One-step low-temperature solid-state synthesis of lead-free cesium copper halide Cs3Cu2Br5 phosphors with bright blue emissions

The development of new efficient and environmental-friendly inorganic phosphors is highly important for phosphor-converted white light-emitting diodes and optoelectronic displays. In this paper, we report a novel and simple one-step solid-state reaction to synthesize lead-free cesium copper halide Cs₃Cu₂Br₅ powders with bright blue emission around 460 nm. The preparation process is performed at low calcination temperatures (290–320 °C) and requires no complex apparatus, reagents, or techniques, and thus it holds great potential for mass production of halide emitters. The Cs₃Cu₂Br₅ powder phosphors are characterized by X-ray diffraction, field-emission scanning electron microscopy, energy dispersive X-ray spectroscopy, photoluminescence excitation and emission spectroscopy, CIE color coordinates, color purity, photoluminescence quantum yield, and temperature-dependent emission spectra. The effect of calcination temperature on the photoluminescence properties of the as-prepared Cs₃Cu₂Br₅ blue phosphors has been investigated and the air-stability property has also been discussed. Impressively, Cs₃Cu₂Br₅ blue phosphors show outstanding stability toward air exposure exceeding one month (30 days). Our work will open a new and simple approach to obtain excellent halide emitters for future solid-state lighting and displays.     

Room-Temperature Synthesized Cd-Doped Cs3Cu2I5: Stable and Excitation-Wavelength Dependent Dual-Color Emission for Advanced Anti-Counterfeiting

With the development of intellectual properties, concern about advanced anti-counterfeiting is accumulating, which conventional single-modal luminescence can no longer satisfy. As one of rapidly developing 0D materials, copper-based perovskite shows great potential to realize multiple luminescent centers for complex emissions. In this work, Cd was doped into Cs₃Cu₂I₅ and showed resultant emission at 560 nm which surprisingly showed a red-shifted excitation peak from that of intrinsic self-trapped emission, resulting in excitation-wavelength dependent emission. When the excitation wavelength increased from 290 nm to 330 nm, the emission of Cd-doped Cs₃Cu₂I₅ changed from deep blue to cold white and finally yellow. Afterwards, Cd-doped Cs₃Cu₂I₅ was mixed with polystyrene to prepare anti-counterfeiting ink for silk-screen printing. Meanwhile, Cd-doped Cs₃Cu₂I₅ maintains outstanding stability after doping, no matter under ambient, continuous UV radiation or high-temperature environment. The intensity can be almost totally recovered after heating-and-cooling cycles. This study lays groundwork for future research into multiple luminescent center manipulation in 0D materials.     

Lead-Free,Water-Stable A3Bi2I9 Perovskites: Crystal Growth and Blue-Emitting Quantum Dots [A=CH3NH3+, Cs+, and (Rb0.05Cs2.95)+]

Despite the great success in the increase in the power conversion efficiency of lead halide perovskite solar cells, the toxicity of lead and the unstable nature of the materials are still major concerns for their wider implementation at the industrial level. Herein, large-size single crystals (SCs) are developed in HI solution by using a temperature lowering method and nanocrystals (NCs) of A₃Bi₂I₉ perovskites [where A=CH₃NH₃⁺ (MA)⁺, Cs⁺, and (Rb_0.05Cs_2.95)⁺] are formed in ethanol (EtOH) and toluene (TOL). The stability of A₃Bi₂I₉ perovskite is investigated by immersing the SCs for 24 h and pellets for 12 h in water. Moreover, the A₃Bi₂I₉ perovskite NCs displays a promising photoluminescence quantum yield of 17.63 % and a long lifetime of 8.20 ns.     

Colloidal Synthesis and Optical Properties of All‐Inorganic Low‐Dimensional Cesium Copper Halide Nanocrystals

Low‐dimensional metal halides have recently attracted extensive attention owing to their unique structure and photoelectric properties. Herein, we report the colloidal synthesis of all‐inorganic low‐dimensional cesium copper halide nanocrystals (NCs) by adopting a hot‐injection approach. Using the same reactants and ligands, but different reaction temperatures, both 1D CsCu₂I₃ nanorods and 0D Cs₃Cu₂I₅ NCs can be prepared. Density functional theory indicates that the reduced dimensionality in 1D CsCu₂I₃ compared to 0D Cs₃Cu₂I₅ makes the excitons more localized, which accounts for the strong emission of 0D Cs₃Cu₂I₅ NCs. Subsequent optical characterization reveals that the highly luminescent, strongly Stokes‐shifted broadband emission of 0D Cs₃Cu₂I₅ NCs arises from the self‐trapped excitons. Our findings not only present a method to control the synthesis of low‐dimensional cesium copper halide nanocrystals but also highlight the potential of 0D Cs₃Cu₂I₅ NCs in optoelectronics.     

