As per expert estimates, by 2025, the worldwide number of IoT, or the Internet of Things, could rise to 75 billion. However, most IoT devices have sensors that run on batteries. Replacing these batteries can be a problem, especially for long-term monitoring.
Researchers at the Massachusetts Institute of Technology have now produced photovoltaic-powered sensors. These sensors can transmit data potentially for several years, before needing a replacement. The researchers achieved this by mounting thin-film perovskite cells as energy harvesters on low-cost RFID or radio-frequency identification tags. Perovskite cells are notoriously inexpensive, highly flexible, and relatively easy to fabricate.
According to the researchers, the future will have billions of sensors all around. Rather than power the sensors with batteries, the photovoltaic-powered sensors could use ambient light. It would be possible to deploy them and then forget them for months at a time or even years.
In a pair of papers the researchers have published, they have described the process of using sensors to monitor indoor and outdoor temperatures continuously over many days. No batteries were necessary for the sensors to transmit a continuous stream of data over a distance greater than five times that traditional RFID tags could. The significance of a long data transmission range means the user can employ one reader for collecting data simultaneously from multiple sensors.
Depending on the presence of moisture and heat in the environment, the sensors can remain under a cover or exposed for months or years before they degrade enough requiring a replacement. This can be valuable for applications requiring long-term sensing indoors as well as outdoors.
For creating self-powered sensors, many other researchers have tried solar cells for IoT devices. However, in most cases, these were the traditional solar cells and not the perovskite type. Although traditional solar cells can be long-lasting, efficient, and powerful under certain conditions, they are rather not suitable for universal IoT sensors.
The reason is, traditional solar cells are expensive and bulky. Moreover, they are inflexible and non-transparent—suitable and useful for monitoring the temperature on windows and car windshields. Most designs of traditional solar cells allow them to effectively harvest energy from bright sunlight, but not from low levels of indoor light.
On the other hand, it is possible to print perovskite cells using easy roll-to-roll manufacturing techniques costing only a few cents each. They can be made into thin, flexible, and transparent sheets. Furthermore, they can be tuned to harvest energy from outdoor or indoors lighting.
Combining a low-cost RFID tag with a low-cost solar power source makes them battery-free stickers. The combination allows for monitoring billions of products all over the world. Adding three to five cents more, it is possible to add tiny antennas working at ultra-high frequencies to the stickers.
Using a communication technique known as backscatter, RFID tags can transmit data. They reflect the modulated wireless signals from the tag and send it back to their reader. The reader is a wireless device, very similar to a Wi-Fi router, and it pings the tag. In turn, the tag powers up and using backscattering, sends a unique signal with information about the product on which it is stuck.