The crystalline structure discovered almost 200 years ago could be key to the solar cell revolution

Solar energy researchers at Oregon State University are focusing their scientific attention on materials with a crystalline structure discovered almost two centuries ago.

Not all materials with the structure, known as perovskites, are semiconductors. But metal-based and halogen-based perovskites are, and have enormous potential as photovoltaic cells that could be much less expensive to manufacture than the silicon-based cells that have owned the market since their inception in the 1950s.

The researchers say there is enough potential to perhaps one day make significant inroads into fossil fuel involvement in the energy sector.

John Labram of the OSU College of Engineering is the corresponding author of two recent articles on perovskite stability, in Communications Physics and the Physical Chemistry Chart Magazineand also contributed to an article published today in Science.

The study in Science, led by researchers at the University of Oxford, revealed that a molecular additive, a salt based on the organic compound piperidine, greatly improves the longevity of perovskite solar cells.

The findings described in all three documents deepen understanding of a promising semiconductor that stems from a discovery long ago by a Russian mineralogist. In the Ural Mountains in 1839, Gustav Rose found a calcium and titanium oxide with an intriguing crystal structure and named it after the Russian nobleman Lev Perovski.

Perovskita now refers to a range of materials that share the original’s crystal lattice. Interest in them began to accelerate in 2009 after a Japanese scientist, Tsutomu Miyasaka, discovered that some perovskites are effective light absorbers.

“Due to their low cost, perovskite solar cells have the potential to undermine fossil fuels and revolutionize the energy market,” said Labram. “However, a poorly understood aspect of this new class of materials is its stability under constant lighting, a problem that represents a barrier to commercialization.”

In the past two years, Labram’s research group at the School of Electrical Engineering and Computer Science has built unique experimental devices to study changes in the conductance of solar materials over time.

“Working as a team with the University of Oxford, we demonstrate that light-induced instability occurs for many hours, even in the absence of electrical contact,” he said. “The findings help clarify similar results seen in solar cells and are the key to improving the stability and commercial viability of perovskite solar cells.”

The efficiency of the solar cell is defined by the percentage of energy from sunlight hitting a cell that is converted into usable electrical energy.

Seven decades ago, Bell Labs developed the first practical solar cell. It had a modest efficiency, by today’s standards, of 6% and was expensive to manufacture, but it found a niche in powering the satellites launched during the nascent days of the space race.

Over time, manufacturing costs decreased and efficiencies increased, even though most cells haven’t changed much, they still consist of two layers of near-pure silicon doped with an additive. By absorbing light, they use its energy to create an electric current through the junction between them.

In 2012, one of Labram’s collaborators Henry Snaith of Oxford discovered that perovskites could be used as the main component of solar cells, rather than just as a sensitizer. This led to a storm of research activity and thousands of scientific articles being published each year on the subject. Eight years of research later, perovskite cells can now operate at 25% efficiency, making them, at least in the laboratory, on par with commercial silicon cells.

Perovskite cells can be manufactured inexpensively from commonly available metal and industrial chemicals and can be printed on flexible, mass-produced plastic films. Silicon cells, by contrast, are rigid and made of thinly sliced ​​wafers of near-pure silicon in an expensive, high-temperature process.

One problem with perovskites is their tendency to be somewhat unstable when temperatures rise, and another is vulnerability to moisture, a combination that can cause cells to break down. That is a problem for a product that needs to last two or three decades outdoors.

“In general, a 25-year warranty is required to sell a solar panel in the United States and Europe,” said Labram. “What that really means is that the solar cell should show no less than 80% of its original performance after 25 years. Current technology, silicon, is good enough for that. But silicon must be produced expensively in temperatures above 2,000 degrees Celsius under controlled conditions, to form flawless, flawless crystals to function properly. “

Perovskites, on the other hand, are highly tolerant of defects, Labram said.

“They can be dissolved in a solvent and then printed at room temperature,” he said. “This means that they could eventually be produced at a fraction of the cost of silicon and thus undermine fossil fuels. However, for this to happen, they must be certifiable with a 25-year warranty. This requires us to understand and improve stability of these materials. “

One way to market is a tandem cell made of silicon and perovskites that could convert more energy from the solar spectrum into energy. Laboratory tests on tandem cells have produced efficiencies of 28%, and efficiencies in the mid-1930s appear realistic, Labram said.

“Tandem cells could allow solar panel producers to deliver performance beyond what silicon alone could achieve,” he said. “The dual approach could help remove the barrier for perovskites entering the market, on the path for perovskites to eventually act as independent cells.”

Perovskite semi-transparent films can also be used one day on windows or in greenhouses, converting some of the incoming sunlight to electricity and letting the rest pass.

“When it comes to power generation, cost is the most important factor,” said Labram. “Silicon and perovskites now show approximately the same efficiency. However, in the long term, perovskite solar cells have the potential to be manufactured at a fraction of the cost of silicon solar cells. And although history has shown us that political action on climate change is largely ineffective, if you can generate electricity from renewable sources at a lower cost than fossil fuels, all you have to do is make the product, then the market will take care of the rest ” .