全无机卤化铅钙钛矿的结构、热力学稳定性和电子性质

有机-无机杂化卤化铅钙钛矿因具有独特的电子和光学特性,已经成为光电领域最有前途的材料。但是,有机-无机钙钛矿材料及器件稳定性差,限制了其实际应用。与杂化钙钛矿相比,全无机卤化物钙钛矿CsPbX₃(X=Cl,Br,I)显示出更强的热稳定性。全无机卤化物钙钛矿CsPbX₃具有多个晶型,在不同的温度下呈不同相结构。目前,关于CsPbX₃的结构和物理性质仍存在争议。本文我们针对三个晶相α-,β-和γ-CsPbX₃的结构,热力学稳定性和电子性质进行了全面的理论研究。第一性原理计算表明,从高温α相到低温β相,然后再到γ相的相变伴随着PbX₆八面体的畸变。零温形成能计算表明,γ相最稳定,这与实验中γ相为低温稳定相的结论一致。电子性质计算表明,所有CsPbX₃钙钛矿都表现出直接带隙性质,并且带隙值从α相到β相再到γ相逐渐增加。这是由于相变发生时,Pb-X成键强度逐渐减弱,使价带顶能量降低,进而带隙增加。在所有相中,α相结构中较强的Pb-X相互作用,导致了较强的带边色散,使其具有较小的载流子有效质量。     

A Review on Cs-Based Pb-Free Double Halide Perovskites: From Theoretical and Experimental Studies to Doping and Applications

Despite the progressive enhancement in the flexibility of Pb-based perovskites for optoelectronic applications, regrettably, they are facing two main challenges; (1) instability, which originates from using organic components in the perovskite structure, and (2) toxicity due to Pb. Therefore, new, stable non-toxic perovskite materials are demanded to overcome these drawbacks. The research community has been working on a wide variety of Pb-free perovskites with different molecular formulas and dimensionality. A variety of Pb-free halide double perovskites have been widely explored by different research groups in search for stable, non-toxic double perovskite material. Especially, Cs-based Pb-free halide double perovskite has been in focus recently. Herein, we present a review of theoretical and experimental research on Cs-based Pb-free double halide perovskites of structural formulas Cs₂M⁺M³⁺X6 (M⁺ = Ag⁺, Na⁺, In⁺ etc.; M³⁺= Bi³⁺, In³⁺, Sb³⁺; X = Cl⁻, Br⁻, I¯) and Cs₂M⁴⁺X₆ (M⁴⁺ = Ti⁴⁺, Sn⁴⁺, Au⁴⁺ etc.). We also present the challenges faced by these perovskite compounds and their current applications especially in photovoltaics alongside the effect of metal dopants on their performance.     

DFT Simulations as Valuable Tool to Support NMR Characterization of Halide Perovskites: the Case of Pure and Mixed Halide Perovskites

Solid state NMR spectroscopy is swiftly emerging as useful tool to characterize the structure, composition and dynamic properties of lead halide perovskites. On the other hand, interpretation of solid state NMR signatures is often challenging, because of the potential presence of many overlapping signals in small range of chemical shifts, hence complicating the extraction of detailed structural features. Here, we demonstrate the reliability of periodic Density Functional Theory in providing theoretical support for the NMR characterization of halide perovskite compounds, considering nuclei with spin I=1/2. For light ¹H and ¹³C nuclei, we predict NMR chemical shifts in good agreement with experiment, further highlighting the effects of motional narrowing. Accurate prediction of the NMR response of ²⁰⁷Pb nuclei is comparably more challenging, but we successfully reproduce the downshift in frequency when changing the halide composition from pure iodine to pure bromine. Furthermore, we confirm NMR as ideal tool to study mixed halide perovskite compounds, currently at the limelight for tandem solar cells and color-tunable light emission.     

