Mitigating halide ion migration by resurfacing lead halide perovskite nanocrystals for stable light-emitting diodes

Monosolvent system for high-purity lead-free perovskite precursors scalable synthesis based on solubility differences

Metal halide perovskites (MHPs) are promising materials for various optoelectronic applications due to their unique properties. However, the presence of lead (Pb) in MHPs raises environmental and health concerns, prompting the search for lead-free alternatives. This study introduces a universal strategy for synthesizing high-purity lead-free perovskite precursors through a methanol monosolvent system that utilizes solubility differences. The synthesis method is scalable and universal, applicable to five lead-free perovskites such as Cs2SnCl6, Cs2TeCl6, Cs3Sb2Cl9, Cs2ZnCl4, and Cs2SnBr6, all maintaining high structural and compositional integrity with purities exceeding 99.985 %. The Cs2SnCl6 perovskite precursors achieve a high yield of 91.7 %. The synthesized Cs2SnCl6 perovskite exhibits superior electron mobility and lower baseline resistance when incorporated into gas sensors, demonstrating a high response (1.98 at 20 ppm) for dimethyl carbonate (DMC) detection due to its high purity. The simplicity and effectiveness of this one-step synthesis method offer a significant advancement for the production of high-quality perovskite materials for commercial applications in sensors and optoelectronics.

Highly Stable Sn─Pb Perovskite Solar Cells Enabled by Phenol-Functionalized Hole Transporting Material

Sn─Pb perovskites, a most promising low bandgap semiconductor for multi-junction solar cells, are often limited by instability due to the susceptibility of Sn2+ to oxidation. Inspired by the antioxidative properties of polyphenolic compounds, we introduce the reductive phenol group and strong electronegative fluorine into an organic conjugated structure and design a multi-functional polymer with fluorine and phenol units (PF─OH). The design of PF─OH allows the effective rise in the energy barrier of Sn2+ oxidation, leading to a significant enhancement in the stability of Sn─Pb perovskite devices from 200 to 8000 h-an improvement of around 100 times. Additionally, the strong binding energy between Sn2+ and the phenol in PF─OH critically influences Sn─Pb perovskite's crystallization and grain growth, resulting in perovskite films with fewer pinholes at the buried interface and extended carrier lifetimes. This enhancement not only boosts the power conversion efficiency (PCE) to 23.61%, but also significantly improves the operational stability of the devices. Ultimately, this design strategy has been proven universal through the phenolization of a series of molecules, marking a milestone in enhancing the stability of Sn─Pb perovskites.

Greatly enhanced ultrafast optical absorption nonlinearities of pyridyl perovskite nanocrystals axially modified by star-shaped porphyrins

The long-chain ligands and weak charge-transport capacity of perovskite nanocrystals (NCs) always hinder their optoelectronic applications. Our study proposes an effective strategy to unlock the optimized nonlinear optical (NLO) properties of perovskite via aromatic ligand-exchange plus porphyrin-axial-coordination. The synthesized porphyrin-pyridine dual-modified CsPbBr3-NC hybrid material, fabricated using 4-(aminomethyl)pyridine (PyMA) and a novel star-shaped zinc-porphyrin trisubstituted triazacoronene compound (ZnPr), exhibits excellent NLO absorption performance under femtosecond (fs) laser irradiation in the visible to near-infrared range. Specifically, its nonlinear absorption coefficient is 10 times higher than that of the pristine CsPbBr3-NC and it also possesses an outstanding optical limiting (OL) capability with an OL threshold as low as 1.8 mJ cm-2. The modification of PyMA ligands reduces the trap state density on the perovskite surface and promotes the electronic coupling between the NC lattices. With the aid of PyMA coordinating with the Zn atoms in ZnPr, large-size planar porphyrins are anchored on the perovskite surface from the axial position, which may enhance the ligand protection capability and thereby significantly facilitate the charge transport between porphyrin components and perovskite NCs. The above two-step modification synergistically contributes to the prominent NLO performance. Our work affords a new viable paradigm for developing multi-field and high-performance perovskite NC photonic materials and devices.

