Tungsten Lattice Structure May Improve Thermal Photovoltaics

A tungsten 3-D photonic crystal developed at Sandia National Laboratories in Albuquerque, N.M., may have applications in thermal photovoltaic power generation for hybrid electric automobiles and for waste-heat-driven generators in industrial plants. In a series of experiments that the Sandia team reported in the July 14 issue of Applied Physics Letters, the tungsten emitter significantly outperformed an ideal blackbody, a phenomenon that the team will address in forthcoming issues of Optics Letters.

The photonic crystals comprise stacks of 500-nm-wide, 750-nm-high parallel tungsten rods spaced 1.5 µm apart and oriented 90° from one layer to the next. When biased at 6.5 V, producing an effective temperature of 1535 K, the structures produced 1.5-µm radiation with a peak power density of 40.5 W/cm² and a spectral linewidth of 900 nm. The researchers note that this emission linewidth is the equivalent of a blackbody at 4000 K and that the peak emission wavelength is a good match for the electronic bandgap of a GaSb photovoltaic cell.

In a thermal photovoltaic system, the photovoltaic cell converts the incoming infrared radiation from a heat generator into electricity. The researchers' models of a planar setup suggest that optical-to-electrical conversion efficiencies as high as 34 percent and electrical power densities of up to 14 W/cm² are possible using a GaSb cell and the photonic crystal emitter.