Kinetics and Mechanism of Iodide-Film Growth on Lead-Effect of Short-Circuiting and Lower-Valent Dopant

Abstract

The influence of Ag as a lower-valent dopant onthe kinetics of lead iodination under normal andshort-circuit conditions in iodine pressures of 0.615 to6.578 kPa in the temperature range of 423 to 523 K was investigated. Like pure Pb, Ag-doped Pbalso follows the parabolic law of film growth. Theisothermal parabolic rate constants are found todecrease in the presence of the dopant. The iodinevapor-pressure dependence of isothermal parabolic rateconstants was observed to be k_P ∝p ^1/2_I2 . Results for normal iodination areexplained in terms of migration of electron holes underthe influence of Cabrera-Mott's electrical field across the film. Theactivation energy for normal iodination of Ag-doped Pbis estimated to be 84 kJ•mol^-1 comparedto that of 64 kJ•mol^-1 for pure lead. Therate of iodide-film growth has been found to decrease further undershort-circuit mode of experiments. Such observationshave been explained with the concept of ion migration asthe ratelimiting step for the film-growth process. The iodine pressure dependence of the rateconstants under short-circuit conditions is observed tobe k_P ∝ p ^1/3_I2 associatedwith an activation energy value of 66kJ•mol^-1. Unlike pure lead, introduction of additional resistances in series to theshort-circuit Pt path during iodination of Ag-doped Pbcaused an increase in the rates with gradual increasedvalue of resistances. Kinetics results are explained by considering the prevalence ofSchottky-Wagner type of point defects in lead iodide.The driving forces for migration of the defect speciesthrough the growing pure PbI₂ films andAg-doped PbI₂ are confirmed to be Wagner's electrochemical potentialgradient and Cabrera-Mott's electrical field,respectively. The iodide films were characterized bySEM, EDS, EPMA, AES, and XRD analyses to substantiatethe kinetic results.