Investigation of narrow bandgap interband cascade thermophotovoltaic cells

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Thermophotovoltaic (TPV) cells based on narrow bandgap interband cascade (IC) structures with discrete type-II (T2) InAs/GaSb superlattice (SL) absorbers are a relatively new type of device for converting radiant infrared photons into electricity. By taking advantage of the broken-gap alignment in a T2 heterostructure, these quantum-engineered IC TPV structures have great flexibility to tailor the bandgap and facilitate carrier transport through interband tunneling with multiple stages for high open-circuit voltage and collection efficiency. Here, we present an investigation of narrow-bandgap (~0.2 eV at 300 K) TPV devices with a varying number of cascade stages and different absorber thicknesses. By comparing the characteristics of five TPV structures with a single absorber or multiple discrete absorbers, it is clearly demonstrated that the device performance of a conventional single-absorber TPV cell is limited mainly by the small collection efficiency associated with a relatively short diffusion length. Furthermore, this work revealed that multi-stage IC TPV structures with thin individual absorbers can circumvent the diffusion length limitation and can achieve a collection efficiency approaching 100% for photo-generated carriers. It is shown that the open-circuit voltage approximately scales with the number of cascade stages, verifying the effectiveness of cascade action. Additionally, the open-circuit voltage, the output power and power conversion efficiency can be significantly increased in IC TPV devices compared to the conventional single-absorber TPV structure. These results have further validated the potential and advantages of narrow bandgap IC structures for TPV cells.