“Indoor solar power” sounds like a contradiction, but it may be coming to a gadget near you.

Advances in photovoltaics, the conversion of light to electricity, mean everyday low-energy devices from keyboards to cameras can be powered by indoor light.

This month the Australian start-up Halocell will begin producing flexible 7 centimetre-long photovoltaic strips it says generate enough power to replace the pair of disposable batteries in a TV remote, or the charger cable for a set of headphones.

It represents the first large-scale manufacturing in Australia of a technology that’s long been touted as the future of solar and a “holy grail” of ultra-cheap energy.

A new class of solar cells made out of a family of materials called perovskites has the potential to be more efficient, lower cost, and perform better in dim indoor light than conventional silicon cells.

But making the cells durable enough to sell has been a long and difficult road for researchers. After years of work, the first commercial perovskite cells are now emerging, designed for niche markets like satellites and consumer electronics.

The Halocell modules will each cost less than a dollar to make and the Wagga Wagga-based company has ambitious plans to produce millions per year, its CEO Paul Moonie said.

Last month two small Australian-made satellites were launched into low-Earth orbit, partly to test the performance of perovskite solar cells developed by the University of Sydney.

Anita Ho-Baillie, a professor of nanoscience who led the team that built the cells, said there was excitement about perovskite solar’s potential, and, despite set backs, a growing sense of optimism.

“Have we solved all the problems? I would say not. Are people still excited? Definitely.”

How perovskite solar works

To appreciate this excitement, let’s look at the basic principles of photovoltaics.

When sunlight strikes the type of standard solar cell we use on rooftops, electrons in the crystalline silicon semiconductor are ejected, creating a difference in electric potential between the top and bottom layers. 

The minimum energy required to excite an electron to jump free of their parent atom is known as the “bandgap”. If the energy of the sunlight is less than the bandgap, nothing will happen. The light will not be absorbed by the electron, and the panel won’t generate electricity.

If the photon’s energy is greater, the excess energy will be lost as heat.

A material’s bandgap ultimately determines what wavelengths of light it can convert to electricity, and therefore its maximum theoretical energy efficiency.

Silicon, which has a bandgap of 1.1 electron volts, has a maximum efficiency of about 32 per cent.

This means about a third of the energy that strikes the panel is converted into electricity. Most commercial solar panels have an efficiency of about 20 per cent.

Given how much power solar panels generate around the world, the efficiency figure is very important. A fraction of a percentage point equates to vast generation output. 

This explains the excitement around perovskite solar cells, which can be tweaked and stacked to achieve about 50 per cent efficiency.

Perovskites are a family of materials with a unique crystal structure that conducts electricity. 

By tinkering with their composition, scientists tune the bandgap, and ultimately optimise different perovskite cells to absorb various ranges of wavelengths. They then stack these complementary cells on top of each other to achieve a higher overall efficiency, Professor Ho-Baillie said.

“We can make the cell more efficient for converting high energy photons or low energy photons. You can tune it to 1.1eV [electronvolts] all the way to 2eV, that’s not a problem.”

“High efficiency means you can get more power with a smaller area.”

A ‘miracle material’ stuck in the lab

Despite their promise, perovskite solar cells are still mostly in research labs.

Since the first perovskite cell in 2009, there’s been speculation the “miracle material” will soon replace silicon and usher in a new era of low-cost, abundant solar power. It was hoped transparent perovskite cells would be pasted onto windows, or added as a flexible spray-on “skin” to clothing.

But commercialisation was held back by ongoing durability problems, with various designs struggling with high temperatures, humidity, UV light and oxygen. Perovskite cells also contain lead, which is toxic.

Meanwhile, production of conventional silicon cells has boomed. Through the 2010s the price of silicon solar fell 90 per cent.

Several perovskite companies folded, including the Queanbeyan-based Greatcell (formerly Dyesol).

Once considered a great hope of the Australian solar industry, Greatcell was among the world’s leading perovskite developers, but could not compete with the low-cost silicon solar panels for full-sun applications despite a $6 million government grant.

Paul Moonie, a former employee of the company, saw an opportunity.

“When Greatcell collapsed myself and some investors bought its equipment and patents and set up operations in Wagga,” Mr Moonie, now Halocell CEO, said.

