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This paper reports recent progress in the development of quaternary III–V thermophotovoltaic (TPV) devices based on MBE grown Ga x In 1− x As y Sb 1− y . TPV is of great interest for a variety of applications (1st and 2nd NREL Conf. on Thermophotovoltaic Generation of Electricity, AIP Conf. Proc. 321 (1994), 358 (1995)). The objective of this work is to develop a TPV cell which is “tunable” to the emission spectrum of a heated blackbody, at temperatures in the range of 1200–1473 K. One aspect of this “tuning” is to match the band gap, E gap , of the photovoltaic device to the peak output of the heat source. An advantage of the quaternary III–V semiconductor systems is that devices can be fabricated by molecular beam epitaxy on a suitable binary substrate, such as GaSb or InAs, and the band gap and lattice constant can be adjusted more or less independently, to match requirements. Quaternary cells, with band-gaps in the 0.5–0.72 eV range, have been fabricated and tested. For 0.54 eV devices we obtained open circuit voltage V OC = 0.3 V and short circuit current I SC = 1.5 A/cm 2 under infrared illumination of a 1200 K blackbody. Under high illumination levels the V OC and I SC ranged from 0.5 V at 3 A/cm 2 for 0.72 eV devices to 0.31 V at 1.2 A/cm 2 for 0.5 eV devices, indicating good photovoltaic device characteristics over the range of bandgaps. The diode ideality factor for 0.54 eV devices ranged from 2.45 at low illumination indicating tunneling-dominated dark current, to 1.7 at high illumination intensity indicating recombination-generation dominated dark currents. |
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