Room temperature precipitated dual phase CsPbBr3-CsPb2Br5 nanocrystals for stable perovskite light emitting diodes

Reaction-controlled shape evolution and insights into the growth mechanism of CsPbBr3 nanocrystals

The correlation between structural transformation and optical characteristics of cesium lead bromide (CsPbBr3) nanocrystals (NCs) suggests insights into their growth mechanism and optical performance. Systematic control of reaction parameters led to the successful fabrication of on-demand shape-morphing CsPbBr3 NCs. Transmission electron microscopy observations showed that the shape transformation from nanocubes to microcrystals could be accelerated by increasing the precursor:ligand molar ratio and reaction time. Further evidence for orthorhombic CsPbBr3 NCs was obtained from their selected-area electron diffraction pattern, which exhibits a twin domain induced by the presence of large NCs. Likewise, we observed a substantial decrease in photoluminescence (PL) intensity of CsPbBr3 due to surface decomposition or surface ligand loss resulting from increased size. In addition, fusion of smaller particles having other dimensionality induced the increase in the PL full-width at half maximum. In particular, existence of larger bulk material caused a reduction in the peak intensity in the absorption spectra and a trend of decreasing tendency in intensity of the absorption bands related to bromoplumbate species provided direct evidence of fully converted Cs-oleate.

One-Step Preparation of High-Stability CsPbX3/CsPb2X5 Composite Microplates with Tunable Emission

The core-shell structure is an effective means to improve the stability and optoelectronic properties of cesium lead halide (CsPbX3 (X = Cl, Br, I)) perovskite quantum dots (QDs). However, confined by the ionic radius differences, developing a core-shell packaging strategy suitable for the entire CsPbX3 system remains a challenge. In this study, we introduce an optimized hot-injection method for the epitaxial growth of the CsPb2X5 substrate on CsPbX3 surfaces, achieved by precisely controlling the reaction time and the ratio of lead halide precursors. The synthesized CsPbX3/CsPb2X5 composite microplates exhibit an emission light spectrum that covers the entire visible range. Crystallographic analyses and density functional theory (DFT) calculations reveal a minimal lattice mismatch between the (002) plane of CsPb2X5 and the (11 ¯ 0) plane of CsPbX3, facilitating the formation of high-quality type-I heterojunctions. Furthermore, introducing Cl- and I- significantly alters the surface energy of CsPb2X5's (110) plane, leading to an evolutionary morphological shift of grains from circular to square microplates. Benefiting from the passivation of CsPb2X5, the composites exhibit enhanced optical properties and stability. Subsequently, the white light-emitting diode prepared using the CsPbX3/CsPb2X5 composite microplates has a high luminescence efficiency of 136.76 lm/W and the PL intensity decays by only 3.6% after 24 h of continuous operation.

Mechanical milling processed highly luminescent Cs-Pb-Br perovskite emitters

We report well-dispersed highly emitting perovskite emitters synthesized via the surfactant-assisted ball-milling method. Both the emitting peaks and the colour purity of the synthesized perovskite emitters can be effectively tuned through additive functionalization and precursor engineering.

Total Luminescence Spectroscopy for Quantification of Temperature Effects on Photophysical Properties of Photoluminescent Materials

Quantification of the temperature effects on the optical properties of photoluminescent (PL) materials is important for a fundamental understanding of both materials optical processes and rational PL materials design and applications. However, existing techniques for studying the temperature effects are limited in their information content. Reported herein is a temperature-dependent total photoluminescence (TPL) spectroscopy technique for probing the temperature dependence of materials optical properties. When used in combination with UV-vis measurements, this TPL method enables experimental quantification of temperature effects on fluorophore fluorescence intensity and quantum yield at any combination of excitation and detection wavelengths, including the fluorophore Stokes-shifted and anti-Stokes-shifted fluorescence. All model polyaromatic hydrocarbon (PAH) and xanthene fluorophores exhibited a strong excitation- and emission-wavelength dependence in their temperature effects. However, the heavy-atom effects used for explaining the strong temperature dependence of brominated anthracenes are not operative with xanthene fluorophores that have heavy atom substitutions. The insights from TPL measurements are important not only for enhancing the fundamental understandings of the materials photophysical properties but also for rational measurement design for applications where the temperature sensitivity of the fluorophore fluorescence is critical. An example application is demonstrated for developing a sensitive and robust ratiometric fluorescence thermometric method for in situ real-time monitoring of sample temperatures inside a fluorescence cuvette placed in a temperature-controlled sample holder.

