Development of a very high efficiency, dot-junction, InGaAs thermophotovoltaic (TPV) converter for deep space missions

Simplified modeling calculations have shown that for a bandgap of 0.6 eV, assuming a conservative 20% planar device conversion efficiency (1800 °F blackbody source), a non-planar In_0.69Ga_0.31As TPV cell with micron size pn dot-junction geometry will result in conversion efficiencies of 30%, or almost four times the efficiency of traditional thermoelectric conversion systems. As an initial proof of principle demonstration of this technology, we have designed, processed, and tested a 0.74 eV In_0.53Ga_0.47As dot-junction cell. The 3.0 μm n-InGaAs dots, separated by a 30.0 μm floating junction region, were photolithographically defined on a p-InGaAs layer on a p-InP substrate. In this initial design, the n-contact, grid fingers, and the bussbar were on the top surface where the light was incident, resulting in a 13.3% grid-shadowing effect. The p-contact was at the bottom surface of the substrate. Tested under AM0 one sun condition using a solar simulator, this first attempt showed an efficiency of 6.93%, with a fill factor of 71.6%. Further improvements are expected with proper passivation of the free surface of the p-type base region. In the final design, both the n- and p-contacts will be defined on the epitaxial layer, with radiation incident through the transparent InP substrate.