Researchers at McGill University have gained new insight into the inner workings of perovskites, especially their ability to function even despite the existence of defects in the materials' crystal structure.
"Historically, people have been using bulk semiconductors that are perfect crystals. And now, all of a sudden, this imperfect, soft crystal starts to work for semiconductor applications, from photovoltaics to LEDs," explains senior author Patanjali Kambhampati, an associate professor in the Department of Chemistry at McGill. "That's the starting point for our research: how can something that's defective work in a perfect way?"
In a recent paper, the researchers reveal that a phenomenon known as quantum confinement occurs within bulk perovskite crystals. Until now, quantum confinement had only been observed in particles a few nanometres in size. When particles are this small, their physical dimensions constrain the movement of electrons in a way that gives the particles distinctly different properties from larger pieces of the same material - properties that can be fine-tuned to produce useful effects such as the emission of light in precise colors.
Using a technique known as state-resolved pump/probe spectroscopy, the researchers have shown a similar type of confinement occurs in bulk caesium lead bromide perovskite crystals. Their experiments revealed quantum dot-like behavior taking place in pieces of perovskite significantly larger than quantum dots.
The work builds on earlier research which established that perovskites, while appearing to be a solid substance to the naked eye, have certain characteristics more commonly associated with liquids. At the heart of this liquid-solid duality is an atomic lattice able to distort in response to the presence of free electrons. Kambhampati draws a comparison to a trampoline absorbing the impact of a rock thrown into its center. Just as the trampoline will eventually bring the rock to a standstill, the distortion of the perovskite crystal lattice - a phenomenon known as polaron formation - is known to have a stabilizing effect on the electron.
While the trampoline analogy would suggest a gradual dissipation of energy consistent with a system moving from an excited state back to a more stable one, the pump/probe spectroscopy data actually revealed the opposite. To the researchers' surprise, their measurements showed an overall increase in energy in the aftermath of polaron formation.
"The fact that the energy was raised shows a new quantum mechanical effect, quantum confinement like a quantum dot," Kambhampati says, explaining that, at the size scale of electrons, the rock in the trampoline is an exciton, the bound pairing of an electron with the space it leaves behind when it is in an excited state.
"What the polaron does is confine everything into a spatially well-defined area. One of the things our group was able to show is that the polaron mixes with an exciton to form what looks like a quantum dot. In a sense, it's like a liquid quantum dot, which is something we call a quantum drop. We hope that exploring the behavior of these quantum drops will give rise to a better understanding of how to engineer defect-tolerant optoelectronic materials."