Why new blue OLEDs die young

Next-generation blue OLED materials degrade early mainly due to the loss of oxygen in molecules at the interface between emission and electron transport layers, according to the UK’s National Physical Laboratory, which teamed up with the Samsung Advanced Institute of Technology to pull OLED junctions apart in minute detail.

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

In the study were two materials that are more efficient than the fluorescence-type blue OLEDs currently commercially-available.

“Two classes of blue OLEDs were made and analysed in this study: with phosphorescent emitters [and] with TADF [thermally-activated delayed fluorescent] emitters. These are highly efficient, however they are prone to degradation,” Trindade told Electronics Weekly.

It was already known that adding cyano groups to the emitting layer’s host material (‘mCBP’) could improve life, but by how much and why?

“Test results showed that, for both phosphorescent and TADF blue OLEDs, a change of host material in the emitting layer presented one order of magnitude longer operational lifetime,” said Trindade. “This longer lifetime correlated with a reduction of the interfacial degradation products found by the OrbiSIMS method, thus providing insight on the degradation mechanisms.”

‘OrbiSIMS’ is a nano-scale mass spectrometry technique invented at NPL in 2017, and it was used to find degraded molecules in the OLEDs with a depth resolution of 7nm (equipment pictured).

‘SIMS’ here stands for secondary ion mass spectrometry, where a stream of ions abrade the surface-of-interest, and the debris is drawn away to be analyses – in this case by a ‘Orbitrap HF’ mass spectrometer.

According to NPL, it was first to make polymer light-emitting diodes, in 1975, using a 2.2μm polymer film between two charge-injecting electrodes. “Since then, developments in red and green OLED technology resulted in these coloured OLEDs being now comparable to conventional LEDs,” it said.

Results are published as ‘Direct identification of interfacial degradation in blue OLEDs using nanoscale chemical depth profiling‘ in Nature Communications.