Non-perovskite Phase Research Articles

Surface-Stabilized CsPbI3 Nanocrystals with Tailored Organic Polymer Ligand Binding.

Perovskite nanocrystals (NCs) exhibit attractive photophysical properties by combining the excellent optoelectronic properties of bulk perovskites with the strong quantum confinement effect at the nanoscale. However, CsPbI3 NCs easily transform into a non-perovskite phase because of the ionic lattice and dynamic ligand binding. Herein, stable black-phase CsPbI3 NCs capped with a new organic ligand, HO-PS-N3 (HOPS), which consists of a polystyrene segment with hydroxyl and azide end groups, are reported. This organic polymer ligand passivated the surface defects and enhanced the stability of CsPbI3 NCs by exposing the linking hydrophobic polystyrene segment. Consequently, the optimized CsPbI3 NCs exhibit significantly improved resistance to moisture or light and maintained 70 % of the original luminous intensity after immersion in water for two months. The theoretical results revealed that the binding energy of the HOPS ligand on the surface of the CsPbI3 NCs is higher than that of the commonly used oleic acid, alleviating the defects-induced degradation during purification. Thus, surface-stabilized CsPbI3 NCs are beneficial for a broad range of optoelectronic applications.

Influence of External Conditions on the Black-to-Yellow Phase Transition of CsPbI3 Based on First-Principles Calculations: Pressure and Moisture

All-inorganic CsPbI3 perovskite has emerged as a promising candidate for next-generation solar cells. However, owing to the poor structural stability of black phase CsPbI3 perovskite at room temperature, it spontaneously transforms to the yellow, photoinactive, nonperovskite phase at room temperature, limiting further development of CsPbI3 perovskite solar cells. To better understand the mechanism driving such undesirable phase transformation, we examine the thermodynamics and kinetics associated with γ-to-δ phase transformation using density functional theory calculation. A solid-state nudged elastic band method was employed to find minimum energy pathways assuming a concerted solid-state phase transformation between these two phases. The gas molecules representing atmospheric (H2O, O2, and N2) and inert (Ar) ambient environments, as well as applied external pressure, were considered to examine the influence of external conditions on the black-to-yellow phase transition. Our calculation reveals that, with increasing pressure, the thermodynamic driving force for converting the γ-phase to the δ-phase increases, whereas a larger activation energy must be overcome for the phase transition to occur. We also investigate the moisture-induced phase transformation with an H2O molecule and its dissociated species (H+/OH–) and demonstrate that the reaction energy barriers can be significantly lowered in the presence of H2O or OH–. On the other hand, other nonpolar species (O2, N2, and Ar) have a negligible effect on the phase transformation kinetics, while they might reduce the thermodynamic driving force for the phase transformation and suppress the undesirable phase transformation. Our theoretical prediction could support recent observations that the γ-to-δ phase transformation occurs rapidly and catalytically in the presence of moisture, whereas in dry argon and dry oxygen atmospheres, the γ-CsPbI3 remains stable.

The impact of the SnI2 purity on the formation of CsSnI3 perovskite modifications as monitored by 127I Nuclear Quadrupole Resonance

The relative content of the perovskite phase in two CsSnI3 samples (of 99.900 and 99.999 wt% SnI2 and 99.999 wt% CsI purity) was estimated using 127I NQR. The results showed that both samples contained the non-perovskite orthorhombic phase as an admixture. In the sample of higher purity (99.999 wt%) its content was insignificant – only trace amounts could be observed at 77 K, whereas at room temperature the presence of the non-perovskite phase was not revealed. However, in the sample of 99.900 wt% purity the content of the non-perovskite phase has reached at least a third of the sample amount, which is due to the probable crystallization of the non-perovskite phase on impurities.

Frontispiece: Water‐Triggered Chemical Transformation of Perovskite Nanocrystals

The interfacial chemistry at the aqueous-non-aqueous interface can be use to trigger chemical transformation of perovskite nanocrystals. In this review, evolution of several dimensional halide (X) perovskites from non-perovskite zero dimensional Cs4PbX6 phase has been revealed through water-assisted approach, which further may use to transform other non-perovskite phase (e.g., CsPb2X5), as well as doping in halide perovskites.

