Analysis of chemical degradation mechanism within sky blue phosphorescent organic light emitting diodes by laser-desorption/ionization time-of-flight mass spectrometry

We present an investigation of the intrinsic chemical degradation mechanisms of phosphorescent OLEDs based on the common sky blue emitter bis(2-(4,6-difluorophenyl)-pyridyl-N,C2')iridium(III)picolinate (FIrpic). The OLEDs are investigated using the laserdesorption/ionization time-of-flight mass spectrometry. The comparison between the collected spectra for electrically aged and unaged OLEDs allows the identification of different reaction products, like fragments which are mainly related to the chemical dissociation of FIrpic molecules during the OLED operation. We present different reaction pathways of the blue emitter FIrpic. One proposed reaction indicates that the short lifetimes of the OLEDs may be related to the irreversible dissociation of the FIrpic molecule by the loss of carbon dioxide (CO2) from the picolinate ligand. Additionally, the formation of chemical complexes between different fragments of the FIrpic molecules with their neighbour materials is visible. The cesium adducts of FIrpic formation indicate a possible contribution of the dopant to the OLED degradation process. Finally, we show rearrangement effects in the Cs doped electron transporting layer. This rearrangement is indicated by the presence of m/z signals of the BPhen dimer and adducts of the dimer with Cs. (C) 2010 Elsevier B.V. All rights reserved.