Indoor solar cells are coming soon to gadgets near you

It’s 3:00 a.m., and you are blissfully deep in slumber. Suddenly, a sharp, short beep rudely wakes you. The low-battery warning of a carbon monoxide alarm going off in the middle of the night is annoying at best, scary at worst. And of course no spare batteries are around when you need them.

What if you never again had to change batteries in your smoke alarm, remote controls, or thermostat?

A future free of disposable batteries is the vision of California-based start-up Ambient Photonics. The company makes solar cells that generate electricity from the artificial light and dim natural light that illuminates homes and offices. “It doesn’t matter if it’s [light-emitting diodes (LEDs)] or incandescents or compact fluorescents or a little bit of light coming in from the window, or a mix of all of the above,” says cofounder and CEO Bates Marshall.

Ambient and a few other start-ups showcased their indoor light-harvesting cells embedded in headphones, bracelet-style health trackers, remote controls, and sensors at the Consumer Electronics Show in Las Vegas this January. Several other research groups and companies around the world are also developing indoor photovoltaic (PV) cells.

These cells aren’t the wimpy ones that ran your (or your parents’) solar-powered calculator in the 1970s. Those low-power devices, which used amorphous silicon, are cousins of the crystalline silicon solar cells that today dot rooftops and cover fields.

Makers of modern indoor PV cells are instead using technologies such as organic photovoltaics (OPVs), perovskite photovoltaics, and dye-sensitized solar cells (DSSCs). These technologies promise to be inexpensive, easy to make, light, and flexible. For a long time, these technologies were no match for silicon. But in the past 5 years, improvements in stability and lifetime, and steady growth in sunlight-to-electricity conversion efficiency have made these devices serious contenders for indoor use. These developments have coincided with the blossoming of the Internet of Things (IoT) era of ubiquitous electronics, connected devices, and smart home gadgets—all of which need power.

“Back in the 2000s, small electronics were just too power hungry to consider being light powered,” says Marina Freitag, a professor of chemistry at Newcastle University in England. “Now we have the best possible photovoltaics for low-light applications. And a much more energy-efficient IoT and communication protocol have come together.”

It’s the right time, in other words, for indoor PV devices to shine.

Billions of battery-powered electronic gadgets are in use around the world. Batteries have limited life-spans, and although they can be recycled, most end up in landfills. The European Union–funded EnABLES project estimates that about 78 million batteries used to power IoT devices will be dumped every day by 2025. Last year, the EU rolled out regulation aimed at making batteries more sustainable and reducing related waste and carbon footprint.

“Battery disposal is an emerging problem that is becoming obvious to electronic device companies,” Marshall says. “There is a fundamental interest in eliminating batteries or reducing battery mass.”

Indoor solar cells could be an answer. But not if they are based on silicon. That’s because of a fundamental property called bandgap. The bandgap determines the wavelengths of light that a PV material absorbs and converts to electricity.

Silicon’s narrow bandgap makes it good at converting the infrared photons in sunlight to electricity. But silicon loses its luster in the white glow of indoor light, which contains higher-energy photons, in the narrow visible wavelength spectrum of 400–700 nm, and just not enough of them. “Energy provided by low light indoors, where you have maybe 1% of the intensity of solar radiation, is just not enough to trigger anything,” Freitag says.

She believes the best technology to harvest indoor light is the modern DSSC. The original version was developed in 1988 by Michael Grätzel and Brian C. O’Regan, chemists at the Swiss Federal Institute of Technology, Lausanne (EPFL). These solar cells feature dye-coated semiconductor particles sandwiched between two electrodes. The dye absorbs light and creates pairs consisting of a negatively charged electron and a positively charged hole. The electrons go through the semiconductor to one electrode, while the positive charges flow through an electrolyte to the other, generating an electric current.

The technology is ideal for indoor use, Freitag says, because you can make a cocktail of dyes to harness light from various artificial sources, and the porous semiconductor structure soaks up light from all angles. Plus, DSSCs do not need high-end fabrication facilities, so they can be made cheaply anywhere in the world.

But the devices of the 1990s were inefficient. They easily deteriorated under humidity and high temperatures, so they remained consigned to the lab. And when lead halide perovskite solar cells made a splash in 2009 with efficiency leaps, many DSSC researchers jumped on the perovskite bandwagon, Freitag says. “But I’m a chemist, and I like to play with molecules. I didn’t want to play the perovskite game.”

Instead, she put DSSCs to use indoors in 2016, as a postdoctoral researcher with chemist Anders Hagfelt at EPFL. With carefully designed dyes and a new solid electrolyte based on copper complexes, she made a DSSC with an efficiency of almost 29% (Nat. Photonics 2017, DOI: 10.1038/nphoton.2017.60). She says her team at Newcastle has reached efficiencies close to 40% and power levels of 150 µW/cm². Those numbers are measured under bright fluorescent light at an intensity of 1,000 lux, however. The power output will be lower in the 500 lux intensity typical of offices or the 200 lux of living rooms.

