Surface Engineering Strategies for High-Performance Perovskite Quantum Dot LEDs: A Comparative Review

Abstract

This article provides a comprehensive comparative analysis of surface engineering approaches for enhancing the performance and stability of perovskite quantum dot light-emitting diodes (PQD-LEDs). Targeting researchers and development professionals, it systematically explores fundamental principles, advanced methodologies including ligand exchange and compositional engineering, and troubleshooting strategies for common challenges like environmental instability and lead toxicity. The review validates techniques through performance metrics and comparative analysis, synthesizing key takeaways to guide future innovations in display technologies and biomedical applications. By integrating the latest research advances, this work establishes a framework for rational surface design to unlock the full potential of PQDs in next-generation optoelectronic devices.

Fundamentals of PQD Surface Physics and Interface Engineering

Perovskite quantum dots (PQDs) are zero-dimensional nanocrystals of metal halide perovskites that exhibit distinct chemical, physical, electrical, and optical properties compared to their bulk counterparts [1]. Their unique crystal structure and pronounced quantum confinement effects make them highly promising materials for a broad range of optoelectronic applications, including light-emitting diodes (LEDs), solar cells, lasers, photodetectors, and quantum technologies [1] [2]. This guide provides a comparative analysis of the fundamental structural and optical properties of PQDs, with a specific focus on how surface engineering approaches modulate these properties to enhance the performance of PQD-based LEDs.

Fundamental Structure and Quantum Confinement in PQDs

Crystal Structure of Perovskite Quantum Dots

The canonical crystal structure for all-inorganic PQDs is CsPbX3 (X = Cl, Br, I), which derives from the ABX3 perovskite configuration. In this structure, cesium (Cs+) ions occupy the A-site, lead (Pb2+) ions the B-site, and halide ions (Cl-, Br-, I-) the X-site [3]. This arrangement forms a three-dimensional network of corner-sharing [PbX6]4- octahedra, with Cs+ ions situated in the cuboctahedral cavities. The integrity of this ionic crystal lattice is crucial for the material's optoelectronic properties.

The optical bandgap, and thus the emission color, of PQDs can be precisely tuned through halide composition. For instance, in mixed-halide systems like CsPbBr3−xIx, the bandgap can be engineered to emit across the visible spectrum [4]. Furthermore, the tolerance factor and octahedral factor are key geometric parameters that determine the stability of the perovskite structure, which can be optimized through ion doping at various lattice sites [3].

Quantum Confinement Effects

When the physical dimensions of a perovskite crystal are reduced to a scale comparable to or smaller than the Bohr exciton diameter (typically 3-10 nm for lead halide perovskites), quantum confinement effects become significant [5]. In this regime, the continuous energy bands of the bulk material transform into discrete, atom-like energy levels, dramatically altering the electronic and optical properties.

• Size-Dependent Bandgap: The bandgap of PQDs increases as their size decreases, leading to a blue shift in both absorption and photoluminescence (PL) emission spectra [5]. This effect is powerfully demonstrated not only in PQDs but also in other semiconducting systems, such as in-plane MoSe2 quantum dots embedded in a WSe2 monolayer, where a clear exciton blue shift of 12-40 meV was observed for dots with widths of ~15-60 nm [6].
• Enhanced Optical Properties: Quantum-confined PQDs exhibit high color purity, characterized by narrow full-width-at-half-maximum (FWHM) emission linewidths (often <20 nm) [7], high photoluminescence quantum yields (PLQY) approaching 100% [2] [8], and a high radiative recombination rate due to the spatial overlap of electron and hole wavefunctions [5].

The following diagram illustrates the fundamental relationship between crystal structure, quantum confinement, and the resulting optical properties in PQDs.

Comparative Analysis of Surface Engineering Approaches for PQD LEDs

The surface of PQDs is a critical determinant of their structural stability and optical performance. Unpassivated surfaces with dangling bonds act as defect sites that trap charge carriers, leading to non-radiative recombination and reduced luminescence efficiency [2]. Surface engineering aims to passivate these defects and protect the ionic crystal lattice from environmental degradation, which is particularly crucial for the operational stability of PQD-LEDs. The table below compares the major surface engineering strategies.

Table 1: Comparison of Surface Engineering Approaches for PQD LEDs

Engineering Approach	Mechanism of Action	Impact on Structural Properties	Impact on Optical Properties	Reported LED Performance
Ligand Exchange/Modification [2] [3]	Replacing long, insulating ligands (e.g., OA, OAm) with shorter or bifunctional ones (e.g., PEABr, DA).	Reduces inter-dot distance, improves charge transport. Stabilizes surface ions.	Reduces trap-assisted recombination; increases PLQY and charge injection efficiency.	High-efficiency pure-red CsPbI3 QLEDs; stable pure-blue LEDs via acid etching-driven ligand exchange [2].
Ion Doping [4] [3]	Incorporation of foreign ions (e.g., Mn2+, Na+, Rb+, Cu+) into the PQD lattice.	Enhances phase stability by optimizing tolerance factor. Passivates surface defects.	Can enhance PL intensity and thermal/air stability. Modifies emission color.	Doping with alkaline metal oxides (e.g., Na2O) reported to enhance PL intensity by up to 200% and PLQY by 29% [4].
Matrix Encapsulation [4] [3]	Embedding PQDs within a protective matrix (e.g., PMMA, oxide glasses, MOFs).	Physically shields PQDs from environmental stressors (H2O, O2, heat).	Maintains high PLQY over time; significantly improves operational longevity.	PMMA encapsulation (2:1 ratio) increased PLQY of CsPbBr3 QDs from 60.2% to 90.1% [3]. Borosilicate glass matrices enable high-stability PQDs for lighting [4].
Bilateral/Composite Passivation [5] [2]	Forming composite structures (e.g., with graphene) or bilateral interfacial layers.	Provides a stable substrate and charge transfer pathways. Suppresses ion migration.	Enhances nonlinear optical properties and device stability. Improves EQE.	Bilateral passivation strategy promoted both efficiency and stability in LEDs [2]. CH3NH3PbBr3-G composites showed enhanced saturable absorption for optical switches [5].

