Contrasting Ultra-Low Frequency Raman and Infrared Modes in Emerging Metal Halides for Photovoltaics

Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination, and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilize ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbI x Br3-x , CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We rule out extrinsic defects, octahedral tilting, cation lone pairs, and "liquid-like" Boson peaks as causes of the debated central Raman peak. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose-Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.

Surface-binding molecular multipods strengthen the halide perovskite lattice and boost luminescence

Reducing the size of perovskite crystals to confine excitons and passivating surface defects has fueled a significant advance in the luminescence efficiency of perovskite light-emitting diodes (LEDs). However, the persistent gap between the optical limit of electroluminescence efficiency and the photoluminescence efficiency of colloidal perovskite nanocrystals (PeNCs) suggests that defect passivation alone is not sufficient to achieve highly efficient colloidal PeNC-LEDs. Here, we present a materials approach to controlling the dynamic nature of the perovskite surface. Our experimental and theoretical studies reveal that conjugated molecular multipods (CMMs) adsorb onto the perovskite surface by multipodal hydrogen bonding and van der Waals interactions, strengthening the near-surface perovskite lattice and reducing ionic fluctuations which are related to nonradiative recombination. The CMM treatment strengthens the perovskite lattice and suppresses its dynamic disorder, resulting in a near-unity photoluminescence quantum yield of PeNC films and a high external quantum efficiency (26.1%) of PeNC-LED with pure green emission that matches the Rec.2020 color standard for next-generation vivid displays.

The Effect of Cesium Incorporation on the Vibrational and Elastic Properties of Methylammonium Lead Chloride Perovskite Single Crystals

Hybrid organic-inorganic lead halide perovskites (LHPs) have emerged as a highly significant class of materials due to their tunable and adaptable properties, which make them suitable for a wide range of applications. One of the strategies for tuning and optimizing LHP-based devices is the substitution of cations and/or anions in LHPs. The impact of Cs substitution at the A site on the structural, vibrational, and elastic properties of MAxCs1-xPbCl3-mixed single crystals was investigated using X-ray diffraction (XRD) and Raman and Brillouin light scattering techniques. The XRD results confirmed the successful synthesis of impurity-free single crystals, which exhibited a phase coexistence of dominant cubic and minor orthorhombic symmetries. Raman spectroscopy was used to analyze the vibrational modes associated with the PbCl6 octahedra and the A-site cation movements, thereby revealing the influence of cesium incorporation on the lattice dynamics. Brillouin spectroscopy was employed to investigate the changes in elastic properties resulting from the Cs substitution. The incorporation of Cs cations induced lattice distortions within the inorganic framework, disrupting the hydrogen bonding between the MA cations and PbCl6 octahedra, which in turn affected the elastic constants and the sound velocities. The substitution of the MA cations with smaller Cs cations resulted in a stiffer lattice structure, with the two elastic constants increasing up to a Cs content of 30%. The current findings facilitate a fundamental understanding of mixed lead chloride perovskite materials, providing valuable insights into their structural and vibrational properties.

Structural Evolution and Photoluminescence Quenching across the FASnI3-xBrx (x = 0-3) Perovskites

One of the primary methods for band gap tuning in metal halide perovskites has been halide (I/Br) mixing. Despite widespread usage of this type of chemical substitution in perovskite photovoltaics, there is still little understanding of the structural impacts of halide alloying, with the assumption being the formation of ideal solid solutions. The FASnI3-xBrx (x = 0-3) family of compounds provides the first example where the assumption breaks down, as the composition space is broken into two unique regimes (x = 0-2.9; x = 2.9-3) based on their average structure with the former having a 3D and the latter having an extended 3D (pseudo 0D) structure. Pair distribution function (PDF) analyses further suggest a dynamic 5s2 lone pair expression resulting in increasing levels of off-centering of the central Sn as the Br concentration is increased. These antiferroelectric distortions indicate that even the x = 0-2.9 phase space behaves as a nonideal solid-solution on a more local scale. Solid-state NMR confirms the difference in local structure yielding greater insight into the chemical nature and local distributions of the FA+ cation. In contrast to the FAPbI3-xBrx series, a drastic photoluminescence (PL) quenching is observed with x ≥ 1.9 compounds having no observable PL. Our detailed studies attribute this quenching to structural transitions induced by the distortions of the [SnBr6] octahedra in response to stereochemically expressed lone pairs of electrons. This is confirmed through density functional theory, having a direct impact on the electronic structure.

