Cambridge, MA--Researchers at the Massachusetts Institute of Technology (MIT) are using 2D photonic crystals (PCs) made out of tungsten to raise the efficiency of thermophotovoltaic (TPV) cells. The PCs are made up of a square array of micron-sized holes in a tungsten film that is one to a few microns thick. The PCs absorb broadband thermal radiation and re-emit it in wavelength bands that are tailored to match the high-efficiency spectral regions of TPV cells.
TPV setup uses two different photonic crystals
In the TPV setup, a second PC made up of alternating layers of silicon and silicon dioxide is placed between the tungsten PC and the TPV cell; the second PC serves as a selective reflector, further optimizing the wavelength band received by the TPV. Radiation rejected by the second PC is reflected back to the tungsten PC, where it is reabsorbed, further boosting efficiency.
Tungsten is required for the primary absorbing PC because it does not degrade at high temperatures. The tungsten film was patterned over a 1 cm diameter using interference lithography and reactive ion etching. Photonic crystals with differing depths, hole diameters, and hole spacings were created for experimentation.
As a practical demo, the researchers created a button-sized "micro-TPV power generator" containing a butane-fueled catalytic microreactor that generates heat. Electricity generated by a TPV is passed to a circuit that dynamically adjusts voltages and currents to extract maximum power from the TPV system while making the output suitable for a cell phone or other device.
Other uses include improving the efficiency of radioisotope-based electricity generators such as those used on space probes; the MIT researchers are collaborating with NASA and with Creare (Hanover, NH) to develop such a generator. The researchers are also teaming up with members of MIT's Solid State Solar Thermal Energy Conversion Center to improve the efficiency of optical solar concentrators for electricity generation.