Highly Stable and Photoluminescent CsPbBr3/Cs4PbBr6 Composites for White-Light-Emitting Diodes and Visible Light Communication

Inorganic lead halide perovskite is one of the most excellent fluorescent materials, and it plays an essential role in high-definition display and visible light communication (VLC). Its photochromic properties and stability determine the final performance of light-emitting devices. However, efficiently synthesizing perovskite with high quality and stability remains a significant challenge. Here, we develop a facile and environmentally friendly method for preparing high-stability and strong-emission CsPbBr₃/Cs₄PbBr₆ composites using ultrasonication and liquid paraffin. Tuning the contents of liquid paraffin, bright-emission CsPbBr₃/Cs₄PbBr₆ composite powders with a maximum PLQY of 74% were achieved. Thanks to the protection of the Cs₄PbBr₆ matrix and liquid paraffin, the photostability, thermostability, and polar solvent stability of CsPbBr₃/Cs₄PbBr₆-LP are significantly improved compared to CsPbBr₃ quantum dots and CsPbBr₃/Cs₄PbBr₆ composites that were prepared without liquid paraffin. Moreover, the fabricated CsPbBr₃/Cs₄PbBr₆-LP-based WLEDs show excellent luminescent performance with a power efficiency of 129.5 lm/W and a wide color gamut, with 121% of the NTSC and 94% of the Rec. 2020, demonstrating a promising candidate for displays. In addition, the CsPbBr₃/Cs₄PbBr₆-LP-based WLEDs were also demonstrated in a VLC system. The results suggested the great potential of these high-performance WLEDs as an excitation light source to achieve VLC.

One-step synthesis of epitaxial 3D/2D metal halide perovskite heterostructures

Facile and scalable synthesis of perovskite heterostructures with well-controlled heterointerfaces remains challenging. Herein, we developed a simple one-step solution method to prepare 3D/2D CsPbBr₃/PEA₂PbBr₄ perovskite heterostructures with a well-defined epitaxial structure in the gram scale. The formation mechanism was detailed by using in situ time-resolved photoluminescence (PL) spectroscopy analysis. In addition, a series of 3D/2D epitaxial heterostructures were also prepared by changing the organic cations or halogen anions. Due to the effective charge separation and transfer, photodetectors based on the type-II 3D/2D CsPbBr₃/PEA₂PbBr₄ heterostructures showed up to 120 times higher photoresponsivities and 50 times higher on/off ratios compared to devices based on single component perovskites.

Highly emissive green CsPbBr3/Cs4PbBr6 composites: formation kinetics, excellent heat, light, and polar solvent resistance, and flexible light-emitting application

Benefit from their near-unity photoluminescence quantum yield (PL QY), narrow emission band, and widely tunable bandgap, metal halide perovskites have shown promising in light-emitting applications. Despite such promise, how to facile, environmentally-friendly, and large-scale prepare solid metal halide perovskite with high emission and stability remains a challenging. Herein, we demonstrate a convenient and environmentally-friendly method for the mass synthesis of solid CsPbBr₃/Cs₄PbBr₆ composites using high-power ultrasonication. Adjusting key experimental parameters, bright emitting CsPbBr₃/Cs₄PbBr₆ solids with a maximum PL QY of 71% were obtained within 30 min. XRD, SEM, TEM, Abs/PL, XPS, and lifetime characterizations provide solid evidence for forming CsPbBr₃/Cs₄PbBr₆ composites. Taking advantage of these composites, the photostability, thermostability, and polar solvent stability of CsPbBr₃/Cs₄PbBr₆ are much improved compared to CsPbBr₃. We further demonstrated CsPbBr₃/Cs₄PbBr₆ use in flexible/stretchable film and high-power WLEDs. After being subjected to bending, folding, and twisting, the film retains its bright emission and exhibits good resistance to mechanical deformation. Additionally, our WLEDs display a superior, durable high-power-driving capability, operating currents up to 300 mA and maintaining high luminous intensity for 50 hours. Such highly emissive and stable metal halide perovskites make them promising for solid-state lighting, lasing, and flexible/stretchable display device applications.

