Tailorable Indirect to Direct Band-Gap Double Perovskites with Bright White-Light Emission: Decoding Chemical Structure Using Solid-State NMR

Advanced Optical Information Encryption Enabled by Polychromatic and Stimuli-Responsive Luminescence of Sb-Doped Double Perovskites

The smart materials with multi-color and stimuli-responsive luminescence are very promising for next generation of optical information encryption and anti-counterfeiting, but these materials are still scarce. Herein, a multi-level information encryption strategy is developed based on the polychromatic emission of Sb-doped double perovskite powders (SDPPs). Cs2NaInCl6:Sb, Cs2KInCl6:Sb, and Cs2AgInCl6:Sb synthesized through coprecipitation methods exhibit broadband emissions with bright blue, cyan, and orange colors, respectively. The information transmitted by specific SDPP is encrypted when different SDPPs are mixed. The confidential information can be decrypted by selecting the corresponding narrowband filter. Then, an encrypted quick response (QR) code with improved security is demonstrated based on this multi-channel selection strategy. Moreover, the three types of SDPPs exhibit three different water-triggered luminescence switching behaviors. The confidential information represented by Cs2NaInCl6:Sb can be erased/recovered through a simple water spray/drying. Whereas, the information collected from the green channel is permanently erased by moisture, which fundamentally avoids information leakage. Therefore, different encryption schemes can be designed to meet a variety of encryption requirements. The multicolor and stimuli-responsive luminescence greatly enrich the flexibility of optical information encryption, which leaps the level of security and confidentiality.

Electronic structure and transport in the potential Luttinger liquids CsNb3Br7S and RbNb3Br7S

The crystal structures of ANb3Br7S (A = Rb and Cs) have been refined by single crystal X-ray diffraction, and are found to form highly anisotropic materials based on chains of the triangular Nb3 cluster core. The Nb3 cluster core contains seven valence electrons, six of them being assigned to Nb-Nb bonds within the Nb3 triangle and one unpaired d electron. The presence of this surplus electron gives rise to the formation of correlated electronic states. The connectivity in the structures is represented by one-dimensional [Nb3Br7S]- chains, containing a sulphur atom capping one face (μ3) of the triangular niobium cluster, which is believed to induce an important electronic feature. Several types of studies are undertaken to obtain deeper insight into the understanding of this unusual material: the crystal structure, morphology and elastic properties are analysed, as well the (photo-)electrical properties and NMR relaxation. Electronic structure (DFT) calculations are performed in order to understand the electronic structure and transport in these compounds, and, based on the experimental and theoretical results, we propose that the electronic interactions along the Nb chains are sufficiently one-dimensional to give rise to Luttinger liquid (rather than Fermi liquid) behaviour of the metallic electrons.

A Robust Anti-Thermal-Quenching Phosphor Based on Zero-Dimensional Metal Halide Rb3InCl6:xSb3

High-power phosphor-converted white light-emitting diodes (hp-WLEDs) have been widely involved in modern society as outdoor lighting sources. In these devices, due to the Joule effect, the high applied currents cause high operation temperatures (>500 K). Under these conditions, most phosphors lose their emission, an effect known as thermal quenching (TQ). Here, we introduce a zero-dimensional (0D) metal halide, Rb3InCl6:xSb3+, as a suitable anti-TQ phosphor offering robust anti-TQ behavior up to 500 K. We ascribe this behavior of the metal halide to two factors: (1) a compensation process via thermally activated energy transfer from structural defects to emissive centers and (2) an intrinsic structural rigidity of the isolated octahedra in the 0D structure. The anti-TQ phosphor-based WLEDs can stably work at a current of 2000 mA. The low synthesis cost and nontoxic composition reported here can herald a new generation of anti-TQ phosphors for hp-WLED.

