LED - Page 13

Two papers have recently been published, reporting on perovskite-based LEDs. The efficiencies with which some perovskite LEDs (PLEDs) produce light from electrons already seem to rival those of OLEDs.

Both papers, by Cao et al. and Lin et al., have developed PLEDs that break an important technological barrier: the external quantum efficiency (EQE) of the devices, which quantifies the number of photons produced per electron consumed, is greater than 20%. There are several similarities between the devices reported by the two groups. Perhaps most notably, the active (emissive) perovskite layer is about 200 nanometres thick in both cases, and is sandwiched between two relatively simple electrodes. This design is called a planar structure, and is the most basic manifestation of diodes made from thin films of materials. The electrodes are appropriately modified to ensure that electrons and holes (quasiparticles formed by the absence of electrons in atomic lattices) are efficiently pumped into the perovskite. As in all LEDs, when electrons meet holes, they can release energy in the form of photons through a process known as radiative recombination.

A next-generation optical material based on perovskite nanoparticles can achieve vivid colors even on very large screens. Due to their high color purity and low cost advantages, it has also gained much interests in industry. A recent study including researchers with UNIST has introduced a simple technique to extract the three primary colors (red, blue, green) from this material.

This innovative work was led by Professor Jin Young Kim in the School of Energy and Chemical Engineering at UNIST. In the study, the research team introduced a simple technique that freely controls light emitting spectra by adjusting the anion halides in perovskite materials. The key is to adjust the anion halides by dissolving them in solvents to achieve red, blue and green lights. Application of this technique to LEDs can result in crystal-clear picture quality.

Researchers at the Adolphe Merkle Institute in Fribourg and the Ecole Polytechnique Fédérale de Lausanne have developed a new technique to replace one of the least stable components in perovskite solar cells, which could be a major step towards commercialization.

Perovskites are seen as promising thin-film solar-cell materials because they can absorb light over a broad range of solar spectrum wavelengths thanks to their tuneable bandgaps. Charge carriers (electrons and holes) can also diffuse through them quickly and over long lengths. The most efficient perovskite solar cells usually contain bromide and MA, which is thermally unstable. To overcome this problem, researchers tried replace MA with FA since it is not only more thermally stable but also has an optimal redshifted bandgap. Unfortunately, because of its large size, FA does distort the perovskite lattice and tends to produce a photoinactive 'yellow' phase at room temperature. The other photoactive 'black phase' can only be seen at high temperatures. However, the researchers in this new work have now found a way to stabilize the black phase of FA at room temperature.

Researchers at Duke University computationally predicted the electrical and optical properties of layered hybrid organic-inorganic perovskites (or HOIPs) - popular materials for light-based devices such as solar cells and light-emitting diodes (LEDs). The ability to build accurate models of these materials atom-by-atom will allow researchers to explore new material designs for next-generation devices.

'Ideally we would like to be able to manipulate the organic and inorganic components of these types of materials independently and create semiconductors with new, predictable properties,' said David Mitzi, Professor of Mechanical Engineering and Materials Science at Duke. 'This study shows that we are able to match and explain the experimental properties of these materials through complex supercomputer simulations, which is quite exciting.'

Researchers from Qatar, Switzerland and Italy have designed a composite perovskite material with a thin surface layer that repels water and protects against moisture-induced degradation. The team has managed to do this by allowing the self-assembly of two-dimensional perovskite on top of a three-dimensional perovskite in an inert atmosphere.

The composite perovskite did not decompose when kept in highly humid air for three days. The top layer of the 2D perovskite blocked water penetration into the 3D perovskite beneath it, preventing its degradation. Bare 3D perovskite completely degrades at a similar humidity.