Structural Diversity and Magnetic Properties of Hybrid Ruthenium Halide Perovskites and Related Compounds

There has been a great deal of recent interest in extended compounds containing Ru³⁺ and Ru⁴⁺ in light of their range of unusual physical properties. Many of these properties are displayed in compounds with the perovskite and related structures. Here we report an array of structurally diverse hybrid ruthenium halide perovskites and related compounds: MA₂RuX₆ (X=Cl or Br), MA₂MRuX₆ (M=Na, K or Ag; X=Cl or Br) and MA₃Ru₂X₉ (X=Br) based upon the use of methylammonium (MA=CH₃NH₃⁺) on the perovskite A site. The compounds MA₂RuX₆ with Ru⁴⁺ crystallize in the trigonal space group and can be described as vacancy‐ordered double‐perovskites. The ordered compounds MA₂MRuX₆ with M⁺ and Ru³⁺ crystallize in a structure related to BaNiO₃ with alternating MX₆ and RuX₆ face‐shared octahedra forming linear chains in the trigonal space group. The compound MA₃Ru₂Br₉ crystallizes in the orthorhombic Cmcm space group and displays pairs of face‐sharing octahedra forming isolated Ru₂Br₉ moieties with very short Ru–Ru contacts of 2.789 Å. The structural details, including the role of hydrogen bonding and dimensionality, as well as the optical and magnetic properties of these compounds are described. The magnetic behavior of all three classes of compounds is influenced by spin–orbit coupling and their temperature‐dependent behavior has been compared with the predictions of the appropriate Kotani models.     

金属卤化物钙钛矿光催化的研究进展

太阳能驱动光催化反应降解污染物、制备化学燃料或其他高附加值产品是绿色化学和可再生能源研究的重要方向.近年来,在传统的金属氧化物半导体材料之外,金属卤化物钙钛矿类化合物凭借其优异的光电特性也被逐步应用于高效光催化反应中.这篇文章综述了以铅卤钙钛矿为主的金属卤化物钙钛矿材料近年来在光催化领域的研究进展,总结了金属卤化物钙钛矿材料在光（电）催化产氢、CO₂还原反应和有机物高附加值转化反应中的应用与反应机制及其关键挑战,最后展望了高效稳定的金属卤化物钙钛矿光催化剂的发展方向和前景.     

Exciton Localization for Highly Luminescent Two-Dimensional Tin-Based Hybrid Perovskites through Tin Vacancy Tuning

Exciton localization is an approach for preparing highly luminescent semiconductors. However, realizing strongly localized excitonic recombination in low-dimensional materials such as two-dimensional (2D) perovskites remains challenging. Herein, we first propose a simple and efficient Sn²⁺ vacancy (V_Sn) tuning strategy to enhance excitonic localization in 2D (OA)₂SnI₄ (OA=octylammonium) perovskite nanosheets (PNSs), increasing their photoluminescence quantum yield (PLQY) to ≈64 %, which is among the highest values reported for tin iodide perovskites. Combining experimental with first-principles calculation results, we confirm that the significantly increased PLQY of (OA)₂SnI₄ PNSs is primarily due to self-trapped excitons with highly localized energy states induced by V_Sn. Moreover, this universal strategy can be applied for improving other 2D Sn-based perovskites, thereby paving a new way to fabricate diverse 2D lead-free perovskites with desirable PL properties.     

Molecular Intercalation and Electronic Two Dimensionality in Layered Hybrid Perovskites

In layered hybrid perovskites, such as (BA)₂PbI₄ (BA=C₄H₉NH₃), electrons and holes are considered to be confined in atomically thin two dimensional (2D) Pb–I inorganic layers. These inorganic layers are electronically isolated from each other in the third dimension by the insulating organic layers. Herein we report our experimental findings that suggest the presence of electronic interaction between the inorganic layers in some parts of the single crystals. The extent of this interaction is reversibly tuned by intercalation of organic and inorganic molecules in the layered perovskite single crystals. Consequently, optical absorption and emission properties switch reversibly with intercalation. Furthermore, increasing the distance between inorganic layers by increasing the length of the organic spacer cations systematically decreases these electronic interactions. This finding that the parts of the layered hybrid perovskites are not strictly electronically 2D is critical for understanding the electronic, optical, and optoelectronic properties of these technologically important materials.     

