Spinning, spinning light could power next-generation electronics

Researchers, led by the University of Cambridge and Eindhoven University of Technology, have made an organic semiconductor that forces electrons to move in a spiral pattern. This could improve the efficiency of OLED displays in televisions and smartphones. Or spinning light could power next-generation computing technologies such as spintronics and quantum computers.

The semiconductor they developed emits circularly polarised light. That means the light contains information about the 'left- or right-handedness' of electrons. The internal structure of most inorganic semiconductors, such as silicon, is symmetric. That means electrons move through it without any preferred direction.

In nature, however, molecules often have a chiral (left- or right-handed) structure: like human hands, chiral molecules are mirror images of each other. Chirality plays an important role in biological processes such as DNA formation. However, it is a difficult phenomenon to exploit and control in electronics.

By using molecular design tricks inspired by nature, the researchers were able to create a chiral semiconductor. They did this by pushing stacks of semiconducting molecules to form ordered right-handed or left-handed spiral columns. Their results are reported in the journal Science.

A promising application for chiral semiconductors is display technology. Current screens often waste a significant amount of energy because of the way screens filter light. The chiral semiconductor developed by the researchers naturally emits light. The semiconductor does this in a way that could reduce these losses, making screens brighter and more energy efficient.

"When I started working on organic semiconductors, many people doubted their potential. But now they dominate display technology," says Professor Sir Richard Friend of the Cavendish Laboratory in Cambridge, who co-led the research.

"Unlike rigid inorganic semiconductors, molecular materials offer incredible flexibility. This allows us to design entirely new structures, such as chiral LEDs. It's like working with a Lego set with every shape you can think of, instead of just rectangular bricks."

The semiconductor is based on a material called triazatruxene (TAT) that self-assembles into a spiral stack. This allows electrons to slide along the structure in a spiral pattern, like along the thread of a screw. "When TAT is activated by blue or ultraviolet light, it emits bright green light with strong circular polarisation. It is an effect that has so far been difficult to achieve in semiconductors," says TU/e co-first author Marco Preuß. "The structure of TAT allows electrons to move efficiently and influences how light is emitted."

By adapting OLED manufacturing techniques, the researchers have successfully incorporated TAT into working OLEDs with circularly-polarised OLEDs (CP-OLEDs). These devices are the best of their kind, as they achieve record levels in efficiency, brightness and polarisation. "We have essentially improved the standard recipe for making OLEDs like we have in our smartphones. We can now capture a chiral structure in a stable, non-crystallising matrix," says co-first author Rituparno Chowdhury of the Cavendish Laboratory in Cambridge. "This provides a practical way to make circularly polarised LEDs. It is something the field has been trying to pull off for a long time."

The work is part of a decades-long collaboration between Friend's research group and Professor Bert Meijer's group at Eindhoven University of Technology. "This is a real breakthrough in making a chiral semiconductor," says Meijer. "By carefully designing the molecular structure, we have linked the chirality of the structure to the motion of the electrons, and this has never been done at this level before." Chiral semiconductors represent a step forward in the world of organic semiconductors, which now represent an industry worth more than USD 60 billion.

Besides displays, this development may also have an impact on quantum computing and spintronics - a field of research that uses the spin, or inherent angular momentum, of electrons to store and process information, potentially leading to faster and more secure computing systems. The research was co-sponsored by the European Union's Marie Curie Training Network and the European Research Council.

Source: TU/e

Opening photo: Microscope image of the green-fluorescent ordered regions after crystallisation in the OLED device. Image: Ritu Chowdhury

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