Researchers from ITMO University, along with colleagues from Sweden, Australia, the United States and Lithuania, have discovered Fano resonance in perovskite nanoparticles and gained control over the resonance spectrum for an array of inorganic nanoparticles. This newly designed method reversibly adjusts the radiation color of nanosized light sources. Previously, radiation color could be specified only during nanoparticle synthesis, but now it can be changed after synthesis. Stability and electromagnetic resonances of the particles are retained during this adjustment. This makes them promising for optical chips, LEDs and optoelectronic devices.

Resonance is the coincidence between frequencies of two oscillations increasing their intensity. A half-century ago, the Italian theoretical physicist Hugo Fano described a special type of resonance with an asymmetric profile arising from the interference of two wave processes. Since then, Fano resonance has been actively used in photonics, for example, to create fast optical switches. The reduction of such switches to nanoscale will dramatically increase the performance of photonic chips by integrating a huge number of elements in one device.

Researchers at AMOLF have found a way of turning calcium carbonate structures, such as a sea urchin skeleton, into perovskite materials, by modifying the composition of the material. The team explained that "the experiment involves no more than dripping two liquids over the calcium carbonate structure. The conversion is complete within a couple of minutes. If you shine a UV lamp on the structure, you can see the conversion taking place in front of your eyes: The sea urchin skeleton, which initially appears blue under the lamp, changes into a bright green structure with each drop".

A sand dollar skeleton gradually converting into a light emitting perovskite

The researchers estimate that the perovskite microstructures made in this process result in more stable materials. They therefore state that solar cells made from this material should last longer. "In addition, we can produce perovskite structures in every desired color. This means that the material could also be used for LEDs in various applications, such as screens," says the research team.

A team of researchers led by Professor Biwu Ma from Florida State University demonstrated a new approach to building efficient and spectrally stable red perovskite LEDs. The team developed a simple solution processing method followed by thermal annealing to prepare highly luminescent ultra-smooth polymer'perovskite composite thin films with tunable emissions from red to deep-red.

Light emitting diodes (LEDs) incorporating inorganic, organic, or nanoscale materials are highly promising for solid-state lighting and displays. Despite the significant progress achieved in green emitting perovskite LEDs in recent years, blue or red emitting LEDs still remain a challenge with regards to their performance and spectral stability during operation.

A new international study led by chemists at the Georgia Institute of Technology has observed that hybrid organic-inorganic perovskites (HOIPs) possessed a 'richness' of semiconducting physics created by what could be described as electrons "dancing" on wobbling chemical underpinnings. That contradicts established semiconductors that rely upon rigidly stable chemical foundations, or quieter molecular frameworks, to produce the desired quantum properties. This could mean that HOIPs may be used in the future as semiconductors with nuanced colors emanating from lasers, lamps, and even window glass.

HOIPs have been reported by the team to be quite challenging to examine, but the researchers from a total of five research institutes in four countries succeeded in measuring a prototypical HOIP and found its quantum properties on par with those of established, molecularly rigid semiconductors, many of which are graphene-based. 'We don't know yet how it works to have these stable quantum properties in this intense molecular motion,' said first author Felix Thouin, a graduate research assistant at Georgia Tech. 'It defies physics models we have to try to explain it. It's like we need some new physics.'

Researchers from the Russian ITMO University have developed effective nanoscale light sources based on a halide perovskite. These nanosources are subwavelength nanoparticles which serve both as emitters and nanoantennas and allow enhancing light emission inherently without additional devices. Moreover, the perovskite enables tuning the emission spectra throughout a visible range by varying the composition of the material. The new nanoparticles are a promising platform for creating compact optoelectronic devices such as optical chips, light-emitting diodes, or sensors.

The nanoscale light sources and nanoantennas have already found a wide range of applications in several areas, such as ultra compact pixels, optical detection, or telecommunications. However, the fabrication of nanostructure-based devices is rather complicated due to the limited luminescence efficiency of the materials used typically as well as non-directional and relatively weak light emission of single quantum dots or molecules. An even more challenging task is placing a nanoscale light source precisely near a nanoantenna.