Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites

Unraveling the excitonics of light emission from metal-halide perovskite quantum dots

Metal halide semicondictor perovskites have been under intense investigation for their promise in light absorptive applications like photovoltaics. They have more recently experienced interest for their promise in light emissive applications. A key aspect of perovskites is their glassy, ionic lattice that exhibits dynamical disorder. One possible result of this dynamical disorder is their strong coupling between electronic and lattice degrees of freedom which may confer remarkable properties for light emission such as defect tolerance. How does the system, comprised of excitons, couple to the bath, comprised of lattice modes? How does this system-bath interaction give rise to novel light emissive properties and how do these properties give insight into the nature of these materials? We review recent work from this group in which time-resolved photoluminescence spectroscopy is used to reveal such insights. Based upon a fast time resolution of 3 ps, energy resolution, and temperature dependence, a wide variety of insights are gleaned. These insights include: lattice contributions to the emission linewidths, multiexciton formation, hot carrier cooling, excitonic fine structure, single dot superradiance, and a breakdown of the Condon approximation, all due to complex structural dynamics in these materials.

Hole Localization in Bulk and 2D Lead-Halide Perovskites Studied by Time-Resolved Infrared Spectroscopy

Scattering and localization dynamics of charge carriers in the soft lattice of lead-halide perovskites impact polaron formation and recombination, which are key mechanisms of material function in optoelectronic devices. In this study, we probe the photoinduced lattice and carrier dynamics in perovskite thin films (CsFAPbX3, X = I, Br) using time-resolved infrared spectroscopy. We examine the CN stretching mode of formamidinium (FA) cations located within the lead-halide octahedra of the perovskite structure. Our investigation reveals the formation of an infrared mode due to spatial symmetry breaking within a hundred picoseconds in 3D perovskites. Experiments at cryogenic temperatures show much-reduced carrier localization, in agreement with a localization mechanism that is driven by the dynamic disorder. We extend our analysis to 2D perovskites, where the precise nature of charge carriers is uncertain. Remarkably, the signatures of charge localization we found in bulk perovskites are not observed for 2D Ruddlesden-Popper perovskites ((HexA)2FAPb2I7). This observation implies that the previously reported stabilization of free charge carriers in these materials follows different mechanisms than polaron formation in bulk perovskites. Through the exploration of heterostructures with electron/hole excess, we provide evidence that holes drive the formation of the emerging infrared mode.

Melting entropy of crystals determined by electron-beam-induced configurational disordering

Upon melting, the molecules in a crystal explore numerous configurations, reflecting an increase in disorder. The molar entropy of disorder can be defined by Boltzmann's formula ΔSd = Rln(Wd), where Wd is the increase in the number of microscopic states, so far inaccessible experimentally. We found that the Arrhenius frequency factor A of the electron diffraction signal decay provides Wd through an experimental equation A = AINTWd, where AINT is an inelastic scattering cross section. The method connects Clausius and Boltzmann experimentally and supplements the Clausius approach, being applicable to a femtogram quantity of thermally unstable and biomolecular crystals. The data also showed that crystal disordering and crystallization of melt are reciprocal, both governed by the entropy change but manifesting in opposite directions.

Topological polarons in halide perovskites

Halide perovskites emerged as a revolutionary family of high-quality semiconductors for solar energy harvesting and energy-efficient lighting. There is mounting evidence that the exceptional optoelectronic properties of these materials could stem from unconventional electron-phonon couplings, and it has been suggested that the formation of polarons and self-trapped excitons could be key to understanding such properties. By performing first-principles simulations across the length scales, here we show that halide perovskites harbor a uniquely rich variety of polaronic species, including small polarons, large polarons, and charge density waves, and we explain a variety of experimental observations. We find that these emergent quasiparticles support topologically nontrivial phonon fields with quantized topological charge, making them nonmagnetic analog of the helical Bloch points found in magnetic skyrmion lattices.

A comparative review of time-resolved x-ray and electron scattering to probe structural dynamics

The structure of molecules, particularly the dynamic changes in structure, plays an essential role in understanding physical and chemical phenomena. Time-resolved (TR) scattering techniques serve as crucial experimental tools for studying structural dynamics, offering direct sensitivity to molecular structures through scattering signals. Over the past decade, the advent of x-ray free-electron lasers (XFELs) and mega-electron-volt ultrafast electron diffraction (MeV-UED) facilities has ushered TR scattering experiments into a new era, garnering significant attention. In this review, we delve into the basic principles of TR scattering experiments, especially focusing on those that employ x-rays and electrons. We highlight the variations in experimental conditions when employing x-rays vs electrons and discuss their complementarity. Additionally, cutting-edge XFELs and MeV-UED facilities for TR x-ray and electron scattering experiments and the experiments performed at those facilities are reviewed. As new facilities are constructed and existing ones undergo upgrades, the landscape for TR x-ray and electron scattering experiments is poised for further expansion. Through this review, we aim to facilitate the effective utilization of these emerging opportunities, assisting researchers in delving deeper into the intricate dynamics of molecular structures.

Rashba and Dresselhaus effects in doped methylammonium lead halide perovskite MAPbI3

Inorganic-organic lead halide perovskites, particularly methylammonium lead halide (MAPbI3) perovskite, have been regarded as promising materials for optoelectronics and spintronics. However, the practical applications of these perovskites are limited by lead toxicity and instability under air and pressure. This study investigates the substitution of Pb with Sn and Ge in cubic MAPbI3 perovskite. The properties of the resulting hybrid perovskites are compared using state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with generalized gradient approximation (PBE) and hybrid functional (HSE06), in conjunction with spin-orbit coupling (SOC). Here, we mainly study the Rashba-Dresselhaus (RD) effect, which arises due to two major mechanisms: (i) the breaking of inversion symmetry (static and dynamic) and (ii) SOC, originating from the presence of heavy elements. We find significant spin-splitting effects in the conduction band minimum and valence band maximum for hybrid perovskites. To gain a deeper understanding of the observed spin-splitting, the spin textures are analyzed, and Rashba coefficients are calculated. We find that the Dresselhaus effect comes into play in substituted hybrid structures in addition to the usual Rashba effect observed in the pristine compound. Additionally, we observe that the strength of Rashba spin-splitting is substantially tuned by the application of uniaxial strain (±5%). Moreover, certain hybrid perovskites exhibit mechanical stability and ductility, making them potential candidates in perovskite-based optoelectronics and spintronics applications.

