Color Variation in Low-Dimensional Perovskites Has Different Origins Than Previously Theorized

Some type of light-emitting diodes, or LEDs, developed from perovskite, a kind of optoelectronic materials, give off light over a long-wavelength range, but the explanation for why it happens has been incorrect all this time.

Now, researchers from the University of Groningen have demonstrated the faulty of the interpretation, which could help them create perovskite LEDs capable of emitting broad-range light.

Quantum Confinement

Low-dimensional (2D or 1D) perovskites give off light in a reduced spectral range and are, therefore, used to create light-emitting diodes of first-rate color purity. Still, sometimes, a large emission spectrum at energy levels below the reduced spectrum has been observed.

This process has been considered as a good tool to produce white light LEDs more easily in comparison to the processes that are now used.

In hybrid perovskites, the ions are organic molecules of various sizes. When the size surpasses a particular dimension, the structure becomes 2D or layered. The produced quantum confinement has massive impacts on the materials’ physical qualities and inter alia, for the 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,” said Maria Loi, Professor of Photophysics and Optoelectronics at the University of Groningen, who also conducted the experiment.

Wide-field photoluminescence micrographs (230×175 µm) 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]
It has been suggested that the vibrations of the octahedron’s ions can ‘catch’ an excited stated in a self-trapped exciton, also known as the self-trapped excited state, making the broad-spectrum photoluminescence, in particular, the 2D systems and in systems where the octahedrons are separated from each other, meaning they are zero-dimensional.

However, research conducted by Loi and her team seems to contradict this hypothesis.

Color Changes Because of Defects in Perovskites

The team suggested that flaws in these perovskites could alter the color of emitted light. Therefore, they decided to examine the mainstream reading with a specific experiment.

Employing laser light of various colors, the researchers studied the emission of the crystals. They observed that when they used photons below the bandgap energy, the large emission still took place. The bandgap is the energy variation between the peak of the valence band and the bottom of the conduction band. According to the classical interpretation, this should not have happened.

The explanation?

“We think that it is a chemical defect in the crystal, probably related to iodide, which causes states inside the band gap,” said Simon Kahmann, a postdoctoral researcher who took part in the research. “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.”

This discovery has deep consequences, Loi explained. If we want to foresee new and better elements that broadly emit light, we need to learn the origin of this particular emission.

The research was published in the journal Nature Communications, and it is named “Extrinsic nature of the broad photoluminescence in lead iodide-based Ruddlesden–Popper perovskites.”

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