A Bright Spot for Solar Windows Powered by Perovskites

To most of us, windows are little more than glass panes that let light in and keep bad weather out. But to some scientists, windows represent possibility-the chance to take passive parts of a building and transform them into active power generators.

Anthony Chesman is one researcher working to develop such solar windows. There are a lot of windows in the world that aren't being used for any other purpose than to allow lighting in and for people to see through," says Chesman, who is from Australia's national science agency, CSIRO. But really, there's a huge opportunity there in turning those windows into a space that can also generate electricity," he says.

The end goal that he and others envision is to create solar cells that are both thin and transparent enough to coat onto glass surfaces, thus enabling windows to absorb and convert energy from the sun into electricity.

With that aim in mind, Chesman and his collaborators from Monash University have invented a next-generation solar cell that he describes as a jump forward" in solar window technology. In a paper recently published in Nano Energy, the team details a new semitransparent perovskite solar cell with conversion efficiencies that match those of conventional silicon-based cells and that are more durable against heat and moisture than previous perovskite solar cells.

Solar windows have the potential to be a vast energy source. If every rooftop in the United States had solar panels installed, those panels could generate roughly 40 percent of the electricity the country needs, says Lance Wheeler, a solar power expert at the National Renewable Energy Laboratory (NREL) in Colorado. Now imagine if you could use not only rooftops but vertical facades such as walls and windows, too. We could reach nearly 100 percent total electricity generation on building-integrated solar alone," he says.

But taking existing solar panels and slapping them onto windows won't do the trick. Crystalline silicon, the type of cells that dominate today's photovoltaic market, are great at absorbing light from the sun. But to do so, they need to be thick and opaque-the very properties you try to avoid when making windows.

Conventional panels are designed to absorb as much light as possible without transmitting any of it," says Wheeler. More transparency means less energy generation. It's a direct trade-off."

That's where new solar cell materials come in, says material chemist Jacek Jasieniak from Monash University, who is a coauthor of the new paper. Solar windows are such an exciting space because it's an area that silicon natively just can't compete in."

In recent years, a handful of new materials has emerged, including quantum dots, organic photovoltaics, copper indium gallium selenide, and cadmium telluride. But perovskites, a type of semiconductor found in devices such as ultrasound machines and memory chips, are seen as a favorable alternative to silicon. Not only do perovskites convert sunlight to electricity at comparable efficiencies, they're also much easier to manufacture and can be made at a fraction of the cost and energy.

But perovskites fail to match up to silicon in one crucial aspect: life span. While most silicon solar cells last between 20 and 25 years, perovskite cells last maybe a few tens of hours," says Jasieniak. That's far from ideal given how disruptive and expensive it is to swap out windows in buildings.

We're only at the very beginning with solar windows." -Ioannis Papakonstantinou

The culprit is Spiro-OMeTAD, a material that's particularly sensitive to moisture and results in a perovskite cell with poor stability. A perovskite solar cell is structured like a sandwich: When light strikes the perovskite-the photo-absorbing middle layer of the cell-the perovskite generates charges that then travel to the cell's respective electrodes through upper and lower layers that surround the semiconductor. Spiro-OMeTAD forms one of these layers.

To overcome the problem with Spiro-OMeTAD, Jasieniak and his collaborators replaced it with VNPB, a material found in organic light-emitting diodes. The move was absolutely" a good idea, says NREL's Wheeler. VNPB improved the durability of the device, which is my largest concern for the technology."

Most important, the new solar cell demonstrated a power conversion efficiency of 17 percent, which is a really good efficiency," says Ioannis Papakonstantinou from the University College London, who is studying how quantum dots can be used in solar windows. Rooftop solar panels, by comparison, convert between 15 to 20 percent of incoming light into electricity.

They have made some quite significant progress," says Papakonstantinou. It's another milestone forward, but we're not talking about a final product. We're only at the very beginning with solar windows."

Chesman and his collaborators don't contest that characterization. They admit it could take as long as a decade for solar windows to hit the market. Scaling up is a major hurdle to commercialization-it's hard to produce uniform coatings over large areas, plus solar cells are made up of a number of very thin layers that are very sensitive, says Chesman. Slight changes to their thickness can cause significant changes to their optical properties.

Researchers will also have to figure out how to carry the electricity generated by solar windows to other parts of the building, as well as how to integrate the glass panes into a building's aesthetics.

Still, those are challenges worth tackling, says Chesman. My hope for the future is to see every window converted to a solar panel in some way," he says, to use the windows and turn an entire building into a power station."