The U.S. National Renewable Energy Laboratory (NREL) worked with the Shanghai Jiao Tong University (SJTU) on hybrid halide perovskite solar cells. Treating them with a specific solution of methyl ammonium bromide (MABr) repairs defects and improves efficiency so that a low-quality perovskite film with pinholes and small grains is converterd into a high-quality film without pinholes and with large grains.
This boosted the efficiency of the perovskite film in converting sunlight to 19% and also made the manufacturing process more reliable. Other process enhancements using vacuum have acheived 20% efficiency while manipulating the grain boundaries has seen efficiency reach 30%.
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, and the fast-crystallization process is regarded as almost an art. 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 such as noncontinuous crystals and nonuniform crystals form. These significantly reduce the effectiveness of a perovskite cell.
The researchers use a new process called Ostwald ripening. This 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 mg/ml.
“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,” say the researchers. “Thus, this new chemical approach enhances processing tolerance to the initial perovskite film quality and improves the reproducibility of device fabrication.”
The improved film quality made the cells more stable, as untreated cells had an efficiency of about 14 percent to 17 percent, while cells treated with the MABr solution had an efficiency of more than 19%.
Details of the research are contained in the paper Facile Fabrication of Large-Grain CH3NH3PbI3-xBrx Film for High-Efficiency Solar Cells via CH3NH3Br Selective Ostwald Ripening are published in Nature Communications. It was funded as part of the US SunShot Initiative to drive down the cost of solar electricity to 6 cents per kilowatt-hour.