Phonon Properties and Lattice Dynamics of Two- and Tri-Layered Lead Iodide Perovskites Comprising Butylammonium and Methylammonium Cations-Temperature-Dependent Raman Studies

Hybrid lead iodide perovskites are promising photovoltaic and light-emitting materials. Extant literature data on the key optoelectronic and luminescent properties of hybrid perovskites indicate that these properties are affected by electron-phonon coupling, the dynamics of the organic cations, and the degree of lattice distortion. We report temperature-dependent Raman studies of BA2MAPb2I7 and BA2MA2Pb3I10 (BA = butylammonium; MA = methylammonium), which undergo two structural phase transitions. Raman data obtained in broad temperature (360-80 K) and wavenumber (1800-10 cm-1) ranges show that ordering of BA+ cations triggers the higher temperature phase transition, whereas freezing of MA+ dynamics occurs below 200 K, leading to the onset of the low-temperature phase transition. This ordering is associated with significant deformation of the inorganic sublattice, as evidenced by changes observed in the lattice mode region. Our results show, therefore, that Raman spectroscopy is a very valuable tool for monitoring the separate dynamics of different organic cations in perovskites, comprising "perovskitizer" and interlayer cations.

Exploring the Influence of Cation and Halide Substitution in the Structure and Optical Properties of CH3NH3NiCl3 Perovskite

This manuscript details a comprehensive investigation into the synthesis, structural characterization, thermal stability, and optical properties of nickel-containing hybrid perovskites, namely CH3NH3NiCl3, CsNiCl3, and CH3NH3NiBrCl2. The focal point of this study is to unravel the intricate crystal structures, thermal behaviors, and optical characteristics of these materials, thereby elucidating their potential application in energy conversion and storage technologies. X-ray powder diffraction measurements confirm that CH3NH3NiCl3 adopts a crystal structure within the Cmcm space group, while CsNiCl3 is organized in the P63/mmc space group, as reported previously. Such structural diversity underscores the complex nature of these perovskites and their potential for tailored applications. Thermal analysis further reveals the stability of CH3NH3NiCl3 and CH3NH3NiBrCl2, which begin to decompose at 260 °C and 295 °C, respectively. The optical absorption properties of these perovskites studied by UV-VIS-NIR spectroscopy revealed the bands characteristic of Ni2+ ions in an octahedral environment. Notably, these absorption bands exhibit subtle shifts upon bromide substitution, suggesting that optical properties can be finely tuned through halide modification. Such tunability is paramount for the design and development of materials with specific optical requirements. By offering a detailed examination of these properties, the study lays the groundwork for future advancements in material science, particularly in the development of innovative materials for sustainable energy technologies.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Magnetic-biased chiral molecules enabling highly oriented photovoltaic perovskites

The interaction between sites A, B and X with passivation molecules is restricted when the conventional passivation strategy is applied in perovskite (ABX3) photovoltaics. Fortunately, the revolving A-site presents an opportunity to strengthen this interaction by utilizing an external field. Herein, we propose a novel approach to achieving an ordered magnetic dipole moment, which is regulated by a magnetic field via the coupling effect between the chiral passivation molecule and the A-site (formamidine ion) in perovskites. This strategy can increase the molecular interaction energy by approximately four times and ensure a well-ordered molecular arrangement. The quality of the deposited perovskite film is significantly optimized with inhibited nonradiative recombination. It manages to reduce the open-circuit voltage loss of photovoltaic devices to 360 mV and increase the power conversion efficiency to 25.22%. This finding provides a new insight into the exploration of A-sites in perovskites and offers a novel route to improving the device performance of perovskite photovoltaics.