Controlled Synthesis of Lead-Free Double Perovskite Colloidal Nanocrystals for Nonvolatile Resistive Memory Devices

Although lead-free double perovskites such as Cs2AgBiBr6 have been widely explored, they still remain a daunting challenge for the controlled synthesis of lead-free double perovskite nanocrystals with highly tunable morphology and band structure. Here, we report the controlled synthesis of lead-free double perovskite colloidal nanocrystals including Cs2AgBiBr6 and Cs2AgInxBi1-xBr6 via a facile wet-chemical synthesis method for the fabrication of high-performance nonvolatile resistive memory devices. Cs2AgBiBr6 colloidal nanocrystals with well-defined cuboidal, hexagonal, and triangular morphologies are synthesized through a facile wet-chemical approach by tuning the reaction temperature from 150 to 190 °C. Further incorporating indium into Cs2AgBiBr6 to synthesize alloyed Cs2AgInxBi1-xBr6 nanocrystals not only can induce the indirect-to-direct bandgap transition with enhanced photoluminescence but also can improve its structural stability. After optimizing the active layers and device structure, the fabricated Ag/polymethylene acrylate@Cs2AgIn0.25Bi0.75Br6/ITO resistive memory device exhibits a low power consumption (the operating voltage is ∼0.17 V), excellent cycling stability (>10 000 cycles), and good synaptic property. Our study would enable the facile wet-chemical synthesis of lead-free double perovskite colloidal nanocrystals in a highly controllable manner for the development of high-performance resistive memory devices.

Structural, electronic, optical, elastic, thermodynamic and thermal transport properties of Cs2AgInCl6 and Cs2AgSbCl6 double perovskite semiconductors using a first-principles study

In this study, we employ the framework of first-principles density functional theory (DFT) computations to investigate the physical, electrical, bandgap and thermal conductivity of Cs2AgInCl6-CAIC (type I) and Cs2AgSbCl6-CASC (type II) using the GGA-PBE method. CAIC possesses a direct band gap energy of 1.812 eV, while CASC demonstrates an indirect band gap energy of 0.926 eV. The CAIC and CASC exhibit intriguingly reduced thermal conductivity, which can be attributed to the notable reduction in their respective Debye temperatures, measuring 182 K and 135 K, respectively. The Raman active modes computed under ambient conditions have been compared with real-world data, showing excellent agreement. The thermal conductivity values of CAIC and CASC compounds exhibit quantum mechanical characteristics, with values of 0.075 and 0.25 W m-1 K-1, respectively, at 300 K. It is foreseen that these outcomes will generate investigations concerning phosphors and diodes that rely on single emitters, with the aim of advancing lighting and display technologies in the forthcoming generations.

A DFT investigation of lead-free TlSnX3 (X = Cl, Br, or I) perovskites for potential applications in solar cells and thermoelectric devices

In the present study, the Density Functional Theory (DFT) was employed to computationally investigate the potential application of newly developed lead-free perovskites with the formula of TlSnX3 (X = Cl, Br, or I) as absorbers in the perovskite solar cells and as thermoelectric materials. The Quantum Espresso code was implemented to optimize the structural configuration of the perovskites and to compute a range of their properties, including their elasticity, electronic behavior, optical characteristics, and thermoelectric attributes. The findings indicated that these perovskite materials exhibit both chemical and structural stability and that TlSnBr3 and TlSnI3 perovskites possess high dynamic stability. The findings additionally revealed direct (R → R) band gap energy values of 0.87 eV for TlSnCl3, 0.52 eV for TlSnBr3, and 0.28 eV for TlSnI3 using the GGA-PBE functional. Further analysis of their elastic properties suggested that these materials are mechanically stable and displayed overall ductile behaviour. They also demonstrated remarkable optical properties, particularly a high absorption coefficient, ranging from 105 cm-1 to 106 cm-1. Consequently, it is reasonable to infer that these materials exhibit considerable potential for utilization in solar cells. Finally, the evaluation of their thermoelectric properties has revealed the highly promising potential of these materials to be employed in thermoelectric applications.

