We systematically investigated the development of film morphology and crystallinity of methyl-ammonium bismuth (III) iodide (MA₃Bi₂I₉) through onestep spin-coating on TiO₂-deposited indium tin oxide (ITO)/glass. The precursor solution concentration and substrate structure have been demonstrated to be critically important in the active-layer evolution of the MA₃Bi₂I₉-based solar cell. This work successfully improved the cell efficiency to 0.42% (average: 0.38%) with the mesoscopic architecture of ITO/compact-TiO₂/mesoscopic-TiO₂ (meso-TiO₂)/MA₃Bi₂I₉/2, 2′, 7, 7′-tetrakis(N, N-di-4-methoxyphenylamino)-9, 9′spiro-bifluorene (spiro-MeOTAD)/MoO₃/Ag under a precursor concentration of 0.45 M, which provided the probability of further improving the efficiency of the Bi³⁺-based lead-free organic–inorganic hybrid solar cells.