MIT researchers have designed low-cost, photovoltaic-powered sensors on RFID tags that work in sunlight
and dimmer indoor lighting, and can transmit data for years before needing replacement.
By 2025, experts estimate the number of “internet of things” devices — including sensors
that gather real-time data about infrastructure and the environment — could rise to 75 billion
worldwide.
As it stands, however, those sensors require batteries that must be replaced frequently,
which can be problematic for long-term monitoring.
MIT researchers have designed photovoltaic-powered sensors that could potentially transmit data
for years before they need to be replaced.
To do so, they mounted thin-film perovskite cells as energy-harvesters on inexpensive
radio-frequency identification (RFID) tags.
Perovskite cells are known for their potential low cost, flexibility, and relative ease of
fabrication.
The cells could power the sensors in both bright sunlight and dimmer indoor conditions.
Moreover, the team found the solar power actually gives the sensors a major power boost that
enables greater data-transmission distances and the ability to integrate multiple sensors
onto a single RFID tag.
Depending on certain factors in their environment, such as moisture and heat, the sensors can
be left inside or outside for months or, potentially, years at a time before they degrade enough
to require replacement.
That can be valuable for any application requiring long-term sensing, indoors and outdoors, including
tracking cargo in supply chains, monitoring soil, and monitoring the energy used by equipment
in buildings and homes.
In recent attempts to create self-powered sensors, other researchers have used solar
cells as energy sources for internet of things (IoT) devices.
But those are basically shrunken-down versions of traditional solar cells — not perovskite.
The traditional cells can be efficient, long-lasting, and powerful under certain conditions ,but
are really infeasible for ubiquitous IoT sensors.
Traditional solar cells are bulky and expensive to manufacture, plus they are inflexible and
cannot be made transparent, which can be useful for temperature-monitoring sensors placed
on windows and car windshields.
They’re also really only designed to efficiently harvest energy from powerful sunlight, not
low indoor light.
Perovskite cells, on the other hand, can be printed using easy roll-to-roll manufacturing
techniques for a few cents each; made thin, flexible, and transparent; and tuned to harvest
energy from any kind of indoor and outdoor lighting.
The idea, then, was combining a low-cost power source with low-cost RFID tags, which are
battery-free stickers used to monitor billions of products worldwide.
The stickers are equipped with tiny, ultra-high-frequency antennas that each cost around three to five
cents to make.
RFID tags rely on a communication technique called “backscatter,” that transmits data
by reflecting modulated wireless signals off the tag and back to a reader.
A wireless device called a reader — basically similar to a Wi-Fi router — pings the tag,
which powers up and backscatters a unique signal containing information about the product
it’s stuck to.
Traditionally, the tags harvest a little of the radio-frequency energy sent by the reader
to power up a little chip inside that stores data, and uses the remaining energy to modulate
the returning signal.
But that amounts to only a few microwatts of power, which limits their communication
range to less than a meter.
The MIT researchers’ sensor consists of an RFID tag built on a plastic substrate.
Directly connected to an integrated circuit on the tag is an array of perovskite solar
cells.
As with traditional systems, a reader sweeps the room, and each tag responds.
But instead of using energy from the reader, it draws harvested energy from the perovskite
cell to power up its circuit and send data by backscattering RF signals.
The key innovations are in the customized cells.
They’re fabricated in layers, with perovskite material sandwiched between an electrode,
cathode, and special electron-transport layer materials.
This achieved about 10 percent efficiency, which is fairly high for still-experimental
perovskite cells.