Indoor solar cells to power the internet of things

Credit: Thor Balkhed

Swedish and Chinese scientists have developed organic solar cells optimised to convert ambient indoor light to electricity. The power they produce is low, but is probably enough to feed the millions of products that the internet of things will bring online.

As the internet of things expands, it is expected that we will need to have millions of products online, both in public spaces and in homes. Many of these will be the multitude of sensors to detect and measure moisture, particle concentrations, temperature and other parameters. For this reason, the demand for small and cheap sources of renewable energy is increasing rapidly, in order to reduce the need for frequent and expensive battery replacements.

This is where organic solar cells come in. Not only are they flexible, cheap to manufacture and suitable for manufacture as large surfaces in a printing press, they have one further advantage: the light-absorbing layer consists of a mixture of donor and acceptor materials, which gives considerable flexibility in tuning the solar cells such that they are optimised for different spectra – for light of different wavelengths.

New combination of materials

Researchers in Beijing, China, led by Jianhui Hou, and Linköping, Sweden, led by Feng Gao, have now together developed a new combination of donor and acceptor materials, with a carefully determined composition, to be used as the active layer in an organic solar cell. The combination absorbs exactly the wavelengths of light that surround us in our living rooms, at the library and in the supermarket.

The researchers describe two variants of an organic solar cell in an article in Nature Energy, where one variant has an area of 1 cm2 and the other 4 cm2. The smaller solar cell was exposed to ambient light at an intensity of 1000 lux, and the researchers observed that as much as 26.1% of the energy of the light was converted to electricity. The organic solar cell delivered a high voltage of above 1 V for more than 1000 hours in ambient light that varied between 200 and 1000 lux. The larger solar cell still maintained an energy efficiency of 23%.

“This work indicates great promise for organic solar cells to be widely used in our daily life for powering the internet of things”, says Feng Gao, senior lecturer in the Division of Biomolecular and Organic Electronics at Linköping University.

Design rules

”We are confident that the efficiency of organic solar cells will be further improved for ambient light applications in coming years, because there is still a large room for optimization of the materials used in this work”, Jianhui Hou, professor at the Institute of Chemistry, Chinese Academy of Sciences, underlines.

The result is a further advance in research within the field of organic solar cells. In the summer of 2018, for example, the scientists, together with colleagues from a number of other universities, published rules for the construction of efficient organic solar cells (see the link given below). The article collected 25 researchers from seven universities, and was published in Nature Materials. The research was led by Feng Gao. These rules have proven to be useful along the complete pathway to efficient solar cell for indoor use.

Spin off company

The Biomolecular and Organic Electronics research group at Linköping University, under the leadership of Olle Inganäs (now professor emeritus), has been for many years a world-leader in the field of organic solar cells. A few years ago, Olle Inganäs and his colleague Jonas Bergqvist, who is co-author of the articles in Nature Materials and Nature Energy, founded, and are now co-owners of a company, which focusses on commercialising solar cells for indoor use.

Learn more: Welcome indoors, solar cells

 

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Smart devices that do not use batteries or require charging move much closer

An RFID tag is modified by cutting out a small part its antenna (silver ribbon) and placing a small push-button switch on it. If several modified tags are mounted on a surface, they can act as a wireless keypad.

Researchers at the University of Waterloo have taken a huge step towards making smart devices that do not use batteries or require charging.

These battery-free objects, which feature an IP address for internet connectivity, are known as Internet of Things (IoT) devices. If an IoT device can operate without a battery it lowers maintenance costs and allows the device to be placed in areas that are off the grid.

Many of these IoT devices have sensors in them to detect their environment, from a room’s ambient temperature and light levels to sound and motion, but one of the biggest challenges is making these devices sustainable and battery-free.

Professor Omid Abari, Postdoctoral Fellow Ju Wang and Professor Srinivasan Keshav from Waterloo’s Cheriton School of Computer Science have found a way to hack radio frequency identification (RFID) tags, the ubiquitous squiggly ribbons of metal with a tiny chip found in various objects, and give the devices the ability to sense the environment.

“It’s really easy to do,” said Wang. “First, you remove the plastic cover from the RFID tag, then cut out a small section of the tag’s antenna with scissors, then attach a sensor across the cut bits of the antenna to complete the circuit.”

In their stock form, RFID tags provide only identification and location. It’s the hack the research team has done — cutting the tag’s antenna and placing a sensing device across it — that gives the tag the ability to sense its environment.

To give a tag eyes, the researchers hacked an RFID tag with a phototransistor, a tiny sensor that responds to different levels of light.

