Research uncovers why water degrades certain solar cell materials

With power efficiencies similar to Si single crystalline solar cells, hybrid inorganic-organic perovskite is a promising material for solar cells. But the material has a problematic weakness – water.

“The material weakens in the environment when exposed to water,” said Masaki Hada, one of the authors of a paper aiming to understand how the material’s degradation occurs. “So we wanted to characterize the reaction between the perovskite material and the water molecules.”

Inside a vacuum chamber, Hada and his collaborators exposed a methylammonium lead iodide perovskite film to water molecules at different temperatures. They recorded the process over the course of an hour using X-ray diffraction and used models to understand the mechanism behind the degradation.

In their findings, Hada et al. report the water molecules were attracted to the perovskite crystal lattices. When a water molecule with sufficient energy penetrates the outer surface of the crystal lattice, it distorts the lattice framework, causing the material to degrade.

The researchers further noticed a degradation dependence on temperature. They characterized the speed of degradation with simple equations based on statistical physics. The experimental results agreed well with the first-principles calculations. According to models, they also found the certain orientation of the material’s crystal structure has a stronger effect on degradation.

“Many researchers are working on fabricating a more robust, stronger hybrid perovskite material to resist moisture,” Hada said. “This experiment can contribute knowledge on how the degradation reaction occurs, which will help the researchers fabricate better solar cell materials.”

Source: “A mechanistic investigation of moisture-induced degradation of methylammonium lead iodide,” by Masaki Hada, Md. Abdullah Al Asad, Masaaki Misawa, Yoichi Hasegawa, Ryota Nagaoka, Hiroo Suzuki, Ryuji Mishima, Hiromi Ota, Takeshi Nishikawa, Yoshifumi Yamashita, Yasuhiko Hayashi, and Kenji Tsuruta, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/5.0021338 .