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

In this paper, a simple thermal evaporation deposition (TED) method is used to obtain high-quality bismuth triiodide (BiI 3 ) films on FTO substrate using BiI 3 powder as an evaporation source. The effects of annealing on the X-ray diffraction analysis and some optical parameters of BiI 3 thin films were investigated. The results show that the annealing temperature on the BiI 3 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 BiI 3 thin films. UV–Vis transmission spectra have been recorded to determine the optical band-gap of BiI 3 films prepared by TED. The obtained data reveal that the indirect optical band-gap of BiI 3 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 BiI 3 thin film as a candidate solar cell absorber such as lead-free perovskite solar cells.