Effects of band gap illumination on surface layers of lead iodide

Abstract

Thin films of lead iodide (PbI 2) decompose under band gap illumination, giving rise to iodine desorption and the formation of lead aggregates. We report on mass spectrometry, conductivity and photoconductivity measurements made in order to define the desorbing species and to elucidate the dissociation mechanism. The desorption rates of atomic and molecular iodine, together with the dark conductivity and the photoconductivity, were measured as functions of the PbI 2 film temperature in the range 300–500 K. The molecular desorption in particular seems to undergo a transition from a diffusion-controlled process (which governs the dark conductivity and the atomic iodine desorption) to a process dominated by the concentration of photoholes at temperatures above 400 K. At these temperatures a striking resemblance appears between the variations in the molecular iodine desorption and the photoconductivity with temperature. This leads us to suggest a model in which the iodine molecules are primarily evolved at special surface sites, e.g. grain boundaries, which also dominate the photoconductivity.