Crystal structures and photoluminescence characteristics of cesium lead bromide perovskite nanoplatelets depending on the antisolvent and ligand used in their syntheses

Cesium lead bromide (CsPbBr3) nanocrystals (NCs) with nanoplatelet shapes and different crystal structures were synthesized via the ligand-assisted reprecipitation (LARP) method using different pairs of ligands and antisolvents, namely oleic acid (OA) or linoleic acid (LA) as the ligand and toluene or chloroform as the antisolvent. The XRD data revealed that the obtained CsPbBr3 NCs have different crystal structures, namely orthorhombic, tetragonal, and cubic, depending on the ligand and antisolvent pair, which exhibited significantly different photoluminescence (PL) characteristics. From the XPS data, these CsPbBr3 nanoplatelets showed two doublet peaks of the Br-3d orbital at different binding energies, representing two different chemical environments of the Br bonds. The doublet peak apparent at a higher binding energy was associated with the Br chemical states at the crystal surface, which appeared because of the distorted crystal structure resulting from the interaction of the solvent and ligand with Br ions. The PL emission consists of three luminescence centers: a PL band peaked at 520 nm (A band), a PL band peaked at 540 nm (B band), and a PL band tail, which can be discussed in terms of exciton models. Stable and intense luminescence was observed in CsPbBr3 nanoplatelets synthesized using a pair of toluene antisolvent and LA ligand, namely CsPbBr3#(Tl/LA). The orthorhombic crystal structure and distorted crystal surface in this sample may lead to confinement of the photogenerated small exciton-polaron and weak phonon interactions, which effectively hinder exciton dissociation, particularly at the crystal surface, resulting in intense PL. The results of this study may provide additional important insights into the role of the antisolvent and ligand in the formation of CsPbBr3 NCs and the exciton behavior in their PL characteristics, which may also be found in other types of halide perovskites.

Breaking the Condon Approximation for Light Emission from Metal Halide Perovskite Nanocrystals

The idea that the electronic transition dipole moment does not depend upon nuclear excursions is the Condon approximation and is central to most spectroscopy, especially in the solid state. We show a strong breakdown of the Condon approximation in the time-resolved photoluminescence from CsPbBr3 metal halide perovskite semiconductor nanocrystals. Experiments reveal that the electronic transition dipole moment increases on the 30 ps time scale due to structural dynamics in the lattice. Ab initio molecular dynamics calculations quantitatively reproduce experiments by considering excitation-induced structural dynamics.

A Strategy for Tuning Electron-Phonon Coupling and Carrier Cooling in Lead Halide Perovskite Nanocrystals

Perovskites have been recognized as a class of promising materials for optoelectronic devices. We intentionally include excessive Cs+ cations in precursors in the synthesis of perovskite CsPbBr3 nanocrystals and investigate how the Cs+ cations influence the lattice strain in these perovskite nanocrystals. Upon light illumination, the lattice strain due to the addition of alkali metal Cs+ cations can be compensated by light-induced lattice expansion. When the Cs+ cation in precursors is about 10% excessive, the electron-phonon coupling strength can be reduced by about 70%, and the carrier cooling can be slowed down about 3.5 times in lead halide perovskite CsPbBr3 nanocrystals. This work reveals a new understanding of the role of Cs+ cations, which take the A-site in ABX3 perovskite and provide a new way to improve the performance of perovskites and their practical devices further.

Small to Large Polaron Behavior Induced by Controlled Interactions in Perovskite Quantum Dot Solids

The polaron is an essential photoexcitation that governs the unique optoelectronic properties of organic-inorganic hybrid halide perovskites, and it has been subject to extensive spectroscopic and theoretical investigation over the past decade. A crucial but underexplored question is how the nature of the photogenerated polarons is impacted by the microscopic perovskite structure and what functional properties this affects. To tackle this question, we chemically tuned the interactions between perovskite quantum dots (QDs) to rationally manipulate the polaron properties. Through a suite of time-resolved spectroscopies, we find that inter-QD interactions open an excited-state channel to form large polaron species, which exhibit enhanced spatial diffusion, slower hot polaron cooling, and a longer intrinsic lifetime. At the same time, polaronic excitons are formed in competition via localized band-edge states, exhibiting strong photoluminescence but are limited by shorter intrinsic lifetimes. This control of polaron type and function through tunable inter-QD interactions not only provides design principles for QD-based materials but also experimentally disentangles polaronic species in hybrid perovskite materials.

Exciton-polaron interactions in metal halide perovskite nanocrystals revealed via two-dimensional electronic spectroscopy

Metal halide perovskite nanocrystals have been under intense investigation for their promise in optoelectronic devices due to their remarkable physics, such as liquid/solid duality. This liquid/solid duality may give rise to their defect tolerance and other such useful properties. This duality means that the electronic states are fluctuating in time, on a distribution of timescales from femtoseconds to picoseconds. Hence, these lattice induced energy fluctuations that are connected to polaron formation are also connected to exciton formation and dynamics. We observe these correlations and dynamics in metal halide perovskite nanocrystals of CsPbI3 and CsPbBr3 using two-dimensional electronic (2DE) spectroscopy, with its unique ability to resolve dynamics in heterogeneously broadened systems. The 2DE spectra immediately reveal a previously unobserved excitonic splitting in these 15 nm NCs that may have a coarse excitonic structure. 2D lineshape dynamics reveal a glassy response on the 300 fs timescale due to polaron formation. The lighter Br system shows larger amplitude and faster timescale fluctuations that give rise to dynamic line broadening. The 2DE signals enable 1D transient absorption analysis of exciton cooling dynamics. Exciton cooling within this doublet is shown to take place on a slower timescale than within the excitonic continuum. The energy dissipation rates are the same for the I and Br systems for incoherent exciton cooling but are very different for the coherent dynamics that give rise to line broadening. Exciton cooling is shown to take place on the same timescale as polaron formation, revealing both as coupled many-body excitation.

Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons

Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

Exciton dissociation in 2D layered metal-halide perovskites

Layered 2D perovskites are making inroads as materials for photovoltaics and light emitting diodes, but their photophysics is still lively debated. Although their large exciton binding energies should hinder charge separation, significant evidence has been uncovered for an abundance of free carriers among optical excitations. Several explanations have been proposed, like exciton dissociation at grain boundaries or polaron formation, without clarifying yet if excitons form and then dissociate, or if the formation is prevented by competing relaxation processes. Here we address exciton stability in layered Ruddlesden-Popper PEA₂PbI₄ (PEA stands for phenethylammonium) both in form of thin film and single crystal, by resonant injection of cold excitons, whose dissociation is then probed with femtosecond differential transmission. We show the intrinsic nature of exciton dissociation in 2D layered perovskites, demonstrating that both 2D and 3D perovskites are free carrier semiconductors and their photophysics is described by a unique and universal framework.

