Electronic metal/polymer interfaces

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

J. L. Bredas, W. R. Salaneck and G. Wegner (Eds), Organic Materials for Electronics Conjugated Polymer Interfaces with Metals and Semiconductors (North Holland, Amsterdam, 1994). 

The experimental spectroscopic methods discussed below are performed in the steady state, i.e., the time average of the nuclei positions is fixed. This justifies the use of the time-independent Schrodinger equation in the calculations. Dynamical systems are also of some interest in the context of metal-polymer interfaces in studies of, for instance, the growth process of the metallic overlayer. Also, in the context of polymer or molecular electronic devices, the dynamics of electron transport, or transport of coupled electron-phonon quasi-particles (polarons) is of fundamental interest for the performance... 

The application we have in mind for the metal-polymer interfaces discussed in this book is primarily that where the polymer serves as the electroactive material (semiconductor) in an electronic device and the metal is the electric contact to the device. Metal-semiconductor interfaces, in general, have been the subject of intensive studies since the pioneering work of Schottky, Stromer and Waibel1, who were the first to explain the mechanisms behind the rectifying behaviour in this type of asymmetric electric contact. Today, there still occur developments in the understanding of the basic physics of the barrier formation at the interface, and a complete understanding of all the factors that determine the height of the (Schottky) barrier is still ahead of us2. 

The aim of this chapter is to present a simple but general band structure picture of the metal-semiconductor interface and compare that with the characteristics of the metal-conjugated polymer interface. The discussion is focused on the polymer light emitting diode (LED) for which the metal-polymer contacts play a central role in the performance of the device. The metal-polymer interface also applies to other polymer electronic devices that have been fabricated, e.g., the thin-film field-effect transistor3, but the role of the metal-polymer interface is much less cruical in this case and... 

P. Dannetun, M. Fahlman, C. Fauquet, K. Kaerijama, Y. Sonoda, R. Lazzaroni, J. L. Bredas and W. R. Salaneck, in Organic Materials for Electronics Conjugated Polymer Interfaces with Metals and Semiconductors, J. L. Br6das, W. R. Salaneck and G. Wegner (Eds) (North Holland, Amsterdam, 1994), p. 113. 

In order make an effort to bring the polyimide-metal adhesion problem to an even more fundamental level, we have previously proposed that model molecules, chosen as representative of selected parts of the polyimide repeat unit, may be used to predict the chemical and electronic structure of interfaces between polyimides and metals (12). Relatively small model molecules can be vapor deposited in situ under UHV conditions to form monolayer films upon atomically clean metal substrates, and detailed information about chemical bonding, charge transfer and molecular orientation can be determined, and even site-specific interactions may be recognized. The result of such studies can also be expected to be relevant in comparison with the results of studies of metal-polymer interfaces. Another very important advantage with this model molecule approach is the possibility to apply a more reliable theoretical analysis to the data, which is very difficult when studying complex polymers such as polyimide. 

The metal polymer interface can be studied in a variety of ways using surface science methods. Recently, much emphasis has been placed on the understanding of the initial stages of metallization of polymers. In particular, the role of metal-organic interactions as they relate to the fundamentals of adhesion mechanisms are of interest. One experimental approach is to examine the first monolayers of metal as they are deposited on a polymer surface (1), i.e the polymer is the substrate. However, the organic polymer-metal interface may be studied in the opposite perspective, via understanding the roles of organic molecular or macromolecular structure and chemistry of the metal surface qua substrate (2). In the present paper, recent ion and electron spectroscopic studies of the... 

Fig. 5.3 Electronic structures of metal and organic when the metal and organic are (a) separated (b) under contact obeying Schottky-Mott model (c) under contact obeying the If aligned approach. The electronic structures of metal/polymer interface are shown in (d).

Adhesive failure Is a problem In solar systems. In the past, polymers have been used to protect the mechanical Integrity of wood and metal structures In severe outdoor environments and to protect sensitive electronic components In relatively benign enclosed environments. Polymers used In solar equipment will have to protect the optical properties of reflectors, thln-fllm electrical conductors, and thln-fllm photovoltalcs from the effects of moisture and atmospheric pollutants In severe outdoor environments while simultaneously maintaining optical, mechanical, and chemical Integrity. In some systems, the prevention of mechanical failure Is Important frequently, adhesive failure at the metal/polymer Interface Is of particular concern because the ensuing corrosion causes optical failure. 

Electrochemical reactions that lead to a degradation of the metal-polymer interface are influenced by the following properties the electron transfer properties at the interface, the redox properties of the oxide between the metal and the polymer and the chemical stability of the interface with respect to those species, which are formed during the electron transfer reaction. 

