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

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 α phase, black and with high photovoltaic efficiency, is metastable against the yellow hexagonal δ phase, it is believed that α−FAPI spontaneously transform into δ−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 α−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 δ 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. Graphical TOC Entry1 arXiv:1905.02992v1 [cond-mat.mtrl-sci] 8 May 2019Although MAPbI 3 (MAPI, MA = methylammonium CH 3 NH 3 ) is the most studied hybrid metal-organic perovskite for photovoltaic applications, 1 better performance in terms of photovoltaic efficiency are found in FAPbI 3 (FAPI, FA = formamidinium CH(NH 2 ) 2 ). This is due both to a smaller bandgap of FAPI and to the fact that the FA + ion, in spite of a smaller electric dipole with respect to MA + , has a much larger quadrupole and faster reorientation dynamics that better screen the photoexcited carriers, enhancing their lifetime. 2 FAPI also has a better stability than MAPI at high temperature but its major flaw is that the black cubic α phase, which has the high photovoltaic efficiency, is metastable at room temperature, where instead the stable phase, and the one obtained by standard chemical methods, is the yellow hexagonal δ phase. For these reasons, major efforts are directed now at trying to stabilize the cubic α phase of FAPI through partial substitutions of FA with MA, Cs, etc. or I with Br, although this approach increases the bandgap. 3,4 It has been discussed, based on neutron diffraction measurements and simulations, that the α → δ transformation is complex and occurs through various intermediate stages, requiring to overcome a free energy barrier estimated in the order of hundreds of milli-electron volt. 5 This explains why the α phase of FAPI is kinetically trapped, resulting in a large thermal hysteresis between the δ → α transition at T h δα = 350 K and the α → δ at T c δα = 290 K. 5 Actually, the barrier for the α → δ transition has not been measured, and there is complete uncertainty on the kinetics of this transition. Indeed, also the reported temperatures for the δ → α transition during heati...

Raman spectroscopy insights into the α- and δ-phases of formamidinium lead iodide (FAPbI3)

“…The yellow (δ) phase is nominally the most stable phase at room temperature, although the black (α) phase can be prepared during the synthesis and effectively quenched to room temperature. The δ phase can also be converted to the α phase on heating with the transition having been previously reported to be at 125°C, 22,29 however, this transition has been found to take place as low as 77°C 30 and up to 185°C, 31 which has been attributed to kinetics of the technique in use and the selected synthesis ramp rate, 32 respectively. A wide variation also exists for the duration of the black phase stability, once formed, ranging from a matter of hours 23,33,34 up to days, 24,[35][36][37][38] with the synthesis approach and the added nanoengineering templating 37,38 appearing to have an effect on stabilization.…”

Structural, electronic, and optical properties of cubic formamidinium lead iodide perovskite: a first-principles investigation

“…Also FAPbI 3 gives better stability than MAPbI 3 at high temperatures. [8][9][10] However, its major aw is that the black cubic a phase, which is responsible for the high photovoltaic efficiency, is metastable toward heat and moisture at normal operating conditions. [11][12][13] The stable phase, which is produced when FAPbI 3 is obtained by standard chemical methods, is the yellow hexagonal d phase.…”

“…Recently, however, Cordero et al found that a-FAPbI 3 can be perfectly stable for at least 100 days unless extrinsic factors induce its degradation. 8 There have been extensive theoretical works studying the structure and electronic, optical, and defect properties of MAPbI 3 , and these have greatly deepened the understanding of MAPbI 3 and accelerated research into its application to devices. [14][15][16][17][18][19] Although there have been some of theoretical works studying FAPbI 3 , 20-24 a systematic and comprehensive study is still absent.…”

Dual Additive for Simultaneous Improvement of Photovoltaic Performance and Stability of Perovskite Solar Cell

“…The intense peaks located at ≈14 o and ≈28 o are corresponding to (001) and (002) planes, respectively. [ 44,45 ] The XRD patterns shown in Figure 4e are measured with the optimal concentration of each additive as shown in Figure S2, Supporting Information, where the single MACl and CsCl additive is found to show best performance at 20 and 10 mol%, respectively, while 15 mol% shows best PCE for the [MACl]/[CsCl] = 2 additive among the investigated concentrations. It is found that the (001) peaks shift to higher angles as compared to the additive‐free perovskite sample, indicative of contraction of c‐axis due to substitution of FA with smaller ions.…”