Novel low-cost synthesis of crystalline SnI4 thin films via anionic replacement: Effect of iodization time on properties

Abstract

SnI4 thin films were deposited for the first time via a novel low-cost route employing rapid iodization of SnS thin films at relatively low temperature in atmospheric conditions. This is the first report on SnI4 thin films obtained via anionic replacement of sulfur in SnS thin films. The effect of iodization time on the structure, morphology, optical and electrical properties of SnI4 thin films were studied. X-ray diffraction analysis revealed the formation of SnI4 thin films which crystallized in cubic Pa-3 structure and all samples were preferentially oriented along the (222) plane. The diffraction pattern of SnI4 thin film was subjected to Rietveld refinement and obtained excellent goodness of fit value of 1.19. The formation of SnI4 was further verified using Raman spectroscopy and intense peaks of SnI4 at 43, 56, 101 and 140 cm−1 are reported. X-ray photoelectron spectroscopy analysis was employed for the determination of chemical composition and the presence of tin and iodine in the Sn4+ and I− states were verified throughout the depth of sample using depth profiling analysis. Elemental mapping from the Energy-dispersive X-ray spectroscopy confirmed uniform distribution of elements (tin and iodine). Surface morphology was modified after iodization and the optical bandgap was shifted from 1.7 to 2.7 eV on the transition of as prepared SnS to SnI4. The best condition SnI4 thin films exhibited crystallite size, refractive index and resistivity values of 63 nm, 2.46 and 3.1 × 105 Ω cm, respectively. SnI4 films obtained by the rapid iodization of SnS films can be used as precursor layer in the two-step deposition of lead-free tin iodide perovskite solar cells for better photovoltaic performance. The obtained results validate the key role of iodization time on the properties of SnI4 films.