Pressure-Suppressed Carrier Trapping Leads to Enhanced Emission in Two-Dimensional Perovskite (HA)2(GA)Pb2I7

A remarkable PL enhancement by 12 fold is achieved using pressure to modulate the structure of a recently developed 2D perovskite (HA)₂(GA)Pb₂I₇ (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)Pb₂I₇.     

How Methylammonium Iodide Reactant Size Affects Morphology and Defect Properties of Mechanochemically Synthesized MAPbI3 Powder

Here, we investigate in detail the impact of the size of the methylammonium iodide (MAI) reactants in the mechanochemical powder synthesis of the halide perovskite methylammonium lead iodide (MAPbI₃). Morphology and structural characterizations by scanning electron microscopy and X-ray diffraction reveal that with increasing MAI reactant size, the particle size of the perovskite powder increases, while its defect density decreases, as suggested by nuclear quadrupole resonance spectroscopy and photoluminescence investigations. The reason for this behavior seems to be associated to the sensitive influence of the MAI size on the time durations of MAPbI₃ synthesis and delayed MAPbI₃ crushing stage during ball milling. Thus, our results emphasize the high importance the reactant properties have on the mechanochemical synthesis of halide perovskites and will contribute to enhance the reproducibility and control of the fabrication of halide perovskites in powder form.     

Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites

Two thin-film 2D organic-inorganic hybrid perovskites, i.e., 2-phenylethylammonium lead iodide (PEPI) and 4-phenyl-1-butylammonium lead iodide (PBPI) were synthesized and investigated by steady-state absorption, temperature-dependent photoluminescence, and temperature-dependent ultrafast transient absorption spectroscopy. PBPI has a longer organic chain (via introducing extra ethyl groups) than PEPI, thus its inorganic skeleton can be distorted, bringing on structural disorder. The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics. In addition, the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy, suggesting its correlation with inorganic skeleton rather than organic chain. Moreover, the photoexcited coherent phonons were observed in both PEPI and PBPI, pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton. This work provides valuable insights into the optical properties, excitonic behaviors and dynamics, as well as coherent phonon effects in 2D hybrid perovskites.     

Lead‐Free Halide Perovskite Nanocrystals: Crystal Structures,Synthesis, Stabilities,and Optical Properties

In recent years, there have been rapid advances in the synthesis of lead halide perovskite nanocrystals (NCs) for use in solar cells, light emitting diodes, lasers, and photodetectors. These compounds have a set of intriguing optical, excitonic, and charge transport properties, including outstanding photoluminescence quantum yield (PLQY) and tunable optical band gap. However, the necessary inclusion of lead, a toxic element, raises a critical concern for future commercial development. To address the toxicity issue, intense recent research effort has been devoted to developing lead‐free halide perovskite (LFHP) NCs. In this Review, we present a comprehensive overview of currently explored LFHP NCs with an emphasis on their crystal structures, synthesis, optical properties, and environmental stabilities (e.g., UV, heat, and moisture resistance). In addition, strategies for enhancing optical properties and stabilities of LFHP NCs as well as the state‐of‐the‐art applications are discussed. With the perspective of their properties and current challenges, we provide an outlook for future directions in this rapidly evolving field to achieve high‐quality LFHP NCs for a broader range of fundamental research and practical applications.     

