Hybrid light emitting diodes based on stable, high brightness all-inorganic CsPbI3 perovskite nanocrystals and InGaN

Vortex flow induced self-assembly in CsPbI3 rods leads to an improved electrical response towards external analytes

We present a facile and versatile strategy for enabling CsPbI3 rods to self-assemble at an air-water interface. The CsPbI3 rods, which float at the air-water interface, align under the influence of the rotational flow field due to the vortex motion of a water subphase. The aligned CsPbI3 rods could be transferred onto various substrates without involving any sophisticated instrumentation. The temperature of the subphase, the concentration of the CsPbI3 aliquot, the rotational speed inducing vortex motion, and the lift-off position and angle of the substrate were optimized to achieve high coverage of the self-assembled rods of CsPbI3 on glass. The Rietveld refinement of the XRD profile confirms that the aligned CsPbI3 is in the pure orthorhombic phase ascribed to the Pnma space group. The hydrophilic carboxylic group of the oleic acid attaches to the Pb atoms of the halide perovskite rods, while their hydrophobic tails encapsulate the rods within their shell, creating a shielding barrier between the water and the perovskite surface like a reverse micelle. The aligned CsPbI3 rods exhibit a nearly 47-fold increment in current upon exposure to ammonia gas (amounting to 5.6 times higher sensitivity in ammonia sensing) compared to the non-aligned CsPbI3 rods.

Multidentate Ligand-Passivated CsPbI3 Perovskite Nanocrystals for Stable and Efficient Red-Light-Emitting Diodes

CsPbI₃ nanocrystals (NCs) have become a research hot spot in the field of light-emitting diodes (LEDs). Whereas, the long chain ligands with weak affinity to CsPbI₃ NCs have prevented their further development and commercialization. Herein, a novel multidentate short ligand tetramethylthiuram disulfide (TMTD) was employed via a ligand exchange process to enhance hole mobility and decrease trap density of the CsPbI₃ NCs film. Therefore, TMTD passivated CsPbI₃ NCs LED exhibited 20.65% maximum external quantum efficiency and 3861 cd/m² maximum luminance. Furthermore, TMTD passivated CsPbI₃ NCs LED exhibited good operational stability with a 128 min half-lifetime. This strategy using multidentate short ligand passivation provides an effective way to promote perovskite LED development and commercialization.

Ligands in Lead Halide Perovskite Nanocrystals: From Synthesis to Optoelectronic Applications

Ligands are indispensable for perovskite nanocrystals (NCs) throughout the whole lifetime, as they not only play key roles in the controllable synthesis of NCs with different sizes and shapes, but also act as capping shell that affects optical properties and electrical coupling of NCs. Establishing a systematic understanding of the relationship between ligands and perovskite NCs is significant to enable many potential applications of NCs. This review mainly focuses on the influence of ligands on perovskite NCs. First of all, the ligands-dominated size and shape control of NCs is discussed. Whereafter, the surface defects of NCs and the bonding between ligands and perovskite NCs are classified, and corresponding post-treatment of surface defects via ligands is also summarized. Furthermore, advances in engineering the ligands towards the high performance of optoelectronic devices based on perovskite NCs, including photodetector, solar cell, light emitting diode (LED), and laser, and finally to potential challenges are also discussed.

Photosensitisation of inkjet printed graphene with stable all-inorganic perovskite nanocrystals

All-inorganic perovskite nanocrystals (NCs) with enhanced environmental stability are of particular interest for optoelectronic applications. Here we report on the formulation of CsPbX₃ (X is Br or I) inks for inkjet deposition and utilise these NCs as photosensitive layers in graphene photodetectors, including those based on single layer graphene (SLG) as well as inkjet-printed graphene (iGr) devices. The performance of these photodetectors strongly depends on the device structure, geometry and the fabrication process. We achieve a high photoresponsivity, R > 10⁶ A W^-1 in the visible wavelength range and a spectral response controlled by the halide content of the perovskite NC ink. By utilising perovskite NCs, iGr and gold nanoparticle inks, we demonstrate a fully inkjet-printed photodetector with R ≈ 20 A W^-1, which is the highest value reported to date for this type of device. The performance of the perovskite/graphene photodetectors is explained by transfer of photo-generated charge carriers from the perovskite NCs into graphene and charge transport through the iGr network. The perovskite ink developed here enabled realisation of stable and sensitive graphene-based photon detectors. Compatibility of inkjet deposition with conventional Si-technologies and with flexible substrates combined with high degree of design freedom provided by inkjet deposition offers opportunities for partially and fully printed optoelectronic devices for applications ranging from electronics to environmental sciences.