Two-Dimensional Cs2 AgInx Bi1- x Cl6 Alloyed Double Perovskite Nanoplatelets for Solution-Processed Light-Emitting Diodes

Lead-free double perovskites have emerged as a promising class of materials with potential to be integrated into a wide range of optical and optoelectronic applications. Herein, the first synthesis of 2D Cs₂ AgIn_x Bi_{1- x} Cl₆ (0 ≤ x ≤ 1) alloyed double perovskite nanoplatelets (NPLs) with well controlled morphology and composition is demonstrated. The obtained NPLs show unique optical properties with the highest photoluminescence quantum yield of 40.1%. Both temperature dependent spectroscopic studies and density functional theory calculation results reveal that the morphological dimension reduction and In-Bi alloying effect together boost the radiative pathway of the self-trapped excitons of the alloyed double perovskite NPLs. Moreover, the NPLs exhibit good stability under ambient conditions and against polar solvents, which is ideal for all solution-processing of the materials in low-cost device manufacturing. The first solution-processed light-emitting diodes is demonstrated using the Cs₂ AgIn_0.9 Bi_0.1 Cl₆ alloyed double perovskite NPLs as the sole emitting component, showing luminance maximum of 58 cd m^-2 and peak current efficiency of 0.013 cd A^-1 . This study sheds light on morphological control and composition-property relationships of double perovskite nanocrystals, paving the way toward ultimate utilizations of lead-free perovskite materials in diverse sets of real-life applications.

Effect of aliovalent bismuth substitution on structure and optical properties of CsSnBr3

Aliovalent substitution of the B component in ABX₃ metal halides has often been proposed to modify the band gap and thus the photovoltaic properties, but details about the resulting structure have remained largely unknown. Here, we examine these effects in Bi-substituted CsSnBr₃. Powder X-ray diffraction (XRD) and solid-state ¹¹⁹Sn, ¹³³Cs and ²⁰⁹Bi nuclear magnetic resonance (NMR) spectroscopy were carried out to infer how Bi substitution changes the structure of these compounds. The cubic perovskite structure is preserved upon Bi-substitution, but with disorder in the B site occurring at the atomic level. Bi atoms are randomly distributed as they substitute for Sn atoms with no evidence of Bi segregation. The absorption edge in the optical spectra shifts from 1.8 to 1.2 eV upon Bi-substitution, maintaining a direct band gap according to electronic structure calculations. It is shown that Bi-substitution improves resistance to degradation by inhibiting the oxidation of Sn.

Seven-photon absorption from Na+/Bi3+-alloyed Cs2AgInCl6 perovskites

Nonlinear multi-phonon (2-7) absorption in the Na⁺/Bi³⁺-alloyed Cs₂AgInCl₆ lead-free double perovskites with ∼100% photoluminescence quantum yield and superior stability is observed for the first time, which can be pumped by a femtosecond laser in a wide spectral range (800-2600 nm). First-principles calculations verify that the parity-forbidden transition from the valence band maximum and conduction band minimum (at the Γ point) is not broken by Na⁺/Bi³⁺ doping, and strong optical band-to-band absorption occurs at the L&X points. Time-resolved emission spectra evidence that single-photon and multi-photon pumping leads to the same self-trapped exciton transition and high-order nonlinear absorption will not induce a remarkable thermal effect. Finally, we demonstrate that the Cs₂Na_0.4Ag_0.6In_0.99Bi_0.01Cl₆ DP shows great potential for next-generation wavelength-selective and highly sensitive multiphoton imaging applications.

