KEYWORDS: Solar cells, Solar energy, Quantum wells, Photovoltaics, Absorption, Germanium, Data modeling, Atmospheric modeling, Concentrated solar cells, Brain-machine interfaces
Triple-junction (3J) solar cells are the world's most efficient photovoltaic conversion devices, hero cells operating >41%
under concentration between 300 and 500 suns. The typical 3J approach has a bandgap combination that limits the cell
efficiency at approximately 49%. Different combinations of bandgaps can increase the theoretical efficiency to closer to
60%, and use of metamorphic materials has attempted to demonstrate still higher efficiencies. Multiple quantum wells
(MQW) can also be used to fabricate materials with different effective bandgaps from the host semiconductor, and can
do so without the attendant lattice constant change and dislocations associated with metamorphics. We show that
sufficiently high absorption in MQWs increases the efficiency of 3J solar cells without incorporating defects during
epitaxy, both in simulations and in practice.
Incorporating quantum wells into multi-junction III-V solar cells provides a means of adjusting the absorption
edge of the component junctions. Further, by using alternating compressive and tensile materials, a strain-balanced
stack of quantum well and barrier layers can be grown, defect free, providing absorption-edge / lattice
parameter combinations that are inaccessible using bulk materials. Incomplete absorption in the quantum wells
has been addressed using a distributed Bragg reflector, extending the optical path length through the cell and
enabling photon recycling to take place. State of the art single-junction quantum well solar cells have now
reached an efficiency of 27.3% under 500X solar concentration and are projected to reach 34% in a double
junction configuration.
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