Spectral utilization in thermophotovoltaic devices Page: 7 of 9
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o 60- Sample 6 ample 8
40S ple 7
0 5 10 15 20 25
Figure 3. Reflectance vs. Wavelength for a series of p-Ino.s3Gao.47As layers grown epitaxially onto InP.
materials (i.e. Samples 2 and 7) display higher energy-weighted parasitic absorptances. This
observation is in agreement with previously published results that have shown that p-type layers display
-17 times the absorptance of comparable n-type layers.4
The reflectances of Sample 8 and the p+ InP approach the reflectance of the gold BSR (-98%) and
the loss due to parasitic absorption in these layers can be considered negligible.
In order to relate the behavior of these layers to that of TPV devices, complete architectures with
various combinations of p-doped emitters and windows and n-doped lateral conduction layers were
grown and characterized. This strategy allows the determination of spectral utilization factors and
device efficiencies and aids in the identification of the layers that contribute most to optical losses.
TPV Cell Architectures
The physical characteristics of two similar architectures grown at two different bandgaps (0.6 eV
and 0.74 eV) are provided in Table 1. Absorptances as a function of wavelength for the four structures
are shown in Figure 4 with the normalized spectral emission of a 1750*F blackbody radiator.
Free-carrier absorption in the highly-doped LCL's is responsible for the peaks at 5.9 pm in Cells 445
and 446. Plasma reflectance from the LCLs and reflectance from the BSRs are responsible for optical
interference that results in the periodicity of the response of these cells in the 5 pm to 10 pm range.
That is, multiple reflections within the device provide many opportunities for radiation to be
parasitically absorbed by the LCL and compositionally-graded layers. Cells 145 and 467 possess LCL's
doped approximately an order of magnitude lower and do not display this effect.
Small differences in the absorptance characteristics in the 2 pm to 5 PM range are primarily due to
absorption by the p-type emitter and top window layers. These layers also contribute to the gradual
increase in absorptance observed for Cells 145 and 467 at long wavelengths. Although it displays a
similar profile at short wavelengths, the absorptance of Cell 446 is -12% higher than that of the other
architectures because of optical losses in the graded layers of this device.
Table 3. Physical Characteristics of Thermophotovoltaic Cell Architectures
TPV Bandgap Emitter Emitter Dopant LCL LCL Dopant
Cell (eV/pLm) Thickness ( m) Density (x 1019c~') Thickness( m) Density (x 108cm-3)
445 0.74/1.69 0.1 1.0 1.0 20
467 0.74/1.69 0.1 1.2 0.15 2.0
446 0.6/2.25 0.3 1.0 1.0 20
145 0.6/2.25 0.3 1.2 0.15 2.0
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Clevenger, M.B. & Murray, C.S. Spectral utilization in thermophotovoltaic devices, article, December 31, 1997; United States. (https://digital.library.unt.edu/ark:/67531/metadc708642/m1/7/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.