Berlin 2008 – scientific programme

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SYSA: Symposium Tayloring Organic Interfaces: Molecular Structures and Applications

SYSA 1: Bandalignment in Organic Materials

SYSA 1.3: Talk

Tuesday, February 26, 2008, 10:15–10:30, H 0105

Measuring the energy level alignment at all interfaces in a complete OLED — Selina Olthof, Rico Meerheim, Karsten Walzer, and •Karl Leo — Institut für Angewandte Photophysik, Technische Universität Dresden, D-01062 Dresden, Germany

As the number of layers used for typical organic semiconductor devices is increasing in the course of optimization, interfaces between those layers become increasingly important for device performance. These interfaces are governed by the relative HOMO and LUMO positions as well as by interface dipoles originating from effects like polarization, chemical reactions and bond formation. As our charge injection layers are electrically doped, band bending and the resulting depletion layers play a significant role as well. To be able to perform simulations of the full device for further optimization it is necessary to measure the energy alignment at all metal/organic and organic/organic interfaces. This can be done by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). We performed such interface resolved studies of a complete OLED by UPS and XPS. As an example, we chose the structure of a long living red OLED: ITO - MeO-TPD:F4-TCNQ - α-NPD - α-NPD:Ir(MDQ)2acac - BAlq - BPhen:Cs - Ag.[1] For photoelectron spectroscopy, the OLED was built stepwise measuring for each interface the thickness dependent photoelectron spectra. The measurements show that the doped layers strongly influence the energetic positions of the intrinsic layers, and that the commonly assumed vacuum level alignment is not fulfilled. [1] R. Meerheim et al., Appl. Phys. Lett. 89, 061111 (2006)