Studies of electronic and chemical structures of interfaces between metals and organic semiconductors

Abstract

Recently considerable attention has been attracted to electronically functional organic semiconductors with respect to various applications of electronic and optoelectronic devices because of their unique properties. The performance of organic-based devices depends critically on interface properties, e.g. electronic and chemical structures and related interface parameters. Despite decades of extensive efforts, understanding the physical mechanisms which determine the interface formation at metal-organic contacts and engineering them for specific device applications are still important theoretical and technological challenges. Chemistries, electronic structures, and electrical behaviors at interfaces between metals and representative organic semiconductors with reverse formation sequence have been systematically investigated using x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and current-voltage (I-V) measurements. It was demonstrated that irrespective to the deposition sequence, chemical reaction occurs between Mg, rare earth metals (e.g., Yb and Sm), and copper phthalocyanine (CuPc) and tris-(8-hydroxyquinoline) aluminum (Alq3), characterized by charge transfer and chemistry-induced gap states. Despite different thicknesses of the reacted zone, both the CuPc-on-Mg and the Mg-on-CuPc interfaces exhibit identical carrier injection barriers, which are confirmed by the symmetric electrical behavior obtained from I-V characteristics of devices with a structure of Mg/CuPc/Mg. These findings contrast with physisorptive noble metal-CuPc interfaces and suggest that strong local chemical bonding is a primary factor determining molecular level alignment at reactive metal-CuPc interfaces. When rare earth metals were used as a cathode in organic light-emitting diodes (OLEDs), electrical characteristics were improved as compared to a control device having a Mg:Ag cathode, while the electroluminescent (EL) efficiency drops considerably. The reduction in EL efficiency is attributed to the low optical reflectance at the rare earth metal-Alq3 interface, rather than to luminescence quenching arising from diffusion of metal atoms into the emissive layer of Alq3. Such kind of cathode can be used to make OLEDs with low reflectance cathode in ambient light reflection for contrast enhancement, which is particularly important to display applications. An electronegativity model is proposed to characterize the variation of carrier injection barrier heights at metal-organic interfaces. In contrast to the traditionally used metal work function, barrier heights at interfaces are shown to be linearly dependent on metal electronegativity for a wide range of organic materials. The physical basis for the better suitability of electronegativity than work function to describe barrier heights is discussed. While barrier formation is caused by charge transfer between metals and organic semiconductors, the variation of barrier height is related to electronegativity difference of metals. The applicability of the electronegativity model to compound or alloy electrodes, such as indium tin oxide having no well-defined electronegativity, is also exemplified. The natural electronic structures at metal-organic interfaces in most cases are not optimized for specific device applications. High-work function metals (e.g., Au and Ag) used as electrodes in OLEDs incurs severe device degradation due to poor carrier injection. Engineering them in a controlled way is of theoretical and technological importance. We introduce a nanometer-scale fluorocarbon coating (CFx) by plasma polymerization to modify the anode-organic interface. OLEDs with a 0.3- nm-thick CFx film on an untreated indium tin oxide (ITO), Au, or Ag anode show superior I-V characteristics and improved operational stability, as compared to the device with a traditional UV-ozone treated ITO anode. The enhancement of hole injection at the anode-organic junction is attributed to the reduction of the barrier height for hole injection, resulting from the interfacial dipole with the negative pole away from the anode induced by the partial charge transfer to CFx. The improvement of device stability is ascribed to the hydrophobic nature of CFx, which protects the anodic contact from atmospheric moisture. The results suggest that an ultrathin CFx coating provides a rather simple and reliable approach to effectively manipulate the energy level offset at an anode-organic junction and to improve the device operational stability. In particular, it is so far the most effective method for metallic anodes such as Au and Ag.

Date of Award	15 Feb 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)