Around the time Freitag was formulating new DSSCs, materials scientist Kethinni Chittibabu was doing the same at the Warner Babcock Institute for Green Chemistry. He and his team developed novel dye and electrolyte molecules that led to DSSCs with three times the power of conventional DSSCs and of old-school amorphous silicon solar cells.

That breakthrough was the start of Ambient Photonics’ story, Marshall says. “Dye-sensitized solar cells are an overnight sensation 30 years in the making,” he jokes. To get them to market for indoor use, he and Chittibabu have pushed for higher power and lower cost since cofounding the start-up in 2019.

A key decision was to use glass substrates instead of flexible plastic ones. Glass is optically clearer, Marshall says, which means more photons in, boosting efficiency and power output. Plus, he says, “glass has tremendous barrier properties. It’s very easy and low cost to keep out the bad stuff, like moisture and oxygen, and keep in the good stuff, which is our unique chemistry and materials.”

At its Scotts Valley factory, funded partly by Amazon’s Climate Pledge Fund, Ambient will soon screen print tens of millions of cells. The devices produce about 29 µW/cm² at 500 lux, Marshall says, with room for improvement unlike amorphous silicon, and a thumb-sized cell costs around $1. “At the end of the day, it’s electrons and dollars that matter,” for the ultimate goal of replacing batteries.

Ambient is working with Google on a new product, and Ambient’s cells will be in remote controls from major manufacturers this year, Marshall says. The PV cells come with a tiny energy-storing supercapacitor to ensure remote controls work in the dark, and going battery-free allows new modern styles. “About 800 million remote controls are sold every year,” he says. “Each typically has two batteries that last a year. We can make a beautiful 5 mm thick remote control that runs for years and doesn’t have to be recharged.”

Sweden-based Exeger is also betting on DSSCs but is going the flexible route to target a different market. The company makes bendable DSSCs that it’s embedding in headphones, dog harnesses, and bike helmets. Its customers include big brands such as 3M and Philips. Exeger’s cells harness both indoor and outdoor light and have a power density of 15.5 µW/cm² at 500 lux; the value of the indoor-only cells is about twice that.

DSSCs aren’t the only players in the indoor PV space. “Organics and perovskites also have a wider bandgap than crystalline silicon, and that ideally overlaps with typical indoor LED illumination,” says Uli Würfel, a physicist who studies organic and perovskite solar cells at the Fraunhofer Institute for Solar Energy Systems.

He and his colleagues recently did a head-to-head comparison of devices that were based on eight PV technologies under artificial indoor light (ACS Appl. Energy Mater. 2023, DOI: 10.1021/acsaem.3c01274). The star performers, with a record efficiency of 39.9% under 500 lux white LED light, were cells made of gallium indium phosphide, which is too expensive to embed into typical IoT devices. The runners-up were perovskites and OPVs, followed by DSSCs.

Start-up Epishine, also in Sweden, uses roll-to-roll printing technology to print its OPV-based indoor solar cells on plastic sheets. In November, the company opened a factory to produce 100 million modules. Epishine is working with various manufacturers on indoor-light-powered smoke detectors, temperature sensors, and electronic shelf labels, the small digital price displays that are used on retail shelving.

Compared with DSSCs and OPVs, perovskite solar cells are “the new kid in town,” says Anand Verma, CEO and cofounder of Perovskia Solar, a spin-off of the Swiss Federal Laboratories for Materials Science and Technology (Empa). “But it’s no longer a kid; it has matured. The material is very efficient.”

A big downside of perovskites is their instability, Verma says. He sought a fix during his tenure as a researcher at Empa, attempting to stabilize the materials with special additives and developing routes to inkjet print the materials on glass. His solution: a special molecule added to the perovskite ink that “bubble wraps the perovskite crystals for protection,” he says, but is thin enough to allow charges to tunnel through.

Perovskia’s soon-to-open factory in Aubonne, Switzerland, will print 1 million solar cells per year, each of which give about 40 µW/cm² at 500 lux, depending on the design. Last year at the Consumer Electronics Show, the company revealed a bracelet-style health tracker developed by French company Baracoda and powered by Perovskia’s cells. Other commercial products are in the works. And Perovskia soon plans to make flexible solar cells on thin, pliable glass films.

Critics point to the use of lead as a shortcoming of perovskites for indoor uses. But Verma says Perovskia’s cells have a mass fraction of less than 0.02% lead, which is under the 0.1% limit of the EU’s rules on the restriction of hazardous substances. “For the US, it’s a leaching test, and our material is a solid, so nothing leaches out,” he says.

The indoor PV market will be much smaller than the behemoth outdoor solar market, Würfel says. Perhaps indoor PV applications could spur development of DSSCs, OPVs, and perovskites for outdoor use. For now, these three technologies are still too unstable to be a match for silicon in the outdoor arena, but they seem poised for success in homes and offices.

“If you think about sensors or other IoT items, after a few years they may be replaced by newer ones because technology evolves,” he says. “So 10–15 years is enough stability for indoor PV, and all three technologies reach this lifetime.”

Indoor PV technologies have nothing more to prove in terms of performance, Freitag says. They are efficient, stable, and scalable. The need of the hour is for the fields of indoor PV and IoT to grow and evolve together.