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

Tuning the Optoelectronic Property of All-Inorganic Lead-Free Perovskite via Finely Microstructural Modulation for Photovoltaics

Bismuth-based halide perovskite materials have attracted extensive attention for optoelectronic applications due to nontoxicity and ambient stability. However, limited by low-dimensional structure and isolate octahedron arrangement, the undesirable photophysical properties of bismuth-based perovskites are still not well modulated. Here, the rational design and synthesis of Cs3 SbBiI9 with improved optoelectronic performance via premeditatedly incorporating antimony atoms with a similar electronic structure to bismuth into the host lattice of Cs3 Bi2 I9 is reported. Compared with Cs3 Bi2 I9 , the absorption spectrum of Cs3 SbBiI9 is broadened from ≈640 to ≈700 nm, the photoluminescence intensity enhances by two orders of magnitude indicating the extremely suppressed carrier nonradiative recombination, and the charge carrier lifetime is further increased from 1.3 to 207.6 ns. Taking representative applications in perovskite solar cells, the Cs3 SbBiI9 exhibits a higher photovoltaic performance benefiting from the improved intrinsic optoelectronic properties. Further structure analysis reveals that the introduced Sb atoms regulate the interlayer spacing between dimers in c-axis direction and the micro-octahedral configuration, which correlate well with the improvement of optoelectronic properties of Cs3 SbBiI9 . It is anticipated that this work will benefit the design and fabrication of lead-free perovskite semiconductors for optoelectronic applications.

Phase transformations and vibrational properties of hybrid organic-inorganic perovskite MAPbI3 bulk at high pressure

The structural stability and internal properties of hybrid organic-inorganic perovskites (HOIPs) have been widely investigated over the past few years. The interplay between organic cations and inorganic framework is one of the prominent features. Herein we report the evolution of Raman modes under pressure in the hybrid organic-inorganic perovskite MAPbI[Formula: see text] by combining the experimental approach with the first-principles calculations. A bulk MAPbI[Formula: see text] single crystal was synthesized via inverse temperature crystallization (ITC) technique and characterized by Raman spectroscopy, while the diamond anvil cells (DACs) was employed to compress the sample. The classification and behaviours of their Raman modes are presented. At ambient pressure, the vibrations of inorganic PbI[Formula: see text] octahedra and organic MA dominate at a low-frequency range (60-760 cm[Formula: see text]) and a fingerprint range (900-1500 cm[Formula: see text]), respectively. The applied pressure exhibits two significant changes in the Raman spectrum and indicates of phase transition. The results obtained from both experiment and calculations of the second phase at 3.26 GPa reveal that the internal vibration intensity of the PbI[Formula: see text] octahedra (< 110 cm[Formula: see text]) reduces as absences of MA libration (150-270 cm[Formula: see text]) and internal vibration of MA (450-750 cm[Formula: see text]). Furthermore, the hydrogen interactions around 1300 cm[Formula: see text] remain strong high pressure up to 5.34 GPa.

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.

Pressure-Tuned Multicolor Emission of 2D Lead Halide Perovskites with Ultrahigh Color Purity

The chemical diversity and structural flexibility of lead halide perovskites (LHPs) offer tremendous opportunities to tune their optical properties through internal molecular engineering and external stimuli. Herein, we report the wide-range and ultrapure photoluminescence emissions in a family of homologous 2D LHPs, [MeOPEA]₂ PbBr_4-4x I_4x (MeOPEA=4-methoxyphenethylammonium; x=0, 0.2, 0.425, 0.575, 1) enabled through internal chemical pressure and external hydrostatic pressure. The chemical pressure, induced by the C-H⋅⋅⋅π interactions and halogen doping/substitution strengthens the structural rigidity to give sustained narrow emissions, and regulates the emission energy, respectively. Further manipulation of physical pressure leads to wide-range emission tuning from 412 to 647 nm in a continuous and reversible manner. This work could open up new pathways for developing 2D LHP emitters with ultra-wide color gamut and high color purity which are highly useful for pressure sensing.

Nanoscale Thermal Strain Engineering-Driven Ferroelastic Domain Evolution in CH3NH3PbI3 Perovskites

A local thermal strain engineering approach via an ac-heated thermal probe was incorporated into methylammonium lead triiodide (MAPbI₃) crystals and acts as a driving force for ferroic twin domain dynamics, local ion migration, and property tailoring. Periodically, striped ferroic twin domains and their dynamic evolutions were successfully induced by local thermal strain and high-resolution thermal imaging, giving decisive evidence of the ferroelastic nature in MAPbI₃ perovskites at room temperature. Local thermal ionic imaging and chemical mappings demonstrate that domain contrasts are from local methylammonium (MA⁺) redistribution into the stripes of chemical segregation in response to the local thermal strain fields. The present results reveal an inherent coupling among local thermal strains, ferroelastic twin domains, local chemical-ion segregations, and physical properties and offer a potential path to improve the functionality of metal halide perovskite-based solar cells.

