Structural Investigation of Cesium Lead Halide Perovskites for High-Efficiency Quantum Dot Light-Emitting Diodes

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.

Advances in Perovskite Light-Emitting Diodes Possessing Improved Lifetime

Recently, perovskite light-emitting diodes (PeLEDs) are seeing an increasing academic and industrial interest with a potential for a broad range of technologies including display, lighting, and signaling. The maximum external quantum efficiency of PeLEDs can overtake 20% nowadays, however, the lifetime of PeLEDs is still far from the demand of practical applications. In this review, state-of-the-art concepts to improve the lifetime of PeLEDs are comprehensively summarized from the perspective of the design of perovskite emitting materials, the innovation of device engineering, the manipulation of optical effects, and the introduction of advanced encapsulations. First, the fundamental concepts determining the lifetime of PeLEDs are presented. Then, the strategies to improve the lifetime of both organic-inorganic hybrid and all-inorganic PeLEDs are highlighted. Particularly, the approaches to manage optical effects and encapsulations for the improved lifetime, which are negligibly studied in PeLEDs, are discussed based on the related concepts of organic LEDs and Cd-based quantum-dot LEDs, which is beneficial to insightfully understand the lifetime of PeLEDs. At last, the challenges and opportunities to further enhance the lifetime of PeLEDs are introduced.

Ligand & band gap engineering: tailoring the protocol synthesis for achieving high-quality CsPbI3 quantum dots

Hot-injection has become the most widespread method used for the synthesis of perovskite quantum dots (QDs) with enormous interest for application in optoelectronic devices. However, there are some aspects of the chemistry involved in this synthesis that have not been completely investigated. In this work, we synthesized ultra-high stable CsPbI₃ QDs for more than 15 months by controlling two main parameters: synthesis temperature and the concentration of capping ligands. By increasing the capping ligand concentration during the QD synthesis, we were able to grow CsPbI₃ in a broad range of temperatures, improving the photophysical properties of QDs by increasing the synthesis temperature. We achieved the maximum photoluminescence quantum yield (PLQY) of 93% for a synthesis conducted at 185 °C, establishing an efficient surface passivation to decrease the density of non-radiative recombination sites. Under these optimized synthesis conditions, deep red LEDs with an External Quantum Efficiency (EQE) higher than 6% were achieved. The performance of these LEDs is higher than that of the reported CsPbI₃ QD-LEDs containing standard capping agents, without additional elements or further element exchange. We show that it is possible to produce stable CsPbI₃ QDs with high PLQY and red emission beyond the requirement of the Rec. 2020 standards for red color.

Structural Transition and Interdigitation of Alkyl Side Chains in the Conjugated Polymer Poly(3-hexylthiophene) and Their Effects on the Device Performance of the Associated Organic Field-Effect Transistor

Direct grazing-angle X-ray scattering evidence of the order-disorder transition and interdigitation of side chains in a conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. The free methyl ends of the side chains exhibit closest packing, as in n-alkane crystallization, and cause a structural mismatch due to the difference between their packing density and the areal density of the attached ends. This mismatch is resolved by increases in the tilt angle of the side chains and local interdigitation. In situ X-ray scattering and electrical measurements show that the structural transition and interdigitation of these side chains strongly affect its surface morphology as well as the charge transport properties of the resulting P3HT-based organic field-effect transistor. Since most conjugated polymers have side chains, the results of this study provide a deeper understanding of the effects of side chains on the structural and electrical properties of conjugated backbones. These results also provide a new perspective on the formation of a metastable polymorph consisting of interdigitated P3HT.

