Breakthrough in OLEDs could lead to organic lasers

An international team of researchers has announced that by adjusting the structures that make up organic light emitting diodes (OLEDs), they were able to produce a brighter, lower-power light source. 

Should this discovery prove scalable, it might very well lead to the long sought-after OLED-based laser diode.

OLEDs are made from carbon-containing materials. They’re considered the future of optoelectronics due to their versatility, especially when it comes to display technologies, where they can be used to make low-power displays so thin they can wrap or fold around other structures.

One thing that OLEDs have not proven useful for is laser technology; that is, until now. 

Researchers from the University of California (Santa Barbara) and Kyushu University announced the key to their discovery was in being able to confine charge transport and recombination to nanoscale areas. This, they found, led to a suppression of heating and a preventing of charge recombination. 

“An important effect of suppressing roll-off is an increase in the efficiency of devices at high brightness,” said Chihaya Adachi of Kyushu University, who is a co-author of the paper. “This results in lower power to obtain the same brightness.”

“For years scientists working in organic semiconductors have dreamed of making electrically-driven organic lasers,” said Thuc-Quyen Nguyen of the University of California, Santa Barbara, another co-author. “Lasers operate in extreme conditions with electric currents that are significantly higher than those used in common displays and lighting. At these high currents, energy loss processes become stronger and make lasing difficult. 

“We see this work, which reduces some loss processes, as one step on the road toward realizing organic lasers,” Nguyen added. 

OLED-based lasers require current densities of thousands of amperes per square centimeter. Until now, development of this particular technology has been limited due to issues with heating; specifically, electrons crashing into one another, forcing them to release their energy in forms other than light. 

In previous work, Adachi and his colleagues showed OLED performance at current densities over 1 kA/cm2 and without the higher level of efficiency necessary for lasers and bright lighting. In their latest paper, the group sows that the efficiency problem can be solved by using electron-beam lithography to produce finely-patterned OLED structures that are capable of supporting charge density injection of 2.8 kA/cm2 while maintaining 100 times higher luminescent efficiency than anything previously observed. 

“As a simple visualization,” Adachi explained, “one can think of an organic semiconductor as a subway train with someone sitting in every seat. The seats represent molecules and the people represent energetic particles, i.e., electrons. When people board the train from one end, they have extra energy and want to go to the relaxed state of sitting. As people board, some of the seated people rise and exit the train at the other end leaving empty seats, or 'holes,' for the standing people to fill. When a standing person sits, the person goes to a relaxed state and releases energy. In the case of OLEDs, the person releases the energy as light.”

He concludes: “In our device structure, we have effectively confined the entrance and exit to the middle of the train. People diffuse to the two less crowded ends of the train and reduce collisions and annihilation.”

The article, “Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm,” was published in the journal Applied Physics Letters on March 2, 2015.

You can access it here.