Photovoltaic Sensors For IoT That Transmit Data Using Less Power

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The sensors are based on low-cost, flexible perovskite cells that operate under bright and dim lighting conditions 

Image courtesy MIT

It is estimated that by 2025, the number of IoT (Internet of Things) devices will rise upto 75 billion worldwide. And that would require frequent battery replacements for the efficient working of various IoT based devices.

In order to reduce this hassle, researchers at the MIT (Massachusetts Institute of Technology) have developed photovoltaic sensors that can transmit data over several years before getting replaced. The cells in these sensors provide power under conditions like bright and dim sunlight and enable data-transmission over greater distances. This feature also allows the integration of multiple sensors onto a single RFID (Radio Frequency Identification) tag.

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“In the future, there could be billions of sensors all around us. With that scale, you’ll need a lot of batteries that you’ll have to recharge constantly. But what if you could self-power them using the ambient light? You could deploy them and forget them for months or years at a time,” says Sai Nithin Kantareddy, a PhD student in the MIT Auto-ID Laboratory. “This work is basically building enhanced RFID tags using energy harvesters for a range of applications.”

Inexpensive energy harvesters

Firstly, several thin-film perovskite cells were placed on inexpensive RFID tags. Perovskite cells are known for their low cost, flexibility and ease of fabrication.

These cells were fabricated in layers and sandwiched between an electrode, cathode and special electron-transport layer materials. This enabled each of the cells to be tuned for optimal “bandgap” (electron-moving property that dictates a cell’s performance in different lighting conditions). All of this helped in achieving an efficiency of 10 per cent, a fairly high number for such perovskite cells that were under development.

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Then these cells were combined with a low-cost power source such as RFID tags. These tags are tiny battery-free stickers that are equipped with ultra-high-frequency antennas. RFID tags operate using “backscatter”, a communication technique through which data is transmitted by reflecting modulated wireless signals off the tag and back to a reader. This reader is similar to a Wi-Fi router which pings the tag and backscatters the information containing signal.

By harvesting energy from the perovskite cells, the custom-built RFID tag got powered and data was sent by backscattering RF signals over distances five times greater than traditional RFID tags – without any batteries required. This ensured simultaneous data collection from multiple sensors spread over a wide range.

Applications include tracking cargo in supply chains, monitoring soil, and monitoring the energy used by equipment in buildings and homes.

Results

The cells were successful in generating 4.3 volts of electricity during sunlight. That’s enough to power up a circuit (about 1.5 volts) and transmit data upto 5 metres. These cells performed equally well in indoor lighting of 45 minutes, achieving efficiencies between 18.5 per cent and 21.4 per cent.

Benefits

As compared to traditional solar cells, that are bulky and expensive to manufacture, Perovskite cells are thin, flexible and cost-effective to manufacture. Perovskite cells also have the advantage to efficiently harvest energy from both sunlight and low indoor light.

Future aims

The researchers aim to create environmental-monitoring sensors, such as humidity, pressure, vibration, and pollution and deploy them at a large scale for long term data collection.

Ian Mathews, Postdoctoral fellow, Department of Mechanical Engineering – MIT, said, “The perovskite materials we use have incredible potential as effective indoor-light harvesters. Our next step is to integrate these same technologies using printed electronics methods, potentially enabling extremely low-cost manufacturing of wireless sensors.”

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The above research has been published in the journal Advanced Functional Materials and IEEE Sensors.

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