McGill University researchers set out to uncover the secrets of perovskite’s ability to act as a semiconductor, even with structural defects. In doing so, they have discovered what amounts to a new state of matter. Perovskites have garnered attention in the last decade due to their ability to act as a semiconductor even with the presence of defects within the material’s crystal structure. Traditionally, semiconductors need near-perfect crystals to be effective, which requires ultraprecise (and costly and stringent) methods for their manufacture. The distortion of the perovskite crystal lattice is followed by the formation of an exciton 'quantum drop.' Courtesy of Colin Sonnichsen. “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,” said senior author Patanjali Kambhampati, an associate professor of chemistry at McGill. “That’s the starting point for our research: How can something that’s defective work in a perfect way?” The work showed a phenomenon present in the material that had previously only been observed in particles a few nanometers in size, known as quantum confinement. In the case of ultrasmall particles such as quantum dots, physical dimensions that constrain the movement of electrons in such a way that the particles possess distinctly different properties form larger pieces of the same material. Those properties can be exploited to produce useful results such as the emission of light in precise colors. Using state-resolved pump/probe spectroscopy, the researchers showed that a similar type of confinement occurs in bulk cesium lead bromide perovskite crystals. In other words, their experiments have uncovered quantum dot-like behavior taking place in pieces of perovskite significantly larger than quantum dots. The work builds on research showing that perovskites blur the line between solids and liquids; though they appear solid, they posses certain characteristics that would typically be expected of liquids, such as an atomic lattice that is able to distort in response to the presence of free electrons. Kambhampati drew a comparison to a trampoline absorbing the impact of a rock thrown to its center: Just as the trampoline eventually brings the rock back to a standstill, the distortion of the perovskite’s crystal lattice — a phenomenon known as polaron formation — is understood to have a stabilizing effect on the electron. The trampoline analogy, however, suggests a gradual dissipation of energy consistent with a system moving from an excited state to a more stable one, which is not the case in the new work. The data from the pump/probe spectroscopy showed the opposite, an overall increase in energy in the aftermath of the polaron formation. “The fact that the energy was raised shows a new quantum mechanical effect, quantum confinement like a quantum dot,” Kambhampati said. At the size scale of electrons, he explained, 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,” Kambhampati said. “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 optoelectronics materials.” The research was published in Physical Review Research (www.doi.org/10.1103/PhysRevResearch.3.023147).