Photoconductivity solutions

The main application today for poly(vinyl carbazole) arises out of its photoconductivity and is in electrostatic dry copying machines. The polymer is applied from solution in thin film (10-15 p.m) layers onto a conductive substrate. 

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. 

Figure 7. Example of space-resolved photoinduced microwave conductivity mapping of semiconductor interface distribution of photoconductivity in natural pyrite (from Murgul, Turkey, surface etched in acid solution). The overflow was adjusted to show patterns of low photoactivity. For color version please see color plates opposite p. 452.

Sham, T.-K. X-Ray Absorption Studies of Liquids Structure and Reactivity of Metal Complexes in Solution and X-Ray Photoconductivity of Hydrocarbon Solutions of Organometallics, 145, 81-106(1987). 

The first apparent report in the open literature of CD PbSe for photoconductive detectors was in 1949 [53], The PbSe was deposited from a solution of PbAci and selenourea onto a predeposited (from PbAci and thiourea) layer of PbS. The PbS layer acted as a seed layer, presumably to obtain faster deposition (it was noted that the PbSe deposition was much slower than that of PbS). The photoconductivity of this film exhibited a broad maximum between 3 and 4 p,m, giving a reasonable response out to beyond 4.5 p,m (PbS drops off at 3 iJim). 

In Refs. 10 and 11, aqueous NaiSiOs was added to SbCls in glacial acetic acid (SbCls hydrolyzes in water unless complexed or the solution is moderately acidic or strongly alkaline). A pH of ca. 3 was optimum below 2.5, adhesion was poor above 4, basic antimony salts precipitated. The solution was kept below room temperature to prevent rapid bulk precipitation. No XRD pattern was found for the as-deposited film, which was presumed to be amorphous. Annealing at 170°C crystallized the film, at least partly. The bandgap of the as-deposited film was reported to be 2.48 eV and that of the annealed film 1.76 eV. Photoconductivity was exhibited by the annealed film but not by the as-deposited one. 

TlS was deposited from a solution of TINO3, ammonia, and thionrea at room temperature (26°C) [96]. X-ray diffraction showed the formation of TIS. Optical spectroscopy (both transmission and diffuse reflection) allowed an approximate bandgap of 1.0 eV to be estimated. The films were p-type, with resistivity of ca. 2 X 10 H-cm. Photoconductivity was measured (although not quantified) with a peak at ca. 1.2 eV (ca. 1 jim) and extending from 1.0 eV to beyond 1.4 eV. 

Films were deposited from solntions of lead and tin salts (the salts used were not specified) with ammoninm acetate, ethylenediamine, and selenonrea at a pH > 9 (probably at least 11) [34]. To obtain thicker films, deposition was repeated a nnmber of times and the films were annealed therefore it is not known if solid so-Intion formation occnrred in as-deposited films. In annealed films, Pbi -j Snj Se solid solntions with x np to 0.11 were verified by XRD. The spectral response of the photoconductivity of the (annealed—as-deposited films were not photosensitive) films shifted from a peak at ca. 4 p,m (pure PbSe) to ca. 7.5 p,m (11% Sn), supporting solid solution formation of the annealed films. The room-temperature, dark resistance of the (probably annealed, but not certain) films varied from 1 to 300 kO, depending on deposition conditions. 

The adsorption of electron acceptors (quinone, chloranil) from the gas phase does not substantially influence the photo-emf of PAC but decreases the dark conductivity and the photoconductivity. The same compounds, however, adsorbed on certain polyacetylenides from solution, increase the photo emf without causing any appreciable change in the spectral distribution. Mercury vapor depresses reversibly the dark conductivity and photoconductivity [276-278]. 

Typical results are shown in Fig. 44. The spectral threshold of the proper photoconductivity and the photo-emf of PAC is situated at 520 nm. The spectral response for the photo emf of PAC itself is shown by curve 1. After PAC has been immersed in an ethanol solution of methylene blue and dried its spectral response is represented by curves 2 and 2. The photo-response appears in the range of the absorption maximum of the dye at 680 nm characteristic of the monomolecular form in the dilute initial solution (curve 3). The observed enhancement of the second maximum at 620 nm in comparison to the solution spectrum is obviously connected with the presence of dye dimers. The shift of the maximum photoresponse to the longer wavelength by 15 nm relatively to the solution is usually the case for the adsorbed state. The sign of the charge carriers both in the proper and sensitized spectra ranges is positive. As seen in Fig. 44 the adsorption of the dye also markedly changes the proper photosensitivity of the PAC. When the monomolecular form of the adsorbed dye dominates, the... 

Fig. 46. Photoconductivity spectra of polycop-perphenyiacetylenide sensitized with methylene blue ethanol solutions of the following concentrations / -10 3 M 2- 1(T4 M 2- 1(T2 M 4 - photoconductivity in the proper photosensitivity range of polycopperphenylacetylenide [20]...

Fig. 47. Photoconductivity spectra of polycopperphenylacetylenide sensitized by polyoxiphenylene with concentrations of the solution in g/cm3 1 - 10 2 - 1(T2,3 - 1CT3 [289]...

At the early stages the photoconductivity of solid solutions of the leucobase of malachite green in various organic media was investigated [285]. In these systems, carrier transport occurs by direct interaction between the leucobase molecules. No direct participation of the organic matrix in the charge transfer was observed. A model was proposed which links charge transfer in these systems with impurity conduction in semiconductors. 

When hydrogen sulfide is passed into cadmium salt solutions, cadmium sulfide is formed as a yellow precipitate with a zinc blende structure (cubic, (i-form). The P-form can be converted into the a-form (e.g., by heating). a-Cadmium sulfide shows photoconductivity due to defects in the crystal lattice (usage in photovoltaic cells) [3.106]. The solubility in water at 25°C is 1.46 x 10 10 mol/L [3.107], Cadmium sulfide forms the basis for all cadmium pigments. 

Transient Photoconductivity. A solution of neutral molecules in a polar solvent shows only ohmic conductivity, but if ions are formed by the action of the photolytic flash these charge carriers generate an additional current which is proportional to the ion concentration. The observation of such transient photocurrents is the most direct experimental evidence for the formation of free, solvated ions in electron transfer reactions. The quantum yield of ion formation can be obtained through proper calibration procedures and the kinetics of ion recombination can be determined. Figure 7.37 gives an example of such transient photocurrent rise and decay. 

IET serves as a theoretical basis not only for fluorescence and photochemistry but also for photoconductivity and for electrochemiluminescence initiated by charge injection from electrodes. These and other related phenomena are considered. The kinetics of luminescence induced by pulse and stationary excitation is elucidated as well as the light intensity dependence of the fluorescence and photocurrent. The variety and complexity of applications proves that IET is a universal key for multichannel reactions in solutions, most of which are inaccessible to conventional (Markovian) chemical kinetics. 

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