Influence of Temperature, Pressure, and Humidity on the Stabilities and Transition Kinetics of the Various Polymorphs of FAPbI₃

Abstract

The phase transitions between the various polymorphs of FAPbI 3 (FAPI, FA = formamidinium [CH(NH 2 ) 2 ] + ) are studied by anelastic, dielectric, and X-ray diffraction measurements on samples pressed from δ-FAPI (2H phase) yellow powder. The samples become orange after application of as little as 0.2 GPa, which has been explained in terms of partial transformations to the other hexagonal polymorphs 4H and 6H. The phenomenon is discussed in the light of what is known about the stability of the various polymorphs of hybrid and inorganic perovskites ABX 3 with large A cations and hence a large tolerance factor t.Remarkably, FAPI at room and higher temperatures behaves like a perovskite with a large t, while just below room temperature it behaves like a perovskite with a small t. The kinetics of the transformations between the polymorphs is enhanced by small amounts of intercalated water. It seems therefore worthy to try improving the atomic diffusion and crystallization during synthesis, and hence the final photovoltaic performance, through controlled small amounts of water that should be thoroughly removed after a sufficiently homogeneous and smooth microstructure is achieved.

Structural Transitions and Stability of FAPbI3 and MAPbI3: The Role of Interstitial Water

“…The mid-point of the step in the Young’s modulus, due to the transition, is at 410 K in this dehydrated condition. That the sample transformed into the phase is confirmed by the subsequent cooling, during which the elastic anomalies at the and transitions [ 19 , 42 ] appeared. FAPI #7 was another freshly pressed -FAPI bar measured during heating again at 1.5 K/min in the same high vacuum equipment filled with 7 mbar water.…”

“…Figure 1 , adapted from Figure 4 of Reference [ 19 ], presents the relative change of the Young’s modulus and the elastic energy loss versus temperature of two bars of FAPI. Sample FAPI #5, pressed from -FAPI, had been measured down to 100 K and kept two days in high vacuum, and therefore was dehydrated with respect to the initial state.…”

“…Exposure to humidity is a major cause of fast degradation, a fact attributed to the aqueous solubility of the MAI and FAI salts [ 16 , 17 ]. On the other hand, it has been found that -FAPI films are perfectly stable for at least 90 days if stored in low humidity [ 18 ] and the loose powder is stable for months even in air, since water uptake occurs mainly at the grain boundaries, which are absent in the powder [ 19 ]. Other strategies for improving the stability are therefore based on additives that should passivate the grain boundaries or various types of encapsulation [ 13 ], but the improvements in stability are generally far from the requirements for commercial applications.…”

“…Much attention has been devoted to the degradation induced by humidity [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ], but much less to the catalytic effects of intercalated water molecules on crystallization and polymorphism [ 19 ]. Actually, beneficial effects of moisture on the crystal morphology and perfection, resulting in improved photovoltaic efficiency and stability, have been often reported [ 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 ], but remain generally unexplained.…”

“…Here, we revisit our studies on FAPI and MAPI, based on anelastic, dielectric and X-ray diffraction measurements [ 19 , 40 , 41 , 42 ], in order to provide insight in the very important issue of the stability of the 3D perovskite phases of MAPI and FAPI and the multiple roles of water in catalyzing decomposition but also the conversions between polymorphs and crystallization. We also analyze unpublished results that we found at first puzzling, but interpret now as evidence of interstitial water in the perovskite and phases.…”