Organic light emitting diodes (OLED) hold promise for being the next generation technology in lighting and large flat panel display applications. OLED devices are fabricated by stacking various layers of organic molecules, namely electron transport layer (ETL), emitter layer (EML), hole transport layer (HTL) and hole injection layer (HIL), which are sandwiched between metal cathode and ITO anode. One of the explanations for the loss of luminescence over time in a working OLED device is the degradation of HTL by decomposition of weak bonds, leading to the molecular fragments, whose electronic properties are markedly different from the parent molecule [1]. It has been demonstrated that, for certain architectures, this chemical degradation occurs at the HTL/EML interface [2], which is the likely position of the recombination zone. In this study, a different degradation route of HTL molecules is explored. This degradation pathway was observed for N2,N2,N10,N10,13,13-hexaphenyl-6H,13H-[1,3,5]diazasilino[1,6-a:3,4-a’]diindol e-2,10-damine (M1), which was being evaluated as an HTL material, whose calculated electronic properties are comparable to typical HTLs, such as NPB. M1 was subjected to photolysis for 2 minutes and the degradation products were analyzed using Mass spectroscopy (MS). Surprisingly, the primary degraded products are formed by eliminating two or four hydrogen atoms from the amine portions of M1. The products, predicted using transition state of the cyclization of the aryl-amine at the Density functional theory (DFT) level, compare well with the MS results. In addition, the calculated electronic properties of the degraded products differ from that of M1, which may be the cause of poor lifetime of the OLED device, fabricated using M1 as HTL. Thus, cyclization of aryl-amines followed by elimination of hydrogen can be a potential degradation route for the HTL molecules, which can lead to poor lifetime of an OLED device. Such decomposition routes must be avoided for molecules, used in commercial OLED stacks for lighting and display applications.
[1] D. Y. Kondakov, J. App. Phys. 104, 084520 (2008) [2] D. Y. Kondakov; R. H. Young, J. App Phys. 108, 074513 (2010)