Radioisotopes have been used as a source of energy for space applications where there is insufficient solar energy to use standard photovoltaic arrays. Thermophotovoltaic (TPV) systems using gallium antimonide (GaSB) photovoltaic cells been proposed as a way to increase the efficiency of the thermal-to-electric conversion. However, GaSB does not absorb most of the blackbody spectrum emitted from the general purpose heat source (GPHS) operating at 1100 degrees centigrade. In order for the system to be efficient, the majority of photons must be reflected back onto the heat source, reabsorbed, and re-emitted at higher energy. This requires that the entire system be highly reflective; any "arasitic absorption" severely degrades the efficiency and lowers the GPHS temperature. TPV cells with a lower bandgap will be fabricated in Phase I from gallium indium antimonide (gainsb) in order to absorb a larger proportion of the blackbody spectrum. The additional energy conversion will be demonstrated to result in an increased efficiency for any system with parasitic absorption of over 5%. For systems with 15% parasitc absorption, the efficiency increases from 16% (using GaSB) to 18% (using gainsb).
