Project Title:

1D and 2D Photonic Crystals for Thermophotovoltaic Applications (poster)

Investigators:

John Kassakian, Thomas Keim, David Perreault
Ivan Celanovic, Natalija Jovanovic, Francis O'Sullivan

Introduction:

Thermophotovoltaic (TPV) devices statically convert heat into electricity using photovoltaic (PV) diodes. The system consists of an emitter, a spectral control component and a PV diode. Good matching between the emitter radiation spectrum and the PV diode characteristics increases the efficiency of the system.

Essential parts of a TPV system.

One-dimensional and two-dimensional photonic crystals are used to modify the emitter spectrum and achieve higher efficiency. Samples of one-dimensional and two-dimensional photonic crystals are produced in the Integrated Circuits Laboratory and the Nanostructures Laboratory.

Theory:

A fuel, e.g. gasoline, is used to heat an emitting surface to approximately 1500K, and the emitter in turn radiates high-energy photons. The PV cells on which this spectrum is incident converts the photons with energies higher than the bandgap of the PV cell into electricity. The remaining portion of the spectrum which cannot be converted is dissipated as heat in the PV cell.

Using a GaSb PV cell, with a bandgap energy of 0.7eV, (1.7µm in wavelength) requires a filter which passes photons whose wavelengths are shorter than 1.7 µm to the PV cell while reflecting the remaining photons back to the emitter.

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. 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.

1D stack scaning electron micrograph (SEM).

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.

1D transmittance measurement and simulation.

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.

System efficiency comparisons.

FTIR setup for emittance measurements.

Fabrication of 2D samples is under way at the Nano-structures Laboratory using interference lithography and chlorine enhanced deep reactive ion etching (dRIE). The process is conducted by Minghao Qi and Christel Zanke of Prof. Hank Smith’s research group.

Publications:

1. F. O’Sullivan, I. Celanovic, S. Akiyama, N. Jovanovic, D. Danielson, K. Wada and J. Kassakian “Optical characteristics of one-dimension Si/SiO₂ photonic crystals for thermophotovoltaic applications.” Journal of Applied Physics 97, 033529, 2005 (PDF).
2. I. Celanovic, F. O’Sullivan, N. Jovanovic, M. Qi, J. Kassakian “1D and 2D Photonic Crystals for Thermophotovoltaic Applications.” presentation at Photonics Europe 2004 Photonic Crystal Materials and Nanostructures, International Society for Optical Engineering, April 2004 (PDF).
3. N. Jovanovic, I. Celanovic, F. O’Sullivan, J. Kassakian “Non-Conventional Electricity Sources for Motor Vehicles.” presentation at MIT/Industry Consortium on Advanced Automotive Electrical/Electronic Components and Systems, San Diego, April 2004.
4. I. Celanovic, F. O’Sullivan, M. Ilak, J. Kassakian, and D. Perreault "Design and optimization of one-dimensional photonic crystals for thermophotovoltaic applications" Optics Letters 29, 8, 863, April 15, 2004 (PDF).

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