Entropy-Suppressed Ferroelectricity in Hybrid Lead-Iodide Perovskites

Emerging Halide Perovskite Ferroelectrics

Halide perovskites have gained tremendous attention in the past decade owing to their excellent properties in optoelectronics. Recently, a fascinating property, ferroelectricity, has been discovered in halide perovskites and quickly attracted widespread interest. Compared with traditional perovskite oxide ferroelectrics, halide perovskites display natural advantages such as structural softness, low weight, and easy processing, which are highly desirable in applications pursuing miniaturization and flexibility. This review focuses on the current research progress in halide perovskite ferroelectrics, encompassing the emerging materials systems and their potential applications in ferroelectric photovoltaics, self-powered photodetection, and X-ray detection. The main challenges and possible solutions in the future development of halide perovskite ferroelectric materials are also attempted to be pointed out.

Long-lived electrets and lack of ferroelectricity in methylammonium lead bromide CH3NH3PbBr3 ferroelastic single crystals

Hybrid lead halides CH₃NH₃PbX₃ (X = I, Br, and Cl) have emerged as a new class of semiconductors for low-cost optoelectronic devices with superior performance. Since their perovskite crystal structure may have lattice instabilities against polar distortions, they are also being considered as potential photo-ferroelectrics. However, so far, research on their ferroelectricity has yielded inconclusive results and the subject is far from being settled. Here, we investigate, using a combined experimental and theoretical approach, the possible presence of electric polarization in tetragonal and orthorhombic CH₃NH₃PbBr₃ (T-MAPB and O-MAPB). We found that T-MAPB does not sustain spontaneous polarization but, under an external electric field, it is projected into a metastable, ionic space-charge electret state. The electret can be frozen on cooling, producing a large and long-lasting polarization in O-MAPB. Molecular dynamics simulations show that the ferroelastic domain boundaries are able to trap charges and segregate ionic point defects, thus playing a favorable role in the stabilization of the electret. At lower temperatures, the lack of ferroelectric behavior is explained using first principles calculations as the result of the tight competition among many metastable states with randomly oriented polarization; this large configurational entropy does not allow a single polar state to dominate at any significant temperature range.

The existence and impact of persistent ferroelectric domains in MAPbI3

Methylammonium lead iodide (MAPbI₃) exhibits exceptional photovoltaic performance, but there remains substantial controversy over the existence and impact of ferroelectricity on the photovoltaic response. We confirm ferroelectricity in MAPbI₃ single crystals and demonstrate mediation of the electronic response by ferroelectric domain engineering. The ferroelectric response sharply declines above 57°C, consistent with the tetragonal-to-cubic phase transition. Concurrent band excitation piezoresponse force microscopy-contact Kelvin probe force microscopy shows that the measured response is not dominated by spurious electrostatic interactions. Large signal poling (>16 V/cm) orients the permanent polarization into large domains, which show stabilization over weeks. X-ray photoemission spectroscopy results indicate a shift of 400 meV in the binding energy of the iodine core level peaks upon poling, which is reflected in the carrier concentration results from scanning microwave impedance microscopy. The ability to control the ferroelectric response provides routes to increase device stability and photovoltaic performance through domain engineering.

Brightness Enhancement in Pulsed-Operated Perovskite Light-Emitting Transistors

Perovskite light-emitting field-effect transistors (PeLEFETs) provide a versatile device architecture to control transport and electroluminescence properties of hybrid perovskites, enabling injection of high charge carrier density and spatial control of the radiative recombination zone. Ionic screening and organic cation polarization effects typical of metal-halide perovskites, however, critically affect PeLEFET efficiency and reliability. In this work, we demonstrate a new device operation mode based on high-frequency modulation of the applied voltages, which allows significant reduction of ionic drift/screening in methylammonium lead iodide light-emitting transistors. In optimized top contact PeLEFETs, AC operation results in brighter and more uniform electroluminescence compared to DC-driven devices, whereas high-frequency modulation enables electroluminescence emission up to room temperature.