Methylammonium-Mediated Crystallization of Cesium-Based 2D/3D Perovskites toward High-Efficiency Light-Emitting Diodes

Two-dimensional (2D)/three-dimensional (3D) perovskites have been successfully applied in high-efficiency light-emitting diodes (LEDs) because of their large exciton binding energy (E_b) caused by the quantum and dielectric confinements. Thermal annealing and antisolvent treatments are usually executed in order to promote the crystallization and film quality of perovskites, which add complexity to the device fabrication process. Here, the cesium-based 2D/3D perovskite was prepared by introducing ammonium halide benzamidine hydrochloride (BMCl) as the additive. By further introducing an appropriate amount of MABr and PbBr₂, BM₂(Cs_1-xMA_xPbBr₃)_n-1PbBr₄ crystals can be formed rapidly without any additional treatments, while inhibiting the formation of the unfavorable Cs₄PbBr₆ phase. The optimized 2D/3D perovskite-based LEDs achieved a maximum luminance of 12 367 Cd/m², a current efficiency of 17.4 Cd/A, and an external quantum efficiency of 5.2%. Our results suggest that appropriate perovskite crystallization can be achieved at room temperature by the regulation of precursor solution, making the perovskite crystallization process easier to control with reduced processing complexity.

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.

Material Design and Optoelectronic Properties of Three-Dimensional Quadruple Perovskite Halides

The discovery of new halide perovskite-type structures could favor the exploration of optoelectronic materials, as in the case of double perovskites applied in solar cells, light-emitting diodes, and X-ray detectors. In this work, we propose a strategy for designing quadruple perovskites by heterovalent cation transmutation from double perovskites. Two stable quadruple perovskite halides, i.e., Cs₄CdSb₂Cl₁₂ and Cs₄CdBi₂Cl₁₂, with a vacancy-ordered three-dimensional (3D) crystal structure were predicted through symmetry analysis and density functional theory (DFT) calculations. The title perovskite halides are also electronically 3D with direct forbidden bandgaps. Following the indication provided by the DFT results, Cs₄CdSb₂Cl₁₂ and Cs₄CdBi₂Cl₁₂ as unique quadruple perovskites were successfully synthesized by a solvothermal method. The steady-state photoluminescence (PL) shows wide emission, while the transient PL exhibits carrier recombination lifetime on the order of microseconds at low temperature. The quadruple perovskite halides provide an alternative platform for promising optoelectronic material design in addition to simple and double perovskites.

Device Engineering for All-Inorganic Perovskite Light-Emitting Diodes

Recently, all-inorganic perovskite light-emitting diodes (PeLEDs) have attracted both academic and industrial interest thanks to their outstanding properties, such as high efficiency, bright luminance, excellent color purity, low cost and potentially good operational stability. Apart from the design and treatment of all-inorganic emitters, the device engineering is another significant factor to guarantee the high performance. In this review, we have summarized the state-of-the-art concepts for device engineering in all-inorganic PeLEDs, where the charge injection, transport, balance and leakage play a critical role in the performance. First, we have described the fundamental concepts of all-inorganic PeLEDs. Then, we have introduced the enhancement of device engineering in all-inorganic PeLEDs. Particularly, we have comprehensively highlighted the emergence of all-inorganic PeLEDs, strategies to improve the hole injection, approaches to enhance the electron injection, schemes to increase the charge balance and methods to decrease the charge leakage. Finally, we have clarified the issues and ways to further enhance the performance of all-inorganic PeLEDs.

Presence of Metal Chloride for Minimizing the Halide Deficiency and Maximizing the Doping Efficiency in Mn(II)-Doped CsPbCl3 Nanocrystals

Pretreatment using metal chlorides during the formation of halide deficient free perovskite nanocrystals is reported. Among several metal chlorides, Cu(II)Cl₂ was observed to be ideal for the synthesis of highly emitting CsPbCl₃ nanocrystals at high reaction temperature. Because high temperature remained more favorable for the dopant insertion, doping of Mn(II) was carried out under this halide-rich system, and nearly 68% photoluminescence quantum yield was recorded. Analysis could not provide strong evidence of insertion of Cu(II) inside the nanocrystals; rather, it was established that Cu(II)Cl₂ in the system helped to stabilize the reaction even at and above 260 °C and provided an adequate chloride source for obtaining the highly emitting host as well as doped nanocrystals. Details of the physical process involved for this metal ion-induced uplifting of the reaction temperature and the consequent impacts on the nanocrystal formation are studied in detail and reported in this Letter.