Experimental Protocols for Key Surface Engineering Methods

To provide a practical research toolkit, here are detailed methodologies for several key surface engineering techniques cited in the comparison table.

1. Ligand Exchange with PEABr (Post-Synthesis) [3]

• Synthesis of Original CsPbBr3 PQDs: Dissolve 0.2 mmol PbBr2 and 0.2 mmol CsBr in 5 mL DMF. Add 0.25 mL oleylamine (OAm) and 0.5 mL oleic acid (OA) and stir until clear. Inject 1 mL of this precursor solution into 10 mL of toluene under vigorous stirring (1200 rpm). The solution turns bright green immediately, indicating PQD formation.
• PEABr Solution Preparation: Dissolve 1.52 mg of PEABr powder and 50 μL of OA in 4 mL of ethyl acetate. Sonicate for 5 minutes to obtain a clear solution.
• Ligand Exchange: After the synthesis of the original PQDs, add the PEABr solution in molar ratios of 25%, 50%, 75%, and 100% relative to the initial OAm amount. Mix thoroughly to allow the exchange of OAm with PEABr on the PQD surface.

2. Mn²⁺ Doping [3]

• Precursor Preparation: Co-dissolve PbBr2 (0.2 mmol), CsBr (0.2 mmol), and varying amounts of MnBr2·4H2O (9 mg, 12 mg, or 15 mg) in 5 mL of DMF. Add 0.25 mL OAm and 0.5 mL OA and stir until clear.
• PQD Synthesis: Follow the same injection and crystallization procedure as for the original CsPbBr3 PQDs. The Mn²⁺ ions are incorporated into the crystal lattice during the nucleation and growth process.

3. PMMA Encapsulation [3]

• PMMA Solution Preparation: Dissolve 50 mg of solid PMMA in 1 mL of toluene by heating and stirring in an 80 °C oil bath until fully dissolved. Allow the solution to cool.
• Encapsulation Process: After the synthesis of the CsPbBr3 PQD solution, add the PMMA solution in varying volume ratios (e.g., QD:PMMA = 1:2, 1:1, 2:1). Mix uniformly to encapsulate the individual PQDs within the transparent polymer matrix.

The workflow for synthesizing and optimizing PQDs for LED applications, integrating these key surface engineering steps, is visualized below.

The Scientist's Toolkit: Essential Research Reagents and Materials

This section details key materials and reagents essential for experimental work in PQD synthesis and surface engineering for LED applications.

Table 2: Essential Research Reagents for PQD Synthesis and Surface Engineering

Reagent/Material	Typical Function	Application Example
Cesium Bromide (CsBr)	Cs+ (A-site) precursor for all-inorganic PQDs [3].	Synthesis of CsPbBr3 QDs.
Lead Bromide (PbBr2)	Pb2+ (B-site) and halide precursor [3].	Synthesis of CsPbBr3 QDs.
Oleic Acid (OA) & Oleylamine (OAm)	Long-chain surface ligands/capping agents [3].	Stabilize nanocrystals during and after synthesis; prevent aggregation.
N,N-Dimethylformamide (DMF)	High-polarity solvent for precursor salts [3].	Dissolving PbBr2 and CsBr before injection into toluene.
Toluene	Low-polarity solvent [3].	Non-solvent for triggering PQD nucleation and growth.
Phenethylammonium Bromide (PEABr)	Short-chain, conductive surface ligand [3].	Ligand exchange to replace OAm, improving charge transport in LED films.
Manganese Bromide (MnBr2)	Dopant source for B-site ion doping [3].	Mn2+ doping to enhance stability and modify optical properties.
Polymethyl Methacrylate (PMMA)	Transparent polymer for matrix encapsulation [3].	Coating PQDs to shield them from moisture and oxygen, enhancing longevity.
Alkaline Metal Oxides (e.g., Na2O)	Glass network modifier and potential dopant [4].	Added to borosilicate glass matrices to enhance crystallization and PL of CsPbBr3−xIx QDs.
Silver Oxide (Ag2O)	Nucleating agent in glass matrices [4].	Promotes the crystallization of PQDs within an oxide glass host.

The optoelectronic performance of perovskite quantum dot LEDs is intrinsically governed by their crystal structure and the quantum confinement effects that arise at the nanoscale. While these properties grant PQDs their exceptional luminescence and color tunability, they also introduce a vulnerability to surface defects and environmental degradation. The comparative analysis presented in this guide underscores that no single surface engineering approach is universally superior. Instead, the choice of strategy—be it ligand engineering for improved charge transport, ion doping for intrinsic lattice stability, or matrix encapsulation for extreme environmental shielding—must be aligned with the specific performance and stability requirements of the target LED application. The future of high-performance PQD-LEDs lies in the rational combination of these strategies, leveraging their synergistic effects to simultaneously achieve high efficiency, color purity, and operational longevity, thereby bridging the gap between laboratory innovation and commercial viability.

The surface chemistry of Perovskite Quantum Dots (PQDs) represents a critical frontier in advancing optoelectronic technologies, particularly for light-emitting diodes (LEDs). Unlike bulk materials, PQDs possess an intrinsically high surface-to-volume ratio, making their optical and electronic properties profoundly dependent on surface conditions [9]. Surface ligands—organic or inorganic molecules bound to the nanocrystal surface—play a dual role: they passivate surface defects to enhance photoluminescence and govern charge transport in solid-state films [10] [11]. The interplay between ligands and surface states directly impacts key performance metrics in PQD-LEDs, including efficiency, stability, and color purity [12]. This guide provides a comparative analysis of surface engineering strategies, offering structured experimental data and protocols to inform research and development in PQD-based devices.

Ligand Engineering Strategies: A Comparative Analysis

Ligand engineering encompasses the selection and modification of molecules coordinated to the PQD surface. Different ligand classes impart distinct effects on the material's properties, as quantitatively compared below.

