These next-gen solar cells could make single-use batteries obsolete

When Bates Marshall, CEO of Ambient Photonics, looks at a square-inch solar cell, he sees a future free of disposable batteries. 

One square inch is roughly the size of the dye-sensitized solar cells that the Mill Valley, California–based startup is embedding in remote controls that Universal Electronics, a major manufacturer of these devices for companies around the world, plans to start selling in 2023.

It’s also small enough to fit on many of the sensors, transmitters and other digital devices that make up the internet-of-things market. While hundreds of millions of remote controls are shipped every year, the market for IOT devices is projected to be in the tens of billions of units over the coming decades, he noted.

Each of these devices needs a battery — and those batteries need to be replaced every year or so, if not more frequently. U.S. households throw away about 3 billion disposable batteries per year, according to Environmental Protection Agency estimates.

While those batteries are supposed to be recycled, more often than not, they’re just tossed in the trash. That’s an enormous if underappreciated amount of waste, not to mention ​“really bad outcomes in terms of landfill disposal issues,” Marshall said.

Ambient Photonics and other providers of an emerging class of next-generation ambient-light-harvesting photovoltaics hope to make disposable batteries obsolete. The trick is to prove that the latest advances on a 25-year-old solar technology can yield cells efficient, reliable and cheap enough for the task.

Dominant photovoltaic technologies like polysilicon only work well in full sunlight. Cheaper amorphous silicon solar cells work better in low light and have powered simple devices like desktop calculators for decades. But they aren’t efficient enough for more sophisticated electronic devices, Marshall said.

The new class of dye-sensitized solar cells (DSSCs) can provide enough power in indoor light conditions to keep more sophisticated devices running without new batteries, he said — or potentially allow manufacturers to replace batteries with capacitors, devices that hold energy in an electrical field and typically don’t require toxic materials.

Ambient Photonics is competing with a handful of other DSSC pioneers around the world to hit these milestones. One notable contender is Swedish startup Exeger, which has raised $170 million from investors including Japan’s SoftBank, according to CEO Giovanni Fili.

Ambient has raised $7.5 million from Mill Valley–based Cthulhu Ventures since its 2019 founding and is now in the midst of raising Series A funding to finance its first production line, Marshall said. In December it won approval to submit an application for a $162 million U.S. Department of Energy loan guarantee that would allow it to build a second factory, expected to be able to produce tens of millions of solar cells per year.

“We land our mission right in the middle of the decarbonization of the electronics industry,” Marshall said. ​“Very high power density and the potential for very low-cost manufacturing could create a billion-dollar market.”

From also-ran solar contender to battery replacement? 

Long-time watchers of solar technology may be forgiven for taking this claim with a grain of salt, given the checkered track record of past attempts to commercialize DSSC technologies. Since their invention in their modern form in 1988, DSSCs have largely been relegated to a class of photovoltaic (PV) technologies that haven’t been able to make the leap from laboratory to real-world application.

“They’ve always held a lot of promise,” Marshall said. ​“They’re low-cost to manufacture and made from low-cost, abundant materials.” But he acknowledges that they’ve also ​“had a very hard time displacing conventional PV.”

There are two main reasons for this. The first is DSSCs’ track record of problems with chemical stability and subpar performance in real-world applications. The second is that there are far more effective technologies for converting direct sunlight to electricity — namely, the polysilicon solar panels that have largely conquered the solar market.

That direct-sunlight world is where Marshall spent most of his 18-year career in the solar industry. But about three years ago, he was introduced to Kethinni Chittibabu, now Ambient Photonics’ CTO and co-founder, and a veteran of Konarka and G24 Innovations, two companies that tried — and ultimately failed — to commercialize DSSC solar cells.

In 2016, Chittibabu joined the Warner Babcock Institute for Green Chemistry, a Lowell, Massachusetts–based lab dedicated to commercializing technologies designed around biological principles, from asphalt-paving technologies to potential cures for the disease ALS. ​“They had the idea of breathing new life into a DSSC” by using a ​“biomimicry” approach to boost the capacity of dye-based cells, Marshall said.

John Warner, the institute’s co-founder and an award-winning chemist, described this breakthrough as something akin to recreating the molecular-level processes that living cells use to transfer electrons in the process of synthesizing energy.

“The unique structure of the chromophore,” which is the part of a molecule that controls the color of the dye used in the solar cell, paired with ​“the unique structure of the electrolyte, creates this unique orientation that allows it to be efficient” beyond the previous boundaries of DSSCs, Warner said in a March interview.

In fact, lab tests done by DSSC co-inventor Michael Grätzel at the Laboratory of Photonics and Interfaces at the Swiss research university École Polytechnique Fédérale de Lausanne ​“found it was one of the highest-performing dye-sensitized solar cell he’s ever evaluated,” Warner said.

