Molecular highway helps blue OLEDs shine brighter, longer

Organic light-emitting diodes (OLEDs) are used widely; for displays, blue OLEDs are required to supplement the primary colours red and green. In blue OLEDs, impurities can lead to electrical losses, which could be circumvented by using complex and expensive device layouts. Now, a team from the Max Planck Institute for Polymer Research has developed a material concept that potentially allows efficient blue OLEDs with a strongly simplified structure.

Light-emitting diodes for blue light are still difficult to manufacture because blue light — physically spoken — has a high energy, which makes the development of materials difficult. The presence of minute quantities of impurities in the material that cannot be removed plays a role in the performance of these materials. These impurities form obstacles for electrons to move inside the diode and participate in the light-generation process. When an electron is captured by such an obstacle, its energy is not converted into light but into heat. This problem, known as ‘charge trapping’, occurs primarily in blue OLEDs and reduces their efficiency.

Researchers led by Paul Blom, Director at the Max Planck Institute for Polymer Research, have tackled charge trapping by using a new class of molecules for this purpose. These consist of two chemical parts; one is responsible for the electron conduction, whereas the other part is not sensitive to impurities. By manipulating the chemical structure of the molecule, a special spatial arrangement is achieved: when several molecules are joined, they form a kind of ‘spiral’, enabling the electron-conducting part of the molecules to form the inner part, which is shielded on the outside by the other part of the molecules. This resembles, in a molecular way, a coaxial cable with an electron-conducting inner core and an outer part shielding the core.

The cladding forms a kind of ‘protective layer’ for the electron-conducting core, shielding it from the intrusion of impurities such as oxygen molecules. Thus, the electrons can move fast along the central axis of the spiral without being trapped by obstacles — similar to cars on a highway without crossings, traffic lights or other obstacles.

“One of the special things about our new material is that the absence of losses due to impurities and resulting efficient electron transport can greatly simplify the design of blue OLEDs, while maintaining a high efficiency,” Blom said.

The researchers hope to take steps towards simpler production of blue light-emitting diodes and have published their results in the journal Nature Materials.

Image credit: iStock.com/Anatoly Morozov