Technical and economic issues for gallium antimonide based thermophotovoltaic systems

Characteristics of photovoltaic cells fabricated from diffused homojunctions in quaternary GalnAsSb are reported for the first time. The unique feature of these quaternary cells is the extended long-wavelength response to 2.2 mircons, enabling the efficient use of blackbody-like thermal sources operating as low as 1073 K in thermophotovoltaic energy conversion systems. Cells were based on a simple structure, and cell fabrication employed low-cost, high-yield, mature microelectronics processes. Specifically, Ga.88In.12As" 11 Sb.89 was grown by liquid-phase-epitaxy lattice matched to n-type GaSb substrates. The junction was formed by zinc doping in a quasi-closed-box diffusion furnace. Silicon nitride served as the anti-reflection coating, and electron-beam deposited metal contacts provided low resistance. Other salient features of these ceils include an internal quantum efficiency exceeding 75% at 299 K, and Voc --0.26 V, a fill factor of 0.55, and an Isc of 1.2 A/cm2, all at 288 K, excellent for a quaternary material whose optical characterization showed a bandgap of 0.58 eV. Process improvement can increase this fill factor to 0.75, as has been previously demonstrated with binary GaSb cells fabricated with similar processes.

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