Photoconductivity silver halides

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. 

Silver halide microcrystals are wide band gap semiconductors which exhibit weak photoconductivity. Early experiments demonstrated that dyes that sensitized silver halide photographic action also sensitized silver halide photoconductivity [6c]. Since the observation of photoconductivity necessitates the movement of free charge within the crystals, dye sensitization processes must inject charge into the silver halide lattice in some way. Initial theories of sensitization were based on the semiconductor view of silver halides, especially as espoused by Gurney and Mott [10]. Current ideas are based on thorough studies of the absorption spectroscopy and luminescence of silver halide emulsions and of adsorbed, sensitizing dyes, and the oxidation-reduction properties of the dyes at silver/silver halide electrodes [11]. 

The suggestion of Mott [190], that photodecomposition of Ba(N3)2 occurs by the same mechanism in silver halides, was disputed by Tompkins and coworkers on the basis of additional observations [80,191,206]. In particular, the photoconductivity was found to be too small to account for the electron motion necessary for the formation of barium colloids [80]. More recently, Marinkas and Bartram were unable to detect photoconductivity in anhydrous crystals [49]. In addition, measurements of the dark conductivity indicated that if it is due to Ba ", it is much too small to account for the observed rate of photodecomposition [80,206]. As a further indication that the photodecomposition of Ba(N3)2 does not take place by the silver hahde process, the energy of formation of a barium interstitial was estimated and found to be much greater than the estimated energy for vacancy formation, thus indicating the possibility of Schottky disorder rather than Frenkel disorder as intrinsic to Ba(N3)2 [206]. Interstitial metal ions are required for the Mott-Gurney mechanism discussed above [167]. 

Decomposition models for silver azide are similar to those proposed for the silver halides. McLaren and Rogers [95] suggested that band to band transitions give rise to electrons and holes in accord with photoconductivity data. Trapped electrons attract interstitial silver atoms which eventually form colloids, and holes lead to the formation of the nitrogen through a less-clearly determined process, possibly a bimolecular reaction of neutral azide molecules near the surface. Such a process requires the presence of discrete band-gap acceptor states, such as cation vacancies, that would serve as reaction sites for holes. The details of the process, however, remain undetermined. 

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