Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide

Abstract

Formamidinium lead iodide (FAPbI3) has the potential to achieve higher performance than established perovskite solar cells like methylammonium lead iodide (MAPbI3), while maintaining a higher stability. The major drawback for the latter material is that it can crystallize at room temperature in a wide bandgap hexagonal symmetry (P63mc) instead of the desired trigonal (P3m1) black phase formed at a higher temperature (130 °C). Our results show that employing a mixture of MAI and FAI in films deposited via a two-step approach, where the MAI content is <20%, results in the exchange of FA molecules with MA without any significant lattice shrinkage. Additionally, we show with temperature-dependent X-ray diffraction that the trigonal phase exhibits no phase changes in the temperature range studied (25 to 250 °C). We attribute the stabilization of the structure to stronger interactions between the MA cation and the inorganic cage. Finally, we show that the inclusion of this small amount of MA also has a positive effect on the lifetime of the photoexcited species and results in more efficient devices.

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…[247] The resulting material exhibits the same lattice parameters, photoluminescence emission peak wavelength, and bandgap as those found for neat FAPbI 3 , meaning that no significant lattice contraction occurred after the inclusion of the smaller MA cation. [247] Combining caesium, FA, and a mixture of halides, additionally allowed for bandgap tuning, which is necessary for inclusion in tandem architectures with a low-bandgap bottom cell, [248] while also enhancing the stability of the system. Finally, the best performing solar cells currently incorporate a 3 or 4 cation mixture, [4] although their stability still needs to be studied in detail.…”

“…[247,249,252] Moreover, this desired phase is stable between room temperature and 220 °C (Figure 15). [247] The resulting material exhibits the same lattice parameters, photoluminescence emission peak wavelength, and bandgap as those found for neat FAPbI 3 , meaning that no significant lattice contraction occurred after the inclusion of the smaller MA cation. [247] Combining caesium, FA, and a mixture of halides, additionally allowed for bandgap tuning, which is necessary for inclusion in tandem architectures with a low-bandgap bottom cell, [248] while also enhancing the stability of the system.…”

“…[247][248][249][250][251][252] For instance, the exchange of approximately 15% of the formamidinium cation with methylammonium leads to the crystallization of a material with the same crystal structure as FAPbI 3 . [247,249,252] Moreover, this desired phase is stable between room temperature and 220 °C (Figure 15). [247] The resulting material exhibits the same lattice parameters, photoluminescence emission peak wavelength, and bandgap as those found for neat FAPbI 3 , meaning that no significant lattice contraction occurred after the inclusion of the smaller MA cation.…”

Lead‐Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?

“…221) space group structure at room temperature and has only one single stable phase up to 200 °C (Figure 3a). This is different from its lead analogue‐FAPbI 3 , which has two competing δ‐phase and α‐phase structure 132, 136, 137. Another unique point is that due to the symmetry reduction of the 3D [SnI 3 ] − framework with larger cations group like FA,132 FASnI 3 has a larger band gap (1.41 eV) value than that of MASnI 3 59.…”

First-Principles Study on Electronic Structures of FAPbX3 (X = Cl, Br, I) Hybrid Perovskites

“…Improving the perovskite stability thus is the key issue for solar cells, and formamidinium (FA, CH(NH 2 ) 2 + ) cations were recently suggested, by plane-wave first-principles calculations, to replace MA inside the inorganic metrix [12,13], due to that the former can interact with the inorganic cage stronger than the latter does, to reduce the release of volatile species [13] or alter the covalent/ionic character of Pb-I bonds [12]. On the other hand, mixing FA with MA is experimentally shown to be a route to stabilize the perovskite [8,14] and improve the power conversion efficiency [14]. However, first-principles calculations with the linear combination of atomic orbital (LCAO) basis-set, which are economical and feasible for charge transport studies, on this issue are still limited to date.…”