Temperature-Dependent Photoluminescence of Hexafluorobenzene-Intercalated Phenethylammonium Tin Iodide 2D Perovskite

Tin halide perovskites are potential alternatives of lead halide perovskites. However, the easy oxidation of Sn²⁺ to Sn⁴⁺ brings in a challenge. Recently, layered two-dimensional hybrid tin halide perovskites have been shown to partially resist the oxidation process because of the presence of hydrophobic organic molecules. Consequently, such layered hybrid perovskites are being explored for optoelectronic applications. The optical properties of layered tin halide perovskites depend on the interlayer separation and the dielectric mismatch between the organic and inorganic layers. Intercalation (insertion) of a molecular species between the layers modifies the interlayer interactions affecting the optical properties of layered hybrid perovskites. We investigated the effect of hexafluorobenzene (HFB) intercalation in phenethylammonium tin iodide [(PEA)₂SnI₄] using temperature-dependent (6 K to 300 K) photoluminescence (PL). HFB intercalation increases the bandgap. A strong PL quenching is observed in pristine (PEA)₂SnI₄ below 150 K, probably because of the presence of non-emissive states. HFB intercalation suppresses the influence of such non-emissive states resulting in an increase in PL intensity at the cryogenic temperatures. Our results highlight that a simple molecular intercalation (non-covalent interaction) into layered hybrid perovskites can significantly tailor the electronic and optical properties.     

Correlating Photophysical Properties with Stereochemical Expression of 6s2 Lone Pairs in Two-dimensional Lead Halide Perovskites

Two-dimensional (2D) lead halide perovskites (LHPs) have shown great promises for light-emitting applications and excitonic devices. Fulfilling these promises demands an in-depth understanding on the relationships between the structural dynamics and exciton-phonon interactions that govern the optical properties. Here, we unveil the structural dynamics of 2D lead iodide perovskites with different spacer cations. Loose packing of an undersized spacer cation leads to out-of-plane octahedral tilting, whereas compact packing of an oversized spacer cation stretches Pb−I bond length, resulting in Pb²⁺ off-center displacement driven by stereochemical expression of the Pb²⁺ 6s² lone pair electrons. Density functional theory calculations indicate that the Pb²⁺ cation is off-center displaced mainly along the direction where the octahedra are stretched the most by the spacer cation. We find dynamic structural distortions associated with either octahedral tilting or Pb²⁺ off-centering lead to a broad Raman central peak background and phonon softening, which increase the non-radiative recombination loss via exciton-phonon interactions and quench the photoluminescence intensity. The correlations between the structural, phonon, and optical properties are further confirmed by the pressure tuning of the 2D LHPs. Our results demonstrate that minimizing the dynamic structural distortions via a judicious selection of the spacer cations is essential to realize high luminescence properties in 2D LHPs.     

Evaluation of Non‐covalent Binding Energies and Optoelectronic Properties of New CuBr2(C6H7N)2 Complex: DFT Approaches

For the first time, the structural and optoelectronic properties of a new complex formulated as CuBr₂(C₆H₇N)₂ ( 1 ) [trans‐dibromidobis(3‐methylpyridine‐κN) copper(II)] were studied by density functional theory (DFT) calculations. They are performed using B3LYP through the Gaussian 09 program and also with full potential linearized augmented plane wave (FP‐LAPW) methods within the Generalized Gradient Approximation (GGA) and Hartree‐Fock (HF) theory by the Wien2k package. The neutral monomeric complex participates in a variety of non‐covalent interactions, including hydrogen bonding and π stacking to create a 2D coordinate plane. The binding energy value of the non‐covalent interactions responsible for the crystalline network formation of 1 were calculated using the method of dispersion corrected density functional theory (DFT‐D). In this method, the independent smallest fragment (monomer) and subsequently the related network, including seven monomers bearing all non‐covalent interactions were optimized. The results demonstrate that hydrogen bonds, especially non‐conventional C–H ··· Br interactions, govern the network formation along the a and c axes. It can be mentioned because of these directed interactions, increasing of charge transfer along x and z directions results in increasement of the absorption and refractive index along y and z directions, and vice versa. The results of band structure show indirectly and directly the nature of the bandgap within GGA and HF, respectively. The bandgap value of CuBr₂(C₆H₇N)₂ is comparable to those of binary semiconductor compounds. DOSs spectra reveal that 3d Cu, 4p Br, and 2p C states play important roles in the optical transitions of the electrons. The calculated electronic absorption of the UV/Vis spectrum shows six major electron‐transition bands derived from d → d (ligand field) n → n, n → π*, π → n, and σ → n MLCT and LMCT transitions. The calculated absorption spectrum of the titled complex through FP‐LAPW within GGA method shows good consistency with the B3LYP/def2‐TZVP/6‐311+G(d,p) method. Our calculated birefringence results show that 1 has capability of nonlinear optical, which can be used in the nonlinear optoelectronic devices.     

