Perovskite phosphor boosts visible light communication: Flashy nanocrystals help LEDs send data in the blink of an eye


Flashy NP Perovskite 1466097172999A green-emitting perovskite nanocrystal phosphor mixed with a red-emitting nitride phosphor looks yellow under ambient light (left). When excited by blue laser light, the phosphor combination produces white light (right).
Credit: Osman Bakr

 

Light-emitting diodes (LEDs) are increasingly used to illuminate homes and offices; soon, the same lights could also transmit data to your computer or smartphone in photon pulses so fast the eye can’t see them. But this form of visible light communication faces two key challenges: The light must flicker fast enough to carry sizeable amounts of data; and at the same time it should provide the warm, balanced color tones needed for pleasant ambient lighting.

 

Nanocrystals of cesium lead bromide (CsPbBr3) could help to solve both problems, according to a team led by Boon S. Ooi and Osman M. Bakr at King Abdullah University of Science & Technology (KAUST). They have found that LEDs coated with the material can reach high data transmission rates of 2 gigabits per second, comparable to the fastest Wi-Fi, while producing a quality of light that matches commercial white-light LEDs (ACS Photonics 2016, DOI: 10.1021/acsphotonics.6b00187).

 

Visible light communication, sometimes called Li-Fi, is already finding real-world applications. Last year, for example, Dutch company Phillips installed a smart LED system in a French supermarket that uses Li-Fi to transmit discount offers to shoppers’ cellphones, based on their location in the store. If data rates could be increased significantly, Li-Fi might add much-needed capacity to congested Wi-Fi networks that rely on radio waves.

 

And since the smart LEDs are doing double duty, by providing both lighting and communication, they offer an economical solution, says Bakr. Ooi adds that these systems do not even need a direct line of sight between LED and computer: “As long as your device can see light, you can detect a signal,” he says.

 

White-light LEDs typically contain a blue LED coated with phosphors that turn some of the light into green and red. But most phosphors take too long to recover between excitation and emission, pulsing no more than a few million times per second. Last year, other researchers showed that polymer semiconductors could reach more than 200 MHz (ACS Photonics2015, DOI: 10.1021/ph500451y).

 

The KAUST team instead turned to CsPbBr3, part of a family of materials known as perovskites that have become the darling of the photovoltaic research community. Perovskite solar cells have seen remarkable efficiency gains over the past seven years, and the materials are cheap and relatively easy to prepare in solution.

 

The team created nanocrystals of the perovskite, roughly 8 nm across, and found that their green emission faded in just seven nanoseconds. This allowed them to pulse reliably at almost 500 MHz, setting what the researchers believe is a new record for LED phosphors. “It is an extremely impressive and important achievement,” says Ted Sargent of the University of Toronto, who works on optoelectronic materials and has collaborated with the KAUST group in the past.

 

The rapid response is partly due to the size of the crystals, Bakr explains. When blue light excites an electron in the material, it forms an electron-hole pair called an exciton. The confines of the tiny crystal change the exciton’s energy levels, making the electron more likely to recombine with its hole and emit a photon.

 

When the researchers teamed the perovskite phosphor with a commercial red-emitting phosphor and a blue gallium nitride LED, the device produced a warm white light with a color rendering index of 89, as good as white LEDs already on the market (natural sunlight itself is rated at 100). “This quality makes this material ideal for low-power indoor illumination,” Sargent says.

 

Jakoah Brgoch of the University of Houston, who develops novel phosphors for LED lighting, says that it is relatively easy to fine-tune the chemistry of perovskites by substituting different halides or metal ions. “That means there’s a lot of potential to improve these properties,” he says.

Chemical & Engineering News ISSN 0009-2347 Copyright © 2016 American Chemical Society

Quantum Dots are ‘Ready for Prime Time’ says Analysts, Yole Development


Yole Développement says revenues “will exceed phosphors by 2020” as adoption into LCD TVs rivals OLED quality.

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Quantum dots’ virtual adoption cycle, according to Yole Développement

Yole Développement (Yole), the Lyon, France-based market research and strategic

consulting company, has published its new LED down converters technology and market

report, entitled Phosphors & Quantum Dots 2015: LED Down Converters for Lighting & Displays.

It presents a detailed review of the industry, especially the impact of the development of

quantum dots on the display and conventional phosphors industry. Yole asks, are quantum dots

now a serious competitor to OLED-based technologies – and its conclusion is: quantum dots

are finally ready for prime time and will exceed traditional phosphor revenue by 2020 by

allowing LCD to compete with OLED in the race for the next generation of displays.”

After the lukewarm reception of 3D and 4K screens, Yole comments that the display

industry needs a “new and disruptive experience improvement” to bring consumers back

to the stores: “image quality perception increases significantly when color gamut and

dynamic contrast ratio are improved.” Yole also notes that “Leading movie studios,

content providers, distributors and display makers have together formed the UHD Alliance

to promote those features.”

Dr Eric Virey, Senior Analyst, LEDs at Yole, commented, “OLED was believed to be

the technology of choice for this next generation of displays. But production challenges

have delayed the availability of affordable OLED TVs. LCD TVs with LED backlights

based on quantum dot down-converters can deliver performance close to, or even

better than OLED in some respects, and at a lower cost.”

 

QD-LCD ‘could pull ahead’ of OLED display

Until OLEDs are ready, says Yole, “QD-LCD technology will have a unique window of

opportunity to try to close enough of the performance gap such that the majority of

consumers will not be able to perceive the difference between the two technologies

so price would become the driving factor in the purchasing decision.” Under this scenario,

the analyst believes that QD-LCD could establish itself as the dominant technology while

struggling OLEDs “would be cornered into the high end of the market.”

Yole acknowledges that OLED-based displays potentially offer more opportunities for

differentiation but the analyst notes, “OLED proponents need to invest massively and

still have to resolve manufacturing yield issues. For tier-2 LCD panel makers who

cannot invest in OLED, Quantum Dots offer an opportunity to boost LCD performance

without imposing additional CAPEX on their fabs.” At this year’s Consumer Electronics

show, as optics.org reported, no fewer than seven leading TV OEMs including

Samsung and LG demonstrated QD-LCD TVs.

 

With tunable and narrowband emissions, QDs offer design flexibility to developers

of new displays. But more is needed to enable massive adoption, including the d

evelopment of cadmium-free formulations. Cole cautions that “traditional phosphors

still have to say their last word”. If PFS could further improve in term of stability and decay

time and a narrow-band green composition was to emerge, traditional phosphors could

also be part of the battle against OLED, Yole concludes.

Yole’s analysis Phosphors & Quantum Dots 2015: LED Down-converters for Lighting & Displays

presents an overview of the quantum dot LED market for display and lighting applications

including quantum dot manufacturing, benefits and drawbacks, quantum dots LCD versus

OLED and detailed market forecast. For more information about this report and other

LED technology & market analysis from Yole, visit i-micronews in its LED Reports section.

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