Engineering Interfacial Charge Transfer in CsPbBr3 Perovskite Nanocrystals by Heterovalent Doping

Ni-Doped CsPbBr3 Perovskite: Synthesis of Highly Stable Nanocubes

A nanocube of Ni-doped CsPbBr₃nanocrystals (NCs) has been successfully synthesized and characterized by various spectroscopy techniques such as High-resolution transmission electron microscopy (HR-TEM), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), etc. The optoelectronic properties of NCs have been investigated with different solvents. HR-TEM and FE-SEM reveal that the obtained Ni-doped CsPbBr₃NCs exhibit cubic-rectangular morphologies with different sizes. Furthermore, the radiative exciton kinetics of NCs was examined using the time-correlated single photon counting (TCSPC) technique. The present study ravels that synthesized nanocubes are highly stable.

Doping Iron in CsPbBr3 Perovskite Nanocrystals for Efficient and Product Selective CO2 Reduction

Lead halide perovskite nanocrystals have recently emerged as an efficient optical material for light harvesting. While these have been extensively studied for obtaining bright emissions, their use as catalysts for enhancing the rate of chemical reactions has been explored little. Considering their importance in catalysis, herein, Fe(II)-doped CsPbBr₃ perovskite nanocrystals have been explored for photocatalytic reduction of CO₂. In comparison to undoped CsPbBr₃, doped nanocrystals showed enhanced catalytic activity and also predominantly led to evolution of CH₄ instead of CO. The observation of a reverse trend of predominated CH₄ evolution in doped nanocrystals rather than CO observed for undoped nanocrystals was correlated to the adsorption/desorption energy of respective products established theoretically earlier. This selective evolution of major products on doping remained unique and also a step forward for understanding more regarding light to chemical energy conversions using perovskite nanocrystals.

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.

Solvothermal synthesis of Mn-doped CsPbCl3 perovskite nanocrystals with tunable morphology and their size-dependent optical properties

Doping metal ions in inorganic halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) endows the NCs with unique optical characteristics, and has thus attracted immense attention. However, controllable synthesis of high-quality doped perovskite NCs with tunable morphology still remains challenging. Here, we report a facile, effective and unified strategy for the controllable synthesis of Mn-doped CsPbCl3 quantum dots (QDs) and nanoplatelets (NPLs) via a single-step solvothermal method. The incorporation of Mn2+ into CsPbCl3 NCs introduces new broad photoluminescence (PL) emission from Mn2+ while maintaining the structure of host CsPbCl3 NCs nearly intact. The PL intensity, emission peak position and size of the NCs can be accurately adjusted by altering the experimental parameters such as Mn-to-Pb feed ratio and reaction time. Especially, by changing the amount of ligands, Mn-doped CsPbCl3 QDs, NPLs or their mixtures can be obtained. Both of the Mn-doped QDs and NPLs exhibit a size-dependent quantum confinement effect, which is confirmed by the relationship between the size of NCs and the exciton emission peaks. The solvothermal reaction condition plays an important role for the precise control of the structure, morphology and PL properties of the Mn-doped NCs. The as-prepared Mn-doped CsPbCl3 NPLs with thickness down to ∼2 nm exhibit a PL quantum yield (PLQY) of more than 22%. This work introduces a new strategy for the controllable synthesis of Mn-doped perovskite NCs, which provides ideas for the in-depth study of the dope-and-grow process and can be extended to approaches of doping other metal ions.

Tuning Hot Carrier Cooling Dynamics by Dielectric Confinement in Two-Dimensional Hybrid Perovskite Crystals

Hot carrier (HC) cooling is a critical photophysical process that significantly influences the optoelectronic performance of hybrid perovskite-based devices. The hot carrier extraction at the device interface is very challenging because of its ultrashort lifetime. Here, ultrafast transient reflectance spectroscopy measurements and time-domain ab initio calculations show how the dielectric constant of the organic spacers can control and slow the HC cooling dynamics in single-crystal 2D Ruddlesden-Popper hybrid perovskites. We find that (EA)₂PbI₄ (EA = HOC₂H₄NH₃⁺) that correspond to a high dielectric constant organic spacer has a longer HC cooling time compared to that of (AP)₂PbI₄ (AP = HOC₃H₆NH₃⁺) and (PEA)₂PbI₄ (PEA = C₆H₅C₂H₄NH₃⁺). The slow HC relaxation process in the former case can be ascribed to a stronger screening of the Coulomb interactions, a small nonradiative internal conversion within the conduction bands, as well as a weak electron-phonon coupling. Our findings provide a strategy to prolong the hot carrier cooling time in low-dimensional hybrid perovskite materials by using organic spacers with reduced dielectric confinement.

Advances in the Stability of Halide Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals are promising candidates for next-generation optoelectronics because of their facile synthesis and their outstanding and size-tunable properties. However, these materials suffer from rapid degradation, similarly to their bulk perovskite counterparts. Here, we survey the most recent strategies to boost perovskite nanocrystals stability, with a special focus on the intrinsic chemical- and compositional-factors at synthetic and post-synthetic stage. Finally, we review the most promising approaches to address the environmental extrinsic stability of perovskite nanocrystals (PNCs). Our final goal is to outline the most promising research directions to enhance PNCs' lifetime, bringing them a step closer to their commercialization.