Table 1: Comparative Performance of Surface Ligands on CsPbI3 PQDs

Ligand Type	Key Functional Group	PL Enhancement	Photostability (After 20 days UV)	Primary Passivation Mechanism
Trioctylphosphine (TOP)	Phosphine	16%	Information Missing	Coordinates undercoordinated Pb²⁺ ions [13]
Trioctylphosphine Oxide (TOPO)	Phosphine Oxide	18%	Information Missing	Coordinates undercoordinated Pb²⁺ ions [13]
L-Phenylalanine (L-PHE)	Amine/Carboxylate	3%	>70% initial PL retained	Suppresses non-radiative recombination [13]

Beyond small molecules, ionic ligands and mixed-ligand systems have emerged as powerful tools. For instance, a bilateral interfacial passivation strategy has been demonstrated to boost both efficiency and operational stability in PQD-LEDs [2]. Similarly, alkyl ammonium iodide-based ligand exchange has been employed to achieve high-efficiency in organic-cation perovskite quantum dot solar cells [2]. These strategies highlight a trend toward multi-functional ligand systems that simultaneously address defect passivation and charge transport.

Impact of Ligands on Electronic Structure

Surface ligands induce changes that extend beyond defect passivation, directly influencing the PQD's ground-state electronic structure. Experimental studies reveal that ligand exchange can alter the optical band gap, absorption coefficient at all wavelengths, and the ionization potential of colloidal QDs [9]. This occurs because the orbitals of the ligands and the inorganic core mix in each other's electric field, making the ligand-core adduct an indecomposable electronic entity. This description moves beyond simple electrostatic models and underscores the importance of treating the ligand and QD as an integrated system for rational design [9].

Advanced Characterization of Ligand-Surface Interactions

Understanding the dynamic interplay between ligands and the PQD surface requires advanced spectroscopic techniques. Mid-IR transient absorption (TA) spectroscopy has enabled the direct probing of ligand vibrational features in the excited states of nanocrystals, providing unprecedented insight into surface dynamics [14].

For example, studies on PbS QDs passivated with oleate ligands revealed a net weakening of Pb-O coordination in the excitonic excited state. This change is attributed to an enhanced electron density on the QD surface, which makes the ligand shell more dynamic and potentially more permeable to charge and energy transfer processes [14]. In contrast, when PbS QDs are passivated with 3-mercaptoproprionic acid (MPA)—which contains both thiol and carboxylate groups—the transient vibrational features indicate a different mechanism. The thiol groups localize hole density at the QD surface, leading to a uniform frequency shift of the carboxylate vibrational features due to changes in surface charge density [14]. These findings demonstrate that the excited-state surface chemistry is highly dependent on the specific ligand structure and its interaction with the nanocrystal.

Table 2: Summary of Key Research Reagent Solutions

Research Reagent	Function in PQD Surface Chemistry	Application Context
Trioctylphosphine (TOP)	Surface passivator for undercoordinated Pb²⁺ ions	CsPbI3 PQD synthesis for enhanced PLQY [13]
Trioctylphosphine Oxide (TOPO)	Surface passivator for undercoordinated Pb²⁺ ions	CsPbI3 PQD synthesis for high PL enhancement [13]
L-Phenylalanine (L-PHE)	Bifunctional ligand for defect suppression	Imparts superior photostability in CsPbI3 PQDs [13]
Oleic Acid (OA)	Common coordinating ligand / Surfactant	Standard in LARP and hot-injection synthesis methods [11]
Oleylamine (OlAm)	Common coordinating ligand / Surfactant	Standard in LARP and hot-injection synthesis methods [11]
3-Mercaptoproprionic Acid (MPA)	Ligand with dual anchoring groups	Model system for studying hole localization at surfaces [14]

Diagram 1: Experimental workflow for studying PQD surface chemistry, covering synthesis, ligand engineering, and advanced characterization.

Machine Learning in Surface Chemistry Optimization

The complex relationship between synthesis parameters, surface chemistry, and final PQD properties presents a formidable challenge for traditional experimental approaches. Machine learning (ML) has emerged as a powerful tool to navigate this multi-dimensional space. For instance, a study on CsPbCl3 PQDs demonstrated that ML models like Support Vector Regression (SVR) and Nearest Neighbour Distance (NND) can accurately predict output properties such as nanocrystal size, absorption, and photoluminescence peaks based on synthesis features as input [15].

The methodology involves creating a comprehensive database from peer-reviewed literature, including parameters such as injection temperature, precursor amounts and sources, ligand volumes (e.g., ODE, OA, OLA), and molar ratios (e.g., Cs-to-Pb, Cl-to-Pb) [15]. These inputs are then used to train various algorithms to predict target properties. This data-driven approach can significantly accelerate the optimization of surface ligand recipes, reducing the reliance on time-consuming and costly trial-and-error experimentation [15].

Experimental Protocols for Key Studies

Protocol 1: Ligand Modification of CsPbI3 PQDs

This protocol is adapted from a study investigating the effect of surface ligand modification on the optical properties of CsPbI3 PQDs [13].

• Synthesis Method: Hot-injection method.
• Precursors: Cesium carbonate (Cs₂CO₃, 99%), Lead(II) iodide (PbI₂, 99%).
• Ligands: Trioctylphosphine (TOP, 99%), Trioctylphosphine oxide (TOPO, 99%), L-Phenylalanine (L-PHE, 98%).
• Reaction Medium: 1-octadecene (ODE) as the non-coordinating solvent.
• Optimized Parameters:
  • Reaction Temperature: 170 °C (found optimal for highest PL intensity and narrowest FWHM).
  • Hot-injection Volume: 1.5 mL.
• Procedure:
  • Dissolve precursors in ODE with ligands.
  • Rapidly inject the precursor mixture into the hot solvent.
  • Maintain the reaction at the target temperature (e.g., 170 °C) for a controlled duration.
  • Purify the resulting PQDs by centrifugation.
• Characterization: Analyze PL intensity, Full Width at Half Maximum (FWHM), and photostability under continuous UV exposure.

Protocol 2: Analyzing Surface Chemical States via Spectroscopy

This protocol outlines the use of photoelectron spectroscopy to correlate surface chemical states with PL characteristics, a method employed in studies of cesium lead bromide NCs [11].

