Stability of Cubic FAPbI₃ from X-ray Diffraction, Anelastic, and Dielectric Measurements

Area of Science:

• Materials Science
• Photovoltaics
• Solid-State Chemistry

Background:

• Hybrid metal-organic perovskites like FAPbI3 (FAPI) offer high photovoltaic efficiency but suffer from contradictory reports on stability.
• The cubic α-phase of FAPI, crucial for high efficiency, is considered metastable against the δ-phase at room temperature.

Purpose of the Study:

• To investigate the intrinsic stability of the α-phase of FAPbI3 (FAPI) under various conditions.
• To identify extrinsic factors influencing FAPI stability and degradation pathways.

Main Methods:

• X-ray diffraction and thermogravimetric analysis of loose FAPI powder.
• Comprehensive dielectric and anelastic spectroscopy of compacted FAPI samples.
• Controlled exposure to environmental factors like humidity.

Main Results:

• The cubic α-FAPI phase demonstrated intrinsic stability for at least 100 days in air under laboratory illumination.
• Degradation was observed upon exposure to humidity, significantly accelerated by grain boundaries and the presence of the δ-phase.
• Loose FAPI powder showed no noticeable degradation under normal laboratory conditions.

Conclusions:

• The α-phase of FAPI is intrinsically stable and does not spontaneously transform into the δ-phase under typical conditions.
• Extrinsic factors, particularly humidity and structural defects, are the primary drivers of FAPI degradation.
• Strategies to enhance FAPI stability for photovoltaic applications should focus on mitigating extrinsic degradation pathways without compromising efficiency.