Lead‐Free,Blue Emitting Bismuth Halide Perovskite Quantum Dots

Lead halide perovskite quantum dots (QDs) are promising candidates for future lighting applications, due to their high quantum yield, narrow full width at half maximum (FWHM), and wide color gamut. However, the toxicity of lead represents a potential obstacle to their utilization. Although tin(II) has been used to replace lead in films and QDs, the high intrinsic defect density and oxidation vulnerability typically leads to unsatisfactory material properties. Bismuth, with much lower toxicity than lead, is promising to constitute lead‐free perovskite materials because Bi³⁺ is isoelectronic to Pb²⁺ and more stable than Sn²⁺. Herein we report, for the first time, the synthesis and optical characterization of MA₃Bi₂Br₉ perovskite QDs with photoluminescence quantum yield (PLQY) up to 12 %, which is much higher than Sn‐based perovskite nanocrystals. Furthermore, the photoluminescence (PL) peaks of MA₃Bi₂X₉ QDs could be easily tuned from 360 to 540 nm through anion exchange.     

Lead‐Free,Blue Emitting Bismuth Halide Perovskite Quantum Dots

Lead halide perovskite quantum dots (QDs) are promising candidates for future lighting applications, due to their high quantum yield, narrow full width at half maximum (FWHM), and wide color gamut. However, the toxicity of lead represents a potential obstacle to their utilization. Although tin(II) has been used to replace lead in films and QDs, the high intrinsic defect density and oxidation vulnerability typically leads to unsatisfactory material properties. Bismuth, with much lower toxicity than lead, is promising to constitute lead‐free perovskite materials because Bi³⁺ is isoelectronic to Pb²⁺ and more stable than Sn²⁺. Herein we report, for the first time, the synthesis and optical characterization of MA₃Bi₂Br₉ perovskite QDs with photoluminescence quantum yield (PLQY) up to 12 %, which is much higher than Sn‐based perovskite nanocrystals. Furthermore, the photoluminescence (PL) peaks of MA₃Bi₂X₉ QDs could be easily tuned from 360 to 540 nm through anion exchange.     

Luminescent Thin Films Enabled by CsPbX3 (X=Cl,Br, I) Precursor Solution

Inorganic cesium lead halide perovskite nanocrystals are candidates for lighting and display materials due to their outstanding optoelectronic properties. However, the dissolution issue of perovskite nanocrystals in polar solvents remains a challenge for practical applications. Herein, we present a newly designed one-step spin-coating strategy to prepare a novel multicolor-tunable CsPbX₃ (X=Cl, Br, I) nanocrystal film, where the CsPbX₃ precursor solution was formed by dissolving PbO, Cs₂CO₃, and CH₃NH₃X into the ionic liquid n-butylammonium butyrate. The as-designed CsPbX₃ nanocrystal films show high color purity with a narrow emission width. Also, the blue CsPb(Cl/Br)₃ film demonstrates an absolute photoluminescence quantum yields (PLQY) of 15.6 %, which is higher than 11.7 % of green CsPbBr₃ and 8.3 % of red CsPb(Br/I)₃ film. This study develops an effective approach to preparing CsPbX₃ nanocrystal thin films, opening a new avenue to design perovskite nanocrystals-based devices for lighting and display applications.     

Lead‐Free Sodium–Indium Double Perovskite Nanocrystals through Doping Silver Cations for Bright Yellow Emission

Lead‐free halide perovskite nanocrystals (NCs) have drawn wide attention for solving the problem of lead perovskites toxicity and instability. Herein, we synthesize the direct band gap double perovskites undoped and Ag‐doped Cs₂NaInCl₆ NCs by variable temperature hot injection. The Cs₂NaInCl₆ NCs have little photoluminescence because of dark self‐trapped excitons (STEs). The dark STEs can be converted into bright STEs by doping with Ag⁺ to produce a bright yellow emission, with the highest photoluminescence quantum efficiency of 31.1 %. The dark STEs has been directly detected experimentally by ultrafast transient absorption (TA) techniques. The dynamics mechanism is further studied. In addition, the Ag‐doped NCs show better stability than the undoped ones. This result provides a new way to enhance the optical properties of lead‐free perovskites NCs for high‐performance light emitters.     

