Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals

Wide-Bandgap Lead Halide Perovskites for Next-Generation Optoelectronics: Current Status and Future Prospects

Over the past decade, lead halide perovskites (LHPs), an emerging class of organic-inorganic ionic-type semiconductors, have drawn worldwide attention, which injects vitality into next-generation optoelectronics. Facilely tunable bandgap is one of the fascinating features of LHPs, enabling them to be widely used in various nano/microscale applications. Notably, wide-bandgap (WBG) LHPs have been considered as promising alternatives to traditional WBG semiconductors owing to the merits of low-cost, solution processability, superior optoelectronic characteristics, and flexibility, which could improve the cost-effectiveness and expand the application scenarios of traditional WBG devices. Herein, we provide a comprehensive review on the up-to-date research progress of WBG LHPs and their optoelectronics in terms of material fundamentals, optoelectronic devices, and their practical applications. First, the features and shortcomings of WBG LHPs are introduced to objectively display their natural features. Then we separately depict three typical optoelectronic devices based on WBG LHPs, including solar cells, light emitting diodes, and photodetectors. Sequentially, the inspiring applications of these optoelectronic devices in integrated functional systems are elaborately demonstrated. At last, the remaining challenges and future promise of WBG LHPs in optoelectronic applications are discussed. This review highlights the significance of WGB LHPs for promoting the development of the next-generation optoelectronics industry.

Internal Electric Field Manipulates Exciton-Phonon Couplings in Single Lead Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have attracted much attention as materials for light-emitting diodes and quantum light sources. A deep understanding of exciton-phonon couplings is essential for obtaining a narrow emission line, weak phonon-sideband photoluminescence (PL), and a long exciton coherence time, which are especially useful for high-color-purity quantum-light-source applications. Here, we report the PL spectra of single CsPbBr3 NCs at 5.5 K as a function of the applied electric field. The exciton peak energy shows an asymmetric parabolic shift for positive and negative biases, implying the presence of a spontaneously generated internal electric field in the NCs when no field is applied. Both the internal electric field and exciton-phonon couplings become larger in smaller NCs, and they have a positive correlation with each other. Our findings show that the exciton-phonon couplings can be manipulated with an electric field, which dominates the PL properties of perovskite NCs.

Fate of Optical Excitons in FAPbI3 Nanocube Superlattices

Understanding the nature of the photoexcitation and ultrafast charge dynamics pathways in organic halide perovskite nanocubes and their aggregation into superlattices is key for potential applications as tunable light emitters, photon-harvesting materials, and light-amplification systems. In this work, we apply two-dimensional coherent electronic spectroscopy (2DES) to track in real time the formation of near-infrared optical excitons and their ultrafast relaxation in CH(NH2)2PbI3 nanocube superlattices. Our results unveil that the coherent ultrafast dynamics is limited by the combination of the inherent short exciton decay time (≃40 fs) and the dephasing due to the coupling with selective optical phonon modes at higher temperatures. On the picosecond time scale, we observe the progressive formation of long-lived localized trap states. The analysis of the temperature dependence of the excitonic intrinsic line width, as extracted by the antidiagonal components of the 2D spectra, unveils a dramatic change of the excitonic coherence time across the cubic to tetragonal structural transition. Our results offer a new way to control and enhance the ultrafast coherent dynamics of photocarrier generation in hybrid halide perovskite synthetic solids.

Band Engineering of Perovskite Quantum Dot Solids for High-Performance Solar Cells

CsPbI3 perovskite quantum dot (PQD) shows high potential for next-generation photovoltaics due to their tunable surface chemistry, good solution-processability and unique photophysical properties. However, the remained long-chain ligand attached to the PQD surface significantly impedes the charge carrier transport within the PQD solids, thereby predominantly influencing the charge extraction of PQD solar cells (PQDSCs). Herein, a ligand-induced energy level modulation is reported for band engineering of PQD solids to improve the charge extraction of PQDSCs. Detailed theoretical calculations and systemic experimental studies are performed to comprehensively understand the photophysical properties of the PQD solids dominated by the surface ligands of PQDs. The results reveal that 4-nitrobenzenethiol and 4-methoxybenzenethiol molecules with different dipole moments can firmly anchor to the PQD surface through the thiol group to modulate the energy levels of PQDs, and a gradient band structure within the PQD solid is subsequently realized. Consequently, the band-engineered PQDSC delivers an efficiency of up to 16.44%, which is one of the highest efficiencies of CsPbI3 PQDSCs. This work provides a feasible avenue for the band engineering of PQD solids by tuning the surface chemistry of PQDs for high-performing solar cells or other optoelectronic devices.

Aromatic 2,2-Diphenylethylamine Ligand Exchange of FA0.9Cs0.1PbBr3 Perovskite Nanocrystals for High-Efficiency Pure Green Light-Emitting Diodes

Perovskite nanocrystals (NCs) with long alkyl ligands cannot easily form high-quality composite films owing to their poor dispersibility in π-conjugated small molecules and polymer host materials. In this study, we demonstrated that the aromatic ligand exchange of mixed-cation FA0.9Cs0.1PbBr3 NCs using 2,2-diphenylethylamine (DPEA) can not only enable the fabrication of high-efficiency light-emitting diodes (LEDs) but also allows dispersibility in host materials. The DPEA-NC film exhibited a pure green wavelength of 530 nm and a full width at half-maximum of 20.9 nm with a photoluminescence quantum yield of 90.9%. A DPEA-NC LED achieved a luminance of 39,700 cd/m2 and an external quantum efficiency of 18.6% even in a thick NC film. Interestingly, the DPEA-NCs formed a composite film with small-molecule tris(4-carbazoyl-9-ylphenyl)amine. The operational stability of this composite LED was eight times higher than that of the DPEA-NC LED owing to enhanced hole-electron charge balance and the suppression of perovskite NC degradation. Therefore, the aromatic DPEA ligand exchange of perovskite NCs is an effective way to improve their electrical properties and operational device stabilities.

Strongly-confined colloidal lead-halide perovskite quantum dots: from synthesis to applications

Colloidal semiconductor nanocrystals enable the realization and exploitation of quantum phenomena in a controlled manner, and can be scaled up for commercial uses. These materials have become important for a wide range of applications, from ultrahigh definition displays, to solar cells, quantum computing, bioimaging, optical communications, and many more. Over the last decade, lead-halide perovskite nanocrystals have rapidly gained prominence as efficient semiconductors. Although the majority of studies have focused on large nanocrystals in the weak- to intermediate-confinement regime, quantum dots (QDs) in the strongly-confined regime (with sizes smaller than the Bohr diameter, which ranges from 4-12 nm for lead-halide perovskites) offer unique opportunities, including polarized light emission and color-pure, stable luminescence in the region that is unattainable by perovskites with single-halide compositions. In this tutorial review, we bring together the latest insights into this emerging and rapidly growing area, focusing on the synthesis, steady-state optical properties (including exciton fine-structure splitting), and transient kinetics (including hot carrier cooling) of strongly-confined perovskite QDs. We also discuss recent advances in their applications, including single photon emission for quantum technologies, as well as light-emitting diodes. We finish with our perspectives on future challenges and opportunities for strongly-confined QDs, particularly around improving the control over monodispersity and stability, important fundamental questions on the photophysics, and paths forward to improve the performance of perovskite QDs in light-emitting diodes.