Studies on the Light-Induced Phase Transition of CsPbBr3 Metal Halide Perovskite Materials

We investigate the internal mechanism of the light-induced phase transition of CsPbBr₃ perovskite materials via density functional theory simulations. Although CsPbBr₃ tends to appear in the orthorhombic structure, it can be changed easily by external stimulus. We find that the transition of photogenerated carriers plays the decisive role in this process. When the photogenerated carriers transit from the valence band maximum to conduction band minimum in the reciprocal space, they actually transit from Br ions to Pb ions in the real space, which are taken away by the Br atoms with higher electronegativity from Pb atoms during the initial formation of the CsPbBr₃ lattice. The reverse transition of valence electrons leads to the weakening of bond strength, which is proved by our calculated Bader charge, electron localization function, and integral value of COHP results. This charge transition releases the distortion of the Pb-Br octahedral framework and expands the CsPbBr₃ lattice, providing possibilities to the phase transition from the orthorhombic structure to tetragonal structure. This phase transition is a self-accelerating positive feedback process, increasing the light absorption efficiency of the CsPbBr₃ material, which is of great significance for the widespread promotion and application of the photostriction effect. Our results are helpful to understand the performance of CsPbBr₃ perovskite under a light irradiation environment.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Nonlinear terahertz control of the lead halide perovskite lattice

Lead halide perovskites (LHPs) have emerged as an excellent class of semiconductors for next-generation solar cells and optoelectronic devices. Tailoring physical properties by fine-tuning the lattice structures has been explored in these materials by chemical composition or morphology. Nevertheless, its dynamic counterpart, phonon-driven ultrafast material control, as contemporarily harnessed for oxide perovskites, has not yet been established. Here, we use intense THz electric fields to obtain direct lattice control via nonlinear excitation of coherent octahedral twist modes in hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites. These Raman-active phonons at 0.9 to 1.3 THz are found to govern the ultrafast THz-induced Kerr effect in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability with potential implications for dynamic charge carrier screening beyond the Fröhlich polaron. Our work opens the door to selective control of LHP's vibrational degrees of freedom governing phase transitions and dynamic disorder.

Understanding and Controlling Photothermal Responses in MXenes

MXenes have the potential for efficient light-to-heat conversion in photothermal applications. To effectively utilize MXenes in such applications, it is important to understand the underlying nonequilibrium processes, including electron-phonon and phonon-phonon couplings. Here, we use transient electron and X-ray diffraction to investigate the heating and cooling of photoexcited MXenes at femtosecond to nanosecond time scales. Our results show extremely strong electron-phonon coupling in Ti₃C₂-based MXenes, resulting in lattice heating within a few hundred femtoseconds. We also systematically study heat dissipation in MXenes with varying film thicknesses, chemical surface terminations, flake sizes, and annealing conditions. We find that the thermal boundary conductance (TBC) governs the thermal relaxation in films thinner than the optical penetration depth. We achieve a 2-fold enhancement of the TBC, reaching 20 MW m^-2 K^-1, by controlling the flake size or chemical surface termination, which is promising for engineering heat dissipation in photothermal and thermoelectric applications of the MXenes.

Light-Induced Transient Lattice Dynamics and Metastable Phase Transition in CH3NH3PbI3 Nanocrystals

Methylammonium lead iodide (MAPbI₃) perovskite nanocrystals (NCs) offer desirable optoelectronic properties with prospective utility in photovoltaics, lasers, and light-emitting diodes (LEDs). Structural rearrangements of MAPbI₃ in response to photoexcitation, such as lattice distortions and phase transitions, are of particular interest, as these engender long carrier lifetime and bolster carrier diffusion. Here, we use variable temperature X-ray diffraction (XRD) and synchrotron-based transient X-ray diffraction (TRXRD) to investigate lattice response following ultrafast optical excitation. MAPbI₃ NCs are found to slowly undergo a phase transition from the tetragonal to a pseudocubic phase over the course of 1 ns under 0.02-4.18 mJ/cm² fluence photoexcitation, with apparent nonthermal lattice distortions attributed to polaron formation. Lattice recovery exceeds time scales expected for both carrier recombination and thermal dissipation, indicating meta-stability likely due to the proximal phase transition, with symmetry-breaking along equatorial and axial directions. These findings are relevant for fundamental understanding and applications of structure-function properties.

Direct Observation of Ultrafast Lattice Distortions during Exciton-Polaron Formation in Lead Halide Perovskite Nanocrystals

The microscopic origin of slow hot-carrier cooling in lead halide perovskites remains debated and has direct implications for applications. Slow hot-carrier cooling of several picoseconds has been attributed to either polaron formation or a hot-phonon bottleneck effect at high excited carrier densities (>1018 cm-3). These effects cannot be unambiguously disentangled with optical experiments alone. However, they can be distinguished by direct observations of ultrafast lattice dynamics, as these effects are expected to create qualitatively distinct fingerprints. To this end, we employ femtosecond electron diffraction and directly measure the sub-picosecond lattice dynamics of weakly confined CsPbBr3 nanocrystals following above-gap photoexcitation. While we do not observe signatures of a hot-phonon bottleneck lasting several picoseconds, the data reveal a light-induced structural distortion appearing on a time scale varying between 380 and 1200 fs depending on the excitation fluence. We attribute these dynamics to the effect of exciton-polarons on the lattice and the slower dynamics at high fluences to slower sub-picosecond hot-carrier cooling, which slows down the establishment of the exciton-polaron population. Further analysis and simulations show that the distortion is consistent with motions of the [PbBr3]- octahedral ionic cage, and closest agreement with the data is obtained for Pb-Br bond lengthening. Our work demonstrates how direct studies of lattice dynamics on the sub-picosecond time scale can discriminate between competing scenarios proposed in the literature to explain the origin of slow hot-carrier cooling in lead halide perovskites.

A spectroscopic overview of the differences between the absorbing states and the emitting states in semiconductor perovskite nanocrystals

Semiconductor perovskites have been under intense investigation for their promise in optoelectronic applications and their novel and unique physical properties. There have been a variety of material implementations of perovskites from thin films to single crystals to nanocrystals. The nanocrystal form, in particular, is attractive as it enables solution processing and also spectroscopically probes both absorptive and emissive transitions. Broadly, the literature is comprised of experiments of either form, but the experiments are rarely performed in concert and are not discussed in a unified picture. For example, absorptive experiments are typically transient absorption measurements, which aim to measure carrier kinetics and dynamics. In contrast, the emissive experiments largely focus on excitonic fine structures and coupling to phonons. The time resolved emission experiments report on excited state lifetimes and their dependence on temperature. There are broad differences in the spectroscopy techniques and the questions asked in both classes of experiments. Yet there is one measure in common that suggests there are mysteries in our understanding of how the absorbing and emitting states are connected. The linewidth of emission spectra is always larger than the linewidth of absorption spectra. The question of the physics underlying linewidths is complex and is one of the central issues in perovskite nanocrystals. So why are the absorptive and emissive linewidths different? At present even this simple question has no clear answer. The more complex questions of the structure and dynamics of absorptive and emissive states are even more ambiguous. Hence there is a need to connect these experiments and the relevant states. Here, we provide an overview of the salient absorptive and emissive spectroscopy techniques in an effort to begin connecting these two disparate areas of inquiry.