Stabilizing RbPbBr3 Perovskite Nanocrystals through Cs+ Substitution

ABX₃-type halide perovskite nanocrystals (NCs) have been a hot topic recently due to their fascinating optoelectronic properties. It has been demonstrated that A-site ions have an impact on their photophysical and chemical properties, such as the optical band gap and chemical stability. The pursuit of halide perovskite materials with diverse A-site species would deepen the understanding of the structure–property relationship of the perovskite family. In this work we have attempted to synthesize rubidium-based perovskite NCs. We have discovered that the partial substitution of Rb⁺ by Cs⁺ help to stabilize the orthorhombic RbPbBr₃ NCs at low temperature, which otherwise can only be obtained at high temperature. The inclusion of Cs⁺ into the RbPbBr₃ lattice results in highly photoluminescent Rb_1−xCs_xPbBr₃ NCs. With increasing amounts of Cs⁺, the band gaps of the Rb_1−xCs_xPbBr₃ NCs decrease, leading to a redshift of the photoluminescence peak. Also, the Rb_1−xCs_xPbBr₃ NCs (x=0.4) show good stability under ambient conditions. This work demonstrates the high structural flexibility and tunability of halide perovskite materials through an A-site cation substitution strategy and sheds light on the optimization of perovskite materials for application in high-performance optoelectronic devices.     

反溶剂法快速合成高效发光二维锡卤钙钛矿材料

铅卤钙钛矿材料由于其优异的光电性质而受到了广泛关注. 但是, 材料中铅的毒性问题极大地阻碍了其大规模应用. 因此, 寻找与铅卤钙钛矿具有相似光电性质的非铅卤化物钙钛矿材料十分重要. 其中, 锡基卤化物钙钛矿被认为是铅基钙钛矿材料最佳的替代材料之一. 本文通过简便的反溶剂方法, 合成了一系列新型二维(RNH₃)₂SnX₄(R为烷基链, X=Br^-, I^-)钙钛矿材料. 研究结果表明, 所合成的材料具有优异的荧光发射性质, 发光量子效率高达98.5%, 比三维ASnX₃[A=Cs⁺, 甲胺(MA⁺), 甲脒(FA⁺)等]型钙钛矿表现出更好的稳定性. 本文所采用的合成方法简单易行, 有利于实现金属卤化物钙钛矿材料的大规模合成及在固态照明器件和显示器件领域的工业应用.     

Efficient Thermally Activated Delayed Fluorescence from All-Inorganic Cesium Zirconium Halide Perovskite Nanocrystals

Thermally activated delayed fluorescence (TADF) is generally observed in solid-state organic molecules or metal-organic complexes. However, TADF in all-inorganic colloidal nanocrystals (NCs) is rare. Herein, we report the first colloidal synthesis of an air-stable all-inorganic lead-free Cs₂ZrCl₆ perovskite NCs. The Cs₂ZrCl₆ NCs exhibit long-lived triplet excited state (138.2 μs), and feature high photoluminescence (PL) quantum efficiency (QY=60.37 %) due to TADF mechanism. The emission color can be easily tuned from blue to green by synthesizing the mixed-halide Cs₂ZrBr_xCl_6−x (0≤x≤1.5) NCs. Femtosecond transient absorption and temperature dependent PL measurements are performed to clarify the emission mechanism. In addition, Bi³⁺ ions are successfully doped into Cs₂ZrCl₆ NCs, which further extends the PL properties. This work not only develops a new lead-free halide perovskite NCs for potential optoelectronic applications, but also offers unique strategies for developing new inorganic phosphors.     

蓝光钙钛矿发光二极管:机遇与挑战

金属卤化钙钛矿由于具有优异的光电性能(如:高电子/空穴迁移率,高荧光量子产率,高色纯度,以及光色可调性等),成为应用于发光二极管(LED)的理想材料。近年来,钙钛矿LED的发展十分迅速,红光和绿光钙钛矿LED的外量子效率(EQE)均已超过20%。然而,蓝光(尤其是深蓝光)钙钛矿LED的EQE以及稳定性依然相对落后,这严重制约了钙钛矿LED在高性能、广色域显示领域和高显色指数白光照明领域的应用。因此,总结现阶段蓝光钙钛矿LED的发展,并剖析其机遇与挑战,对未来蓝光甚至整个钙钛矿LED领域的发展至关重要。本文将蓝光钙钛矿LED根据光色细分为天蓝光、纯蓝光、深蓝光三大部分进行总结,回顾了三种LED器件的发展历程,并详细阐述了现阶段实现他们的主要手段以及相关的基础原理,最后分析了它们各自的问题并提出了相应的解决思路。     