Ultranarrow Deep-Blue Luminescence of Perovskite Nanocrystals by A-Site Cation Control

Metal-halide perovskite nanocrystals (NCs) are one of the most promising emitters for the application of display and nanolight sources. The full width at half-maximum (FWHM) of photoluminescence (PL) emission is essential for color purity, which however remains a difficulty to further reduce the FWHM of the perovskite NCs at room temperature. Here, we show the quasi-sphere perovskite NCs with narrow PL emission at a deep-blue wavelength of ∼430 nm; its PL FWHM reaches ∼11 nm at room temperature, owing to the monodispersion in size distribution as well as the symmetric quasi-sphere morphology of NCs releasing the fine structure splitting-induced inhomogeneous broadening. Through regulating A cations with respect to the ratio of FA (or MA)-to-Cs and Cs-to-Pb, the PL emission of the NCs could be tuned from ∼505 to ∼430 nm combined with varied morphologies from large cube to small quasi-sphere. Such spectroscopic and morphological discrepancies are supposed to be attributed to the different crystalline kinetics that is strongly dependent on the synthetic condition. To be specific, in the case of increasing FA (or MA)-to-Cs, the growth rate of CsPbBr3 and FAPbBr3 (or MAPbBr3) perovskites is determined by the reactivity of transient species, while in the case of decreasing the Cs-to-Pb ratio, the growth rate of perovskites is slowed down by the serious reduction of Cs+ in the precursor. This study provides an effective strategy to adjust the emission across from green to deep-blue color and promotes the perovskite NCs with a narrow FWHM, and tunable PL emission facilitates in application of optoelectronic devices.

Enhancing Optical and Thermal Stability of Blue-Emitting Perovskite Nanocrystals through Surface Passivation with Sulfonate or Sulfonic Acid Ligands

This study aims to enhance the optical and thermal properties of cesium-based perovskite nanocrystals (NCs) through surface passivation with organic sulfonate (or sulfonic acid) ligands. Four different phenylated ligands, including sodium β-styrenesulfonate (SbSS), sodium benzenesulfonate (SBS), sodium p-toluenesulfonate (SPTS), and 4-dodecylbenzenesulfonic acid (DBSA), were employed to modify blue-emitting CsPbBr1.5Cl1.5 perovskite NCs, resulting in improved size uniformity and surface functionalization. Transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the successful anchoring of sulfonate or sulfonic acid ligands on the surface of perovskite NCs. Moreover, the photoluminescence quantum yield increased from 32% of the original perovskite NCs to 63% of the SPTS-modified ones due to effective surface passivation. Time-resolved photoluminescence decay measurements revealed extended PL lifetimes for ligand-modified NCs, indicative of reduced nonradiative recombination. Thermal stability studies demonstrated that the SPTS-modified NCs retained nearly 80% of the initial PL intensity when heated at 60 °C for 10 min, surpassing the performance of the original NCs. These findings emphasize the optical and thermal stability enhancement of cesium-based perovskite NCs through surface passivation with suitable sulfonate ligands.

In Situ Synthesis of CsPbX3/Polyacrylonitrile Nanofibers with Water-Stability and Color-Tunability for Anti-Counterfeiting and LEDs

Inorganic CsPbX3 (X = Cl, Br, I) perovskite quantum dots (PQDs) have attracted widespread attention due to their excellent optical properties and extensive application prospects. However, their inherent structural instability significantly hinders their practical application despite their outstanding optical performance. To enhance stability, an in situ electrospinning strategy was used to synthesize CsPbX3/polyacrylonitrile composite nanofibers. By optimizing process parameters (e.g., halide ratio, electrospinning voltage, and heat treatment temperature), all-inorganic CsPbX3 PQDs have been successfully grown in a polyacrylonitrile (PAN) matrix. During the electrospinning process, the rapid solidification of electrospun fibers not only effectively constrained the formation of large-sized PQDs but also provided effective physical protection for PQDs, resulting in the improvement in the water stability of PQDs by minimizing external environmental interference. Even after storage in water for over 100 days, the PQDs maintained approximately 93.5% of their photoluminescence intensity. Through the adjustment of halogen elements, the as-obtained composite nanofibers exhibited color-tunable luminescence in the visible light region, and based on this, a series of multicolor anti-counterfeiting patterns were fabricated. Additionally, benefiting from the excellent water stability and optical performance, the CsPbBr3/PAN composite film was combined with red-emitting K2SiF6:Mn4+ (KSF) on a blue LED (460 nm), producing a stable and efficient WLED device with a color temperature of around 6000 K and CIE coordinates of (0.318, 0.322). These results provide a general approach to synthesizing PQDs/polymer nanocomposites with excellent water stability and multicolor emission, thereby promoting their practical applications in multifunctional optoelectronic devices and advanced anti-counterfeiting.

Identification and Dynamics of Microsecond Long-Lived Charge Carriers for CsPbBr3 Perovskite Quantum Dots, Featuring Ambient Long-Term Stability

We analyze the stability and photophysical dynamics of CsPbBr3 perovskite quantum dots (PeQDs), fabricated under mild synthetic conditions and embedded in an amorphous silica (SiOx) matrix (CsPbBr3@SiOx), underscoring their sustained performance in ambient conditions for over 300 days with minimal optical degradation. However, this stability comes at the cost of a reduced photoluminescence efficiency. Time-resolved spectroscopic analyses, including flash-photolysis time-resolved microwave conductivity and time-resolved photoluminescence, show that excitons in CsPbBr3@SiOx films decay within 2.5 ns, while charge carriers recombine over approximately 230 ns. This longevity of the charge carriers is due to photoinduced electron transfer to the SiOx matrix, enabling hole retention. The measured hole mobility in these PeQDs is 0.880 cm2 V-1 s-1, underscoring their potential in optoelectronic applications. This study highlights the role of the silica matrix in enhancing the durability of PeQDs in humid environments and modifying exciton dynamics and photoluminescence, providing valuable insights for developing robust optoelectronic materials.

High-Quality Solution-Processed Quasi-2D Perovskite for Low-Threshold Lasers

Spin-coated quasi-two-dimensional halide perovskite films, which exhibit superior optoelectronic properties and environmental stability, have recently been extensively studied for lasers. Crystallinity is of great importance for the laser performance. Although some parameters related to the spin-coating process have been studied, the in-depth understanding and effective control of the acceleration rate on two-dimensional perovskite crystallization during spin-coating are still unknown. Here we investigate the effect of solvent evaporation on the microstructure of the final perovskite films during the spin-coating process. The crystallization quality of the film can be significantly improved by controlling solvent evaporation. As a result, the prepared quasi-2D perovskite film exhibits a stimulated emission threshold (pump: 343 nm, 6 kHz, 290 fs) of 550 nm as low as 16.2 μJ/cm2. Transient absorption characterization shows that the radiative biexciton recombination time is reduced from 738.5 to 438.3 ps, benefiting from the improved crystallinity. The faster biexciton recombination significantly enhanced the photoluminescence efficiency, which is critical for population inversion. This work could contribute to the development of low-threshold lasers.

