Formamidinium lead iodide (FAPbI3), which has a narrower band gap close to the Shockley-Queisser limit, offers higher power conversion efficiency (PCE) than other perovskite compositions, surpassing 27% [1]–[3]. However, under external stressors like moisture in ambient air processing conditions, its high tolerance factor value causes phase instability because of the larger ion size of FA+ [4]–[6]. Additionally, a higher annealing temperature (>390 K) is needed to reach the cubic α-phase of FAPbI3, whereas lower temperatures result in the formation of a non-photoactive δ-phase [7].

In recent years, FAPbI3 perovskite ink has been extensively incorporated with volatile methylammonium chloride (MACl) as a transitional stabilizer [8]. This substance effectively provides FAPbI3 black phase without annealing by decreasing the formation energy. However, the advantageous effects of MACl as an α-phase FAPbI3 inducer and stabilizer at room temperature are neutralized under ambient conditions and in the presence of non-volatile coordinating DMSO, which is frequently used as a co-solvent with toxic DMF to regulate the crystallization process [9]. DMSO accelerates the α-to-δ phase transition in air by displacing MACl from the intermediate film through the formation of stronger bonds with PbI2.

In this work, we use recently emerged Triethyl Phosphate (TEP) as a green solvent to dissolve FAPbI3 precursors and in-situ study its crystallization kinetics in the presence of MACl and excess PbI2 under ambient condition using transmission wide angle x-ray scattering (T-WAXS) [10] and steady-state photoluminescence (PL) techniques. Our results show that, unlike DMSO containing solvent systems, TEP with appropriate coordination ability allows for direct solvent extraction during anti-solvent quenching process avoiding intermediate phase formation. Furthermore, it has been found that the addition of excess PbI2 to the perovskite solution, along with MACl, not only regulates the pre-nucleation stage, leading to larger and more ordered crystals, as opposed to MACl alone as an additive, but also accelerates the formation of α-phase FAPbI3 at room temperature and stabilizes it under ambient condition during spin casting. This study paves the way for achieving high efficiency FAPbI3 solar cells using a non-toxic solvent system and under ambient conditions.