Redox Chemistry of the Subphases of α-CsPbI2Br and β-CsPbI2Br: Theory Reveals New Potential for Photostability

The logic in the design of a halide-mixed APb(I1−xBrx)3 perovskite is quite straightforward: to combine the superior photovoltaic qualities of iodine-based perovskites with the increased stability of bromine-based perovskites. However, even small amounts of Br doped into the iodine-based materials leads to some instability. In the present report, using first-principles computations, we analyzed a wide variety of α-CsPbI2Br and β-CsPbI2Br phases, compared their mixing enthalpies, explored their oxidative properties, and calculated their hole-coupled and hole-free charged Frenkel defect (CFD) formations by considering all possible channels of oxidation. Nanoinclusions of bromine-rich phases in α-CsPbI2Br were shown to destabilize the material by inducing lattice strain, making it more susceptible to oxidation. The uniformly mixed phase of α-CsPbI2Br was shown to be highly susceptible towards a phase transformation into β-CsPbI2Br when halide interstitial or halide vacancy defects were introduced into the lattice. The rotation of PbI4Br2 octahedra in α-CsPbI2Br allows it either to transform into a highly unstable apical β-CsPbI2Br, which may phase-segregate and is susceptible to CFD, or to phase-transform into equatorial β-CsPbI2Br, which is resilient against the deleterious effects of hole oxidation (energies of oxidation >0 eV) and demixing (energy of mixing <0 eV). Thus, the selective preparation of equatorial β-CsPbI2Br offers an opportunity to obtain a mixed perovskite material with enhanced photostability and an intermediate bandgap between its constituent perovskites.

Lasing in Two-Dimensional Tin Perovskites

Two-dimensional (2D) perovskites have been proposed as materials capable of improving the stability and surpassing the radiative recombination efficiency of three-dimensional perovskites. However, their luminescent properties have often fallen short of what has been expected. In fact, despite attracting considerable attention for photonic applications during the last two decades, lasing in 2D perovskites remains unclear and under debate. Here, we were able to improve the optical gain properties of 2D perovskite and achieve optically pumped lasing. We show that the choice of the spacer cation affects the defectivity and photostability of the perovskite, which in turn influences its optical gain. Based on our synthetic strategy, we obtain PEA₂SnI₄ films with high crystallinity and favorable optical properties, resulting in amplified spontaneous emission (ASE) with a low threshold (30 μJ/cm²), a high optical gain above 4000 cm^-1 at 77 K, and ASE operation up to room temperature.

Large Exciton Polaron Formation in 2D Hybrid Perovskites via Time-Resolved Photoluminescence

We find evidence for the formation and relaxation of large exciton polarons in 2D organic-inorganic hybrid perovskites. Using ps-scale time-resolved photoluminescence within the phenethylammonium lead iodide family of compounds, we identify a red shifting of emission that we associate with exciton polaron formation time scales of 3-10 ps. Atomic substitutions of the phenethylammonium cation allow local control over the structure of the inorganic lattice, and we show that the structural differences among materials strongly influence the exciton polaron relaxation process, revealing a polaron binding energy that grows larger (up to 15 meV) in more strongly distorted compounds.

Nonequilibrium Lattice Dynamics in Photoexcited 2D Perovskites

Interplay between structural and photophysical properties of metal halide perovskites is critical to their utility in optoelectronics, but there is limited understanding of lattice response upon photoexcitation. Here, 2D perovskites butylammonium lead iodide, (BA)₂ PbI₄ , and phenethylammonium lead iodide, (PEA)₂ PbI₄ , are investigated using ultrafast transient X-ray diffraction as a function of optical excitation fluence to discern structural dynamics. Both powder X-ray diffraction and time-resolved photoluminescence linewidths narrow over 1 ns following optical excitation for the fluence range studied, concurrent with slight redshifting of the optical bandgaps. These observations are attributed to transient relaxation and ordering of distorted lead iodide octahedra stimulated mainly by electron-hole pair creation. The c axis expands up to 0.37% over hundreds of picoseconds; reflections sampling the a and b axes undergo one tenth of this expansion with the same timescale. Post-photoexcitation appearance of the (110) reflection in (BA)₂ PbI₄ would suggest a transient phase transition, however, through new single-crystal XRD, reflections are found that violate glide plane conditions in the reported Pbca structure. The static structure space group is reassigned as P2₁ 2₁ 2₁ . With this, a nonequilibrium phase transition is ruled out. These findings offer increased understanding of remarkable lattice response in 2D perovskites upon excitation.

Probing the Disorder Inside the Cubic Unit Cell of Halide Perovskites from First-Principles

Strong deviations in the finite temperature atomic structure of halide perovskites from their average geometry can have profound impacts on optoelectronic and other device-relevant properties. Detailed mechanistic understandings of these structural fluctuations and their consequences remain, however, limited by the experimental and theoretical challenges involved in characterizing strongly anharmonic vibrational characteristics and their impact on other properties. We overcome some of these challenges by a theoretical characterization of the vibrational interactions that occur among the atoms in the prototypical cubic CsPbBr3. Our investigation based on first-principles molecular dynamics calculations finds that the motions of neighboring Cs-Br atoms interlock, which appears as the most likely Cs-Br distance being significantly shorter than what is inferred from an ideal cubic structure. This form of dynamic Cs-Br coupling coincides with very shallow dynamic potential wells for Br motions that occur across a locally and dynamically disordered energy landscape. We reveal an interesting dynamic coupling mechanism among the atoms within the nominal unit cell of cubic CsPbBr3 and quantify the important local structural fluctuations on an atomic scale.

Diverging Expressions of Anharmonicity in Halide Perovskites

Lead-based halide perovskite crystals are shown to have strongly anharmonic structural dynamics. This behavior is important because it may be the origin of their exceptional photovoltaic properties. The double perovskite, Cs₂ AgBiBr₆ , has been recently studied as a lead-free alternative for optoelectronic applications. However, it does not exhibit the excellent photovoltaic activity of the lead-based halide perovskites. Therefore, to explore the correlation between the anharmonic structural dynamics and optoelectronic properties in lead-based halide perovskites, the structural dynamics of Cs₂ AgBiBr₆ are investigated and are compared to its lead-based analog, CsPbBr₃ . Using temperature-dependent Raman measurements, it is found that both materials are indeed strongly anharmonic. Nonetheless, the expression of their anharmonic behavior is markedly different. Cs₂ AgBiBr₆ has well-defined normal modes throughout the measured temperature range, while CsPbBr₃ exhibits a complete breakdown of the normal-mode picture above 200 K. It is suggested that the breakdown of the normal-mode picture implies that the average crystal structure may not be a proper starting point to understand the electronic properties of the crystal. In addition to our main findings, an unreported phase of Cs₂ AgBiBr₆ is also discovered below ≈37 K.