Enhanced Photostability of Lead Halide Perovskite Nanocrystals with Mn3+ Incorporation

Recently, lead halide perovskite nanocrystals (NCs) have shown great potential and have been widely studied in lighting and optoelectronic fields. However, the long-term stability of perovskite NCs under irradiation is an important challenge for their application in practice. Mn2+ dopants are mostly proposed as substitutes for the Pb site in perovskite NCs synthesized through the hot-injection method, with the aim of improving both photo- and thermal stability. In this work, we employed a facile ligand-assisted reprecipitate strategy to introduce Mn ions into perovskite lattice, and the results showed that Mn3+ instead of Mn2+, even with a very low level of incorporation of 0.18 mol % as interstitial dopant, can enhance the photostability of perovskite binder film under the ambient conditions without emission change, and the photoluminescent efficiency can retain 70% and be stable under intensive irradiation for 12 h. Besides, Mn3+ incorporation could prolong the photoluminescent decay time by passivating trap defects and modifying the distortion of the lattice, which underscores the significant potential for application as light emitters.

Chemical Aspects of Halide Perovskite Nanocrystals

Halide perovskite nanocrystals (HPNCs) are currently among the most intensely investigated group of materials. Structurally related to the bulk halide perovskites (HPs), HPNCs are nanostructures with distinct chemical, optical, and electronic properties and significant practical potential. One of the keys to the effective exploitation of the HPNCs in advanced technologies is the development of controllable, reproducible, and scalable methods for preparation of materials with desired compositions, phases, and shapes and low defect content. Another important condition is a quantitative understanding of factors affecting the chemical stability and the optical and electronic properties of HPNCs. Here we review important recent developments in these areas. Following a brief historical prospective, we provide an overview of known chemical methods for preparation of HPNCs and approaches used to control their composition, phase, size, and shape. We then review studies of the relationship between the chemical composition and optical properties of HPNCs, degradation mechanisms, and effects of charge injection. Finally, we provide a short summary and an outlook. The aim of this review is not to provide a comprehensive summary of all relevant literature but rather a selection of highlights, which, in the subjective view of the authors, provide the most significant recent observations and relevant analyses.

Cyanuric Acid-Functionalized Perovskite Nanocrystals toward Low Interface Impedance, High Environmental Stability, and Superior Electrochemiluminescence

Perovskite nanocrystals (PNs) have received much attention as luminescence materials in the field of electrochemiluminescence (ECL). However, as one key factor for determining the optoelectronic properties of the surface state of PNs, the surface passivation layer of PNs has enormous difficulty in simultaneously meeting the requirements of high ECL efficiency, conductivity, and stability. Herein, an effective surface modification strategy with cyanuric acid (CA) is used to solve such issue. As confirmed, the CA molecules are chemically anchored onto the surface of PNs via the Lewis interaction between π electrons of the triazine ring and the empty orbit of Pb2+. Benefiting from the above interaction, the electrochemical impedance of PNs is decreased greatly without the loss of light-emitting efficiency. Moreover, the stability of PNs under O2 exposure is improved by almost sixfold. These improvements are confirmed to be beneficial for enhancing the ECL behaviors of PNs under electrochemical operation. Upon cathode ECL driving conditions in aqueous media, the ECL intensity and efficiency of PNs are increased to 200 and 170%, respectively. This work provides a new modification strategy to holistically improve the ECL performance of PNs, which is instructive to exploring robust perovskite nanomaterials for electrochemical applications.

Core-Shell Perovskite Quantum Dots for Highly Selective Room-Temperature Spin Light-Emitting Diodes

Circularly polarized light (CPL) is a crucial light source with a wide variety of potential applications such as magnetic recording, and 3D display. Here, core-shell heterostructured perovskite quantum dots (QDs) for room-temperature spin-polarized light-emitting diodes (spin-LEDs) are developed. Specifically, a 2D chiral perovskite shell is deposited onto the achiral 3D inorganic perovskite (CsPbBr3 ) core. Owing to the chiral-induced spin selectivity effect, the spin state of the injected charge carriers is biased when they are transmitted through the 2D chiral shell. The spin-controlled carriers then radiatively recombine inside the CsPbBr3 emissive core, resulting in CPL emission. It is demonstrated that the (R)- and (S)-1-(2-(naphthyl)ethylamine) (R-/S-NEA) 2D chiral cations enhance the spin polarization degree due to their strong chiroptical properties. Systematical defect analyses confirm that 2D chiral cations (i.e., R-/S-NEA) successfully passivate halide vacancies at the surface of the CsPbBr3 QDs, thereby attaining a high photoluminescence quantum yield of 78%. Moreover, the spin-LEDs prepared with core-shell QDs achieve a maximum external quantum efficiency of 5.47% and circularly polarized electroluminescence with a polarization degree (PCP-EL ) of 12% at room temperature. Finally, various patterns fabricated by inkjet printing the core-shell QDs emit strong CPL, highlighting their potential as an emitter for next-generation displays.

Rejuvenating Aged Perovskite Quantum Dots for Efficient Solar Cells

Perovskite quantum dots (PQDs) have emerged as one of the most promising candidates for next-generation solar cells owing to its remarkable optoelectronic properties and solution processability. However, the optoelectronic properties of PQDs suffer from severe degradation in storage due to the dynamically binding ligands, predominantly affecting photovoltaic applications. Herein, an in situ defect healing treatment (DHT) is reported to effectively rejuvenate aged PQDs. Systematically, experimental studies and theoretical calculations are performed to fundamentally understand the causes leading to the recovered optoelectronic properties of aged PQDs. The results reveal that the I3 - anions produced from tetra-n-octylammonium iodide and iodine could strongly anchor on the surface matrix defects of aged PQDs, substantially diminishing the nonradiative recombination of photogenerated charge carriers. Meanwhile, an DHT could also renovate the morphology of aged PQDs and thus improve the stacking orientation of PQD solids, substantially ameliorating charge carrier transport within PQD solids. Consequently, by using a DHT, the PQD solar cell (PQDSC) yields a high efficiency of up to 15.88%, which is comparable with the PQDSCs fabricated using fresh PQDs. Meanwhile, the stability of PQDSCs fabricated using the rejuvenated PQDs is also largely improved.

Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons

Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

Enhanced Higher Harmonic Generation in Modified MAPbBr3-xClx Single Crystal by Additive Engineering

Mixed-halide perovskite materials (MHSCs) hold significant interest in photonics applications owing to their inherent advantages, including tunable bandgap properties, remarkable defect tolerance characteristics, and facile processability. These attributes position MHSCs as up-and-coming materials for various applications. However, the commercialization of these materials is severely affected by external factors, such as humidity and oxygen. The current work studies change in higher harmonics generation (HHG) in MAPbBr3-xClx single crystals (MHSC) with changing nitrogen-based additives. These additives act as a passivating layer and improve the nanolevel crystallinity. The additive engineering strategy impacts morphological and optical properties, depending on the additive's interaction.

