Effects of annealing temperature on bismuth oxyiodide thin film for leadfree perovskite solar cells

Perovskite solar cells based on lead (Pb) halide has demonstrated the rapid increased in efficiency and advanced in photovoltaic technology in the 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 were synthesized and deposited using Successive Ionic Layer Adsorption and Reaction (SILAR) on the glass substrates, the BiOI films were characterized with all these characterizations prior to find the optimum annealing temperature and used it for fabrication of solar cells device and accessed its perfomance. The samples were annealed at various annealing temperature for optimization of the performance which influenced on crystallization, morphology and leads to improving the electrical properties of BiOI films from 250°C, 350 °C, 450 °C and 550 °C, for 20 minutes. The physical observation, microstructure, thickness, optical, structural, and electrical properties of BiOI thin films were characterized. From the physical observation, the colour 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, Eg in range 1.59 to 2.10 eV. From XRD characterization, sample annealed below 350 °C demonstrated single crystalline structure, but with further higher annealing, BiOI film changed to polycrystalline with mixed phases of BiOI. The crystallite size was calculated in range from 27.74 to 29.31 nm. Finally the BiOI thin films were measured its conversion efficiency using I-V measurement. The sample annealed with 350 °C shows the highest efficiency with 1.67 %. By referring to all the results in this study, we conclude that the optimum annealing temperature for BiOI thin film is 350 °C with 1.67%. This study provided the clue on BiOI thin film properties for active layer in low-toxicity perovskite solar cells.
Keywords:
Bismuth Oxyiodide, SILAR, Thin Film, Lead-free Perovskite Solar Cell, Annealing Temperature.