Visualizing Light-Induced Microstrain and Phase Transition in Lead-Free Perovskites Using Time-Resolved X-Ray Diffraction

Metal halide perovskites have emerged as promising materials for optoelectronic applications in the last decade. A large amount of effort has been made to investigate the interplay between the crystalline lattice and photoexcited charge carriers as it is vital to their optoelectronic performance. Among them, ultrafast laser spectroscopy has been intensively utilized to explore the charge carrier dynamics of perovskites, from which the local structural information can only be extracted indirectly. Here, we have applied a time-resolved X-ray diffraction technique to investigate the structural dynamics of prototypical two-dimensional lead-free halide perovskite Cs₃Bi₂Br₉ nanoparticles across temporal scales from 80 ps to microseconds. We observed a quick recoverable (a few ns) photoinduced microstrain up to 0.15% and a long existing lattice expansion (∼a few hundred nanoseconds) at mild laser fluence. Once the laser flux exceeds 1.4 mJ/cm², the microstrain saturates and the crystalline phase partially transfers into a disordered phase. This photoinduced transient structural change can recover within the nanosecond time scale. These results indicate that photoexcitation of charge carriers couples with lattice distortion, which fundamentally affects the dielectric environment and charge carrier transport.

Photo-induced enhancement of lattice fluctuations in metal-halide perovskites

The optoelectronic properties of metal-halide perovskites (MHPs) are affected by lattice fluctuations. Using ultrafast pump-probe spectroscopy, we demonstrate that in state-of-the-art mixed-cation MHPs ultrafast photo-induced bandgap narrowing occurs with a linear to super-linear dependence on the excited carrier density ranging from 10¹⁷ cm^-3 to above 10¹⁸ cm^-3. Time-domain terahertz spectroscopy reveals carrier localization increases with carrier density. Both observations, the anomalous dependence of the bandgap narrowing and the increased carrier localization can be rationalized by photo-induced lattice fluctuations. The magnitude of the photo-induced lattice fluctuations depends on the intrinsic instability of the MHP lattice. Our findings provide insight into ultrafast processes in MHPs following photoexcitation and thus help to develop a concise picture of the ultrafast photophysics of this important class of emerging semiconductors.

Photophysics of Two-Dimensional Semiconducting Organic-Inorganic Metal-Halide Perovskites

Two-dimensional organic-inorganic hybrid perovskites (2DHPs) consist of alternating anionic metal-halide and cationic organic layers. They have widely tunable structural and optical properties. We review the role of the organic cation in defining the structural and optical properties of 2DHPs through example lead iodide 2DHPs. Even though excitons reside in the metal-halide layers, the organic and inorganic frameworks cannot be separated-they must be considered as a single unit to fully understand the photophysics of 2DHPs. We correlate cation-induced distortion and disorder in the inorganic lattice with the resulting optical properties. We also discuss the role of the cation in creating and altering the discrete excitonic structure that appears at cryogenic temperatures in some 2DHPs, including the cation-dependent presence of hot-exciton photoluminescence. We conclude our review with an outlook for 2DHPs, highlighting existing gaps in fundamental knowledge as well as potential future applications. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Dynamic Exciton Polaron in Two-Dimensional Lead Halide Perovskites and Implications for Optoelectronic Applications

ConspectusThe past few years have witnessed an exciting revival of the research interest in two-dimensional (2D) lead halide perovskites. The renaissance is strongly motivated by the great success of their three-dimensional (3D) counterparts in optoelectronic applications. Different from 3D lead halide perovskites where free carriers are generated upon photoexcitation, 2D lead halide perovskites experience weaker dielectric screening and stronger quantum confinement effects. Therefore, strongly bound excitons with binding energy of up to a few hundreds of meV are considered to be the main excited-state species responsible for optoelectronic processes in 2D perovskites. In addition to strong excitonic effects, polaronic effects are also inherent in the soft and anharmonic lattice of lead halide perovskites, and polaronic structural relaxation is found to strongly renormalize carrier excited-state behaviors. For example, ferroelectric large polaron formation and liquid-like solvation of band edge carriers are proposed to account for the exceptional properties of 3D lead halide perovskites. As for 2D lead halide perovskites, polaronic characteristics have also been observed in exciton spectral characters, but how the interplay between excitonic effect and polaronic effect redefines the nature of exciton polarons and their excited-state behaviors still remains largely unexplored.In this Account, we discuss our recent experimental findings about the excited-state properties of exciton polarons in 2D lead halide perovskites. We begin our discussion by introducing a conventional view of strongly bound excitons in 2D lead halide perovskites with large exciton binding energy, which is typically estimated from steady-state absorption spectra. However, owing to the soft and anharmonic lattice, excitons in 2D lead halide perovskites exhibit significant polaronic characters and exist as exciton polarons. It is still unclear how polaronic effects would affect the exciton properties in 2D lead halide perovskites, especially in their excited-state dynamics. By probing exchange interaction, we found that both intra- and inter-exciton Coulomb interaction strengths are substantially weakened by the polaronic screening effect, which is manifested as (1) a counterintuitively longer exciton spin lifetime by almost an order of magnitude or a smaller intraexcitonic interaction strength with temperature increasing from 80 to 340 K and (2) an order of magnitude smaller interexcitonic interaction strength compared to another prototypical 2D semiconductor named transition-metal dichalcogenides (TMDCs) with a comparable steady-state exciton binding energy. We further discuss the interplay between the long- and short-range exciton-phonon interaction and conclude that the exciton-phonon interaction strength is in an intermediate regime and the exciton polaron is momentarily trapped in 2D perovskites, that is, a dynamic exciton polaron.Finally, we highlight prospective opportunities with ligand and cation engineering to regulate the exciton-phonon interaction and exciton polaron properties in 2D perovskites, which have strong implications toward future rational design for 2D perovskite-based efficient photovoltaics or light-emitting devices with high color purity.

