Photoconduction mechanism

In the case of undoped PVCa films, impurities and surface states dominate the photoconduction mechanism (6) leading one to question any study of intrinsic pKotoconduction in organic polymers. Poly(N-vinylcarbazole) films yellow under ambient laboratory conditions. Work in our laboratory (7) has shown that ageing of a purified sample of PVCa leads to an increase in photoresponse in the 350-450 nm region while there is an initial drop in photoresponse in the 250-... 

Identical methods for investigation of photoconductivity can be used for inorganic and organic semiconductors. Polymer semiconductors as a rule have very high resistance. For such materials the main information about the photoconductive mechanisms and properties may be obtained by two methods electrophotographic (or discharge method) and time of flight (or transit method). Both methods are successfully applied for materials with low mobilities, less than lO-4m2 V-1 s-1, which are the usual values for polymer semiconductors. 

Later on a lot of research into the photosensitivity of the polymer matrix with various inserted molecules was carried out. A great deal of attention was focused on molecule doped polycarbonates. The photoconduction mechanism in such systems has many common features with the same mechanism for doped PVC. 

Figure 6.2 The photoconductive mechanism of an OPC (G = charge-generating molecule, T = charge-transport molecule)...

The study of monolayer assemblies containing periodically arranged dye molecules may assist in the understanding of photoconduction mechanisms in organic matrices. Cells containing several layers of arachidate and incorporating... 

The area of photoinduced electron transfer in LB films has been estabUshed (75). The abiUty to place electron donor and electron acceptor moieties in precise distances allowed the detailed studies of electron-transfer mechanism and provided experimental support for theories (76). This research has been driven by the goal of understanding the elemental processes of photosynthesis. Electron transfer is, however, an elementary process in appHcations such as photoconductivity (77—79), molecular rectification (79—84), etc. 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

West, W. Correlations between photographic and photoconductance sensitivity of silver halides.. Mitchell, J. W. Fundamental mechanisms of photographic sensitivity, p. 99. London Butterworth s Sci. Publ. 1951. 

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. 

The experimental data presented show that sNPS can be used as transducers, which are stable for a long time after the construction of an immune biosensor. The specific immune complex formed on the sNPS surface may be registered by measuring its photoluminescence or photoconductivity. Such immune biosensors can be applied for control of T2 mycotoxin. The biosensors developed are sensitive and simple and allows for rapid analysis and analysis in field conditions. This approach may be applied for detection of any biochemical substances which can form an immune complex. Further investigations should be directed towards studying the mechanism of the biochemical signal detection by the sNPS and characterization of all the steps of analysis. 

The resistivity of the films increased from 10 O-cm for very low Cd content to a maximum of ca. 10 U-cm at 6% Cd and then slowly dropped again with increasing Cd. This maximum correlates with the minimum crystal size, suggesting a dominant role of grain boundaries in the conduction mechanism. The spectral response of the photoconductivity blue-shifted with increase Cd content up to a peak response at 1.35 p,m for 8.4% Cd. 

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

In systems in which the charge-transfer excitation band differs from the action spectrum of photoconductivity, the doping effect may be due to a change of recombination path that results in an enhancement of carrier liefetime (e.g. holes in merocyanines and phthalocyanines). (Details on the mechanism are given in 10,11,74).)... 

Polymers with saturated bonds, heteroatoms, heterostructures and poly-conjugated ones are available now as photosensitive materials. Really one cannot expect a single mechanism to be reponsible for photoconductivity in so many diverse systems. However, there are a lot of verified factors which permit us to explain the main features of the photoconductive processes in polymers. The status and prospects of the application of polymeric photoconductors as prospective new electronic materials will be also analyzed for various types of photoconductors. 

Now the research in the photoconductive properties of the polyconjugated materials is growing fast. Heterocycle or heteroatom-containing polymers are involved in this process due to their excellent mechanical and electric properties. The sensitized photoeffect in polyconjugated materials was first observed in 1964 [19,20] and the high significance of the increase in the photosensitivity of these compounds became apparent... 

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