One of the main barriers to a wider adoption of OLED technology resides in its lack of efficiency compared to fluorescent lamps or Light-emitting diodes (LED). The SOLED project hoped to solve this problem using chiral organic semiconductor structures. The difference is undisputable: when put side by side with an LED display, its OLED counterpart will stand out thanks to its sharper images, better contrast and crisp colours.
Energy efficiency, however, is a key concern for consumers, and OLED is still lagging behind other technologies in this regard. In fact, the only type of display it can top is LCD, but only marginally.
To solve this problem, the Weizmann Institute kicked off the SOLED (Chiral organic semiconductor structures) project in January 2016. They aimed to tackle the OLED efficiency problem at its source:
‘The low efficiency of OLED technology is a result of low light emission yield due to the formation of triplet electronic states, in which the two electrons have the same orientation,’ explains Prof. Ron Naaman, coordinator of SOLED.
The project’s plan was to use electrons’ spin control with a view to reducing the probability of producing triplet states. This is known as the spin-LED/OLED concept: electrons injected into and from the light-emitting species have a predetermined spin, which helps avoid the formation of ‘dark’, non-emitting triplet states.
The team had already benefitted from past experience in this field. They could capitalise on their earlier research on the Chiral-induced spin selectivity (CISS) effect, and proposed to develop chiral organic semiconductor structures to control the spin state of injected electrons and holes in OLEDs.
As they initiated the SOLED project, they expected this effect to be able to increase the energy efficiency of OLED devices by a factor of four.
‘The chiral-induced spin selectivity effect is supposed to allow full control of the electrons’ spin orientation by ensuring that the electron that leaves the emitting molecule has the same spin orientation as the electron entering into the molecule,’ says Prof. Naaman.
Whilst the concept was successfully demonstrated in principle, the team quickly realised that further research would be required to reach their objective. ‘In collaboration with the group of Richard Friend from Cambridge and E. W. (Bert) Meijer from Eindhoven, we could demonstrate our ability to affect the spin orientation in the OLED, but the efficiency of the process was not very high,’ Prof. Naaman explains.
‘The reason for it is the organisation of the molecules in the OLED. Now, we pursue this work with our collaborators towards better control of material organisation.’
Until this problem is solved, the team has had to postpone the pre-commercialisation measures they had originally planned for. However, Prof. Naaman is still hopeful that the technology will help OLED technology spread throughout European homes in the form of flexible light emitters. He also underlines the realisation that material organisation is the key factor in achieving spin control as a major outcome for the project.
‘We intend to study molecules that self-assemble into three dimensional organised structures, like micro-crystals. We hope to do that under either the FET-OPEN programme or other specific programmes,’ Prof. Naaman concludes.