THE EFFECTS OF ANNEALING ON MICROSTRUCTURE, OPTICAL AND STRUCTURAL PROPERTIES OF BISMUTH OXYIODIDE THIN FILMS

Abstract

Perovskite solar cells based on lead (Pb) halide has demonstrated the rapid increase in efficiency and advanced in photovoltaic technology during this last decade, but the toxicity issue impede the large-scale industrial production. Bismuth oxyiodide (BiOI) has been recognized as suitable candidate of less-toxic material to replace Pb in conventional Pb-halide perovskite solar cells (PSCs), without adversely impacting performance in perovskite solar cells. Thin films of BiOI was synthesized and deposited using Successive Ionic Layer Adsorption and Reaction (SILAR) on glass substrates. The same molar ratios of bismuth (III) nitrate pentahydrate (Bi(NO₃)5H₂O) and potassium iodide (KI) were used in this experiment. The samples were annealed at various annealing temperature from 250 °C, 350 °C, 450 °C and 550 °C, for 20 minutes. The physical observation, microstructure, average thickness, optical and structural properties of BiOI thin films were characterized using macroscopic examination, field-emission scanning electron microscope (FESEM), surface profilometer (SP), ultraviolet-visible (UV-Vis) and X-ray diffraction (XRD), respectively. From the physical observation, the color of the films changed from the orange-yellow to yellowish with increasing annealing temperature. The microstructure study demonstrated the BiOI thin films have flakes morphology structure. The average thicknesses of BiOI films were in ranged of 3.479 - 8.082 µm with the optical band gap, E_g in range 1.607 to 2.150 eV. From XRD characterization, sample annealed below 350 °C demonstrated single crystalline structure, but at higher annealing, BiOI film changed to polycrystalline with mixed phases of BiO. The crystallite size was calculated in range from 27.747 to 29.314 nm. This study provided some clue on BiOI thin film properties for active layer in low-toxicity perovskite solar cells.