NREL researcher Mengjin Yang examines hybrid perovskite solar cells in his lab. Photo: Dennis Schroeder/NREL.
NREL researcher Mengjin Yang examines hybrid perovskite solar cells in his lab. Photo: Dennis Schroeder/NREL.

Scientists at the US Department of Energy's National Renewable Energy Laboratory (NREL), in collaboration with researchers at Shanghai Jiao Tong University (SJTU) in China, have devised a method to improve perovskite solar cells, making them more efficient and reliable with higher reproducibility. They describe this method in a paper in Nature Communications.

The paper reveals that treating hybrid halide perovskite solar cells with a specific solution of methyl ammonium bromide (MABr) can repair defects, thereby 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. In doing so, they boosted the efficiency with which the perovskite film could convert sunlight to electricity up to 19%.

The conversion efficiency of perovskites has jumped from slightly less than 4% in 2009, when the first tests were done, to more than 22% today. In practice, however, this conversion efficiency generally varies between 15% and 20%, depending on the skills of the researchers making perovskites at different laboratories.

Perovskite films are typically grown using a solution of precursor chemicals that form the crystals, which are then exposed to an anti-solvent that removes the precursor solvent. This fast-crystallization process is almost an art. NREL researchers found that it is easy to miss the narrow time window for properly adding the anti-solvent, resulting in the creation of perovskite crystals with defects. These defects, such as non-continuous crystals and non-uniform crystals with relatively small crystallite sizes and pinholes, can significantly reduce the effectiveness of a perovskite solar cell.

The scientists from NREL and SJTU have now come up with a better crystallization process, using what's called the Ostwald ripening process, which involves small crystals dissolving and then redepositing onto larger crystals. The researchers were able to induce the Ostwald ripening process in a perovskite crystal by treating it with a MABr solution. In order for this to work, however, the solution need to have just the right concentration of MABr, which proved to be about 2mg/mL.

"With the Ostwald ripening process, different-sized [perovskite] nanocrystals formed with different film qualities could then grow into pinhole-free perovskite films with similar large crystal sizes," the researchers say. "Thus, this new chemical approach enhances processing tolerance to the initial perovskite film quality and improves the reproducibility of device fabrication."

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%. In addition, the improved film quality made the perovskite solar cells more stable.

This story is adapted from material from the National Renewable Energy Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.