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Bismuth-based perovskites are potentially a promising alternative for lead-free perovskites. During bond formation, however, trivalent ions on <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>Cs</mi></mrow><mn>3</mn></msub><msub><mrow><mi>Bi</mi></mrow><mn>2</mn></msub><msub><mi mathvariant="normal">I</mi><mn>9</mn></msub><mtext> </mtext></mrow></semantics></math></inline-formula>with CsI/BiI₃ ratio of 1.5/1 form 0D-neutral charged compounds with higher bandgap (>2.0 eV) and poor absorption capacity. Mixed 0/2-dimensional structures are potentially suitable substitutes due to their low bandgap. So far, the reported CsI/BiI₃ ratios for 0D/2D structures are 1:1, 1:2 and 1:3. Herein, a new ratio of 1/1.5 is reported. Caesium bismuth iodide at a ratio of CsI/BiI₃ of 1/1.5 was synthesised using a one-step processing method with/without solvent vapour annealing. During solvent annealing, a 1/4 (<i>v</i>/<i>v</i>) mixture of DMF/methanol was used as a solvent. The crystal structure formed at a ratio of 1/1.5 is more similar to 1.5/1 than to 1/3. The XRD pattern revealed additional characteristics peaks at 009, 012, 209 and 300, indicating the growth of another phase. The formed heterogeneous mixed 0D/2D structure has an extended light absorption capacity greater than 720 nm. Solvent vapour annealing improved film morphology by enhancing grain size and packing density. When cells with and without solvent vapour annealing are compared, the power conversion efficiency of caesium bismuth iodide increases from 0.26% without solvent annealing to 0.98% with solvent vapour annealing. This study establishes a new route for future research on crystal configuration, nomenclature, film and morphology, quality tailoring and applications toward the goal of lead-free perovskite solar cells.