Just the right amount of water makes perovskite solar better

The change, noticed when cell are left out in air, is due to improved doping distribution in the hole transport layer –  spiro-MeOTAD doped with lithium TFSI – which increases its conductivity.

“It is known that the dopant of the hole transport layer plays a key role in perovskite solar cells’ performance,” said researcher Zafer Hawash. “We found is that oxygen and nitrogen do not have any role in the redistribution of the dopants, but 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.”

However, moisture is also the enemy of the photon-converting perovskite layer, making short work of destroying it – something that would happen slightly slower if it were not for pin-holes in the spiro-MeOTAD. These give water molecules a fast-track from outside to the perovskite, and perovskite decomposition products a route back to attack the cell electrodes.

But a balance can be struck. ”With the right amount of exposure to moisture, the electric proprieties are irreversibly enhanced,” said Hawash.

He previously did much to improve the understanding of these pin-holes, which are so small they cannot be seen by optical microscope.

During their work on moisture-induced dopant movement, Okenama researchers also noticed some changes due to oxygen. According to Harwash: “Oxygen enhances the electrical conductivity of the transport layer as well, but this effect does not last long.”

This work is published in Advanced Materials Interfaces as ‘Moisture and oxygen enhance conductivity of LiTFSI-doped spiro-MeOTAD hole transport layer in perovskite solar cells‘.