Additive-Free, Low-Temperature Crystallization of Stable α-FAPbI3 Perovskite

Formamidinium lead triiodide (FAPbI(3)) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure alpha-FAPbI(3) conventionally requires high-temperature thermal annealing at 150 degrees C whilst the obtained alpha-FAPbI(3) is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow delta-FAPbI(3) into black alpha-FAPbI(3) at only 100 degrees C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained alpha-FAPbI(3) exhibits remarkably enhanced stability compared to the 150 degrees C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable alpha-FAPbI(3). This overcomes the strain-induced lattice expansion that is known to cause the metastability of alpha-FAPbI(3). Accordingly, pure FAPbI(3) p-i-n solar cells are reported, facilitated by the low-temperature (<= 100 degrees C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 degrees C annealed films.