Perovskite quantum dots for light-emitting devices

Perovskite quantum dots (QDs) have been hotly pursued in recent decades owing to their quantum confinement effect and defect-tolerant nature. Their unique optical properties, such as high photoluminescence quantum yield (PLQY) approaching unity, narrow emission bandwidth, tunable wavelength spanning the entire visible spectrum, and compatibility with flexible/stretchable electronics, render perovskite QDs promising for next-generation solid lighting sources and information displays. Herein, the advances in perovskite QDs and their applications in LEDs are reviewed. Strategies to fabricate efficient perovskite QDs and device configuration, including material composition design, synthetic methods, surface engineering, and device optimization, are investigated and highlighted. Moreover, the main challenges in perovskite QDs of instability and toxicity (lead-based) are identified, while the solutions undertaken with respect to composition engineering, device encapsulation, and lead-replacement QDs are demonstrated. Meanwhile, perspectives for the further development of perovskite QDs and corresponding LEDs are presented.

CsPbBr3 Perovskite Nanocrystal Grown on MXene Nanosheets for Enhanced Photoelectric Detection and Photocatalytic CO2 Reduction

All-inorganic CsPbX₃ (X = Cl, Br or I) perovskite nanocrystals have attracted extensive interest recently due to their exceptional optoelectronic properties. In an effort to improve the charge separation and transfer following efficient exciton generation in such nanocrystals, novel functional nanocomposites were synthesized by the in situ growth of CsPbBr₃ perovskite nanocrystals on two-dimensional MXene nanosheets. Efficient excited state charge transfer occurs between CsPbBr₃ NCs and MXene nanosheets, as indicated by significant photoluminescence (PL) quenching and much shorter PL decay lifetimes compared with pure CsPbBr₃ NCs. The as-obtained CsPbBr₃/MXene nanocomposites demonstrated increased photocurrent generation in response to visible light and X-ray illumination, attesting to the potential application of these heterostructure nanocomposites for photoelectric detection. The efficient charge transfer also renders the CsPbBr₃/MXene nanocomposite an active photocatalyst for the reduction of CO₂ to CO and CH₄. This work provides a guide for exploration of perovskite materials in next-generation optoelectronics, such as photoelectric detectors or photocatalyst.

Strategies to Improve Luminescence Efficiency of Metal-Halide Perovskites and Light-Emitting Diodes

Metal-halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light-emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence-limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next-generation displays and solid-state lightings are suggested.

Journey of Making Cesium Lead Halide Perovskite Nanocrystals: What's Next

Understanding physical insights of making different lead halide perovskite nanocrystals remains in limelight of current research because of their strong emission that is tunable in the entire visible spectrum. Optimizing reaction parameters, intensifying the emission, modulating A, B, and X sites to bring optical and phase stability, understanding the interface and ligand chemistry, investigating the growth kinetics, tuning the dimension, making heterostructures, etc. were intensively studied. In addition, several aspects of colloidal nanocrystals such as time-dependent growth and obtaining size-tunable nanocrystals as a function of time from one reaction, controlling anisotropic growth or stabilizing other than the six facets of cubes, formation of various heterostructures with epitaxial growths, etc. could not be established largely. Hence, while the field is emerging among efficient optically active materials, addressing these issues by summarizing different directions of research became important. Under these prospects, this Perspective focuses on top developments in the field of synthesis modulations where success has been achieved and also summarizes several directions of research where physical insights still could not be understood broadly.

Tuning the bandgap of Cs2AgBiBr6 through dilute tin alloying

Sn alloying tunes a halide double perovskite to absorb visible light, in a nontoxic composition.

The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs₂AgBiBr₆ (1) represents one such alternative, offering long carrier lifetimes and greater stability under ambient conditions. However, the large and indirect 1.95 eV bandgap hinders its potential as a solar absorber. Here we report that alloying crystals of 1 with up to 1 atom% Sn results in a bandgap reduction of up to ca. 0.5 eV while maintaining low toxicity. Crystals can be alloyed with up to 1 atom% Sn and the predominant substitution pathway appears to be a ∼2 : 1 substitution of Sn²⁺ and Sn⁴⁺ for Ag⁺ and Bi³⁺, respectively, with Ag⁺ vacancies providing charge compensation. Spincoated films of 1 accommodate a higher Sn loading, up to 4 atom% Sn, where we see mostly Sn²⁺ substitution for both Ag⁺ and Bi³⁺. Density functional theory (DFT) calculations ascribe the bandgap redshift to the introduction of Sn impurity bands below the conduction band minimum of the host lattice. Using optical absorption spectroscopy, photothermal deflection spectroscopy, X-ray absorption spectroscopy, ¹¹⁹Sn NMR, redox titration, single-crystal and powder X-ray diffraction, multiple elemental analysis and imaging techniques, and DFT calculations, we provide a detailed analysis of the Sn content and oxidation state, dominant substitution sites, and charge-compensating defects in Sn-alloyed Cs₂AgBiBr₆ (1:Sn) crystals and films. An understanding of heterovalent alloying in halide double perovskites opens the door to a wider breadth of potential alloying agents for manipulating their band structures in a predictable manner.

