New molecules will improve screen displays

A new research project from Harvard University has isolated more than 1000 new blue-light emitting molecules for organic light-emitting diodes (OLEDs) that could lead to dramatic improvements in displays for televisions, phones and tablets.

OLED screens use organic molecules that emit light when an electric current is applied. Unlike ubiquitous liquid crystal displays (LCDs), OLED screens don’t require a backlight, meaning the display can be as thin and flexible as a sheet of plastic. Individual pixels can be switched on or entirely off, dramatically improving the screen’s colour contrast and energy consumption. OLEDs are already replacing LCDs in high-end consumer devices but a lack of stable and efficient blue materials has made them less competitive in large displays such as televisions.

An interdisciplinary team of Harvard researchers has collaborated with others from MIT and Samsung to develop a large-scale, computer-driven screening process, called the Molecular Space Shuttle. They say that the process incorporates theoretical and experimental chemistry, machine learning and cheminformatics to identify new OLED molecules that perform as well as, or better than, industry standards.

The research is described in the current issue of Nature Materials, according to a report on Science Daily.

Like LCDs, OLEDs rely on green, red and blue subpixels to produce full colour, but the biggest challenge in the manufacture of affordable OLED is emission of the colour blue as it has proven to be difficult to find organic molecules that efficiently emit blue light. The report says that OLED producers have previously created organometallic molecules with expensive transition metals like iridium to enhance the molecule through phosphorescence and, apart from being an expensive solution, it is yet to achieve a stable outcome.

To that end, research head Alán Aspuru-Guzik and his team sought to replace these organometallic systems with entirely organic molecules.

They began by building libraries of more than 1.6 million candidate molecules. Then, to narrow the field, a team of researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), led by Ryan Adams, Assistant Professor of Computer Science, developed new machine learning algorithms to predict which molecules were likely to have good outcomes, and prioritise those to be virtually tested. This effectively reduced the computational cost of the search by at least a factor of 10 and is what researchers describe as a ‘natural collaboration between chemistry and machine learning’.

“Machine learning tools are really coming of age and starting to see applications in a lot of scientific domains,” said Adams.

“This collaboration was a wonderful opportunity to push the state of the art in computer science, while also developing completely new materials with many practical applications. It was incredibly rewarding to see these designs go from machine learning predictions to devices that you can hold in your hand,” he said.

“Molecules are like athletes,” Aspuru-Guzik said. “It’s easy to find a runner, it’s easy to find a swimmer, it’s easy to find a cyclist, but it’s hard to find all three. Our molecules have to be triathletes. They have to be blue, stable and bright.”

Tim Hirzel, senior software engineer in the Department of Chemistry and Chemical Biology and co author of the paper, said that finding the molecules is more than raw computing power, it also requires human intuition. Hirzel and the team built a web application for collaborators to explore the results of more than half a million quantum chemistry simulations. The 2500 most promising molecules were identified and collaborators then voted via a tool the team nicknamed ‘molecular Tinder’.

“We facilitated the social aspect of the science in a very deliberate way,” said Hirzel.

After this accelerated design cycle, the team was left with hundreds of molecules that perform as well as, if not better than, state-of-the-art, metal-free OLEDs.

The researchers say that applications of this type of molecular screening also extend far beyond OLEDs.

“This research is an intermediate stop in a trajectory towards more and more advanced organic molecules that could be used in flow batteries, solar cells, organic lasers and more,” said Aspuru-Guzik.

“The future of accelerated molecular design is really, really exciting,” he said.

Image credit: © iStockphoto.com/Vladgrin