Ostwald Ripening sounds like a DC Comics villain, but it refers to a common method for growing crystals and it could become the hero of the perovskite solar cell field. A research team at the National Renewable Energy Laboratory has joined with China’s Shanghai Jiao Tong University to demonstrate that the Ostwald method can be deployed to create perovskite solar cells with greater stability, fewer defects, and improved efficiency.
Perovskite Solar Cells: Art, Meet Science
The new research is significant because perovskites are emerging as a low-cost, high-efficiency alternative to silicon for solar cells. Their crystalline structure is based on the naturally occurring mineral perovskite, and they can be synthesized with relative ease.
However, the “ease” part is a bit of misdirection. Perovskites are fairly simple to create in the lab, but they come with built-in defects that chip away at efficiency and limit the lifespan of the solar cell.
That’s because conventional perovskite film is “grown” using one-step methods that admit plenty of opportunity for error. As described by NREL, there is as much art to the process as science.
The one-step method consists of engaging a solution of precursor chemicals, then adding an anti-solvent to start forming the crystals. The advantage is that it’s quick, and it’s a room temperature process that saves energy. The downside is that the window for adding the anti-solvent is relatively narrow:
Because of the general complex processes involved in perovskite formation, the processing for making high-quality perovskite films has become an art owing to the very narrow time window for properly adding the antisolvent; this could potentially be responsible for some reproducibility problems, especially the large variations among different groups.
So, miss that window, and your perovskite film will grow with defects that inhibit efficiency and limit stability. The defects include gaps, differing crystal sizes including some that are undesirably small, and pinholes, all of which undermine the efficiency and stability of perovskite solar cells.
Non-Artsy Perovskite Solar Cells
The new perovskite solar cell research takes the guesswork — or art, as the case may be — out of the process.
That’s where Ostwald ripening comes in. Loosely speaking, Ostwald ripening refers to dissolving small crystals and re-depositing them onto larger crystals. As applied to perovskite solar cells, it could be used to smooth out or repair defective perovskite films.
Here’s the happy recap from the research team:
With the Ostwald ripening process, different-sized nanocrystals formed with different film qualities could then grow into pinhole-free perovskite films with similar large crystal sizes…Thus, this new chemical approach enhances processing tolerance to the initial perovskite film quality and improves the reproducibility of device fabrication.
The tricky part was to kick-start the process. Apparently the research team realized early on that treating the film with a solution of methyl ammonium bromide (MABr) would be effective, but it took a lot of trial-and-error before they settled on the exact proportions:
…[The Ostwald ripening process is] strongly affected by MABr concentration and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concentration enhances I–Br anion exchange reaction, yielding poorer device performance.
Fans of Sherlock Holmes will be disappointed to know that the ideal proportion is not 7 percent. It is only 0.2 percent, or two milligrams per milliliter.
As for the results, the untreated solar cells used by the team came in at 14 to 17 percent efficiency. After their MABr bath, the efficiency improved to 19 percent.
That’s a little shy of the 21.1 percent perovskite solar cell mark hit by Switzerland’s EPFL last spring, but it’s a good start (for the record, EPFL is also working on lowering the cost of perovskite solar cells).
The research team notes that while conventional perovskite solar cells have gotten up to the 18 to 20 percent range at some labs, more typically the results have hovered around 15 to 16 percent.
The research team anticipates that fine-tuning the Ostwald strategy will yield improved results.
You can get full details about the new research from the journal Nature under the crunchy mouthful of a title, “Facile fabrication of large-grain CH3NH3PbI3−xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening.”
Image (cropped): via NREL.
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