# Stability of Cubic FAPbI$_3$ from X-ray Diffraction, Anelastic, and Dielectric Measurements

## TL;DR

This study demonstrates that the cubic alpha phase of FAPbI$_3$ remains stable for at least 100 days under normal conditions, with degradation only occurring due to humidity and grain boundary effects, informing stability improvement strategies.

## Contribution

First comprehensive dielectric and anelastic spectra of FAPI, clarifying its stability and degradation factors, challenging previous reports of rapid spontaneous transformation.

## Key findings

- Alpha-FAPI is stable for at least 100 days in air.
- Degradation occurs due to humidity and grain boundaries.
- Loose powder remains stable under normal conditions.

## Abstract

Among the hybrid metal-organic perovskites for photovoltaic applications FAPbI_3 (FAPI) has the best performance regarding efficiency and the worst regarding stability, even though the reports on its stability are highly contradictory. In particular, since at room temperature the cubic alpha phase, black and with high photovoltaic efficiency, is metastable against the yellow hexagonal delta phase, it is believed that alpha-FAPI spontaneously transform into delta-FAPI within a relatively short time. We performed X-ray diffraction and thermogravimetric measurements on loose powder of FAPI, and present the first complete dielectric and anelastic spectra of compacted FAPI samples under various conditions. We found that alpha-FAPI is perfectly stable for at least 100 days, the duration of the experiments, unless extrinsic factors induce its degradation. In our tests, degradation was detected after exposure to humidity, strongly accelerated by grain boundaries and the presence of delta phase, but it was not noticeable on the loose powder kept in air under normal laboratory illumination. These findings have strong implications on the strategies for improving the stability of FAPI without diminishing its photovoltaic efficiency through modifications of its composition.