• Synthesis Methods: Employ parallel synthesis using LARP and URSOA methods with the same precursor composition to produce NCs with and without ligands.
• Ligand Removal: Purify NCs using a mixture of ethanol and toluene (1:1 v/v) with sonication to remove surface ligands.
• Surface Analysis:
  • X-ray Photoelectron Spectroscopy (XPS) and Hard X-ray Photoelectron Spectroscopy (HAXPES): Analyze the core-level orbitals to determine chemical states at the NC surface. Look for evidence of surface defect species such as Pb atoms with zero oxidation state (Pb⁰), unbonded Br atoms, and Br vacancies.
  • Fourier Transform Infrared (FTIR) Spectroscopy: Confirm the presence or absence of ligand binding.
• Correlation with Optical Properties:
  • Measure photoluminescence (PL) decay dynamics (time-resolved PL).
  • Directly correlate changes in surface chemical states (from XPS/HAXPES) with observed PL characteristics and decay lifetimes to understand the role of surface defects influenced by ligands.

Diagram 2: Logical relationships showing how intrinsic/extrinsic factors and surface components determine the final optoelectronic properties of PQDs.

The strategic engineering of surface ligands is paramount for harnessing the full potential of perovskite quantum dots in optoelectronic devices. Comparative analysis demonstrates that different ligand classes—from small molecules like TOP and L-Phenylalanine to complex mixed systems—impart distinct effects on PL enhancement, photostability, and charge transport. The emerging paradigm treats the ligand-PQD core as an indecomposable electronic entity, where surface chemistry directly dictates the ground-state and excited-state properties. Integrating advanced characterization like transient spectroscopy with data-driven machine learning models provides a robust framework for accelerating the development of high-performance, stable PQD-LEDs, pushing the boundaries of display and lighting technologies.

Perovskite quantum dots (PQDs) have emerged as a revolutionary class of semiconducting nanomaterials with exceptional optoelectronic properties, including high absorption coefficients, tunable bandgaps, and high photoluminescence quantum yield (PLQY) [2]. These characteristics make them particularly promising for applications in light-emitting diodes (LEDs), solar cells, and other advanced optoelectronic devices. However, the practical implementation and commercial viability of PQD-based technologies are significantly hampered by the presence of surface defects, primarily halide vacancies and uncoordinated Pb²⁺ ions [16] [17] [18]. These defects act as non-radiative recombination centers, deteriorating both the efficiency and operational stability of the resulting devices.

The "defect-tolerant" nature of metal halide perovskites is a double-edged sword. While many defects form shallow levels not detrimental to emission, halide vacancies and uncoordinated lead ions can create deep-level traps that facilitate non-radiative recombination and ion migration [16]. In mixed-halide systems, such as CsPb(Br/I)₃ PQDs developed for pure-red LEDs, halide vacancies significantly lower the activation energy for ion migration, leading to electric-field-driven phase segregation. This segregation manifests as undesirable emission broadening and spectral shifts, directly undermining color purity—a critical parameter for display applications [17]. Similarly, uncoordinated Pb²⁺ ions, often found on lead-rich surfaces, provide sites for exciton quenching and can initiate degradation pathways, reducing the PQDs' lifetime [17] [19]. Therefore, understanding and mitigating these specific defects through advanced surface engineering is a central focus in contemporary PQD research and development.

Defect Classification and Impact on PQD Performance

Origin and Classification of Defects

The soft ionic lattice of metal halide perovskites, combined with their typical low-temperature solution processing, makes them prone to the rapid formation of point defects during crystal growth [16]. These structural imperfections interrupt the perfect crystal periodicity and profoundly influence the material's electronic properties. The primary point defects in PQDs can be categorized as follows:

• Vacancies: Empty lattice sites where an atom is missing. For PQDs, iodide/bromide vacancies (Vᵢ/VBr) and lead vacancies (V_Pb) are common.
• Interstitials: Atoms that occupy positions outside the regular lattice sites.
• Anti-site substitutions: Atoms swapping their positions in the crystal lattice [16].

Among these, halide vacancies (Vᵢ/VBr) are notably prevalent due to their low formation energy. They are a major source of hole traps and serve as pathways for ion migration. Uncoordinated Pb²⁺ ions often arise from the absence of a passivating ligand or halide ion, leaving the Pb²⁺ ion under-saturated and creating a strong electron trap [17]. The formation energy of a defect determines its concentration in the lattice, and fortunately, many defects in MHPs have high formation energies for deep-level traps, resulting in a low non-radiative recombination rate—a property known as "defect tolerance" [16].

Performance-Limiting Mechanisms

The detrimental impact of halide vacancies and uncoordinated Pb²⁺ ions on PQD LEDs manifests through several physical mechanisms, as shown in the diagram below.

• Non-Radiative Recombination: Both halide vacancies and uncoordinated Pb²⁺ ions create electronic states within the bandgap that capture photogenerated charge carriers (electrons and holes). Instead of recombining to emit a photon, the carriers lose their energy as heat through these trap states. This process directly lowers the external quantum efficiency (EQE) and photoluminescence quantum yield (PLQY) of PQD LEDs [16] [17].

• Ion Migration and Phase Segregation: Halide vacancies act as hopping sites for halide ions, especially under the influence of an electric field or light illumination. In mixed-halide PQDs (e.g., CsPb(Br/I)₃), this migration leads to the separation into iodine-rich and bromine-rich domains. The iodine-rich regions, with a narrower bandgap, become low-energy traps for charge carriers, causing the emission spectrum to red-shift and broaden over time. This phenomenon severely compromises the spectral stability and color purity of the LEDs, which is critical for display applications [17].

• Reduced Charge Transport: Long-chain insulating ligands used in synthesis, such as oleic acid and oleylamine, are often partially removed during purification to improve electrical conductivity. However, aggressive ligand removal can expose uncoordinated Pb²⁺ sites, creating a high density of charge traps. This results in poor film conductivity, increased hysteresis, and significant open-circuit voltage (V_OC) losses in devices [19].

Comparative Analysis of Surface Engineering Strategies

A range of surface engineering strategies has been developed to passivate these defects. The table below provides a comparative overview of the primary approaches, their mechanisms, and performance outcomes.