Phase stability and electronic structure of CsPbCl3 under hydrostatic stress and anion substitution

Perovskites based on CsPbX₃ (X = Cl, Br, I) have promising applications in solar cells, light-emitting diodes, and photodetectors. In this paper, the phase stability of inorganic metal halide perovskite CsPbCl₃ under hydrostatic pressure and anion substitution is studied using density functional theory (DFT), and this modification is explained by the interaction of the octahedrons and transformation of the bond-orbital coupling. In addition, two space groups, P4/mbm and Amm2, which are stable under stress, are subjected to anion substitution; then, the structural stability and band gap change of CsPbCl_3-yX_y (X = Br, I; y = 0, 1, 2, 3) are analyzed after applying stress; finally, the electronic structures and optical properties of the six most stable components are presented. The effect of stress and anions on the components' optoelectronic properties is closely linked with the crystal's structural alteration mechanism. These results show that stress and anion modulation can significantly change the optoelectronic properties of materials, which make these materials have broad application prospects. Furthermore, stress can be used as an effective tool for screening the most stable material structure.

Pressure-Induced Metallization of Lead-Free Halide Double Perovskite (NH4 )2 PtI6

Metallization has recently garnered significant interest due to its ability to greatly facilitate chemical reactions and dramatically change the properties of materials. Materials displaying metallization under low pressure are highly desired for understanding their potential properties. In this work, the effects of the pressure on the structural and electronic properties of lead-free halide double perovskite (NH4 )2 PtI6 are investigated systematically. Remarkably, an unprecedented bandgap narrowing down to the Shockley-Queisser limit is observed at a very low pressure of 0.12 GPa, showing great promise in optoelectronic applications. More interestingly, the metallization of (NH4 )2 PtI6 is initiated at 14.2 GPa, the lowest metallization pressure ever reported in halide perovskites, which is related to the continuous increase in the overlap between the valence and conduction band of I 5p orbital. Its structural evolution upon compression before the metallic transition is also tracked, from cubic Fm-3m to tetragonal P4/mnc and then to monoclinic C2/c phase, which is mainly associated with the rotation and distortions within the [PtI6 ]2- octahedra. These findings represent a significant step toward revealing the structure-property relationships of (NH4 )2 PtI6 , and also prove that high-pressure technique is an efficient tool to design and realize superior optoelectronic materials.

Pressure-stimulus-responsive behaviors of core-shell InP/ZnSe nanocrystals: remarkable piezochromic luminescence and structural assembly

Piezochromic luminescence materials with optical properties can be adjusted (the colors most sensitive to the human eye range from red to green) to provide powerful means for information acquisition in various applications. Inorganic quantum dots, typically based on heavy metals such as cadmium and lead, have congenital advantages as luminescence materials, including strong inoxidizability and excellent photoelectric properties. However, small band-gap shifts under pressure have hindered the development of inorganic-based piezochromic materials. Herein, we combined in situ high-pressure photoluminescence (PL) and absorption measurements with synchrotron X-ray scattering spectra to elucidate the remarkable modulation of optical properties and morphologies by pressure, particularly that of the piezochromic luminescence, in all-inorganic core-shell InP/ZnSe nanocrystals (NCs). We observed a stepwise PL color change from red to green, and an ultrabroad bandgap tunability of 0.46 eV was observed from 1.99 to 2.45 eV in the pressure range of 14.2 GPa for InP/ZnSe NCs. Moreover, two-dimensional (2D) InP/ZnSe nanosheets were synthesized by the stress-driven attachment of nanoparticles. These results demonstrate the ability of the pressure-stimulus response to trigger remarkable piezochromic luminescence and 2D nanosheet assembly in InP/ZnSe NCs, which paves the way for new applications of all-inorganic InP-based semiconductor NCs.

Pressure-induced bandgap engineering of lead-free halide double perovskite (NH4)2SnBr6

Lead-free halide double perovskites (HDPs) have recently been proposed as potential stable and environment-friendly alternatives to lead-based halide perovskites. Bandgap engineering plays a vital role in the optoelectronic applications of HDP materials. In this study, methods combining high-pressure techniques with density functional theory calculations were employed to implement the bandgap engineering of a classic HDP-based (NH₄)₂SnBr₆. Under high pressure, (NH₄)₂SnBr₆ exhibits a redshift of the bandgap with increasing pressure up to 6.3 GPa and a sudden blueshift up to 20.2 GPa, followed by a redshift at higher pressures, which is relevant to the cubic-tetragonal phase transition, direct-indirect transition, and amorphization, respectively. Our results enrich the understanding of the structural-optical properties of (NH₄)₂SnBr₆ and reveal the special role of NH₄⁺ cations in pressure-induced bandgap engineering, thus providing important information for application in optoelectronic devices and helping to design ideal materials with higher efficiency.

