Pressure-induced dramatic changes in organic-inorganic halide perovskites

The effective prolongation of the excited-state carrier lifetime of CsPbI2Br with applying strain

In recent years, all-inorganic perovskites CsPbX3 (X = Cl, Br, I) have emerged as excellent candidates for solar cells due to their remarkable thermal stability and suitable bandgaps. Among them, CsPbI2Br is a hotspot in perovskite material research currently. Non-radiative electron-hole recombination often leads to significant energy losses, impacting the efficiency of solar cells, so a thorough understanding of carrier recombination mechanisms is crucial. Our work investigated the carrier recombination dynamics in detail and proved that strains can effectively reduce nonradiative recombination. In this study, using first-principles calculations combined with nonadiabatic (NA) molecular dynamics (MD), we demonstrate that applying 2% tensile and 2% compressive strains to CsPbI2Br can modify the bandgap, induce moderate disorder, reduce the overlap of electron-hole wavefunctions, decrease NA coupling, and shorten decoherence time, thereby minimizing non-radiative recombination and extending the carrier lifetime. Especially the 2% tensile strain exhibits more effective control performance, significantly reducing non-radiative electron-hole recombination and extending the charge carrier lifetime to 14.59 ns, nearly five times that of the pristine CsPbI2Br system (3.12 ns). This study reveals the impact mechanism of strain on carrier behavior in perovskite solar cells, providing a new non-chemical strategy for modulating the lifetime of photo-generated carriers and enhancing the efficiency of all-inorganic perovskite solar cells.

Orthorhombic lead-free hybrid perovskite CH3NH3SnI3 under strain: an ab initio study

We report a computational study where we explore the possibility of tuning the electronic properties of orthorhombic methylammonium tin iodide CH3NH3SnI3 using strains. According to our findings, a moderate [001] strain, smaller than 2%, would open the band gap up to 1.25 eV and enhance the exciton binding energy, opening up new possibilities for the use of CH3NH3SnI3 in technological applications. To better understand the impact of strain, we also examined its influence on bonding properties. The results reveal that the directional pnictogen and the hydrogen bonding are not altered by strains and that the tuning of the electronic properties is the result of changes induced in the orbital contributions to states near the Fermi level and the tilting of the SnI6 octahedral units.

Solvent-free preparation and thermocompression self-assembly: an exploration of performance improvement strategies for perovskite solar cells

Perovskite solar cells (PSCs) exhibit sufficient technological efficiency and economic competitiveness. However, their poor stability and scalability are crucial factors limiting their rapid development. Therefore, achieving both high efficiency and good stability is an urgent challenge. In addition, the preparation methods for PSCs are currently limited to laboratory-scale methods, so their commercialization requires further research. Effective packaging technology is essential to protect the PSCs from degradation by external environmental factors and ensure their long-term stability. The industrialization of PSCs is also inseparable from the preparation technology of perovskite thin films. This review discusses the solvent-free preparation of PSCs, shedding light on the factors that affect PSC performance and strategies for performance enhancement. Furthermore, this review analyzes the existing simulation techniques that have contributed to a better understanding of the interfacial evolution of PSCs during the packaging process. Finally, the current challenges and possible solutions are highlighted, providing insights to facilitate the development of highly efficient and stable PSC modules to promote their widespread application.

Band gap tuning of perovskite solar cells for enhancing the efficiency and stability: issues and prospects

The intriguing optoelectronic properties, diverse applications, and facile fabrication techniques of perovskite materials have garnered substantial research interest worldwide. Their outstanding performance in solar cell applications and excellent efficiency at the lab scale have already been proven. However, owing to their low stability, the widespread manufacturing of perovskite solar cells (PSCs) for commercialization is still far off. Several instability factors of PSCs, including the intrinsic and extrinsic instability of perovskite materials, have already been identified, and a variety of approaches have been adopted to improve the material quality, stability, and efficiency of PSCs. In this review, we have comprehensively presented the significance of band gap tuning in achieving both high-performance and high-stability PSCs in the presence of various degradation factors. By investigating the mechanisms of band gap engineering, we have highlighted its pivotal role in optimizing PSCs for improved efficiency and resilience.