Table 1: Comparison of Surface Engineering Strategies for Defect Passivation in PQDs

Strategy	Passivation Mechanism	Key Reagents / Conditions	Reported Performance Outcomes	Key Advantages & Challenges
Pseudohalide Passivation	SCN⁻ ions strongly coordinate with uncoordinated Pb²⁺; K⁺/GA⁺ cations fill A-site vacancies.	KSCN, GASCN in acetonitrile; post-synthetic treatment [17].	EQE: 22.1%; Luminance: 31,000 cd/m²; T₅₀: 1020 min (5x improvement) [17].	Adv: Strong binding, suppresses ion migration. Chal: Solubility in processing solvents.
Multiligand Exchange	Replaces long-chain insulating ligands with short-chain conductive ones; reduces inter-dot distance.	3-mercaptopropionic acid (MPA), Formamidinium Iodide (FAI) in methyl acetate [19].	Jₛc: ~2 mA cm⁻² increase; PCE: 28% improvement; Reduced hysteresis [19].	Adv: Enhances charge transport. Chal: Complex multi-step process.
Lewis Acid-Base Passivation	Molecular modifiers donate/accept electrons to neutralize undercoordinated ions and halide vacancies.	4-guanidinobenzyl aminoguanidine hydrochloride (GBACl) [20].	V_OC: 1.208 V (1.67 eV PSC); PCE: 21.10% (from 18.72%) [20].	Adv: Multi-functional passivation. Chal: Molecular design complexity.
Machine Learning (ML) Guided Synthesis	ML models predict optimal synthesis parameters to minimize defect formation during growth.	SVR, NND models using synthesizing features (temp., precursor amounts, etc.) [15].	High R², low RMSE/MAE in predicting PQD size, absorbance, and PL properties [15].	Adv: Reduces trial-and-error; high prediction accuracy. Chal: Requires large, high-quality dataset.

In-Depth Protocol: Pseudohalide Passivation for PQD LEDs

The following diagram and protocol detail the pseudohalide passivation strategy, which has demonstrated exceptional results for pure-red PQD LEDs.

Experimental Objective: To simultaneously passivate uncoordinated Pb²⁺ ions and halide vacancies in CsPb(Br/I)₃ PQDs via a post-synthetic treatment, thereby enhancing the efficiency and stability of pure-red PeLEDs [17].

Materials:

• Source PQDs: CsPbI₂Br QDs synthesized via a modified hot-injection method.
• Passivation Reagents: Potassium thiocyanate (KSCN, >99%) and/or Guanidinium thiocyanate (GASCN, >99%).
• Solvent: Anhydrous acetonitrile (ACN, 99.8%).
• Other Solvents: Hexane, ethyl acetate, toluene.

Detailed Procedure:

• PQD Purification: The as-synthesized CsPbI₂Br PQDs are precipitated and centrifuged. The pellet is redispersed in a minimal amount of hexane.
• Passivation Solution Preparation: A 1 mg/mL solution of KSCN (or GASCN) in anhydrous acetonitrile is prepared. Note: Acetonitrile is chosen for its ability to coordinate with and gently etch lead-rich surfaces without destroying the QD structure [17].
• Ligand Exchange Reaction: The passivation solution is added dropwise to the PQD dispersion in hexane under vigorous stirring. The mixture is stirred for 10-15 minutes at room temperature.
• Purification and Isolation: The treated PQDs are precipitated by adding ethyl acetate, followed by centrifugation. The supernatant, containing excess ligands and by-products, is discarded. The pellet is redispersed in toluene to form a stable colloidal solution for film fabrication.
• Film Formation and Device Fabrication: The passivated PQD ink is spin-coated onto substrates. Subsequent layers (e.g., hole transport layer, electrodes) are deposited to complete the LED device structure.

Key Characterization Techniques:

• Photoluminescence Quantum Yield (PLQY): Measured using an integrating sphere to quantify the enhancement in emission efficiency after passivation.
• Transmission Electron Microscopy (TEM): To confirm that the QD morphology and size distribution are maintained after treatment.
• X-ray Photoelectron Spectroscopy (XPS): To verify the successful binding of SCN⁻ species to the PQD surface.
• FT-IR Spectroscopy: To monitor the replacement of original long-chain ligands (OA/OAm) with thiocyanate ligands.
• Electroluminescence Testing: To evaluate the final LED performance metrics (EQE, luminance, operational stability).

The Scientist's Toolkit: Essential Research Reagents

Successful surface engineering relies on a specific set of chemical reagents. The following table lists key materials used in the featured protocols.

Table 2: Essential Research Reagents for PQD Surface Passivation

Reagent Name	Chemical Function	Role in Defect Passivation
Potassium Thiocyanate (KSCN)	Inorganic pseudohalide salt	SCN⁻ anion acts as a bidentate ligand, strongly coordinating with uncoordinated Pb²⁺ ions to suppress electron trapping [17].
Guanidinium Thiocyanate (GASCN)	Organic pseudohalide salt	Provides SCN⁻ for Pb²⁺ passivation; the guanidinium cation (GA⁺) can interact with the perovskite surface, potentially improving stability [17].
3-Mercaptopropionic Acid (MPA)	Short-chain organic acid with thiol group	Thiol (-SH) group has a high affinity for Pb²⁺, forming stable bonds. The short chain enhances inter-dot charge transport [19].
Formamidinium Iodide (FAI)	Organic halide salt	Used in ligand exchange to provide halide ions (I⁻) that fill halide vacancies, reducing hole traps and ion migration [19].
4-Guanidinobenzyl aminoguanidine hydrochloride (GBACl)	Multifunctional molecular modifier	The carboxyl group acts as a Lewis base to undercoordinated Pb²⁺, while the protonated amine neutralizes halide vacancies as a Lewis acid [20].
Acetonitrile (ACN)	Polar aprotic solvent	Medium for pseudohalide dissolution; selectively etches poorly coordinated surface species (e.g., lead oxides) without degrading the QD core [17].
Methyl Acetate (MeOAc)	Polar solvent	Anti-solvent for PQD purification; used in liquid/solid-phase ligand exchange to remove long-chain ligands like OA and OAm [19].