Specifically, the cells were able to generate as much as three times the power produced by similarly sized amorphous silicon solar cells, the kind of solar cells that have adorned calculators and other portable electronics for years. They also outperformed conventional DSSC cells under dim light conditions below 100 lux, about the brightness of well-lit rooms in homes, as this chart from Ambient Photonics indicates.

Marshall highlighted other benefits of Ambient Photonics’ technology. These include its ability to generate electricity from light across the visible spectrum — a feature of its use of a combination of what are called ​“organic sensitizer molecules” in its dyes — and to continue to produce power even when part of the miniature solar cell is shaded, which is a quality of the ​“single monolithic” structure of the cells, he said.

Similar advances are being made by DSSC researchers around the world, Grätzel said in a February interview. For a long time, the best cells were only able to convert about 11 percent of the light they absorbed into electricity, he noted. But the latest cells are reaching between 30 and 40 percent conversion efficiencies in lab tests, he said.

“The technology has a lot of versatility,” Grätzel said, particularly for low-light environments where other technologies tend to perform poorly. 

DSSCs can also be designed to be flexible, he said. One example is Exeger’s approach: ​“You take titanium powder,” which is a good electricity conductor, ​“and put it into one of the paper mills they have in Sweden and compress it, and it makes a porous film” that can be converted to a photovoltaic cell that can be bent and shaped to fit a variety of form factors, he said.

Exeger is building its second factory in Stockholm and is embedding its PowerFoyle solar material in headphones, light-equipped bike helmets and wireless-tracking-enabled dog harnesses, CEO Giovanni Fili said in a February interview.

“We have free-form printing, in any form and any shape, and they work both indoors and outdoors,” he said. That’s led Exeger to focus on consumer electronics, a market where devices that don’t have to be charged or have their batteries replaced can command a premium price.

“I think with time, with consumer products that you use every day and are important to you, you will see a change in behavior,” Fili said. ​“People will become light-conscious. They will put their remote control on the windowsill” to charge up when they’re not using it.

Getting into real-world markets 

Ambient Photonics was spun out of the Warner Babcock Institute in 2019. CEO Marshall sees its deal with Universal Electronics as the first test of a mass-market application of DSSCs that are effective enough to reduce or replace the use of batteries.

The Scottsdale, Arizona–based company provides remote controls for many top-tier TV and set-top box brands, and sells about 130 million units per year, he said. The company’s latest remote controls come with backlit displays, voice recognition and processing capability that need quite a bit more electricity to operate reliably. That puts pressure on Ambient Photonics’ solar cells to meet the promise of controllers that can ​“work without battery replacement, for up to the useful life of the product,” as the company’s CES 2022 press materials claim.

Ambient Photonics has also been ​“very deliberate in our manufacturing strategy,” Marshall said. The startup has contracted with Manz, the German solar module manufacturing machinery producer, to design and produce its manufacturing and assembly lines. That relationship should allow it to scale up production quickly and efficiently to meet growing customer demand, according to the CEO.

The company intends to build its factories in the U.S. in order to protect its intellectual property, he said. That also puts it in the position of being eligible to seek a loan guarantee from the DOE’s Loan Programs Office, which has about $46 billion in lending authority to support a wide range of clean-energy technologies.

“It turns out there’s a huge sustainability win with the application of our products,” he said, in the form of ​“offsetting disposable batteries, offsetting lithium-ion, offsetting toxic material in landfills” and reducing the greenhouse gas emissions associated with battery manufacturing.

DSSCs are also environmentally safer than competing low-light solar cell technologies such as gallium arsenide or metal halide perovskite, said Marina Freitag, a Royal Society university research fellow studying DSSC materials at Newcastle University in the U.K. Those latter solar cells contain highly toxic materials ​“that limit the technologies tremendously” for use in consumer electronics or other devices handled by humans, she said.

Batteries, while ubiquitous, carry similar environmental drawbacks, she noted. ​“Even if we use rechargeable batteries, 95 percent of those end up in landfills.” And if batteries end up being used to power the tens of billions of IOT devices projected to be installed over the coming decade, ​“that will be a disaster for energy sustainability worldwide.”

Whether or not IOT devices can substitute ambient-light solar power for batteries will depend not only on the capability of the solar cells in question but also on how those devices are designed, Freitag said. A typical IOT sensor in ​“sleep mode” needs a steady amount of electricity equivalent to that provided from a square-centimeter of DSSC, she said. ​“To be operational, you’d need 10 square centimeters.”

In some cases, adding solar charging could actually allow devices to become more capable, if they’re designed to take advantage of it, Freitag noted. Most IOT devices today ​“process and transmit [as little] information…as possible to save battery life,” she said. ​“With indoor photovoltaics, you have to rethink energy consumption [because]…you don’t want to waste it.”

Grätzel agreed that DSSCs that are efficient, durable and cheap enough to meet a device’s power needs ought to be built into their design. 

”If you can do it with ambient light, why would you want to drag a battery everywhere you go?” he said. ​“This gives you, basically, eternal battery life.”