Spectral utilization in thermophotovoltaic devices Page: 5 of 9
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For architectures such as that shown in Figure 1, the p-doped emitter and window layers and the n-
doped lateral conduction layer dominate parasitic absorption. Optically, it would be advantageous to
incorporate only thin, lowly-doped layers into cell architectures. Unfortunately, this strategy results in
electrical losses in the form of high series resistances. To combat this trade-off, it is important to
understand the dependence of the total absorptance within TPV devices on each layer. This permits the
accurate decoupling of the absorptances of the spectrally dominant layers within TPV cell architectures
and enables the use of predictive capabilities that allow each layer to be grown at an optimum dopant
density and thickness. Consequently, n- and p-type InGaAs layers of varying thickness and dopant
density were grown epitaxially onto InP substrates in an effort to simulate the effects individual layers
have on the total optical response of cells and to correlate these effects to optical response and device
efficiency. In this work, these layer are characterized individually to determine their contribution to
device absorptance and techniques employed to evaluate the spectral utilization factors and project
device efficiencies are presented.
Individual n- and p-type InGaAs layers were grown epitaxially onto semi-insulating InP substrates
by organometallic vapor phase epitaxy -(OMVPE).- The precursor materials were trimethyl indium,
trimethyl gallium, arsine and phosphine. Diethyl zinc and silane were employed for p- and n-type
doping, respectively. Back-surface reflectors (BSRs) consisting of gold or gold on a thin MgF2 layer
were deposited by thermal evaporation.
Measurements of the optical responses of the individual layers and of complete grown structures
were completed on a Nicolet 760 Fourier-transform infrared spectrophotometer fitted with in-
compartment assemblies for independently measuring spectral reflectance at an incidence angle of 120
and total diffuse reflectance at normal incidence.
Measurements of sample dopant densities and electron mobilities were completed on a Lakeshore
7504 Hall Effect / Electronic Transport Measurement System capable of measuring carrier
concentrations up to 1020 cm-3 and sample resistances in excess of 1011 92 at room temperature and 77 K.
Because of its thickness and dopant density, the lateral conduction layer contributes to optical losses
significantly more than other n-type layers in TPV cells. In order to evaluate the degree of parasitic
absorption in the lateral conduction layer, a series of n-Ino.53Ga0.47As layers were grown epitaxially onto
semi-insulating InP. Physical and spectral properties of these layers are summarized in Table 2.
Reflectance as a function of wavelength was measured for the samples in order to determine the energy-
weighted parasitic absorptance exhibited by each as shown in Figure 3.
Table 1. Physical Properties of n-Ino.53Gao.47As layers on InP.
Sample Carrier Density Carrier Mobility Thickness % Power Absorbed (PA) X > 2 pm*
(x1017 cm-3) (cm2/Vs) ( m) 1750*F Radiator 2250*F Radiator
1 279 1636 2.35 26.9 27.3
2 120 2029 2.67 16.1 14.6
3 84.8 2337 2.50 11.6 10.2
4 9.45 3949 2.95 2.8 2.5
5 287 1586 0.81 9.5 9.3
*2 pm corresponds to a bandgap of approximately 0.6 eV.
Free-carrier absorption in thick, highly-doped n-In0.53Ga0.47As can constitute a considerable loss in
TPV devices. However, as the dopant density is decreased, the free-carrier absorption decreases in
magnitude and shifts to higher wavelengths where less energy is available from a blackbody radiator.
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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/m1/5/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.