Solar energy researchers at Oregon State University shine their scientific spotlight on materials with a crystal structure that was discovered nearly two centuries ago.
Not all materials with the structure, known as perovskites, are semiconductors. But perovskites based on a metal and a halogen are, and they have enormous potential as photovoltaic cells that can be much cheaper to make than the silicon-based cells that have owned the market since its inception in the 1
Enough potential, say researchers, to perhaps one day cut significantly into the share of fossil fuels in the energy sector.
John Labram from OSU College of Engineering is the corresponding author of two new articles on perovskite stability, in 2003 communication Physics and that Journal of Physical Chemistry Lettersand also contributed to a paper published on July 3, 2020, Science.
The study i Science, is led by researchers at Oxford University, revealed that a molecular additive – a salt based on the organic compound piperidine – improves the longevity of perovskite solar cells.
The results described in all three articles deepen the understanding of a promising semiconductor stemming from a discovery long ago of a Russian mineralogist. In the Ural Mountains in 1839, Gustav Rose came upon an oxide of calcium and titanium with an intriguing crystal structure and named it in honor of the Russian nobleman Lev Perovski.
Perovskite now refers to a variety of materials that share the original crystal lattice. Interest in them began to accelerate in 2009 after a Japanese scientist, Tsutomu Miyasaka, discovered that some perovskites are effective light absorbers.
“Because of their low cost, perovskite solar cells have the potential to underestimate fossil fuels and revolutionize the energy market,” Labram said. “However, a poorly understood aspect of this new class of materials is their stability under constant illumination, an issue that is a barrier to commercialization.”
Over the past two years, Labram’s research team at the School of Electrical Engineering and Computer Science has built unique experimental devices to study changes in conductivity for solar materials over time.
“In collaboration with the University of Oxford, we demonstrated that light-induced instability occurs for many hours, even in the absence of electrical contact,” he said. “The results help clarify similar results observed in solar cells and are key to improving the stability and commercial viability of perovskite solar cells.”
Solar cell efficiency is defined by the percentage of power from sunlight that hits a cell that is converted into useful electrical power.
Seven decades ago, Bell Labs developed the first practical solar cell. It had modest, by today’s standards, efficiency of 6% and was costly to do, but it found a niche in operating the satellites launched during the new days of space travel.
Over time, manufacturing costs decreased and efficiency increased, although most cells have not changed much – they still consist of two layers of almost pure silicon doped with an additive. Absorbing light, they use the energy from it to create an electric current across the intersection between them.
In 2012, one of Labram’s collaborators, Henry Snaith from Oxford, made a breakthrough discovery that perovskites could be used as the main component of solar cells, rather than as a sensitizer. This led to a storm of research activity and thousands of scientific articles were published each year on the subject. Eight years later, perovskite cells can now operate at 25% efficiency – making them, at least in the lab, at the level of commercial silicon cells.
Perovskite cells can be cheaply made from widely available industrial chemicals and metals and can be printed on flexible films of plastic and mass-produced. Conversely, silicon cells are rigid and made from thinly sliced slices of almost pure silicon in an expensive high temperature process.
One problem with perovskites is their tendency to be somewhat unstable as the temperature rises, and another is a moisture vulnerability – a combination that can cause cells to decompose. It is a problem for a product that must last for two or three decades in the open air.
“Generally, it requires a 25-year warranty to be able to sell a solar panel in the US and Europe,” Labram said. “What this really means is that the solar cell should show no less than 80% of its original performance after 25 years. The current technology, silicon, is pretty good for that. But silicon must be expensive to produce at temperatures above 2000 degrees Celsius under controlled conditions to form perfect, defective crystals so that they function properly. “
Perovskites, on the other hand, are very defective tolerants, Labram said.
“They can be dissolved in a solvent and then pressed at near room temperature,” he said. “This means that they could eventually be produced at a fraction of the cost of silicon and thus underestimate fossil fuels. But for this to happen, they must be certified with a 25-year warranty. This requires that we understand and improve the stability of these materials. “
One way to market is a tandem cell made of both silicon and perovskite that can transform more of the sunlight’s spectrum into energy. Lab tests on tandem cells have produced 28% efficiency, and efficiency in the mid-30s seems realistic, Labram said.
“Tandem cells can allow solar panel manufacturers to offer a performance beyond anything silicon alone can achieve,” he said. “The dual approach can help remove the barriers to perovskites entering the market, enabling perovskites to eventually function as standalone cells.”
Semi-transparent, perovskite films can also one day be used on windows or in greenhouses and convert some of the incoming sunlight into electricity while allowing the rest to pass through.
“When it comes to energy production, cost is the most important factor,” Labram said. “Silicone and perovskite now show about the same efficiency. In the long term, however, perovskite solar cells can be made at a fraction of the cost of silicon solar cells. And although history has shown us that climate change policy measures are largely ineffective, if you can produce electricity from renewable sources at a lower cost than fossil fuels, all you have to do is manufacture the product, then the market will take care of the rest. “
Reference: “A piperidinium salt stabilizes efficient metal halide-perovskite solar cells” by Yen-Hung Lin, Nobuya Sakai, Peimei Da, Jiaying Wu, Harry C. Sansom, Alexandra J. Ramadan, Suhas Mahesh, Junliang Liu, Robert DJ Oliver, Jongchul Lim, Lee Aspitarte, Kshama Sharma, PK Madhu, Anna B. Morales-Vilches, Pabitra K. Nayak, Sai Bai, Feng Gao, Chris RM Grovenor, Michael B. Johnston, John G. Labram, James R. Durrant, James M. Ball , Bernard Wenger, Bernd Stannowski and Henry J. Snaith, July 3, 2020, Science.
DOI: 10.1126 / science.aba1628