This Small Business Innovation Research (SBIR) Phase I project aims to develop strain-relieved GaSb thin films grown directly on GaAs for near-field thermophotovoltaic (TPV) devices. Thin films of direct low-bandgap materials will outperform germanium, an indirect bandgap material, in both power density and efficiency. In this project, TPV devices will be fabricated and tested in the near-field using thin films of GaSb grown directly on GaAs via molecular beam epitaxy (MBE). It has been shown that the well-behaved 90-degree misfit dislocations at the heterojunction result in strain-relieved, low defect density thin films of GaSb grown on relatively inexpensive GaAs substrates. These substrates also have the optical and electrical properties required for fabrication of monolithically integrated TPV devices that are compatible with operation in the near-field of a hot thermal radiator.
The broader/commercial impact of this project will be the potential to provide an economically viable solution in waste heat harvesting. Of all power produced in the world, over half is rejected in the form of waste heat. This has led research teams worldwide to search for solid state solutions to waste heat recovery, but with very limited success. This project will develop the TPV devices, and demonstrate that their operation in the near field of hot objects may lead to more than a tenfold increase in power density over existing technologies. This technology is expected to be used for industrial waste heat, methane flaring, solar, vehicle power, satellite power, and personal portable power applications.
The main purpose of this NSF Program was to evaluate the feasibility of using GaSb epi-layers grown on Semi-insulating (SI) GaAs for fabricating Monolithic Integrated Modules (MIM) of PV cells connected in series which could be used for the conversion of low temperature heat radiating sources into electrical energy. One of the advantages of using GaSb epi-layers on SI GaAsinstead of on GaSb wafers, is that the problem of free carrier absorption is eliminated withou the need of using interference filters and back surface reflectors can be used to return the below bandgap radiation back to the heat source. In addition SI GaAs wafers are available in diameters larger than 4" at reasonable cost. Computer simulations showed that assuming an SRH recombination lifetime of 100 nanoseconds, in addition th Intrinsic Radiative and Auger recombination inGaSb, it is possible to design PV cells that placed at submicron distance from a 900°C radiating source are able to convert the heat into electrical energy at power density of 1.5 to 3 watts/cm2 using GaSb epi-layers grown on SI GaAs. The two figures show a Bird's Eye view and Cross Section of the proposed MIM of PV cells; consisting of 6 isolated strings of 3 cells each for the figures shown. In the program the MIM fabricated consisted of 24 cells connected in series to build the output voltage and reduce the current per cell but conserving the same power density. The cross sectional view shows that each cell consists of an N-GaAs lateral conducting layer, an N GaSb base and P+ and P++ emitter layers. All the necessary processing steps for realizing this structure were developed in the program