Hybrid-DFT study of halide perovskites, an energy-efficient material under compressive pressure for piezoelectric applications

Recent studies have reported that lead-halide perovskites are the most efficient energy-harvesting materials. Regardless of their high-output energy and structural stability, lead-based products have risk factors due to their toxicity. Therefore, lead-free perovskites that offer green energy are the expected alternatives. We have taken CsGeX3(X = Cl, Br, and I) as lead-free halide perovskites despite knowing the low power conversion rate. Herein, we have tried to study the mechanisms of enhancement of energy-harvesting capabilities involving an interplay between structure and electronic properties. A density functional theory simulation of these materials shows a decrease in the band gaps, lattice parameters, and volumes with increasing applied pressure. We report the high piezoelectric responses and high electro-mechanical conversion rates, which are intriguing for generating electricity through mechanical stress.

Time-Resolved Dynamics of Metal Halide Perovskite under High Pressure: Recent Progress and Challenges

Metal halide perovskites have garnered significant attention in the scientific community for their promising applications in optoelectronic devices. The application of pressure engineering, a viable technique, has played a crucial role in substantially improving the optoelectronic characteristics of perovskites. Despite notable progress in understanding ground-state structural changes under high pressure, a comprehensive exploration of excited-state dynamics influencing luminescence remains incomplete. This Perspective delves into recent advances in time-resolved dynamics studies of photoexcited metal halide perovskites under high pressure. With a focus on the intricate interplay between structural alterations and electronic properties, we investigate electron-phonon interactions, carrier transport mechanisms, and the influential roles of self-trapped excitons (STEs) and coherent phonons in luminescence. However, significant challenges persist, notably the need for more advanced measurement techniques and a deeper understanding of the phenomena induced by high pressure in perovskites.

Towards Long-Term Stable Perovskite Solar Cells: Degradation Mechanisms and Stabilization Techniques

It is certain that perovskite materials must be a game-changer in the solar industry as long as their stability reaches a level comparable with the lifetime of a commercialized Si photovoltaic. However, the operational stability of perovskite solar cells and modules still remains unresolved, especially when devices operate in practical energy-harvesting modes represented by maximum power point tracking under 1 sun illumination at ambient conditions. This review article covers from fundamental aspects of perovskite instability including chemical decomposition pathways under light soaking and electrical bias, to recent advances and techniques that effectively prevent such degradation of perovskite solar cells and modules. In particular, fundamental causes for permanent degradation due to ion migration and trapped charges are overviewed and explain their interplay between ions and charges. Based on the degradation mechanism, recent advances on the strategies are discussed to slow down the degradation during operation for a practical use of perovskite-based solar devices.

Photophysical Behavior of Triethylmethylammonium Tetrabromoferrate(III) under High Pressure

The pressure-induced properties of hybrid organic-inorganic ferroelectrics (HOIFs) with tunable structures and selectable organic and inorganic components are important for device fabrication. However, given the structural complexity of polycrystalline HOIFs and the limited resolution of pressure data, resolving the structure-property puzzle has so far been the exception rather than the rule. With this in mind, we present a collection of in situ high-pressure data measured for triethylmethylammonium tetrabromoferrate(III), ([N(C2H5)3CH3][FeBr4]) (EMAFB) by unraveling its flexible physical and photophysical behavior up to 80 GPa. Pressure-driven X-ray diffraction and Raman spectroscopy disclose its soft and reversible structural distortion, creating room for delicate band gap modulation. During compression, orange turns dark red at ∼2 GPa, and further compression results in piezochromism, leading to opaque black, while decompressed EMAFB appears in an orange hue. Assuming that the mechanical softness of EMAFB is the basis for reversible piezochromic control, we present alternations in the electronic landscape leading to a 1.22 eV band narrowing at 20.3 GPa while maintaining the semiconducting character at 72 GPa. EMAFB exhibits an emission enhancement, manifested by an increase of photoluminescence up to 17.3 GPa, correlating with the onsets of structural distortion and amorphization. The stimuli-responsive behavior of EMAFB, exhibiting stress-activated modification of the electronic structure, can enrich the physical library of HOIFs suitable for pressure-sensing technologies.