How Photogenerated I2 Induces I-Rich Phase Formation in Lead Mixed Halide Perovskites

Bandgap tunability of lead mixed halide perovskites (LMHPs) is a crucial characteristic for versatile optoelectronic applications. Nevertheless, LMHPs show the formation of iodide-rich (I-rich) phase under illumination, which destabilizes the semiconductor bandgap and impedes their exploitation. Here, it is shown that how I2 , photogenerated upon charge carrier trapping at iodine interstitials in LMHPs, can promote the formation of I-rich phase. I2 can react with bromide (Br- ) in the perovskite to form a trihalide ion I2 Br- (Iδ- -Iδ+ -Brδ- ), whose negatively charged iodide (Iδ- ) can further exchange with another lattice Br- to form the I-rich phase. Importantly, it is observed that the effectiveness of the process is dependent on the overall stability of the crystalline perovskite structure. Therefore, the bandgap instability in LMHPs is governed by two factors, i.e., the density of native defects leading to I2 production and the Br- binding strength within the crystalline unit. Eventually, this study provides rules for the design of chemical composition in LMHPs to reach their full potential for optoelectronic devices.

Raman Spectroscopy of Formamidinium-Based Lead Mixed-Halide Perovskite Bulk Crystals

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. Nonetheless, the stability of perovskite films and associated solar cells remains a source of uncertainty and necessitates sophisticated characterization techniques. Here, we report low- to mid-frequency resonant Raman spectra of formamidinium-based lead mixed-halide perovskites. The assignment of the different Raman lines in the measured spectra is assisted by DFT simulations of the Raman spectra of suitable periodic model systems. An important result of this work is that both experiment and theory point to an increase of the stability of the perovskite structure with increasing chloride doping concentration. In the Raman spectra, this is reflected by the appearance of new lines due to the formation of hydrogen bonds. Thus, higher chloride doping results in less torsional motion and lower asymmetric bending contributing to higher stability. This study yields a solid basis for the interpretation of the Raman spectra of formamidinium-based mixed-halide perovskites, furthering the understanding of the properties of these materials, which is essential for their full exploitation in solar cells.

Polarized Raman Study of Internal Vibrations of the Organic Cation in 3-Cyanopyridinium Lead Tribromide Post-perovskite

In this work, we apply polarized Raman spectroscopy for study of internal vibrations of the 3-cyanopyridinium cation in the halide post-perovskite (3cp)PbBr₃ (3cp = 3-CN-C₅H₅NH⁺). For a single cation, the vibrational frequencies and intensities of the Raman signal were calculated using the density functional theory. Selection rules were established for vibrations of cations in the crystal. These rules together with modeling results were used to identify the internal vibrations of the cation in the Raman spectrum of the crystal. Narrow and isolated internal vibrations of cations could be used as spectators of the crystalline environment.

Phonon-driven intra-exciton Rabi oscillations in CsPbBr3 halide perovskites

Coupling electromagnetic radiation with matter, e.g., by resonant light fields in external optical cavities, is highly promising for tailoring the optoelectronic properties of functional materials on the nanoscale. Here, we demonstrate that even internal fields induced by coherent lattice motions can be used to control the transient excitonic optical response in CsPbBr₃ halide perovskite crystals. Upon resonant photoexcitation, two-dimensional electronic spectroscopy reveals an excitonic peak structure oscillating persistently with a 100-fs period for up to ~2 ps which does not match the frequency of any phonon modes of the crystals. Only at later times, beyond 2 ps, two low-frequency phonons of the lead-bromide lattice dominate the dynamics. We rationalize these findings by an unusual exciton-phonon coupling inducing off-resonant 100-fs Rabi oscillations between 1s and 2p excitons driven by the low-frequency phonons. As such, prevailing models for the electron-phonon coupling in halide perovskites are insufficient to explain these results. We propose the coupling of characteristic low-frequency phonon fields to intra-excitonic transitions in halide perovskites as the key to control the anharmonic response of these materials in order to establish new routes for enhancing their optoelectronic properties.

Synthesis, Photoluminescence and Vibrational Properties of Aziridinium Lead Halide Perovskites

Three-dimensional lead halide perovskites are known for their excellent optoelectronic properties, making them suitable for photovoltaic and light-emitting applications. Here, we report for the first time the Raman spectra and photoluminescent (PL) properties of recently discovered three-dimensional aziridinium lead halide perovskites (AZPbX₃, X = Cl, Br, I), as well as assignment of vibrational modes. We also report diffuse reflection data, which revealed an extended absorption of light of AZPbX₃ compared to the MA and FA counterparts and are beneficial for solar cell application. We demonstrated that this behavior is correlated with the size of the organic cation, i.e., the energy band gap of the cubic lead halide perovskites decreases with the increasing size of the organic cation. All compounds show intense PL, which weakens on heating and shifts toward higher energies. This PL is red shifted compared to the FA and MA counterparts. An analysis of the PL data revealed the small exciton binding energy of AZPbX₃ compounds (29-56 meV). Overall, the properties of AZPbX₃ are very similar to those of the well-known MAPbX₃ and FAPbX₃ perovskites, indicating that the aziridinium analogues are also attractive materials for light-emitting and solar cell applications.

