InGaAsSb/GaSb Thermophotovoltaic Cells for 2.5 Micron Application

Satellites, submarines and remote field units all need efficient and economical sources of electrical power. Our proposed solution is to convert thermal energy directly into electricity through the use of thermophotovoltaics. In Phase I we will demonstrate the growth and fabrication techniques for a metastable InGaAsSb-based thermophotovoltaic cell (TPV) operating out to 2.5 micron (~1000 C), which will enable efficient electrical power production from either exothermic reactions (fuel combustion) or radioisotope thermal sources. The TPV cell will be grown by molecular beam epitaxy (MBE) growth techniques and consist of an InGaAsSb double-heterostructure fabricated on a 2" GaSb substrate. The InGaAsSb composition will be lattice matched to the GaSb substrate to eliminate dislocations at the growth interface, a common problem with InGaAs based TPV cell technologies. A p-on-n configuration will be used to reduce overall free carrier absorption for the incorporation of a back surface reflector. TPV cells comprising InGaAsSb quaternary material systems and GaSb substrates are expected to have better performance characteristics than existing cells at this operating wavelength as a result of decreased dark currents and the ability to incorporate effective back surface reflectors for reduced operating temperatures. The Phase I efforts will utilize Sarnoff's previous experience developing AlGaAsSb/InGaAsSb quantum-well lasers operating out to 2.7 micron, to establish the growth parameters for appropriate material compositions and doping profiles followed by cell manufacturing and testing at SMI. SMI will optimize the front and back contacts for high current extraction and transmission at the surface and maximize reflection at the back plane. The cells will be evaluated for both material and performance characteristics. In Phase II, the structures will be further refined for increased efficiency by modeling the performance characteristics of the Phase I cells. Characteristics such as the spectral response and series resistance will enable optimization of crucial structure parameters including optimum layer thickness and doping. We will also test the cells using a prototype fuel module.