Conversion layers lead to an increased adhesion strength of organic coatings on metals xmder dry and wet conditions. In addition, the kinetics of ion and electron transfer processes at the metal-polymer interface are slowed down. In case of iron and zinc, especially the oxygen reduction rate, which strongly influences the delamination kinetics of the coating, is reduced. 

Another theory claims that a protective complex between the metal and the CP is formed in the metal-polymer interface. Kinlen et al. [73] found by electron spectroscopy chemical analysis (ESCA) that an iron-PANl complex in the intermediate layer between the steel surface and the polymer coating is formed. By isolating the complex, it was found that the complex has an oxidation potential 250 mV more positive than PANI. According to Kinlen et al. [73], this complex more readily reduces oxygen and produces a more efficient electrocatalyst. 

The development of an adequate equivalent circuit has been controversially discussed in the literature. Gabrielli et al. considered the polymer primarily as a non-porous layer. Transport processes in the polymer matrix dominated the impedance. Vorotyntsev et al. developed a model that took into account the electron transfer at the metal—polymer interface, transport of charge carriers in the film, and ion transfer at the polymer-electrolyte interface (Figure 11.16). 

After a brief description of the fundanietitals of High Resolution Electron Energy Loss Spectroscopy (HREELS), its potentialities in elucidating chemical reactions at a metal-polymer interface are illustrated by the well-known case of alunuRium evaporated onto polyimide (PMDA-OOA). Then the diHkuldes (but also the new promises) in roudnely applying this new spectroscopy to any metal-polymer sysKm will be shown for the copper-polyphenylquinoxaline interface. 

Uloa-high-vacuum environment is mandatory for HREELS experiments, in order to prevent attenuation/scaitcring of the electron beam, and sample contamination by residual gas. For studies described in this wotk, evsqioration of metal onto the polymer surfoce was p onned in situ, using a Knudsen celt with a very low and well controll evaporation rate, calibrated with a quartz microbalance (typical evaporation rate lA/minute). This is required to grow and characterize stepwise, in the submonolayer coverage range, the metal-polymer interface. 

In this contribution, we report on the surface modifications of polymers by a dual frequency electron cyclotron resonance (ECR) plasma and their influence on the formation of the metal-polymer interfaces. The surface modifications are studied with respect to different parameters of the plasma treatment including the influence of an atmospheric contact. The interface of an evaporated metal film with a polymer surface is characterized in terms of the observed growth mode of the film as a function of the polymer surface properties. 

Two charge-transfer semicircles are expected, which correspond to two RC parallel combinations of the doublelayer capacitance and charge-transfer resistance at the electrode-polymer interface and the double-layer capacitance and charge-transfer resistance at the polymer-electrolyte interface. At the metal-polymer interface, electron transfer would occur while at the polymer-electrolyte interface anion transfer is expected. 

Only one semicircle is predicted for the metal-polymer interfaces, since only electron transfer can occur. 

Since electron transfer occurs at both metal-polymer interfaces, a dc current can flow through the system, that is, the low-frequency limit is not a capacitance as in the case of the metalconducting polymer-electrolyte system but the impedance response bends over to the real axis after the Warburg region. 

Both electronic and medical applications of plasma polymers have been reported [54-61]. Most of these investigations are on the interface between polymers and inorganic materials, for instance, metal/polymer interfaces in structural adhesive joints, and cation diffusion along polymer/metal interfaces under an applied electric potential. In another reference, more specific aspects for electrical and electronic applications [59] were treated, wherein protective films for microcircuitry, and for wettability were explained. The use of such film for surface treatment has also been examined. 

However, it is important to pay attention to more than just the electronic charging of the polymer film (i.e., to electron exchange at the metal polymer interface and electron transport through the smface layer), since ions will cross the film solution interface in order to preserve electroneutrality within the film. The movement of counterions (or less frequently that of co-ions) may also be the ratedetermining step. 

Fig. 6.1 A schematic picture of a polymer film electrode. In an electrochemical experiment the electron transfer occurs at the metal polymer interface that initiates the electron propagation through the film via an electron exchange reaction between redox couples A and B or electronic conduction through the polymer backbone. (When the polymer reacts with an oxidant or reductant added to the solution, the electron transfer starts at the polymerjsolution interface.) Ion-exchange processes take place at the polymer solution interface in the simplest case counterions enter the film and compensate for the excess charge of the polymer. Neutral (solvent) molecules (O) may also be incorporated into the film (resulting in swelling) or may leave the polymer layer...

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