Lead-Free Solid State Mechanochemical Synthesis of Cs2NaBi1-xFexCl6 Double Perovskite: Reduces Band Gap and Enhances Optical Properties

Efficient and stable lead-free halide double perovskites (DPs) have attracted great attention for the future generation of electronic devices. Herein, we have developed a doping approach to incorporate Fe³⁺ ions into the Cs₂NaBiCl₆ crystal unit and reveal a crystallographic and optoelectronic study of the Cs₂NaBi_1-xFe_xCl₆ double perovskite. We report a simple solid-state mechanochemical method that has a solvent-free, one-step, green chemistry approach for the synthesis of Cs₂NaBi_1-xFe_xCl₆ phosphor. The analysis of powder X-ray diffraction (XRD) data determines the contraction of the lattice due to the incorporation of Fe³⁺ cations, and this effect is well supported by X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and solid-state nuclear magnetic resonance spectroscopy (ss-NMR). The band gap is reduced with increasing Fe content owing to the strong overlap of the Fe-3d orbitals with Cl-3p orbitals and shift of the valence band maxima (VBM) toward higher energies, as confirmed by ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) analyses. Photoluminescence (PL) studies of Cs₂NaBi_1-xFe_xCl₆ phosphors exhibit a large Stokes shift, broadband emission, and increased PL intensity more than ten times for 15% of Fe content phosphor with enhancement in the average decay lifetimes (up to 38 ns) compared to pristine Cs₂NaBiCl₆ DP. These results indicate that the transition of dark self-trapping of excitons (STEs) into bright STEs enhances yellow emission. XRD, UV, and thermo-gravimetric analysis (TGA) confirmed that the Cs₂NaB_1-xFe_xCl₆ DPs have good structural and thermal stabilities. Our findings indicate that the doping of Fe³⁺ cations into the Cs₂NaBiCl₆ lattice is a constructive strategy to enhance significantly the optoelectronic properties of these phosphors.

Enhanced Luminescence of Dye-Decorated ZIF-8 Composite Films via Controllable D-A Interactions for White Light Emission

Metal-organic frameworks (MOFs) constructed by metal ions/clusters and organic linkers are used to encapsulate fluorescent guest species with aggregation-caused quenching (ACQ) effects to enhance fluorescence properties due to their porous structures and high specific surface areas. However, there would be a problem of matching between MOF pores and guest molecules' sizes. In this paper, amorphous ZIF-8 was modified by carboxyl functional groups (H₃BTC-ZIF-8) via introducing the 1,2,4-benzenetricarbonic acid (H₃BTC) ligand into the ZIF-8 sol system. Moreover, H₃BTC-ZIF-8 was used for the loading of organic fluorescent dyes rhodamine 6G (R6G) and coumarin 151 (C151) to prepare R6G/C151/H₃BTC-ZIF-8 composite films. A white-light-emitting composite film (R6G/C151/H₃BTC-ZIF-8) with CIE coordinates of (0.323, 0.347) was successfully prepared by compounding fluorescent dyes (R6G and C151) with H₃BTC-modified ZIF-8, whose photoluminescence quantum yield (PLQY) can reach 64.0%. It was higher than the PLQY of the composite films prepared by crystalline ZIF-8 (40.2%) or amorphous ZIF-8 without H₃BTC (48.0%) compounded with the same concentrations of dyes. The fluorescence enhancement was probably attributed to an increased amount of active sites of H₃BTC-modified ZIF-8 interacting with dyes C151 and R6G. This can form hydrogen bonds between H₃BTC-ZIF-8 and C151, and weak electron donor-acceptor (D-A) interactions between H₃BTC-ZIF-8 and R6G molecules, respectively, thus enhancing the interactions between dyes and ZIF-8 and reducing the ACQ effect existing between dye molecules. Therefore, this strategy could provide an important guidance to develop white-light-emissive materials.

