Shedding light on the development of efficient blue-emitting semiconductors

September 17, 2020

Artificial light accounts for approximately 20% of the total electricity consumed globally. Considering the present environmental crisis, this makes the discovery of energy-efficient light-emitting materials particularly important, especially those that produce white light. Over the last decade, technological advances in solid-state lighting, the subfield of semiconductors research concerned with light-emitting compounds, has led to the widespread use of white LEDs. However, most of these LEDs are actually a blue LED chip coated with a yellow luminescent material; the emitted yellow light combined with the remaining blue light produces the white color.

Therefore, a way to reduce the energy consumption of modern white LED lights is to find better blue-emitting semiconductors. Unfortunately, no known blue-emitting compounds were simultaneously highly efficient, easily processible, durable, eco-friendly, and made from abundant materials--until now.

In a recent study, published in Advanced Materials, a team of scientists from Tokyo Institute of Technology, Japan, discovered a new alkali copper halide, Cs5Cu3Cl6I2, that fills all the criteria. Unlike Cs3Cu2I5, another promising blue-emitting candidate for future devices, the proposed compound has two different halides, chloride and iodide. Although mixed-halide materials have been tried before, Cs5Cu3Cl6I2 has unique properties that emerge specifically from the use of I? and Cl? ions.

It turns out that Cs5Cu3Cl6I2 forms a one-dimensional zigzag chain out of two different subunits, and the links in the chain are exclusively bridged by I? ions. The scientists also found another important feature: its valence band, which describes the energy levels of electrons in different positions of the material's crystalline structure, is almost flat (of constant energy). In turn, this characteristic makes photo-generated holes--positively charged pseudoparticles that represent the absence of a photoexcited electron--"heavier." These holes tend to become immobilized due to their strong interaction with I? ions, and they easily bond with nearby free electrons to form a small system known as an exciton.

Excitons induce distortions in the crystal structure. Much like the fact that one would have trouble moving atop a suspended large net that is considerably deformed by one's own weight, the excitons become trapped in place by their own effect. This is crucial for the highly efficient generation of blue light. Professor Junghwan Kim, who led the study, explains: "The self-trapped excitons are localized forms of optically excited energy; the eventual recombination of their constituting electron-hole pair causes photoluminescence, the emission of blue light in this case."

In addition to its efficiency, Cs5Cu3Cl6I2 has other attractive properties. It is exclusively composed of abundant materials, making it relatively inexpensive. Moreover, it is much more stable in air than Cs3Cu2I5 and other alkali copper halide compounds. The scientists found that the performance of Cs5Cu3Cl6I2 did not degrade when stored in air for three months, while similar light-emitting compounds performed worse after merely days. Finally, Cs5Cu3Cl6I2 does not require lead, a highly toxic element, making it eco-friendly overall.

Excited about the results of the study, Prof. Kim concludes: "Our findings provide a new perspective for the development of new alkali copper halide candidates and demonstrate that Cs5Cu3Cl6I2 could be a promising blue-emitting material." The light shed by this team of scientists will hopefully lead to more efficient and eco-friendly lighting technology.

Tokyo Institute of Technology

Related Crystal Structure Articles from Brightsurf:

Getting single-crystal diamond ready for electronics
Researchers from Osaka University and collaborating partners polished single-crystal diamond to near-atomic smoothness without damaging it.

Crystal structure of SARS-CoV-2 papain-like protease
The pandemic of coronavirus disease 2019 (COVID-19) is changing the world like never before.

Crystal structure of SARS-CoV-2 nucleocapsid protein RNA binding domain reveals potential unique drug targeting sites
Crystal structure of SARS-CoV-2 nucleocapsid protein RNA binding domain reveals potential unique drug targeting sites.

Photonic crystal light converter
Spectroscopy is the use of light to analyze physical objects and biological samples.

Crystal structure discovered almost 200 years ago could hold key to solar cell revolution
Solar energy researchers are shining their scientific spotlight on materials with a crystal structure discovered nearly two centuries ago.

Crystal wars
Scientists at The University of Tokyo and Fudan University researched the process of crystallization in which competing structural forms coexist.

Melting a crystal topologically
Physicists at EPFL have successfully melted a very thin crystal of magnetic quasi-particles controllably, as turning ice into water.

The makings of a crystal flipper
Hokkaido University scientists have fabricated a crystal that autonomously flips back and forth while changing its flipping patterns in response to lighting conditions.

Crystal power
Scientists at the US Department of Energy's Argonne National Laboratory have created and tested a single-crystal electrode that promises to yield pivotal discoveries for advanced batteries under development worldwide.

Pressing 'pause' on nature's crystal symmetry
From snowflakes to quartz, nature's crystalline structures form with a reliable, systemic symmetry.

Read More: Crystal Structure News and Crystal Structure Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to