Antireflection Coated Refractory Mletal Matched Emitters for Use with GaSb Thermophotovoltaic Generators Lewis Fraas, John Samaras, James Avery, Leonid Minkin JX Crystals Inc, 1105 12’h Ave NW, Suite A2, Issaquah, WA 98027, USA ABSTRACT GaSb thermophotovoltaic cells can be combined with infrared emitters to produce electric power. In this application, both lpower density and efficiency are important. High power density requires a practical target emitter temperature of 1600°K. In order to reach this temperature, spectral efficiency becomes extremely important. Radiation with wavelengths greater than 1.8 microns cannot be converted by the GaSb cells; instead, this long wavelength radiation overheats the cells, limiting power density and efficiency. A solution is to use refractory-metal coated emitters, because metals have low emittance at long wavelengths. Further, an antireflection (AR) coating on the metal can enhance the emittance in the cell convertible band. A spectral efficiency of 75% has been demonstrated for an AR coated tungsten emitter and a GaSb cell power density of 1.5 Watts / cm2 has been measured with an AR coated tungsten emitter operating at 155!YK. !lNTRODUCTlON GaSb photovoltaic cells responding to wavelengths out to 1.8 microns can be combined with combustion heated ceramic infrared emitters to produce electric power. In this application, both power density and efficiency are important. Power densities are higher for higher emitter temperatures. JX Crystals has established a target emitter temperature of 1600°K. However, in order to reach this temperature, spectral efficiency becomes extremely important. Spectral efficiency is defined as the emitted radiant power at cell convertible wavelengths divided by the total eimitted radiant power. The above problem can be quantified as follows. The total radiated power from a blackbody at 1600’K is 37 W/cm’. For this radiated power, 25% or about 9.2 W/cm’ falls in the band with wavelengths below 1.8 microns, 47% is in an intermediate band between 1.8 and 3.6 microns, and the last 28% falls in the wavelength interval beyond 3.6 microns. Since a simple dielectric filter can be used to reflect the mid-band radiation between 1.8 and 3.6 microns back to the emitter, at first, it would seem that a spectral efficiency of nearly 50% could be obtained with a simple dielectric filter. The problem with the above argument is that the remaining power density of 19.6 W/cm* must be transferred to the ernitter by the combustion gases and also removed from the cells by the cooling system. This power density is too large for either of these heat transfer requirements. In fact, the 10.4 W/cm* associated with the 28% of the radiant energy beyond 3.6 microns is difficult to handle. The conclusion is that one must reduce the size of the long wavelength radiant term before high temperatures, high power densities, or high spectral efficiencies can be obtained. The above argument has led to the use of refractory- metal coated emitters because metals have low emittance at long wavelengths. To enhance the emittance of the metal In the cell convertible band, an antireflection coat was applied to the metal with a thickness such that the minimum reflection falls in the middle of the cell converliible band. Fig. 1. shows this new AR coated refractory metal matched emitter concept. SIC substrate \ ;;; “& 1 ?*a,,: Tungsten coating I- ,:$i& : I - Convertible .AR coating Infrared Fig. 1. Emitter Concept. In a first practical embodiment of this matched emitter concept, a refractory oxide with a refractive index of 2.0 was chosen for the AR coating material and tungsten as the refractory metal. The reflectivity (R) of this pair as a function of wavelength has been modeled theoretically using TF CALC given n and k values for tungsten. The results are shown in figure 2. This figure plots (I-R) for both bare tungsten and AR coated tungsten for various AR coating thicknesses. Note that (I-R) is equivalent to absorptance or emittance for a non-transparent system. These theoretical results indicate a peak emittance of over 0.8 for cell convertible wavelengths with low emittance at long wavelengths. 0-7803-5772-S/00/$10.00 02000 IEEE 1020