Degradation mechanisms of perovskite nanocrystals in color-converted InGaN micro-light-emitting diodes

The metal halide perovskite nanocrystals (NCs) have attracted much attention because of their excellent optical properties and potential for application in optoelectronic devices. However, their photo- and thermostability are still practical challenges and need further optimization. Here, we have studied the degradation behaviors of CsPbI₃ NCs utilized as optical conversion layer in InGaN based blue micro-LEDs in situ. Furthermore, the effects of temperature and light irradiation on perovskite NCs were investigated respectively. The results indicate that both blue light irradiation and high temperature can cause the increased nonradiative recombination rate, resulting in the degradation of perovskite NCs and reduction of the photoluminescence quantum yield (PLQY). Especially in high-temperature condition, both the single-exciton nonradiative recombination rate and the biexciton nonradiative recombination rate are increased, causing the significant reduction of PLQY of perovskite NCs in high temperature environment than blue light irradiation. Our work provides a detailed insight about the correlation between the light irradiation and temperature consequences for CsPbI₃ NCs and may help to pave the way toward optoelectronic device applications.

Cesium Lead Iodide Perovskites: Optically Active Crystal Phase Stability to Surface Engineering

Among perovskites, the research on cesium lead iodides (CsPbI₃) has attracted a large research community, owing to their all-inorganic nature and promising solar cell performance. Typically, the CsPbI₃ solar cell devices are prepared at various heterojunctions, and working at fluctuating temperatures raises questions on the material stability-related performance of such devices. The fundamental studies reveal that their poor stability is due to a lower side deviation from Goldschmidt's tolerance factor, causing weak chemical interactions within the crystal lattice. In the case of organic-inorganic hybrid perovskites, where their stability is related to the inherent chemical nature of the organic cations, which cannot be manipulated to improve the stability drastically whereas the stability of CsPbI₃ is related to surface and lattice engineering. Thus, the challenges posed by CsPbI₃ could be overcome by engineering the surface and inside the CsPbI₃ crystal lattice. A few solutions have been proposed, including controlled crystal sizes, surface modifications, and lattice engineering. Various research groups have been working on these aspects and had accumulated a rich understanding of these materials. In this review, at first, we survey the fundamental aspects of CsPbI₃ polymorphs structure, highlighting the superiority of CsPbI₃ over other halide systems, stability, the factors (temperature, polarity, and size influence) leading to their phase transformations, and electronic band structure along with the important property of the defect tolerance nature. Fortunately, the factors stabilizing the most effective phases are achieved through a size reduction and the efficient surface passivation on the delicate CsPbI₃ nanocrystal surfaces. In the following section, we have provided the up-to-date surface passivating methods to suppress the non-radiative process for near-unity photoluminescence quantum yield, while maintaining their optically active phases, especially through molecular links (ligands, polymers, zwitterions, polymers) and inorganic halides. We have also provided recent advances to the efficient synthetic protocols for optically active CsPbI₃ NC phases to use readily for solar cell applications. The nanocrystal purification techniques are challenging and had a significant effect on the device performances. In part, we summarized the CsPbI₃-related solar cell device performances with respect to the device fabrication methods. At the end, we provide a brief outlook on the view of surface and lattice engineering in CsPbI₃ NCs for advancing the enhanced stability which is crucial for superior optical and light applications.

Quasi-Type II Core-Shell Perovskite Nanocrystals for Improved Structural Stability and Optical Gain

In recent years, core-shell lead halide perovskite nanocrystals (PeNCs) and their devices have attracted intensive attention owing to nearly perfect optoelectronic properties. However, the complex photophysical mechanism among them is still unclear. Herein, monodispersed core-shell PeNCs coated with an all-inorganic cesium lead bromide (CsPbBr₃) shell epitaxially grown on the surface of formamidinium lead bromide (FAPbBr₃) PeNCs were synthesized. Through power- and temperature-dependent photoluminescence (PL) measurements, it is found that the electronic structure of the core-shell FAPbBr₃/CsPbBr₃ PeNCs has a quasi-type II band alignment. The presence of Cs⁺ in the shell limits ion migration and helps to stabilize structural and optical properties. On this basis, after being exposed to pulsed nanosecond laser for a period, an amplified spontaneous emission (ASE) can be observed, which is attributed to the effective passivation induced by laser irradiation on defects at the interface. The ASE threshold of the core-shell PeNCs showing high structural and optical stability is 447 nJ/cm² under pulsed nanosecond optical pumping. The results that are demonstrated here provide a new idea and perspective for improving the stability of perovskite and can be of practical interest for the utilization of the core-shell PeNCs in optoelectronic devices.