Triangulating Dopant-Level Mn(II) Insertion in a Cs2NaBiCl6 Double Perovskite Using Magnetic Resonance Spectroscopy

Lead-free metal halide double perovskites are gaining increasing attention for optoelectronic applications. Specifically, doping metal halide double perovskites using transition metals enables broadband tailorability of the optical bandgap for these emerging semiconducting materials. One candidate material is Mn(II)-doped Cs₂NaBiCl₆, but the nature of Mn(II) insertion on chemical structure is poorly understood due to low Mn loading. It is critical to determine the atomic-level structure at the site of Mn(II) incorporation in doped perovskites to better understand the structure-property relationships in these materials and thus to advance their applicability to optoelectronic applications. Magnetic resonance spectroscopy is uniquely qualified to address this, and thus a comprehensive three-pronged strategy, involving solid-state nuclear magnetic resonance (NMR), high-field dynamic nuclear polarization (DNP), and electron paramagnetic resonance (EPR) spectroscopies, is used to identify the location of Mn(II) insertion in Cs₂NaBiCl₆. Multinuclear (²³Na, ³⁵Cl, ¹³³Cs, and ²⁰⁹Bi) one-dimensional (1D) magnetic resonance spectra reveal a low level of Mn(II) incorporation, with select spins affected by paramagnetic relaxation enhancement (PRE) induced by Mn(II) neighbors. EPR measurements confirm the oxidation state, octahedral symmetry, and low doping levels of the Mn(II) centers. Complementary EPR and NMR measurements confirm that the cubic structure is maintained with Mn(II) incorporation at room temperature, but the structure deviates slightly from cubic symmetry at low temperatures (<30 K). HYperfine Sublevel CORrelation (HYSCORE) EPR spectroscopy explores the electron-nuclear correlations of Mn(II) with ²³Na, ¹³³Cs, and ³⁵Cl. The absence of ²⁰⁹Bi correlations suggests that Bi centers are replaced by Mn(II). Endogenous DNP NMR measurements from Mn(II) → ¹³³Cs (<30 K) reveal that the solid effect is the dominant mechanism for DNP transfer and supports that Mn(II) is homogeneously distributed within the double-perovskite structure.

Tailoring Photophysical Dynamics in a Hybrid Gallium-Bismuth Heterometallic Halide by Transferring from an Indirect to a Direct Band Structure

Low-dimensional lead-free metal halides have emerged as novel luminous materials for solid-state lighting, remote thermal imaging, X-ray scintillation, and anticounterfeiting labeling applications. However, the influence of band structure on the intriguing optical property has rarely been explored, especially for low-dimensional hybrid heterometallic halides. In this study, we have developed a lead-free zero-dimensional gallium-bismuth hybrid heterometallic halide, A₈(GaCl₄)₄(BiCl₆)₄ (A = C₈H₂₂N₂), that is photoluminescence (PL)-inert because of its indirect-band-gap character. Upon rational composition engineering, parity-forbidden transitions associated with the indirect band gap have been broken by replacing partial Ga³⁺ with Sb³⁺, which contains an active outer-shell 5s² lone pair, resulting in a transition from an indirect to a direct band gap. As a result, broadband yellow PL centered at 580 nm with a large Stokes shift over 200 nm is recorded. Such an emission is attributed to the radiative recombination of an allowed direct transition from triplet ³P₁ states of Sb³⁺ based on experimental characterizations and theoretical calculations. This study provides not only important insights into the effect of the band structure on the photophysical properties but a guidance for the design of new hybrid heterometallic halides for optoelectronic applications.

Compact ultrabroadband light-emitting diodes based on lanthanide-doped lead-free double perovskites

