Inter-Conversion between Different Compounds of Ternary Cs-Pb-Br System

Challenges in Developing Perovskite Nanocrystals for Commercial Applications

Zero-dimensional lead halide perovskite nanocrystals (NCs) exhibit size-dependent bandgap and carrier confinement compared to bulk counterparts due to the quantum confinement effect, making them essential for achieving wide-color-gamut displays, studying excitonic spin relaxation, and constructing superlattices. Despite their promising potential, they face a variety of technical bottlenecks, such as insufficient color reproducibility, limited large-scale production, low stability, and toxicity. An outline of a research roadmap is provided in the review, which highlights key challenges in developing perovskite NCs for commercial applications.

Phase Control of Cs-Pb-Br Derivatives to Suppress 0D Cs4 PbBr6 for High-Efficiency and Stable All-Inorganic CsPbBr3 Perovskite Solar Cells

The precise phase control of Cs-Pb-Br derivatives from 3D CsPbBr₃ to 0D Cs₄ PbBr₆ highly determines the photovoltaic performance of all-inorganic CsPbBr₃ perovskite solar cells (PSCs). Herein, the preferred phase conversion from precursor to Cs-Pb-Br derivatives is revealed by theoretically calculating the Gibbs free energies (∆G) of various phase conversion processes, allowing for a simplified multi-step solution-processable spin-coating method to hinder the formation of detrimental 0D Cs₄ PbBr₆ phase and enhance the photovoltaic performance of a PSC because of its large exciton binding energy, which is regarded as a recombination center. By further accelerating the interfacial charge extraction with a novel 2D transition metal dichalcogenide ReSe₂ , the hole-free CsPbBr₃ PSC achieves a champion efficiency of 10.67% with an impressive open-circuit voltage of 1.622 V and an excellent long-term stability. This work provides an in-depth understanding on the precise Cs-Pb-Br perovskite phase control and the effect of derivatives on photovoltaic performance of advanced CsPbBr₃ PSCs.

Pulsed Laser Deposition of CsPbBr3 Films: Impact of the Composition of the Target and Mass Distribution in the Plasma Plume

All-inorganic cesium lead bromine (CsPbBr3) perovskites have gained a tremendous potential in optoelectronics due to interesting photophysical properties and much better stability than the hybrid counterparts. Although pulsed laser deposition (PLD) is a promising alternative to solvent-based and/or thermal deposition approaches due to its versatility in depositing multi-elemental materials, deep understanding of the implications of both target composition and PLD mechanisms on the properties of CsPbBr3 films is still missing. In this paper, we deal with thermally assisted preparation of mechano-chemically synthesized CsPbBr3 ablation targets to grow CsPbBr3 films by PLD at the fluence 2 J/cm2. We study both Cs rich- and stoichiometric PbBr2-CsBr mixture-based ablation targets and point out compositional deviations of the associated films resulting from the mass distribution of the PLD-generated plasma plume. Contrary to the conventional meaning that PLD guarantees congruent elemental transfer from the target to the substrate, our study demonstrates cation off-stoichiometry of PLD-grown CsPbBr3 films depending on composition and thermal treatment of the ablation target. The implications of the observed enrichment in the heavier element (Pb) and deficiency in the lighter element (Br) of the PLD-grown films are discussed in terms of optical response and with the perspective of providing operative guidelines and future PLD-deposition strategies of inorganic perovskites.

Chemically Spiraling CsPbBr3 Perovskite Nanorods

Light emitting lead halide perovskite nanocrystals are currently emerging as the workhorse in quantum dot research. Most of these reported nanocrystals are isotropic cubes or polyhedral; but anisotropic nanostructures with controlled anisotropic directions still remain a major challenge. For orthorhombic CsPbBr₃, the 1D shaped nanostructures reported are linear and along either of the axial directions ⟨100⟩. In contrast, herein, spiral CsPbBr₃ perovskite nanorods in the orthorhombic phase are reported with unusual anisotropy having (101) planes remaining perpendicular to the major axis [201]. While these nanorods are synthesized using the prelattice of orthorhombic Cs₂CdBr₄ with Pb(II) diffusion, the spirality is controlled by manipulation of the compositions of alkylammonium ions in the reaction system which selectively dissolve some spiral facets of the nanorods. Further, as spirality varied with facet creation and elimination, these nanorods were explored as photocatalysts for CO₂ reduction, and the evolution of methane was also found to be dependent on the depth of the spiral nanorods. The entire study demonstrates facet manipulation of complex nanorods, and these results suggest that even if perovskites are ionic in nature, their shape could be constructed by design with proper reaction manipulation.

