Convert light to electricity with an ultra-small antenna

We developed a green laser for visible light rectification antenna and measured its conversion efficiency.

Main structure of manufactured rectenna for visible light

Researchers at Georgia Tech in the United States have developed a rectenna that receives visible light and converts it into electricity. The rectifying antenna is a device that combines rectifying diodes on the antenna to convert electromagnetic waves into direct current. The principle of power generation is completely different from solar cells using semiconductor pn junctions.

At present, the conversion efficiency of the visible light rectifying antenna is less than 1%, but developers believe that, with further optimization in the future, a photovoltaic power generation element with a conversion efficiency of 40% can be manufactured at a cost of 1/10 of the existing silicon solar cell. If this can be achieved, the world's energy landscape will change dramatically. Detailed development results have been published in the academic magazine Nature Nanotechnology.

The rectifying antenna can achieve conversion efficiency of up to 90% or more using microwaves having a wavelength of about 10 cm. Its attempts to miniaturize and convert light into electricity began in the 1960s. However, due to limitations in manufacturing technology, it was difficult to manufacture rectennas for infrared rays and visible light with a wavelength of 10 μm or less.

This time, Baratunde Cola, an associate professor at the Georgia Institute of Technology in the United States, used the metal multi-walled carbon nanotube (CNT) as an antenna and the MIM (Metal-Insulator-Metal) element as a diode to prototype a visible light rectifier antenna. . Although it is inefficient, it can generate electricity. Cola pointed out that "With the improvement of manufacturing technology, it is ripe to challenge the use of rectenna for visible light."

CNT doubles as antenna and MIM structure diode "M"

The outline of the manufacturing process is as follows. First, multiple layers of CNTs are grown vertically on a conductive substrate to form a "CNT forest." Then, the CNT forest was coated with a very thin alumina (Al2O3) film using chemical vapor deposition (CVD). Finally, calcium (Ca) and aluminum (Al) films are formed thereon by sputtering.

The material laminated on the CNT forest is fully transparent to visible light, so that when the light is irradiated, light will reach the CNT. As a result, CNTs first function as a rod antenna. Also, the CNT itself, the Al2O3 layer, and the Ca/Al layer will constitute the diode of the MIM structure.

Since the work function of CNT and Ca is different, electrons flow from CNT to Ca side in one direction. In addition, the diode charge and discharge time constant of this MIM structure is very small and has a high-speed response of femtoseconds (10-15 seconds). This is fully responsive to visible light frequencies - hundreds of THz. Therefore, the finished element can be used as a rectenna for visible light.

Related technologies have applied for patents in the United States, Europe, Japan, and China. (Reporter: Nozawa Tetsuo)