The majority of high-performance solar cells under development use a combination of materials including perovskite and spiro-MeOTAD. The cells are cheaper than traditional silicon-based solar cells, and their efficiency has been increasing in the past few years. However, perovskite, which is the layer that converts sunlight to electricity, degrades quickly.
The OIST researchers believe that miniscule pinholes in the spiro-MeOTAD layer are the source of the problem. The pinholes, which are so small they cannot be seen even with a light microscope, look like they may be creating easy pathways for water and other gas molecules in air to diffuse through the thin film and degrade the perovskite.
“These pinholes may play a major role in the degradation of the lifetime of the solar cells,” explained Zafer Hawash, a PhD student at OIST who discovered the pinholes. The findings are published in the journal Chemistry of Materials.
Hawash noticed the pinholes while analyzing how independent components of air, like water, oxygen and nitrogen, interact with spiro-MeOTAD. At first Hawash did not think they were important.
“No one has really mentioned this,” said Hawash, who works in OIST’s Energy Materials and Surface Sciences Unit. “I started realizing it was something important to report, to let people know these pinholes exist and that we should get rid of them to get better lifetime.”
The pinholes appear to be related to how the spiro-MeOTAD layer is usually made – a solution is spin-coated onto a base layer to create a thin film a fraction of the thickness of a human hair. Another preparation method, vacuum evaporation, does not produce the pinholes, but is less convenient to use, explained Dr. Luis Ono, an Energy Materials and Surface Sciences Unit group leader and paper co-author.