Earlier studies had concluded that exposing perovskite films to ambient air was detrimental because moisture reacted with perovskite, which degraded over time. As a result it was believed the material had to be prepared using a heat treatment called annealing in a water-free environment.
OIST researchers set out to investigate the effects of moisture on perovskite formation during 45 minutes of annealing, at temperatures between 105 and 125 degrees centigrade. The researchers grew a type of perovskite that has been shown to work better for solar cells. Then, they compared the perovskite film's formation in a nitrogen atmosphere with its formation in humid air and found that the films actually receive a growth improvement resulting in larger grain sizes than usual in the presence of moisture. The film grows slowly, so larger grains can form.
"Larger grain sizes mean the crystals on the film is more continuous, and the electrons passing through the film face fewer interruptions," said Sonia Ruiz-Raga, the study’s first author in a paper published in Chemistry of Materials.
Larger grains make perovskite solar cells more efficient. The highest efficiency achieved by the study was 12.7 percent. While other teams have achieved higher
efficiencies, the OIST result ensures that future industries need not invest in expensive climate control machinery to keep the moisture down to one part per million.
Overall, 12.7 percent efficiency is by no means the ceiling for this fabrication technique and the researchers believe it is possible to obtain even larger grain
Chemistry of Materials, "Influence of Air Annealing on High Efficiency Planar Structure Perovskite Solar Cells", Sonia R. Raga , Min-Cherl Jung , Michael V. Lee,
Matthew R. Leyden , Yuichi Kato , and Yabing Qi. http://pubs.acs.org/doi/abs/10.1021/cm5041997
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