Blue quantum dot tech improves power efficiency and colour accuracy of LEDs

Quantum dots are nanoscale crystals capable of emitting light of different colours which can bring greater power efficiency, brightness and colour purity to displays than previous generations.

Major manufacturers such as Samsung are already using the tech in some of their premium displays to improve their HDR capabilities.

Now, a team of researchers from the University of Tokyo have developed a way to construct blue quantum dots – typically the most difficult colour to manufacture – to be more accurate, efficient and cost effective than existing quantum dots.

The new method is based on self-organising chemical structures and a cutting-edge imaging technique.

While pixels can appear almost any colour to the human eye, they are not actually the smallest element on your screen as they are typically made up of subpixels which are red, green and blue.

The variable intensity of these subpixels gives the individual pixels the appearance of a single colour from a palette of billions.

Displays based on QD-LEDs already exist, but the technology is still maturing, and current options have some drawbacks, specifically regarding the blue subpixels within them.

Blue subpixels are the most important, because, through a process called down-conversion, blue light is used to generate green and red light. This means that blue quantum dots require more tightly controlled physical parameters making them highly complex and costly to produce.

“Previous design strategies for blue quantum dots were very top down, taking relatively large chemical substances and putting them through a series of processes to refine them into something that worked,” said team leader professor Eiichi Nakamura.

“Our strategy is bottom up. We built on our team’s knowledge of self-organising chemistry to precisely control molecules until they form the structures we want. Think of it like building a house from bricks rather than carving one from stone. It’s much easier to be precise, design the way you want, and is more efficient and cost effective too.”

When exposed to ultraviolet light, the newly developed quantum dots produce nearly perfect blue light, according to the international standard for measuring colour accuracy, known as BT.2020.

This is due to the unique chemical makeup of their dot, a hybrid mix of organic and inorganic compounds including lead perovskite, malic acid and oleylamine. Through self-organisation these can be coaxed into the form required, which is a cube of 64 lead atoms, four to a side.

“Surprisingly, one of our biggest challenges was in finding that malic acid was a key piece of our chemical puzzle. It took over a year methodically trying different things to find it,” said Nakamura.

“Perhaps less surprising is that our other main challenge was to determine the structure of our blue quantum dot. At 2.4 nanometres, 190 times smaller than the wavelength of the blue light we sought to create with it, the structure of a quantum dot cannot be imaged by conventional means. So, we turned to an imaging tool pioneered by some of our team known as SMART-EM, or ‘cinematic chemistry’ as we like to call it.”

Cinematic chemistry is an evolution of electron microscope imaging that is more akin to shooting a video than taking a still image.

For capturing details of the structure of the blue quantum dot, this is essential, as the nanocrystal is actually quite dynamic, so any single image of it would only tell a small piece of its story.

Unfortunately, the new blue quantum dots are quite short-lived, although the researchers expected this and are now aiming to improve their stability.