On the Ion Coordination and Crystallization of Metal Halide Perovskites by In Situ Dynamic Optical Probing

Controlling the crystallization to achieve high-quality homogeneous perovskite film is the key strategy in developing perovskite electronic devices. Here, an in situ dynamic optical probing technique is demonstrated that can monitor the fast crystallization of perovskites and effectively minimize the influence of laser excitation during the measurement. This study finds that the typical static probing technique would damage and induce phase segregation in the perovskite films during the excitation. These issues can be effectively resolved with the dynamic probing approach. It also found that the crystallization between MAPbI3 and MAPbI2 Br is strikingly different. In particular, MAPbI2 Br suffers from inefficient nucleation during the spin-coating that strongly affects the uniform crystal growth in the annealing process. The commonly used pre-heating process is found at a lower temperature not only can further promote the nucleation but also to complete the crystallization of MAPbI2 Br. The role of further annealing at a higher temperature is to facilitate ion-dissociation on the crystal surface to form a passivation layer to stabilize the MAPbI2 Br lattices. The device performance is strongly correlated with the film formation mechanism derived from the in situ results. This work demonstrates that the in situ technique can provide deep insight into the crystallization mechanism, and help to understand the growth mechanism of perovskites with different compositions and dimensionalities.

Velocity field distribution control in antisolvent flow realizing highly stable and efficient perovskite nanocrystals

Achieving highly stable and efficient perovskite nanocrystals (NCs) without applying functional additives or encapsulation, particularly sustaining the stability in ultra-dilute solution, is still a formidable challenge. Here, we show the FAPbI₃ perovskite NCs with achieved ∼100 % photoluminescence quantum yield (PLQY) and low defect density (∼0.2 cm^-3 per NCs), which is obtained by controlling the velocity field distribution of antisolvent flow in ligand-assisted reprecipitation process. The NCs show incredible reproducibility with narrow deviation of PLQY and linewidth between batch by batch, as well as remarkable stability of maintaining over 80 % PLQY, either in an ultra-diluted solution (9.3 × 10^-6 mg/mL), or storing in ambient condition after 90 days with concentration of 0.09 mg/mL. The results in this work demonstrate the interplay of fluid mechanics and crystallization kinetics of perovskite, which pioneers a novel and unprecedent understanding for improving the stability of perovskite NCs for efficient quantum light source.

Antisolvent-Assisted In Situ Cation Exchange of Perovskite Quantum Dots for Efficient Solar Cells

Cesium-formamidinium lead iodide perovskite quantum dots (FA_x Cs_{1- x} PbI₃ PQDs) show high potential for next-generation photovoltaics due to their outstanding optoelectronic properties. However, achieving composition-tunable hybrid PQDs with desirable charge transport remains a significant challenge. Herein, by leveraging an antisolvent-assisted in situ cation exchange of PQDs, homogeneous FA_x Cs_{1- x} PbI₃ PQDs with controllable stoichiometries and surface ligand chemistry are realized. Meanwhile, the crystallographic stability of PQDs is substantially improved by substituting the cations of the PQDs mediated by surface vacancies. Consequently, PQD solar cell delivers an efficiency of 17.29%, the highest value among the homostructured PQD solar cells. The high photovoltaic performance is attributed to the broadened light harvesting spectra, flattened energy landscape, and rationalized energy levels of highly oriented PQD solids, leading to efficient charge carrier extraction. This work provides a feasible approach for the stoichiometry regulation of PQDs to finely tailor the optoelectronic properties and tolerance factors of PQDs toward high-performing photovoltaics.

Photoinduced interfacial electron transfer from perovskite quantum dots to molecular acceptors for solar cells

Bandgap-engineered inorganic and hybrid halide perovskite (HP) films, nanocrystals, and quantum dots (PQDs) are promising for solar cells. Fluctuations of photoinduced electron transfer (PET) rates affect the interfacial charge separation efficiencies of such solar cells. Electron donor- or acceptor-doped perovskite samples help analyze PET and harvest photogenerated charge carriers efficiently. Therefore, PET in perovskite-based donor-acceptor (D-A) systems has received considerable attention. We analyzed the fluctuations of interfacial PET from MAPbBr₃ or CsPbBr₃ PQDs to classical electron acceptors such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 1,2,4,5-tetracyanobenzene (TCNB) at single-particle and ensemble levels. The significantly negative Gibbs free energy changes of PET estimated from the donor-acceptor redox potentials, the donor-acceptor sizes, and the solvent dielectric properties help us clarify the PET in the above D-A systems. The dynamic nature of PET is apparent from the decrease in photoluminescence (PL) lifetimes and PL photocounts of PQDs with an increase in the acceptor concentrations. Also, the acceptor radical anion spectrum helps us characterize the charge-separated states. Furthermore, the PL blinking time and PET rate fluctuations (10⁸ to 10⁷ s^-1) provide us with single-molecule level information about interfacial PET in perovskites.

Surface Passivation for Promotes Bi-Excitonic Amplified Spontaneous Emission in CsPb(Br/Cl)3 Perovskite at Room Temperature

Perovskite-type lead halides exhibit promising performances in optoelectronic applications, for which lasers are one of the most promising applications. Although the bulk structure has some advantages, perovskite has additional advantages at the nanoscale owing to its high crystallinity given by a lower trap density. Although the nanoscale can produce efficient light emission, its comparatively poor chemical and colloidal stability limits further development of devices based on this material. Nevertheless, bulk perovskites are promising as optical amplifiers. There has been some developmental progress in the study of optical response and amplified spontaneous emission (ASE) as a benchmark for perovskite bulk phase laser applications. Therefore, to achieve high photoluminescence quantum yields (PLQYs) and large optical gains, material development is essential. One of the aspects in which these goals can be achieved is the incorporation of a bulk structure of high-quality crystallization films based on inorganic perovskite, such as cesium lead halide (CsPb(Br/Cl)₃), in polymethyl methacrylate (PMMA) polymer and encapsulation with the optimal thickness of the polymer to achieve complete surface coverage, prevent degradation, surface states, and surface defects, and suppress emission at depth. Sequential evaporation of the perovskite precursors using a single-source thermal evaporation technique (TET) effectively deposited two layers. The PL and ASEs of the bare and modified films with a thickness of 400 nm PMMA were demonstrated. The encapsulation layer maintained the quantum yield of the perovskite layer in the air for more than two years while providing added optical gain compared to the bare film. Under a picosecond pulse laser, the PL wavelength of single excitons and ASE wavelength associated with the stimulated decay of bi-excitons were achieved. The two ASE bands were highly correlated and competed with each other; they were classified as exciton and bi-exciton recombination, respectively. According to the ASE results, bi-exciton emission could be observed in an ultrastable CsPb(Br/Cl)₃ film modified by PMMA with a very low excitation energy density of 110 µJ/cm². Compared with the bare film, the ASE threshold was lowered by approximately 5%. A bi-exciton has a binding energy (26.78 meV) smaller than the binding energy of the exciton (70.20 meV).

