Researchers created nanomaterial-based white LEDs
that exhibit a record high efficiency thanks to quantum dots that are
suspended in solution rather than embedded in a solid. The new LEDs could
offer an energy-efficient lighting source for homes, offices and televisions.
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.
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.
Source: The Optical Society
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