Surface ligand engineering of CsPbBr3 perovskite nanowires for high-performance photodetectors

Surface ligand engineering is of great importance for the preparation of one-dimensional (1D) CsPbBr₃ nanowires for high-performance photodetectors. The traditional long-chain terminated ligands such as oleylamine/oleic acid (C18) used in the preparation of CsPbBr₃ nanowires will form an electrically insulating layer on the surface of the nanowires, which hinders the effective transport of charge carriers in optoelectronic devices. In this paper, short-chain ligands, including dodecylamine/dodecanoic acid (C12), octylamine/octanoic acid (C8) and hexylamine/hexanoic acid (C6), are introduced to partially replace long-chain ligands (C18) to successfully prepare various CsPbBr₃ nanowires via a solvothermal method. Microstructure characterization indicates that the four kinds of nanowires before/after surface ligand engineering, which are named as C18-CsPbBr₃, C12/18-CsPbBr₃, C8/18-CsPbBr₃ and C6/18-CsPbBr₃, all have high aspect ratio and purity. As compared with CsPbBr₃ with long-chain terminated ligands, the C8/18-CsPbBr₃ and C6/18-CsPbBr₃ nanowires with shorter chain ligands exhibit superior photoluminescence (PL) performance and stability under adverse conditions such as ultraviolet irradiation and high temperature. The constructed photodetectors based on C8/18-CsPbBr₃ and C6/18-CsPbBr₃ nanowires have shown improved performances. This work provides a new idea for the preparation of CsPbBr₃ nanowires with high optical properties, stability and charge transport, and the prepared CsPbBr₃ nanowires have potential application prospects in optoelectronic devices.

Current status on synthesis, properties and applications of CsPbX3(X = Cl, Br, I) perovskite quantum dots/nanocrystals

The quantum confinement effect and interesting optical properties of cesium lead halide (CsPbX₃; X = Cl, Br, I) perovskite quantum dots (QDs) and nanocrystals (NCs) have given a new horizon to lighting and photonic applications. Given the exponential rate at which scientific results on CsPbX₃NCs are published in the last few years, it can be expected that the research in CsPbX₃NCs will further receive increasing scientific interests in the near future and possibly lead to great commercial opportunities to realize these materials based practical applications. With the rapid progress in the single-photon emitting CsPbX₃QDs and NCs, practical applications of the quantum technologies such as single-photon emitting light-emitting diode, quantum lasers, quantum computing might soon be possible. But to reach at cutting edge of stable perovskite QDs/NCs, the study of fundamental insight and theoretical aspects of crystal design is yet insufficient. Even more, it has aroused many unanswered questions related to the stability, optical and electronic properties of the CsPbX₃QDs. Aim of the present review is to illustrate didactically a precise study of recent progress in the synthesis, properties and applications of CsPbX₃QDs and NCs. Critical issues that currently restrict the applicability of these QDs will be identified and advanced methodologies currently in the developing queue, to overcome the roadblock, will be presented. And finally, the prospects for future directions will be provided.

Intermittency of CsPbBr3 Perovskite Quantum Dots Analyzed by an Unbiased Statistical Analysis

We analyze intermittency in intensity and fluorescence lifetime of CsPbBr3 perovskite quantum dots by applying unbiased Bayesian inference analysis methods. We apply change-point analysis (CPA) and a Bayesian state clustering algorithm to determine the timing of switching events and the number of states between which switching occurs in a statistically unbiased manner, which we have benchmarked particularly to apply to highly multistate emitters. We conclude that perovskite quantum dots display a plethora of gray states in which brightness, broadly speaking, correlates inversely with decay rate, confirming the multiple recombination centers model. We leverage the CPA partitioning analysis to examine aging and memory effects. We find that dots tend to return to the bright state before jumping to a dim state and that when choosing a dim state, they tend to explore the entire set of states available.

Recent Advances in Ligand Design and Engineering in Lead Halide Perovskite Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) have recently garnered enhanced development efforts from research disciplines owing to their superior optical and optoelectronic properties. These materials, however, are unlike conventional quantum dots, because they possess strong ionic character, labile ligand coverage, and overall stability issues. As a result, the system as a whole is highly dynamic and can be affected by slight changes of particle surface environment. Specifically, the surface ligand shell of LHP NCs has proven to play imperative roles throughout the lifetime of a LHP NC. Recent advances in engineering and understanding the roles of surface ligand shells from initial synthesis, through postsynthetic processing and device integration, finally to application performances of colloidal LHP NCs are covered here.

