Stabilizer-free pure α-phase FAPbI₃ perovskite through seed-assisted growth yields efficient photovoltaics

The α-phase of formamidinium lead iodide (α-FAPbI₃) perovskite has a promising bandgap for efficient single-junction photovoltaics. However, its large A-site FA⁺ cation destabilizes the perovskite lattice, favoring transformation into non-perovskite δ-phases under ambient conditions. To overcome this instability, compositional A-site engineering with Cs⁺, Rb⁺, and MA⁺ has been widely explored, but such α-FAPbI₃ stabilizers inherently alter the bandgap and complicate crystallization dynamics. Here, we report an α-FAPbI₃ stabilizer-free seeded growth strategy that directly incorporates FAPbI₃ seeds (SDs) into the PbI₂ precursor, lowering, in this way, the nucleation barrier and guiding directional crystallization. Multiscale simulations showed that the persistent structural characteristics of the dissolving SDs pertain more to the α-phase thereby promoting growth of the photoactive α-phase while suppressing the formation of the competing photoinactive δ-phase. This approach yields highly crystalline films with compact morphology and markedly reduced surface roughness, as confirmed by SEM, AFM, and XRD analyses. Electroluminescence measurements (EL) and optical-electrical device simulations reveal that the combined effects of enhanced carrier lifetime and improved charge transport suppress non-radiative recombination, translating into a champion PCE of 23.51% (compared to a 15.5% PCE for the SD-free device). Importantly, the seeded devices preserve 99% of their initial performance after 3000 h under ambient conditions and one sun illumination at 35 ± 5 °C without encapsulation, whereas control devices rapidly degrade, exhibiting a 15% drop in PCE under identical conditions. Our comprehensive investigation concluded that these results establish seeded growth as a practical and effective pathway for realizing efficient and durable FAPbI₃-based solar cells without the need for additional α-phase stabilizers, redefining the potential of FA-based perovskite via seeded growth strategy.