Impurity doping is an effective approach to tuning the optoelectronic performance of host materials by imparting extrinsic electronic channels. Herein, a family of lanthanide (Ln³⁺) ions was successfully incorporated into a Bi:Cs₂AgInCl₆ lead-free double-perovskite (DP) semiconductor, expanding the spectral range from visible (Vis) to near-infrared (NIR) and improving the photoluminescence quantum yield (PLQY). After multidoping with Nd, Yb, Er and Tm, Bi/Ln:Cs₂AgInCl₆ yielded an ultrabroadband continuous emission spectrum with a full width at half-maximum of ~365 nm originating from intrinsic self-trapped exciton recombination and abundant 4f-4f transitions of the Ln³⁺ dopants. Steady-state and transient-state spectra were used to ascertain the energy transfer and emissive processes. To avoid adverse energy interactions between the various Ln³⁺ ions in a single DP host, a heterogeneous architecture was designed to spatially confine different Ln³⁺ dopants via a "DP-in-glass composite" (DiG) structure. This bottom-up strategy endowed the prepared Ln³⁺-doped DIG with a high PLQY of 40% (nearly three times as high as that of the multidoped DP) and superior long-term stability. Finally, a compact Vis-NIR ultrabroadband (400~2000 nm) light source was easily fabricated by coupling the DiG with a commercial UV LED chip, and this light source has promising applications in nondestructive spectroscopic analyses and multifunctional lighting.

Boosting the Self-Trapped Exciton Emission in Alloyed Cs2 (Ag/Na)InCl6 Double Perovskite via Cu+ Doping

Fundamental understanding of the effect of doping on the optical properties of 3D double perovskites (DPs) especially the dynamics of self-trapped excitons (STEs) is of vital importance for their optoelectronic applications. Herein, a unique strategy via Cu⁺ doping to achieve efficient STE emission in the alloyed lead-free Cs₂ (Ag/Na)InCl₆ DPs is reported. A small amount (1.0 mol%) of Cu⁺ doping results in boosted STE emission in the crystals, with photoluminescence (PL) quantum yield increasing from 19.0% to 62.6% and excitation band shifting from 310 to 365 nm. Temperature-dependent PL and femtosecond transient absorption spectroscopies reveal that the remarkable PL enhancement originates from the increased radiative recombination rate and density of STEs, as a result of symmetry breakdown of the STE wavefunction at the octahedral Ag⁺ site. These findings provide deep insights into the STE dynamics in Cu⁺ -doped Cs₂ (Ag/Na)InCl₆ , thereby laying a foundation for the future design of new lead-free DPs with efficient STE emission.

Solid-state nuclear magnetic resonance of spin-9/2 nuclei 115 In and 209 Bi in functional inorganic complex oxides

Indium and bismuth are technologically important elements, in particular as oxides for optoelectronic applications. ¹¹⁵ In and ²⁰⁹ Bi are both I = 9/2 nuclei with high natural abundances and moderately high frequencies but large nuclear electric quadrupole moments. Leveraging the quadrupolar interaction as a measure of local symmetry and polyhedral distortions for these nuclei could provide powerful insights on a range of applied materials. However, the absence of reported nuclear magnetic resonance (NMR) parameters on these nuclei, particularly in oxides, hinders their use by the broader materials community. In this contribution, solid-state ¹¹⁵ In and ²⁰⁹ Bi NMR of three recently discovered quaternary bismuth or indium oxides are reported, supported by density functional theory calculations, numerical simulations, diffraction and additional multinuclear (²⁷ Al, ^69,71 Ga, and ¹²¹ Sb) solid-state NMR measurements. The compounds LiIn₂ SbO₆ , BiAlTeO₆ , and BiGaTeO₆ are measured without special equipment at 9.4 T, demonstrating that wideline techniques such as the QCPMG pulse sequence and frequency-stepped acquisition can enable straightforward extraction of quadrupolar tensor information in I = 9/2 ¹¹⁵ In and ²⁰⁹ Bi even in sites with large quadrupolar coupling constants. Relationships are described between the NMR observables and local site symmetry. These are amongst the first reports of the NMR parameters of ¹¹⁵ In, ¹²¹ Sb, and ²⁰⁹ Bi in oxides.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of ¹H, ¹³C, ¹⁵N, ¹⁴N, ¹³³Cs, ⁸⁷Rb, ³⁹K, ²⁰⁷Pb, ¹¹⁹Sn, ¹¹³Cd, ²⁰⁹Bi, ¹¹⁵In, ¹⁹F and ²H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

