Directly converting electrical power to heat is easy; however, converting heat into electrical power is not as easy. To address this issue, a tiny silicon-based device was developed that can harness what was previously called waste heat and turn it into DC power. The device could be used as a compact infrared power supply that could replace radioisotope thermoelectric generators (RTGs) that are used for such tasks as powering sensors for space missions that don’t get enough direct sunlight to power solar panels. The device is made of common and abundant materials such as aluminum, silicon, and silicon dioxide — or glass — combined in uncommon ways.
The device is about 1/8“ by 1/8”, half as thick as a dime, and metallically shiny. The top is aluminum etched with stripes roughly 20 times smaller than the width of a human hair. This pattern, though far too small to be seen by eye, serves as an antenna to catch the infrared radiation. Between the aluminum top and the silicon bottom is a very thin layer of silicon dioxide. This layer is about 20 silicon atoms thick, or 16,000 times thinner than a human hair. The patterned and etched aluminum antenna channels the infrared radiation into this thin layer.
The infrared radiation trapped in the silicon dioxide creates very fast electrical oscillations — about 50 trillion times a second. This pushes electrons back and forth between the aluminum and the silicon in an asymmetric manner. This process, called rectification, generates net DC electrical current. The infrared rectenna (rectifying antenna) is a solidstate device with no moving parts to jam, bend, or break, and doesn’t have to directly touch the heat source, which can cause thermal stress.
Because the infrared rectenna is made with the same processes used by the integrated circuit industry, it is readily scalable. In theory, any commercial integrated circuit fabrication facility could make the rectenna. One of the biggest fabrication challenges was inserting small amounts of other elements into the silicon, or doping it, so that it would reflect infrared light like a metal.
The prototype version of the infrared rectenna produces 8 nanowatts of power per square centimeter from a specialized heat lamp at 840 °F. For context, a typical solar-powered calculator uses about 5 microwatts, so a sheet of infrared rectennas slightly larger than a standard piece of paper would be required to power a calculator. To maximize power, the rectenna’s top pattern could be changed to 2D x’s instead of 1D stripes, in order to absorb infrared light over all polarizations. The rectifying layer also could be designed as a full-wave rectifier instead of the current half-wave rectifier, and it could be made on a thinner silicon wafer to minimize power loss due to resistance. Through improved design and greater conversion efficiency, the power output per unit area will increase.