title: Degradation Mechanisms in Organic Light-Emitting Diodes with Polyethylenimine as a Solution-Processed Electron Injection Layer.
abstract: In this work, we investigate the performance and operational stability of solution-processed organic light-emitting diodes (OLEDs), which comprise polyethylenimine (PEI) as an electron injection layer (EIL). We show that the primary degradation mechanism in these OLEDs depends on the cathode metal that is used in contact with the EIL. In the case of Al, the deterioration in OLED performance during electrical driving is mainly caused by excitons which reach and subsequently degrade the emitter/PEI interface. In contrast, in the case of Ag, device performance degradation occurs due to an additional mechanism: hole accumulation at the emitter/PEI interface and a consequent drop in the emitter quantum yield. As a result, the operational lifetime of OLEDs that use PEI as EIL can vary significantly with the cathode material, and at a current density of 20 mA cm-2, LT50 lifetimes of ∼200 h and <10 h are obtained for Al and Ag, respectively. Finally, we show that the first degradation mechanism can be significantly slowed by using a mixture of PEI and ZnO nanoparticles as EIL. As a result, the operational lifetime of OLEDs with an Al cathode is increased to more than 1000 h, without adversely affecting device performance. This lifetime is significantly longer than that of a LiF/Al reference OLED.
authors.affiliation: Karlsruhe Institute of Technology , Light Technology Institute, Engesserstr. 13, 76131 Karlsruhe, Germany.|InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany.
authors.forename: Sebastian
authors.identifier: http://orcid.org/0000-0001-8804-0207
authors.initials: S
authors.lastname: Stolz
authors.affiliation: University of Waterloo , Department of Electrical and Computer Engineering & Waterloo Institute for Nanotechnology, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada.
authors.forename: Yingjie
authors.identifier: http://orcid.org/0000-0002-6468-8583
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Karlsruhe Institute of Technology , Light Technology Institute, Engesserstr. 13, 76131 Karlsruhe, Germany.|Karlsruhe Institute of Technology , Institute of Microstructure Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Uli
authors.initials: U
authors.lastname: Lemmer
authors.affiliation: Karlsruhe Institute of Technology , Light Technology Institute, Engesserstr. 13, 76131 Karlsruhe, Germany.|InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany.
authors.forename: Gerardo
authors.initials: G
authors.lastname: Hernandez-Sosa
authors.affiliation: University of Waterloo , Department of Electrical and Computer Engineering & Waterloo Institute for Nanotechnology, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada.
authors.forename: Hany
authors.initials: H
authors.lastname: Aziz
keywords: OLEDs; electron injection layers; lifetime; polyethylenimine; solution-processing