Indoor solar cells could replace all the batteries in your gadgets

We hear a lot about how solar panels and solar energy solutions are being used outdoors to help make global energy production more sustainable and fight climate change, but what about solar solutions for indoors where it’s darker?

Billions of internet-connected devices now adorn our walls and ceilings, sensing, monitoring, and transmitting data to smartphones and far-flung servers, and as the number of gadgets we all have proliferate so too does our electricity demand and our hunger for household batteries - most of which wind up in landfills polluting the Earth. To combat this enormous amount of waste researchers have now created a new type of indoor solar cell that can harvest energy from the indoor lights we’re already using.

Obviously the dominant material used in today’s solar cells, crystalline silicon, doesn’t perform as well under lamps as it does beneath the blazing sun. But emerging alternatives such as perovskite solar cells may prove to be significantly more efficient at converting artificial lighting to electrical power.

Now a group of researchers from Italy, Germany, and the US have managed to develop a new form of flexible perovskite solar cell that’s been made specifically for indoor devices. In recent tests, their thin-film solar cell delivered power-conversion efficiencies of more than 20 percent under 200 lux, the typical amount of illuminance in homes. That’s about triple the indoor efficiency of polycrystalline silicon, according to Thomas Brown, a project leader and engineering professor at the University of Rome Tor Vergata.

The results, published in late April, are the highest reported efficiencies for any indoor flexible solar cell technology, Brown said. By comparison so called Amorphous silicon technology, which is used in commercial products like calculators, is only about 9 percent efficient on flexible layers.

“There’s a revolution happening with the Internet of Things,” Brown said. He imagined integrating the ultralight cells into wireless thermostats, fire alarms, security cameras, and other low-power sensors. When solar cells are paired with an energy storage device, “that would diminish or eliminate the use of replaceable batteries,” he says.

Perovskites, a rapidly growing area of photovoltaic research, are a hybrid of organic compounds, metals, and halides, with crystal structures mirroring those of the mineral calcium titanium oxide. Companies and universities are racing to turn the low-cost, easy-to-produce materials into durable and stable solar cells, in the hopes of making renewable energy more affordable and abundant worldwide.

In recent years, Brown’s team in Rome has made flexible perovskite solar cells using low-temperature techniques and in 2018 they began applying the solar cells to a 100-micrometer-thick piece of flexible glass coated with Indium Tin Oxide (ITO). The Italian group then exposed the solar cells to varying intensities of indoor lighting.

They found that, in shelf-life tests, un-encapsulated solar cells retained 80 percent of their initial power-conversion efficiency for more than 100 days. However, performance declined more rapidly after that period. In order to integrate into IoT devices, indoor photovoltaic cells will need to last at least five to ten years, around the time people typically upgrade electronic devices, Brown said.

Perovskite solar cells in general can degrade when exposed to moisture, harsh temperatures, ultraviolet light, oxygen, and other elements. Such challenges are amplified when the technology is used outside, where the environment is less forgiving, and where solar panels are expected to last for decades.

“I often tell people, in the context of photovoltaics, your iPhone is a wonderful device, but if you put it on your roof for 30 years, I don’t think you expect that it’s still going to be functioning,” said Joe Berry, a senior research scientist at the US Department of Energy’s National Renewable Energy Laboratory. “That’s what we do with solar cells, and we expect them to survive for 30 years.”

Berry is director of the US Manufacturing of Advanced Perovskites Consortium, which brings government labs, academic institutions, and companies together to speed up development of the novel solar technology. With small indoor devices likely to hit shelves first, Berry said early uses of perovskites could guide researchers and manufacturers in developing cells for higher-volume, larger-scale outdoor applications.

“As soon as you’re able to get to market, the things you learn are just very different than what you can learn being in the lab,” he said.

If and when the technology advances from the lab the teams behind them will undoubtedly have ample opportunities to harvest energy in the real world. By some estimates, about 75 billion Internet of Things devices will have been installed worldwide by 2025.