0.52eV Quaternary InGaAsSb Thermophotovoltaic Diode Technology Page: 3 of 12
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THEORY AND ASSUMPTIONS
The analysis presented here illustrates the limiting effect of Auger recombination on TPV
diode conversion efficiency, and the competing influences of poor photon recycling and defect-
assisted recombination. Auger recombination of photogenerated carriers sets a fundamental limit
to conversion efficiency below the thermodynamic limit, which is determined by the radiative
heat transfer between the radiator and diode of a given bandgap.'' Auger coefficients for low
bandgap III-V materials (0.5-0.6eV) have recently been measured via minority carrier lifetime in
the range of 1x10-28cm/s'10.
The thermal to electric conversion efficiency of TPV is equal to the diode efficiency (electric
output power to the above -bandgap radiation absorbed in the diode) multiplied by the spectral
efficiency (above-bandgap radiation absorbed in the diode to the total heat absorbed at the cold
side). The optimal diode bandgap, when considering 95-97% reflection of all below-bandgap
photons is in the range of 0.5 to 0.6eV1 for a 9500C radiator. Figure 1 shows the measured
reflection and absorption of today's 0.52eV front surface filter technology" corresponding to
80% spectral efficiency. Figure 1 also shows an idealized step function reflection assumed for
the modeling in this work(dashed lines). The radiator is also assumed to have a uniform
emissivity of 0.9. The familiar effective emissivity equation for infinite parallel plates is given as
equation (1):
1 1 0.78, X < X Eg
ef 1 -11 -10.03, X > XEg
+ -1 -+ -1 E
E radiator E diode 0.9 1- R (X)
100
tRE<Eg = 9704
R80 measured
.
y 60
0
spectral = 80% (measured)
O 40 spectral = 87% (dotted)
N
N
20 - 4* RE>Eg = 15% Amodeled
1 5 wavelength (pm)10 15
FIGURE 1. Angle-of-incidence ( 0 = 11, 30 , 450, 60 and 80 ) weighted filter reflectance (triangles)
designed for glue adhesive, but measured in air11. Filter absorption (solid) is calculated for the actual filter
design with OptiLayer software. Measured above band gap transmission of filters is 79% and spectral
efficiency is 80%. The performance projections in the modeling studies of this paper assume a step
function reflectance (15% to 97%) and 3% absorption (87% spectral efficiency).
These effective emissivity and reflection values assumed in the equation (1) correspond to an
87% spectral efficiency for a 9500C radiator. The integrated above-bandgap transmissivity (T)
includes the effective cavity emissivity, as wells as, above-bandgap absorption in the filter and is
equal to T=79%. Thus maximum short-circuit current density for the 0.52 eV device and 950 0C
radiator considered here is 3.6 A/cm2 .
For the remainder of the discussion, the focus is on the diode efficiency, which can be
expressed in terms of the following four parameters (FF = fill factor, QE = quantum efficiency,
Fo = overexcitation factor, and qVoc/Eg = voltage factor, see reference [1]).
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Dashiell, M. W.; Beausang, J. F.; Nichols, G.; Depoy, D. M.; Danielson, L. R.; Ehsani, H. et al. 0.52eV Quaternary InGaAsSb Thermophotovoltaic Diode Technology, report, June 9, 2004; Schenectady, New York. (https://digital.library.unt.edu/ark:/67531/metadc788112/m1/3/: accessed May 30, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.