Deciphering the Ultrafast Nonlinear Optical Properties and Dynamics of Pristine and Ni-Doped CsPbBr3 Colloidal Two-Dimensional Nanocrystals

While the unabated race persists in achieving record efficiencies in solar cells and other photonic/optoelectronic devices using lead halide perovskite absorbers, a comprehensive picture of the correlated third-order nonlinear optical (NLO) properties is yet to be established. The present study is aimed at deciphering the role of dopants in multiphoton absorption properties of intentionally engineered CsPbBr₃ colloidal nanocrystals (NCs). The charge separation of the plasmon-semiconductor conduction band owing to the hot electron transfer at the interface was demystified using the dynamics of the bleached spectral data from femtosecond (fs) transient absorption spectroscopy with broadband capabilities. The NLO properties studied through the fs Z-scan technique revealed that Ni-doped CsPbBr₃ NCs exhibited strong third-order NLO susceptibility of ∼10^-10 esu. The exotic photophysical phenomena in these pristine and Ni-doped CsPbBr₃ colloidal two-dimensional (2D) NCs reported herein are believed to provide the avenues to address the critical variables involved in the structural differences and their correlated optoelectronic properties.

High-Performance CsPbIBr2 Perovskite Solar Cells: Effectively Promoted Crystal Growth by Antisolvent and Organic Ion Strategies

Growing attention has been paid to CsPbIBr₂ perovskite solar cells (PSCs) after balancing the band gap and stability features of the interested full-inorganic perovskites. However, their power-conversion efficiency (PCE) still lags behind that of the PSCs using hybrid halide perovskite and how to increase the corresponding PCE is still a challenge. Herein, antisolvents and organic ion surface passivation strategies were systematically applied to precisely control the growth of CsPbIBr₂ crystals for constructing a high-quality full-inorganic perovskite film. Through careful adjustments, a CsPbIBr₂ film with a pure phase, full coverage, and high crystallinity with preferable (100) orientation was successfully obtained by introducing diethyl ether as the antisolvent followed by guanidinium surface passivation. The optimal CsPbIBr₂ film was composed by a large grain with an average size of 950 nm, few grain boundaries, and higher hydrophobic property. Planer PSC using the optimal CsPbIBr₂ film and electron-beam-deposited TiO₂ compact layer exhibits a PCE of 9.17%, which ranks among the highest PCE range of the reported CsPbIBr₂ PSCs. Besides, the designed CsPbIBr₂ PSC exhibited good long-term stability, which could maintain 90% of the initial PCE in 40% humidity ambient, which remained constant after heat treatment at 100 °C for 100 h. Based on the optimal CsPbIBr₂ film, the flexible and large-area (up to 225 mm²) PSCs were further fabricated. The adopted film improvement methods were further extended to other kinds of full-organic PSCs, which demonstrated the universality of this strategy.

Tracking the dimensional conversion process of semiconducting lead bromide perovskites by mass spectroscopy, powder X-ray diffraction, microcalorimetry and crystallography

The structural information of a material in both the solid state and solution state is essential to the in-depth understanding of the properties of inorganic-organic hybrid materials. A one-dimensional (1D) lead bromide formulated as [H][NH₃(CH₂)₂SS(CH₂)₂NH₃][H₂O][PbBr₅] (1) could be converted into a new two-dimensional (2D) complex, [NH₃(CH₂)₂SS(CH₂)₂NH₃][PbBr₄] (2), by soaking the crystals in water. The isolated 2D compound showed single-layer lead-halide perovskite structures. Electrospray ionization mass spectrometry (ESI-MS) analyses of the reaction solution revealed that the [PbBr₃]^- fragments are initially formed from the rapid decomposition of the 1D [PbBr₅]^3- chains and subsequently reassemble into 2D [PbBr₄]^2- layers, which was verified by powder X-ray diffraction (PXRD) and microcalorimetry. Because of the decomposition and reassembly process, complex 1 could be used as a precursor to synthesize M²⁺-doped 2D lead bromide perovskites, namely, Mn@2, Ni@2 and Cd@2. In addition, preliminary tests indicated that complex 2 exhibited a lower optical band gap (3.25 eV) and higher electrical conductivity (3.2 × 10^-11 S cm^-1) than complex 1 (3.38 eV, 5.4 × 10^-12 S cm^-1).

Role of Halogen Atoms on High-Efficiency Mn2+ Emission in Two-Dimensional Hybrid Perovskites

Doped halide pervoskites as highly efficient light emitters have recently fascinated the research community, while the influence of halogen atoms X (X = Cl, Br, I) on the hybrid energy levels and photoluminescence properties remains a challenge. Here, the role of X compositions in the two-dimensional hybrid perovskite BA₂PbX₄ (BA = C₄H₉NH₃) on the doped Mn²⁺ emission is identified, wherein Mn²⁺ reveals a strong luminescence dependence on the nature of the halogen, and optimum Mn²⁺ emission with a record quantum yield of 60.1% has been achieved in BA₂PbBr₄. Density functional theory calculations show that BA₂PbBr₄ holds low Br vacancy concentration and unique coupled states of the Mn-3d level and Pb-6p level at the conduction band minimum, leading to efficient energy transfer from the host to Mn²⁺. Our work sheds new light on the methods to realize strong exciton-dopant exchange coupling for achieving high-efficiency dopant luminescence.