Zero-Dimensional Hybrid Organic-Inorganic Lead Halides and Their Post-Synthesis Reversible Transformation into Three-Dimensional Perovskites

Zero-dimensional (0D) mixed-halide hybrid organic-inorganic MA₄PbX₆·2H₂O (MA = CH₃NH₃⁺; X = Br_1 - xI_x with 0 < x < 1) has been synthesized by a solvent-free mechanochemical approach. It has been shown that this 0D phase with sharp absorption features in the near-UV is a hydrated structure, which can be reversibly transformed into the three-dimensional perovskite phase MAPbX₃ by simple thermal annealing (dehydration) in air. This work reveals a new approach to hybrid organic-inorganic perovskites and related 0D structures, which have so far only been thoroughly studied for the inorganic Cs₄PbX₆ compounds.

All-Inorganic CsPbBr3 Perovskite Films Prepared by Single Source Thermal Ablation

Hybrid organo-lead halide perovskites are becoming the benchmark material for next generation photovoltaics and a very important player for other applications such as photodetectors and light emitting diodes. Nevertheless, the most important issue hindering the large-scale application of these materials remains their intrinsic instability due to the organic cation. Although the substitution with inorganic cesium (Cs) enhances stability, in most cases solution deposition methods of fully inorganic perovskites result in high surface roughness and poor surface coverage. This work reports on the evaporation of the CsPbBr₃ precursor by Single Source Thermal Ablation, showing that just after deposition films consist of a mixture of CsPbBr₃, CsPb₂Br₅, and Cs₄PbBr₆ due to a vertical composition gradient. We point out that mild post deposition treatments lead to the conversion of CsPb₂Br₅ and Cs₄PbBr₆ into CsPbBr₃ due to its higher thermodynamic stability. Conversion results into smooth and pinhole-free CsPbBr₃ films with good light absorption and emission properties. We demonstrate the suitability of obtained films for planar devices by preparing perovskite-based pure-green light emitting diodes, thus promoting Single Source Thermal Ablation as a promising alternative deposition technique for all-inorganic perovskite-based devices.

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.

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.

All-inorganic lead halide perovskite nanohexagons for high performance air-stable lithium batteries

All-inorganic Cs4PbBr6 perovskite nanohexagons, pre-synthesized by a room temperature co-precipitation method, have been electrochemically investigated in a conventional aqueous electrolyte for potential application as an anode material in Li-ion batteries. The nanohexagons were uniformly deposited on ITO precoated glass substrate and subsequently annealed at ambient air to form a mechanically stable perovskite layer. These perovskite layers showed excellent performance during continuous Li-ion intercalation/deintercalation scans in an aqueous electrolyte, exhibiting a diffusion coefficient of 7.34 × 10-8 cm2 s-1, a specific discharge capacity of 377 mA h g-1, a capacity retention of 75% and coulombic efficiency that deteriorated to 98% after 100 scans. A water-triggered transformation of the Cs4PbBr6 to the CsPb2Br5 was initially observed followed by a reversible Li intercalation/deintercalation in the CsPb2Br5 structure for 40 consecutive scans. Following this period, an irreversible conversion reaction of CsPb2Br5 to CsBr and PbBr2 took place. The excellent electrochemical performance observed is promising towards the potential application of all-inorganic perovskite nanocrystals for air-stable, lithium storage applications.

Brightly luminescent and color-tunable green-violet-emitting halide perovskite CH3NH3PbBr3 colloidal quantum dots: an alternative to lighting and display technology

Organic-inorganic hybrid perovskite (CH3NH3PbX3, X = Cl, Br, or I) quantum dots have become one of the most promising materials for optoelectronic applications. We controllably synthesized CH3NH3PbBr3 quantum dots with a tunable spectrum with the emission peaks covering the range from green (523.6 nm), blue and eventually to deep violet (409.4 nm), which is wider than that of quantum dots obtained without changing the halide component. The mechanism of the blueshift was investigated. The purified quantum dots have allowed the fabrication of efficient electroluminescence devices having a simple glass/ITO/PEDOT:PSS/TFB/CH3NH3PbBr3 quantum dot/TPBi/LiF/Al structure. CH3NH3PbBr3 quantum dots with 5-30 μL n-octylamine showed an ideal color-saturated green emission with Commission Internationale de l'Eclairage color coordinates of (0.123, 0.744) and a narrow full width at half-maximum of 19-24 nm. The photoluminescence quantum yield was up to 90.2%. In addition, it is also worth noting that the chromaticity coordinates (x, y) of CH3NH3PbBr3 quantum dots with 50-100 μL n-octylamine are (0.300, 0.344), (0.305, 0.314) and (0.323, 0.318) in the white region. All these properties indicate that these MAPbBr3 quantum dots can provide effective data support for the application of white LEDs, and may potentially be used as single-component multicolor-emitting materials, which can be applied to lighting and display technology.