Invited Paper OF+EL+TF-ThA3
Dependence of Electrical Properties of Metal/Organic/Metal Systems on Interface Electronic Structure, Morphology and Chemistry
Interactions between metals and organic molecules are varied, and depend sensitively on the nature of the metal, of the organic molecule, and on the mode of the formation of the interface. In general, an interface formed by deposition of a metal on an organic film is broader than the interface formed by reverse deposition sequence. Metal atoms diffuse and/or react in the organic layer, acting as electrically active dopant and altering the electronic properties and the structure of the molecular film. These interactions have a profound impact on the injection of charge carriers across these organic interfaces. We present here a comprehensive investigation of these effects performed on a series of metal/organic/metal structures. All experiments are performed in ultra-high vacuum to eliminate extrinsic effects. The organic materials are Alq@sub 3@ (tris-(8-hydroxy quinoline) aluminum), amorphous emissive material extensively used in molecular OLEDs; and F@sub 16@-CuPc (hexadecafluoro copper phthalocyanine), a good electron-transport material that forms molecular stacks. The metals are Al and Au. Combinations of structures including permutations of Al and Au as top and bottom contacts, and even ultra-thin metal layers inserted into the bulk of the organic film, are used to investigate the effects. Key results are: 1. The predominant role of chemistry-induced electronic gap states in leading to identical metal/organic barriers for systems like Al-on-Alq@sub 3@ and Alq@sub 3@-on-Al; 2. the "p-like" doping induced by Au atoms deposited on, and diffused deep into, organic films like Alq@sub 3@ and F@sub 16@-CuPc; and the structural disruption in the F@sub 16@-CuPc stacks resulting from Al-F@sub 16@-CuPc chemical reaction, leading to considerable decrease in electron mobility through the material. General conclusions concerning metal/organic contacts will be discussed. Work supported by the MRSEC program of the National Science Foundation (DMR-9809483) and the NJCOE.