Small practical cluster models for perovskites based on the similarity criterion of central location environment and their applications

Developing universal theoretical models for perovskites (often denoted as ABX₃) can contribute to the rational design of novel perovskite photovoltaic materials. However, few models can be successfully applied to study the intrinsic electronic structure due to the poor accuracy and unaffordable computational cost. Herein, we report the innovative construction of small practical cluster models through the similarity criterion of the central location environment, which retains only the central A-site as the original cation while the others are substituted by Cs to keep the clusters electrically neutral. The central cation has a chemical environment similar to that of the bulk perovskite. The binding energy between A and the BX framework, geometric structures (B-X distances and B-X-B angles), and the electronic structures (the gap and the spatial distribution of HOMO and LUMO, electron distribution) of these clusters have been investigated and compared with the corresponding properties of bulk materials. The results suggest that the cluster model with twelve B-atoms suitably describes these properties. The geometric structures and gaps are closer to the bulk situations than the quasi-one-dimensional and quasi-two-dimensional cluster models with all-primitive cations, respectively. Other organic cations, such as NH₃(CH₂)_nCH₃ (n = 1, 2, and 3 for EA, PA, and BA, respectively), and (NH₂)₂CH (FA) can, therefore, mimic perovskite materials. Clusters with different sizes of A indicate that PA and BA will distort the quasi-cubic structures, which is consistent with the judgment of the tolerance factor of bulk materials. The reliable cluster model provides the research foundation for some basic issues of perovskites, such as vibrational spectroscopy and hydrogen bonding strength, to gain detailed insight into the interactions between A and the BX framework.

Tailoring Photoluminescence by Strain-Engineering in Layered Perovskite Flakes

Strain is an effective strategy to modulate the optoelectronic properties of 2D materials, but it has been almost unexplored in layered hybrid organic-inorganic metal halide perovskites (HOIPs) due to their complex band structure and mechanical properties. Here, we investigate the temperature-dependent microphotoluminescence (PL) of 2D (C₆H₅CH₂CH₂NH₃)₂Cs₃Pb₄Br₁₃ HOIP subject to biaxial strain induced by a SiO₂ ring platform on which flakes are placed by viscoelastic stamping. At 80 K, we found that a strain of <1% can change the PL emission from a single peak (unstrained) to three well-resolved peaks. Supported by micro-Raman spectroscopy, we show that the thermomechanically generated strain modulates the bandgap due to changes in the octahedral tilting and lattice expansion. Mechanical simulations demonstrate the coexistence of tensile and compressive strain along the flake. The observed PL peaks add an interesting feature to the rich phenomenology of photoluminescence in 2D HOIPs, which can be exploited in tailored sensing and optoelectronic devices.

Scalable and Blue Photoluminescence Emissions of (C4H9NH3)2PbBr4 2D Perovskite Fabricated by the Dip-Coating Method Using a Co-Solvent System

The improved efficiency of perovskite-related photovoltaic devices, such as light-emitting diodes (LEDs), is related to film uniformity, the compactness of each layer, and thickness. Herein, we improved the traditional single-solvent, solution-processed method and developed a co-solvent method to prepare a two-dimensional (2D) (C4H9NH3)2PbBr4 perovskite film for blue photoluminescence (PL) emissions. A poor film-forming uniformity was observed for the use of the single-solvent, dimethylformamide (DMF) method. In adding 1,2-dichlorobenzene (ODCB) of a smaller polarity to DMF, the co-solvent engineering dramatically changed the film-forming properties. Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometer (XRD), and time-resolved PL (TR-PL) spectroscopy analyses confirmed that the perovskite film prepared by the co-solvent system had a good crystallinity, fewer defects, and a longer carrier lifetime. These experimental results show a simple, scalable (1.23 × 1.23 cm2), and stable reproducibility method for preparing 2D perovskite of 415 nm wavelength PL emissions that might be beneficial for the development of ultraviolet (UV) photodetectors, blue LEDs, and high-resolution displays.

