Effect of annealing on photo-physical properties of BiI₃ thin films via vacuum thermal evaporation deposition for photovoltaic applications

Abstract

In this paper, a simple thermal evaporation deposition (TED) method is used to obtain high-quality bismuth triiodide (BiI3) films on FTO substrate using BiI3 powder as an evaporation source. The effects of annealing on the X-ray diffraction analysis and some optical parameters of BiI3 thin films were investigated. The results show that the annealing temperature on the BiI3 thin film has a great influence on the preferred orientation of the films. When the annealing temperature is 100 °C, for a deposition thickness of 400 nm, the thin films have high crystallinity and rhombohedral morphology. Scanning electron microscopy observation showed high special resolution of BiI3 thin films. UV–Vis transmission spectra have been recorded to determine the optical band-gap of BiI3 films prepared by TED. The obtained data reveal that the indirect optical band-gap of BiI3 thin films decreases from about 1.7 to 1.6 eV, while the Urbach tail width decreases from 484 to 236 meV, by annealing. We discuss the outstanding annealing effect on photo-physical characterizations of BiI3 thin film as a candidate solar cell absorber such as lead-free perovskite solar cells.