Fluorine cleavage of the light blue heteroleptic triplet emitter FIrpic

The lifetime stability of devices containing FIrpic as emitter has been a major concern for organic blue light emitting devices (OLEDs). To gain a deeper knowledge about the purity of FIrpic (bis[2-(4,6-difluorophenyl)pyridyl-N,C2']iridium (III)) emitters and how the purity is influenced by sublimation steps, non-sublimated and sublimated FIrpic material was analyzed via liquid chromatography coupled with electron spray ionization mass spectrometry (LC/ESI/MS). Cleavage of an electron-withdrawing group from one of the ligands of the heteroleptic phosphorescent emitter could be identified in sublimated FIrpic material via LC/ESI/MS. A detailed chemical analysis using LC/ESI/MS was carried out for complete blue emitting devices of the following structure: indium-tin-oxide (ITO)/50 nm (alpha-4,4'-bis[(1-naphthyl)phenylamino]-1,1'-biphenyl) (alpha-NPD)/10 nm 4,4',4 ''-tris(carbazol-9-yl)triphenylamine (TCTA)/100 nm TCTA:8% FIrpic/50 nm 1,1'-biphenyl-4'-oxy)-bis(8-hydroxy-2-methylquinolinato)-aluminum (BAlq)/1 nm LiF/100 nm AI. Two isomers of (FIrpic-1F) could be detected in an aged OLED. Changes in the ligand systems of FIrpic, especially the loss of fluorine during the deposition process can alter the emissive properties of the blue phosphorescent emitter. Beside isomer formation and chemical degradation of FIrpic, substantial degradation was observed for the hole transport material alpha-NPD in driven OLEDs. (C) 2009 Elsevier B.V. All rights reserved.