Highly multicolored light-emitting arrays for compressive spectroscopy

Miniaturized, multicolored light-emitting device arrays are promising for applications in sensing, imaging, computing, and more, but the range of emission colors achievable by a conventional light-emitting diode is limited by material or device constraints. In this work, we demonstrate a highly multicolored light-emitting array with 49 different, individually addressable colors on a single chip. The array consists of pulsed-driven metal-oxide-semiconductor capacitors, which generate electroluminescence from microdispensed materials spanning a diverse range of colors and spectral shapes, enabling facile generation of arbitrary light spectra across a broad wavelength range (400 to 1400 nm). When combined with compressive reconstruction algorithms, these arrays can be used to perform spectroscopic measurements in a compact manner without diffractive optics. As an example, we demonstrate microscale spectral imaging of samples using a multiplexed electroluminescent array in conjunction with a monochrome camera.

Two-dimensional metal halide perovskites and their heterostructures: from synthesis to applications

Size- and shape-dependent unique properties of the metal halide perovskite nanocrystals make them promising building blocks for constructing various electronic and optoelectronic devices. These unique properties together with their easy colloidal synthesis render them efficient nanoscale functional components for multiple applications ranging from light emission devices to energy conversion and storage devices. Recently, two-dimensional (2D) metal halide perovskites in the form of nanosheets (NSs) or nanoplatelets (NPls) are being intensively studied due to their promising 2D geometry which is more compatible with the conventional electronic and optoelectronic device structures where film-like components are usually employed. In particular, 2D perovskites exhibit unique thickness-dependent properties due to the strong quantum confinement effect, while enabling the bandgap tuning in a wide spectral range. In this review the synthesis procedures of 2D perovskite nanostructures will be summarized, while the application-related properties together with the corresponding applications will be extensively discussed. In addition, perovskite nanocrystals/2D material heterostructures will be reviewed in detail. Finally, the wide application range of the 2D perovskite-based structures developed to date, including pure perovskites and their heterostructures, will be presented while the improved synergetic properties of the multifunctional materials will be discussed in a comprehensive way.

Continuous manufacturing of highly stable lead halide perovskite nanocrystals via a dual-reactor strategy

Lead halide perovskite nanocrystals possess incredible potential as next generation emitters due to their stellar set of optoelectronic properties. Unfortunately, their instability towards many ambient conditions and reliance on batch processing hinder their widespread utilities. Herein, we address both challenges by continuously synthesizing highly stable perovskite nanocrystals via integrating star-like block copolymer nanoreactors into a house-built flow reactor. Perovskite nanocrystals manufactured in this strategy display significantly enhanced colloidal, UV, and thermal stabilities over those synthesized with conventional ligands. Such scaling up of highly stable perovskite nanocrystals represents an important step towards their eventual use in many practical applications in optoelectronic materials and devices.

Energy and Charge Transfer Dynamics in Red-Emitting Hybrid Organo-Inorganic Mixed Halide Perovskite Nanocrystals

We report time-resolved spectral properties of highly stable and efficient red-emitting hybrid perovskite nanocrystals with the composition FA_0.5MA_0.5PbBr_0.5I_2.5 (FAMA PeNC) synthesized by using the hot-addition method. The PL spectrum of the FAMA PeNC shows a broad asymmetric band covering 580 to 760 nm with a peak at 690 nm which can be deconvoluted into two bands corresponding to the MA and FA domains. The interactions between the MA and FA domains are shown to affect the relaxation dynamics of the PeNCs from the subpicosecond to tens of nanoseconds scale. Time-correlated single-photon counting (TCSPC), femtosecond PL optical gating (FOG), and femtosecond transient absorption spectral (TAS) techniques were employed to study the intercrystal energy transfer (photon recycling) and intracrystal charge transfer processes between the MA and the FA domains of the crystals. These two processes are shown to increase the radiative lifetimes for the PLQYs exceeding 80%, which may play a key role in enhancing the performance of PeNC-based solar cells.

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.

Hybrid Organic-Inorganic Perovskite Superstructures for Ultrapure Green Emissions

All inorganic CsPbBr₃ superstructures (SSs) have attracted much research interest due to their unique photophysical properties, such as their large emission red-shifts and super-radiant burst emissions. These properties are of particular interest in displays, lasers and photodetectors. Currently, the best-performing perovskite optoelectronic devices incorporate organic cations (methylammonium (MA), formamidinium (FA)), however, hybrid organic-inorganic perovskite SSs have not yet been investigated. This work is the first to report on the synthesis and photophysical characterization of APbBr₃ (A = MA, FA, Cs) perovskite SSs using a facile ligand-assisted reprecipitation method. At higher concentrations, the hybrid organic-inorganic MA/FAPbBr₃ nanocrystals self-assemble into SSs and produce red-shifted ultrapure green emissions, meeting the requirement of Rec. 2020 displays. We hope that this work will be seminal in advancing the exploration of perovskite SSs using mixed cation groups to further improve their optoelectronic applications.

Scale-up synthesis of high-quality solid-state-processed CsCuX (X = Cl, Br, I) perovskite nanocrystal materials toward near-ultraviolet flexible electronic properties

High-quality CsCu₂X₃ and Cs₃Cu₂X₅ (X = Cl, Br, I) nanocrystals (NCs) exhibit excellent optoelectronic, physical, and chemical properties for detection of UV radiation due to large carrier mobility and lifetime, and heavy atoms. The nanocrystal materials can be prepared via a low-cost and simple solid-state synthesis. However, poor reproducibility and complex synthesis methods of obtaining perovskite NC thin films represent a drawback for the fabrication of the commercial photoelectric device. To address these issues, we develop highly stable CsCu₂X₃ and Cs₃Cu₂X₅ NC materials using a facile solid-state reaction method for the scale-up production of halogen lead-free perovskites. We suggest a distinctive way to design a series of nanocrystalline perovskites using short-term synthesis and study the mechanism of perovskite formation using thermal solid-state synthesis. These all-inorganic and lead-free CsCu₂X₃ and Cs₃Cu₂X₅ exhibit large photoluminescence quantum yields (PLQYs) up to 95.2%. Moreover, flexible paper photodetectors based on this series of lead-free perovskites show strong photoselectivity and bending stability at 254 nm, 365 nm, and 405 nm wavelengths. High-quality responses with a responsivity of 1.1 × 10^-3 A W^-1 and detectivity of 2.71 × 10⁹ jones under UV illumination (10 μW cm^-2) at a bias voltage of 5 mV are demonstrated. These results open prospects for designing photodetectors, LEDs, and other photosensitive devices.

Deciphering the Role of Water in Promoting the Optoelectronic Performance of Surface-Engineered Lead Halide Perovskite Nanocrystals

Lead halide perovskites are promising candidates for high-performance light-emitting diodes (LEDs); however, their applicability is limited by their structural instability toward moisture. Although a deliberate addition of water to the precursor solution has recently been shown to improve the crystallinity and optical properties of perovskites, the corresponding thin films still do not exhibit a near-unity quantum yield. Herein, we report that the direct addition of a minute amount of water to post-treated formamidinium lead bromide (FAPbBr₃) nanocrystals (NCs) substantially enhances the stability while achieving a 95% photoluminescence quantum yield in a NC thin film. We unveil the mechanism of how moisture assists in the formation of an additional NH₄Br component. Alongside, we demonstrate the crucial role of moisture in assisting localized etching of the perovskite crystal, facilitating the partial incorporation of NH₄⁺, which is key for improved performance under ambient conditions. Finally, as a proof-of-concept, the application of post-treated and water-treated perovskites is tested in LEDs, with the latter exhibiting a superior performance, offering opportunities toward commercial application in moisture-stable optoelectronics.

