Green Solvent Engineering and Additive Modulation in Stabilized Black FAPbI3 Perovskite Single Crystals for High-Performance Photodetectors
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 [PbIx]2−x 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 α 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 FA0.9MA0.1PbI3 single crystal thin films exhibit a low defect density of 8.3 × 10^11 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 × 10^10 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.