Materials and structures allowing very-low-bandgap thermophotovoltaic energy conversion – LOW-GAP-TPV

The objective of the project is to propose, fabricate and evaluate new materials and structures allowing very-low-bandgap (from 0.36 down to 0.17 eV) thermophotovoltaic (TPV) conversion of thermal energy from medium-grade heat sources (< 1000 °C). As main requirements, the infrared photovoltaic cell should be able to operate at room temperature and the emitter-cell structures should be designed in order to maximize spectral and power conversion efficiencies.
Thermophotovoltaic cells convert photon energy into electrical energy, but with radiation coming from a hot body instead of the Sun. To date, the best TPV converters operate with emitter temperatures higher than 1000 °C and cells with bandgaps larger than 0.53 eV. However, several major issues come with high emitter temperatures: long-term stability of the emitter material, losses toward the environment, power needed for cooling the cell, since it must remain at ambient temperature to avoid efficiency drop. The main research hypothesis of this project is that by harvesting thermal radiation from medium-grade heat sources (< 1000 °C), the aforementioned issues are mitigated. However, with this strategy new scientific and technological challenges come for designing, fabricating and operating the optimum couples of emitter - TPV cell in such conditions: low (0.36 eV) to very low (0.17 eV) bandgap cells able to operate at room temperature, spectral matching between emitter and cell,…
An analysis of the state-of-the-art reveals that research works on low-bandgap TPV cells are very scarce. Recently, indium arsenide and indium antimonide cells have been fabricated but with the requirement to be cooled down to cryogenic temperatures. Interestingly, a new architecture for low bandgap (~0.2-0.4 eV at 300 K) TPV cells has been proposed. Based on interband cascade (IC) structures with multi-stage Ga-containing InAs/GaSb type-II superlattice (T2SL) individual absorbers, these cells overcome the Eg/q (bandgap / elementary charge) limitation for the open circuit voltage while operating at room temperature. About optimal radiation exchange between the emitter and the cell, key for thermophotovoltaic converters, spectral selectivity is usually achieved by tuning the reflectance of the cell and/or the emittance of the emitter. However, to date the overwhelming majority of works is for high temperature emitters and cells having relatively large bandgaps.
In this context, to achieve its main objective, the project is based on the development of new low bandgap IC-TPV cells with gallium-free T2SL InAs/InAsSb absorbers, having better diffusion lengths than the gallium-containing structures mentioned above. The first stage of the work-program will consist in designing the TPV cells, finding the optimum combinations of emitter and cell, and collecting optical, electrical and thermal properties. Then the emitters and TPV cells having the optimum parameters will be fabricated, using PVD sputtering, deposition and etching techniques for the emitters, and Molecular Beam Epitaxy followed by standard clean-room processing techniques for the cells. Specific samples will be built for measuring required properties unavailable in the literature. In the third stage, performances of the fabricated components will be characterized: spectral emittance and stability for the emitters; reflectance, spectral response and current - voltage characteristics for the TPV cells. The performances will be analyzed in detail, by searching for reasons explaining the possible differences with the expectations. Possible improvements will be proposed in the frame of an iterative process comprising three cycles of design, fabrication and tests.