Berlin 2015 – wissenschaftliches Programm
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DS: Fachverband Dünne Schichten
DS 6: Organic photovoltaics and electronics - mostly cell design (jointly with CPP,DS)
DS 6.5: Vortrag
Montag, 16. März 2015, 16:00–16:15, EW 202
Understanding thickness dependant onset voltage shifts in OLED IV-characteristics II: Equilibrium band alignments and their impact on the steady-state situation — •Eric Mankel1,2, Maybritt Kühn1,2, Christof Pflumm3, Thomas Mayer1,2, and Wolfram Jaegermann1,2 — 1TU Darmstadt, Darmstadt — 2InnovationLab GmbH, Heidelberg — 3Merck KGaA, Darmstadt
Steady-state IV measurements are used as important characterization method for organic light-emitting diodes (OLEDs). Surprisingly, in some OLED devices the current onset voltage increases significantly depending on the emission layer (EML) thickness by approximately 2,7 V/100 nm. In part 1 we used two isomers - one showing changes in onset voltage (EML-A) the other (EML-B) not - and concluded that enhanced internal electric fields explain this unexpected behavior. Here in part 2 we present how interface band offsets (a), interface dipoles (b) and space charge regions (c) influence the field distribution under steady-state conditions. First we show the impact of a, b and c on the electric and the electrochemical potential distribution as well as the IV-characteristic of a two layer model device. Then we allocate these results to the measured interface discontinuities of the electric field shown in part 1. Finally we present the interface band diagrams of the EML layers and the respective contact layers measured by XPS layer-by-layer experiments. The values for a, b and c were used to model the IV-characteristics of both, EML-A and EML-B containing devices. The described onset voltage shift can successfully be reproduced.