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Structural effects on the luminescence properties of CsPbI3 nanocrystals

Metal halide perovskite nanocrystals (NCs) are promising for photovoltaic and light-emitting applications. Due to the softness of their crystal lattice, structural modifications have a critical impact on their optoelectronic properties. Here we investigate the size-dependent optoelectronic properties of CsPbI3 NCs ranging from 7 to 17 nm, employing temperature and pressure as thermodynamic variables to modulate the energetics of the system and selectively tune the interatomic distances. By temperature-dependent photoluminescence spectroscopy, we have found that luminescence quenching channels exhibit increased non-radiative losses and weaker exciton-phonon coupling in bigger particles, in turn affecting the luminescence efficiency. Through pressure-dependent measurements up to 2.5 GPa, supported by XRD characterization, we revealed a NC-size dependent solid-solid phase transition from the γ-phase to the δ-phase. Importantly, the optical response to these structural changes strongly depends on the size of the NC. Our findings provide an interesting guideline to correlate the size and structural and optoelectronic properties of CsPbI3 NCs, important for engineering the functionalities of this class of soft semiconductors.

The role of atmospheric conditions in the nonradiative recombination in individual CH3NH3PbI3 perovskite crystals

Organic-inorganic metal halide perovskites have been emerging as potential candidates for lightweight photovoltaic applications in space. However, fundamental physics concerning the effect of atmosphere on the radiative and nonradiative recombination in perovskites remains far from well understood. Here, we investigate the creation and annihilation of nonradiative recombination centers in individual CH₃NH₃PbI₃ perovskite crystals by controlling the atmospheric conditions. We find that the photoluminescence (PL) of individual perovskite crystals can be quenched upon exposure from air to vacuum, while the subsequent PL enhancement in air shows a pressure dependence. Further analysis attributes the PL decline in vacuum to the activation of nonradiative trap sites, which is likely due to the lattice distortion caused by the variation of local strain on perovskites. With a gradual increase of the air pressure, the light-assisted chemisorption of oxygen on perovskite will passivate these nonradiative trap sites while simultaneously restoring the lattice imperfection, leading to PL enhancement. The present findings suggest that placing the perovskite in an environment with moderate oxygen content can protect the material from photophysical losses that can be pronounced under inert conditions.

Room-Temperature Doping of CsPbBr3 Nanocrystals with Aluminum

B-site doping is an emerging strategy for tuning the emission wavelength of cesium lead halide ABX₃ nanocrystals. We present a simple method for the postsynthetic doping of CsPbBr₃ nanocrystals with aluminum at room temperature by exposing them to a solution of AlBr₃ in dibromomethane. Despite the much smaller ionic radius of Al³⁺ compared to that of Pb²⁺, nominal doping levels in a range from 8.1% to 24.3% were obtained when increasing the Al/Pb feed ratio from 1 to 4.5. Al³⁺ introduction leads to a hypsochromic shift of the photoluminescence (PL) emission of the CsPbBr₃ nanocrystals. The PL peak position is highly stable over at least 6 months and tunable in a range of 510 to 480 nm by increasing the doping level. Structural analyses revealed a linear correlation between the PL energy and the lattice parameter with a slope of -1.96 eV/Å.

Regulating Exciton-Phonon Coupling to Achieve a Near-Unity Photoluminescence Quantum Yield in One-Dimensional Hybrid Metal Halides

Low-dimensional hybrid metal halides are emerging as a highly promising class of single-component white-emitting materials for their unique broadband emission from self-trapped excitons (STEs). Despite substantial progress in the development of these metal halides, many challenges remain to be addressed to obtain a better fundamental understanding of the structure-property relationship and realize the full potentials of this class of materials. Here, via pressure regulation, a near 100% photoluminescence quantum yield (PLQY) of broadband emission is achieved in a corrugated 1D hybrid metal halide C5 N2 H16 Pb2 Br6 , which possesses a highly distorted structure with an initial PLQY of 10%. Compression reduces the overlap between STE states and ground state, leading to a suppressed phonon-assisted non-radiative decay. The PL evolution is systematically demonstrated to be controlled by the pressure-regulated exciton-phonon coupling which can be quantified using Huang-Rhys factor S. Detailed studies of the S-PLQY relation for a series of 1D hybrid metal halides (C5 N2 H16 Pb2 Br6 , C4 N2 H14 PbBr4 , C6 N2 H16 PbBr4 , and (C6 N2 H16 )3 Pb2 Br10 ) reveal a quantitative structure-property relationship that regulating S factor toward 28 leads to the maximum emission.