Intrinsic defects on α, γ and δ-CsPbI3 (001) surfaces and implications for the α/γ to δ phase transition.

Compared with traditional organic-inorganic hybrid perovskites, CsPbI3 is considered to be a better solar photovoltaic absorption material. However, under environmental conditions, it will undergo the phase transition from the α phase to the γ phase and finally to the non-perovskite phase (δ), especially in a humid environment. Considering the important role of surface intrinsic defects in the phase transition process, we investigated the intrinsic defects on α, γ and δ-CsPbI3 (001) surfaces by first-principles calculations based on density functional theory (DFT). The formation energy of most defects on the surface is similar to that in the bulk in all three phases except for VPb and VI. The formation energy of VPb and VI on the α-CsPbI3 (001) surface is significantly increased, and the formation energy of VPb on the γ-CsPbI3 (001) surface is also increased, due to the relaxation and distortion of the surface Cs and the Pb-I octahedron. The formation energy of interstitial defects on the α-CsPbI3 (001) surface is the lowest due to the remaining large dodecahedral void, even though the Pb-I octahedron distortion has largely enhanced the stability of the α-CsPbI3 (001) surface. The formation energy of VCs is the lowest in all three phases, indicating that Cs ions in CsPbI3 are indeed flexible in CsPbI3. The results are expected to provide a theoretical basis and guidance for the stability improvement of all-inorganic halide perovskites especially in a humid environment.

Lewis Acid-Base Adducts for Efficient and Stable Cesium-Based Lead Iodide-Rich Perovskite Solar Cells.

All-inorganic cesium-lead-iodide (CsPbI3 Br3- x (2 < x< 3)) perovskite presents preeminent photovoltaic performance and chemical stability. Unfortunately, this kind of material suffers from phase transition to a nonperovskite phase under oxidative chemical stresses. Herein, the introduction of a low concentration of Lewis acid-base adducts (LABAs) is reported to synergistically reduce defect density, optimize interfacial energy alignment, and improve device stability of CsPbI2.75 Br0.24 Cl0.01 (CsPbTh3 ) solar cells. Both theoretical simulations and experimental measurements reveal that the noncoordinating anions, PF6 - , as a Lewis base can more effectively bind with undercoordinated Pb2+ to passivate iodide vacancy defects than the BF4 - and absorbed I- , and thus the point defects are well suppressed. In addition, N-propyl-methyl piperidinium (NPMP+ ) is selected to assemble with PF6 - in CsPbTh3 film. The NPMP+ can regulate the crystal growth and finally homogenize the grain size and decrease the trap density. Consequently, the LABAs strategy can improve the power conversion efficiency of CsPbTh3 solar cells to 19.02% under 1-sun illumination (100mW cm-2 ). Fortunately, the NPMP+ and PF6 - -treated CsPbTh3 film shows great phase stability after storage in ambient air for 250 days, and the power conversion efficiency of corresponding solar cells is almost 76% of the initial value after 60 days aging under ambient conditions.

The effect of CO2-doped spiro-OMeTAD hole transport layer on FA(1−x)CsxPbI3 perovskite solar cells

Black-phase formamidinium lead iodine with 1.48 eV bandgap is considered to be the most promising material for improving the near-theoretical limit efficiency of perovskite solar cells, but at room temperature, black-phase formamidinium lead iodine easily transforms into the yellow non-perovskite phase formamidinium lead iodine. Here, different ratios of Cs+-incorporated formamidinium lead iodine prepared by one-step processing with the stability and power conversion efficiency of formamidinium lead iodine perovskite solar cells are investigated. FA0.85Cs0.15PbI3 shows the highest power conversion efficiency of 10.63% (Voc = 1.04 V, Jsc = 16.81 mA cm−2, and fill factor = 0.60), and the unencapsulated device maintained 60% of the initial power conversion efficiency after storage in air with 40% humidity for 186 h with an active area of 0.1 cm2, when the ratios of Cs+ reached 15% ( x = 0.15) in formamidinium lead iodine. However, the efficiency of perovskite solar cell–based formamidinium lead iodine is still low. In this work, a simple but an effective strategy was carried out to rapidly and fully oxidize hole transport layer solution by doping CO2 or O2 under ultraviolet light irradiation to increase the conductivity of hole transport layer, thereby improving the power conversion efficiency of solar cells. The results show that FA0.85Cs0.15PbI3 solar cells by CO2-doped hole transport layer for 90 s exhibited the highest power conversion efficiency of 16.11% (VOC = 1.11 V, JSC = 19.73 mA cm−2, and fill factor = 0.74). The improved photovoltaic performance is attributed to CO2-doped spiro-OMeTAD increasing charge carrier density and accelerating charge separation, thereby inducing higher conductivity. CO2 or O2 doped can rapidly and fully oxidize spiro-OMeTAD, and reduce the solar cell fabrication time; it is beneficial to the commercial use of perovskite solar cells.

