The growing demand for sustainable energy solutions in urban environments has driven the development of building-integrated photovoltaics (BIPV), an approach in which solar modules are incorporated directly into architectural elements such as roofs, facades, windows, and railings. By bringing photovoltaic (PV) generation closer to demand centers, BIPV offers a compelling strategy to reduce costs and maximize solar energy utilization within the built environment. Among the various BIPV-enabling technologies, semi-transparent solar cells are particularly attractive because they allow partial light transmission while generating electricity, making them ideally suited for integration into buildings. Wide-bandgap (WBG) perovskites (E_ɡ > 1.7 eV) are commonly employed in semi-transparent architectures to balance transmittance and absorption; however, these bromide-rich compositions often suffer from halide segregation, photo-instability, and reduced long-term operational stability under continuous illumination. In this work, we introduce an alternative strategy based on thickness engineering of formamidinium lead iodide (FAPbI₃), a narrow-bandgap perovskite typically used in opaque device architectures. By precisely tuning the absorber thickness, we achieve high optical transparency and competitive power conversion efficiency, circumventing the intrinsic limitations of wide-bandgap perovskites and the limited performance of quasi-2D systems. Optical modeling reveals that thinning the FAPbI₃ layer effectively balances visible transmittance and device performance: absorbers of 140 nm and 160 nm deliver average visible transmittances (AVTs) of 24.2% and 18.2% alongside power conversion efficiencies (PCEs) of 15.9% and 18.0%, respectively. Surface passivation with 4-fluorophenylethylammonium iodide enhances the open-circuit voltage (V_OC) by over 100 mV and raises efficiency by more than 1%, as confirmed by photoluminescence and transient optoelectronic analyses showing reduced non-radiative recombination losses and extended carrier lifetimes. Crucially, both passivated and unpassivated devices exhibit excellent operational stability despite the reduced absorber thickness. These results establish thin FAPbI₃ as a robust and versatile platform for efficient, stable semi-transparent solar cells targeting BIPV applications.