Efficiencies and limiting factors of narrow bandgap thermophotovoltaic cells (Conference Presentation)

We identified the factors that limit the conversion efficiency of narrow bandgap thermophotovoltaic (TPV) cells and investigated how these factors affect key performance aspects such as quantum efficiency (QE), fill factor and open-circuit voltage. Calculations are made for narrow bandgap InAs/GaSb superlattice materials to elucidate how the conversion efficiency is limited by these factors for specific material parameters such as the product of absorption coefficient a and diffusion length L. It is shown that the multi-stage interband cascade (IC) structure is able to solve the problem of low QE in single-absorber TPV cells, therefore, increase conversion efficiency by about 10% in a wide range of aL. Also, the dependence of conversion efficiency on the illumination source is investigated, which shows the flexibility and advantage of multi-stage IC structure to achieve its maximum efficiency with the incident photon energy near the bandgap.