Integration of Highly Luminescent Lead Halide Perovskite Nanocrystals on Transparent Lead Halide Nanowire Waveguides through Morphological Transformation and Spontaneous Growth in Water

The integration of highly luminescent CsPbBr₃ quantum dots on nanowire waveguides has enormous potential applications in nanophotonics, optical sensing, and quantum communications. On the other hand, CsPb₂ Br₅ nanowires have also attracted a lot of attention due to their unique water stability and controversial luminescent property. Here, the growth of CsPbBr₃ nanocrystals on CsPb₂ Br₅ nanowires is reported first by simply immersing CsPbBr₃ powder into pure water, CsPbBr_{3- γ}  X_γ (X = Cl, I) nanocrystals on CsPb₂ Br_{5 -γ}  X_γ nanowires are then synthesized for tunable light sources. Systematic structure and morphology studies, including in situ monitoring, reveal that CsPbBr₃ powder is first converted to CsPb₂ Br₅ microplatelets in water, followed by morphological transformation from CsPb₂ Br₅ microplatelets to nanowires, which is a kinetic dissolution-recrystallization process controlled by electrolytic dissociation and supersaturation of CsPb₂ Br₅ . CsPbBr₃ nanocrystals are spontaneously formed on CsPb₂ Br₅ nanowires when nanowires are collected from the aqueous solution. Raman spectroscopy, combined photoluminescence, and SEM imaging confirm that the bright emission originates from CsPbBr_{3 -γ}  X_γ nanocrystals while CsPb₂ Br_{5 -γ}  X_γ nanowires are transparent waveguides. The intimate integration of nanoscale light sources with a nanowire waveguide is demonstrated through the observation of the wave guiding of light from nanocrystals and Fabry-Perot interference modes of the nanowire cavity.

Fabrication Strategies and Optoelectronic Applications of Perovskite Heterostructures

Metal halide perovskites (MHPs) are emerging low‐cost and multifunctional semiconductor materials. They have been widely used in optoelectronic devices such as perovskite solar cells, light‐emitting diodes, photodetectors, memristors, and lasers. Developing new MHPs, defects passivation, optimizing device structures, and packaging techniques are all effective methods to improve photoelectric performance and stability of perovskite devices. Particularly, the fabrication of perovskite/perovskite heterostructures (PPHSs) is a novel and arresting method to obtain stable and high‐performing optoelectronic perovskite devices since it can passivate defects, regulate energy gaps, and provide new carrier transmission modes of MHPs for multiple semiconductor applications. In this paper, representative fabrication strategies of PPHSs including films and single‐crystal heterostructures are reviewed, and their applications in optoelectronic devices are summarized. Furthermore, the challenges and prospects of PPHSs are discussed based on the current status.

Thermal and photo stability of all inorganic lead halide perovskite nanocrystals

Inorganic lead halide perovskite (ILHP) nanocrystals (NCs) show great potential in solid state lighting and next generation display technology due to their excellent optical properties. However, almost all ILHP NCs are still facing the problem of unstable luminescence properties caused by heating and/or UV illumination. Further improving the thermal and photo stability of ILHP NCs has become the most urgent challenge for their practical application. This Perspective review specifically focuses on the thermal and photo stability of ILHP NCs, discusses and analyzes the factors that affect the thermal and photo stability of ILHP NCs from the perspective of surface ligands and structure composition, summarizes the current strategies to improve the thermal and photo stability of ILHP NCs, and presents the key challenges and perspectives on the research for the improvement of thermal and photo stability of ILHP NCs.