Thermochromism Perovskite (COOH(CH2)3NH3)2PbI4 Crystals: Single-Crystal to Single-Crystal Phase Transition and Excitation-Wavelength-Dependent Emission

As a potential multifunctional phase transition material, the organic-inorganic hybrid perovskite has attracted extensive attention in recent years. Here, we report the single-crystal to single-crystal phase transition and excitation-wavelength-dependent emission (EDE) of layered perovskite (COOH(CH₂)₃NH₃)₂PbI₄. Single-crystal X-ray diffraction indicated that the crystal structure changes from layered Ruddlesden-Popper (RP) at 302 K to "X" network composed of face-sharing and corner-sharing [PbX₆]^4- octahedra at 425 K. The material exhibits thermochromic change from orange to yellow at higher temperature. Considering the thermochromism of the material, we apply it for anticounterfeiting and information encryption. The material exhibits EDE properties with a fluorescence color changing from green to red upon 420 and 546 nm excitation, respectively. Time-dependent density functional theory indicated that this phenomenon is mainly related to the laser-induced crystal structural transfer. Our research shows that the (COOH(CH₂)₃NH₃)₂PbI₄ crystal has a potential application for multifunctional devices.

Exploring the influence of polymorphism and chromophore co-ligands on linkage isomer photoswitching in [Pd(bpy4dca)(NO2)2]

The polymorphic Pd(II)-nitrite complex [Pd(bpy4dca)(NO2)2] (1) (bpy4dca = 2,2’-bipyridine-4,4’-dicarboxylic acid methyl ester) is shown to undergo photoinduced nitro → nitrito linkage isomer switching in two crystal forms, to varying excited...

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

Impedance spectroscopy (IS) provides a detailed understanding of the dynamic phenomena underlying the operation of photovoltaic and optoelectronic devices. Here we provide a broad summary of the application of IS to metal halide perovskite materials, solar cells, electrooptic and memory devices. IS has been widely used to characterize perovskite solar cells, but the variability of samples and the presence of coupled ionic-electronic effects form a complex problem that has not been fully solved yet. We summarize the understanding that has been obtained so far, the basic methods and models, as well as the challenging points still present in this research field. Our approach emphasizes the importance of the equivalent circuit for monitoring the parameters that describe the response and providing a physical interpretation. We discuss the possibilities of models from the general perspective of solar cell behavior, and we describe the specific aspects and properties of the metal halide perovskites. We analyze the impact of the ionic effects and the memory effects, and we describe the combination of light-modulated techniques such as intensity modulated photocurrent spectroscopy (IMPS) for obtaining more detailed information in complex cases. The transformation of the frequency to time domain is discussed for the consistent interpretation of time transient techniques and the prediction of features of current-voltage hysteresis. We discuss in detail the stability issues and the occurrence of transformations of the sample coupled to the measurements.

Two-dimensional overdamped fluctuations of the soft perovskite lattice in CsPbBr3

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr₃ crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties.

Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskite Nanocrystals

The development of next-generation perovskite-based optoelectronic devices relies critically on the understanding of the interaction between charge carriers and the polar lattice in out-of-equilibrium conditions. While it has become increasingly evident for CsPbBr3 perovskites that the Pb-Br framework flexibility plays a key role in their light-activated functionality, the corresponding local structural rearrangement has not yet been unambiguously identified. In this work, we demonstrate that the photoinduced lattice changes in the system are due to a specific polaronic distortion, associated with the activation of a longitudinal optical phonon mode at 18 meV by electron-phonon coupling, and we quantify the associated structural changes with atomic-level precision. Key to this achievement is the combination of time-resolved and temperature-dependent studies at Br K and Pb L3 X-ray absorption edges with refined ab initio simulations, which fully account for the screened core-hole final state effects on the X-ray absorption spectra. From the temporal kinetics, we show that carrier recombination reversibly unlocks the structural deformation at both Br and Pb sites. The comparison with the temperature-dependent XAS results rules out thermal effects as the primary source of distortion of the Pb-Br bonding motif during photoexcitation. Our work provides a comprehensive description of the CsPbBr3 perovskites' photophysics, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.

Strongly Anharmonic Octahedral Tilting in Two-Dimensional Hybrid Halide Perovskites

Recent investigations of two-dimensional (2D) hybrid organic-inorganic halide perovskites (HHPs) indicate that their optical and electronic properties are dominated by strong coupling to thermal fluctuations. While the optical properties of 2D-HHPs have been extensively studied, a comprehensive understanding of electron-phonon interactions is limited because little is known about their structural dynamics. This is partially because the unit cells of 2D-HHPs contain many atoms. Therefore, the thermal fluctuations are complex and difficult to elucidate in detail. To overcome this challenge, we use polarization-orientation Raman spectroscopy and ab initio calculations to compare the structural dynamics of the prototypical 2D-HHPs [(BA)2PbI4 and (PhE)2PbI4] to their three-dimensional (3D) counterpart, MAPbI3. Comparison to the simpler, 3D MAPbI3 crystal shows clear similarities with the structural dynamics of (BA)2PbI4 and (PhE)2PbI4 across a wide temperature range. The analogy between the 3D and 2D crystals allows us to isolate the effect of the organic cation on the structural dynamics of the inorganic scaffold of the 2D-HHPs. Furthermore, using this approach, we uncover the mechanism of the order-disorder phase transition of (BA)2PbI4 (274 K) and show that it involves relaxation of octahedral tilting coupled to anharmonic thermal fluctuations. These anharmonic fluctuations are important because they induce charge carrier localization and affect the optoelectronic performance of these materials.

Picosecond Charge Localization Dynamics in CH3NH3PbI3 Perovskite Probed by Infrared-Activated Vibrations

Hybrid metal halide perovskites exhibit well-defined semiconducting properties and efficient optoelectronic performance considering their soft crystal structure and low-energy lattice motions. The response of such a crystal lattice to light-induced charges is a fundamental question, for which experimental insight into ultrafast time scales is still sought. Here, we use infrared-activated vibrations (IRAV) of the organic components within the hybrid perovskite lattice as a sensitive probe for local structural reorganizations after photoexcitation, with femtosecond resolution. We find that the IRAV signal response shows a delayed rise of about 3 ps and subsequent decay of pronounced monomolecular character, distinguishing it from absorption associated with free carriers. We interpret our results as a two-step carrier localization process. Initially, carriers localize transiently in local energy minima formed by lattice fluctuations. A subpopulation of these can then fall into deeper trapped states over picoseconds, likely due to local reorganization of the organic molecules surrounding the carriers.

Coherent vibrational dynamics reveals lattice anharmonicity in organic-inorganic halide perovskite nanocrystals

The halide ions of organic-inorganic hybrid perovskites can strongly influence the interaction between the central organic moiety and the inorganic metal halide octahedral units and thus their lattice vibrations. Here, we report the halide-ion-dependent vibrational coherences in formamidinium lead halide (FAPbX₃, X = Br, I) perovskite nanocrystals (PNCs) via the combination of femtosecond pump–probe spectroscopy and density functional theory calculations. We find that the FAPbX₃ PNCs generate halide-dependent coherent vibronic wave packets upon above-bandgap non-resonant excitation. More importantly, we observe several higher harmonics of the fundamental modes for FAPbI₃ PNCs as compared to FAPbBr₃ PNCs. This is likely due to the weaker interaction between the central FA moiety and the inorganic cage for FAPbI₃ PNCs, and thus the PbI₆⁴⁻ unit can vibrate more freely. This weakening reveals the intrinsic anharmonicity in the Pb-I framework, and thus facilitating the energy transfer into overtone and combination bands. These findings not only unveil the superior stability of Br–based PNCs over I–based PNCs but are also important for a better understanding of their electronic and polaronic properties.

