Degradation of Ir(ppy) 2 (dtb-bpy)PF 6 iTMC OLEDs
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Degradation of Ir(ppy)2(dtb-bpy)PF6 iTMC OLEDs Velda Goldberg,1 Michael D. Kaplan,1 Leonard Soltzberg,1 Dolly Armira,1 Megan Bigelow,1 Stephanie Bitzas,1 Rachel Brady,1 Shannon Browne,1 Bianca Dichiaro,1 Heather Foley,1 Lauren Hutchinson,1 Alison Inglis,1 Nicole Kawamoto,1 Amanda McLaughlin,1 Caitlin Millett,1 Hanah Nasri,1 Sarah Newsky,1 Tram Pham,1 Cassandra Saikin,1 Mary Scharpf,1 Melissa Trieu,1 George G. Malliaras,2 and Stefan Bernhard3 1 Chemistry and Physics Departments, Simmons College, Boston, Massachusetts; 2Materials Science & Engineering Department, Cornell University, Ithaca, New York; 3 Chemistry Department, Princeton University, Princeton, New Jersey ABSTRACT Simplicity of construction and operation are advantages of iTMC (ionic transition metal complex) OLEDs (organic light emitting diodes) compared with multi-layer OLED devices. Unfortunately, lifetimes do not compare favorably with the best multi-layer devices. We have previously shown for Ru(bpy)3(PF6)2 based iTMC OLEDs that electrical drive produces emission-quenching dimers of the active species. We report evidence here that a chemical process may also be implicated in degradation of devices based on Ir(ppy)2(dtb-bpy)PF6 albeit by a very different mechanism. It appears that degradation of operating devices made with this Irbased complex is related to current-induced heating of the organic layer, resulting in loss of the dtb-bpy ligand. (The dtb-bpy ligand is labile compared with the cyclometallated ppy ligands.) Morphological changes observed in electrically driven Ir(ppy)2(dtb-bpy)PF6 OLEDs provide evidence of substantial heating during device operation. Evidence from UV-vis spectra in the presence of an electric field as well as MALDI-TOF mass spectra of the OLED materials before and after electrical drive add support for this model of the degradation process. INTRODUCTION As OLED-based displays begin to appear in major mainstream consumer devices, such as television sets, the importance of understanding the mechanisms of OLED degradation increases. Chemical processes, such as decomposition or other reactions of the active compounds, and physical processes, such as delamination of metal contacts have been implicated in various studies. In either case, electrical drive is likely to cause, or at least accelerate, device degradation. Iridium-based iTMC compounds are among the brightest emitters in the OLED palette. However, like all other known OLED materials, these compounds are also subject to performance degradation during operation. We have accumulated increasing evidence that degradation in the bright yellow emitter Ir(ppy)2(dtb-bpy)PF6 (Figure 1) results from chemical decomposition caused by substantial heating during electrical drive [ppy = 2-phenylpyridyl and dtb-bpy = 4,4’-di-tert-butyl-2,2’-dipyridyl]. Our approach to these studies has been to correlate electrical and light emission measurements with MALDI-TOF mass spectra of the OLED material before and after device operation. These observations have been supplemented, and
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