3D-to-2D Transition of Anion Vacancy Mobility in CsPbBr3 under Hydrostatic Pressure

Anion vacancy migration in the orthorhombic Pnma phase of the lead-halide perovskite CsPbBr₃ under hydrostatic pressure is studied computationally. Density functional theory calculations are used to determine transition states, activation enthalpies, and attempt frequencies for vacancies to hop between nearby lattice sites, under pressure in the range 0.0-2.0 GPa. The resulting data are used to parametrize a kinetic model of vacancy migration under the influence of an electric field, which is solved in the steady state to determine the anion vacancy mobility tensor as a function of pressure. It is found that the mobility tensor becomes increasingly anisotropic with increasing pressure, such that at 2.0 GPa, the mobility within the (010) lattice plane is 3 orders of magnitude greater than the mobility normal to it. The results demonstrate the potentially significant influence of pressure, and by extension, other forms of stress, on defect migration in lead-halide perovskites.

High-Pressure Structural Phase Transformation of Ferroelectric Bis-benzylammonium Lead Tetrachloride Studied by Raman Spectroscopy and X-ray Diffraction

Hybrid organic-inorganic 2-D perovskite bis-benzylammonium lead tetrachloride (BALC) is a room-temperature ferroelectric semiconductor. A structural phase transformation from the ambient Cmc2₁ structure is evident at 1.8 GPa from the Raman spectra, and this is confirmed by our high-pressure X-ray diffraction studies that point to a centrosymmetric structure Cmcm at 1.7 GPa. The ambient phase is recoverable on decompression. Using density functional theory calculations, we have studied the intermolecular and intramolecular vibrations to get an idea of the structural changes as a function of pressure. The high-pressure transition is identified to be due to a distortion in the PbCl₆ octahedra and a conformation change in the molecule. There are several discontinuities, broadening, and splitting of the Raman bands, corresponding to NH₃ units above 1.8 GPa that point to rearrangements in the hydrogen bond network in the new phase. The ambient structure shows anisotropic compressibility, with a bulk modulus of 14.5 ± 0.33 GPa. As the new phase is a centrosymmetric structure, BALC is expected to lose its ferroelectricity above ∼1.8 GPa.

Ultra-flexible and waterproof perovskite photovoltaics for washable power source applications

A washable perovskite solar cell with high efficiency (over 11%) and outstanding crumpling durability (maintaining 81.2% after 100 cycles crumpling) is demonstrated herein by combining the flexible self-encapsulation method with a waterproof glue coated substrate.

Microscopic Picture of Electron-Phonon Interaction in Two-Dimensional Halide Perovskites

Perovskites have attracted much attention due to their remarkable optical properties. While it is well established that excitons dominate their optical response, the impact of higher excitonic states and formation of phonon sidebands in optical spectra still need to be better understood. Here, we perform a theoretical study of excitonic properties of monolayered hybrid organic perovskites-supported by temperature-dependent photoluminescence measurements. Solving the Wannier equation, we obtain microscopic access to the Rydberg-like series of excitonic states including their wave functions and binding energies. Exploiting the generalized Elliot formula, we calculate the photoluminescence spectra demonstrating a pronounced contribution of a phonon sideband for temperatures up to 50 K, in agreement with experimental measurements. Finally, we predict temperature-dependent line widths of the three energetically lowest excitonic transitions and identify the underlying phonon-driven scattering processes.

