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Band alignment in organic light emitting diodes - On the track of thickness dependent onset voltage shifts |
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| Institution: | 1. InnovationLab GmbH, Speyerer Straße 4, 69115 Heidelberg, Germany;2. Technische Universität Darmstadt, Materials Science Department, Surface Science Division, Jovanka-Bontschits-Straße 2, 64287 Darmstadt, Germany;3. Kirchhoff-Institut für Physik, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany;4. Merck KGaA, Frankfurter Straße 250, 64293 Darmstadt, Germany;1. Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova St., 28, 119991, Moscow, Russian Federation;2. Carl von Ossietzky University of Oldenburg, 26129, Oldenburg, Germany;3. Department of Physics, The LNM Institute for Information Technology, Jamdoli, Jaipur, Rajasthan 302031, India;1. Department of Physics, Beijing Technology and Business University, Beijing 100048, PR China;2. Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China;1. School of Physics, Shandong University, Jinan, 250100, People''s Republic of China;2. Department of Physics, Jining University, Qufu, 273155, People''s Republic of China;1. Department of Materials Science and Engineering, National Chiao Tung University, 1001 University Road, Hsinchu, 30010, Taiwan;2. Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, No. 70, Lienhai Road, Kaohsiung, 80424, Taiwan |
| Abstract: | In this paper we report on an unclear effect in the IV characteristics of organic light emitting diodes (OLEDs). When the thickness of the emitter layer based on a modified phenyl carbozole triplet host material (TH) in the device is increased, a significant shift of the onset voltage to higher values can be noticed. The voltage shift is not observed if the TH is substituted by an isomer with only minor variation of the molecular structure. In a previous publication we could already show that an electric interface field is necessary to describe the onset voltage behaviour. To find the origin of this interface field in the present publication the two isomers are characterized and the band alignment at the interfaces to the emitter layer is investigated using photoelectron spectroscopy. The interface energy diagrams have been measured on stepwise prepared model interfaces. A further simplification of the bipolar to a hole only device stack proofs, that band bending at the hole injecting interface to the TH layer is the origin of the interface field. In contrast an entire flat band situation is measured in case of the device using the other isomer showing no onset voltage shift. |
| Keywords: | Organic light emitting diodes Interphase experiments Onset voltage shift Photoelectron spectroscopy Angle dependent IR-measurements |
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