Defect Passivating Hole Transporting Material for Large-Area and Stable Perovskite Quantum-Dot Light-Emitting Diodes

Organic hole-transporting materials (HTMs) with high hole mobility and a defect passivating ability are critical for improving the performance and stability of perovskite optoelectronics, including perovskite quantum dot light-emitting diodes (Pe-QLEDs) and perovskite solar cells. In this study, we designed two small-molecule HTMs, termed X13 and X15, incorporating the methylthio group (SMe) as defect-passivating sites to enhance the interaction between HTMs and the perovskite layer for Pe-QLED applications. Our study highlights that X15, featuring SMe groups at the para-position of the carbazole unit, demonstrates a strong interaction and superior passivation effects with perovskite quantum dots. Consequently, Pe-QLEDs (0.09 cm2) incorporating X15 as the HTM achieve a maximum external quantum efficiency (EQE) of 22.89%. Moreover, employing X15 in large-area Pe-QLEDs (1 cm2) yields an EQE of 21.10% with uniform light emission, surpassing the PTAA-based devices (EQE ∼ 15.03%). Our finding provides crucial insights into the molecular design of defect-passivating small-molecule HTMs for perovskite light-emitting diodes and related optoelectronic devices.

Heteroatomic molecules for coordination engineering towards advanced Pb-free Sn-based perovskite photovoltaics

As an ideal eco-friendly Pb-free optoelectronic material, Sn-based perovskites have made significant progress in the field of photovoltaics, and the highest power conversion efficiency (PCE) of Sn-based perovskite solar cells (PSCs) has been currently approaching 16%. In the course of development, various strategies have been proposed to improve the PCE and stability of Sn-based PSCs by solving the inherent problems of Sn2+, including high Lewis acidity and easy oxidation. Notably, the recent breakthrough comes from the development of heteroatomic coordination molecules to control the characteristics of Sn-based perovskites, which are considered to be vital for realizing efficient PSCs. In this review, the up-to-date advances in the design of heteroatomic molecules and their key functions in the fabrication of Sn-based perovskite films are comprehensively summarized. Firstly, the design principles of heteroatomic coordination molecules and their impact on the colloidal chemistry, crystallization dynamics, and defect properties of Sn-based perovskites are introduced. Then, state-of-the-art heteroatomic coordination molecules for efficient Sn-based PSCs are discussed in terms of their heteroatom types and functional groups. Lastly, we shed some light on the current challenges and future perspectives regarding the rational design of heteroatomic coordination molecules for further boosting the performance of Sn-based PSCs.

Light-Emitting Diodes Based on Metal Halide Perovskite and Perovskite Related Nanocrystals

Light-emitting diodes (LEDs) based on halide perovskite nanocrystals have attracted extensive attention due to their considerable luminescence efficiency, wide color gamut, high color purity, and facile material synthesis. Since the first demonstration of LEDs based on MAPbBr3 nanocrystals was reported in 2014, the community has witnessed a rapid development in their performances. In this review, a historical perspective of the development of LEDs based on halide perovskite nanocrystals is provided and then a comprehensive survey of current strategies for high-efficiency lead-based perovskite nanocrystals LEDs, including synthesis optimization, ion doping/alloying, and shell coating is presented. Then the basic characteristics and emission mechanisms of lead-free perovskite and perovskite-related nanocrystals emitters in environmentally friendly LEDs, from the standpoint of different emission colors are reviewed. Finally, the progress in LED applications is covered and an outlook of the opportunities and challenges for future developments in this field is provided.

Manipulating Electron-Phonon Coupling for Efficient Tin Halide Perovskite Blue LEDs

Low-dimensional perovskites have opened up a new frontier in light-emitting diodes (LED) due to their excellent properties. However, concerns regarding the potential toxicity of Pb limited their commercial development. Sn-based perovskites are regarded as a promising candidate to replace Pb-based counterparts, while they generally exhibit strong electron-phonon coupling and consequently blue emission quenching at room temperature (RT), thus the Sn-based perovskite blue LED devices have not yet been reported. Herein, the luminescence properties are regulated by assembling a rigid organic skeleton within perovskite structure, and the protonated 4-bromobenzylamine (BrPMA+ = C7H9BrN+) is employed as A site cation to synthesize a 100-oriented 2D perovskite (BrPMA)2SnBr4, which exhibits a strong lattice rigidity via strong intermolecular interaction and consequently weak electron-phonon coupling, achieving the excellent blue PL emission at RT. The high quality (BrPMA)2SnBr4 perovskite thin films are obtained by further inhibiting oxidation and promoting crystallization. Finally, the Sn-based perovskite blue emission LED is successfully fabricated for the first time at 467 nm with a champion EQE of 1.3% and a maximum brightness of 800 cd m-2. This work gives insights into the luminescence mechanism of Sn-based perovskites and provides a new theoretical basis for the development of lead-free blue LEDs.

