LED - Page 8

A joint research team, affiliated with Korea's UNIST, has developed a novel method capable of controlling the brightness and wavelength of quantum dots (QDs). The work was led by Professor Kyoung-Duck Park in the Department of Physics at UNIST, in collaboration with Professor Sohee Jeong in the Department of Energy Science from Sungkyunkwan University (SKKU).

The research team demonstrated the tip-induced dynamic control of strain, bandgap, and quantum yield of single CsPbBr_xI_3'x pQDs by using a controllable plasmonic nanocavity combined with tip-enhanced photoluminescence (TEPL) spectroscopy.

A research team, led by Prof. Di Dawei from the Zhejiang University College of Optical Science and Engineering, recently discovered that by using germanium (Ge), an environmentally friendly group-IV element, to partially substitute lead in the perovskite, it is possible to create highly luminescent perovskite materials and devices.

Schematic of the Ge'Pb PeLED device structure. Image from Nature Communications

To resolve the toxicity problem that arises from the use of lead, an effective method has been the use of tin (Sn) as a partial or full replacement of lead in the perovskite materials. This strategy has been particularly successful for perovskite solar cells. However, tin-based (including tin-lead) perovskite materials are generally very poor light emitters, causing unsatisfactory performance of tin-based perovskite light-emitting devices (LEDs).

A team of researchers from the universities of Marburg, Giessen and Paderborn has combined the advantages of two-dimensional materials and hybrid perovskites, to create new materials to benefit computer chips, light-emitting diodes and solar cells.

The team explains that the development of new two-dimensional materials has, to date, been rather limited to structures with layers of rigid chemical bonds in two spatial directions - like a sheet of paper in a stack. Now, for the first time, the research team led by Dr. Johanna Heine (Inorganic Chemistry, Philipps University of Marburg) has overcome this limitation by using an innovative concept. The researchers developed an organic-inorganic hybrid crystal which consists of chains in a single direction, yet still forms two-dimensional layers in spite of this. This makes it possible to combine different material components, like pieces in a construction set, to create tailored materials with innovative properties.

AMOLF researchers Lukas Helmbrecht and Wim Noorduin have developed a reactive ink that can be painted on an equally reactive canvas. The ink reacts with the material on the canvas to become a semiconductor that emits colored light, an essential part of electronic components such as LEDs.

Schematic illustration showing the fabrication of a spatially patterned perovskite film. Image from Advanced Materials

The AMOLF team's new ion exchange lithography technique enables one to paint a canvas by making the canvas itself change to a different color instead of brushing paint on it. In this technique, the 'ink' reacts with the 'canvas' by means of ion exchange. To demonstrate the technique, the team used it to airbrush an image of Madame Curie. 'I find it fascinating to see: the green image forms as soon as you start spraying, despite both the ink and the canvas being colorless', said Helmbrecht.

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A team of researchers from the National Renewable Energy Laboratory (NREL) and the University of Utah has developed a new type of LEDs that utilizes spintronics without needing a magnetic field, magnetic materials or cryogenic temperatures.

'The companies that make LEDs or TV and computer displays don't want to deal with magnetic fields and magnetic materials. It's heavy and expensive to do it,' said Valy Vardeny, distinguished professor of physics and astronomy at the University of Utah. 'Here, chiral molecules are self-assembled into standing arrays, like soldiers, that actively spin polarize the injected electrons, which subsequently lead to circularly polarized light emission. With no magnetic field, expensive ferromagnets and with no need for extremely low temperatures. Those are no-nos for the industry.'

A joint research project by scientists from several China-based universities and laboratories has developed a 2D perovskite material for highly efficient LEDs.

Effects of MeS on phase distribution of perovskite films. The yellow path indicates the exciton energy transfer between nanosheets of different thicknesses. Credit: Nature

LEDs are ubiquitous, but current high-quality LEDs still need to be processed at high temperatures and require elaborate deposition technologies ' which makes their production cost expensive. Scientists have recently realized that metal halide perovskites can be extremely promising candidates for next generation LEDs. These perovskites can be processed into LEDs from solution at room temperature, thus largely reducing their production cost. However, the electro-luminescence performance of perovskites in LEDs still has room for improvements.

Researchers from North Carolina State University and the University of Texas have developed and demonstrated a new approach for designing photonic devices. The new method enabled the team to control the direction and polarization of light from thin-film LEDs, overcoming the widely known obstacles of beam shaping that arise from their Lambertian nature. Such LEDs with directional and polarized light emission could be useful for many photonic applications.

'This is a fundamentally new device architecture for photonic devices,' says Franky So, corresponding author of a paper describing the work and Professor of Materials Science and Engineering at NC State. 'And we've demonstrated that, using our approach, directional and polarized emissions from an organic LED or a perovskite LED without external optical elements can be realized'.

Researchers at Linköping University in Sweden have developed efficient blue LEDs based on halide perovskites. The new LEDs could open the door to cheap and energy-efficient illumination.

Schematic of the PeLED structure and the HAADF cross-sectional device image. Image from Nature Communications

Illumination is responsible for approximately 20 percent of global electricity consumption, a figure that could be reduced significantly if all light sources consisted of light-emitting diodes (LEDs). The blue-white LEDs currently in use, however, need complicated manufacturing methods and are expensive, which makes it more difficult to achieve a global transition. LEDs manufactured from halide perovskites could be a cheaper and more eco-friendly alternative for both illumination and LED-based monitors.

A collaboration between University of Pennsylvania, Seoul National University, the Korea Advanced Institute of Science and Technology, the Ecole Polytechnique Fédérale de Lausanne, the University of Tennessee, the University of Cambridge, the Universitat de Valencia, the Harbin Institute of Technology, and the University of Oxford has yielded an understanding of how a class of electroluminescent perovskite materials can be designed to work more efficiently.

This latest work is based on a past endeavor by Penn theoretical chemist Andrew M. Rappe and Tae-Woo Lee at Seoul National University to develop a theory to help explain experimental results. The material that was studied was formamidinium lead bromide, a type of metal-halide perovskite nanocrystal (PNC). Results collected by the Lee group seemed to indicate that green LEDs made with this material were working more efficiently than expected. 'As soon as I saw their data, I was amazed by the correlation between the structural, optical, and light-efficiency results. Something special had to be going on,' says Rappe.