Transferable Classical Force Field for Pure and Mixed Metal Halide Perovskites Parameterized from First-Principles

Many key features in photovoltaic perovskites occur in relatively long time scales and involve mixed compositions. This requires realistic but also numerically simple models. In this work we present a transferable classical force field to describe the mixed hybrid perovskite MAxFA1-xPb(BryI1-y)3 for variable composition (∀x, y ∈ [0, 1]). The model includes Lennard-Jones and Buckingham potentials to describe the interactions between the atoms of the inorganic lattice and the organic molecule, and the AMBER model to describe intramolecular atomic interactions. Most of the parameters of the force field have been obtained by means of a genetic algorithm previously developed to parametrize the CsPb(BrxI1-x)3 perovskite (Balestra et al. J. Mater. Chem. A. 2020, DOI: 10.1039/d0ta03200j). The algorithm finds the best parameter set that simultaneously fits the DFT energies obtained for several crystalline structures with moderate degrees of distortion with respect to the equilibrium configuration. The resulting model reproduces correctly the XRD patterns, the expansion of the lattice upon I/Br substitution, and the thermal expansion coefficients. We use the model to run classical molecular dynamics simulations with up to 8600 atoms and simulation times of up to 40 ns. From the simulations we have extracted the ion diffusion coefficient of the pure and mixed perovskites, presenting for the first time these values obtained by a fully dynamical method using a transferable model fitted to first-principles calculations. The values here reported can be considered as the theoretical upper limit, that is, without grain boundaries or other defects, for ion migration dynamics induced by halide vacancies in photovoltaic perovskite devices under operational conditions.

Halide Perovskite Crystallization Processes and Methods in Nanocrystals, Single Crystals, and Thin Films

Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High-performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (C_min ). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation-dominated process by rapidly increasing the concentration over C_min , single crystals are mainly prepared via the growth-dominated process by keeping the concentration between solubility and C_min , while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed.

A Critical Review of the Use of Bismuth Halide Perovskites for CO2 Photoreduction: Stability Challenges and Strategies Implemented

Inspired by natural photosynthesis, the photocatalytic CO2 reduction reaction (CO2RR) stands as a viable strategy for the production of solar fuels to mitigate the high dependence on highly polluting fossil fuels, as well as to decrease the CO2 concentration in the atmosphere. The design of photocatalytic materials is crucial to ensure high efficiency of the CO2RR process. So far, perovskite materials have shown high efficiency and selectivity in CO2RR to generate different solar fuels. Particularly, bismuth halide perovskites have gained much attention due to their higher absorption coefficients, their more efficient charge transfer (compared to oxide perovskites), and their required thermodynamic potential for CO2RR. Moreover, these materials represent a promising alternative to the highly polluting lead halide perovskites. However, despite all the remarkable advantages of bismuth halide perovskites, their use has been limited, owing to instability concerns. As a consequence, recent reports have offered solutions to obtain structures highly stable against oxygen, water, and light, promoting the formation of solar fuels with promising efficiency for CO2RR. Thus, this review analyzes the current state of the art in this field, particularly studies about stability strategies from intrinsic and extrinsic standpoints. Lastly, we discuss the challenges and opportunities in designing stable bismuth halide perovskites, which open new opportunities for scaling up the CO2RR.

Engineering the Optical Properties of CsPbBr3 Nanoplatelets through Cd2+ Doping

Lead halide perovskite nanoplatelets (NPls) attract significant attention due to their exceptional and tunable optical properties. Doping is a versatile strategy for modifying and improving the optical properties of colloidal nanostructures. However, the protocols for B-site doping have been rarely reported for 2D perovskite NPls. In this work, we investigated the post-synthetic treatment of CsPbBr₃ NPls with different Cd²⁺ sources. We show that the interplay between Cd²⁺ precursor, NPl concentrations, and ligands determines the kinetics of the doping process. Optimization of the treatment allows for the boosting of linear and nonlinear optical properties of CsPbBr₃ NPls via doping or/and surface passivation. At a moderate doping level, both the photoluminescence quantum yield and two-photon absorption cross section increase dramatically. The developed protocols of post-synthetic treatment with Cd²⁺ facilitate further utilization of perovskite NPls in nonlinear optics, photonics, and lightning.

Colloidal FAPbBr3 perovskite nanocrystals for light emission: what's going on?

Semiconducting nanomaterials have been widely explored in diverse optoelectronic applications. Colloidal lead halide perovskite nanocrystals (NCs) have recently been an excellent addition to the field of nanomaterials, promising an enticing building block in the field of light emission. In addition to the notable optoelectronic properties of perovskites, the colloidal NCs exhibit unique size-dependent optical properties due to the quantum size effect, which makes them highly attractive for light-emitting diodes (LEDs). In the past few years, perovskite-based LEDs (PeLEDs) have demonstrated a meteoritic rise in their external quantum efficiency (EQE) values, reaching over 20% so far. Among various halide perovskite compositions, FAPbBr3 and its variants remain one of the most interesting and sought-after compounds for green light emission. This review focuses on recent progress in the design and synthesis protocols of colloidal FAPbBr3 NCs and the emerging concepts in tailoring their surface chemistry. The structural and physicochemical features of lead halide perovskites along with a comprehensive discussion on their defect-tolerant properties are briefly outlined. Later, the prevalent synthesis, ligand, and compositional engineering strategies to boost the stability and photoluminescence quantum yield (PLQY) of FAPbBr3 NCs are extensively discussed. Finally, the fundamental concepts and recent progress on FAPbBr3-based LEDs, followed by a discussion of the challenges and prospects that are on the table for this enticing class of perovskites, are reviewed.

Exciton-Phonon and Trion-Phonon Couplings Revealed by Photoluminescence Spectroscopy of Single CsPbBr3 Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have outstanding photoluminescence (PL) properties and excellent potential for light-emitting diodes and single-photon sources. Here, we report the multiple-peak structures originating from excitons, trions, and biexcitons in low-temperature PL spectra of single CsPbBr₃ NCs. We found fine-structure splitting in the PL peaks of bright excitons and biexcitons and also in the longitudinal-optical (LO)-phonon replicas of excitons. LO-phonon replicas of trions are clearly observed under strong photoexcitation, which do not show fine-structure splitting. From size-dependent analyses of these replicas, we clarified that both exciton-phonon and trion-phonon couplings become larger for smaller NCs and the coupling strengths of trions are larger than those of excitons in large NCs. These behaviors can be explained by the spatial distributions of the electron and hole wave functions in the NCs. Our findings provide essential information on electron-phonon couplings in perovskites and for the design of high-purity single-photon sources.