Synthesis,Structures, Thermal and Luminescence Properties of Zn and Cd Halide Coordination Polymers with 2-Cyanopyrazine

Reaction of MX₂ (M = Cd, Zn; X = Cl, Br, I) with 2-cyanopyrazine leads to the formation of compounds with the composition CdX₂(2-cyanopyrazine)₂ (X = Cl; CdCl , X = Br; CdBr and X = I; CdI ) and ZnX₂(2-cyanopyrazine)₂ (X = Cl; ZnCl , X = Br; ZnBr and X = I; ZnI/I ). In the crystal structures of the Cd compounds and in ZnCl , the metal cations are octahedrally coordinated and are linked into chains by the halide anions via common edges. In contrast, in the crystal structures of ZnBr and ZnI/I the metal cations are tetrahedrally coordinated into discrete complexes. Further investigations show that a second modification of ZnCl₂(2-cyanopyrazine)₂ exists ( ZnI/II ), which is formed by kinetic control. The thermal properties of the 2-cyanopyrazine rich compounds were investigated by TG-DTA and temperature dependent XRPD measurements. Upon heating the Cd compounds, all 2-cyanopyrazine ligands are removed in a single step with no indication of the formation of a 2-cyanopyrazine deficient phase. A similar behavior is observed for ZnI , whereas for ZnCl and ZnBr , TG-DTA measurements suggest the formation of a 2-cyanopyrazine deficient phase that, in case of ZnBr , cannot be isolated and, for ZnCl , cannot be obtained pure. The emission of these compounds is shifted from the blue to orange depending on the crystal structure and the nature of the halide anion.     

Fully sp2-Carbon-Linked Crystalline Two-Dimensional Conjugated Polymers: Insight into 2D Poly(phenylenecyanovinylene) Formation and its Optoelectronic Properties

Cyano-substituted polyphenylene vinylenes (PPVs) have been the focus of research for several decades owing to their interesting optoelectronic properties and potential applications in organic electronics. With the advent of organic two-dimensional (2D) crystals, the question arose as to how the chemical and optoelectronic advantages of PPVs evolve in 2D compared with their linear counterparts. In this work, we present the efficient synthesis of two novel 2D fully sp²-carbon-linked crystalline PPVs and investigate the essentiality of inorganic bases for their catalytic formation. Notably, among all bases screened, cesium carbonate (Cs₂CO₃) plays a crucial role and enables reversibility in the first step with subsequent structure locking by formation of a C=C double bond to maintain crystallinity, which is supported by density functional theory (DFT) calculations. A quantifiable energy diagram of a “quasi-reversible reaction” is proposed, which allows the identification of further suitable C−C bond formation reactions for 2D polymerizations. Moreover, the narrowing of the HOMO–LUMO gap is delineated by expanding the conjugation into two dimensions. To enable environmentally benign processing, the post-modification of 2D PPVs is further performed, which renders stable dispersions in the aqueous phase.     

Pressure‐Suppressed Carrier Trapping Leads to Enhanced Emission in Two‐Dimensional Perovskite (HA)2(GA)Pb2I7

A remarkable PL enhancement by 12 fold is achieved using pressure to modulate the structure of a recently developed 2D perovskite (HA)₂(GA)Pb₂I₇ (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)Pb₂I₇.