2D Materials and Heterostructures at Extreme Pressure

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Pressure-Suppressed Carrier Trapping Leads to Enhanced Emission in Two-Dimensional Perovskite (HA)2 (GA)Pb2 I7

A remarkable PL enhancement by 12 fold is achieved using pressure to modulate the structure of a recently developed 2D perovskite (HA)₂ (GA)Pb₂ I₇ (HA=n-hexylammonium, GA=guanidinium). This structure features a previously unattainable, extremely large cage. In situ structural, spectroscopic, and theoretical analyses reveal that lattice compression under a mild pressure within 1.6 GPa considerably suppresses the carrier trapping, leading to significantly enhanced emission. Further pressurization induces a non-luminescent amorphous yellow phase, which is retained and exhibits a continuously increasing band gap during decompression. When the pressure is released to 1.5 GPa, emission can be triggered by above-band gap laser irradiation, accompanied by a color change from yellow to orange. The obtained orange phase could be retained at ambient conditions and exhibits two-fold higher PL emission compared with the pristine (HA)₂ (GA)Pb₂ I₇ .

Pressure-Assisted Fabrication of Perovskite Solar Cells

This paper presents the results of a combined experimental and analytical/computational study of the effects of pressure on photoconversion efficiencies of perovskite solar cells (PSCs). First, an analytical model is used to predict the effects of pressure on interfacial contact in the multilayered structures of PSCs. The PSCs are then fabricated before applying a range of pressures to the devices to improve their interfacial surface contacts. The results show that the photoconversion efficiencies of PSCs increase by ~40%, for applied pressures between 0 and ~7 MPa. However, the photoconversion efficiencies decrease with increasing pressure beyond ~7 MPa. The implications of the results are discussed for the fabrication of efficient PSCs.

Pressure-Induced Structural Evolution and Bandgap Optimization of Lead-Free Halide Double Perovskite (NH4)2SeBr6

Lead-free halide double perovskites (HDPs) are promising candidates for high-performance solar cells because of their environmentally-friendly property and chemical stability in air. The power conversion efficiency of HDPs-based solar cells needs to be further improved before their commercialization in the market. It requires a thoughtful understanding of the correlation between their specific structure and property. Here, the structural and optical properties of an important HDP-based (NH4)2SeBr6 are investigated under high pressure. A dramatic piezochromism is found with the increase in pressure. Optical absorption spectra reveal the pressure-induced red-shift in bandgap with two distinct anomalies at 6.57 and 11.18 GPa, and the energy tunability reaches 360 meV within 20.02 GPa. Combined with structural characterizations, Raman and infrared spectra, and theoretical calculations using density functional theory, results reveal that, the first anomaly is caused by the formation of a Br-Br bond among the [SeBr6]2- octahedra, and the latter is attributed to a cubic-to-tetragonal phase transition. These results provide a clear correlation between the chemical bonding and optical properties of (NH4)2SeBr6. It is believed that the proposed strategy paves the way to optimize the optoelectronic properties of HDPs and further stimulate the development of next-generation clear energy based on HDPs solar cells.

Band Gap Engineering in MASnBr3 and CsSnBr3 Perovskites: Mechanistic Insights through the Application of Pressure

Here we report on the first structural and optical high-pressure investigation of MASnBr₃ (MA = [CH₃NH₃]⁺) and CsSnBr₃ halide perovskites. A massive red shift of 0.4 eV for MASnBr₃ and 0.2 eV for CsSnBr₃ is observed within 1.3 to 1.5 GPa from absorption spectroscopy, followed by a huge blue shift of 0.3 and 0.5 eV, respectively. Synchrotron powder diffraction allowed us to correlate the upturn in the optical properties trend (onset of blue shift) with structural phase transitions from cubic to orthorhombic in MASnBr₃ and from tetragonal to monoclinic for CsSnBr₃. Density functional theory calculations indicate a different underlying mechanism affecting the band gap evolution with pressure, a key role of metal-halide bond lengths for CsSnBr₃ and cation orientation for MASnBr₃, thus showing the impact of a different A-cation on the pressure response. Finally, the investigated phases, differently from the analogous Pb-based counterparts, are robust against amorphization showing defined diffraction up to the maximum pressure used in the experiments.