Reversible Photochromism in ⟨110⟩ Oriented Layered Halide Perovskite

Extending halide perovskites' optoelectronic properties to stimuli-responsive chromism enables switchable optoelectronics, information display, and smart window applications. Here, we demonstrate a band gap tunability (chromism) via crystal structure transformation from three-dimensional FAPbBr₃ to a ⟨110⟩ oriented FA_n+2Pb_nBr_3n+2 structure using a mono-halide/cation composition (FA/Pb) tuning. Furthermore, we illustrate reversible photochromism in halide perovskite by modulating the intermediate n phase in the FA_n+2Pb_nBr_3n+2 structure, enabling greater control of the optical band gap and luminescence of a ⟨110⟩ oriented mono-halide/cation perovskite. Proton transfer reaction-mass spectroscopy carried out to precisely quantify the decomposition product reveals that the organic solvent in the film is a key contributor to the structural transformation and, therefore, the chromism in the ⟨110⟩ structure. These intermediate n phases (2 ≤ n ≤ ∞) stabilize in metastable states in the FA_n+2Pb_nBr_3n+2 system, which is accessible via strain or optical or thermal input. The structure reversibility in the ⟨110⟩ perovskite allowed us to demonstrate a class of photochromic sensors capable of self-adaptation to lighting.

Polaronic Signatures in Doped and Undoped Cesium Lead Halide Perovskite Nanocrystals through a Photoinduced Raman Mode

Lead halide perovskites (LHPs) are promising candidates for photovoltaic applications as they exhibit large carrier diffusion lengths and long carrier lifetimes among many other interesting properties. One of the widely accepted mechanisms for these properties is polaron formation, which is mainly driven by octahedral distortions of the inorganic framework. Since structure modifications of the framework largely affect associated distortions, we investigated Mn-doped and undoped CsPbX₃ (where X = Cl, Br, Cl/Br) using a local probe via micro-Raman spectroscopy and density functional theory (DFT) calculations for polaron formation. Our results highlight a new vibrational lattice mode at 132 cm^-1 due to polaronic distortion upon photoinduction. From the DFT studies, we have shown that the polaronic states are dominated by the B-site cation in the perovskite structure, but it is the strong covalent overlap of the halide which determines its stability. This elucidation to map polaronic signatures with excellent spatial resolution using traditional Raman spectroscopy can be used as a simple tool to understand the structural changes and their impacted electronic properties and thus design superior devices using its in situ applications.

Impact of the polar optical phonon and alloy scattering on the charge-carrier mobilities of FA0.83Cs0.17Pb(I1-xBrx)3 hybrid perovskites

Lead mixed-halide perovskites are promising absorption materials that are suitable for applications in tandem solar cells using existing silicon technology. Charge-carrier mobility is an important factor that affects the performance of tandem solar cells. However, a detailed understanding of the fundamental mechanisms of lead mixed-halide perovskites remains elusive. Here, we used LO (longitudinal optical) phonons and alloy scattering to the elucidate charge-carrier mobilities in the FA_0.83Cs_0.17Pb(I_1-xBr_x)₃ hybrid perovskite system. It was found that these scattering mechanisms provided very good quantitative agreement with the experimental results, between 11-40 cm² V^-1 s^-1. Our findings provide new insights into charge transport scattering in lead mixed-halide hybrid perovskites and pave the way toward design of novel semiconductor alloys for solar cell applications.

Tailored Lattice "Tape" to Confine Tensile Interface for 11.08%-Efficiency All-Inorganic CsPbBr3 Perovskite Solar Cell with an Ultrahigh Voltage of 1.702 V

The crystal distortion such as lattice strain and defect located at the surfaces and grain boundaries induced by soft perovskite lattice highly determines the charge extraction-transfer dynamics and recombination to cause an inferior efficiency of perovskite solar cells (PSCs). Herein, the authors propose a strategy to significantly reduce the superficial lattice tensile strain by means of incorporating an inorganic 2D Cl-terminated Ti3 C2 (Ti3 C2 Clx ) MXene into the bulk and surface of CsPbBr3 film. Arising from the strong interaction between Cl atoms in Ti3 C2 Clx and the under-coordinated Pb2+ in CsPbBr3 lattice, the expanded perovskite lattice is compressed and confined to act as a lattice "tape", in which the PbCl bond plays a role of "glue" and the 2D Ti3 C2 immobilizes the lattice. Finally, the defective surface is healed and a champion efficiency as high as 11.08% with an ultrahigh open-circuit voltage up to 1.702 V is achieved on the best all-inorganic CsPbBr3 PSC, which is so far the highest efficiency record for this kind of PSCs. Furthermore, the unencapsulated device demonstrates nearly unchanged performance under 80% relative humidity over 100 days and 85 °C over 30 days.