α-FAPbI3 phase stabilization using aprotic trimethylsulfonium cation for efficient perovskite solar cells

Stabilization of the black-phase formamidinium lead triiodide (α-FAPbI3) perovskite by inhibiting its phase transition to the yellow non-perovskite phase (δ-FAPbI3) under ambient conditions is one of the major concerns for developing stable and highly efficient perovskite solar cells (PSCs). Moreover, the protic characteristic of the formamidinium (FA+) cation leads to the facile degradation of the corresponding α-FAPbI3 induced by deprotonation in moisture. This work demonstrates the phase stabilization of α-FAPbI3 with enhanced moisture stability by introducing a moisture-stable and aprotic organosulfonium cation, namely trimethylsulfonium (TMS+), to form a mixed-cation (TMS)x(FA)1-xPbI3 perovskite. The optimized doping of TMS+ into the FAPbI3 increased the crystallinity with compact and pinhole-free morphology and reduced the defect states and leakage current. Concurrently, the power conversion efficiency (PCE) of the optimized (TMS)0.02(FA)0.98PbI3-based PSCs reached 17.69%, compared to 15.41% for the FAPbI3-based PSC. Further, the mixed-cation (TMS)x(FA)1-xPbI3 PSCs showed improved device stability under continuous one-sun illumination and storage in ambient conditions. This study paves the way for developing new cations that can be applied to PSCs to minimize the phase transition of α-FAPbI3 with high moisture stability.

Accurately Determining the Phase Transition Temperature of CsPbI3 via Random-Phase Approximation Calculations and Phase-Transferable Machine Learning Potentials

While metal halide perovskites (MHPs) have shown great potential for various optoelectronic applications, their widespread adoption in commercial photovoltaic cells or photosensors is currently restricted, given that MHPs such as CsPbI3 and FAPbI3 spontaneously transition to an optically inactive nonperovskite phase at ambient conditions. Herein, we put forward an accurate first-principles procedure to obtain fundamental insight into this phase stability conundrum. To this end, we computationally predict the Helmholtz free energy, composed of the electronic ground state energy and thermal corrections, as this is the fundamental quantity describing the phase stability in polymorphic materials. By adopting the random phase approximation method as a wave function-based method that intrinsically accounts for many-body electron correlation effects as a benchmark for the ground state energy, we validate the performance of different exchange-correlation functionals and dispersion methods. The thermal corrections, accessed through the vibrational density of states, are accessed through molecular dynamics simulations, using a phase-transferable machine learning potential to accurately account for the MHPs’ anharmonicity and mitigate size effects. The here proposed procedure is critically validated on CsPbI3, which is a challenging material as its phase stability changes slowly with varying temperature. We demonstrate that our procedure is essential to reproduce the experimental transition temperature, as choosing an inadequate functional can easily miss the transition temperature by more than 100 K. These results demonstrate that the here validated methodology is ideally suited to understand how factors such as strain engineering, surface functionalization, or compositional engineering could help to phase-stabilize MHPs for targeted applications.

Kinetic pathway of γ-to-δ phase transition in CsPbI3

<h2>Summary</h2> All-inorganic photoactive CsPbI₃ perovskites easily transform into a photo-inactive non-perovskite phase, but the transition kinetics at the atomic level are currently unknown. In this study, we used first-principle-based stochastic surface walking (SSW) pathway sampling to resolve the phase evolution of the CsPbI₃ transition from γ to δ. The lowest-energy pathway of γ (3D) → <i>Pm</i> (3D) → <i>Cmcm</i> (2D) → <i>Pmcn</i> (1D) → δ (1D) was found to have a transition barrier as low as ∼31 meV/atom. The γ-to-<i>Pm</i> transition was identified as the performance-controlling step. Furthermore, volcano-shaped transition barriers were obtained depending on the ionic size of the substitution dopants, as explained by Pauling's rule and the cohesive energy. The [010] plane has the largest strain variation during phase transition, implying that the strain involving the [010] axis has a more significant effect on increasing the transition barriers. These results provide rational suggestions and guidance for achieving stable, long-term, all-inorganic halide CsPbI₃ perovskites.