Surface pre-optimization of a mixed halide perovskite toward high photoluminescence quantum yield in the blue spectrum range

The photoluminescence quantum yields (PLQYs) of all-inorganic halide perovskites in the green and red spectral ranges have approached over 90%, overwhelmingly arousing burgeoning interests for creating a revolution in next-generation high-definition displays. However, obtaining pure blue-emitting perovskites with high PLQYs still remains a challenge. Herein, we designed a novel strategy to pre-optimize CsPbCl3 quantum dots (QDs) using praseodymium(iii) chloride (PrCl3), and then efficient blue-emitting CsPbBrxCl3-x QDs were obtained through halide exchange between the optimized CsPbCl3 and efficient CsPbBr3 QDs. Specifically, the PrCl3 optimization simultaneously and efficiently passivated the surface vacancy defects and appropriately reduced the surface long-chain organic ligands of the CsPbCl3 QDs, synergistically eliminating the deep trap states, and hence considerably suppressing nonradiative recombination. As a result, the radiative recombination rate was enhanced by more than one order of magnitude from 4.3 to 79 μs-1. Benefiting from this, the blue-emitting CsPbBrxCl3-x QDs exhibited an admirable PLQY of up to 89%, which is competitive compared with that of the state-of-the-art red and green-emitting perovskites. This strategy provides a unique understanding regarding the low PLQY of blue-emitting perovskites and an efficient method to boost it, which is especially attractive for constructing efficient blue and white light-emitting diodes.

Perovskite-Based Artificial Multiple Quantum Wells

Semiconductor quantum well structures have been critical to the development of modern photonics and solid-state optoelectronics. Quantum level tunable structures have introduced new transformative device applications and afforded a myriad of groundbreaking studies of fundamental quantum phenomena. However, noncolloidal, III-V compound quantum well structures are limited to traditional semiconductor materials fabricated by stringent epitaxial growth processes. This report introduces artificial multiple quantum wells (MQWs) built from CsPbBr₃ perovskite materials using commonly available thermal evaporator systems. These perovskite-based MQWs are spatially aligned on a large-area substrate with multiple stacking and systematic control over well/barrier thicknesses, resulting in tunable optical properties and a carrier confinement effect. The fabricated CsPbBr₃ artificial MQWs can be designed to display a variety of photoluminescence (PL) characteristics, such as a PL peak shift commensurate with the well/barrier thickness, multiwavelength emissions from asymmetric quantum wells, the quantum tunneling effect, and long-lived hot-carrier states. These new artificial MQWs pave the way toward widely available semiconductor heterostructures for light-conversion applications that are not restricted by periodicity or a narrow set of dimensions.

Ultrastable CsPbBr3 Perovskite Quantum Dot and Their Enhanced Amplified Spontaneous Emission by Surface Ligand Modification

The poor stability and aggregation problem of CsPbBr₃ quantum dots (QDs) in air are great challenges for their future practical application. Herein, a simple and effective ligand-modification strategy is proposed by introducing 2-hexyldecanoic acid (DA) with two short branched chains to replace oleic acid (OA) with long chains during the synthesis process. These two short branched chains not only maintain their colloidal stability but also contribute to efficient radiative recombination. The calculations show that CsPbBr₃ QDs with DA modification (CsPbBr₃ -DA QDs) have larger binding energy than CsPbBr₃ QDs with OA (CsPbBr₃ -OA QDs), resulting in significantly enhanced stability. Due to the strong binding energy between DA ligands and QDs, CsPbBr₃ -DA QDs exhibit no aggregation phenomenon even after stored in air for more than 70 d, and CsPbBr₃ -DA QDs films can maintain 94.3% of initial PL intensity after 28 d, while in CsPbBr₃ -OA QDs films occurs a rapid degradation of PL intensity. Besides, the enhanced amplified spontaneous emission (ASE) performance of CsPbBr₃ -DA QDs films has been demonstrated under both one- and two-photon laser excitation. The ASE threshold of CsPbBr₃ -DA QDs films is reduced by more than 50% and their ASE photostability is also improved, in comparison to CsPbBr₃ -OA QDs films.

From Nonluminescent to Blue-Emitting Cs4 PbBr6 Nanocrystals: Tailoring the Insulator Bandgap of 0D Perovskite through Sn Cation Doping

All-inorganic cesium lead halide perovskite nanocrystals (NCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic performance such as tunable bandgaps over the entire visible spectral region. However, compared to well-developed 3D CsPbX₃ perovskites (X = Cl, Br, and I), the bandgap tuning in 0D Cs₄ PbX₆ perovskite NCs remains an arduous task. Herein, a simple but valid strategy is proposed to tailor the insulator bandgap (≈3.96 eV) of Cs₄ PbBr₆ NCs to the blue spectral region by changing the local coordination environment of isolated [PbBr₆ ]^4- octahedra in the Cs₄ PbBr₆ crystal through Sn cation doping. Benefitting from the unique Pb²⁺ -poor and Br^- -rich reaction environment, the Sn cation is successfully introduced into the Cs₄ PbBr₆ NCs, forming coexisting point defects comprising substitutional Sn_Pb and interstitial Br_i , thereby endowing these theoretically nonluminescent Cs₄ PbBr₆ NCs with an ultranarrow blue emission at ≈437 nm (full width at half maximum, ≈12 nm). By combining the experimental results with first-principles calculations, an unusual electronic dual-bandgap structure, comprising the newly emerged semiconducting bandgap of ≈2.87 eV and original insulator bandgap of ≈3.96 eV, is found to be the underlying fundamental reason for the ultranarrow blue emission.

