Spectral utilization in thermophotovoltaic devices Page: 4 of 9
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SPECTRAL UTILIZATION IN THERMOPHOTOVOLTAIC DEVICES
MARVIN B. CLEVENGER, CHRISTOPHER S. MURRAY AND DAVID R. RILEY
Westinghouse Electric Company, West Mifflin, PA 15122
Multilayer assemblies of epitaxially-grown, III-V semiconductor materials are being investigated
for use in thermophotovoltaic (TPV) energy conversion applications. It has been observed that thick,
highly-doped semiconductor layers within cell architectures dominate the parasitic free-carrier
absorption (FCA) of devices at wavelengths above the bandgap of the semiconductor material. In this
work, the wavelength-dependent, free-carrier absorption of p- and n-type InGaAs layers grown
epitaxially onto semi-insulating (SI) InP substrates has been measured and related to the total absorption
of long-wavelength photons in thermophotovoltaic devices. The optical responses of the TPV cells are
then used in the calculation of spectral utilization factors and device efficiencies.
Thermophotovoltaic devices have received a great deal of attention due to their potential utility in
space and other power generation applications. In contrast to silicon- and gallium arsenide-based
photovoltaics (solar cells), material systems for TPV applications must be tailored to utilize the radiant
spectrum from one of a limited number of blackbody-type radiators. Traditionally, front-surface
spectral control elements, such as selective radiators and plasma or dielectric filters, have been
employed to expose TPV devices only to the portion of a radiator's spectrum with energy greater than
the cell bandgap. However, these types of bandpass front-surface spectral control methods generally
result in systems that display lower output power densities.
An alternative approach is to design TPV cells that transmit all incident radiation, effectively utilize
the portion of the incident light with energy above the bandgap and efficiently reflect lower-energy
photons back to the radiator. The reflection of unabsorbed light is made possible by the use of back-
surface reflectors (BSRs) and semi-insulating substrates. A thermophotovoltaic device that meets these
criteria can display a high spectral utilization factor that is generally defined by the relationship:
absorbed energy > he / X in device
F = (1)
total absorbed energy in device
In order to produce TPV devices with high spectral utilization factors, it is necessary to utilize
materials that do not parasitically absorb long-wavelength, below-bandgap energy. As a result, InGaAs
heterostructures grown epitaxially onto semi-insulating InP have emerged as useful material systems for
the fabrication of low-bandgap TPV devices.! A gold. BSR is employed for the reflection of low-energy
radiation not absorbed in the device.2 A schematic diagram of a typical TPV device architecture is
shown in Figure 1.
0.1 sm p+ InP Window
0.1 -0.3 pm p+ InGaAs Emitter
2- 3 pm n InGaAs Base
0.1 pm n+ InP Back Surface Field
1-3 pm n++ InGaAs Lateral Conduction Layer (LCL)
3 pm InPXAsI., or InXGaI , As Graded Layers*
350 m Semi-insulating InP substrate
0.1 -0.2 pm Back-surface reflector (BSR)
Figure 1. Schematic, cross-sectional diagram depicting the layered architecture of a typical
thermophotovoltaic device. (* For lattice-mismatched devices only.)
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Clevenger, M.B. & Murray, C.S. Spectral utilization in thermophotovoltaic devices, article, December 31, 1997; United States. (digital.library.unt.edu/ark:/67531/metadc708642/m1/4/: accessed September 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.