Effect of silica encapsulation on the stability and photoluminescence emission of FAPbI3 nanocrystals for white-light-emitting perovskite diodes

The α-FAPbI 3 perovskite nanocrystals exhibit interesting optical properties, which make them attractive for versatile optoelectronic applications; however, the spontaneous phase transformation to the non-perovskite phase (δ-FAPbI 3 ) in humid conditions requires a new strategy to improve the phase stability of the material. We employed 3-Aminopropyl triethoxysilane (APTES)-assisted reprecipitation and sol-gel methods at ambient temperature to prepare stable α-FAPbI 3 nanocrystals embedded in a silica matrix. Transmission electron microscopy (TEM) and X-ray diffraction analysis determined that ultrafine perovskite nanocrystals (11.5 ± 3 nm) were uniformly distributed in a silica matrix. Investigations of the optical properties of the synthesized materials revealed that about 94% of the photoluminescence (PL) emission of silica-encapsulated FAPbI 3 nanocrystals was preserved under humid conditions (60%) after at least a month, while the PL emission of untreated FAPbI 3 was significantly decayed after few days. The thermal analysis also determined that the thermal stability of FAPbI 3 nanocrystals was improved by silica encapsulation. The red-emitting silica-encapsulated FAPbI 3 nanocrystals with a wavelength of 720 nm were used to fabricate white light-emitting diodes (LEDs) in a vertical structure consisting of a blue-chip, PMMA/CsPbBr 3 QDs, and PMMA/FAPbI 3 -SiO 2 QDs. Stable perovskite-based white LEDs with a color gamut of 144% NTSC were fabricated, which was better than many phosphor-based and perovskite-based LEDs. The synthesized composite perovskite nanocrystals could be used for next-generation optoelectronic devices. • Synthesis of silica-encapsulated FAPbI 3 quantum dots at ambient temperature by a combined LARP and sol-gel process. • Minor PL intensity loss in the humidified atmosphere after few months. • WLEDs with a high color gamut of 144% NTSC. • A pure red LED with coordination (0.729, 0.270).

Refined GFN1-xTB Parameters for Engineering Phase-Stable CsPbX3 Perovskites

Halide perovskites are a promising class of materials for optoelectronic applications due to their excellent optoelectronic performance. However, they suffer from several dynamical degradation problems, the characterization of which is challenging. Atomic scale simulations can provide valuable insights; however, the high computational cost of first-principles methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present refined GFN1-xTB parameters for the accurate description of structural and dynamical properties of CsPbBr3, CsPbI3, and CsPb(I1–xBrx)3. Our molecular dynamics simulations show that the phase stability is strongly correlated to the displacement of ions in the perovskites. In the low temperature orthorhombic phase, directional movement of the Cs cations increases their distance to the surrounding halides, initiating a transition to the nonperovskite phase. However, once enough thermal energy is available, the increased Cs–halide distance can be compensated by increased halide fluctuations, resulting in a transition to the phase-stable tetragonal or cubic phases. Furthermore, we find the mixing of halides increases halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.

Phase‐Pure Engineering for Efficient and Stable Formamidinium‐Based Perovskite Solar Cells

