Cesium Bismuth Iodide Solar Cells from Systematic Molar Ratio Variation of CsI and BiI₃

Metal halide compounds with photovoltaic properties prepared from solution have received increased attention for utilization in solar cells. In this work, low-toxicity cesium bismuth iodides are synthesized from solution, and their photovoltaic and optical properties as well as electronic and crystal structures are investigated. The X-ray diffraction patterns reveal that a CsI/BiI₃ precursor ratio of 1.5:1 can convert pure rhombohedral BiI₃ to pure hexagonal Cs₃Bi₂I₉, but any ratio intermediate of this stoichiometry and pure BiI₃ yields a mixture containing the two crystalline phases Cs₃Bi₂I₉ and BiI₃, with their relative fraction depending on the CsI/BiI₃ ratio. Solar cells from the series of compounds are characterized, showing the highest efficiency for the compounds with a mixture of the two structures. The energies of the valence band edge were estimated using hard and soft X-ray photoelectron spectroscopy for more bulk and surface electronic properties, respectively. On the basis of these measurements, together with UV-vis-near-IR spectrophotometry, measuring the band gap, and Kelvin probe measurements for estimating the work function, an approximate energy diagram has been compiled clarifying the relationship between the positions of the valence and conduction band edges and the Fermi level.