Unravelling the effects of oxidation state of interstitial iodine and oxygen passivation on charge trapping and recombination in CH3NH3PbI3 perovskite: a time-domain ab initio study

Understanding nonradiative charge recombination mechanisms is a prerequisite for advancing perovskite solar cells. By performing time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics simulations, we show that electron-hole recombination in perovskites strongly depends on the oxidation state of interstitial iodine and oxygen passivation. The simulations demonstrate that electron-hole recombination in CH3NH3PbI3 occurs within several nanoseconds, agreeing well with experiment. The negative interstitial iodine delays charge recombination by a factor of 1.3. The deceleration is attributed to the fact that interstitial iodine anion forms a chemical bond with its nearest lead atoms, eliminates the trap state, and decreases the NA electron-phonon coupling. The positive interstitial iodine attracts its neighbouring lattice iodine anions, resulting in the formation of an I-trimer and producing an electron trap. Electron trapping proceeds on a very fast timescale, tens of picoseconds, and captures the majority of free electrons available to directly recombine with free holes while inhibiting the recombination of free electrons and holes, delaying the recombination by a factor of 1.5. However, the positive interstitial iodine easily converts to a neutral iodine defect by capturing an electron, giving rise to a singly occupied state above the valence band maximum and acting as a hole trap. The photoexcitation valence band hole becomes trapped by the hole trap state very rapidly, followed by acceleration of recombination with the conduction band free electron by a factor of 1.6. Surprisingly, molecular oxygen interacting with interstitial iodine anion forms a stable IO3 -1 species, which inhibits ion migration, stabilizes perovskites, and suppresses the electron-hole recombination by a factor of 2.7. Our simulations reveal the microscopic effects of the oxidation state of interstitial iodine defects and oxygen passivation in perovskites, suggesting an effective way to improve perovskite photovoltaic and optoelectronic devices.

Pressure-induced transformation of CH3NH3PbI3: the role of the noble-gas pressure transmitting media

The photovoltaic perovskite, methylammonium lead triiodide [CH3NH3PbI3 (MAPbI3)], is one of the most efficient materials for solar energy conversion. Various kinds of chemical and physical modifications have been applied to MAPbI3 towards better understanding of the relation between composition, structure, electronic properties and energy conversion efficiency of this material. Pressure is a particularly useful tool, as it can substantially reduce the interatomic spacing in this relatively soft material and cause significant modifications to the electronic structure. Application of high pressure induces changes in the crystal symmetry up to a threshold level above which it leads to amorphization. Here, a detailed structural study of MAPbI3 at high hydrostatic pressures using Ne and Ar as pressure transmitting media is reported. Single-crystal X-ray diffraction experiments with synchrotron radiation at room temperature in the 0-20 GPa pressure range show that atoms of both gaseous media, Ne and Ar, are gradually incorporated into MAPbI3, thus leading to marked structural changes of the material. Specifically, Ne stabilizes the high-pressure phase of NexMAPbI3 and prevents amorphization up to 20 GPa. After releasing the pressure, the crystal has the composition of Ne0.97MAPbI3, which remains stable under ambient conditions. In contrast, above 2.4 GPa, Ar accelerates an irreversible amorphization. The distinct impacts of Ne and Ar are attributed to differences in their chemical reactivity under pressure inside the restricted space between the PbI6 octahedra.

Facile synthesis of 1D organic-inorganic perovskite micro-belts with high water stability for sensing and photonic applications

The development of low-dimensional perovskite micro/nanostructures with high water stability for novel photonic/electronic applications is highly desirable. Herein, one-dimensional (1D) organic-inorganic hybrid perovskite micro-belts [(AD)Pb2Cl5] (OIHP-AD, AD = acridine) were facilely synthesized through fast precipitation in aqueous solution at room temperature without any organic solvent and expensive alkyl halide. Luminescent properties and water stability are efficiently enhanced due to the highly regular arrangement of the protonated AD dyes with larger steric hindrance distributed in the perovskite host-guest system, which can afford denser crystal packing to prevent water erosion. The OIHP-AD micro-belts present upconversion fluorescence, polarized photoemission and optical waveguide performances with a low loss coefficient (0.004 dB μm-1) during propagation, thus extending the applications of 1D perovskite micro/nanostructures to potential optical communication micro-devices.

