Multi-junction solar cell with dilute nitride sub-cell having graded doping
A lattice-matched solar cell having a dilute nitride-based sub-cell has exponential doping to thereby control current-carrying capacity of the solar cell. Specifically a solar cell with at least one dilute nitride sub-cell that has a variably doped base or emitter is disclosed. In one embodiment, a lattice matched multi junction solar cell has an upper sub-cell, a middle sub-cell and a lower dilute nitride sub-cell, the lower dilute nitride sub-cell having doping in the base and/or the emitter that is at least partially exponentially doped so as to improve its solar cell performance characteristics. In construction, the dilute nitride sub-cell may have the lowest bandgap and be lattice matched to a substrate, the middle cell typically has a higher bandgap than the dilute nitride sub-cell while it is lattice matched to the dilute nitride sub-cell. The upper sub-cell typically has the highest bandgap and is lattice matched to the adjacent sub-cell. In further embodiments, a multi junction solar cell according to the invention may comprise four, five or more sub-cells in which the one or more sub-cells may each comprise exponentially doped dilute nitride alloys.
1. A multi-junction solar cell comprising a plurality of sub-cells, wherein:
each of the plurality of sub-cells is lattice matched to adjacent sub-cells; and
at least one of the plurality of sub-cells comprises a GaInNAsSb emitter, a GaInNAsSb base adjacent the GaInNAsSb emitter, and an emitter-base junction at an interface between the GaInNAsSb emitter and the GaInNAsSb base, wherein,
the GaInNAsSb base comprises a base doping profile characterized by a base dopant concentration that increases with distance from the emitter-base junction; and
the GaInNAsSb emitter comprises no doping or an emitter doping profile characterized by an emitter dopant concentration that increases with distance from the emitter-base junction.
2. The multi-junction solar cell of claim 1 , wherein the base dopant concentration is between 1×10 15 /cm 3 and 1×10 19 /cm 3 .
3. The multi-junction solar cell of claim 1 , wherein the base doping profile increases exponentially over at least a portion of the distance from the emitter-base junction.
4. The multi-junction solar cell of claim 1 , wherein the base doping profile increases linearly over at least a portion of the distance from the emitter-base junction.
5. The multi-junction solar cell of claim 1 , wherein the base doping profile is selected to enhance the efficiency of the multi-junction solar cell compared to a multi-junction solar cell without the base doping profile.
6. The multi-junction solar cell of claim 1 , wherein the emitter doping profile is selected to enhance the efficiency of the multi-junction solar cell compared to a multi-junction solar cell without the emitter doping profile.
7. The multi-junction solar cell of claim 1 , wherein the plurality of sub-cells comprises a first sub-cell, a second sub-cell, and a third sub-cell, wherein,
the first sub-cell comprises a GaInNAsSb sub-cell and is lattice matched to an underlying substrate, and is characterized by a first band gap;
the second sub-cell overlies the first GaInNAsSb sub-cell and is lattice matched to the first sub-cell and is characterized by a second; and
the third sub-cell overlies the second sub-cell and is lattice matched to the second sub-cell, and is characterized by a third bandgap; and
the second band gap is higher than the first band gap and the third band gap is higher than the second bandgap.
8. The multi-junction solar cell of claim 1 , wherein the GaInNAsSb base comprises a first doping subregion and a second doping subregion, wherein,
the first doping subregion is adjacent the emitter-base junction and is characterized by a constant doping profile, and
the second doping subregion is adjacent the first doping region and is characterized by a doping concentration that increases with distance from the emitter-base junction.
9. The multi-junction solar cell of claim 1 , wherein the plurality of sub-cells comprises at least a first sub-cell, a second sub-cell, a third sub-cell and a fourth sub-cell, wherein,
the first sub-cell is characterized by a first bandgap;
the second sub-cell overlies the first sub-cell and comprises a GaInNAsSb base, wherein the GaInNAsSb base is lattice matched to the first sub-cell; and is characterized by a second bandgap that is greater than the first bandgap;
the third sub-cell overlies the second sub-cell, is lattice matched to the second sub-cell, and is characterized by a third bandgap that is greater than the second bandgap; and
the fourth sub-cell overlies the third sub-cell, is lattice matched to the third sub-cell, and is characterized by a fourth bandgap that is greater than the third bandgap.
10. A multi-junction solar cell comprising a plurality of sub-cells, wherein:
each of the plurality of sub-cells is lattice matched to adjacent sub-cells; and
at least one of the plurality of sub-cells comprises a GaInNAsSb emitter, a GaInNAsSb base adjacent the GaInNAsSb emitter, and an emitter-base junction at an interface between the GaInNAsSb emitter and the GaInNAsSb base, wherein,
the GaInNAsSb base comprises a first subregion and a second subregion,
the first subregion is adjacent the emitter-base junction and is not doped, and
the second subregion is adjacent the first subregion and comprises a doping profile characterized by a doping concentration that increases with distance from the emitter-base junction.
11. A method for forming a GaInNAsSb subcell comprising a GaInNAsSb emitter, a GaInNAsSb base adjacent the GaInNAsSb emitter, and an emitter-base junction at an interface between the GaInNAsSb emitter and the GaInNAsSb base, wherein the method comprises:
growing the GaInNAsSb base in an atmosphere containing a dopant, under growth conditions selected to provide a doping profile of the GaInNAsSb base characterized by a doping concentration that increases with distance from the emitter-base junction;
and
growing the GaInNAsSb emitter comprising no doping or an emitter doping profile characterized by an emitter dopant concentration that increases with distance from the emitter-base junction.
12. The method of claim 11 comprising annealing the GaInNAsSb sub-cell.
13. A method for forming a graded doping profile comprising:
growing a GaInNAsSb base of a GaInNAsSb sub-cell of a multi-junction solar cell in an atmosphere containing a dopant, under growth conditions selected to provide a doping profile of the GaInNAsSb base characterized by a dopant concentration that increases with a distance from the emitter-base junction; and
growing a GaInNAsSb emitter of a GaInNAsSb subcell of a multi-junction solar cell after growing the GaInNAsSb base, the GaInNAsSb emitter being grown in an atmosphere containing a dopant, the GaInNAsSb emitter comprising no doping or an emitter doping profile characterized by an emitter dopant concentration that increases with distance from the emitter-base junction.
14. The method of claim 13 comprising annealing the multi-junction solar cell containing the GaInNAsSb sub-cell.
15. The multi-junction solar cell of claim 1 , wherein each of the plurality of sub-cells is lattice matched to each of the other sub-cells.
16. The multi-junction solar cell of claim 1 , wherein at least one of the GaInNAsSb emitter and the GaInNAsSb base comprises a doping profile characterized by a dopant concentration that is lowest at the emitter-base junction and increases continuously with distance from the emitter-base junction.