Ultrafast carrier dynamics of metal halide perovskite nanocrystals and perovskite-composites

Perovskite nanocrystals (NCs), especially those based on cesium lead halides, have emerged in recent years as highly promising materials for efficient solar cells and photonic applications. The key to realization of full potential of these materials lies however in the molecular level understanding of the processes triggered by light. Herein we highlight the knowledge gained from photophysical investigations on these NCs of various sizes and compositions employing primarily the femtosecond pump-probe technique. We show how spectral and temporal characterization of the photo-induced transients provide insight into the mechanism and dynamics of relaxation of hot and thermalized charge carriers through their recombination and trapping. We discuss how the multiple excitons including the charged ones (trions), generated using high pump fluence or photon energy, recombine through the Auger-assisted process. We discussed the harvesting of hot carriers prior to their cooling and band-edge carriers from these perovskite NCs to wide band-gap metal oxides, metal chalcogenide NCs and molecular acceptors. How perovskites can influence the charge carrier dynamics in composites of organic and inorganic semiconductors is also discussed.

Reducing Defects in Halide Perovskite Nanocrystals for Light-Emitting Applications

The large specific surface area of perovskite nanocrystals (NCs) increases the likelihood of surface defects compared to that of bulk single crystals and polycrystalline thin films. It is thus crucial to comprehend and control their defect population in order to exploit the potential of perovskite NCs. This Perspective describes and classifies recent advances in understanding defect chemistry and avenues toward defect density reduction in perovskite NCs, and it does so in the context of the promise perceived in light-emitting devices. Several pathways for decreasing the defect density are explored, including advanced NC syntheses, new surface-capping strategies, doping with metal ions and rare earths, engineering elemental compensation, and the translation of core-shell heterostructures into the perovskite materials family. We close with challenges that remain in perovskite NC defect research.

Trivalent ion mediated abnormal growth of all-inorganic perovskite nanocrystals and their divergent emission properties

In this work, a new trivalent ion-mediated one-pot synthetic protocol is reported to create two well-defined optical absorbance and photoluminescence (PL) emissions in all-inorganic halide perovskite nanocrystals (NCs). The foreign M3+ cations (M = Bi, Al, In), typically from BiBr3, BiFeO3, BiCl3, AlBr3 or InBr3, function as capping ligands for generating a growth-constrained thinner nanoplatelet (NPL) population displaying the quantum confinement effect. The formation mechanism of the growth-constrained NPLs is proposed based on density functional theory (DFT) on the different slab energy of the representative NPLs achieved in the presence of Bi3+ ions and the density of states (DOS) of the supposed bulk perovskites. Notably, the formation of two groups of NCs with different sizes allows for the generation of dual optical absorbance and PL emissions. The influence of the M : Pb molar ratios on the precursors is systematically elucidated, from which the relative intensity and position of each PL emission can be fine-tuned. By virtue of the representative NPLs with well-defined green and blue emissions, the M3+-assisted synthetic protocol provides a facile and cost-effective route for producing unique NCs and nanostructures for optoelectronic device applications.

Doping Mn(II) in All-Inorganic Ruddlesden-Popper Phase of Tetragonal Cs2PbCl2I2 Perovskite Nanoplatelets

Doping Mn(II) in inorganic Ruddlesden-Popper phase Cs₂PbCl₂I₂ perovskite nanoplatelets is reported. The host nanostructures were prepared with a calculative protocol taking the exact required composition of Cs(I) and Pb(II) and injecting the preformed mixed oleylammonium chlorides and iodides at optimized reaction temperature. Reactions were optimized with various halides and their mixtures, but the stable phase of the Cs₂PbX₄ system was obtained only for the chloride-iodide mixed-halide system. Introduction of Mn(II) along with Pb(II), resulted in successful light-emitting doped nanocrystals. Measuring the photoluminescence and the decay lifetimes at room and liquid nitrogen temperatures, the variations in the excitonic, self-trapped, and Mn dopant emission properties were compared with those of the chalcogenide and perovskite nanocrystals.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Grinding Synthesis of APbX3 (A = MA, FA, Cs; X = Cl, Br, I) Perovskite Nanocrystals

Currently, metal halide perovskite nanocrystals have been extensively explored due to their unique optoelectronic properties and wide application prospects. In the present work, a facile grinding method is developed to prepare whole-family APbX₃ (A = MA, FA, and Cs; X = Cl, Br, and I) perovskite nanocrystals. This strategy alleviates the harsh synthesis conditions of precursor dissolution, atmosphere protection, and high temperature. Impressively, the as-prepared perovskite nanocrystals are evidenced to have halogen-rich surfaces and yield visible full-spectral emissions with maximal photoluminescence quantum yield up to 92% and excellent stability. Additionally, the grinding method can be extended to synthesize widely concerned Mn²⁺-doped CsPbCl₃ nanocrystals with dual-modal emissions of both excitons and dopants. As a proof-of-concept experiment, the present perovskite nanocrystals are demonstrated to be applicable as blue/green/red color converters in UV-excitable white-light-emitting diodes.

