Collaboration identifies degradation of blue OLEDs

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The degradation of blue organic light-emitting diodes (OLEDs) is better understood following research by scientists at the National Physical Laboratory (NPL) and the Samsung Advanced Institute of Technology (SAIT).

Close-up of the OrbiSIMS instrument’s vacuum chamber showing the nozzles of the ion beams and electrode that extract ionised molecules for analysis
Close-up of the OrbiSIMS instrument’s vacuum chamber showing the nozzles of the ion beams and electrode that extract ionised molecules for analysis - NPL

Understanding the degradation mechanism of blue OLEDs is essential to improve their performance and stability, but the degradation mechanisms that cause blue OLEDs to fail are still not yet fully understood. This limits the stability of blue OLEDs, and by extension the lifetime of OLED technology in full colour display screens and lighting.

The first polymer light-emitting diode (PLED), created at NPL in 1975, used a polymer film of up to 2.2µm thick located between two charge-injecting electrodes. Since then, developments in red and green OLED technology resulted in these coloured OLEDs being comparable now to conventional LEDs.

OLEDs are formed of very thin layers of organic molecules, and chemically sampling nanoscale organic layers and interfaces with enough analytical information is challenging.

To tackle this, the NPL/SAIT team used OrbiSIMS, a mass spectrometry imaging technique invented at NPL in 2017. The team used OrbiSIMS’ nanoscale mass spectrometry to identify, for the first time, degradation molecules of blue OLEDs with exceptional sensitivity and localise them with 7nm depth resolution within the OLEDs multi-layered architecture.

In their study published in Nature Communications, the team found that chemical degradation is mainly related to loss of oxygen in molecules at the interface between emission and electron transport layers.

According to NPL, the OrbiSIMS results also showed approximately one order of magnitude increase in the lifetime of OLED devices that use slightly different host materials.

The results and the method described in the study can inform and drive future efforts on improving the performance of new blue OLED architectures and help display technology manufacturers to develop better quality displays with longer product lifetimes.

In a statement, Dr Gustavo Trindade, one of NPL’s lead authors of the study, said, “Our research enabled us to identify degradation molecules that are reaction products localised at the interface between emission and electron transport layers.

“The presence of these degradation molecules correlates negatively with the blue OLEDs lifetime. Furthermore, we showed that devices with subtly modified host materials have much reduced intensities of the interfacial degradation products and exhibit superior lifetime.”

blue OLEDs