Electronic Structure of Organic/Metal Interfaces Studied by UPS and Kelvin Probe
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ABSTRACT
The interfaces of organic materials with other solids play important roles in the function of various organic devices such as organic light-emitting diodes (OLEDs), spectral sensitization in photography, organic solar cells, and electrophotography. Also they should be important in future molecular devices, both in the central part of the device and at the connection with outside circuits. However, serious experimental examination of such interfaces has started only recently. In this talk we focus our attention on the organic/metal interfaces, and summarize our understanding about (1) the energy level alignment right at the interface, and (2) possible band bending within an organic layer, mainly using the techniques of UV photoemission spectroscopy (UPS) and Kelvin probe method. As for (1), the formation of electric dipole layer was observed in most organic/metal interfaces, and its origin is discussed. As for (2), recent examination of the existence/absence of band bending in ultrahigh vacuum will be reported. It is also pointed out that there can be much effects of (i) atmosphere at sample preparation and measurements, and (2) chemistry and interdiffusion at the interface.
INTRODUCTION
Recently there has been much interest in the electronic devices using electronically functional organic materials. Organic photoconductors are already used in the real application of electrophotography, and active studies are currently carried out worldwide in the field of organic electroluminescent devices (OLEDs). Such investigations are also aimed at devices at molecular level, as extensively discussed in this session. In many of these devices, the function originates in the interface formed by the organic material with other solids, such as metal electrode or another organic material. Thus the elucidation of the interfacial electronic structure is very important for understanding and improving the function. However, serious experimental examination of the electronic structures has started just recently.
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Figure 1. The energy level alignment at organic / metal interface. (a)Mott-Schottky model. (b) observed energy diagram by UPS.
The interfacial electronic structure has been often estimated by lining up the separately observed electronic structures of an organic material and an electrode metal, with the assumption of a common vacuum level at the interface, according to Mott-Schottky (MS) model [1] as shown in Fig.1(a). In this model, the vacuum level of an organic semiconductor coincides to that of a metal electrode at their interface, while the energy positions of electronic levels of the organic semiconductor changes with the distance from the interface to achieve the Fermi level alignment between the organic semiconductor and the electrode metal. This spatial change of the energy levels is called band bending. By using this model, for example, the barrier height for
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