Inverse Temperature Crystallization of Formamidinium Tin Iodide: Indirect Transition State and Restriction of Cation Motion

Inverse temperature crystallization (ITC) growth of formamidinium tin iodide (FASnI(3)) is realized using gamma-valerolactone (GVL) solvent, which possesses a low Gutmann acceptor number. The high-crystalline ITC-grown FASnI(3) film was fabricated via a dip-coating method featuring a high-temperature substrate and a precursor solution using the GVL solvent. Transmittance measurements and photoluminescence spectroscopy revealed the presence of a narrow band gap in the FASnI(3) ITC-grown film, contrasting with the band gap of a conventional single crystal and spin-coated film. Solid-state proton nuclear magnetic resonance spectroscopy demonstrated that formamidinium cations in the FASnI(3) ITC-grown film were strongly bound with the SnI6 octahedron and that the motion of the formamidinium molecules was restricted, resulting in the stabilization of a metastable bonding state in the FASnI(3) ITC-grown film. On the basis of these results and previous reports, the narrow band gap observed in the FASnI(3) ITC-grown film was attributed to an indirect transition state induced by a Rashba splitting effect.