Using a combination of femtosecond pump-probe spectroscopy and first-principles calculations, Debnath et al. elucidated the halide-dependence of the excited state vibrational coherences in hybrid organic-inorganic perovskite nanocrystals. The study revealed an intrinsic anharmonicity of lead-halide framework, which correlates with perovskite stability and is influenced by the interaction between the framework and the organic molecules.

Visualization of dynamic polaronic strain fields in hybrid lead halide perovskites

Excitation localization involving dynamic nanoscale distortions is a central aspect of photocatalysis¹, quantum materials² and molecular optoelectronics³. Experimental characterization of such distortions requires techniques sensitive to the formation of point-defect-like local structural rearrangements in real time. Here, we visualize excitation-induced strain fields in a prototypical member of the lead halide perovskites⁴ via femtosecond resolution diffuse X-ray scattering measurements. This enables momentum-resolved phonon spectroscopy of the locally distorted structure and reveals radially expanding nanometre-scale strain fields associated with the formation and relaxation of polarons in photoexcited perovskites. Quantitative estimates of the magnitude and shape of this polaronic distortion are obtained, providing direct insights into the dynamic structural distortions that occur in these materials^5-9. Optical pump-probe reflection spectroscopy corroborates these results and shows how these large polaronic distortions transiently modify the carrier effective mass, providing a unified picture of the coupled structural and electronic dynamics that underlie the optoelectronic functionality of the hybrid perovskites.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals

Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.

Charge trapping can lead to severe nonradiative losses in colloidal semiconductor nanocrystals (NCs). The authors report femtosecond electron diffraction measurements on photoexcited NCs to reveal atomic-scale insights into how localization of charges at trap sites induce surface deformations.

Dynamic structural property of organic-inorganic metal halide perovskite

Unique organic-inorganic hybrid semiconducting materials have made a remarkable breakthrough in new class of photovoltaics (PVs). Organic-inorganic metal (Pb and/or Sn) halides (-I, -Br, and -Cl) are the semiconducting absorber with the crystal structure of the famous "Perovskite". It is widely called "perovskite solar cells (PSCs)" in PV society. Now, the power conversion efficiency (PCE) of PSCs is recorded in 25.5%. Prototypical composition of the absorbers is (A = methylammonium [MA], formamidinium [FA], and Cs), (M = Pb and/or Sn), and (X = I, Br, and Cl) in the form of perovskite AMX₃. Since the report on the stable all solid-state PSCs in 2012, the average annual growth rate of PCE is well over ∼10%. Such an outstanding PV performance attracts huge number of scientists in our research society. Their chemical as well as physical properties are dramatically different from monocrystalline Si, GaAs, other III-IV semiconductors, and many oxides with the crystal structure of perovskite. In this review, different fundamental aspects, in particular, the dynamic properties of A site cationic molecules and PbI₆ octahedrons linked with their corners, from other semiconducting and dielectric materials are reviewed and summarized. Upon discussing unique properties, perspectives on the promising PV applications based on the comprehension in dynamic nature of the orientation in A site molecule and PbI₆ octahedron tilting will be given.

Static Rashba Effect by Surface Reconstruction and Photon Recycling in the Dynamic Indirect Gap of APbBr3 (A = Cs, CH3NH3) Single Crystals

Recently, halide perovskites have gained significant attention from the perspective of efficient spintronics owing to the Rashba effect. This effect occurs as a consequence of strong spin-orbit coupling under a noncentrosymmetric environment, which can be dynamic and/or static. However, there exist intense debates on the origin of broken inversion symmetry since the halide perovskites typically crystallize into a centrosymmetric structure. In order to clarify the issue, we examine both dynamic and static effects in the all-inorganic CsPbBr₃ and organic-inorganic CH₃NH₃PbBr₃ (MAPbBr₃) perovskite single crystals by employing temperature- and polarization-dependent photoluminescence excitation spectroscopy. The perovskite single crystals manifest the dynamic effect by photon recycling in the indirect Rashba gap, causing dual peaks in the photoluminescence. However, the effect vanishes in CsPbBr₃ at low temperatures (<50 K) accompanied by a striking color change of the crystal, arising presumably from lower degrees of freedom for inversion symmetry breaking associated with the thermal motion of the spherical Cs cation compared with the polar MA cation in MAPbBr₃. We also show that the static Rashba effect occurs only in MAPbBr₃ below 90 K, presumably due to surface reconstruction via MA-cation ordering, which likely extends across a few layers from the crystal surface to the interior. We further demonstrate that this static Rashba effect can be completely suppressed upon surface treatment with polymethyl methacrylate (PMMA) coating. We believe that our results provide a rationale for the Rashba effects in halide perovskites.

Transmission electron microscopy of organic-inorganic hybrid perovskites: myths and truths

Organic-inorganic hybrid perovskites (OIHPs) have attracted extensive research interest as a promising candidate for efficient and inexpensive solar cells. Transmission electron microscopy (TEM) characterizations that can benefit the fundamental understanding and the degradation mechanism are widely used for these materials. However, their sensitivity to the electron beam illumination and hence structural instabilities usually prevent us from obtaining the intrinsic information or even lead to significant artifacts. Here, we systematically investigate the structural degradation behaviors under different experimental factors to reveal the optimized conditions for TEM characterizations of OIHPs by using low-dose electron diffraction and imaging techniques. We find that a low temperature (-180 °C) does not slow down the beam damage but instead induces a rapid amorphization for OIHPs. Moreover, a less severe damage is observed at a higher accelerating voltage. The beam-sensitivity is found to be facet-dependent that a (1 0 0) exposed CH₃NH₃PbI₃ (MAPbI₃) surface is more stable than a (0 0 1) surface. With these guidance, we successfully acquire the atomic structure of pristine MAPbI₃ and identify the characterization window that is very narrow. These findings are helpful to guide future electron microscopy characterizations of these beam-sensitive materials, which are also useful for finding strategies to improve the stability and performance of the perovskite solar cells.

