Scientists at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL), in collaboration with researchers at Shanghai Jiao Tong University (SJTU), say they have devised a method to improve perovskite solar cells, making them more efficient and reliable with higher reproducibility.
Funded by the DOE’s SunShot Initiative, the research involved hybrid halide perovskite solar cells and revealed treating them with a specific solution of methyl ammonium bromide (MABr) would repair defects, improving efficiency. The scientists converted a low-quality perovskite film with pinholes and small grains into a high-quality film without pinholes and with large grains. Doing so boosted the efficiency of the perovskite film in converting sunlight to 19%, according to NREL.
The efficiency of perovskites in converting sunlight into electricity has jumped from slightly less than 4% in 2009, when the first tests were done, to more than 22% today. However, NREL says the efficiency can fluctuate according to the skills of the researchers making perovskites at different laboratories, to somewhere between 15% and 20%.
As the national lab explains, perovskite films are typically grown using a solution of precursor chemicals that form the crystals, which are then exposed to a second anti-solvent that removes the precursor solvent. The fast-crystallization process is almost an art, and NREL researchers found that, because of the narrow time window for properly adding the anti-solvent, it is easy to miss that window and perovskite crystals with defects could form. Defects, such as noncontinuous crystals and nonuniform crystals with relatively small crystallite sizes and pinholes, can significantly reduce the effectiveness of a perovskite cell.
The scientists from NREL and SJTU say they came up with a better method, using what’s called the Ostwald ripening process. The process involves small crystals dissolving and then redepositing onto larger crystals. The researchers were able to induce the Ostwald ripening process by treating the perovskite with a MABr solution. The amount of the solution proved key, as the ideal was proven to be about 2 milligrams per milliliter.
“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,” the researchers note in paper published in Nature Communications. “Thus, this new chemical approach enhances processing tolerance to the initial perovskite film quality and improves the reproducibility of device fabrication.”
According to NREL, the improved film quality made the cells more stable. The perovskite cells treated with MABr were shown to be more efficient than those without the treatment. Untreated cells had an efficiency of about 14% to 17%, while cells treated with the MABr solution had an efficiency of more than 19%, the lab adds.
Photo courtesy of Dennis Schroeder/NREL: NREL researcher Mengjin Yang examines hybrid perovskite solar cells in his lab.