Thermally Stable Copper(II)-Doped Cesium Lead Halide Perovskite Quantum Dots with Strong Blue Emission

All-inorganic perovskite quantum dots (QDs) have emerged as potentially promising materials for lighting and displays, but their poor thermal stability restricts their practical application. In addition, optical characteristics of the blue-emitting CsPbX₃ QDs lag behind their red- and green-emitting counterparts. Herein, we addressed these two issues by doping divalent Cu²⁺ ions into the perovskite lattice to form CsPb_{1- x}Cu _xX₃ QDs. Extended X-ray absorption fine structure (EXAFS) measurements reveal that doping smaller Cu²⁺ guest ions induces a lattice contraction and eliminates halide vacancies, which leads to an increased lattice formation energy and improved short-range order of the doped perovskite QDs. This results in the improvement of both the thermal stability and the optical performance of CsPb_{1- x}Cu _x(Br/Cl)₃ QDs, which exhibit bright blue photoluminescence at 450-460 nm, with a high quantum yield of over 80%. The CsPb_{1- x}Cu _xX₃ QD films maintain stable luminescence performance even when annealed at temperatures of over 250 °C.

Perfection of Perovskite Grain Boundary Passivation by Eu-Porphyrin Complex for Overall-Stable Perovskite Solar Cells

The formation of defects at surfaces and grain boundaries (GBs) during the fabrication of solution-processed perovskite film are thought to be responsible for its instability. Herein, Eu-porphyrin complex (Eu-pyP) is directly doped into methylammonium lead triiodide (MAPbI3) precursor, perfectly fabricating 2D (Eu-pyP)0.5MA n -1Pb n I3 n +1 platelets inlaying the GBs of 3D polycrystalline interstices in this protocol. The device based on Eu-pyP doped perovskite film possesses a champion efficiency of 18.2%. More importantly, the doped perovskite solar cells device shows beyond 85% retention of its pristine efficiency value, whereas the pure MAPbI3 device has a rapid drop in efficiency down to 10% within 100 h under 45% humidity at 85 °C in AM 1.5 G. The above acquired perovskite films reveal an unpredictable thermodynamic self-healing ability. Consequently, the findings provide an avenue for defect passivation to synchronously improve resistibility to moisture, heat, and solar light including UV.

Engineering ultrafast charge transfer in a bismuthene/perovskite nanohybrid

In this work, 0-dimensional (0D) CsPbBr3 QDs were integrated with 2D bismuthene having ultrafast carrier mobility, to obtain a 0D/2D nanohybrid. Moreover, an excellent charge transfer efficiency (0.53) and an appreciable quenching constant of 2.3 × 105 M-1 were observed. Tuning the ratio of bismuthene in the Bi/perovskite nanohybrid achieved the quantified control of charge transfer efficiency and quenching performance at the interface.

An overview on enhancing the stability of lead halide perovskite quantum dots and their applications in phosphor-converted LEDs

Beyond the unprecedented success achieved in photovoltaics (PVs), lead halide perovskites (LHPs) have shown great potential in other optoelectronic devices. Among them, nanometer-scale perovskite quantum dots (PQDs) with fascinating optical properties including high brightness, tunable emission wavelength, high color purity, and high defect tolerance have been regarded as promising alternative down-conversion materials in phosphor-converted light-emitting diodes (pc-LEDs) for lighting and next-generation of display technology. Despite the promising applications of perovskite materials in various fields, they have received strong criticism for the lack of stability. The poor stability has also attracted much attention. Within a few years, numerous strategies towards enhancing the stability have been developed. This review summarizes the mechanisms of intrinsic- and extrinsic-environment-induced decomposition of PQDs. Simultaneously, the strategies for improving the stability of PQDs are reviewed in detail, which can be classified into four types: (1) compositional engineering; (2) surface engineering; (3) matrix encapsulation; (4) device encapsulation. Finally, the challenges for applying PQDs in pc-LEDs are highlighted, and some possible solutions to improve the stability of PQDs together with suggestions for further improving the performance of pc-LEDs as well as the device lifetime are provided.

Dopant-induced localized light absorption in CsPbX3 (X = Cl, Br, I) perovskite quantum dots

Doping is known to play an important role in the optoelectronic properties of semiconducting materials.

Influence of shape on the carrier relaxation dynamics of CsPbBr3 perovskite nanocrystals

Shape dependent carrier relaxation dynamics of lead halide perovskite nanocrystal (NCs) is an important issue for efficient light harvesting system.

Stable Sn2+ doped FAPbI3 nanocrystals for near-infrared LEDs

This communication reports Sn²⁺ doping to stabilize the α-perovskite phase of FAPbI₃ NCs and their application in NIR-LEDs with EQE ∼ 1.7%.