Competition between Polar and Antiferrodistortive Modes and Correlated Dynamics of the Methylammonium Molecules in MAPbI3 from Anelastic and Dielectric Measurements

The mechanisms behind the exceptional photovoltaic properties of the metallorganic perovskites are still debated and include a ferroelectric (FE) state from the ordering of the electric dipoles of the organic molecules. We present the first anelastic (complex Young's modulus) and new dielectric measurements on CH₃NH₃PbI₃, which provide new insight into the reorientation dynamics of the organic molecules and the reason why they do not form a FE state. The permittivity is fitted within the tetragonal phase with an expression that includes the coupling between FE and octahedral tilt modes, indicating that the coupling is competitive and prevents FE ordering. The onset of the orthorhombic phase is accompanied by sharp stiffening, analogous to the drop of permittivity, due to the hindered molecular dynamics. On further cooling, an intense anelastic relaxation process without a dielectric counterpart suggests the reorientation of clusters of molecules with strong antiferroelectric correlations.

Exploring the orthorhombic-tetragonal phase transition in CH3NH3PbI3: the role of atom kinetics

Methylammonium lead tri-iodide is a polymorphic material with two temperature-induced phase transitions at 165 K and 327 K, accompanied by an orthorhombic-to-tetragonal and a tetragonal-to-cubic lattice modification. Understanding the origins of these transitions as well as their implications on the crystal structure of the material is fundamental for its technological optimization. Here, we use the density functional theory along with ab initio molecular dynamics to study the low-temperature phase transition of CH₃NH₃PbI₃. Considering two kinetically robust models for the orthorhombic and the tetragonal phase, we show that the vibrational features of the material can be strongly affected by the orientations of the methylammonium ions. We argue that the orthorhombic-tetragonal transition is characterized by a partial rearrangement of the organic cations that locally relaxes the stress induced by the thermal movement of atoms. We finally propose a macroscopic model for the tetragonal phase that consists of rotated noncentrosymmetric domains, where the methylammonium ions are quasi-two-dimensionally confined around the a-b crystallographic plane.

Unraveling the Exciton Binding Energy and the Dielectric Constant in Single-Crystal Methylammonium Lead Triiodide Perovskite

We have accurately determined the exciton binding energy and reduced mass of single crystals of methylammonium lead triiodide using magneto-reflectivity at very high magnetic fields. The single crystal has excellent optical properties with a narrow line width of ∼3 meV for the excitonic transitions and a 2s transition that is clearly visible even at zero magnetic field. The exciton binding energy of 16 ± 2 meV in the low-temperature orthorhombic phase is almost identical to the value found in polycrystalline samples, crucially ruling out any possibility that the exciton binding energy depends on the grain size. In the room-temperature tetragonal phase, an upper limit for the exciton binding energy of 12 ± 4 meV is estimated from the evolution of 1s-2s splitting at high magnetic field.

Modeling hybrid perovskites by molecular dynamics

The topical review describes the recent progress in the modeling of hybrid perovskites by molecular dynamics simulations. Hybrid perovskites and in particular methylammonium lead halide (MAPI) have a tremendous technological relevance representing the fastest-advancing solar material to date. They also represent the paradigm of an organic-inorganic crystalline material with some conceptual peculiarities: an inorganic semiconductor for what concerns the electronic and absorption properties with a hybrid and solution processable organic-inorganic body. After briefly explaining the basic concepts of ab initio and classical molecular dynamics, the model potential recently developed for hybrid perovskites is described together with its physical motivation as a simple ionic model able to reproduce the main dynamical properties of the material. Advantages and limits of the two strategies (either ab initio or classical) are discussed in comparison with the time and length scales (from pico to microsecond scale) necessary to comprehensively study the relevant properties of hybrid perovskites from molecular reorientations to electrocaloric effects. The state-of-the-art of the molecular dynamics modeling of hybrid perovskites is reviewed by focusing on a selection of showcase applications of methylammonium lead halide: molecular cations disorder; temperature evolution of vibrations; thermally activated defects diffusion; thermal transport. We finally discuss the perspectives in the modeling of hybrid perovskites by molecular dynamics.

Understanding charge transport in lead iodide perovskite thin-film field-effect transistors

Fundamental understanding of the charge transport physics of hybrid lead halide perovskite semiconductors is important for advancing their use in high-performance optoelectronics. We use field-effect transistors (FETs) to probe the charge transport mechanism in thin films of methylammonium lead iodide (MAPbI₃). We show that through optimization of thin-film microstructure and source-drain contact modifications, it is possible to significantly minimize instability and hysteresis in FET characteristics and demonstrate an electron field-effect mobility (μ_FET) of 0.5 cm²/Vs at room temperature. Temperature-dependent transport studies revealed a negative coefficient of mobility with three different temperature regimes. On the basis of electrical and spectroscopic studies, we attribute the three different regimes to transport limited by ion migration due to point defects associated with grain boundaries, polarization disorder of the MA⁺ cations, and thermal vibrations of the lead halide inorganic cages.

