Stabilizing the pure phase of FAPbI3 perovskites enabled via a solid–liquid low entropy ink strategy

Abstract

Phase-pure α-formamidinium lead iodide (α-FAPbI₃) perovskites that exclude additional cations or anions (for example, methylammonium (MA⁺), Cs⁺, or Br⁻) hold great promise for high-efficiency and thermally stable perovskite solar cells. However, strongly coordinated solvents generate a disordered coordination network, increasing entropy in the ink while creating inefficient phase transition pathways. Here, we report a solid–liquid low entropy ink (LEI) composed of 2-methoxyethanol (2-Me) and formamidine acetate (FAAc), enabling the fabrication of phase-pure α-FAPbI₃ thin films in ambient air. We found that FAAc strongly coordinates with Pb²⁺, while hydrogen bonding between 2-Me and FAI enhances FA⁺ dispersion, thereby suppressing undesired [PbI₃]⁻ complex formation and promoting rapid phase-pure crystallization with minimal residual solvent. Moreover, this strategy mitigates solvation-induced phase impurities and degradation at buried interfaces. As a result, perovskite solar cells achieved a power conversion efficiency of 25.23% (0.049 cm²) and 23.05% for 5 × 5 cm² modules, representing the highest value reported for phase-pure α-FAPbI₃. Furthermore, perovskite solar modules retain 90% of their initial efficiency over 1000 h of continuous illumination under maximum power point tracking (ISOS-L2).