Synthesis and luminescent properties of polymer-silica multilayer-encapsulated perovskite quantum dots for optoelectronics

Hydrophobic all-inorganic perovskite quantum dots (IPQDs) CsPbX₃ (X = Cl, Br, and I) were synthesized using conventional solution methods. IPQDs are potential materials for lasers, solar cells, and displays because of their narrow full width at half-maximum, optically tunable wavelength, and great photoluminescence quantum yield. Their shortcomings are structurally unstable and moisture sensitive. To overcome the poor stability, we have developed a chemical modification to form GQD + PMAO@SiO₂ that enhances their photoluminescence quantum yield (from 60 to ~90%), thermal stability (from 60 to ~90°C), and photochemical stability. The combination of the green-emitting GQD + PMAO@SiO₂ and red-emitting RQD + PMAO with a 450 nm blue InGaN-based LED chip was used to fabricate a white light emitting diode with the correlated color temperature (CCT) value of 6,943 K and Commission Internationale del'Eclairage of (0.312, 0.290).     

Aqueous Synthesis of Methylammonium Lead Halide Perovskite Nanocrystals

Methylammonium lead halide perovskite nanocrystals offer attractive optoelectronic properties but suffer from fast degradation in the presence of water. In contradiction to this observation, we demonstrate the possibility of a direct aqueous synthesis of CH₃NH₃PbX₃ (X=Br or Cl/Br) nanocrystals through the reaction between the lead halide complex and methylamine when the pH is maintained in the range of 0–5. Under these synthetic conditions, the positively charged surface of the perovskite nanocrystals and the proper ionic balance help to prevent their decomposition in water. Additional surface capping with organic amine ligands further improves the photoluminescence quantum yield of the perovskite nanocrystals to values close to 40 %, ensures their stability under ambient conditions for several months, and their photoluminescence performance under continuous 0.1 W mm^?2 405 nm light irradiation for over 250 hours.     

Impact of Zn-doping on the composition,stability, luminescence properties of silica coated all-inorganic cesium lead bromide nanocrystals and their biocompatibility

Cesium lead halide (CsPbX₃: X = I, Br, Cl) nanocrystals (NCs) are believed to be potential candidates for bioimaging applications. However, their low structural stability against polar solvents remains as a major limitation. To improve the NCs stability and maintain high emission intensity, we synthesized silica coated Zn-doped core@shell perovskite NCs via modified ligand assisted reprecipitation (LARP) synthetic method under relatively high humid condition. We systemically varied the composition inside the perovskite structure and then studied their photophysical properties and stability. Interestingly, the Zn-doping amount controls the ratio of CsPbBr₃ to Cs₄PbBr₆ perovskites inside the core and also facilitates the growth of (OA)₂PbBr₄ shell, enables overall increase in NCs emission intensity and stability. We observed green color emission from these NCs in the spectral range of 494-506 nm with a maximum photoluminescence quantum yield (PLQY) up to 88%. The optimized Zn-doped NCs exhibited nearly four times better water stability compared to the bare NCs and retain emission properties for several months even in highly polar solvents. Finally, we performed biocompatibility test of the NCs generated on biological samples and hydroponics test in a gardenia leaf for their potential bioimaging applications.     

Achieving Color-Tunable Long Persistent Luminescence in Cs2CdCl4 Ruddlesden-Popper Phase Perovskites

Two-dimensional (2D)-halide perovskites have been enriched over recent years to offer remarkable features from diverse chemical structures and environmental stability endowed with exciting functionalities in photoelectric detectors and phosphorescence systems. However, the low conversion efficiency of singlet to triplet in 2D hybrid halide perovskites reduces phosphorescence lifetimes. In this study, the long persistent luminescence of 2D all-inorganic perovskites with a self-assembled 2D interlayer galleries structure is investigated. The results show that the decay time of the long persistent luminescence increases from 450 s to 600 s, and the luminescence color changes from cyan to orange, and the thermal stability of photoluminescence enhances dramatically after replacing Cd²⁺ by appropriate Mn²⁺ ions in 2D Cs₂CdCl₄ Ruddlesden-Popper phase perovskites. Furthermore, diversified anti-counterfeiting modes are fabricated to highlight the promising applications of Cs₂CdCl₄ perovskite systems with tunable persistent luminescence in advanced anti-counterfeiting. Therefore, our studies provide a novel model for realizing tunable long persistent luminescence of perovskite with 2D self-assembled layered structure for advanced anti-counterfeiting.     