Growth Kinetics and Optical Properties of CsPbBr3 Perovskite Nanocrystals

We synthesized CsPbBr3 perovskite nanocrystals (NCs) at different reaction temperatures and tracked their growth kinetics on the basis of their optical properties and estimated size. The energies of the absorption and fluorescence (FL) peaks with increasing reaction temperature for the CsPbBr3 perovskite NCs were tuned within the regions of 2.429–2.570 eV and 2.391–2.469 eV, respectively, depending on size of the NCs (9.9–12.5 nm). The Stokes shifts of CsPbBr3 perovskite NCs with increasing NC size decreased from 101 meV to 38 meV. The full-width at half-maximum of the FL peaks for the CdSe NCs decreased from 150 meV to 90 meV because of the improved size uniformity of the CsPbBr3 perovskite NCs. The energy spacing of CsPbBr3 perovskite NCs synthesized at various reaction temperatures was calculated from Tauc plots; this information is critical for determining the bandgap energy and enables the size of the CsPbBr3 perovskite NCs to be estimated using the effective mass approximation.

Self-defect-passivation by Br-enrichment in FA-doped Cs1-xFAxPbBr3 quantum dots: towards high-performance quantum dot light-emitting diodes

Halide vacancy defect is one of the major origins of non-radiative recombination in the lead halide perovskite light emitting devices (LEDs). Hence the defect passivation is highly demanded for the high-performance perovskite LEDs. Here, we demonstrated that FA doping led to the enrichment of Br in Cs_1−xFA_xPbBr₃ QDs. Due to the defect passivation by the enriched Br, the trap density in Cs_1−xFA_xPbBr₃ significantly decreased after FA doping, and which improved the optical properties of Cs_1−xFA_xPbBr₃ QDs and their QD-LEDs. PLQY of Cs_1–xFA_xPbBr₃ QDs increased from 76.8% (x = 0) to 85.1% (x = 0.04), and L_max and CE_max of Cs_1–xFA_xPbBr₃ QD-LEDs were improved from L_max = 2880 cd m⁻² and CE_max = 1.98 cd A⁻¹ (x = 0) to L_max = 5200 cd m⁻² and CE_max = 3.87 cd A⁻¹ (x = 0.04). Cs_1–xFA_xPbBr₃ QD-LED device structure was optimized by using PVK as a HTL and ZnO modified with b-PEI as an ETL. The energy band diagram of Cs_1–xFA_xPbBr₃ QD-LEDs deduced by UPS analyses.

In Situ Growth of 3D/2D (CsPbBr3/CsPb2Br5) Perovskite Heterojunctions toward Optoelectronic Devices

Two-dimensional (2D) CsPb₂Br₅ exhibits intriguing functions in enhancing the performance of optoelectronic devices in terms of environmental stability and luminescence properties when composited with other perovskites in different dimensionalities. We built a type I three-dimensional (3D) CsPbBr₃/2D CsPb₂Br₅ heterojunction through phase transition where CsPbBr₃ quantum dots in situ grew into 2D CsPb₂Br₅. A thorough growth mechanism study in combination with excited state dynamic investigations via femtosecond spectroscopy and first-principles calculations revealed that the type I hierarchy enhanced the stability of the heterojunction and spurred its luminous quantum yield by prolonging the lifetime of photogenerated carriers. Mixing the heterojunction with other phosphors yielded white-light-emitting diodes with a color rendering index of 94%. The work thus not only offered one new avenue for building heterojunctions by using the "soft crystal" nature of perovskites but also disentangled the enhanced luminescence mechanism of the heterojunction that can be harnessed for promising applications in the luminescence and display fields.

Curing the fundamental issue of impurity phases in two-step solution-processed CsPbBr3 perovskite films

Inorganic lead halide perovskite CsPbBr₃ offers attractive photophysical properties and phase stability for high-performance optoelectronic devices. However, CsPbBr₃ films produced by the classic solution-based two-step method are always accompanied with impurity phases of CsPb₂Br₅ and Cs₄PbBr₆, which represents a major efficiency-limiting factor for future advances of CsPbBr₃-based devices. The challenge lies in the complexity of the Cs-Pb-Br phase system, requiring both spatially and temporally precise control of the precursor stoichiometry during solution-phase growth of CsPbBr₃ films. By adopting 2-methoxyethanol as the solution conversion medium instead of commonly applied methanol, the reaction between CsBr and PbBr₂ can be finely controlled to yield single phase CsPbBr₃ films within a few minutes; extending the solution-conversion step to 24 h does not alter the phase purity of resulting CsPbBr₃ films. The present work paves the way to regulate the crystal growth behaviors of two-step solution-processed CsPbBr₃ films by simple solvent engineering.

