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Among the hybrid metal-organic perovskites for photovoltaic applicationsFAPbI_3 (FAPI) has the best performance regarding efficiency and the worstregarding stability, even though the reports on its stability are highlycontradictory. In particular, since at room temperature the cubic alpha phase,black and with high photovoltaic efficiency, is metastable against the yellowhexagonal delta phase, it is believed that alpha-FAPI spontaneously transforminto delta-FAPI within a relatively short time. We performed X-ray diffractionand thermogravimetric measurements on loose powder of FAPI, and present thefirst complete dielectric and anelastic spectra of compacted FAPI samples undervarious conditions. We found that alpha-FAPI is perfectly stable for at least100 days, the duration of the experiments, unless extrinsic factors induce itsdegradation. In our tests, degradation was detected after exposure to humidity,strongly accelerated by grain boundaries and the presence of delta phase, butit was not noticeable on the loose powder kept in air under normal laboratoryillumination. These findings have strong implications on the strategies forimproving the stability of FAPI without diminishing its photovoltaic efficiencythrough modifications of its composition.

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