The Photophysics of Perovskite Emitters: from Ensemble to Single Particle

Halide perovskite emitters are a groundbreaking class of optoelectronic materials possessing remarkable photophysical properties for diverse applications. In perovskite light emitting devices, they have achieved external quantum efficiencies exceeding 28%, showcasing their potential for next-generation solid-state lighting and ultra high definition displays. Furthermore, the demonstration of room temperature continuous-wave perovskite lasing underscores their potential for integrated optoelectronics. Of late, perovskite emitters are also found to exhibit desirable single-photon emission characteristics as well as superfluorescence or superradiance phenomena for quantum optics. With progressive advances in synthesis, surface engineering, and encapsulation, halide perovskite emitters are poised to become key components in quantum optical technologies. Understanding the underpinning photophysical mechanisms is crucial for engineering these novel emergent quantum materials. This review aims to provide a condensed overview of the current state of halide perovskite emitter research covering both established and fledging applications, distill the underlying mechanisms, and offer insights into future directions for this rapidly evolving field.

Microstructural stiffness engineering of low dimensional metal halide perovskites for efficient X-ray imaging

Low dimensional metal halide perovskites (MHPs) have a soft lattice, leading to strong exciton phonon coupling and exciton localization. Microstructural stiffness engineering is an effective tool for modulating the mechanical and electrical properties of materials, but its complex effects on the luminescence of low dimensional MHPs remain lacking. Here, we report microstructural stiffness engineering of low dimensional MHPs by halogen replacement in Ag-X bonds and [AgX4]3- (X = Br, Cl) units to increase the Young's modulus from 15.6 to 18.3 GPa, resulting in a 10-fold enhancement of X-ray excited luminescence (XEL) intensity and a 16-fold enhancement of photoluminescence quantum yield (PLQY), from 2.8% to 44.3%. Spectroscopic analysis reveals that high stiffness in Rb2AgCl3 facilitates the radiative pathway of defect-bound excitons and efficiently decreases the non-radiative transitions. The projected crystal orbital Hamilton population shows that the shorter Ag-Cl bonds impart Rb2AgCl3 with superior anti-deformation ability upon photoexcitation, leading to enhanced radiation resistance performance. A scintillation screen based on Rb2AgCl3@PDMS achieves zero self-absorption, an ultra-low detection limit of 44.7 nGyair s-1, and a high resolution of 20 lp mm-1, outperforming most reported X-ray detectors. This work sheds light on stiffness engineering for the rational design of efficient emitters.

In Situ Formation of Iodide Precursor for Perovskite Quantum Dots with Application in Efficient Solar Cells

Perovskite quantum dots (PQDs) become a kind of competitive material for fabricating high-performance solar cells due to their solution processability and outstanding optoelectronic properties. However, the current synthesis method of PQDs is mostly based on the binary-precursor method, which results in a large deviation of the I/Pb input ratio in the reaction system from the stoichiometric ratio of PQDs. Herein, a ternary-precursor method with an iodide source self-filling ability is reported for the synthesis of the CsPbI3 PQDs with high optoelectronic properties. Systematically experimental characterizations and theoretical calculations are conducted to fundamentally understand the effects of the I/Pb input molar ratio on the crystallographic and optoelectronic properties of PQDs. The results reveal that increasing the I/Pb input molar ratio can obtain ideal cubic structure PQDs with iodine-rich surfaces, which can significantly reduce the surface defects of PQDs and realize high orientation of PQD solids, facilitating charge carrier transport in the PQD solids with diminished nonradiative recombination. Consequently, the PQD solar cells exhibit an impressive efficiency of 15.16%, which is largely improved compared with that of 12.83% for the control solar cell. This work provides a feasible strategy for synthesizing high-quality PQDs for high-performance optoelectronic devices.

Anion regulation for surface passivation enables ultrahigh-stability perovskite nanocrystals

All-inorganic perovskite CsPbBr3 nanocrystals (NCs) display high photoluminescence quantum yield and narrow emission, which show great potential application in optoelectronic devices. However, the poor environment stability of NCs will hinder their practical application. Herein, a series of ionic liquids with different anions (BF4-, Br-, and NO3-) were used as a sole capping ligand to synthesize NCs. Among the three samples, 1-hexadecyl-3-methylimidazolium tetrafluoroborate ([C16MIM]BF4) capped NCs have the highest stability in light, thermal, and water, possibly attributing to the in situ passivation of bromine vacancy via pseudohalogen BF4- and tight binding of ionic liquid ligands and lead atoms. In addition, green-emission [C16MIM]BF4 NCs were used to assemble a white light-emitting diode device, and it possessed a wide National Television System Committee color gamut of 124.5% and a stable emission peak at high driving currents of 380 mA. This work paves the way for resurfacing perovskite NCs with ultrahigh stability, thereby driving the perovskite NC display industry closer to real-world application.