The journey toward high-performance and commercially viable PQD LEDs is intrinsically linked to the effective management of surface defects, particularly halide vacancies and uncoordinated Pb²⁺ ions. As this comparison guide illustrates, sophisticated surface engineering strategies—ranging from pseudohalide passivation and multiligand exchange to ML-guided synthesis—have proven highly effective in suppressing these defects. The experimental data confirms that such interventions directly translate to superior device metrics, including enhanced EQE, improved color stability, and extended operational lifetimes.

The choice of passivation strategy is not one-size-fits-all; it depends on the specific PQD composition, the target application, and the scale of production. Pseudohalide treatment currently stands out for its efficiency in producing high-performance red LEDs, while multiligand exchange offers a compelling route to improve charge transport in photovoltaic devices. Looking forward, the integration of high-throughput experimentation with machine learning models presents a promising pathway to accelerate the discovery of next-generation passivants, potentially moving beyond defect suppression to active functionalization of the PQD surface.

Impact of Surface Properties on Non-Radiative Recombination and PLQY

Perovskite quantum dots (PQDs) have emerged as leading semiconductor nanomaterials for next-generation light-emitting diodes (LEDs), photocatalysis, and solar cells due to their outstanding photophysical properties, including high absorption cross-sections, efficient charge separation, and tunable emission wavelengths [21] [22]. However, the performance and stability of PQD-based optoelectronic devices are fundamentally limited by non-radiative recombination losses occurring at surface defects. The photoluminescence quantum yield (PLQY) serves as a critical metric quantifying the efficiency of radiative recombination processes, directly reflecting the extent of surface-mediated non-radiative losses [21]. This comparative analysis examines recent surface engineering methodologies aimed at suppressing non-radiative recombination pathways by passivating surface defects, thereby enhancing PLQY for high-performance PQD LEDs.

Surface Defects in PQDs: Origins and Recombination Mechanisms

Perovskite quantum dots possess high surface-to-volume ratios, making their optical properties exceptionally susceptible to surface chemistry. The primary origins of non-radiative recombination include:

• Halide Vacancies: Under-coordinated lead atoms and halide vacancies create deep-level traps that promote non-radiative Shockley-Read-Hall recombination, significantly reducing PLQY [21] [23]. These defects exhibit low formation energies, making them prevalent even in high-quality syntheses [21].
• Insufficient Ligand Coverage: Dynamic binding of native ligands like oleic acid (OA) and oleylamine (OLA) leads to frequent desorption, creating surface defects that quench photoluminescence [24] [22].
• Stoichiometric Imbalances: Non-ideal I/Pb ratios in precursor materials (e.g., >2.000) generate iodide interstitials that act as non-radiative recombination centers [21].

The following diagram illustrates how surface defects promote non-radiative recombination and how passivation strategies counteract these pathways:

Comparative Analysis of Surface Engineering Approaches

Ligand Engineering Strategies

Ligand exchange and modification represent the most direct approach to suppress surface defect-mediated non-radiative recombination. Different ligand engineering strategies yield substantially varied outcomes in PLQY enhancement and defect suppression, as compared in Table 1.

Table 1: Performance comparison of ligand engineering strategies for PQD surface passivation

Strategy	Mechanism	PLQY Improvement	Key Findings	Limitations
Ultrasonic-Assisted Chiral Ligand Exchange [24]	Enhanced chiral ligand (R-/S-MBA) coverage via ultrasonic desorption of native OA/OAm ligands	Not explicitly quantified, but gEL ~0.28 and EQE ~16.8% achieved	Simultaneously imparts chirality/spin selectivity and passivates surface defects	Requires specialized chiral ligands; complex optimization
Acid-Assisted Ligand Passivation [25]	Replaces weak long-chain ligands with stable Pb-S-P coordination bonds	Up to 96% PLQY for CsPbBr₃ NPLs	Enhances deep-blue emission (461 nm); meets Rec. 2020 standard	Acid concentration critical to prevent degradation
Pseudohalogen Engineering [25]	In-situ etching of Pb-rich surfaces and defect passivation with pseudohalogen ligands	Significant improvement (specific values not provided)	Suppresses halide migration in mixed-halide PeQDs; improves film conductivity	Requires post-treatment processing steps
Dual-Interface Molecular Passivation [25]	Solvent-free rub-on transfer of passivation molecules preserves film integrity	Not explicitly quantified	Prevents secondary defects from solution-based processing	Novel technique requiring specialized equipment

Precursor Engineering and Synthetic Control

Beyond post-synthetic treatments, precursor quality and synthesis conditions fundamentally influence surface defect formation. Controlled recrystallization of PbI₂ precursors to achieve optimal I/Pb stoichiometry (2.000-2.012) significantly reduces iodide interstitials that otherwise function as non-radiative recombination centers [21]. This approach demonstrates that internal defect suppression complements surface passivation in maximizing overall PLQY.

Compositional and Dimensional Engineering

Low-dimensional perovskite structures offer alternative pathways to suppress non-radiative recombination:

• Quasi-2D Perovskites: Mixed phase systems with energy funneling structures that concentrate excitons in smaller-bandgap domains, reducing access to surface traps [24].
• Chiral Perovskite QDs: Exhibit chiral-induced spin selectivity (CISS) effect, generating spin-polarized carriers that form spin excitons with rapid radiative recombination, effectively competing with non-radiative pathways [24].

Experimental Protocols for Surface Engineering

Objective: Enhance chiral ligand exchange efficiency to simultaneously impart spin selectivity and improve surface passivation.

Methodology:

• Synthesize achiral CsPbBr₃ PQDs using conventional hot-injection method with OA/OAm ligands
• Prepare chiral ligand solution (R-/S-MBA in suitable solvent)
• Apply ultrasonic treatment during ligand exchange to assist desorption of native OA/OAm ligands
• Purify resulting chiral PQDs via centrifugation and redispersion
• Characterize using CD spectroscopy, CPL measurements, and mCP-AFM for spin selectivity

Key Parameters: Ultrasonic power density, treatment duration, ligand concentration, solvent system.

