<p>Formamidine lead-based perovskites (FAPbI₃) exhibit significant potential in optoelectronic applications. Nevertheless, they encounter challenges related to δ-phase instability and reliance on toxic solvents. In this study, we present a green solvent-additive synergy strategy that employs γ-valerolactone (GVL) in conjunction with reductive acids like oxalic acid (OA), to stabilize α-FAPbI₃ single crystals (SCs). GVL enhances the stability of the precursors through the formation of FA⁺-GVL hydrogen bonds and high-valence [PbI_<i>x</i>]^2−<i>x</i> clusters, resulting in ambient-stable α-phase SCs, yielding a marked improvement in stability compared to SCs prepared with the toxic solvent γ-butyrolactone (GBL). Additive modulation demonstrates that H⁺ and reductive groups play a critical role in regulating crystallization, suppressing the δ-phase by promoting FA⁺ dissociation and inhibiting MA⁺ deprotonation. A solvent-involved intermediate, δ-FAPbI₃-GVL, has been identified; this intermediate evolves into α-FAPbI₃ at a low temperature of 60 °C, thereby reducing the energy barriers associated with the a to δ phase transition. In contrast, non-reductive acids and reductive ionic liquids do not inhibit δ-phase formation, with the latter even promoting the crystallization of pure δ-FAPbI₃. By utilizing low-volatility OA as an additive, optimized FA_0.9MA_0.1PbI₃ single crystal thin films exhibit a low defect density of 8.3 × 10¹¹ cm⁻³. Subsequently, a photodetector was fabricated. Under zero bias voltage and 780 nm illumination, the device exhibited a responsivity of 14.5 mA/W, a detectivity of 3.75 × 10¹⁰ Jones, and a response speed of 149/65 µs. Moreover, without any encapsulation, the device’s performance diminished by only 17% after 30 days of storage in ambient conditions, indicating remarkable stability.</p>