Liquid-Based White LEDs Achieve High Luminous Efficiency

Quantum dot (QD) white LEDs that show a luminous efficiency of 105 lm/W have been developed. The QDs are liquid-based and, according to researchers, could help the LEDs achieve an efficiency double that of LEDs that incorporate quantum dots in solid films. With further development, researchers say the new LEDs could reach efficiencies over 200 lm/W.

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. Courtesy of Sedat Nizamoglu, Koç University.

The new LEDs use commercially available blue LEDs combined with flexible lenses filled with QDs. Light from the blue LED causes the QDs to emit green and red, which combine with the blue emission to create white light.

To make the white LEDs, researchers from Koç University filled the space between a polymer lens and LED chip with a solution of QDs that were synthesized by mixing cadmium, selenium, zinc, and sulfur at high temperatures. The researchers used silicone to make the lens because its elasticity would allow them to inject the solution into the lens without any solution leaking out, and because silicone’s transparency would enable light transmission.

The new LEDs use flexible lenses, such as the one shown here, filled with a solution of quantum dots. Courtesy of Sedat Nizamoglu, Koç University.

The team carried out more than 300 synthesis reactions to identify the best conditions, such as temperature and time of the reaction, for making QDs that would 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 researcher Sedat 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.”

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

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

The team said that the synthesis and fabrication methods it used for making the QDs and the new LEDs were inexpensive and applicable for mass production.

Pictured from left to right are Mohammad Mohammadi Aria, Sedat Nizamoglu, Sadra Sadeghi, and Rustamzhon Melikov from Koç University in Turkey. Courtesy of Sedat Nizamoglu, Koç University.

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.

“Efficient LEDs have a strong potential for saving energy and protecting the environment. 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, " said Nizamoglu. "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.”

Unlike the phosphors that are used to create white light with today’s LEDs, QDs 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 QDs that generate different colors with a blue LED. Quantum dots are also easy to make, and the color of their emission can be easily changed by increasing the size of the semiconductor particle. By changing their concentration, QDs can be used to generate warm white light sources (e.g., incandescent light bulbs) or cool white sources (e.g., typical fluorescent lamps).

The research was published in Optica, a publication of OSA, The Optical Society (doi:10.1364/OPTICA.5.000793).