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field-Effect Transistors

The past several years have witnessed tremendous developments of metal halide perovskite (MHP)-based optoelectronics. Particularly, the intensive research of MHP-based light-emitting diodes, photodetectors, and solar cells could probably reform the optoelectronic semiconductor industry. In comparison, in spite of the large intrinsic charge carrier mobility of MHPs, the development of MHP-based field-effect transistors (MHP-FETs) is relatively slow, which is essentially due to the gate-field screening effect induced by the ion migration and accumulation in MHP-FETs. This work mainly aims to summarize the recent important work on MHP-FETs and propose solutions in terms of the development bottleneck of perovskite-based transistors, in an attempt to boost the research of MHP transistors further. First, the advantages and potential applications of MHP-FETs are briefly introduced, which is followed by a detailed description of the MHP crystalline structure and various material fabrication techniques. Afterward, MHP-FETs are discussed, including transistors based on hybrid organic-inorganic perovskites, all-inorganic perovskites, and lead-free perovskites.

Solvent-Assisted Surface Engineering for High-Performance All-Inorganic Perovskite Nanocrystal Light-Emitting Diodes

All-inorganic cesium halide perovskite nanocrystals have attracted much interest in optoelectronic applications for the sake of the readily adjustable band gaps, high photoluminescence quantum yield, pure color emission, and affordable cost. However, because of the ineluctable utilization of organic surfactants during the synthesis, the structural and optical properties of CsPbBr₃ nanocrystals degrade upon transforming from colloidal solutions to solid thin films, which plagues the device operation. Here, we develop a novel solvent-assisted surface engineering strategy, producing high-quality CsPbBr₃ thin films for device applications. A good solvent is first introduced as an assembly trigger to conduct assembly in a one-dimensional direction, which is then interrupted by adding a nonsolvent. The nonsolvent drives the adjacent nanoparticles connecting in a two-dimensional direction. Assembled CsPbBr₃ nanocrystal thin films are densely packed and very smooth with a surface roughness of ∼4.8 nm, which is highly desirable for carrier transport in a light-emitting diode (LED) device. Meanwhile, the film stability is apparently improved. Benefiting from this facile and reliable strategy, we have achieved remarkably improved performance of CsPbBr₃ nanocrystal-based LEDs. Our results not only enrich the methods of nanocrystal surface engineering but also shed light on developing high-performance LEDs.

All-Ambient Processed Binary CsPbBr3-CsPb2Br5 Perovskites with Synergistic Enhancement for High-Efficiency Cs-Pb-Br-Based Solar Cells

All-inorganic CsPbBr₃ perovskite solar cells display outstanding stability toward moisture, light soaking, and thermal stressing, demonstrating great potential in tandem solar cells and toward commercialization. Unfortunately, it is still challenging to prepare high-performance CsPbBr₃ films at moderate temperatures. Herein, a uniform, compact CsPbBr₃ film was fabricated using its quantum dot (QD)-based ink precursor. The film was then treated using thiocyanate ethyl acetate (EA) solution in all-ambient conditions to produce a superior CsPbBr₃-CsPb₂Br₅ composite film with a larger grain size and minimal defects. The achievement was attributed to the surface dissolution and recrystallization of the existing SCN^- and EA. More specifically, the SCN^- ions were first absorbed on the Pb atoms, leading to the dissolution and stripping of Cs⁺ and Br^- ions from the CsPbBr₃ QDs. On the other hand, the EA solution enhances the diffusion dynamics of surface atoms and the surfactant species. It is found that a small amount of CsPb₂Br₅ in the composite film gives the best surface passivation, while the Br-rich surface decreases Br vacancies (V_Br) for a prolonged carrier lifetime. As a result, the fabricated device gives a higher solar cell efficiency of 6.81% with an outstanding long-term stability.