Ligand exchange engineering of FAPbI3 perovskite quantum dots for solar cells

Formamidinium lead triiodide (FAPbI₃) perovskite quantum dots (PQDs) show great advantages in photovoltaic applications due to their ideal bandgap energy, high stability and solution processability. The anti-solvent used for the post-treatment of FAPbI₃ PQD solid films significantly affects the surface chemistry of the PQDs, and thus the vacancies caused by surface ligand removal inhibit the optoelectronic properties and stability of PQDs. Here, we study the effects of different anti-solvents with different polarities on FAPbI₃ PQDs and select a series of organic molecules for surface passivation of PQDs. The results show that methyl acetate could effectively remove surface ligands from the PQD surface without destroying its crystal structure during the post-treatment. The benzamidine hydrochloride (PhFACl) applied as short ligands of PQDs during the post-treatment could fill the A-site and X-site vacancies of PQDs and thus improve the electronic coupling of PQDs. Finally, the PhFACl-based PQD solar cell (PQDSC) achieves a power conversion efficiency of 6.4%, compared to that of 4.63% for the conventional PQDSC. This work provides a reference for insights into the surface passivation of PQDs and the improvement in device performance of PQDSCs.

Oleic acid/oleylamine ligand pair: a versatile combination in the synthesis of colloidal nanoparticles

A variety of colloidal chemical approaches has been developed in the last few decades for the controlled synthesis of nanostructured materials in either water or organic solvents. Besides the precursors, the solvents, reducing agents, and the choice of surfactants are crucial for tuning the composition, morphology and other properties of the resulting nanoparticles. The ligands employed include thiols, amines, carboxylic acids, phosphines and phosphine oxides. Generally, adding a single ligand to the reaction mixture is not always adequate to yield the desired features. In this review, we discuss in detail the role of the oleic acid/oleylamine ligand pair in the chemical synthesis of nanoparticles. The combined use of these ligands belonging to two different categories of molecules aims to control the size and shape of nanoparticles and prevent their aggregation, not only during their synthesis but also after their dispersion in a carrier solvent. We show how the different binding strengths of these two molecules and their distinct binding modes on specific facets affect the reaction kinetics toward the production of nanostructures with tailored characteristics. Additional functions, such as the reducing function, are also noted, especially for oleylamine. Sometimes, the carboxylic acid will react with the alkylamine to form an acid-base complex, which may serve as a binary capping agent and reductant; however, its reducing capacity may range from lower to much lower than that of oleylamine. The types of nanoparticles synthesized in the simultaneous presence of oleic acid and oleylamine and discussed herein include metal oxides, metal chalcogenides, metals, bimetallic structures, perovskites, upconversion particles and rare earth-based materials. Diverse morphologies, ranging from spherical nanoparticles to anisotropic, core-shell and hetero-structured configurations are presented. Finally, the relation between tuning the resulting surface and volume nanoparticle properties and the relevant applications is highlighted.

Impact of bismuth-doping on enhanced radiative recombination in lead-free double-perovskite nanocrystals

Lead-free double-perovskite nanocrystals (NCs) have received considerable attention as promising candidates for environmentally friendly optical applications. Furthermore, double-perovskite nanostructures are known to be physically stable compared to most other inorganic halide perovskites, with a generic chemical formula of ABX₃ (e.g., A = Cs⁺; B = Sn²⁺ or Ge²⁺; X = Cl^-, Br^-, I^-, or their combination). However, relevant experimental studies on the photophysical properties are still insufficient for Pb-free double-perovskite NCs. Herein, we synthesized Cs₂Ag_0.65Na_0.35InCl₆ NCs doped with bismuth (Bi³⁺) ions and investigated their photophysical properties to reveal the role of the dopant on the enhanced photoemission properties. Specifically, it was found that the photoluminescence quantum yield (PLQY) increased up to 33.2% by 2% Bi-doping. The optical bandgap of the NCs decreased from 3.47 eV to 3.41 eV as the amount of the dopant increased from 2% to 15%. To find out the effect of Bi-doping, the temperature-dependent PL properties of the undoped and doped NCs were investigated by utilizing steady-state and time-resolved PL spectroscopy. With increasing the temperature from 20 K to 300 K, the PL intensities of the doped NCs decreased slower than the undoped ones. The correlated average PL lifetimes of both the bismuth-doped and undoped NCs decreased with increasing the temperature. The experimental results revealed that all the NC samples showed thermal quenching with the temperature increasing, and the PL quenching was suppressed in bismuth-doped NCs.

On direct synthesis of high quality APbX3 (A = Cs+, MA+ and FA+; X = Cl-, Br- and I-) nanocrystals following a generic approach

Direct synthesis of APbX₃ [A = Cs⁺, methylammonium (MA⁺) or formamidinium (FA⁺) and X = Cl^-, Br^- or I^-] perovskite nanocrystals (NCs) following a generic approach is a challenging task even today. Motivated by our recent success in obtaining directly high-quality red/NIR-emitting APbI₃ NCs employing 1,3-diiodo-5,5-dimethylhydantoin (DIDMH) as an iodide precursor, we explore here whether violet/green-emitting APbCl₃ and APbBr₃ NCs can also be obtained using the chloro- and bromo-analog of DIDMH keeping in mind that a positive outcome will provide the generic protocol for direct synthesis of all APbX₃ NCs using similar halide precursors. It is shown that green-emitting APbBr₃ NCs with near-unity PLQY and violet-emitting CsPbCl₃ NCs with an impressive PLQY of ∼70%, mixed-halide NCs, CsPb(Cl/Br)₃ and CsPb(Br/I)₃, emitting in the blue and yellow-orange region with PLQYs of 87-95% and 68-98%, respectively can indeed be obtained employing the bromo- and chloro-analog of DIDMH. These NCs exhibit remarkable stability under different conditions including the polar environment. Femtosecond pump-probe studies show no ultrafast carrier trapping in these systems. The key elements of the halide precursors that facilitated the synthesis and the factors contributing to the excellent characteristics of the NCs are determined by careful analysis of the data. The results are of great significance because a direct method of obtaining highly luminescent and stable APbX₃ NCs (except violet-emitting hybrid NCs) is eventually identified and the work provides valuable insight into the selection of appropriate halide precursors for the development of superior systems.

Harnessing of Spatially Confined Perovskite Nanocrystals Using Polysaccharide-based Block Copolymer Systems

Metal halide perovskite nanocrystals (PVSK NCs) are generally unstable upon their transfer from colloidal dispersions to thin film devices. This has been a major obstacle limiting their widespread application. In this study, we proposed a new approach to maintain their exceptional optoelectronic properties during this transfer by dispersing brightly emitting cesium lead halide PVSK NCs in polysaccharide-based maltoheptaose-block-polyisoprene-block-maltoheptaose (MH-b-PI-b-MH) triblock copolymer (BCP) matrices. Instantaneous crystallization of ion precursors with favorable coordination to the sugar (maltoheptaose) domains produced ordered NCs with varied nanostructures of controlled domain size (≈10-20 nm). Confining highly ordered and low dimension PVSK NCs in polysaccharide-based BCPs constituted a powerful tool to control the self-assembly of BCPs and PVSK NCs into predictable structures. Consequently, the hybrid thin films exhibited excellent durability to humidity and stretchability with a relatively high PL intensity and photoluminescence quantum yield (>70%). Furthermore, stretchable phototransistor memory devices were produced and maintained with a good memory ratio of 10⁵ and exhibited a long-term memory retention over 10⁴ s at a high strain of 100%.

