Internet of Things devices require batteries that must be replaced frequently, which can be problematic for long-term monitoring. Photovoltaic-powered sensors were developed that could potentially transmit data for years before they need to be replaced. Thin-film perovskite cells — known for their potential low cost, flexibility, and relative ease of fabrication — were mounted as energy-harvesters on inexpensive radio frequency identification (RFID) tags.
The cells could power the sensors in both bright sunlight and dimmer indoor conditions. 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. The sensors transmit data continuously at distances five times greater than traditional RFID tags, with no batteries required. Longer data-transmission ranges mean that one reader can be used to collect data from multiple sensors simultaneously.
Depending on factors 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.
Traditional solar cells are bulky and expensive to manufacture and are inflexible and cannot be made transparent, which can be useful for temperature-monitoring sensors placed on windows and car windshields. They are also designed to efficiently harvest energy only from powerful sunlight, not low indoor light. Perovskite cells, on the other hand, can be printed using 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 technology combines a low-cost power source with low-cost RFID tags — battery-free stickers used to monitor billions of products. 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 “back-scatter” 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 to which it is stuck. Traditionally, the tags harvest a little of the radio frequency energy sent by the reader to power up a 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 new 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.