The degradation mechanisms - whether physical, chemical or something else - that cause blue OLEDs to fail are yet to be fully understood and this limits the stability of blue OLEDs which in turn can impact the lifetime of OLED technology in full colour display screens and lighting.
The first polymer light-emitting diode (PLED) was created at NPL in 1975. It used a polymer film of up to 2.2 micrometres thick located between two charge-injecting electrodes. Since then, developments in red and green OLED technology have resulted in these coloured OLEDs being now comparable to conventional LEDs.
Understanding the degradation mechanism of blue OLEDs is essential to improve their performance and stability. However, 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 longstanding problem, the NPL/SAIT team used OrbiSIMS, an innovative 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 unprecedented sensitivity and localise them with seven nanometres depth resolution within the OLEDs multi-layered architecture.
The team found that chemical degradation is mainly related to loss of oxygen in molecules at the interface between emission and electron transport layers. 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. The method has already been used in another study led by Samsung and the Korea Advanced Institute of Science & Technology (KAIST), which also published in Nature Communications.
Commenting Dr Gustavo Trindade, one of NPL 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 (ETL/EML). 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.”
Professor Ian Gilmore, NPL corresponding author of the study, said, “OrbiSIMS allows high confidence in the identification of complex molecules with attomole sensitivity and simultaneous localisation to a layer of less than seven nanometres. This cannot be achieved using traditional High-performance LC-MS methods requiring dissolution of the device. OrbiSIMS as a diagnostic tool for degradation of OLEDs can play a vital role in providing insight for future material and device architecture development.”
“The degradation of organic light emitting diodes (OLED) is currently one of the major obstacles for the OLED industry,” said Dr Soohwan Sul and Dr Joonghyuk Kim, SAIT lead authors of the study. “Thanks to the development of the OrbiSIMS with its unprecedented depth/mass resolution and the ability of the intact analysis of organic molecules, we can now diagnose and answer a variety of outstanding problems in organic electronic devices such as OLEDs.”