Three‐Dimensional Cuprous Lead Bromide Framework with Highly Efficient and Stable Blue Photoluminescence Emission

Considering the instability and low photoluminescence quantum yield (PLQY) of blue‐emitting perovskites, it is still challenging and attractive to construct single crystalline hybrid lead halides with highly stable and efficient blue light emission. Herein, by rationally introducing d¹⁰ transition metal into single lead halide as new structural building unit and optical emitting center, we prepared a bimetallic halide of [(NH₄)₂]CuPbBr₅ with new type of three‐dimensional (3D) anionic framework. [(NH₄)₂]CuPbBr₅ exhibits strong band‐edge blue emission (441 nm) with a high PLQY of 32 % upon excitation with UV light. Detailed photophysical studies indicate [(NH₄)₂]CuPbBr₅ also displays broadband red light emissions derived from self‐trapped states. Furthermore, the 3D framework features high structural and optical stabilities at extreme environments during at least three years. To our best knowledge, this work represents the first 3D non‐perovskite bimetallic halide with highly efficient and stable blue light emission.     

Aqueous Precursor Induced Morphological Change and Improved Water Stability of CsPbBr3 Nanocrystals

In the literature, lead halide perovskites are very notable for their degradation in the presence of polar solvents, such as water. In contrast, in this research, it is observed that adding a minor amount of water into the precursor solution can improve the stability and photoluminescence quantum yield of CsPbBr₃ nanocrystals through a ligand-assisted reprecipitation (LARP) method. In this way, the shape and phase transformation from CsPbBr₃ nanoplates to CsPbBr₃/Cs₄PbBr₆ nanorods and Cs₄PbBr₆ nanowires can be controlled with increasing water content in the precursor solution. Upon adding water up to an ideal amount, CsPbBr₃ maintains its phase and nanoplate morphology. The key role of water amount for tuning the crystallinity, stability, morphology, optical properties, and phase transformation of cesium lead halide perovskite nanocrystals will be beneficial in the future commercialization of optoelectronics.     

Bi3+‐Er3+ and Bi3+‐Yb3+ Codoped Cs2AgInCl6 Double Perovskite Near‐Infrared Emitters

Bi³⁺ and lanthanide ions have been codoped in metal oxides as optical sensitizers and emitters. But such codoping is not known in typical semiconductors such as Si, GaAs, and CdSe. Metal halide perovskite with coordination number 6 provides an opportunity to codope Bi³⁺ and lanthanide ions. Codoping of Bi³⁺ and Ln³⁺ (Ln=Er and Yb) in Cs₂AgInCl₆ double perovskite is presented. Bi³⁺‐Er³⁺ codoped Cs₂AgInCl₆ shows Er³⁺ f‐electron emission at 1540 nm (suitable for low‐loss optical communication). Bi³⁺ codoping decreases the excitation (absorption) energy, such that the samples can be excited with ca. 370 nm light. At that excitation, Bi³⁺‐Er³⁺ codoped Cs₂AgInCl₆ shows ca. 45 times higher emission intensity compared to the Er³⁺ doped Cs₂AgInCl₆. Similar results are also observed in Bi³⁺‐Yb³⁺ codoped sample emitting at 994 nm. A combination of temperature‐dependent (5.7 K to 423 K) photoluminescence and calculations is used to understand the optical sensitization and emission processes.     

Bi3+-Er3+ and Bi3+-Yb3+ Codoped Cs2AgInCl6 Double Perovskite Near-Infrared Emitters

Bi³⁺ and lanthanide ions have been codoped in metal oxides as optical sensitizers and emitters. But such codoping is not known in typical semiconductors such as Si, GaAs, and CdSe. Metal halide perovskite with coordination number 6 provides an opportunity to codope Bi³⁺ and lanthanide ions. Codoping of Bi³⁺ and Ln³⁺ (Ln=Er and Yb) in Cs₂AgInCl₆ double perovskite is presented. Bi³⁺-Er³⁺ codoped Cs₂AgInCl₆ shows Er³⁺ f-electron emission at 1540 nm (suitable for low-loss optical communication). Bi³⁺ codoping decreases the excitation (absorption) energy, such that the samples can be excited with ca. 370 nm light. At that excitation, Bi³⁺-Er³⁺ codoped Cs₂AgInCl₆ shows ca. 45 times higher emission intensity compared to the Er³⁺ doped Cs₂AgInCl₆. Similar results are also observed in Bi³⁺-Yb³⁺ codoped sample emitting at 994 nm. A combination of temperature-dependent (5.7 K to 423 K) photoluminescence and calculations is used to understand the optical sensitization and emission processes.