Tailoring Interface Energies via Phosphonic Acids to Grow and Stabilize Cubic FAPbI₃ Deposited by Thermal Evaporation

Coevaporation of formamidinium lead iodide (FAPbI₃) is a promising route for the fabrication of highly efficient and scalable optoelectronic devices, such as perovskite solar cells. However, it poses experimental challenges in achieving stoichiometric FAPbI₃ films with a cubic structure (α-FAPbI₃). In this work, we show that undesired hexagonal phases of both PbI₂ and FAPbI₃ form during thermal evaporation, including the well-known 2H-FAPbI₃, which are detrimental for optoelectronic performance. We demonstrate the growth of α-FAPbI₃ at room temperature via thermal evaporation by depositing phosphonic acids (PAc) on substrates and subsequently coevaporating PbI₂ and formamidinium iodide. We use density-functional theory to develop a theoretical model to understand the relative growth energetics of the α and 2H phases of FAPbI₃ for different molecular interactions. Experiments and theory show that the presence of PAc molecules stabilizes the formation of α-FAPbI₃ in thin films when excess molecules are available to migrate during growth. This migration of molecules facilitates the continued presence of adsorbed organic precursors at the free surface throughout the evaporation, which lowers the growth energy of the α-FAPbI₃ phase. Our theoretical analyses of PAc molecule-molecule interactions show that ligands can form hydrogen bonding to reduce the migration rate of the molecules through the deposited film, limiting the effects on the crystal structure stabilization. Our results also show that the phase stabilization with molecules that migrate is long-lasting and resistant to moist air. These findings enable reliable formation and processing of α-FAPbI₃ films via vapor deposition.