Multijunction solar cells with graded buffer Bragg reflectors
Distributed Bragg reflectors are incorporated into the compositionally graded buffers of metamorphic solar cells, adding functionality to the buffer without adding cost. The reflection aids in collection in subcells that are optically thin due to low diffusion length, high bulk recombination, radiation hardness, partially-absorbing quantum structures, or simply for cost savings. Performance enhancements are demonstrated in GaAs subcells with QWs, which is beneficial when GaAs is not the ideal bandgap.
1. An optoelectronic device comprising at least one graded buffer Bragg reflector comprising a compositionally graded buffer comprising a distributed Bragg reflector comprising layers of alloy wherein the lattice constant of each layer of alloy is different than the adjacent layer of alloy; and
wherein each layer of alloy has a refractive index that is different than an adjacent layer of alloy; and
wherein the graded buffer Bragg reflector increases in In-fraction as between alternating layers of Al 0.3 Ga 0.7-x In x As/Al 0.7 Ga 0.3-x In x As.
2. The optoelectronic device of claim 1 wherein the optoelectronic device of claim 1 comprises a light emitting diode.
3. The optoelectronic device of claim 1 wherein the optoelectronic device of claim 1 comprises a laser.
4. The optoelectronic device of claim 1 wherein the optoelectronic device of claim 1 comprises a solar cell.
5. The optoelectronic device of claim 1 comprising a solar cell with an efficiency of at least 36.5 percent when taken under a one-sun illuminated global spectrum at 1000 W/m 2 at 25° C.
6. The optoelectronic device of claim 1 comprising a solar cell with an efficiency of at least 36.6 percent when taken under a one-sun illuminated direct spectrum at 1000 W/m 2 at 25° C.
7. The optoelectronic device of claim 1 comprising a solar cell with an efficiency of at least 32 percent when taken under a one-sun illuminated AM0 spectrum at 1000 W/m 2 at 25° C.
8. The optoelectronic device of claim 1 wherein the graded buffer Bragg reflector has at least 0.5 percent lattice mismatch to a growth substrate of the optoelectronic device.
9. The optoelectronic device of claim 1 comprising at least a subcell that has a relative voltage (W oc ) that is lower than about 0.42 V and comprises a graded buffer Bragg reflector that is at least 2 μm thick.
10. The optoelectronic device of claim 1 comprising at least 91 percent internal reflectance within at least one graded buffer Bragg reflector.
11. The optoelectronic device of claim 10 comprising at least a subcell that has a relative voltage (W oc ) that is lower than about 0.39 V and comprises a graded buffer Bragg reflector that is at least 4 μm thick.
12. The optoelectronic device of claim 1 comprising at least 98 percent internal reflectance within at least one graded buffer Bragg reflector.
13. The optoelectronic device of claim 12 comprising at least a subcell that has a relative voltage (W oc ) that is lower than about 0.38 V and comprises a graded buffer Bragg reflector that is at least 6 μm thick.
14. The optoelectronic device of claim 1 wherein the internal reflectance within at least one graded buffer Bragg reflector is at least 72 percent.