Formamidinium lead triiodide (FAPbI3) with a narrow bandgap, broad light absorption spectra, and high thermal stability has emerged as one of the promising active materials for perovskite solar cells. To date, the certified power conversion efficiency of FAPbI3‐based solar cells has reached 25.7%, comparable with that of monocrystalline silicon solar cells (26.7%). However, FAPbI3 tends to form an undesirable metastable nonperovskite phase (α‐FAPbI3), which is the most fatal issue for the commercialization development of FAPbI3‐based perovskite solar cells. Many efforts are committed to stabilizing the α‐FAPbI3 phase. In this review, the strategies involving composition engineering in A‐site (including double‐cation, triple‐cation, quadruple‐cation systems) and X‐site ions (halides and pseudohalides) to stabilize FA‐based perovskites are summarized. To realize higher efficiencies and avoid the increase in bandgap and phase segregation issue induced by the multicomponent elements, the corresponding strategies for preparing a pure α‐FAPbI3 perovskite with various functional materials are discussed. Moreover, the perovskite crystal redissolution strategy to prepare perovskite films with high purity, precise stoichiometric ratio, high crystallinity, ideal crystal orientation, and low defect density is described for highly efficient FAPbI3‐based perovskite solar cells. Finally, the perspective for future research directions toward highly reproducible and large‐area FAPbI3‐based photovoltaics is raised.

Controlling the Phase Transition in CsPbI3 Nanowires.

Cesium lead iodide (CsPbI3) is a promising semiconductor with a suitable band gap for optoelectronic devices. CsPbI3 has a metastable perovskite phase that undergoes a phase transition into an unfavorable nonperovskite phase in an ambient environment. This phase transition changes the optoelectronic properties of CsPbI3 and hinders its potential for device applications. Therefore, it is of central importance to understand the kinetics of such instability and develop strategies to control and stabilize the perovskite phase. Here, we use ultralong CsPbI3 nanowires as a model platform to investigate the phase transition kinetics. Our results depict the role of environmental stressors (moisture and temperature) in controlling the phase transition dynamics of CsPbI3, which can serve as guiding principles for future phase transition studies and the design of related photovoltaics. Furthermore, we demonstrate the controllability of phase propagation on individual nanowires by varying the moisture level and temperature.

Electrodeposition of lead dioxide induces the fabrication of perovskite FAPbI3 film and electron-transport-layer-free solar cells

Formamidinum based lead halide (α-FAPbI3) perovskite solar cells have refreshed the highest power conversion efficiency several times in the best Research-Cell Efficiency Chart due to its suitable bandgap. However, the non-perovskite yellow phase δ-FAPbI3 is easily formed in the process of synthesizing the black phase α-FAPbI3. To solve this problem, we provide a new method to grow pure perovskite α-FAPbI3 film through electrodeposited lead dioxide (PbO2). The as-prepared α-FAPbI3 shows an obvious absorption edge despite some pinholes formation during the reaction of PbO2 and FAI. By adding extra MABr during the synthesis of α-FAPbI3, the size and number of pinholes can be effectively suppressed. Thus, the optimal electron-transport-layer (ETL)-free perovskite FAPbI3 solar cells show a drastically elevated power conversion efficiency (PCE) of 10.73%, which is nearly 20 times higher than that of PbI2 based perovskite solar cells with the same device structure. These results demonstrate that the growth of PbO2 film by the electrodeposition method provides a new way for the preparation of high-efficiency perovskite solar cells. On this basis, combined with the introduction of the ETL (TiO2, SnO2, etc.) and the optimization of device fabrication processes, the PCE of the perovskite solar cells obtained by this method may be further improved in the future.

Crystal Structure and Concentration-Driven Phase Transitions in Lu(1-x)ScxFeO3 (0 ≤ x ≤ 1) Prepared by the Sol-Gel Method.

The structural state and crystal structure of Lu(1−x)ScxFeO3 (0 ≤ x ≤ 1) compounds prepared by a chemical route based on a modified sol–gel method were investigated using X-ray diffraction, Raman spectroscopy, as well as scanning electron microscopy. It was observed that chemical doping with Sc ions led to a structural phase transition from the orthorhombic structure to the hexagonal structure via a wide two-phase concentration region of 0.1 < x < 0.45. An increase in scandium content above 80 mole% led to the stabilization of the non-perovskite bixbyite phase specific for the compound ScFeO3. The concentration stability of the different structural phases, as well as grain morphology, were studied depending on the chemical composition and synthesis conditions. Based on the data obtained for the analyzed samples, a composition-dependent phase diagram was constructed.