Formamidinium Lead Halide Perovskite Nanocomposite Scintillators

While there is great demand for effective, affordable radiation detectors in various applications, many commonly used scintillators have major drawbacks. Conventional inorganic scintillators have a fixed emission wavelength and require expensive, high-temperature synthesis; plastic scintillators, while fast, inexpensive, and robust, have low atomic numbers, limiting their X-ray stopping power. Formamidinium lead halide perovskite nanocrystals show promise as scintillators due to their high X-ray attenuation coefficient and bright luminescence. Here, we used a room-temperature, solution-growth method to produce mixed-halide FAPbX3 (X = Cl, Br) nanocrystals with emission wavelengths that can be varied between 403 and 531 nm via adjustments to the halide ratio. The substitution of bromine for increasing amounts of chlorine resulted in violet emission with faster lifetimes, while larger proportions of bromine resulted in green emission with increased luminescence intensity. By loading FAPbBr3 nanocrystals into a PVT-based plastic scintillator matrix, we produced 1 mm-thick nanocomposite scintillators, which have brighter luminescence than the PVT-based plastic scintillator alone. While nanocomposites such as these are often opaque due to optical scattering from aggregates of the nanoparticles, we used a surface modification technique to improve transmission through the composites. A composite of FAPbBr3 nanocrystals encapsulated in inert PMMA produced even stronger luminescence, with intensity 3.8× greater than a comparative FAPbBr3/plastic scintillator composite. However, the luminescence decay time of the FAPbBr3/PMMA composite was more than 3× slower than that of the FAPbBr3/plastic scintillator composite. We also demonstrate the potential of these lead halide perovskite nanocomposite scintillators for low-cost X-ray imaging applications.

Solvent-Free Method for Defect Reduction and Improved Performance of p-i-n Vapor-Deposited Perovskite Solar Cells

As perovskite-based photovoltaics near commercialization, it is imperative to develop industrial-scale defect-passivation techniques. Vapor deposition is a solvent-free fabrication technique that is widely implemented in industry and can be used to fabricate metal-halide perovskite thin films. We demonstrate markably improved growth and optoelectronic properties for vapor-deposited [CH(NH₂)₂]_0.83Cs_0.17PbI₃ perovskite solar cells by partially substituting PbI₂ for PbCl₂ as the inorganic precursor. We find the partial substitution of PbI₂ for PbCl₂ enhances photoluminescence lifetimes from 5.6 ns to over 100 ns, photoluminescence quantum yields by more than an order of magnitude, and charge-carrier mobility from 46 cm²/(V s) to 56 cm²/(V s). This results in improved solar-cell power conversion efficiency, from 16.4% to 19.3% for the devices employing perovskite films deposited with 20% substitution of PbI₂ for PbCl₂. Our method presents a scalable, dry, and solvent-free route to reducing nonradiative recombination centers and hence improving the performance of vapor-deposited metal-halide perovskite solar cells.

MAPbBr3nanocrystals from aqueous solution for poly(methyl methacrylate)-MAPbBr3nanocrystal films with compression-resistant photoluminescence

In this work, we develop an environmental-friendly approach to produce organic-inorganic hybrid MAPbBr₃(MA = CH₃NH₃) perovskite nanocrystals (PeNCs) and PMMA-MAPbBr₃NC films with excellent compression-resistant PL characteristics. Deionized water is used as the solvent to synthesize MAPbBr₃powder instead of conventionally-used hazardous organic solvents. The MAPbBr₃PeNCs derived from the MAPbBr₃powder exhibit a high photoluminescence quantum yield (PLQY) of 93.86%. Poly(methyl methacrylate) (PMMA)-MAPbBr₃NC films made from the MAPbBr₃PeNCs retain ∼97% and ∼91% of initial PL intensity after 720 h aging in ambient environment at 50 °C and 70 °C, respectively. The PMMA-MAPbBr₃NC films also exhibit compression-resistant photoluminescent characteristics in contrast to the PMMA-CsPbBr₃NC films under a compressive stress of 1.6 MPa. The PMMA-MAPbBr₃NC film integrated with a red emissive film and a blue light emitting source achieves an LCD backlight of ∼114% color gamut of National Television System Committee (NTSC) 1953 standard.

Colloidal Metal-Halide Perovskite Nanoplatelets: Thickness-Controlled Synthesis, Properties, and Application in Light-Emitting Diodes

Colloidal metal-halide perovskite nanocrystals (MHP NCs) are gaining significant attention for a wide range of optoelectronics applications owing to their exciting properties, such as defect tolerance, near-unity photoluminescence quantum yield, and tunable emission across the entire visible wavelength range. Although the optical properties of MHP NCs are easily tunable through their halide composition, they suffer from light-induced halide phase segregation that limits their use in devices. However, MHPs can be synthesized in the form of colloidal nanoplatelets (NPls) with monolayer (ML)-level thickness control, exhibiting strong quantum confinement effects, and thus enabling tunable emission across the entire visible wavelength range by controlling the thickness of bromide or iodide-based lead-halide perovskite NPls. In addition, the NPls exhibit narrow emission peaks, have high exciton binding energies, and a higher fraction of radiative recombination compared to their bulk counterparts, making them ideal candidates for applications in light-emitting diodes (LEDs). This review discusses the state-of-the-art in colloidal MHP NPls: synthetic routes, thickness-controlled synthesis of both organic-inorganic hybrid and all-inorganic MHP NPls, their linear and nonlinear optical properties (including charge-carrier dynamics), and their performance in LEDs. Furthermore, the challenges associated with their thickness-controlled synthesis, environmental and thermal stability, and their application in making efficient LEDs are discussed.

Research progress of ABX3-type lead-free perovskites for optoelectronic applications: materials and devices

We summarize the development and application of ABX3-type lead-free halide perovskite materials, especially in optoelectronic devices.

Surface Chemistry Engineering of Perovskite Quantum Dots: Strategies, Applications, and Perspectives

The presence of surface ligands not only plays a key role in keeping the colloidal integrity and non-defective surface of metal halide perovskite quantum dots (PQDs), but also serves as a knob to tune their optoelectronic properties for a variety of exciting applications including solar cells and light-emitting diodes. However, these indispensable surface ligands may also deteriorate the stability and key properties of PQDs due to their highly dynamic binding and insulating nature. To address these issues, a number of innovative surface chemistry engineering approaches have been developed in the past few years. Based on an in-depth fundamental understanding of the surface atomistic structure and surface defect formation mechanism in the tiny nanoparticles, a critical overview focusing on the surface chemistry engineering of PQDs including advanced colloidal synthesis, in-situ surface passivation, and solution-phase/solid-state ligand exchange is presented, after which their unprecedented achievements in photovoltaics and other optoelectronics are presented. The practical hurdles and future directions are critically discussed to inspire more rational design of PQD surface chemistry toward practical applications.