AsianScientist (Aug. 2, 2016) – Researchers in Japan have discovered why perovskite solar cells increase in efficiency when exposed to ambient air for several hours after fabrication. Their results, published in Advanced Materials Interfaces, finally explains a common practice whose effectiveness was previously mostly anecdotal.
Perovskite solar cells are the rising star in the photovoltaic landscape, being lighter, cheaper, and more flexible than traditional crystalline silicon-based cells. Since their invention less than ten years ago, their efficiency has doubled twice and it is now over 22 percent—an astonishing result in the renewable energy sector.
“It’s intriguing: why do we need ambient air to enhance the effectiveness of perovskite solar cells?” asked PhD student Mr. Zafer Hawash from the Okinawa Institute of Science and Technology Graduate University, who is the first author of the study. “Which component of ambient air is linked to this phenomenon?”
To address these questions, the researchers, led by Professor Yabing Qi from the Energy Materials and Surface Sciences Unit, focused on the top layer of the solar cells, which is the one in direct contact with ambient air. This layer is called the hole transport layer and has a dopant, which is a substance that 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—that is, 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 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.”
According to Hawash, moisture is the air component that causes the redistribution of the dopant across the material, and thus the enhancement of the electrical properties of the solar cells.
Furthermore, the researchers found that while oxygen enhances the electrical conductivity of the transport layer, this effect does not last long. Therefore, exposing the device to the right amount of moisture after fabrication is still the most effective way to enhance the solar cells’ performance, as their electric properties are irreversibly enhanced.
The article can be found at: Hawash et al. (2016) Moisture and Oxygen Enhance Conductivity of LiTFSI-Doped Spiro-MeOTAD Hole Transport Layer in Perovskite Solar Cells.
Source: OIST; Photo: Shutterstock.
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