Electrical and Photoconductive Properties

Electrochemical studies of several polymetallaynes have also been reported [70, 71, 79]. Reduction of organic spacer groups can be achieved, and the oxidation of substituents (e. g. ferrocene) can be reversible. However, electrochemical oxidation of the platinum centers in at least some polymetallaynes appears to occur in two irreversible redox steps (presumably involving Pt /Pt and Pt 7Pt couples). This observation suggests that the chemical doping experiments that were used to increase the electrical conductivities of the polymers described above may not involve simple processes. 

The first members of an interesting new dass of organometallic t onjugated polymers that contain metallocydopentadiene units in the main chain were reported in 1993 [90, 91). The addition of acetylenes to CpCo(PPh3)2 allowed the preparation of polymers 5.36 (Eq. 5.15). These materials were found to be intractable in common organic solvents their degree of polymerization was estimated to be ca. 20 by IR spectroscopy [90]. 

A similar synthetic approach was used to prepare Co polymers 5.37 (Eq. 5.16), which possessed improved solubility as a result of the presence of flexible aliphatic spacers in the main chain [91]. 

Cobaltacyclopentadiene polymers undergo thermal rearrangement to the more stable ( / -cyclobutadiene)cobalt derivatives, and reaction with isocyanates affords new polymers containing 2-pyridone moieties in the polymer backbone (for details, see Chapter 4, Section 4,4.1) [98-100], 

A novel extension of the metal-induced coupling of diynes involves the preparation of the red, moisture-sensitive zirconocene-containing polymer 5.39 (Eq. 5.18) with Mw=37,000 (PDI = 2.1) by GPC (relative to polystyrene) [101], Related organo-silicon polymers and macrocycles derived from the zirconocene coupling of MeC = CSiMe2C6H4SiMe2C = CMe have also been reported [102, 103], 

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Electrical and Photoconductive Properties of Orthorhombic Sulfur Crystals... 

We can conclude this section with the insight, gained from this overview of the electrical and photoconductivity properties of these films, that, in spite of the many studies already carreid out, a comprehensive and systematic study of these properties and their correlation with a wide range of deposition parameters is still needed in order to understand what determines these properties. These studies should also include postdeposition treatments— not so much annealing, which has been carried out, but surface treatments (e.g., immersion in triethanolamine), which could show the importance (or lack of it) of the crystal surface condition. 

Clearly, work in this area will continue at a rapid pace because of the unusual electrical and photoconductive properties of these systems. 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

Nowadays, polymeric photoconductors may be used in electrophotography, microfilms, photothermoplastic recording, spatial light modulators, and nonlinear elements. The combination of photosensitivity with high quality electrical and mechanical properties permits the use of such materials in optoelectronics, holography, laser recording and information processes. The applications of the various types of polymers were reported in the final parts of the relevant items in the earlier sections. Here, we will briefly analyze the common features of photoconductive polymer applications. The separate questions of each type have been dealt with in some books and papers [3, 11, 14, 329]. 

The electrical and optical properties of fully conjugated linear polymers have been of considerable research interest during the past several decades(l-3). The motivation has been to utilize the enormous variety of possible backbone and sidegroup structures to obtain new semicon-ductive, photoconductive, metallic, and superconductive materials. 

A. Torres-Filho and R. W. Lenz, Electrical, thermal, and photoconductive properties of poly(phenylene vinylene) precursors I. Laser-induced elimination reactions in precursor solutions, J. Polym. Sci. Part B Polym. Phys. 57 959 (1993). 

The electrical properties (dark conductivity and photoconductivity) are reported to first decrease and then increase upon increasing power [361]. The optical bandgap increases with increasing power, due to the increase of the hydrogen content [63, 82, 362, 363]. However, at very high power levels, microcrystalline silicon is formed [364], which causes the hydrogen content (and, consequently, the bandgap) to decrease. 

The focus of this section is charge transport in PS, and electrical properties such as resistivity, carrier mobility, capacitance and photoconductivity are discussed. 

Polysilanes can be regarded as one-dimensional analogues to elemental silicon, on which nearly all of modern electronics is based. They have enormous potential for technological uses [1-3]. Nonlinear optical and semiconductive properties, such as high hole mobility, photoconductivity, and electrical conductivity, have been investigated in some detail. However, their most important commercial use, at present, is as precursors to silicon carbide ceramics, an application which takes no advantage of their electronic properties. 

The reaction of solid porphyrin films with light in the presence of oxygen by producing MgTPP must affect electrical properties, in particular semi conduct on, photoconduction, and photovoltaic properties. We have provided evidence for "photodoping" by light and oxygen, a phenomenon that must be clearly understood if these materials are to have device applications. 

A very common and useful approach to studying the plasma polymerization process is the careful characterization of the polymer films produced. A specific property of the films is then measured as a function of one or more of the plasma parameters and mechanistic explanations are then derived from such a study. Some of the properties of plasma-polymerized thin films which have been measured include electrical conductivity, tunneling phenomena and photoconductivity, capacitance, optical constants, structure (IR absorption and ESCA), surface tension, free radical density (ESR), surface topography and reverse osmosis characteristics. So far relatively few of these measurements were made with the objective of determining mechanisms of plasma polymerization. The motivation in most instances was a specific application of the thin films. Considerable emphasis on correlations between mass spectroscopy in polymerizing plasmas and ESCA on polymer films with plasma polymerization mechanisms will be given later in this chapter based on recent work done in this laboratory. 

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