Tuning morphological features of lead iodide by low pressure vapor phase deposition

Abstract

Lead iodide (PbI2) is a semiconductor with extensive use as an active layer for X-ray detectors and as a precursor for perovskite solar cells. Here we present a low vacuum method to obtain very uniform PbI2 films with full substrate coverage. This method consists in the sublimation of PbI2 inside a hot zone and its transport by an Ar flow to a substrate held at a controlled temperature. Using scanning electron microscopy combined with focused ion beam and X-ray diffraction we studied the morphology and crystallographic structure of the PbI2 films with different deposition parameters: substrate and source evaporation temperature, deposition time and substrate material. At high substrate temperature (80 °C) and low evaporation temperature (310 °C) onto a glass sample, we obtained dense and smooth PbI2 films showing hexagonal crystals, or platelets, stacked parallel to the substrate. The choice of the substrate material has a significant impact on the film morphology yielding porous-like structures with voids within the films for some substrates. A bandgap Eg = 2.42 eV and Urbach energy EU = 34.7 meV were obtained by absorbance measurements, which are comparable to films evaporated in high vacuum. Photoluminescence studies showed a dependence of the emission energies on the crystal orientation of the platelets which grow differently depending on the deposition conditions. The results show the ability of the low pressure vapor phase deposition technique to obtain good film properties, suitable for sensors and optoelectronic devices.