Stabilization of Lead-Tin-Alloyed Inorganic-Organic Halide Perovskite Quantum Dots

Recently, lead-tin-based alloyed halide perovskite quantum dots (QDs) with improved stability and less toxicity have been introduced. However, the perovskite QDs containing tin are still unstable and exhibit low photoluminescence quantum yields (PLQYs), owing to the presence of defects in the alloyed system. Here, we have attempted to introduce sulfur anions (S^2-) into the host lattice (MAPb_0.75Sn_0.25Br₃) as a promising route to stable alloyed perovskite QDs with improved stability and PLQY. In this study, we used elemental sulfur as a sulfur precursor. The successful incorporation of sulfur anions into the host lattice resulted in a highly improved PLQY (>75% at room temperature), which is believed to be due to a reduction in the defect-related non-radiative recombination centers present in the host lattice. Furthermore, we found that the emission property could be tuned between the bright green and cyan-bluish regions without compromising on color quality. This work invigorates the perovskite research community to prepare stable, bright, and color-tunable alloyed inorganic-organic perovskite QDs without compromising on their phases and color quality, which can lead to considerable advances in display technology.

CsPbBr3 Nanocrystal/MO2 (M = Si, Ti, Sn) Composites: Insight into Charge-Carrier Dynamics and Photoelectrochemical Applications

Though coating CsPbBr₃ nanocrystal (NC) with an outer layer has been regarded as an effective strategy to address its instability issues, deep investigations into the electronic interaction between CsPbBr₃ NC and coating layer have yet to be conducted. In this study, the dynamics of hot carrier and charge carrier of CsPbBr₃ nanocrystal with various MO₂ (M = Si, Ti, Sn) coating layers have been comprehensively studied. Combined transient optical characterizations (time-resolved photoluminescence and ultrafast transient absorption) and photoelectrochemical measurements reveal that coating with insulating SiO₂ accelerates the hot carrier relaxation and enhances the radiative recombination by passivating surface traps, whereas efficient charge-carrier separation and extraction are observed after coating with SnO₂ and TiO₂. The electron injection from CsPbBr₃ NC to SnO₂ (1.14 × 10⁸ s^-1) is 2-fold faster than to TiO₂ (5.4 × 10⁷ s^-1) owing to the lower conduction band edge and the higher electron mobility of SnO₂. Particularly, the first time fabricated CsPbBr₃ NC/SnO₂ composite exhibits superior stability against UV light and moisture, as well as the best photocurrent response in this study. This work has implied that rational design of the coating layer for perovskite NC can not only improve the stability but also tailor the electronic and optoelectronic properties for various applications.

Simultaneous Strontium Doping and Chlorine Surface Passivation Improve Luminescence Intensity and Stability of CsPbI3 Nanocrystals Enabling Efficient Light-Emitting Devices

A method is proposed to improve the photo/electroluminescence efficiency and stability of CsPbI₃ perovskite nanocrystals (NCs) by using SrCl₂ as a co-precursor. The SrCl₂ is chosen as the dopant to synthesize the CsPbI₃ NCs. Because the ion radius of Sr²⁺ (1.18 Å) is slightly smaller than that of Pb²⁺ (1.19 Å) ions, divalent Sr²⁺ cations can partly replace the Pb²⁺ ions in the lattice structure of perovskite NCs and cause a slight lattice contraction. At the same time, Cl^- anions from SrCl₂ are able to efficiently passivate surface defect states of CsPbI₃ nanocrystals, thus converting nonradiative trap states to radiative states. The simultaneous Sr²⁺ ion doping and surface Cl^- ion passivation result in the enhanced photoluminescence quantum yield (up to 84%), elongated emission lifetime, and improved stability. Sr²⁺ -doped CsPbI₃ NCs are employed to produce light-emitting devices with a high external quantum yield of 13.5%.

Ultrathin-Film Titania Photocatalyst on Nanocavity for CO2 Reduction with Boosted Catalytic Efficiencies

Photocatalytic CO₂ reduction with water to hydrocarbons represents a viable and sustainable process toward greenhouse gas reduction and fuel/chemical production. Development of more efficient catalysts is the key to mitigate the limits in photocatalytic processes. Here, a novel ultrathin-film photocatalytic light absorber (UFPLA) with TiO₂ films to design efficient photocatalytic CO₂ conversion processes is created. The UFPLA structure conquers the intrinsic trade-off between optical absorption and charge carrier extraction efficiency, that is, a solar absorber should be thick enough to absorb majority of the light allowable by its bandgap but thin enough to allow charge carrier extraction for reactions. The as-obtained structures significantly improve TiO₂ photocatalytic activity and selectivity to oxygenated hydrocarbons than the benchmark photocatalyst (Aeroxide P25). Remarkably, UFPLAs with 2-nm-thick TiO₂ films result in hydrocarbon formation rates of 0.967 mmol g^-1 h^-1, corresponding to 1145 times higher activity than Aeroxide P25. This observation is confirmed by femtosecond transient absorption spectroscopic experiments where longer charge carrier lifetimes are recorded for the thinner films. The current work demonstrates a powerful strategy to control light absorption and catalysis in CO₂ conversion and, therefore, creates new and transformative ways of converting solar energy and greenhouse gas to alcohol fuels/chemicals.

