LED - Page 12

Scientists at Linköping University (LiU), along with colleagues from China, have shown how to achieve efficient perovskite light-emitting diodes (LEDs). The researchers provide guidelines on fabricating high-quality perovskite light emitters, and consequently high-efficiency perovskite LEDs.

Different metal oxide layers affect the properties of the thin perovskite films. Credit: Charlotte Perhammar

The halide perovskites can be easily prepared by low-cost solution processing from precursor solution comprising metal halides and organic halides. The resulting perovskites reportedly possess excellent optical and electrical properties, making them promising candidates for various kinds of optoelectronic devices, such as solar cells, LEDs and photodetectors.

Penn State researchers have found a new class of 2D perovskite materials with edges that are conductive like metals and cores that are insulating. The researchers said these unique properties may have applications in solar cells and nanoelectronics.

'This observation of the metal-like conductive states at the layer edges of these 2D perovskite materials provides a new way to improve the performance of next-generation optoelectronics and develop innovative nanoelectronics,' said Kai Wang, assistant research professor in materials science and engineering at Penn State and lead author on the study.

An international research team led by the University of Houston researchers have tackled a lingering question about how a two-dimensional perovskite crystal composed of cesium, lead and bromine emits a strong green light. Crystals that produce light on the green spectrum are desirable because green light, while valuable in itself, can also be relatively easily converted to other forms that emit blue or red light, making it especially important for optical applications ranging from light-emitting devices to sensitive diagnostic tools.

There was, however, confusion as to how the crystal, CsPB₂Br₅, produced the green photoluminescence. Several theories emerged, without a definitive answer. Now, the researcher team from the United States, Mexico and China, led by the University of Houston, have reported that they have used sophisticated optical and high-pressure diamond anvil cell techniques to determine not only the mechanism for the light emission but also how to replicate it.

Researchers at Linköping University have developed efficient perovskite near-infrared (NIR) light-emitting diodes. The external quantum efficiency is a record 21.6%. The work was led by Linköping scientist Feng Gao, in close collaboration with colleagues in China, Italy, Singapore and Switzerland.

The external quantum efficiency (the ratio of charge carriers emitted as light over all of those fed into the materials) of light-emitting diodes based on perovskites has until now been limited by defects that arise in the material during manufacture. The defects act as traps for the charge carriers and thus cause energy losses.

A team of scientists from Washington University in St. Louis, Oak Ridge National Laboratory and University of Missouri studied the structure and properties of the commonly occurring planar defects at the atomic scale of lead halide perovskite.

When these materials are made, defects can occur where different crystals meet, known as grain boundaries. In conventional semiconductors, these defects can decrease their electrical conductivity and the solar energy-to-electricity conversion efficiency; however, in lead-halide perovskites, there are differing experimental reports on the activity of grain boundaries. In some cases, they are found to be harmful, while in other cases they either have no impact on performance or are even beneficial. But, to date, no one understood why. The research team in this work set out to discover these reasons.

A recent joint-research co-led by City University of Hong Kong (CityU) and Shanghai University has developed an efficient fabrication approach for all-inorganic perovskite films with better optical properties and stability, enabling the development of high color-purity and low-cost perovskite LEDs with a high operational lifetime.

The team has found that using cesium trifluoroacetate (TFA) as the cesium source in the one-step solution coating, instead of the commonly used cesium bromide (CsBr), enables fast crystallization of small-grained CsPbBr₃ perovskite crystals, forming the smooth and pinhole-free perovskite films. This is because the interaction of TFA- anions with Pb2+ cations in the CsPbX3 precursor solution greatly improves the crystallization rate of perovskite films and suppresses surface defects.

Researchers from the Ulsan National Institute of Science and Technology (UNIST) have developed perovskite LEDs (PeLED) which are flexible enough to be folded. A transparent material was used in the electrode of the device as a replacement for metal to ensure translucency.

Flexible translucent PeLED maintains performance even when bending curvature is small

According to the team, PeLED is a kind of light emitting diode (LED) that emits light by injecting current into a compound. This device uses a perovskite material as an active layer that emits light by receiving electricity, and its advantages include high electron mobility, good color purity, and easy color control. However, conventional PeLEDs are low in flexibility and opaque due to limitations of metal electrodes.

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Researchers at Huazhong University of Science and Technology (HUST) in China, University of Toledo in the U.S, Monash University in Australia, Jilin University and Tsinghua University in China, the Dalian Institute in China and the University of Toronto in Canada have examined a lead-free double perovskite that exhibited stable and efficient white light emission. In its mechanism of action, the material produced self-trapped excitons (STEs) due to Jahn-Teller distortion of the AgCl₆ octahedron in the excited state of the complex, observed when investigating exciton-phonon coupling in the crystal lattice.

The research team stated that a fifth of global electricity consumption is based on lighting, and efficient and stable white-light emission with single materials is ideal for such applications. Photon emission that covers the entire visible spectrum is, however, difficult to attain with a single material. Metal halide perovskites, for instance, have outstanding emission properties but contain lead, and so yield unsatisfactory stability. The perovskite in this study is, therefore, lead-free.

Researchers at the University of Cambridge have announced a new efficiency record for LEDs based on perovskite semiconductors, reportedly rivaling that of the best organic LEDs (OLEDs).

The team stated that compared to OLEDs, which are widely used in high-end consumer electronics, the perovskite-based LEDs can be made at much lower costs, and can be tuned to emit light across the visible and near-infrared spectra with high color purity.