Objective: Achieve high PLQY in deep-blue emitting CsPbBr₃ nanoplatelets through enhanced surface passivation.

Methodology:

• Synthesize CsPbBr₃ NPLs using established methods
• Prepare acid-assisted passivation solution with short-chain ligands
• Treat NPLs to replace weak long-chain ligands with stable Pb-S-P coordination bonds
• Purify passivated NPLs and fabricate LED devices
• Characterize using PLQY measurements, EL spectroscopy, and device performance testing

Key Parameters: Acid concentration, ligand type, reaction time, temperature control.

Objective: Correlate halide deposition on PQD surfaces with PLQY enhancement through electrochemical methods.

Methodology:

• Prepare CsPbBr₃ perovskite films on conductive substrates
• Employ electrochemical setup with controlled potential application
• Simultaneously monitor photoluminescence intensity during electrodeposition
• Characterize bromide deposition on surface using complementary techniques
• Correlate PLQY enhancement with halide coverage

Key Parameters: Applied potential, electrolyte composition, deposition time, in-situ monitoring setup.

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key research reagents for PQD surface engineering and defect passivation studies

Reagent/Material	Function	Application Context
R-/S-Methylbenzylamine (MBA)	Chiral ligand for spin-selective passivation	Imparts chirality and enhances surface coverage via ultrasonic-assisted exchange [24]
Octylammonium Iodide (OAI)	Dual-role additive for crystallization control and defect passivation	Modulates perovskite crystallization while passivating defects in pure red PeLEDs [25]
Pseudohalogen Inorganic Ligands	Defect passivation via halide vacancy suppression	Post-treatment strategy for mixed-halide PeQDs to suppress halide migration [25]
Cetyl Trimethylammonium Bromide	Stabilizing ligand suppressing inter-ligand proton transfer	Replaces oleylamine to enhance thermal and moisture stability [22]
Carboxyl-functionalized Polystyrene	Matrix encapsulation for enhanced stability	Composites with PQDs via chemical interactions to inhibit environmental degradation [22]
Phenanthroline-based Compounds	Vacuum-deposited passivation agents	Co-evaporated with perovskite precursors for in-situ passivation of halide vacancies [25]

Surface property optimization through advanced engineering approaches presents a critical pathway for suppressing non-radiative recombination and maximizing PLQY in PQDs. Comparative analysis demonstrates that ligand engineering strategies—including ultrasonic-assisted chiral ligand exchange, acid-assisted passivation, and pseudohalogen treatments—effectively address surface defect challenges while enabling additional functionalities like spin selectivity and enhanced stability. The experimental protocols and reagent toolkit outlined provide researchers with practical methodologies for implementing these surface engineering approaches. As PQD LED technology advances toward commercialization, synergistic combination of these surface passivation strategies with precise synthetic control and dimensional engineering will be essential for achieving both high performance and operational stability in next-generation optoelectronic devices.

Metal halide perovskites (MHPs) have emerged as revolutionary semiconducting materials for optoelectronic devices, particularly perovskite quantum dot light-emitting diodes (PQD LEDs). Their exceptional performance stems from a fundamental property known as defect tolerance, which distinguishes them from conventional semiconductors like silicon, GaAs, or CdTe [26]. In traditional semiconductors, even parts-per-billion levels of defects create deep-level traps within the bandgap that severely quench photoluminescence and reduce charge-carrier lifetimes [26]. In contrast, the most common native point defects in MHPs (A-site and X-site vacancies) typically form shallow-level defects that do not actively promote non-radiative recombination [26]. This inherent tolerance arises from the electronic structure of MHPs, where the valence band maximum comprises antibonding Pb(6s)-I(5p) orbitals, and the conduction band minimum primarily involves Pb(6p) orbitals [27].

However, this defect tolerance has limitations. Despite the favorable intrinsic point defect properties, surface defects and grain boundaries remain critically detrimental to device performance [28] [26]. Undercoordinated Pb²⁺ ions at nanocrystal surfaces, halide vacancies, and structural discontinuities at grain boundaries create charge traps that significantly impact optical properties and device stability [13] [26]. These defects become particularly problematic in PQDs due to their high surface-to-volume ratio, where surface atoms constitute a substantial fraction of the total material [26]. Consequently, sophisticated surface modification strategies have become essential for developing high-performance PQD LEDs, forming the foundation of modern perovskite optoelectronics.

Defect Types and Their Impacts on PQD Performance

Classification of Defects in Metal Halide Perovskites

Defects in metal halide perovskites manifest across multiple scales, from atomic point defects to extended structural imperfections. Point defects include vacancies (atoms missing from lattice sites), interstitials (atoms occupying non-lattice positions), and anti-site defects (atoms swapping positions) [26]. Among these, Pb²⁺ vacancies and halide vacancies are most common, with formation energies suggesting halide vacancies are most prevalent under standard conditions [27]. Structural defects encompass grain boundaries between crystalline domains, dislocations, and surface defects where the periodic lattice terminates [26]. In colloidal PQDs, surface defects dominate due to the nanoscale dimensions, with undercoordinated Pb²⁺ ions representing the most significant non-radiative recombination centers [13] [26].

Table: Common Defect Types in Metal Halide Perovskites and Their Characteristics

Defect Type	Formation Energy	Trap Depth	Impact on Optoelectronic Properties
Halide Vacancy (Vₓ)	Low	Shallow	Moderate impact on non-radiative recombination
A-site Cation Vacancy (Vₐ)	Low	Shallow	Minimal non-radiative recombination
Pb²⁺ Vacancy (V_Pb)	Medium	Shallow to Medium	Contributes to non-radiative losses
Undercoordinated Pb²⁺	Surface defect	Deep	Strong non-radiative recombination center
Grain Boundaries	N/A	Varies	Charge trapping, ion migration pathways

Influence of Defects on Optical and Electronic Properties

Defects undermine PQD LED performance through multiple mechanisms. Non-radiative recombination at defect sites reduces photoluminescence quantum yield (PLQY) and external quantum efficiency (EQE) in devices [28] [26]. Even with high initial PLQY, defects become critical under LED operating conditions where injected charge carrier densities are typically lower than trap densities, resulting in actual PLQY during operation being significantly lower than measured optically [27]. Additionally, defects initiate and accelerate degradation processes by acting as entry points for environmental factors like moisture and oxygen, and by facilitating ion migration across interfaces [27]. This ion migration leads to electrode corrosion and operational instability, presenting fundamental challenges for commercial applications.

