Flexible Rectenna Converts WiFi Signals into Electricity for Tiny Devices

• A new type of rectenna made of a thin, two-dimensional semiconductor can convert WiFi signals to electricity. 
• Researchers were able to generate 40 microwatts of power when they exposed rectenna to normal WiFi signals.
• That is more than enough power to drive silicon chips or light up an LED. 

Wireless power transmission dates back to the late 19th century when Nikola Tesla was working on methods for transferring electrical energy without wires. A few years later, an Italian inventor Guglielmo Marconi invented the first, complete, commercially successful wireless telegraphy system.

Efficient radiofrequency energy harvesters were later established (in the mid 20th century) on rigid substrates like silicon. However, it has been very difficult to extend this technology to electronic systems that are used on daily basis. Although a wide range of flexible semiconductors has been analyzed to enable truly ubiquitous sensing, scientists haven’t achieved any significant milestone in this field.

Recently, a research team MIT came up with a first completely flexible device that can transform WiFi signals into electrical energy to power electronics. It’s a new type of rectenna — an instrument for converting AC electromagnetic waves into DC electricity — that uses a flexible radiofrequency antenna to capture electromagnetic waves as AC waveforms.

This antenna is then attached to a unique device made of a 2D semiconductor. The device converts the AC signal into DC voltage that could recharge batteries and power electronic circuits.

What’s This Unique Device?

Conventional rectennas use either gallium arsenide or silicon for rectifier that converts the AC signal into DC power. Although these semiconductor cover WiFi band, they are rigid. Using them to fabricate vast areas (walls, surfaces of buildings) would be extremely expensive.

Scientists have been trying to fix these issues for decades. They have developed some efficient rectennas but they work at low frequencies and cannot transform signals in gigahertz frequencies, where most WiFi and mobile phone signals lie.

In this study, researchers used a novel two-dimensional material named molybdenum disulfide that is only three atoms thick. When atoms of this material come in contact with certain chemicals, they reposition themselves in a way that they act as a switch. This forces the material to alter its phase from a semiconductor to a metal.

Reference: Nature | doi:10.1038/s41586-019-0892-1 | MIT

This results in a junction of a semiconductor with a metal, called a Schottky diode, which minimizes the parasitic capacitance and series resistance at the same time.

The Schottky diode parasitic capacitance is much lower than that of existing state-of-the-art flexible rectifiers’ parasitic capacitance. Thus, it can capture and convert wireless signals at much faster rates (up to 10 gigahertz), covering radiofrequency bands used by cellular LTE, Bluetooth, and Wi-Fi.

Courtesy of researchers

Results and Applications

The maximum output efficiency of the device is about 40%, and it varies according to the input of WiFi signal. To put this into content, conventional rectennas made of gallium arsenide and silicon have efficiency around 50% to 60%. The team plans to develop more complex systems with higher efficiency.

The early applications of this new device include powering handheld electronics, medical instruments, and sensors for ‘internet of things’. In this study, researchers were able to generate 40 microwatts of power when the rectenna was exposed to normal WiFi signals (that usually have 150 microwatts of power).

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Nowadays, scientists are developing swallowable pills that can stream health data back to computers for diagnostics. The new rectenna can also be used to power such implantable medical devices in the future.