Photoconductivity electron conduction

Measurements of photoconductivity and of the Hall potential [367] are accurate and unambiguous methods of detecting electronic conduction in ionic solids. Kabanov [351] emphasizes, however, that the absence of such effects is not conclusive proof to the contrary. From measurements of thermal potential [368], it is possible to detect solid-solution formation, to distinguish between electronic and positive hole conductivity in semi-conductors and between interstitial and vacancy conductivity in ionic conductors. 

Photoconductivity in zinc oxide, on the other hand, appears to be influenced by the surface through a different effect. Absorption of light effectively excites the electrons trapped in surface levels into the conduction band. This chapter will be primarily devoted to a consideration of this concept, proposed by Melnick (11), that photoconducting electrons are produced through the ionization of surface levels, specifically the adsorbed oxygen levels on zinc oxide. The decay of the photoconductivity. 

Electronic conduction Thermionic emission Photoelectric emission Photoconduction. Q QI2+X Q+X Q R RI2+X R+X R... 

Szent Gyorgyi s hypothesis on electronic conduction in biopolymers induced both theoreticians and experimentalists to carry out investigations on biopolymers. In 1946, he reported photoconductive effects for protein films containing dye molecules [4] and gave a new dimension to the concept of biological semi-conduction. A review of the various experimental and theoretical studies carried out on biomolecules such as proteins and DNA are presented. 

Application of high pressure changes the position of the electronic conduction level, Vq (see Section 6.9), and it increases the dielectric constant of the liquid. Both effects have an influence on the ionization energy of a solute. The dependence of Vo(p) is complicated and experimental data must be used. The effect of pressure on the dielectric constant is due to the increase in density and it is well described by the Clausius-Mossotti equation (see Section 1.6). In Figure 8a the photoconductivity spectrum of TMPD in neohexane is shown as a function of pressure. The variation of the photoconductivity threshold with pressure is depicted in Figure 8b. Evaluation of the data by means of Bom s formula (Chapter 7, Equation 94) led to the hypothesis that an additional increase of liquid density around the solute molecule due to fluctuations is responsible for the observed shifts (Katoh et al., 1995). 

Table 2 Electron Emission Threshold, Ej Cvac) Gas Phase Ionization Energy, Eth(gas) Polarization Energy of the Positive Ion, P+ Photoconductivity Threshold, Eth(pc) Energy of the Electronic Conduction Level, Vq, Refractive Index, n Positive Ionic Radius for Silicone Fluids, R t = (26 2fC...

Roth, S., "Conductive polymers in molecular electronics Conductivity and photoconductivity", p. 129 in Salaneck, W.R. Clark, D.T. Samuelsen, E.J. (Eds.), Science and Applications of Conducting Polymers, Adam Hilger, New York, USA (1991). 

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

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