Formation and Near-Infrared Emission of CsPbI3 Nanoparticles Embedded in Cs4PbI6 Crystals

Cs₄PbI₆, as a rarely investigated member of the Cs₄PbX₆ (X is a halogen element) family, has been successfully synthesized at low temperatures, and the synthetic conditions have been optimized. Metal iodides such as LiI, KI, NiI₂, CoI₂, and ZnI₂, as additives, play an important role in enhancing the formation of the Cs₄PbI₆ microcrystals. ZnI₂ with the lowest dissociation energy is the most efficient additive to supply iodide ions, and its amount of addition has also been optimized. Strong red to near-infrared (NIR) emission properties have been detected, and its optical emission centers have been identified to be numerous embedded perovskite-type α-CsPbI₃ nanocrystallites (∼5 nm in diameter) based on investigations of temperature- and pressure-dependent photoluminescent properties. High-resolution transmission electron microscopy was used to detect these hidden nanoparticles, although the material was highly beam-sensitive and confirmed a "raisin bread"-like structure of the Cs₄PbI₆ crystals. A NIR mini-LED for the biological application has been successfully fabricated using as-synthesized Cs₄PbI₆ crystals. This work provides information for the future development of infrared fluorescent nanoscale perovskite materials.

Lead-Free Halide Double Perovskite Nanocrystals for Light-Emitting Applications: Strategies for Boosting Efficiency and Stability

Lead-free halide double perovskite (HDP) nanocrystals are considered as one of the most promising alternatives to the lead halide perovskite nanocrystals due to their unique characteristics of nontoxicity, robust intrinsic thermodynamic stability, rich and tunable optoelectronic properties. Although lead-free HDP variants with highly efficient emission are synthesized and characterized, the photoluminescent (PL) properties of colloidal HDP nanocrystals still have enormous challenges for application in light-emitting diode (LED) devices due to their intrinsic and surface defects, indirect band, and disallowable optical transitions. Herein, recent progress on the synthetic strategies, ligands passivation, and metal doping/alloying for boosting efficiency and stability of HDP nanocrystals is comprehensive summarized. It begins by introducing the crystalline structure, electronic structure, and PL mechanism of lead-free HDPs. Next, the limiting factors on PL properties and origins of instability are analyzed, followed by highlighting the effects of synthesis strategies, ligands passivation, and metal doping/alloying on the PL properties and stability of the HDPs. Then, their preliminary applications for LED devices are emphasized. Finally, the challenges and prospects concerning the development of highly efficient and stable HDP nanocrystals-based LED devices in the future are proposed.

Influence of hidden halogen mobility on local structure of CsSn(Cl1-x Br x )3 mixed-halide perovskites by solid-state NMR

Tin halide perovskites are promising candidates for lead-free photovoltaic and optoelectronic materials, but not all of them have been well characterized. It is essential to determine how the bulk photophysical properties are correlated with their structures at both short and long ranges. Although CsSnCl₃ is normally stable in the cubic perovskite structure only above 379 K, it was prepared as a metastable phase at room temperature. The transition from the cubic to the monoclinic phase, which is the stable form at room temperature, was tracked by solid-state ¹³³Cs NMR spectroscopy and shown to take place through a first-order kinetics process. The complete solid solution CsSn(Cl_1−xBr_x)₃ (0 ≤ x ≤ 1) was successfully prepared, exhibiting cubic perovskite structures extending between the metastable CsSnCl₃ and stable CsSnBr₃ end-members. The NMR spectra of CsSnBr₃ samples obtained by three routes (high-temperature, mechanochemical, and solvent-assisted reactions) show distinct chemical shift ranges, spin-lattice relaxation parameters and peak widths, indicative of differences in local structure, defects and degree of crystallinity within these samples. Variable-temperature ¹¹⁹Sn spin-lattice relaxation measurements reveal spontaneous mobility of Br atoms in CsSnBr₃. The degradation of CsSnBr₃, exposed to an ambient atmosphere for nearly a year, was monitored by NMR spectroscopy and powder X-ray diffraction, as well as by optical absorption spectroscopy.

