Degradation of HTL layers during device operation in PhOLEDs

Different analytical tools and methodologies are currently employed to determine degradation products of organic blue light emitting devices in order to identify the failure mechanisms which determine the lifetime of these devices. This article provides a deeper understanding of degradation mechanisms of organic light emitting diodes (OLEDs) during device operation. Degradation products of blue emitting devices containing 8% of the phosphorescent emitter iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2′ picolinate (FIrpic) in a matrix containing bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminium (BAlq) as electron transport layer (ETL), 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (α-NPD) were investigated using laser desorption ionization (LDI) coupled with a time of flight mass spectrometry (TOF/MS). Especially chemical degradation pathways of the hole transport materials TCTA and α-NPD were investigated. The comparison of experimental data of unstressed and stressed device revealed that new reaction products are formed during the device operation. The linkage of TCTA fragments to the α-NPD core in an interfacial reaction as well as a dimerization of TCTA itself was observed. Ten new reaction products could be characterized via LDI-TOF-MS. Some of these compounds might possess a negative influence on the drop of efficiency and lifetime of blue light emitting devices based on FIrpic.

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