Solution-Phase Halide Exchange and Targeted Annealing Kinetics in Lead Chloride Derived Hybrid Perovskites

Hybrid perovskites are a promising class of materials for a range of optoelectronic applications. Many material properties are dictated by the details of the synthetic process, yet a mechanistic understanding is lacking for the majority of these materials. We have studied the formation of methylammonium lead iodide films derived from a lead chloride precursor to understand both the casting solution chemistry and its influence on the final, largely chlorine-free, film. Using solution-phase extended X-ray absorption spectroscopy, we observe a halide exchange with the primary solution plumbate species identified as PbI_2.5Cl_0.33. The mixed halide plumbate solution species leads to formation of the crystalline intermediate phase of (CH₃NH₃)₂PbI₃Cl. Using in situ synchrotron X-ray diffraction, we show that compositional control of the casting solution can control the annealing kinetics of film formation. Our study demonstrates the importance of solution-phase chemistry and its impact on lead halide perovskite synthesis.

Single Ensemble Non-exponential Photoluminescent Population Decays from a Broadband White-Light-Emitting Perovskite

The mechanism of white-light emission from layered Pb-X (X = Cl or Br) perovskites following UV excitation has generated considerable interest. Prior time-dependent studies indicated that the broadband photoluminescence (PL) from (110) perovskites arises from a distribution of self-trapped excitonic sites emitting in different regions of the visible spectrum with different decay dynamics. Here, using time-correlated single photon counting to study single crystals, we show that the white-light emission decay from the (110) perovskite (EDBE)PbBr₄ (EDBE = 2,2'-(ethylenedioxy)bis(ethylammonium)) behaves as a single ensemble. Following the rapid decay (0.6 ns) of a small spectral side band, the broad emission line shape is constant to 100 ns. We propose that rapid local structural fluctuations cause the self-trapped excitons (STEs) to experience a wide range of energies, resulting in the very broad PL. The STEs sample fluctuating local environments on time scales fast compared to the PL, which averages the PL decay at all emission wavelengths, yielding single ensemble PL dynamics. Although emission occurs from a very wide, inhomogeneously broadened spectral line with time-averaged single ensemble luminescence dynamics, the decay is tri-exponential. Two heuristic models for the tri-exponential decay involving defects are discussed. Spin-coated films show faster non-exponential decays with the slowest component of the crystal PL absent. Like the crystals, the film PL decays as a single ensemble. These results demonstrate that the broadband emission decay of (EDBE)PbBr₄ arises from a time-averaged single ensemble and not from a set of excited states emitting with distinct luminescence decays at different wavelengths.

Photoinduced Vibrations Drive Ultrafast Structural Distortion in Lead Halide Perovskite

The success of organic–inorganic perovskites in optoelectronics is dictated by the complex interplay between various underlying microscopic phenomena. The structural dynamics of organic cations and the inorganic sublattice after photoexcitation are hypothesized to have a direct effect on the material properties, thereby affecting the overall device performance. Here, we use ultrafast heterodyne-detected two-dimensional (2D) electronic spectroscopy to reveal impulsively excited vibrational modes of methylammonium (MA) lead iodide perovskite, which drive the structural distortion after photoexcitation. Vibrational analysis of the measured data allows us to monitor the time-evolved librational motion of the MA cation along with the vibrational coherences of the inorganic sublattice. Wavelet analysis of the observed vibrational coherences reveals the coherent generation of the librational motion of the MA cation within ∼300 fs complemented with the coherent evolution of the inorganic skeletal motion. To rationalize this observation, we employed the configuration interaction singles (CIS), which support our experimental observations of the coherent generation of librational motions in the MA cation and highlight the importance of the anharmonic interaction between the MA cation and the inorganic sublattice. Moreover, our advanced theoretical calculations predict the transfer of the photoinduced vibrational coherence from the MA cation to the inorganic sublattice, leading to reorganization of the lattice to form a polaronic state with a long lifetime. Our study uncovers the interplay of the organic cation and inorganic sublattice during formation of the polaron, which may lead to novel design principles for the next generation of perovskite solar cell materials.

Efficient Lone-Pair-Driven Luminescence: Structure-Property Relationships in Emissive 5s2 Metal Halides

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of hybrid lead halide perovskites as optoelectronic materials. In particular, the light emission of low-dimensional halides based on the 5s2 cations Sn2+ and Sb3+ has found utility in a variety of applications complementary to those of the three-dimensional halide perovskites because of its unusual properties such as broadband character and highly temperature-dependent lifetime. These properties derive from the exceptional chemistry of the 5s2 lone pair, but the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials that would lead to the development of efficient optoelectronic devices. In this Perspective, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s2 lone pair to identify the structural features that enable strong emission. We unite the different theoretical models that have been able to explain the success of these bright 5s2 emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We highlight those state-of-the-art materials and applications that demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s2 metal halides.

Optomechanical Effects Occurring in a Hybrid Metal-Halide Perovskite Single Crystal Based on Photoinduced Resonant Ultrasound Spectroscopy

This letter reports optomechanical effects occurring in a hybrid metal-halide perovskite single crystal (MAPbBr₃) based on resonant ultrasound spectroscopy (RUS) measurements under continuous wave (CW) laser illumination. The optomechanical effects are a new phenomenon in hybrid perovskite single crystals where the elastic constant of a single crystal is measured by RUS probed under varying excitation conditions. Our studies show that applying a CW laser (405 nm) to the single-crystal face shifts the RUS peaks to higher frequencies by about 1-4% in the perovskite single crystal at room temperature. The light-induced shift of the RUS peaks can be observed only when photoexcitation is occurring, rather than during heating, by positioning the laser wavelength within the optical absorption spectrum. In contrast, positioning the laser wavelength outside of the optical absorption spectrum leads to an absence of RUS peak shifting. Clearly, the laser-light-induced RUS peak shifts shows that the crystal elastic moduli can be changed by photoexcitation, leading to an optomechanical phenomenon via excited states. Essentially, the observed optomechanical phenomenon reflects the fact that the mechanical properties can be optically changed through internal repulsive and attractive force constants by external photoexcitation in a hybrid perovskite single crystal.

General Decomposition Pathway of Organic-Inorganic Hybrid Perovskites through an Intermediate Superstructure and its Suppression Mechanism

Organic-inorganic hybrid perovskites (OIHPs) have generated considerable excitement due to their promising photovoltaic performance. However, the commercialization of perovskite solar cells (PSCs) is still plagued by the structural degradation of the OIHPs. Here, the decomposition mechanism of OIHPs under electron beam irradiation is investigated via transmission electron microscopy, and a general decomposition pathway for both tetragonal CH₃ NH₃ PbI₃ and cubic CH₃ NH₃ PbBr₃ is uncovered through an intermediate superstructure state of CH₃ NH₃ PbX_2.5 , X = I, Br, with ordered vacancies into final lead halides. Such decomposition can be suppressed via carbon coating by stabilization of the perovskite structure framework. These findings reveal the general degradation pathway of OIHPs and suggest an effective strategy to suppress it, and the atomistic insight learnt may be useful for improving the stability of PSCs.