Quantum dot LEDs achieve record luminous efficiency

Researchers from Turkey’s Koç University have developed nanomaterial-based white light-emitting diodes (LEDs) that exhibit a luminous efficiency of 105 lumens per watt (lm/W) — a higher efficiency level than any other quantum dot-based white LEDs.

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. They have been described in the research journal of The Optical Society, Optica.

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 colours 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 colour temperatures and optical properties by combining quantum dots that generate different colours with a blue LED.

Quantum dots are also easy to make and the colour of their emission can be easily changed by increasing the size of the semiconductor particle. Moreover, quantum dots can be 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 Koç University 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 colours 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 research leader 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 colours 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 synthesised by mixing cadmium, selenium, zinc and sulfur at high temperatures. They 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 new LEDs use commercially available blue LEDs combined with flexible lenses, like the one shown here, filled with a solution of quantum dots. Image credit: Sedat Nizamoglu, Koç University.

Demonstrating their liquid-based white LEDs by using them to illuminate a 7″ display, the researchers showed that they could achieve an efficiency double that of LEDs that incorporate quantum dots in solid films. Quantum dots thus hold “great promise for efficient lighting applications”, said Nizamoglu, who added that “the synthesis and fabrication methods for making the quantum dots and the new LEDs are easy, inexpensive and applicable for mass production”.

The researchers are now 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, with Nizamoglu claiming, “There is still significant room for technology development that would generate more efficient approaches to lighting.” He and his fellow researchers believe the LEDs could eventually reach efficiencies over 200 lm/W, 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 Nizamoglu. “Replacing conventional lighting sources with LEDs with an efficiency of 200 lm/W would decrease the global electricity consumed for lighting by more than half. That reduction is equal to the electricity created by 230 typical 500 MW coal plants and would reduce greenhouse gas emissions by 200 million tons.”

Top image credit: Sedat Nizamoglu, Koç University.