Rectannas : Will They Make Solar Cells Obsolete?

Professor Baratunda Cola and colleagues at the Georgia Institute of Technology, Atlanta, claims to have improved on the solar cells available. They have reported their findings in Nature Nanotechnology. The new type of solar cell is actually a rectenna – half antenna and half rectifier that can be tuned to any frequency as a detector, while generating electricity from solar and infrared light falling on it.

The team claims they can achieve a broad-spectrum efficiency of 40 percent with their new cell, although the efficiency they have achieved so far is only one percent. Comparatively, conventional solar cells such as the silicon and multi-junction gallium arsenide types have a maximum efficiency of 20 percent. The team also claims their rectenna can achieve an upper limit of 90 percent efficiency for single wavelength conversion at only a one-tenth the cost of conventional solar cells.

The theory of rectennas is not new, but was discovered more than 50 years ago. However, so far, technology was not advanced enough to fabricate them. According to Professor Baratunda Cola, with currently available technology, it is now possible to make cheap solar-to-electricity converters from carbon nanotubes with ends turned into a special tunnel diode. Cola says the concept is well suited for mass production.

Rectennas are made by growing fields of vertical carbon nanotubes. Their length roughly matches the wavelength of the energy source – for solar radiation, it is one micron. An insulating dielectric such as aluminum oxide caps the carbon nanotubes on the tethered end of the bundles. On the dielectric grows a low-work function of metal – calcium/aluminum. This arrangement makes each nanotube a rectenna with a two electron-volt potential when collecting sunlight and converting it to direct current.

According to Cola, the process uses three steps. In the first step, they grow a large array of vertical nanotube bundles. Then one end of the tubes is coated with a dielectric, while a layer of metal is deposited. One end of the nanotubes changes to a super-fast metal-insulator-metal type of tunnel diode by this process. This method is eminently suitable for mass production, and up to ten times cheaper than making crystalline silicon cells.

With its metal-insulator-metal form, the structure resembles a capacitor with a rating of a few attofarads (1aF = 10-18F). Each nanotube bundle is only 10-20 microns in diameter and consequently, the area of the capacitor plates is so small that the electrical field concentration at the end of the nanotube is very high. With the low work function of the metal, the device behaves just as a tunnel diode does in the peta-hertz (1015 Hertz) region when excited by solar energy and emits electrons in bursts of femtoseconds (10-15 seconds).

Commercialization will require several trillions of nanotube bundles growing side-by-side. Once optimized for higher efficiency, this bunch of nanotube bundles could ramp the power output well into the megawatt range. According to Cola, increasing the efficiency can be achieved by lowering the contact resistance between the antenna and diode. The team expects to improve the efficiency up to 40 percent in only a few years.