Semiconductor to metallic transition under induced pressure in Cs2AgBiBr6 double halide perovskite: a theoretical DFT study for photovoltaic and optoelectronic applications

Inorganic double halide perovskites have a wide range of applications in low-cost photovoltaic and optoelectronic devices. In this manuscript, we have studied their structural, electronic, mechanical and optical properties using density functional theory (DFT) simulations. In this work, hydrostatic pressure is induced from 0 to 50 GPa. Disordered Ag and Bi atoms have a large impact on band gap energy; in this case, the indirect band gap is transferred towards a direct band gap. We have seen that pressure-driven samples have transformed a band energy semiconductor into a metallic one. Under the induced hydrostatic pressure, the covalent bond is transformed into a metallic bond and the bond lengths are reduced. Meanwhile, pressure-induced samples enhance symmetry breaking in [AgBr₆]⁵⁻ and [BiBr₆]³⁻ octahedra, which reduces the density of states of the Fermi surface and lowers the total energy. The mechanical behaviors demonstrated that the studied materials are mechanically stable as well as ductile and their ductile nature is enhanced by the driving pressure. The absorption peak is shifted towards the low energy region with increased hydrostatic pressure. The absorptivity and dielectric constant values are also increased with driving pressure. Phase transformed double halide perovskites triggered by outside stimuli produce several outstanding materials properties, giving great scope for a broad range of applications. This type of pristine and disordered double halide perovskite with pressure-driven semiconductor-to-metal phase transition samples may have potential applications in optoelectronic and photovoltaic devices.

Inorganic double halide perovskites have a wide range of applications in low-cost photovoltaic and optoelectronic devices.

Harvesting High-Quality White-Light Emitting and Remarkable Emission Enhancement in One-Dimensional Halide Perovskites Upon Compression

The pressure induced emission (PIE) behavior of halide perovskites has attracted extensive interest due to its potential application in pressure sensors and trademark security. However, the PIE phenomenon of white-light-emitting hybrid perovskites (WHPs) is rare, and that at pressures above 10.0 GPa has never been reported. Here, we effectively adjusted the perovskite to emit high-quality "cold" or "warm" white light and successfully realized pressure-induced emission (PIE) upon even higher pressure up to 35.1 GPa in one-dimensional halide perovskite C₄N₂H₁₄PbCl₄. We reveal that the degree of structural distortion and the rearrangement of the multiple self-trapped states position are consistent with the intriguing photoluminescence variation, which is further supported by in situ high-pressure synchrotron X-ray diffraction experiments and time-resolved photoluminescence decay dynamics data. The underlying relationship between octahedron behavior and emission plays a key role to obtain high-quality white emission perovskites. We anticipate that this work enhances our understanding of structure-dependent self-trapped exciton (STE) emission characteristics and stimulates the design of high-performance WHPs for next generation white LED lighting devices.

Pressure-Induced Emission toward Harvesting Cold White Light from Warm White Light

The pressure-induced emission (PIE) behavior of halide perovskites has attracted widespread attention and has potential application in pressure sensing. However, high-pressure reversibility largely inhibits practical applications. Here, we describe the emission enhancement and non-doping control of the color temperature in two-dimensional perovskite (C₆ H₅ CH₂ CH₂ NH₃ )₂ PbCl₄ ((PEA)₂ PbCl₄ ) nanocrystals (NCs) through high-pressure processing. A remarkable 5 times PIE was achieved at a mild pressure of 0.4 GPa, which was highly associated with the enhanced radiative recombination of self-trapped excitons. Of particular importance is the retention of the 1.6 times emission of dense (PEA)₂ PbCl₄ NCs upon the complete release of pressure, accompanied by a color change from "warm" (4403 K) to "cold" white light with 14295 K. The irreversible pressure-induced structural amorphization, which facilitates the remaining local distortion of inorganic Pb-Cl octahedra with respect to the steric hindrance of organic PEA⁺ cations, should be greatly responsible for the quenched high-efficiency photoluminescence.

Chemical Control of Spin-Orbit Coupling and Charge Transfer in Vacancy-Ordered Ruthenium(IV) Halide Perovskites

Vacancy-ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of Ru^IV halides are presented; A₂ RuCl₆ and A₂ RuBr₆ , where A is K, NH₄ , Rb or Cs. We show that the optical properties and spin-orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand-to-metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non-magnetic ground state with J_eff =0 caused by SOC. Ru-X covalency, and consequently, the delocalization of metal d-electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion.