Why watering solar cells can actually improve the technology’s power efficiency

While perovskite solar cells are a relatively new form of renewable energy technology, they are gaining considerable attention. That’s because it has doubled its efficiency to 22% since first being introduced less than a decade ago. Additionally, it is lighter, cheaper, and more flexible than the traditional crystalline silicon-based cells. 

Perovskite solar cells

The terms perovskite comes from the light-harvesting layer that differentiates it from other types of solar cell. When they’re manufactured, they are exposed to ambient air for a few hours after fabrication has been completed. Doing this improves the cells’ efficiency, though no one had a clear explanation as to why—manufacturers just knew that it works. 

Now, it appears as though it’s understood, and with this explanation, a recommendation for improvement. It comes from a team of researchers at the Okinawa Institute of Science and Technology (OIST) Graduate University, who were led by Professor Yabing Qui. Their results were published in the journal Advanced Materials Interfaces. 

“It's intriguing: why do we need ambient air to enhance the effectiveness of perovskite solar cells?” said Zafer Hawash, first author of the study and an OIST PhD student. “Which component of the ambient air is linked to this phenomenon?” 

The team used these questions as the foundation for their study, and began by looking at the top layer of the solar cells — a logical choice given that perovskite solar cells have within them several layers. The top layer is the one that comes in contact with ambient air; as such, it’s the layer most likely to be affected by the external environment. 

Before getting into the team’s findings, it is worth explaining that this layer is referred to as the “hole transport layer” because it has a dopant, or a substance added to enhance the electrical conductivity of the material. Researchers have gotten as far as understanding that the dopant of the hole transport layer plays an important role in improving the solar cells’ performance, but not why. 

What the team ended up doing is exposing the layer to a variety of environmental gases — specifically focusing on oxygen, nitrogen, and moisture (water in a gas state). After each exposure, they checked the electrical properties of the layer to determine 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.”

Expounding on the discovery a bit, the team said the improvement in efficiency has to do with the structure of the transport layer; specifically, the fact that it has many pinholes within it. They are meant to allow the passage of gasses between the ambient air and the material that lies underneath it. The dopant on this layer is Lithium TFSI — a salt. This is the eureka moment — you see, given that it is a salt, it has a hygroscopic nature or, in layman’s terms, it absorbs water. So, when the solar cells are exposed to moisture, the water that is absorbed by the transport layer causes the dopant to redistribute, thus improving the cell’s overall performance and efficiency. 

Worth noting — overexposure can have a negative impact on the solar cells, so time exposed to moisture needs to be monitored. 

The team did also document the role of oxygen within the solar cells, and how it impacted the technology’s performance.

“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.”

So, in conclusion, exposing perovskite solar cells to moisture after fabrication is the most effective way to improve the technology’s performance; but not too much moisture. 

To explore the discovery a bit more, check out the team’s published paper, entitled “Moisture and Oxygen Enhance Conductivity of LiTFSI-Doped Spiro-MeOTAD Hole Transport Layer in Perovskite Solar Cells”.

Via Okinawa Institute of Science and Technology Graduate University