Influence of short circuiting on the kinetics and mechanism of iodide film growth on Ag and Cd-doped Ag

Abstract

Results of iodination studies on pure Ag and Ag with 6000 ppm of Cd under normal and shortcircuit conditions in the temperature and iodine pressure ranges of 333 to 373 K and 0.067 to 6.078 kPa, respectively, are reported. Under all experimental conditions, the iodide film growth kinetics conform to parabolic rate law. The iodide films have been characterized by scanning electron microscopy (SEM), electron probe microanalysis (EPM), X-ray diffraction (XRD), and Auger electron spectroscopy (AES) analyses. The effect of a higher valent dopant like Cd in Ag is observed to decrease the rate of normal iodination, which suggests that the film growth process is controlled by migration of electron holes across the iodide layers. The presence of a short-circuit path enhances the rate of iodination for pure Ag. However, the iodination rate of Cd-doped Ag under short-circuit mode is found to be further enhanced compared to that for pure Ag. This has been explained on the basis of the ion migration mechanism. The pressure dependence of rate constants is found to follow a relation like k P ␣ for pure Ag and Cd-doped Ag under normal conditions. The corresponding 1/2 P I2 relations under short-circuit mode are observed to be k P ␣ and k P ␣ for Ag and Cd-doped 1/3 1/4 P P I I 2 2Ag, respectively. Arrhenius plots have yielded activation energy values of 25.8 and 15.2 kJ⅐mol Ϫ1 under normal and short-circuit conditions, respectively. The kinetic results of parabolic film growth have been explained with the help of defect equilibria, considering the predominance of Frenkel defects in pure and doped AgI lattices. The mechanism of film growth processes has been confirmed to be Wagner's electrochemical potential gradient-induced migration of defect species across the iodide layer.

Steady-State Photoluminescence Characteristics of Sb-Doped AgI Thin Films

“…Iodide film growth is mainly controlled by the migration of electron holes across the growing film on both Ag-and Sb-doped Ag systems. 23 The luminescence spectra exhibit five prominent emission bands, the first of its kind to the best of our knowledge obtained in AgI thin films under ambient conditions. The first weak fluorescence band at around 440 nm is red shifted from the excitonic absorption peak and does not appear to be from the bottom of the conduction band, thus possibly corresponding to a shallow trap state or intrinsic near band edge state slightly below the conduction band.…”

“…This is in accordance with the fact that presence of higher valent dopant cations strongly reduces the iodination rate of silver under normal conditions. Iodide film growth is mainly controlled by the migration of electron holes across the growing film on both Ag- and Sb-doped Ag systems 3 PL emission spectra of the AgI- and Sb-doped AgI thin films (same as that in Figure ) for four different excitation energies.…”

Interfacial control on microstructure, morphology and optics of beta-AgI nanostructures fabricated on sputter-disordered Ag-Sn bilayers

“…Moreover, doping could stabilize the crystal structure by strengthening the cation (or anion) sublattice of the ionocovalent semiconductor (CdS or AgI) and introducing a certain number of donors/acceptors in the forbidden gap of the host semiconductor thereby impacting the electrical and optical properties of the host semiconductor [12]. Sn -with valences 2 and 4-was chosen because it is a covalent metal and mixes well with Ag and could controls the iodization kinetics [13] enabling realization of desired optimized nanostructure even for a single Ag/Sn ratio.…”