Hamburg 2001 – scientific programme

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SYOF: Organische Festkörper

SYOF 2: Surfaces and Interfaces

SYOF 2.1: Invited Talk

Tuesday, March 27, 2001, 11:10–11:50, S6

Electronic Properties of π-Conjugated Organic Molecular Semiconductor Interfaces — •Antoine Kahn — Dept. of Electrical Engineering, Princeton University, Princeton, NJ, USA

This talk presents recent results obtained via direct and inverse photoemission spectroscopy and in-situ transport measurements, which clarify key aspects of interfaces between organic molecular thin films and metals. First, we address the important issue of the determination of the transport gap E_t, as opposed to the optical gap E_opt, in molecular films. We present a determination of E_t and of the exciton binding energy in six molecular materials (PTCDA, CuPc, Alq₃, α-NPD, α-6T and pentacene). We find E_t to be significantly larger, i.e. by the exciton binding energy, than E_opt usually used to describe interface and bulk molecular level diagrams. The exciton binding energy ranges from 0.4eV to 1.4 eV in the materials investigated. These measurements lead to a more accurate picture of the transport levels and energy barriers at interfaces of these materials. Second, we show how chemical reaction leads to the formation of electronic gap states, which play a central role in setting the injection barriers of electrons and holes at interfaces like Mg:Ag- and Al-Alq₃. We show why these interfaces are electrically symmetrical, i.e. I-V characteristics are nearly identical for metal-on-organic and organic-on-metal interfaces, whereas other less reactive interfaces such as Au-F₁₆CuPc show asymmetric electrical characteristics. Work supported by the MRSEC program of the National Science Foundation (DMR-9809483