  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.