By exposing the phototransistor to light, it changed the characteristics of the RFID’s antenna, which in turn caused a change in the signal going to the reader. They then developed an algorithm on the reader side that monitors change in the tag’s signal, which is how it senses light levels.

Among the simplest of hacks is adding a switch to an RFID tag so it can act as a keypad that responds to touch.

“We see this as a good example of a complete software-hardware system for IoT devices,” Abari said. “We hacked simple hardware — we cut RFID tags and placed a sensor on them. Then we designed new algorithms and combined the software and hardware to enable new applications and capabilities.

“Our main contribution is showing how simple it is to hack an RFID tag to create an IoT device. It’s so easy a novice could do it.”

Learn more: Batteryless smart devices closer to reality

 

 

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Turning everyday objects into smart, connected devices inexpensively

Printed thin, flexible LiveTag tags in comparison with a piece of photo paper (far left). Photos courtesy of Xinyu Zhang

Engineers have developed printable metal tags that could be attached to everyday objects and turn them into “smart” Internet of Things devices.

The metal tags are made from patterns of copper foil printed onto thin, flexible, paper-like substrates and are made to reflect WiFi signals. The tags work essentially like “mirrors” that reflect radio signals from a WiFi router. When a user’s finger touches these mirrors, it disturbs the reflected WiFi signals in such a way that can be remotely sensed by a WiFi receiver, like a smartphone.

The tags can be tacked onto plain objects that people touch and interact with every day, like water bottles, walls or doors. These plain objects then essentially become smart, connected devices that can signal a WiFi device whenever a user interacts with them. The tags can also be fashioned into thin keypads or smart home control panels that can be used to remotely operate WiFi-connected speakers, smart lights and other Internet of Things appliances.

“Our vision is to expand the Internet of Things to go beyond just connecting smartphones, smartwatches and other high-end devices,” said senior author Xinyu Zhang, a professor of electrical and computer engineering at the UC San Diego Jacobs School of Engineering and member of the Center for Wireless Communications at UC San Diego. “We’re developing low-cost, battery-free, chipless, printable sensors that can include everyday objects as part of the Internet of Things.”

Zhang’s team named the technology “LiveTag.” These metal tags are designed to only reflect specific signals within in the WiFi frequency range. By changing the type of material they’re made of and the pattern in which they’re printed, the researchers can redesign the tags to reflect either Bluetooth, LTE or cellular signals.

The tags have no batteries, silicon chips, or any discrete electronic components, so they require hardly any maintenance—no batteries to change, no circuits to fix.

The team presented their work at the recent USENIX Symposium on Networked Systems Design and Implementation Conference.

Smart tagging

LiveTag music player controller

As a proof of concept, the researchers used LiveTag to create a paper-thin music player controller complete with a play/pause button, next track button and sliding bar for tuning volume. The buttons and sliding bar each consist of at least one metal tag so touching any of them sends signals to a WiFi device. The researchers have so far only tested the LiveTag music player controller to remotely trigger a WiFi receiver, but they envision that it would be able to remotely control WiFi-connected music players or speakers when attached to a wall, couch armrest, clothes, or other ordinary surface.

The researchers also adapted LiveTag as a hydration monitor. They attached it to a plastic water bottle and showed that it could be used to track a user’s water intake by monitoring the water level in the bottle. The water inside affects the tag’s response in the same way a finger touch would—as long as the bottle is not made of metal, which would block the signal. The tag has multiple resonators that each get detuned at a specific water level. The researchers imagine that the tag could be used to deliver reminders to a user’s smartphone to prevent dehydration.

Future applications

On a broader scope, Zhang envisions using LiveTag technology to track human interaction with everyday objects. For example, LiveTag could potentially be used as an inexpensive way to assess the recovery of patients who have suffered from stroke.

“When patients return home, they could use this technology to provide data on their motor activity based on how they interact with everyday objects at home—whether they are opening or closing doors in a normal way, or if they are able to pick up bottles of water, for example. The amount, intensity and frequency of their activities could be logged and sent to their doctors to evaluate their recovery,” said Zhang. “And this can all be done in the comfort of their own homes rather than having to keep going back to the clinic for frequent motor activity testing,” he added.

Another example is tagging products at retail stores and assessing customer interest based on which products they touch. Rather than use cameras, stores could use LiveTag as an alternative that offers customers more privacy, said Zhang.

Next steps

The researchers note several limitations of the technology. LiveTag currently cannot work with a WiFi receiver further than one meter (three feet) away, so researchers are working on improving the tag sensitivity and detection range. Ultimately, the team aims to develop a way to make the tags using normal paper and ink printing, which would make them cheaper to mass produce.

Learn more: These tags turn everyday objects into smart, connected devices

 

 

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