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

Solar perovskites have received phenomenal attention and success over the past decade, due to their high power conversion efficiencies (PCE), ease of fabrication and low cost which has enabled the prospect of them being a real commercial contender to the traditional silicon technology. In one of the several developments on the archetypal MAPbI3 perovskite absorber layer, FAPbI3 was found to obtain a higher PCE, likely due to its more optimum band gap, with doping strategies focusing on the inclusion of MA+/Cs+ cations to avoid the unfavourable phase transformation to a photoinactive phase. To better understand the phase change from the photoactive cubic (Pm3[combining macron]m) black (α) phase to the unwanted photoinactive (P63/mmc) yellow (δ) phase, we make use of variable temperature Raman spectroscopy to probe the molecular species and its relationship to the inorganic framework. We show for the first time there to be no Raman active modes for the α phase up to 4000 cm-1, which can be correlated to the Pm3[combining macron]m cubic symmetry of that phase. Our detailed studies suggest that previous reports of the observation of Raman peaks for this phase are likely associated with degradation reactions from the localised laser exposure and the formation of Raman active lead oxide. In addition, we have identified water as a contributing factor to the transformation, and observed a corresponding signal in the Raman spectra, although confirmation of its exact role still remains inconclusive.

Hidden polymorphism of FAPbI3 discovered by Raman spectroscopy

Formamidinium lead iodide (FAPbI3) can be used in its cubic, black form as a light absorber material in single-junction solar cells. It has a band-gap (1.5 eV) close to the maximum of the Shockley-Queisser limit, and reveals a high absorption coefficient. Its high thermal stability up to 320 °C has also a downside, which is the instability of the photo-active form at room temperature (RT). Thus, the black α-phase transforms at RT with time into a yellow non-photo-active δ-phase. The black phase can be recovered by annealing of the yellow state. In this work, a polymorphism of the α-phase at room temperature was found: as-synthesized (αi), degraded (αδ) and thermally recovered (αrec). They differ in the Raman spectra and PL signal, but not in the XRD patterns. Using temperature-dependent Raman spectroscopy, we identified a structural change in the αi-polymorph at ca. 110 °C. Above 110 °C, the FAPbI3 structure has undoubtedly cubic Pm3[combining macron]m symmetry (high-temperature phase: αHT). Below that temperature, the αi-phase was suggested to have a distorted perovskite structure with Im3[combining macron] symmetry. Thermally recovered FAPbI3 (αrec) also demonstrated the structural transition to αHT at the same temperature (ca. 110 °C) during its heating. The understanding of hybrid perovskites may bring additional assets in the development of new and stable structures.

Observation of Charge Generation via Photoinduced Stark Effect in Mixed-Cation Lead Bromide Perovskite Thin Films

Extensive transient absorption studies on hybrid organic-inorganic lead halide perovskites have elucidated many optical properties important for their device performance. Despite the enormous progress, the derivative shaped photoinduced absorption feature in transient spectra that is above the bandgap has many explanations, including the photoinduced Stark effect, where the bandgap is blue shifted due to a local electric field generated by charges. In this work, we employ broad band transient absorption and two-dimensional electronic spectroscopy (2DES) to examine the early transient events after photoexcitation of [CH(NH₂)₂]_0.83Cs_0.17PbBr₃ (FA_0.83Cs_0.17PbBr₃). 2DES resolves a photomodulation feature at the excitation energy of the exciton, suggesting the presence of a dipole field created by a polaron pair shifting the exciton transition to higher energies. As this polaron pair dissociates over 200 fs, the exciton transition shifts to higher energies over the same time scale, evidenced by the 2DES diagonal energy spectra. Given that the observations are well explained in terms of the Stark effect, our work provides extra grounds to support the Stark effect assignment of the above-gap photoinduced absorption. Furthermore, our study reports on the time scale of charge generation, contributing to the fundamental understanding of mixed-cation lead bromide perovskite photophysics.