Unravelling the atomic-level chemical structure, slow phase conversion or degradation pathways and rapid halogen hopping of cesium tin(ii) halide perovskites using solid-state ¹¹⁹Sn and ¹³³Cs NMR spectroscopy.

The atomic-level structure of bandgap engineered double perovskite alloys Cs2AgIn1-x Fe x Cl6

Although lead-free halide double perovskites are considered as promising alternatives to lead halide perovskites for optoelectronic applications, state-of-the-art double perovskites are limited by their large bandgap. The doping/alloying strategy, key to bandgap engineering in traditional semiconductors, has also been employed to tune the bandgap of halide double perovskites. However, this strategy has yet to generate new double perovskites with suitable bandgaps for practical applications, partially due to the lack of fundamental understanding of how the doping/alloying affects the atomic-level structure. Here, we take the benchmark double perovskite Cs₂AgInCl₆ as an example to reveal the atomic-level structure of double perovskite alloys (DPAs) Cs₂AgIn_1−xFe_xCl₆ (x = 0–1) by employing solid-state nuclear magnetic resonance (ssNMR). The presence of paramagnetic alloying ions (e.g. Fe³⁺ in this case) in double perovskites makes it possible to investigate the nuclear relaxation times, providing a straightforward approach to understand the distribution of paramagnetic alloying ions. Our results indicate that paramagnetic Fe³⁺ replaces diamagnetic In³⁺ in the Cs₂AgInCl₆ lattice with the formation of [FeCl₆]³⁻·[AgCl₆]⁵⁻ domains, which show different sizes and distribution modes in different alloying ratios. This work provides new insights into the atomic-level structure of bandgap engineered DPAs, which is of critical significance in developing efficient optoelectronic/spintronic devices.

Through Fe³⁺-alloying, the bandgap of benchmark double perovskite Cs₂AgInCl₆ can be tuned from 2.8 eV to 1.6 eV. The atomic-level structure of Cs₂AgIn_1−xFe_xCl₆ was revealed by solid-state nuclear magnetic resonance (ssNMR).

Unique Behavior of Halide Double Perovskites with Mixed Halogens

Engineering halide double perovskite (A₂M⁺M³⁺X^VII₆) by mixing elements is a viable way to tune its electronic and optical properties. In spite of many emerging experiments on halide double perovskite alloys, the basic electronic properties of the alloys have not been fully understood. In this work, we chose Cs₂AgBiCl₆ as an example and systematically studied electronic properties of its different site alloys Cs₂Na_xAg_1-xBiCl₆, Cs₂AgSb_xBi_1-xCl₆, and Cs₂AgBi(Br_xCl_1-x)₆ (x = 0.25, 0.5, 0.75) by first-principles calculations. Interestingly, the halogen site alloy shows opposite behavior to M⁺ and M³⁺ cation site alloys; that is, Cs₂AgBi(Br_xCl_1-x)₆ displays virtual crystal behavior without substantial broadening, while Cs₂Na_xAg_1-xBiCl₆ and Cs₂AgSb_xBi_1-xCl₆ show split-band behaviors with substantial broadening, which indicates that lifetimes of electrons and holes in Cs₂AgBi(Br_xCl_1-x)₆ would be longer than those in Cs₂Na_xAg_1-xBiCl₆ and Cs₂AgSb_xBi_1-xCl₆. We further found that long lifetimes of electrons and holes are common for mixed halide perovskites. Moreover, the band alignment is provided to determine the band gap change of alloys and to understand the transport of electrons and holes when these pure compounds form heterostructures. Our systematical studies should be helpful for future optoelectronic applications of halide perovskites.