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Thermophotovoltaics
Photonic Crystals:

Of many ways to exert spectral control in a TPV system, two have been pursued: a 1D photonic crystal as a filter and a 2D photonic crystal as a selective emitter.

The 1D photonic crystal is a dielectric stack composed of multiple alternating layers of Si and SiO2. Simulations of this kind of a structure have shown it produces a large stop band, the limits of which can be engineered to provide excellent matching to the diode requirements. A genetic algorithm was used to design these filters—comprised of alternating Si and SiO2 layers—for use with GaSb PV cells.
 


1D stack scaning electron micrograph (SEM).
 

For the stop band starting at 1.7µm it was found that the appropriate thicknesses are 170nm and 390nm for Si and SiO2, respectively. The preliminary design simulations for a ten layer filter showed a stop band from 1.75µm to 3.2µm.

Initially, a stack was fabricated using the e-beam evaporation technique, within MIT’s Technology Research Laboratory. It was concluded that the layer uniformity and thickness precision produced by this technique were not satisfactory. A different approach at producing the filter using low pressure chemical vapor deposition (LPCVD) techniques yielded significantly improved fabrication results.

A broadband spectrophotometer was used to characterize the filters’ spectral performance. The optical properties of the deposited materials were evaluated using a spectroscopic ellipsometer. Effective filter fabrication is a complex and iterative process requiring careful experimental validation of theoretical predictions, and evaluation of various techniques with a view to identifying the optimal technique, in terms of cost, complexity and, of course, product quality.
 

 

The 2D photonic crystal consists of a 2D photonic crystal acting as the spectral control component on the surface of a solid. Introduction of a pattern onto the surface of an emitter can modify its emission properties. We choose the circular hole hexagonal pattern, so called “honeycomb”, because of its selective properties for both transverse electric (TE) and transverse magnetic (TM) modes.

 


2D 'honeycomb' structure.		  
    2D normal emittance simulation.
     
A metal-coated array of low-permittivity holes can be modeled as an array of parallel-plate wave guides. In such a wave guide, the propagating modes are determined by the dimensions of the wave guide. The modes that do not propagate are evanescent, and they attenuate exponentially with the length of the guide. Therefore, by controlling the lateral dimensions of the wave guide array we define the emitter’s selectivity, while the depth of each feature enhances the selective properties.


 
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