Using atomic force microscopy (AFM), a researcher from the Okinawa Institute of Science and Technology (OIST) in Japan has discovered that the top layers of next-generation solar cells are covered in pinholes.
Most of these promising, inexpensive devices are made from spiro-MeOTAD and perovskite. However, the perovskite layer deteriorates quicker, and the OIST discovery may finally reveal what the problem is. Pinholes in the spiro-MeOTAD layer allow water and gas molecules to enter, spread through the thin film and degrade the perovskite, significantly compromising the cell’s longevity.
PhD student Zafer Hawash, who works in OIST’s Energy Materials and Surface Sciences Unit, first observed the pinholes while testing how air interacts with the spiro-MeOTAD. He quickly realized that scientific literature had not previously alluded to this problem. “Many researchers avoid this type of research and instead focus on the main component of the solar cells,” he explains why possibly no other researcher had discovered the pinholes before him. He knew this was something significant to report.
“I discovered the pinholes after observing that there is a phase-segregated concentration of spiro-MeOTAD (hole transport layer, or HTL) and LiTFSI (the dopant) within the prepared films of the LiTFSI-doped spiro-MeOTAD, which currently prevails as HTL in the perovskite solar cells,” Hawash explains. Initially, LiTFSI was highly concentrated at the bottom of the film and upon exposure to air segregated to the top. The researcher observed this via X-ray photoelectron spectroscopy (XPS). He wondered why and how the dopant diffuses to the top surface and began looking at the topography using atomic force microscopy (AFM). That’s when he found the pinholes. “I was not sure if the pinholes make channels or were just on the top surface, but after different trials with cross-sectional scanning electron microscopy (SEM), I found out that these pinholes form channels across the film.”
From Hawashe’s observations, the pinholes appeared to be connected to the way the spiro-MeOTAD layer was manufactured — it was spin-coated onto a base layer to create a thin film. The alternative method of vacuum evaporation, on the other hand, did not generate pinholes.
Perovskite being the solar material du jour, the discovery could have a major impact on the design of future generations of this type of thin-film photovoltaics. “I hope this discovery will help in achieving a better stability and longer life for perovskite solar cells, whether by optimizing this HTL preparation method or by using suitable alternative materials,” Hawash says.
The question remains, how to prevent the pinholes while keeping production costs low. The researcher proposes experimenting with different preparation methods for the same material, adding other components to the material or using alternative materials.
Abating the pinhole problem could pay off in optimizing these solar cells. “Since we already have quite high efficiency from perovskites solar cells, the only thing that we currently need is better stability and longer lifetime,” Hawash says. “And I believe that fixing the pinhole problem will do so, because these pinholes have several unwanted effects, including facilitating moisture or any other molecules from ambient air, migration through the doped spiro-MeOTAD film as well as facilitating the migration of some of the perovskite component elements to the top surface.”
Hawash’s findings are detailed in the paper “Air-Exposure Induced Dopant Redistribution and Energy Level Shifts in Spin-Coated Spiro-MeOTAD Films,” co-authored by Hawash and published in the journal Chemistry of Materials.
Written by Sandra Henderson, Research Editor, Solar Novus Today