Additive-free, Low-temperature Crystallization of Stable α-FAPbI3 Perovskite.

Formamidinium lead triiodide (FAPbI3 ) 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 α-FAPbI3 conventionally requires high-temperature thermal annealing at 150°C whilst the obtained α-FAPbI3 is metastable at room temperature Here we report aerosol assisted crystallization (AAC) to convert yellow δ-FAPbI3 into black α-FAPbI3 at only 100°C using precursor solutions containing only lead iodide (PbI2 ) and formamidinium iodide (FAI) with no chemical additives. The obtained α-FAPbI3 exhibits remarkably enhanced stability compared to the 150°C annealed counterparts, in combination with improvement in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering and density functional theory simulation, we identify 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 α-FAPbI3 . This overcomes strain-induced lattice expansion that is known to cause the metastability of α-FAPbI3 . Accordingly, we report pure FAPbI3 p-i-n solar cells for the first time, facilitated by the low-temperature (≤ 100°C) AAC processing, which demonstrate increases of both power conversion efficiency and operational stability compared to devices fabricated using 150°C annealed films. This article is protected by copyright. All rights reserved.