Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites

Area of Science:

• Materials Science
• Solid-State Physics
• Crystallography

Background:

• Formamidinium lead iodide (FAPbI3) perovskites are crucial for solar cell technology.
• Understanding their phase stability and vibrational dynamics is key to improving device performance.
• The local structure and cation dynamics influence the overall material properties.

Purpose of the Study:

• To investigate the vibrational dynamics and local structure of pure and methylammonium-doped FAPbI3.
• To elucidate the origin of phase metastability in the tetragonal structure of FAPbI3.
• To explore how doping with organic cations stabilizes the perovskite framework.

Main Methods:

• High-resolution neutron spectroscopy (inelastic neutron scattering).
• First-principles electronic-structure calculations.
• Analysis of cation vibrations and local structure.

Main Results:

• Direct evidence for the formation of a low-temperature orientational glass in FAPbI3.
• Accurate analysis of cation vibrations and local structure around organic moieties.
• Doping strengthens hydrogen-bonding interactions of formamidinium cations due to cage deformation.
• Weakening of methylammonium interactions with the perovskite framework.

Conclusions:

• The formation of an orientational glass explains the phase metastability in tetragonal FAPbI3.
• Doping-induced strengthening of formamidinium hydrogen bonds stabilizes the perovskite structure.
• Synergistic effects between cation interactions and framework deformation are crucial for perovskite stabilization.