Compositional degradation with Br content in Cesium lead halide CsPbBrxI3-x

The inorganic cesium lead halide CsPbX3 (where X ​= ​I, Br, Cl) are reported to possess good stability than that of organic-inorganic hybrid halides. CsPbI3 is reported to have four phases: perovskite black phase (cubic (α), tetragonal (β), orthorhombic(γ)) and non-perovskite yellow orthorhombic (δ). With band gap of ∼ 1.73eV, CsPbI3-α is most suitable phase among all-inorganic lead halide Perovskites. However, in ambient atmosphere (almost instantly) the perovskite black phase convert into non-perovskite yellow orthorhombic phase. At the same time CsPbBr3 remains in the orthorhombic phase at room temperature. Thus, in the present investigation, compositional variation of Br in Cesium lead iodide CsPbBrxI3-x (x ​= ​0.0 to 3.0 ​at the step of 0.5) samples synthesized by cold sintering technique is studied. The XRD patterns showed the orthorhombic phase for CsPbBr3. Further, CsPbBrxI3-x (x ​≤ ​1.0) have shown double phase corresponding to CsPbI3 (Pnma) ​+ ​PbI2 ( R3¯m). However, the compound with 1.5 ≤ x ​< ​3.0 showed the presence of various phases with and without oxygen including the CsPbBr3 (orthorhombic, Pnma) phase which has also been confirmed through UV–visible spectroscopy, impedance and Schottky anomaly. This compositional failure is also suggested by deviation from Vegards law showing volumetric expansion. The role of oxygen vacancy has been probed for the compositional failure for 1.5 ≤ x ​< ​3.0 using XPS characterization and Raman spectroscopy.

Phase evolution of all-inorganic perovskite nanowires during its growth from quantum dots

All-inorganic lead-halide perovskites have emerged as an exciting material owing to their excellent optoelectronic properties and high stability over hybrid organometallic perovskites. Nanowires of these materials, in particular, have shown great promise for optoelectronic applications due to their high optical absorption coefficient and low defect state density. However, the synthesis of the most promising alpha-Cesium lead iodide (α-CsPbI3) nanowires is challenging as it is metastable and spontaneously converts to a non-perovskite δ-phase. The hot-injection method is one of the most facile, well-controlled, and commonly used approaches for synthesizing CsPbX3 nanostructures. But the exact mechanism of growing these nanowires in this technique is not clear. Here, we show that the hot-injection method produces photoactive phases of quantum dots (QDs) and nanowires of CsPbBr3, and QDs of CsPbI3, but CsPbI3 nanowires are grown in their non-perovskite δ-phase. Monitoring the nanowire growth during the hot-injection technique and through detailed characterization, we establish that CsPbI3 nanowires are formed in the non-perovskite phase from the beginning rather than transforming after its growth from perovskite to a non-perovskite phase. We have discussed a possible mechanism of how non-perovskite nanowires of CsPbI3 grow at the expense of photoactive perovskite QDs. Our findings will help to synthesize nanostructures of all-inorganic perovskites with desired phases, which is essential for successful technological applications.

Low-temperature processing of polyvinylpyrrolidone modified CsPbI2Br perovskite films for high-performance solar cells

Owing to the inevitability of moisture/thermal instability in organic-inorganic hybrid perovskites, pure inorganic perovskites such as CsPbI2Br perovskite have surfaced as promising options for commercial perovskite solar cells (PSCs) due to their high photovoltaic performance and excellent inherent stability. However, designing additive engineering approaches to mitigate defect-induced crystalline phase transitions from a photosensitive perovskite phase to a non-perovskite phase has been a difficult task for researchers. In this work, using a one-step spin coating approach, we have prepared a polyvinylpyrrolidone (PVP) polymer-incorporated stable phase of CsPbI2Br at a low temperature (120 ​°C). Examinations using structural, morphological, and photo-physical measurements revealed that the optimum amount of PVP can greatly improve the optoelectronic properties of the film, which facilitates in reducing the trap states and defect in perovskite film, thus enabling charge carrier separation and suppressing charge recombination. As a result, the device based on 5 ​mg-PVP achieves a champion power conversion efficiency (PCE) of 10.47%, with a fill factor (FF) of 56.35%, a short-circuit current (JSC) of 18.47 ​mA ​cm2, and an open-circuit voltage (VOC) of 1.01 ​V, which is significantly higher than the device without PVP (6.36%). These findings suggest that PVP-CsPbI2Br has tremendous promise for future research and application in photovoltaic devices.

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase