Record Efficiency for Quantum Dot White LED

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. (Source: S. Nizamoglu, Koç Univ.)

Researchers have demonstrated nano­material-based white-light-emitting diodes that exhibit a record luminous effi­ciency of 105 lumens per watt. 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 new LEDs use commer­cially available blue LEDs combined with flexible lenses filled with a solution of nano-sized semi­conductor 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.”

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: OSA)

Reference: S. Sadeghi et al.: Quantum dot white LEDs with high luminous efficiency, Optica 5, 793 (2018); DOI: 10.1364/OPTICA.5.000793

Link: Graduate School of Materials Science and Engineering, Koç University, Istanbul, Turkey

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