Spectral utilization in thermophotovoltaic devices Page: 8 of 9
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Normalized 1750"F Blackbody
o ~Cell 445 1
63d) Cell 145
0 2 4 6 8 10 12
Figure 4. Absorptance vs. Wavelength for four thermophotovoltaic cell architectures. The overlay is the
nomialized emission of a 17500 F blackbody radiator.
Thus, when designing TPV devices, the optical properties of the cells can be approximated by
measuring the degree of parasitic absorptance -in individual layers of the architecture. Successful
implementation of this strategy allows the accurate prediction of the optical losses associated with
various TPV cell architectures and results in the development of more efficient devices.
Spectral Utilization and Device Efficiencv
In order to evaluate the spectral utilization of TPV cells, it is necessary to characterize the infrared
reflectance of architectures such as that shown in Figure 1. Since the gold BSR prevents infrared
transmittance through the rear of the device, long-wavelength absorptance can be determined with the
a(X)=l-p() for -i(,)=0 (2)
The spectral utilization factor can then be evaluated from the measured reflectance as follows:.
f [1- p(%)]Nph (,)dk
Fu = 0 (3)
f[1- p(k)]N ph (X
where X is the wavelength and Nph is the energy flux given by:
27 h c2
N h (2) = (4)
Ph A5 [exp(hc/XkT)-1]
Spectral utilization is the single most important factor controlling TPV device efficiency.
Efficiency can be approximated through knowledge of the voltage factor (Voc/Eg), the energy-weighted
quantum efficiency (QE), the fill factor (FF), the over-excitation factor (Fo) and the spectral utilization
factor (FU) as follows:'
I=-- --QE-FF-FO - Fu (5)
Typical experimentally-determined values for these parameters measured on fully-processed TPV
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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/m1/8/: accessed March 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.