Greatcell’s collapse taught him one lesson: don’t compete with silicon outdoors, so Halocell focused on indoor applications.

“We knew the indoor market was a much easier environment to operate in than full-sun outdoor.”

‘We want to be around 70 million units a year’

Conventional silicon solar’s narrow bandgap is good at converting the infrared photons in sunlight to electricity, but works terribly under the narrow spectra of indoor light.

The cells operate at less than 5 per cent efficiency in the white glow of indoor light, whereas perovskite cells can be tuned to generate electricity optimally in these conditions.

Halocell’s perovskite cells operate at 27 per cent efficiency in low indoor light (50 lux) and 22 per cent in bright indoor light (1000 lux), according to the company’s product specifications.

The modules are less than a millimetre thick, can be printed for low-cost on long continuous rolls of plastic, operate from -10 to 60 degrees Celsius, and come with a five-year warranty for indoor use. Durability has been improved (to the standard for indoor use) with the addition of a protective coating.

According to Mr Moonie, a single module produces enough power to replace the batteries in a TV remote or power a set of headphones, and had an acceptable lead content based on guidance from the US Environmental Protection Agency.

“We’re starting out at 8,000 a year and will be constantly increasing that output,” he said. 

“Eventually we want to be around 70 million units a year.

Hongxia Wang, a perovskites expert at the Queensland University of Technology who led a government-funded co-operative research project with Halocell, said it was the only Australian company manufacturing perovksite cells at scale.

“I think the initiative of development of indoor power by Halocell is quite significant,” Professor Wang said.

Professor Ho-Baillie’s team at the University of Sydney is also working to commercialise perovskite solar cells with the start-up SunDrive, but they are yet to announce manufacturing plans.

A low-light alternative to perovskites

Perovskites aren’t the only emerging technology for indoor solar.

Dye-sensitised solar cells (DSSC) contain a porous layer of titanium dioxide nanoparticles covered in a dye that absorbs incoming photons from the sun.

The excited electrons in the dye flow through an external circuit, generating electricity.

It’s analogous to how plants use photosynthetic pigments like chlorophylls to broaden the range of light they can absorb.

DSSCs are relatively inefficient in full sunlight, but perform better than conventional silicon cells in low light. They can be printed as a thin film on a range of materials, and are low-cost and relatively durable.

A number of start-ups as well as companies such as Google and Adidas are integrating the cells into a range of consumer products such as bags, bracelet-style health trackers, bike helmets, remote controls, sensors and “self-charging” headphones. 

More efficient solar panels means cheaper electricity

But for solar cell makers the real prize is success under the sun.

Mr Moonie hopes that once perovskite has proved itself indoors, and solved some of its remaining durability issues, it will compete with low-cost silicon solar outdoors.

“I think we’ll be pumping out rooftop solar by 2030,” Mr Moonie said.

Martin Green, the University of NSW professor credited with partly inventing the technology used in most of today’s solar cells, said low-cost commercial perovskite-silicon tandem cells were the “holy grail” for many researchers. 

Researchers are stacking perovksite cells with conventional silicon cells, to make a perovskite-silicon tandem cell that works better in full sunlight.

The thin film of perovskite cells absorb wavelengths of light the silicon does not, to boost the tandem cell’s efficiency. In theory, these cells can be up to 40 per cent efficient.

“Triple-junction” cells with an extra stack of perovskite could be up to 50 per cent efficient.

The UK start-up Oxford PV aims to move to high-volume manufacturing of silicon-perovskite tandem cells in coming years.

MIT-spinoff Swift Solar in the US also plans to build a factory to manufacture perovskite tandem photovoltaics.

After a long, hard road, stable, durable perovskites may be almost here.

“There are at least 20 companies around the world that are seriously looking at stacking silicon and perovskite, including the big solar manufacturers,” Professor Ho-Baillie said.

Professor Ho-Baillie’s University of Sydney team last month became the first Australian group to develop a perovskite-silicon tandem cell that can hit 30 per cent efficiency.

Her next challenge is a tandem cell that hits 40 per cent efficiency. The cutting-edge design may initially be used for satellites, and then, ultimately, on rooftops.

“If I have enough resources for funding and [we’re] well supported, I’m pretty optimistic we can do it,” she said.