Unexpected Cation Dynamics in the Low-Temperature Phase of Methylammonium Lead Iodide: The Need for Improved Models

High-resolution inelastic neutron scattering and extensive first-principles calculations have been used to explore the low-temperature phase of the hybrid solar-cell material methylammonium lead iodide up to the well-known phase transition to the tetragonal phase at ca. 160 K. Contrary to original expectation, we find that the Pnma structure for this phase can only provide a qualitative description of the geometry and underlying motions of the organic cation. A substantial lowering of the local symmetry inside the perovskite cage leads to an improved atomistic model that can account for all available spectroscopic and thermodynamic data, both at low temperatures and in the vicinity of the aforementioned phase transition. Further and detailed analysis of the first-principles calculations reveals that large-amplitude distortions of the inorganic framework are driven by both zero-point-energy fluctuations and thermally activated cation motions. These effects are significant down to liquid-helium temperatures. For this important class of technological materials, this work brings to the fore the pressing need to bridge the gap between the long-range order seen by crystallographic methods and the local environment around the organic cation probed by neutron spectroscopy.

Vibrational Response of Methylammonium Lead Iodide: From Cation Dynamics to Phonon-Phonon Interactions

The dynamic evolution of the vibrational interactions in the prototypical CH₃ NH₃ PbI₃ was studied through a comprehensive experimental and theoretical investigation with a focus on the interactions between the organic cations and the inorganic cage. To date, no clear picture has emerged on the critical and fundamental interactions between the two perovskite components, despite the relevance of phonons to the electronic properties of several classes of perovskites. For the first time, we have monitored the IR and nonresonant Raman response in the broad frequency range 30-3400 cm^-1 and in the temperature interval 80-360 K. Strong changes in the energies of different vibrational modes with temperature are observed and examined in the framework of phonon-phonon interactions considering a significant anharmonic contribution to the phonon relaxation process. The vibrational relaxation of the bending modes and their reorientation activation energies identify that such mechanisms are governed by medium-to-strong hydrogen bonds in the orthorhombic phase; however, any ferroelectric ordering in the orthorhombic phase is governed mostly by dipole interactions. These changes imply that charge localization mechanisms play a primary role, and our study enriches the fundamental knowledge of phonon interactions and charge transport in CH₃ NH₃ PbI₃ for the further development of optoelectronic applications.

Origin of unusual bandgap shift and dual emission in organic-inorganic lead halide perovskites

Emission characteristics of metal halide perovskites play a key role in the current widespread investigations into their potential uses in optoelectronics and photonics. However, a fundamental understanding of the molecular origin of the unusual blueshift of the bandgap and dual emission in perovskites is still lacking. In this direction, we investigated the extraordinary photoluminescence behavior of three representatives of this important class of photonic materials, that is, CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃, which emerged from our thorough studies of the effects of temperature on their bandgap and emission decay dynamics using time-integrated and time-resolved photoluminescence spectroscopy. The low-temperature (<100 K) photoluminescence of CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ reveals two distinct emission peaks, whereas that of CH(NH₂)₂PbBr₃ shows a single emission peak. Furthermore, irrespective of perovskite composition, the bandgap exhibits an unusual blueshift by raising the temperature from 15 to 300 K. Density functional theory and classical molecular dynamics simulations allow for assigning the additional photoluminescence peak to the presence of molecularly disordered orthorhombic domains and also rationalize that the unusual blueshift of the bandgap with increasing temperature is due to the stabilization of the valence band maximum. Our findings provide new insights into the salient emission properties of perovskite materials, which define their performance in solar cells and light-emitting devices.