Ligand-Solvent Coordination Enables Comprehensive Trap Passivation for Efficient Near-Infrared Quantum Dot Light-Emitting Diodes

Near-infrared light-emitting diodes (NIR LEDs) based on perovskite quantum dots (QDs) have produced external quantum efficiency (EQE) of ~15 %. However, these high-performance NIR-QLEDs suffer from immediate carrier quenching because of the accumulation of migratable ions at the surface of the QDs. These uncoordinated ions and carriers-if not bound to the nanocrystal surface-serve as centers for exciton quenching and device degradation. In this work, we overcome this issue and fabricate high-performance NIR QLEDs by devising a ligand anchoring strategy, which entails dissolving the strong-binding ligand (Guanidine Hydroiodide, GAI) in the mediate-polar solvent. By employing the dye-sensitized device structure (phosphorescent indicator), we demonstrate the elimination of the interface defects. The treated QDs films exhibit an exciton binding energy of 117 meV: this represents a 1.5-fold increase compared to that of the control (74 meV). We report, as a result, the NIR QLEDs with an EQE of 21 % which is a record among NIR perovskite QLEDs. These QLEDs also exhibit a 7-fold higher operational stability than that of the best previously reported NIR QLEDs. Furthermore, we demonstrate that the QDs are compatible with large-area QLEDs: we showcase 900 mm2 QLEDs with EQE approaching 20 %.

Highly efficient blue light-emitting diodes based on mixed-halide perovskites with reduced chlorine defects

Perovskite light-emitting diodes (PeLEDs) provide excellent opportunities for low-cost, color-saturated, and large-area displays. However, the performance of blue PeLEDs lags far behind that of their green and red counterparts. Here, we show that the external quantum efficiencies (EQEs) of blue PeLEDs scale linearly with the photoluminescence quantum yields (PL QYs) of CsPb(BrxCl1-x)3 nanocrystals emitting at 460 to 480 nm. The recombination efficiency of carriers is highly sensitive to the chlorine content and the related deep-level defects in nanocrystals, causing notable EQE drops even with minor increases in chlorine defects. Minor adjustments of chlorine content through rubidium compensation on the A-site effectively suppress the formation of nonradiative defects, improving PL QYs while retaining desirable bandgaps for blue-emitting nanocrystals. Our PeLEDs with record-high efficiencies span the blue spectrum, achieving peak EQEs of 12.0% (460 nm), 16.7% (465 nm), 21.3% (470 nm), 24.3% (475 nm), and 26.4% (480 nm). This work exemplifies chlorine-defect control as a key design principle for high-efficiency blue PeLEDs.

Modification strategies of lead halide perovskite nanocrystals for efficient and stable LEDs

Lead halide perovskite nanocrystals (PNCs) hold immense promise in high-performance light-emitting diodes (LEDs) for future high-definition displays. Their adjustable bandgaps, vivid colors, and good carrier mobility are key factors that make them a potential game-changer. However, to fully harness their potential, the efficiency and long-term stability of PNCs-based light-emitting diodes (PNC-LEDs) must be enhanced. Recent material research results have shed light on the leading cause of performance decline in PNC-LEDs, which is ionic migration linked to surface defects and grain boundary imperfections. This review aims to present recent advancements in the modification strategies of PNCs, focusing on obtaining high-quality PNCs for LEDs. The PNC modification strategies are first summarized, including crystal structure regulation, nanocrystal size tuning, ligand exchange, and surface passivation. Then, the effects of these material design aspects on LED device performances, such as efficiency, brightness, and stability, are presented. Based on the efficient modification strategies, we propose promising material design insights for efficient and stable PNC-LEDs.

