Perovskite solar cells could be the next big thing to touch off another downward surge in the cost of solar energy, and a Swiss research team is reporting that it is on track to make it happen. The team has developed a low cost way to manufacture thin film solar cells that use a layer of “molecular soccer balls” to hold perovskite in place.
The new perovskite solar cell comes from Empa, the Swiss Federal Laboratories for Materials Science and Technology. When the Empa team incorporated it into a “tandem” thin film solar cell, it reported a solar conversion efficiency of 20.5 percent. While that’s peanuts compared to the best silicon solar cells, the relative simplicity and low cost of manufacturing the new cell would make it a bargain.
Perovskite Solar Cells
For those of you new to the topic, perovskite refers to a class of synthetic crystals based on the structure of the naturally occurring mineral perovskite. Perovskites are relatively cheap and easy to manufacture, so if their solar conversion abilities could be harnessed, perovskite solar cells could give their silicon cousins a run for the money.
The problem is that perovskite tends to fall apart, especially in humid conditions. In order to use perovskite in a solar cell exposed to the elements you have to figure out how to protect it, which translates into higher costs. So far the conventional solution has been to use lead as a platform for perovskite solar cells. Lead happens to be cheap, but it raises a raft of environmental and public health issues — (remember lead paint and leaded gasoline) that could limit the widespread application of perovskite solar cells.
The good news is that “unleaded” perovskite solutions are emerging. CleanTechnica has also been following the use of tin as an enabler for perovskite solar cells, and last month we noticed that work is progressing on so-called third generation “hot-carrier” solar cells using perovskite.
Lead-enabled perovskite solar cell research is still a hot topic, though, and that brings us to the latest development.
The Empa Perovskite Solar Cell Solution
In order to get a grasp on the significance of this development, first we need to take a step back and look at the economics of the aforementioned tandem solar cells.
Tandem solar cells are composed of layers of different materials, each of which is particularly good at capturing a different part of the light spectrum. Whatever isn’t caught by the top layer can pass through to the next.
According to the Empa team, the practical maximum efficiency of a single-layer thin film solar cell is about 25 percent. A tandem cell, in contrast, could reach 30 percent.
Here’s a rundown from Empa on the way that works:
Stacking two solar cells one on top of the other, where top cell is semi-transparent, which efficiently converts large energy photons into electricity, while the bottom cell converts the remaining or transmitted low energy photons in an optimum manner. This allows a larger portion of the light energy to be converted to electricity.
The trade-off is that tandem solar cells are more complex and they are more expensive to manufacture, so the financials work out only for a limited set of applications:
Up to now, the sophisticated technology needed for the procedure was mainly confined to the realm of Space or Concentrated Photovoltaics (CPV). These “tandem cells” grown on very expensive single crystal wafers are considered not attractive for mass production and low cost solar electricity.
The Empa research aims to show that an assist from perovskite can lower the cost of tandem solar cells, thereby broadening their application to everyday use. The key is a manufacturing process that relies on standard roll-to-roll systems to produce thin films, used in a variety of industries including the solar industry.
The team is already anticipating that the new tandem solar cell could be based on an ultra-cheap substrate (substrate is fancyspeak for base) made of plastic or metal foil, but for this particular study they fabricated the perovskite solar cell in the form of nanoscale crystals made of methylammonium lead iodide, which they grew on a thin film of PCBM.
In case you were wondering where those”molecular soccer balls” were going to show up, that’s the structural form of PCBM. PCBM is short for phenyl-C61-butyric acid methyl ester, a molecule that encompasses 61 carbon atoms arranged in a soccer ball-ish structure.
The Empa team used conventional low cost thin film processes — vapor deposition and spin coating — to layer the perovskite film onto the PCBM film. Also helping to reduce costs is the use of a low heat (described as “lukewarm”) process to bind the two together.
The result is a perovskite solar cell that absorbs the blue and yellow parts of the light spectrum. The Empa team reports that the cell’s transparency is about 72 percent, so a good deal of red and infrared light pass through instead of bouncing off the surface. This is collected by a second solar cell based on the now-familiar CIGS (copper indium gallium diselenide) formulation.
What About CIGS?
In case you’re wondering why the team chose CIGS, that’s a no-brainer. CIGS solar cells are an Empa specialty, and teams of researchers at the institution have gradually raised the solar conversion efficiency bar for individual cells from 12.8 percent in 1999 to 20.4 percent in 2013.
At a conversion efficiency of 20.5 percent the new perovskite/CIGS solar cell may not look that much of an improvement, but that’s just for starters. In addition to lowering manufacturing costs with cheaper substrates, the team is eyeing that practical maximum mark of 30 percent or more.
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Photo credit (cropped): “Highly efficient semi-transparent Perovskite solar cell partners with CIGS thin film solar cell” via Empa Pictures/flickr.com.
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