Thermally Activated Point Defect Diffusion in Methylammonium Lead Trihalide: Anisotropic and Ultrahigh Mobility of Iodine

We study the diffusion of point defects in crystalline methylammonium lead halide (MAPI) at finite temperatures by using all-atoms molecular dynamics. We find that, for what concerns intrinsic defects, iodine diffusion is by far the dominant mechanism of ionic transport in MAPI, with diffusivities as high as 7.4 × 10(-7) and 4.3 × 10(-6) cm(2) s(-1) at 300 K and single activation energies of 0.24 and 0.10 eV, for interstitials and vacancies, respectively. The comparison with common covalent and oxide crystals reveals the ultrahigh mobility of defects in MAPI. Though at room temperature the vacancies are about 1 order of magnitude more diffusive, the anisotropic interstitial dynamics increases more rapidly with temperature, and it can be dominant at high temperatures. Present results are fully consistent with the involvement of iodide ions in hysteresis and have implications for improvement of the material quality by better control of defect diffusion.

Role of Polar Phonons in the Photo Excited State of Metal Halide Perovskites

The development of high efficiency perovskite solar cells has sparked a multitude of measurements on the optical properties of these materials. For the most studied methylammonium(MA)PbI₃ perovskite, a large range (6–55 meV) of exciton binding energies has been reported by various experiments. The existence of excitons at room temperature is unclear. For the MAPbX₃ perovskites we report on relativistic Bethe-Salpeter Equation calculations (GW-BSE). This method is capable to directly calculate excitonic properties from first-principles. At low temperatures it predicts exciton binding energies in agreement with the reported ‘large’ values. For MAPbI₃, phonon modes present in this frequency range have a negligible contribution to the ionic screening. By calculating the polarization in time from finite temperature molecular dynamics, we show that at room temperature this does not change. We therefore exclude ionic screening as an explanation for the experimentally observed reduction of the exciton binding energy at room temperature and argue in favor of the formation of polarons.

Spontaneous bidirectional ordering of CH3NH3(+) in lead iodide perovskites at room temperature: The origins of the tetragonal phase

CH₃NH₃PbI₃ is a hybrid organic-inorganic material with a perovskite structure and a temperature-dependent polymorphism whose origins are still unclear. Here we perform ab initio molecular dynamics simulations in order to investigate the structural properties and atom dynamics of CH₃NH₃PbI₃ at room temperature. Starting from different initial configurations, we find that a single-crystalline system undergoes a spontaneous ordering process which brings the ions to alternately point towards the center of two out of the six faces of the cubic framework, i.e. towards the 〈100〉 and 〈010〉 directions. This bidirectional ordering gives rise to a preferential distortion of the inorganic lattice on the a-b plane, shaping the observed tetragonal symmetry of the system. The process requires tens of picoseconds for CH₃NH₃PbI₃ supercells with just eight ions.

Temperature Evolution of Methylammonium Trihalide Vibrations at the Atomic Scale

The temperature evolution of vibrations of CH3NH3PbI3 (MAPI) is studied by combining first principles and classical molecular dynamics and compared to available experimental data. The work has a fundamental character showing that it is possible to reproduce the key features of the vibrational spectrum by the simple physical quantities included in the classical model, namely the ionic-dispersive hybrid interactions and the mass difference between organic and inorganic components. The dynamics reveals a sizable temperature evolution of the MAPI spectrum along with the orthorhombic-to-tetragonal-to-cubic transformation and a strong dependence on molecular confinement and order. The thermally induced weakening of the H-I interactions and the anharmonic mixing of modes give two vibrational peaks at 200-250 cm(-1) that are not present at zero temperature and are expected to have detectable infrared activity. The infrared inactive vibrational peak at ∼140 cm(-1) due to molecular spinning disappears abruptly at the orthorhombic-to-tetragonal transition and forms a broad molecular band red-shifting progressively with temperature. This trend is correlated to the reduced confinement of the rotating cations due to thermal expansion of the lattice.

Tuning the thermal conductivity of methylammonium lead halide by the molecular substructure

It is shown by molecular dynamics that the substructure of organic molecules can tailor the thermal conductivity of MAPI.

Low electron-polar optical phonon scattering as a fundamental aspect of carrier mobility in methylammonium lead halide CH3NH3PbI3 perovskites

Calculated mobility of CH₃NH₃PbI₃ in two temperature regions, characterized by the dominance of electron-acoustic phonon scattering (left) and electron-polar optical phonon scattering (right).

Domain-dependent electronic structure and optical absorption property in hybrid organic–inorganic perovskite

Hybrid organic–inorganic perovskites, represented by materials in the CH₃NH₃PbI₃ series, have become one of the most promising materials for solar cells with a high power conversion efficiency and low cost.