Engineers at MIT are developing a way to turn the humble RFID tag into a light-powered sensor for the internet of things. Based on thin-film perovskite cells, the goal is to create inexpensive, internet-connected sensors that can operate without batteries or other outside power sources for months or even years.
According to MIT, experts predict internet of things devices will number in the neighborhood of 75 billion by the year 2025. Whether this turns out to be a good thing or not, it does mean that there will be a lot of sensors and other data-gathering devices that will need a lot of energy to keep working.
Since it would be impractical to wire up all these devices to the mains and the prospect of constantly changing millions of tiny batteries is unappealing, MIT researchers have designed photovoltaic versions that can run on sunlight or even dim indoor lighting.
It isn't a new idea. The researchers from the MIT Auto-ID Laboratory and MIT Photovoltaics Research Laboratory admit that tiny solar-powered devices have been tried before, but these have relied on conventional solar technology, which is bulky, expensive, inflexible, and cannot be made transparent. In contrast, the perovskite cells are inexpensive, printable, flexible, and can be made see-through.
The MIT team's approach is to take the perovskite cells and merge them with RFID tags equipped with multiple sensors for monitoring various environmental factors, such as temperature and humidity. These tags can be printed in rolls with the photovoltaic cells incorporated on them and can even be made see-through, so they can be mounted on window glass. They also have tiny, ultra-high-frequency antennas that only cost pennies to manufacture.
The new sensor tags work on the same principle as the RFID tags commonly used to mark retail goods. An RFID tag is essentially an electronic circuit without a power source, but when it comes within range of a reader device that is transmitting a radio signal, the tag picks up energy from the backscatter effect – essentially, it gets its electricity from the radio signal itself. It then transmits the information that is stored in the tag's chip, allowing it to be used for pricing, stock taking, security, tracking, and other applications.
The problem is that the tag can only produce a few microwatts of power and only when the reader is scanning it within a range of a few meters. If it's to be used as a practical sensor, it needs to be powered for much longer periods of time.
For the new tags, the MIT team sandwiched perovskite between an electrode, cathode, and special electron-transport layer materials. This allowed the engineers to tune each cell so it operated as desired under different lighting conditions. These were then made into modules of four cells each, which could generate 4.3 volts each in direct sunlight and transmit data up to 5 m (16 ft) over a 1.5-volt circuit.
Other tests showed the cells reaching efficiencies of up to 21.4 percent under fluorescent lighting and 45 minutes of light exposure could charge them for up to three hours, allowing the sensor tags to monitor temperatures indoor and out for several days while continuously transmitting data five times farther than conventional RFID tags. This would make it possible for one reader to simultaneously glean data from multiple sensors.
When the technology is matured, the team sees the new tags as a way to monitor the environment for months or even years before they deteriorate too much to function. They could be used not only for temperature monitoring, but also cargo tracking, soil monitoring, and energy use monitoring as they expand to include the ability to measure humidity, pressure, vibration, and pollution.
"The perovskite materials we use have incredible potential as effective indoor-light harvesters," says Department of Mechanical Engineering postdoc Ian Mathews. "Our next step is to integrate these same technologies using printed electronics methods, potentially enabling extremely low-cost manufacturing of wireless sensors,"
The research was published in Advanced Functional Materials and IEEE Sensors.
Source: MIT