Short-branched alkyl sulfobetaine-passivated CsPbBr3 nanocrystals for efficient green light emitting diodes

Inorganic cesium lead bromide nanocrystals (CsPbBr3 NCs) hold promising prospects for high performance green light-emitting diodes (LEDs) due to their exceptional color purity and high luminescence efficiency. However, the common ligands employed for passivating these indispensable NCs, such as long-chain organic ligands like oleic acid and oleylamine (OA/OAm), display highly dynamic binding and electronic insulating issues, thereby resulting in a low efficiency of the as-fabricated LEDs. Herein, we report a new zwitterionic short-branched alkyl sulfobetaine ligand, namely trioctyl(propyl-3-sulfonate) ammonium betaine (TOAB), to in situ passivate CsPbBr3 NCs via a feasible one-step solution synthesis, enabling efficiency improvement of CsPbBr3 NC-based LEDs. The zwitterionic TOAB ligand not only strengthened the surface passivation of CsPbBr3 NCs with a high photoluminescence quantum yield (PLQY) of 97%, but also enhanced the carrier transport in the fabricated CsPbBr3 NC thin films due to the short-branched alkyl design. Consequently, CsPbBr3 NCs passivated with TOAB achieved a green LED with an external quantum efficiency (EQE) of 7.3% and a maximum luminance of 5716 cd m-2, surpassing those of LEDs based on insulating long-chain ligand-passivated NCs. Our work provides an effective surface passivation ligand design to enhance the performance of CsPbBr3 NC-based LEDs.

Synchronously Polishing the Lead-Rich Surface and Passivating Surface Defects of CsPb(Br/I)3 Quantum Dots for High-Performance Pure-Red PeLEDs

Mixed-halide CsPb(Br/I)3 perovskite quantum dots (QDs) are regarded as one of the most promising candidates for pure-red perovskite light-emitting diodes (PeLEDs) due to their precise spectral tuning property. However, the lead-rich surface of these QDs usually results in halide ion migration and nonradiative recombination loss, which remains a great challenge for high-performance PeLEDs. To solve the above issues, we employ a chelating agent of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid hydrate (DOTA) to polish the lead-rich surface of the QDs and meanwhile introduce a new ligand of 2,3-dimercaptosuccinic acid (DMSA) to passivate surface defects of the QDs. This synchronous post-treatment strategy results in high-quality CsPb(Br/I)3 QDs with suppressed halide ion migration and an improved photoluminescence quantum yield, which enables us to fabricate spectrally stable pure-red PeLEDs with a peak external quantum efficiency of 23.2%, representing one of the best performance pure-red PeLEDs based on mixed-halide CsPb(Br/I)3 QDs reported to date.

Perovskite Nanocrystals: Superior Luminogens for Food Quality Detection Analysis

With the global limited food resources receiving grievous damage from frequent climate changes and ascending global food demand resulting from increasing population growth, perovskite nanocrystals with distinctive photoelectric properties have emerged as attractive and prospective luminogens for the exploitation of rapid, easy operation, low cost, highly accurate, excellently sensitive, and good selective biosensors to detect foodborne hazards in food practices. Perovskite nanocrystals have demonstrated supreme advantages in luminescent biosensing for food products due to their high photoluminescence (PL) quantum yield, narrow full width at half-maximum PL, tunable PL in the entire visible spectrum, easy preparation, and various modification strategies compared with conventional semiconductors. Herein, we have carried out a comprehensive discussion concerning perovskite nanocrystals as luminogens in the application of high-performance biosensing of foodborne hazards for food products, including a brief introduction of perovskite nanocrystals, perovskite nanocrystal-based biosensors, and their application in different categories of food products. Finally, the challenges and opportunities faced by perovskite nanocrystals as superior luminogens were proposed to promote their practicality in the future food supply.

Perspectives on development of optoelectronic materials in artificial intelligence age

Optoelectronic devices, such as light-emitting diodes, have been demonstrated as one of the most demanded forthcoming display and lighting technologies because of their low cost, low power consumption, high brightness, and high contrast. The improvement of device performance relies on advances in precisely designing novelty functional materials, including light-emitting materials, hosts, hole/electron transport materials, and yet which is a time-consuming, laborious and resource-intensive task. Recently, machine learning (ML) has shown great prospects to accelerate material discovery and property enhancement. This review will summarize the workflow of ML in optoelectronic materials discovery, including data collection, feature engineering, model selection, model evaluation and model application. We highlight multiple recent applications of machine-learned potentials in various optoelectronic functional materials, ranging from semiconductor quantum dots (QDs) or perovskite QDs, organic molecules to carbon-based nanomaterials. We furthermore discuss the current challenges to fully realize the potential of ML-assisted materials design for optoelectronics applications. It is anticipated that this review will provide critical insights to inspire new exciting discoveries on ML-guided of high-performance optoelectronic devices with a combined effort from different disciplines.