Photoconduction correlation

Work is continuing to correlate the formation of surface oxidation states with changes in the photoconduction of films of PVCa. The relationship between energy transfer and photoconductivity is being investigated. 

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. 

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. 

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. 

A principal obstacle to identification of defects is the difficulty of comparing the results from EPR, luminescence, absorption, and deep state experiments. Probably the least ambiguous is that between EPR and luminescence when, as for transition metal impurities, it is possible for optical Zeeman measurements of a sharp luminescence line to determine the ground state g factor. If the optical and EPR measurements give the same value, then the correlation is made (Watts, 1977). In some cases, when optical excitation enhances or quenches the EPR signal, there may be a similar response in the photoconductivity or luminescence excitation spectrum. 

The evidence that adsorption has an influence on the luminescence of zinc oxide is not as clear-cut as it is in the case of photoconductivity and conductivity. No experiments have been carried out, to the author s knowledge, directly correlating adsorption to luminescence, such as the work by Ewles and Heap (7) on silica, which showed correlations between its fluorescence and the adsorption of the hydroxyl radical. 

At present the studies of the luminescence of zinc oxide give little precise information on the adsorption properties of this substance, in contrast to the more fruitful studies of its photoconductivity and conductivity. But there exists a possible correlation of the 3950 A. fluorescence with the adsorption of oxygen. Further study of the fluorescent properties may clarify this correlation. 

The impurity generation route is shown in Fig. 5 to the right of the line. The interaction of the relaxed states formed from m ami unexcited impurity t results in exiplex t formation. This exiplex can decay thermally or form coupled ion-radical pairs t, which may dissociate in the electric field. For explanation of the absorption and photoconductivity spectrum correlation it is necessary to assume a very high concentration of exiplex sites. 

Some types of the polymers were investigated in detail. The photoconductivity of polyethylene with quantum efficiency 10 5-10 10 is caused by impurities, Schottky type contact injection, and hole transport [82,83], The crystallinity increase is accompanied by a photocurrent increase. There is no clear correlation between the chemical structure and the photocurrent. 

In our experiments with the low-temperature sensitivity of monodisperse silver bromide emulsions having 0.5 ym cubic grains, the sensitivity of the (S+Au)-sensitized emulsion differed from that of the S-sensitized emulsion both with respect to the dependence on the degree of sensitization and the correlation with microwave photoconductivity measurements (16). 

Figure 4. Correlations between the Pt(A, C) or Ni (B) content and (i) the logarithm of the photoconductance a at equilibrium under vacuum (A, B) or (ii) the amounts of oxygen photo-adsorbed (C).

Photocurrent spectra of a p-type GaN epilayer have also been measured [7], From the onset of photoconductivity spectra, it was suggested that metastable centres at 1.1, 1.40, and 2.04 eV above the valence bandedge were responsible for the PPC in Mg-doped GaN, and that Ga vacancies may be responsible for PPC in n-type GaN [7], The spectral dependence of the optical cross section of the PPC related impurities in n-type epilayers has also been measured and the results are shown in FIGURE 4, from which an optical ionisation energy of about 2.7 eV was obtained [8], The correlation between the... 

Polyethylene In Section 2.2.2, HF and CIS (and TDHF) are shown to describe the photoconduction, photoemission, and optical absorption thresholds of polyethylene qualitatively correctly. There are, however, quantitative errors of a few electron volts, which are speculated to be electron-correlation effects. To prove or disprove this speculation, correlated CIS calculations at the Dyson(2) or MP2 level are performed [55] on the basis of Eqs. (2-46) and (2-47) or (2-48) with W(r1,r2 w)= ri—r2 1 ... 

However, disorder appears to be crucial in the occurrence of photoexcited paramagnetic centers since irradiation of single crystal samples, e.g. poly TS, has no effect. The energy levels in the low energy absorption tail of the disordered samples play an important part. It is not clear if these derive from excitonic or interband transitions but it should be noted that in principle any distortion of the polymer chain can lead to defect states in the optical gap ( ) and that the weak absorption tail in crystals is a major factor determining photoconduction (9 ). Thus, it appears that a basis exists for the correlation of structure and the intrinsic absorption spectra but that further efforts are required to obtain a better understanding of defect states and their spectroscopic properties. 

If carrier generation originates from exciton dissociation, a linear correlation should exist between Aa(E)/o and -AIl(E)/Il. This is not the case the onset fields are different (the nonlinearity in the photoconductivity turns on at a lower field see Fig. VD-1), and even above the onset, AIp[Pg.150]

The observation of photocurrent response at low fields, the onset of photoconductivity at a photon energy that coincides with the absorption edge, and the absence of correlation between Aa(E)/o- and - AIl(E)/1l all suggest... 

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