From televisions to smartphones, organic light-emitting diodes (OLEDs) are finding their way into many everyday devices. For use in displays, blue OLEDs are also required to supplement the primary colors — red and green. Especially in blue OLEDs, impurities give rise to strong electrical losses, which could be partly circumvented by using highly complex and expensive device layouts.
Now, a team from the Max Planck Institute for Polymer Research (MPI-P) has developed a new material concept that could allow efficient blue OLEDs with a strongly simplified structure.
Minute quantities of impurities in the material that cannot be removed play a decisive role in the performance of these materials. These impurities — oxygen molecules, for example — 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 significantly reduces their efficiency.
The team, led by Paul Blom, used a new class of molecules to tackle the problem of charge trapping. It consists of two chemical parts: one is responsible for the electron conduction, whereas the other 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” — meaning the electron-conducting part of the molecules forms 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 thus forms a kind of “protective layer” for the electron-conducting core, shielding it from the intrusion of oxygen molecules. Thus, the electrons can move fast and freely 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,” said Paul Blom.
The researchers hope their work — which was published in the journal Nature Materials — is an important step toward simpler production of blue light-emitting diodes.
For more information, contact Dr. Christian Schneider at