Template-Free Synthesis of High-Yield Fe-Doped Cesium Lead Halide Perovskite Ultralong Microwires with Enhanced Two-Photon Absorption

Doping in perovskite is challenging and competitive due to the inherently fast growth mechanism of perovskite structure. Here, we demonstrate successful synthesis of high-yield Fe-doped cesium lead halide perovskite ultralong microwires (MWs) that have diameters up to ∼5 μm and lengths up to millimeters via an antisolvent vapor-assisted template-free method. Microstructure characterization confirms the uniformly doped Fe in the high-quality crystal perovskite MWs. Significantly, doping the Fe(III) concentration can affect both the MW morphology and photoluminescence (PL). The band edge emission of the MW at variable excitation has been accounted for by the superposition and combination of optical transitions of nearby singlet, triplet, and magnetic polaronic excitons. High-quality two-photon PL emission and the enhanced nonlinear absorption coefficient of Fe-doped MWs have been experimentally demonstrated. This superhigh nonlinear absorption coefficient and high-quality optical properties endow it with promising applications in spin-related optical switching and optical limiting devices.

The Benefit and Challenges of Zero-Dimensional Perovskites

To break free of the limitations imposed by three-dimensional (3D) perovskites, such as their lackluster stability, researchers have opened new frontiers into lower-dimensional perovskite derivatives. Thanks to advances in solvent-based synthesis methods, zero-dimensional (0D) inorganic perovskites, mainly Cs₄PbBr₆, have recently reemerged in various forms (from single crystals to nanocrystals) as materials with properties that bridge organic molecules and inorganic semiconductors. These properties include intrinsic Pb²⁺ ion emission, large exciton binding energy, and small polaron formation upon photoexcitation, in addition to anomalous green photoluminescence with improved stability and high quantum yield. Moreover, the demonstration of Cs₄PbBr₆-based light-emitting diode (LED) devices highlights the accelerating efforts toward their applications and motivates further investigations of these emerging materials. This Perspective summarizes the progress in the field of Cs₄PbBr₆ perovskites, focusing on their molecular-electronic properties and hotly debated green photoluminescence. We conclude by presenting the implications of the unique findings and suggesting opportunities for the future development and applications of these 0D perovskites.

Spontaneous Silver Doping and Surface Passivation of CsPbI3 Perovskite Active Layer Enable Light-Emitting Devices with an External Quantum Efficiency of 11.2

Lead halide perovskite nanocrystals are currently under intense investigation as components of solution-processed light-emitting devices (LEDs). We demonstrate LEDs based on Ag doped-passivated CsPbI3 perovskite nanocrystals with external quantum efficiency of 11.2% and an improved stability. Ag and trilayer MoO3/Au/MoO3 structure were used as cathode and anode, respectively, which reduce the electron injection barrier and ensure the high transparency and low resistance of the anode. Silver ions diffuse into perovskite film from the Ag electrode, as confirmed by the elemental mapping, the presence of Ag 3d peaks in the X-ray photoelectron spectrum, and the peak shift in the X-ray diffraction patterns of CsPbI3. In addition to doping, silver ions play the beneficial role of passivating surface defect states of CsPbI3 nanocrystals, which results in increased photoluminescence quantum yield, elongated emission lifetime, and improved stability of perovskite films.

All-Inorganic Metal Halide Perovskite Nanocrystals: Opportunities and Challenges

The past decade has witnessed the growing interest in metal halide perovskites as driven by their promising applications in diverse fields. The low intrinsic stability of the early developed organic versions has however hampered their widespread applications. Very recently, all-inorganic perovskite nanocrystals have emerged as a new class of materials that hold great promise for the practical applications in solar cells, photodetectors, light-emitting diodes, and lasers, among others. In this Outlook, we first discuss the recent developments in the preparation, properties, and applications of all-inorganic metal halide perovskite nanocrystals, with a particular focus on CsPbX₃, and then provide our view of current challenges and future directions in this emerging area. Our goal is to introduce the current status of this type of new materials to researchers from different areas and motivate them to explore all the potentials.

Pick a Color MARIA: Adaptive Sampling Enables the Rapid Identification of Complex Perovskite Nanocrystal Compositions with Defined Emission Characteristics

Recent advances in the development of hybrid organic-inorganic lead halide perovskite (LHP) nanocrystals (NCs) have demonstrated their versatility and potential application in photovoltaics and as light sources through compositional tuning of optical properties. That said, due to their compositional complexity, the targeted synthesis of mixed-cation and/or mixed-halide LHP NCs still represents an immense challenge for traditional batch-scale chemistry. To address this limitation, we herein report the integration of a high-throughput segmented-flow microfluidic reactor and a self-optimizing algorithm for the synthesis of NCs with defined emission properties. The algorithm, named Multiparametric Automated Regression Kriging Interpolation and Adaptive Sampling (MARIA), iteratively computes optimal sampling points at each stage of an experimental sequence to reach a target emission peak wavelength based on spectroscopic measurements. We demonstrate the efficacy of the method through the synthesis of multinary LHP NCs, (Cs/FA)Pb(I/Br)₃ (FA = formamidinium) and (Rb/Cs/FA)Pb(I/Br)₃ NCs, using MARIA to rapidly identify reagent concentrations that yield user-defined photoluminescence peak wavelengths in the green-red spectral region. The procedure returns a robust model around a target output in far fewer measurements than systematic screening of parametric space and additionally enables the prediction of other spectral properties, such as, full-width at half-maximum and intensity, for conditions yielding NCs with similar emission peak wavelength.