Synergistic function of doping and ligand engineering to enhance the photostability and electroluminescence performance of CsPbBr3quantum dots

The photostability issue of CsPbX₃(X = Cl, Br, I) quantum dots (QDs) is one of the key origins for the degradation of their luminescence performance, which hinders their application in lighting and displays. Herein, we report a new method combining doping and ligand engineering, which effectively improves the photostability of CsPbBr₃QDs and the performance of QD light-emitting diodes (QLEDs). In this method, ZnBr₂is doped into CsPbBr₃QDs to reduce surface anion defects; didodecyldimethyl ammonium bromide (DDAB) and tetraoctylammonium bromide (TOAB) hybrid ligands, which have strong adsorption with QDs, are employed to protect the surface and enhance the conductivity of QD layer in QLEDs. The photoluminescence (PL) and transmission electron microscopy measurements prove the effectively improved photostability of CsPbX₃QDs. Moreover, reduced defects and improved conductivity by doping and hybrid ligands treatment also enable the improved electroluminescence performance of CsPbX₃QDs. The maximum luminance and external quantum efficiency of the QLED with optimized CsPbX₃QDs are 3518.9 cd m^-2and 5.07%, which are 3.6 and 2.1 times than that of the control device, respectively. Combining doping and hybrid ligands makes perovskite QDs have an extremely promising prospect in future applications of high-definition displays, high-quality lighting, as well as solar cells.

Revealing the Exciton Fine Structure in Lead Halide Perovskite Nanocrystals

Lead-halide perovskite nanocrystals (NCs) are attractive nano-building blocks for photovoltaics and optoelectronic devices as well as quantum light sources. Such developments require a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. In this review, we give an overview of recent magneto-optical spectroscopic studies revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of their inhomogeneities in morphology and crystal structure. We highlight the prominent role of the electron-hole exchange interaction in the order and splitting of the bright triplet and dark singlet exciton sublevels and discuss the effects of size, shape anisotropy and dielectric screening on the fine structure. The spectral and temporal manifestations of thermal mixing between bright and dark excitons allows extracting the specific nature and strength of the exciton–phonon coupling, which provides an explanation for their remarkably bright photoluminescence at low temperature although the ground exciton state is optically inactive. We also decipher the spectroscopic characteristics of other charge complexes whose recombination contributes to photoluminescence. With the rich knowledge gained from these experiments, we provide some perspectives on perovskite NCs as quantum light sources.

Metal Halide Perovskite Nanorods: Shape Matters

Quasi-1D metal halide perovskite nanorods (NRs) are emerging as a type of materials with remarkable optical and electronic properties. Research into this field is rapidly expanding and growing in the past several years, with significant advances in both mechanistic studies of their growth and widespread possible applications. Here, the recent advances in 1D metal halide perovskite nanocrystals (NCs) are reviewed, with a particular emphasis on NRs. At first, the crystal structures of perovskites are elaborated, which is followed by a review of the major synthetic approaches toward perovskite NRs, such as wet-chemical synthesis, substrate-assisted growth, and anion exchange reactions, and discussion of the growth mechanisms associated with each synthetic method. Then, thermal and aqueous stability and the linear polarized luminescence of perovskite NRs are considered, followed by highlighting their applications in solar cells, light-emitting diodes, photodetectors/phototransistors, and lasers. Finally, challenges and future opportunities in this rapidly developing research area are summarized.

Memories in the photoluminescence intermittency of single cesium lead bromide nanocrystals

Single cesium lead bromide (CsPbBr₃) nanocrystals show strong photoluminescence intermittency, with on- and off- dwelling times following power-law distributions. We investigate the correlations for successive on-times and successive off-times, and find a memory effect in the photoluminescence intermittency of such inorganic perovskite nanocrystals. This memory effect is not sensitive to the nature of the surface capping ligand and the embedding polymer. These observations suggest that photoluminescence intermittency and its memory are mainly controlled by intrinsic traps in the nanocrystals. Our findings will help optimizing light-emitting devices based on these perovskite nanocrystals.

Direct Synthesis of Quaternary Alkylammonium-Capped Perovskite Nanocrystals for Efficient Blue and Green Light-Emitting Diodes

Cesium lead halide nanocrystals (CsPbX₃ NCs) are new inorganic light sources covering the entire visible spectral range and exhibiting near-unity efficiencies. While the last years have seen rapid progress in green and red electroluminescence from CsPbX₃ NCs, the development of blue counterparts remained rather stagnant. Controlling the surface state of CsPbX₃ NCs had proven to be a major factor governing the efficiency of the charge injection and for diminishing the density of traps. Although didodecyldimethylammonium halides (DDAX; X = Br, Cl) had been known to improve the luminescence of CsPbX₃ NCs when applied postsynthetically, they had not been used as the sole long-chain ammonium ligand directly in the synthesis of these NCs. Herein we report a facile, direct synthesis of DDAX-stabilized CsPbX₃ NCs. We then demonstrate blue and green light-emitting diodes, characterized by the electroluminescence at 463-515 nm and external quantum efficiencies of 9.80% for green, 4.96% for sky-blue, and 1.03% for deep-blue spectral regions.