Hidden polymorphism of FAPbI₃ discovered by Raman spectroscopy

Area of Science:

• Materials Science
• Solid State Chemistry
• Photovoltaics

Background:

• Formamidinium lead iodide (FAPbI3) is a promising light absorber for single-junction solar cells due to its optimal band-gap and high absorption coefficient.
• The photo-active black α-phase of FAPbI3 is unstable at room temperature, transforming into a non-photo-active yellow δ-phase, hindering device performance.
• Understanding the phase behavior and polymorphism of FAPbI3 is critical for developing stable and efficient perovskite solar cells.

Purpose of the Study:

• To investigate the room-temperature polymorphism of formamidinium lead iodide (FAPbI3).
• To characterize the structural and thermal properties of different FAPbI3 phases.
• To elucidate the phase transitions of FAPbI3 under varying temperatures.

Main Methods:

• Temperature-dependent Raman spectroscopy was employed to analyze structural changes.
• X-ray diffraction (XRD) patterns were used to assess crystal structure.
• Photoluminescence (PL) signals were measured to evaluate photo-activity.

Main Results:

• Three distinct room-temperature polymorphs of FAPbI3 were identified: as-synthesized (αi), degraded (αδ), and thermally recovered (αrec).
• These polymorphs exhibit differences in Raman spectra and PL signals but share identical XRD patterns.
• A structural transition from a distorted Im3[combining macron] symmetry to a cubic Pm3[combining macron]m symmetry (αHT) was observed at approximately 110 °C for both αi and αrec phases.

Conclusions:

• The study reveals a previously unidentified polymorphism of FAPbI3 at room temperature, impacting its stability and performance.
• A key structural transition temperature of ~110 °C was identified, differentiating low-temperature distorted phases from the high-temperature cubic phase.
• Further understanding of these hybrid perovskite structures is essential for designing novel and stable materials for solar cell applications.