Quantum Dot White LEDs Achieve Record Efficiency

Researchers have demonstrated nanomaterial-based white-light-emitting
diodes (LEDs) that exhibit a record luminous efficiency of 105 lumens
per watt. Luminous efficiency is a measure of how well a light source
uses power to generate light. With further development, the new LEDs
could reach efficiencies over 200 lumens per watt, making them a
promising energy-efficient lighting source for homes, offices and
televisions.

“Efficient LEDs have strong potential for saving energy and protecting
the environment,” said research leader Sedat Nizamoglu, Koç University,
Turkey. “Replacing conventional lighting sources with LEDs with an
efficiency of 200 lumens per watt would decrease the global electricity
consumed for lighting by more than half. That reduction is equal to the
electricity created by 230 typical 500-megawatt coal plants and would
reduce greenhouse gas emissions by 200 million tons.”

The researchers describe how they created the high-efficiency white LEDs
in Optica,
The Optical Society’s journal for high impact research. The new LEDs use
commercially available blue LEDs combined with flexible lenses filled
with a solution of nano-sized semiconductor particles called quantum
dots. Light from the blue LED causes the quantum dots to emit green and
red, which combines with the blue emission to create white light.

“Our new LEDs reached a higher efficiency level than other quantum
dot-based white LEDs,” said Nizamoglu. “The synthesis and fabrication
methods for making the quantum dots and the new LEDs are easy,
inexpensive and applicable for mass production.”

Advantages of quantum dots

To create white light with today’s LEDs, blue and yellow light are
combined by adding a yellowish phosphor-based coating to blue LEDs.
Because phosphors have a broad emission range, from blue to red, it is
difficult to sensitively tune the properties of the generated white
light.

Unlike phosphors, quantum dots generate pure colors because they emit
only in a narrow portion of the spectrum. This narrow emission makes it
possible to create high-quality white light with precise color
temperatures and optical properties by combining quantum dots that
generate different colors with a blue LED. Quantum dots also bring the
advantage of being easy to make and the color of their emission can be
easily changed by increasing the size of the semiconductor particle.
Moreover, quantum dots can be advantageously used to generate warm white
light sources like incandescent light bulbs or cool white sources like
typical fluorescent lamps by changing the concentration of incorporated
quantum dots.

Although quantum dots embedded in a film are currently used in LED
televisions, this lighting approach is not suitable for widespread use
in general lighting applications. Transferring the quantum dots in a
liquid allowed the researchers to overcome the problematic drop in
efficiency that occurs when nanomaterials are embedded into solid
polymers.

Making efficient white LEDs requires quantum dots that efficiently
convert blue light to red or green. The researchers carried out more
than 300 synthesis reactions to identify the best conditions, such as
temperature and time of the reaction, for making quantum dots that emit
at different colors while exhibiting optimal efficiency.

“Creating white light requires integrating the appropriate amount of
quantum dots, and even if that is accomplished, there are an infinite
number of blue, green and red combinations that can lead to white,” said
Nizamoglu. “We developed a simulation based on a theoretical approach we
recently reported and used it to determine the appropriate amounts and
best combinations of quantum dot colors for efficient white light
generation.”

To make the new LEDs, the researchers filled the space between a polymer
lens and LED chip with a solution of quantum dots that were synthesized
by mixing cadmium, selenium, zinc and sulfur at high temperatures. The
researchers used a type of silicone to make the lens because its
elasticity allowed them to inject solutions into the lens without any
solution leaking out, and the material’s transparency enabled the
necessary light transmission.

The researchers showed that their liquid-based white LEDs could achieve
an efficiency double that of LEDs that incorporate quantum dots in solid
films. They also demonstrated their white LEDs by using them to
illuminate a 7-inch display.

“Quantum dots hold great promise for efficient lighting applications,”
said Nizamoglu. “There is still significant room for technology
development that would generate more efficient approaches to lighting.”

As a next step, the researchers are working to increase the efficiency
of the LEDs and want to reach high efficiency levels using
environmentally friendly materials that are cadmium- and lead-free. They
also plan to study the liquid LEDs under different conditions to ensure
they are stable for long-term application.

Paper: S. Sadeghi, B. G. Kumar, R. Melikov, M. M. Aria, H. B.
Jalali, S. Nizamoglu “Quantum Dot White LEDs with High Luminous
Efficiency,” Optica, Volume 5, Issue 7Pages: 793-802
(2018). DOI: https://doi.org/10.1364/OPTICA.5.000793

About Optica

Optica is an open-access, online-only journal dedicated to the
rapid dissemination of high-impact peer-reviewed research across the
entire spectrum of optics and photonics. Published monthly by The
Optical Society (OSA), Optica provides a forum for pioneering
research to be swiftly accessed by the international community, whether
that research is theoretical or experimental, fundamental or applied. Optica
maintains a distinguished editorial board of more than 50 associate
editors from around the world and is overseen by Editor-in-Chief Alex
Gaeta, Columbia University, USA. For more information, visit Optica.

About The Optical Society

Founded in 1916, The Optical Society (OSA) is the leading professional
organization for scientists, engineers, students and business leaders
who fuel discoveries, shape real-life applications and accelerate
achievements in the science of light. Through world-renowned
publications, meetings and membership initiatives, OSA provides quality
research, inspired interactions and dedicated resources for its
extensive global network of optics and photonics experts. For more
information, visit osa.org.

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