Mr. Zafer Hawash setting up the hanging mercury drop electrode system for conductivity measurement. Credit: Image courtesy of Okinawa Institute of Science and Technology Graduate University – OIST
Researchers have clarified the relationship between air exposure and enhanced electric proprieties in perovskite solar cells. Perovskite solar cells are the rising star in the photovoltaic landscape. Since their invention, <10 years ago, their efficiency has doubled twice and it is now over 22% – an astonishing result in the renewable energy sector. Taking the name ‘perovskite’ from the light-harvesting layer that characterizes them, these solar cells are lighter, cheaper, and more flexible than the traditional crystalline silicon-based cells.
Perovskite solar cells are usually exposed to ambient air for several hours after fabrication. This procedure increases their efficiency, even if the reason behind the phenomenon was unclear. The scientific explanation of this practice has now been discovered. The top layer is called ‘hole transport layer’, and it has a dopant, which enhances the electrical conductivity of the material.
The scientists performed controlled exposure of the hole transport layer to environmental gasses, focusing on oxygen, nitrogen, and moisture – water that is in a gas state. Then, they checked the electrical properties of the hole transport layer, using a variety of methods, to see if and how the inside of the transport layer changed. “What we found is that oxygen and nitrogen do not have any role in the redistribution of the dopants,” Hawash explained. “But in the case of moisture, the solar cells’ efficiency increases. This is the discovery: moisture is the air component that causes the redistribution of the dopant across the material, and thus the enhancement of the electric properties of the solar cells.”
Moisture and Oxygen Enhance Conductivity of LiTFSI-Doped Spiro-MeOTAD Hole Transport Layer in Perovskite Solar Cell
The scientists explain this phenomenon with the structure of the transport layer, which has many pinholes that allow the passage of gasses between the ambient and the underneath material. The dopant in the transport layer is a salt – Lithium TFSI. Being a salt, the dopant has a hygroscopic nature: it absorbs water. When the solar cells are exposed to moisture, the water absorbed causes the dopant to redistribute. However, long time exposure to moisture has a detrimental effect on the solar cells.
“Oxygen enhances the electrical conductivity of the transport layer as well, but this effect does not last long,” Hawash commented. “But with the right amount of exposure to moisture, the electric proprieties are irreversibly enhanced.”
Exposing the device to moisture after fabrication is then the most effective way to enhance the solar cells’ performance. This finding is of high importance for the perovskite solar cells’ future, as it finally explains a common practice whose effectiveness was mostly anecdotic, and thus could now lead the way to further improvement in the perovskite solar cells’ performance. http://www.oist.jp/news-center/news/2016/7/27/watering-solar-cells-makes-them-grow%E2%80%A6-power
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