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.
Interband cascade lasers (ICLs) are becoming a leading semiconductor laser technology for the mid-infrared because of their high efficiency and low power consumption, especially as compared with conventional diode lasers and intersubband quantum cascade lasers (QCLs) in the wavelength range from 3-5 μm. Although a greater effort has been directed towards GaSb-based ICLs in the ~3-5μm range, recent work has highlighted the exciting potential for InAs-based ICLs for reaching longer emission wavelengths.
In this work we report the development of low-threshold InAs-based ICLs with a room-temperature emission wavelength of 6.3μm. The devices were grown on n+-InAs (100) substrates by solid-source molecular beam epitaxy in a custom V90 system using valved crackers for Sb2 and As2. The ICL structures employ an improved waveguide design using intermediate AlAs/AlSb/InAs strain-balanced superlattice cladding layers surrounded by heavily-doped n+-InAs plasmonic claddings. The active region includes 15-stages with AlSb/InAs/In(0.35)Ga(0.65)Sb/InAs/AlSb type-II “W” quantum wells and optimized electron injector doping.
In pulsed mode, broad-area devices lased at 300 K at a lasing wavelength of 6.26 μm and a threshold current density of 395 A/cm2 which is the lowest ever reported among semiconductor lasers at similar wavelengths. The broad-area devices lased up to 335K in pulsed mode at a wavelength of 6.45 μm. These results provide strong evidence of the potential for InAs-based ICLs as efficient sources in the mid-IR.
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.
Interband cascade IR photodetectors (ICIPs) are multi-stage detectors with discrete absorbers separated by unipolar electron and hole barriers. In this multi-stage detector architecture, the absorber in each stage is designed with a thickness that is thinner than the carrier diffusion length so that photo-generated carriers can be collected efficiently and quickly. This advantage has been validated with the operation of ICIPs at high temperatures (>400 K) and high frequencies (up to 1.3 GHz). Recently, electrical gain exceeding unity was observed in ICIPs. Also, negative differential conductance (NDC) was observed at high temperatures. In this paper, we will discuss the underlying physics of the NDC and a new mechanism that should well explain the electrical gain in ICIPs, as well as our progress towards improved understanding and high temperature performance.
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