Chemical Transformation of Lead Halide Perovskite into Insoluble, Less Cytotoxic, and Brightly Luminescent CsPbBr3/CsPb2Br5 Composite Nanocrystals for Cell Imaging

Lead halide perovskite nanocrystals (NCs) have been widely investigated owing to their potential applications as optoelectronic devices. However, these materials suffer from poor water stability, which make them impossible to be applied in biomedicine. Here, insoluble CsPbBr₃/CsPb₂Br₅ composite NCs were successfully synthesized via simple water-assisted chemical transformation of perovskite NCs. Water plays two key roles in this synthesis: (i) stripping CsBr from CsPbBr₃/Cs₄PbBr₆ and (ii) modifying the coordination number of Pb²⁺ (six in CsPbBr₃ and Cs₄PbBr₆ vs eight in CsPb₂Br₅). The as-prepared CsPbBr₃/CsPb₂Br₅ composite NCs not only retain the photoluminescence quantum yield (up to 80%) and a narrow full width to half-maximum of 16 nm, but also present excellent water stability and low cytotoxicity. With these properties, the CsPbBr₃/CsPb₂Br₅ composite NCs were demonstrated as efficient fluorescent probes in live HeLa cells. We believe that our finding not only provides a new method to prepare insoluble, narrow-band, and brightly luminescent CsPbBr₃/CsPb₂Br₅ composite NCs, but also extend the potential applications of lead halides in biomedicine.

LEDs using halide perovskite nanocrystal emitters

The emerging family of lead-halide perovskite (LHP) nanocrystal emitters has shown impressive achievements in solid-state light-emitting applications. With luminous efficiency comparable to that of organic light-emitting diodes, LHP light-emitting diodes (PeLEDs) have demonstrated a wide colour gamut with high colour purity and a widely tunable range of emissive wavelengths across the whole visible range. Herein, the understanding of LHP nanocrystals in light emission and the resulting PeLEDs are reviewed. Additionally, key features of LHP nanocrystal emitters applied in PeLEDs and guidelines towards realizing high-performance devices are discussed.

The photoluminescence mechanism of CsPb2Br5 microplates revealed by spatially resolved single particle spectroscopy

CsPb2Br5 is a new member of the all-inorganic lead halide perovskite family with unique structures and optoelectronic properties for various applications. As an indirect band gap semiconductor, the photoluminescence (PL) mechanism of CsPb2Br5 is still under debate. To resolve this issue, CsPb2Br5 microplates with strong green PL have been prepared by a hot-injection method. Characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis and ultraviolet-visible (UV-vis) absorption spectroscopy indicates the existence of a small amount of embedded CsPbBr3 phase. The removal of the embedded CsPbBr3 phase by treatment with water containing ethanol solvent resulted in complete PL quenching, suggesting the origin of PL due to the embedded CsPbBr3 phase. Spatially resolved PL and time-resolved PL mapping have been further employed to directly visualize the spatial distribution of different emission centers. Our single particle spectroscopic studies indicated the existence of three different emission centers with different PL lifetimes: two types of embedded CsPbBr3 phases (clumped and randomly distributed CsPbBr3 nanocrystals) and intrinsic defects of CsPb2Br5. The embedded CsPbBr3 phases with fast and intermediate PL lifetimes are the primary contributors to PL of the CsPb2Br5 microplates while their intrinsic defects with slow PL lifetimes make only a minor contribution. These studies have unambiguously clarified the PL mechanisms of the CsPb2Br5 microplates and provided the direct mapping of different emission centers, which resolve the contradictory explanation and debate about the PL mechanism of the CsPb2Br5 microplates.