Wide-field photoluminescence micrographs show how some perovskite flakes appear bright green over their entire area (left panel), whilst other flakes exhibit a distinctly red-shifted emission (right panel). Image: Loi lab, University of Groningen.Some light-emitting diodes (LEDs) created from perovskite, a class of optoelectronic material, emit light over a broad range of wavelengths. Scientists from the University of Groningen in the Netherlands have now shown that, in some cases, the explanation for why this happens is incorrect. Their new explanation, reported in a paper in Nature Communications, should help scientists to design perovskite LEDs capable of broad-range light emission.
Low-dimensional (2D or 1D) perovskites emit light over a narrow spectral range and are therefore used to make light-emitting diodes of superior color purity. However, scientists have noted that low-dimensional perovskites can also produce a broad emission spectrum at energy levels below the narrow spectrum. This process has attracted great interest as it could offer a way for LEDs to emit white light more efficiently than currently possible. To design perovskites for specific purposes, however, it is necessary to understand why some perovskites produce broad-spectrum emissions while others emit over a narrow spectrum.
Perovskites are a versatile group of materials with a very distinctive crystal structure, known as the perovskite structure. In an idealized cubic unit cell, anions form an octahedron around a central cation, while the corners of the cube are occupied by other, larger cations. Different ions can be used to create different perovskites.
In hybrid perovskites, the cations are organic molecules of different sizes. When the size exceeds a certain dimension, the structure becomes two-dimensional or layered. The resulting quantum confinement has major effects on the materials' physical properties and, in particular, on their optical properties.
“There are many reports in the literature where, in addition to the narrow emission of these low dimensional systems, there is a broad low energy spectrum. And this is thought to be an intrinsic property of the material,” says Maria Loi, professor of photophysics and optoelectronics at the University of Groningen.
It has been proposed that the vibrations of the octahedron's atoms can 'trap' an excited state, producing a self-trapped exciton. This exciton then causes the broad-spectrum photoluminescence, especially in 2D systems and in systems where the octahedrons are isolated from each other (zero-dimensional).
But observations made in Loi's laboratory appear to contradict this theory, says Simon Kahmann, a postdoctoral researcher in her team. “One of our students studied single crystals of a lead-iodide-based 2D perovskite and noticed that some crystals emitted green light and others emitted red light. This is not what you would expect if the broad red emission were an intrinsic property of this material.”
The research team proposed that defects in these perovskites could change the color of the emitted light, and decided to test the mainstream explanation with an ad hoc experiment. “In the accepted theoretical explanation, the excitations should be larger than the bandgap to produce broad emission,” explains Loi. The bandgap is the energy difference between the top of the valence band and the bottom of the conduction band.
Using laser light of different colors, and therefore of different energies, the scientists studied the emission of the crystals. “We noted that when we used photons below the bandgap energy, the broad emission still occurred,” says Loi. “This should not have happened according to the mainstream interpretation.”
The scientists’ alternative explanation is that a defect state with an energy level inside the bandgap is governing the broad emission and the large color variation of the crystals. “We think that it is a chemical defect in the crystal, probably related to iodide, which causes states inside the band gap,” says Kahmann.
Thus, the broad emissions are not an intrinsic property of the material but are caused by an extrinsic effect. “At this point, we cannot totally rule out that this is a quirk of lead iodide perovskites, but it is likely to be a general property of low-dimensional perovskites,” says Kahmann.
This finding has profound consequences, adds Loi: “If we want to predict new and better compounds that broadly emit light, we need to understand the origin of this emission. We should not be tricked by this chameleon.”
This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.