Chameleon Materials: The Origin Of Colour Variation In Low-Dimensional Perovskites

Some light-emitting diodes (LEDs) created from perovskite, a class of optoelectronic materials, emit light over a broad wavelength range. Scientists from the University of Groningen have now shown that in some cases, the explanation of why this happens is incorrect. Their new explanation should help scientists to design perovskite LEDs capable of broad-range light emission. The study was published in the journal Nature Communications on 11 May.

Low-dimensional (2D or 1D) perovskites emit light in a narrow spectral range and are therefore used to make light-emitting diodes of superior colour purity. However, in some cases, a broad emission spectrum at energy levels below the narrow spectrum has been noted. This process has attracted great interest as it could be used to produce white light LEDs more easily compared to the processes that are currently being used. To design perovskites for specific purposes, however, it is necessary to understand why some perovskites produce broad spectrum emissions while others emit a narrow spectrum.

Quantum confinement
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 large consequences for the materials’ physical properties and, in particular, for the optical properties.

Emissions
‘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 stated in a self-trapped exciton, or self-trapped excited state, causing the broad-spectrum photoluminescence, especially in these two-dimensional systems and in systems where the octahedrons are isolated from each other (zero-dimensional).

However, 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.’

Colour
The research team proposed that defects in these perovskites could change the colour of emitted light,. Therefore, they decided to test the mainstream interpretation with an ad hoc experiment. Loi: ‘In the accepted theoretical explanation, the excitations should be larger than the band gap to produce broad emission.’ The band gap is the energy difference between the top of the valence band and the bottom of the conduction band.

Using laser light of different colours, and therefore of different energies, they studied the emission of the crystals. ‘We noted that when we used photons below the band gap energy, the broad emission still occurred,’ says Loi. ‘This should not have happened according to the mainstream interpretation.’

Consequences
Their explanation is that a defect state with an energy level inside the band gap is governing the broad emission and the large colour 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. Kahmann: ‘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 finding has profound consequences, explains 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.’