Dresden 2014 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 23: Organic Electronics and Photovoltaics III (DS jointly with CPP, HL, O)
O 23.2: Vortrag
Montag, 31. März 2014, 18:15–18:30, CHE 91
Correlation of electric properties and interface band alignment in organic light-emitting diodes — •Maybritt Kühn1,2, Eric Mankel1,2, Christof Pflumm3, Thomas Mayer1,2, and Wolfram Jaegermann1,2 — 1Technische Universität Darmstadt, Institute of Materials Science, Surface Science Division — 2InnovationLab GmbH, Heidelberg — 3Merck KGaA, Darmstadt
Organic light-emitting diodes consist of several functional organic layers sandwiched between two electrodes with different work functions. At the current onset voltage the applied electric field is high enough that charge carrier injection and transport begins. In some device structures the onset voltage increases with increasing thickness of the emission layer. We present a detailed study on this up to now unpredictable phenomenon by combining IV-measurements and interface investigations using photoelectron spectroscopy (XPS/UPS). We focus on two isomers synthesized by Merck that serve as matrix material in the emission layer-one showing the changes in onset voltage the other not. The complex device architecture was reduced to a model device system using NPB as hole-transport layer and the undoped isomers as emission layer. The shift in onset voltage can still be observed in the model devices as well in hole-only devices derived from the model system. By stepwise evaporation of the respective isomer onto NPB we performed PES-interface experiments and analyzed the band alignment between NPB and the isomers in an integrated UHV system. It was found that the hole injection barrier is larger by about 200 meV in case of the isomer showing the observed shift in onset voltage.