defined in claim 1 , wherein the lower solar subcell comprise a germanium substrate including a first photoactive junction and forming a bottom solar subcell, and the upper solar subcell comprises InGaP, InGaP/InGaAs, or InGaAs.
3. A multijunction solar cell as defined in claim 1 , wherein the first DBR structure includes alternating layers of lattice mismatched materials and includes a first DBR sublayer composed of a plurality of n type or p type Al x Ga 1-x As layers, and a second DBR sublayer disposed over the first DBR sublayer 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.
4. A multijunction solar cell as defined in claim 3 , wherein the sequence of first and second different semiconductor sublayers comprises compressively strained and tensionally strained layers, respectively.
5. A multijunction solar cell as defined in claim 3 , wherein each of the first and second DBR sublayers is approximately 300 to 800 angstroms thick.
6. A multijunction solar cell as defined in claim 1 , further comprising:
a second distributed Bragg reflector (DBR) structure disposed directly beneath 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 said light in the first spectral wavelength range can be reflected back into the upper solar subcell by the second DBR structure, and a second portion of said light in a third spectral wavelength range can be transmitted through the second DBR structure to the lower solar subcell disposed beneath the second DBR structure.
7. A multijunction solar cell as defined in claim 1 , wherein the radiative cooling of the solar cell is in excess of 4 degrees Centigrade.
8. A multijunction solar cell as defined in claim 6 , wherein the second DBR structure includes alternating layers of lattice mismatched materials and includes a first DBR sublayer composed of a plurality of n type or p type Al x Ga 1-x As layers, and a second DBR sublayer disposed over the first DBR sublayer 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, wherein the sequence of first and second different semiconductor sublayers comprises compressively strained and tensionally strained layers, respectively.