Electrical Conductivity and Photoconductivity

As charge transport materials, the polysilanes are unique in that the active sites are on the polymer backbone itself. In other electrophotographic materials, charged sites 

In addition to photoconductivity, polysilanes have been found to exhibit marked nonlinear optical properties,95-97 suggesting that they may eventually be useful in laser and other optical technology. The third-order non-linear susceptibility, X3, is a measure of the strength of this effect. The non-linear properties of polysilanes, like the absorption spectra, seem to be dependent on chain conformation and are enhanced for polymers having an extended, near anti conformation (Table 5.5). The value of 11 x 10 12 esu observed for (n-Hex2Si) below its transition temperature is the largest ever observed for a polymer which is transparent in the visible region. 

---

The electrical conductivity and photoconductivity of metal polyynes have been studied in some detail.For instance, it has been demonstrated that in the unoxidized state 166 (M = Gu) is an insulator (cr= 10 Scm ) and, upon doping with I2, the conductivity is increased to 10 S cm Polyynes such as 165 (M = Ni, Pd, Pt L = E Bus E = P or As) can either be oxidized with nitric acid or doped with Measurements on undoped films showed low conductivity values of ca. 10 Scm which improved to ca. 10 Scm in the l2-doped samples." The Pt-acetylide pyridine polymers 188 showed conductivities of 2.5 x 10 Scm upon doping with iodine. Quaternization followed by iodine doping gave a similar value (3.4 x 10 S cm ). These values appear to be the highest reported for soluble polymetallaynes. 

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. 

Electrical data are shown in Figure 59 as a function of deposition rate for all frequencies, using the relation between deposition rate and power density as depicted in Figure 54. Both dark conductivity and photoconductivity decrease exponentially with increasing deposition rate. The data in this range of deposition rates can be fitted with (Td = 9 x 10 -exp( —1.5r[Pg.142]

On the other hand, they found that doping AgN3 with Pb " (5 mole %) decreased the electrical conductivity and rate of photodecomposition but had no effect on the rate of thermal decomposition [98]. Small amounts of Ag" and Cu " in lead azide (0.1-3.0 mole %) had the possible effect of reducing anion vacancies, as indicated by a decrease in conductivity. However, at 5 mole % Ag" and 10 mole % Cu " the trend was reversed, possibly because at these higher concentrations AgN3 and Cu(N3)2 precipitated as a second phase [99]. The effects of Ag" and Cu " on the decomposition rate of lead azide are in contrast to the effects on photoconductivity Ag" (5 mole %) increases both the rate of thermal decomposition and photoconductivity of lead azide, and Cu " ... 

Elemental semiconductor clusters encaged in zeolites provide a valuable opportunity for gaining a fundamental understanding of semiconductor clusters because stoichiometry is not a concern in the synthesis. Selenium is of interest because it has an intermediate electrical conductivity and a negative coefficient of resistivity in the dark hence it is markedly photoconductive. It has uses in, for example, photoelectric devices and xerography. When Se is sorbed into a molecular sieve, it gives markedly different optical absorption spectra from those of the bulk material. 

Figure 19 Mobility of carriers in P-rhombohedral boron obtained by different methods and different authors. 1, From space-charge limited currents 2 and 3, (1h 4, field effect 6, thermally activated hopping O, from electrical conductivity and spin density , (Xh. > from electrical conductivity and ESR magnetoresistance , from ESR line width +, band mobility A, hopping mobUity A, from photoconductivity V, from high-field conductivity, I, Hall mobility and photoconductivity. (See Ref. 2 and references therein.)...

With the present knowledge of the vibrational spectra of many polyconjugated systems (oligomers and polymers) in their pristine (insulating), doped (electrically conducting) and photoexcited (photoconducting) states it is possible to list a set of characteristic spectroscopic manifestations of general validity. Since it has been shown [14,80] that most of these features are related to the existence of a network of delocalized tt electrons, their interpretation should be made with a unified theoretical approach,independently from the molecule considered. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Kabanov [351] has provided an excellent review of the application of measurements of electrophysical effects in studies of the thermal decomposition of solids, including surveys of electrical conductivity, photoconductivity, dielectric measurements and interface (contact), Hall and thermal (Seebeck) potentials. Care must be exercised in applying the results obtained in such studies to the interpretation of data for thermal decomposition in the absence of an applied electric field since many examples have been given [352] in which such a field markedly influences the course of decomposition. 

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. 

---