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

Revealing Photoluminescence Modulation from Layered Halide Perovskite Microcrystals upon Cyclic Compression

Halide perovskites show promise for high-efficiency solar energy conversion and light-emitting diode devices owing to their bandgap, which falls within the visible optical range. However, due to their rigidity, GPa pressures are necessary to control the complex interplay between their electronic and crystallographic structure. Layered perovskites are likely to be controlled using much lower pressures by exploiting the optical anisotropy of the embedded organic molecules in the structure. This work introduces layered perovskite microplatelets and demonstrates the extreme sensitivity of their emission to cyclic mechanical loading in the range of tens of MPa. A drastic change in their emission is observed in situ, from near-white to an enhanced blue color. This process is reversible, as is evident from a hysteresis loop in the photoluminescence (PL) intensity of the microplatelets. A combination of experimental analysis and computational modelling shows that such behavior cannot be attributed to changes in the crystallographic structure of the flakes. Instead, it suggests that, thanks to their structural anisotropy, microplate alignment and reorientation are responsible for the observed PL modulation. The possibility to tune the optical emission of layered perovskite crystals via low pressures makes them highly interesting as active materials in applications where stress sensing or light modulation is desired.

Two-Dimensional Hybrid Halide Perovskites: Principles and Promises

Hybrid halide perovskites have become the "next big thing" in emerging semiconductor materials, as the past decade witnessed their successful application in high-performance photovoltaics. This resurgence has encompassed enormous and widespread development of the three-dimensional (3D) perovskites, spearheaded by CH₃NH₃PbI₃. The next generation of halide perovskites, however, is characterized by reduced dimensionality perovskites, emphasizing the two-dimensional (2D) perovskite derivatives which expand the field into a more diverse subgroup of semiconducting hybrids that possesses even higher tunability and excellent photophysical properties. In this Perspective, we begin with a historical flashback to early reports before the "perovskite fever", and we follow this original work to its fruition in the present day, where 2D halide perovskites are in the spotlight of current research, offering characteristics desirable in high-performance optoelectronics. We approach the evolution of 2D halide perovskites from a structural perspective, providing a way to classify the diverse structure types of the materials, which largely dictate the unusual physical properties observed. We sort the 2D hybrid halide perovskites on the basis of two key components: the inorganic layers and their modification, and the organic cation diversity. As these two heterogeneous components blend, either by synthetic manipulation (shuffling the organic cations or inorganic elements) or by application of external stimuli (temperature and pressure), the modular perovskite structure evolves to construct crystallographically defined quantum wells (QWs). The complex electronic structure that arises is sensitive to the structural features that could be in turn used as a knob to control the dielectric and optical properties the QWs. We conclude this Perspective with the most notable achievements in optoelectronic devices that have been demonstrated to date, with an eye toward future material discovery and potential technological developments.

Pressure-Induced Phase Transformation and Band-Gap Engineering of Formamidinium Lead Iodide Perovskite Nanocrystals

Formamidinium lead halide (FAPbX₃, X = Cl, Br, I) perovskite materials have recently drawn an increased amount of attention owing to their superior optoelectronic properties and enhanced material stability as compared with their methylammonium-based (MA-based) analogues. Herein, we report a study of the pressure-induced structural and optical evolutions of FAPbI₃ hybrid organic-inorganic perovskite nanocrystals (NCs) using a synchrotron-based X-ray scattering technique coupled to in situ absorption and photoluminescence spectroscopies. As a result of their unique structural stability and soft nature, FAPbI₃ NCs exhibit a wide range of band-gap tunability (1.44-2.17 eV) as a function of pressure (0-13.4 GPa). The study presented here not only provides an efficient and chemically orthogonal means to controllably engineer the band gap of FAPbI₃ NCs using pressure but more importantly sheds light on how to strategically design the band gaps of FA-based hybrid organic-inorganic perovskites for various optoelectronic applications.