Methylammonium iodide bismuthate ((CH₃NH₃)₃Bi₂I₉) (MBI) perovskite was introduced as a new lead-free and air-stable absorber for hole conductor-free perovskite solar cells. The two-step soaking-assisted sequential solution (2-S) method was adopted to fabricate MBI films for the first time. We compared the formation processes and final morphologies of the MBI films fabricated using the 1-S and 2-S methods on planar and mesoporous TiO₂ layers, respectively. We also investigated the effects of the morphologies of MBI films and device architectural design on device performance. Results showed that the MBI films fabricated using the 2-S method achieved a superior coverage both on the compact TiO₂ and mesoporous TiO₂ layers. The mesoporous structure devices presented higher power conversion efficiencies than the planar structure devices. In addition, all devices exhibited excellent thermal and long-term stabilities. The presented architectural design and solution-processable approach could inspire further research and practical applications on lead-free organic–inorganic hybrid perovskite solar cells.