Fundamental Passivation Mechanisms

Surface passivation strategies for PQDs employ distinct chemical approaches to address different types of defects. The four primary mechanisms include:

Ionic Bonding

Ionic bonding passivation utilizes charged species that electrostatically interact with surface ions. For example, halide anions (such as I⁻, Br⁻) can fill halide vacancy sites, while alkylammonium cations (such as oleylammonium) can occupy A-site cation vacancies [26]. This mechanism effectively reduces halide vacancy concentrations and provides better surface coverage, improving environmental stability [27].

Coordinate Bonding

Coordinate bonding represents one of the most effective passivation mechanisms for addressing the most problematic defects in PQDs—undercoordinated Pb²⁺ ions. Lewis base donors (such as phosphine oxides, thiols, or amines) donate electron pairs to undercoordinated Pb²⁺ surface sites, forming stable coordination complexes [13] [28]. Trioctylphosphine oxide (TOPO) demonstrates exceptional passivation efficacy, with studies showing PL intensity enhancements of 18% in CsPbI₃ PQDs through coordination with undercoordinated Pb²⁺ ions and surface defects [13].

Hydrogen Bonding

Hydrogen bonding interactions can stabilize surface species and improve crystallinity. Molecules containing amine groups or hydroxyl functionalities form hydrogen bonds with surface halide ions, helping to maintain structural integrity and reduce ion migration [28] [27]. While generally providing weaker passivation than coordinate bonding, hydrogen bonding often works synergistically with other mechanisms in multi-functional ligand systems.

Core-Shell Structures

Creating core-shell architectures represents a physical passivation approach where the PQD core is encapsulated within a protective shell [28]. The shell material (typically wider bandgap semiconductors or stable oxides) physically isolates the core from environmental factors and reduces surface defect density. For blue QDs, sophisticated CdZnSe/ZnSe/ZnSeS/ZnS core-shell structures have achieved PLQYs up to 90% by effectively confining carriers and passivating interface defects [29].

Figure 1: Fundamental passivation mechanisms in perovskite quantum dots and their corresponding target defects. Each mechanism addresses specific defect types through distinct chemical interactions.

Comparative Analysis of Passivation Methods

Ligand Engineering Approaches

Ligand exchange represents the most widely employed passivation strategy for PQDs, with systematic studies revealing significant performance variations between different ligand types. Research on CsPbI₃ PQDs demonstrates that the choice of passivation ligand directly impacts optical properties and environmental stability [13].

Table: Comparative Performance of Surface Ligands on CsPbI₃ PQDs

Ligand	Chemical Type	Passivation Mechanism	PL Enhancement	Photostability Retention
Trioctylphosphine Oxide (TOPO)	Phosphine oxide	Coordinate bonding	18%	~60% (20 days)
Trioctylphosphine (TOP)	Phosphine	Coordinate bonding	16%	Data not reported
L-Phenylalanine (L-PHE)	Amino acid	Mixed coordinate/ionic	3%	>70% (20 days)
Oleic Acid/Oleylamine	Carboxylic acid/amine	Ionic/hydrogen bonding	Baseline	~30% (20 days)

The data reveal important structure-property relationships: coordinate bonding via P=O groups (TOPO) provides superior initial PL enhancement, while multi-functional ligands like L-phenylalanine offer better long-term photostability despite lower immediate PL gains [13]. The enhanced stability of L-phenylalanine-modified PQDs (retaining >70% initial PL intensity after 20 days of UV exposure) highlights the importance of considering both initial performance and long-term stability in passivation design [13].

Aromatic Ligand Systems for Blue PQDs

Blue-emitting PQDs present particular challenges due to their smaller particle size and consequently higher surface-to-volume ratio. Research demonstrates that aromatic ligands with shorter chain lengths offer distinct advantages for blue QD applications [29]. Comparing oleic acid (OA) and 3-fluorocinnamate (3-F-CA) ligands reveals dramatic performance differences:

• PLQY Enhancement: 3-F-CA modification increased PLQY from 90% to 93% for blue QDs [29]
• Carrier Lifetime: Transient PL lifetime increased from 12.1 ns (OA) to 16.9 ns (3-F-CA) [29]
• Device Performance: QLEDs with 3-F-CA showed EQE of 16.4% versus 7.7% with OA—a 2.1-fold improvement [29]

The superior performance of aromatic ligands stems from dual mechanisms: stronger binding energy to the QD surface (more effective defect passivation) and π-π interactions between adjacent ligands that enhance inter-QD attraction and facilitate long-range ordered assembly [29]. Density functional theory calculations confirm higher binding energy and significantly more negative interaction energy (−0.64 eV for 3-F-CA vs −0.04 eV for OA) between modified QDs [29].

Experimental Protocols for Defect Passivation

Synthesis and Ligand Modification of CsPbI₃ PQDs

The synthesis of high-quality PQDs requires precise control over reaction parameters to minimize intrinsic defect formation [13]. A standardized protocol for CsPbI₃ PQD synthesis includes:

• Precursor Preparation: Cesium carbonate (Cs₂CO₃, 99%) and lead iodide (PbI₂, 99%) are combined with ligand modifiers (TOP, TOPO, or L-PHE) in 1-octadecene solvent [13].

• Temperature Optimization: Reaction temperatures between 140°C and 180°C are systematically evaluated, with 170°C identified as optimal for achieving highest PL intensity and narrowest emission linewidth (FWHM 24-28 nm) [13].

• Hot-Injection Volume Control: A hot-injection volume of 1.5 mL provides enhanced PL intensity while maintaining narrow emission profile [13].

• Ligand Exchange Procedure: Purified PQDs are treated with specific ligand solutions (0.1 M concentration in hexane) at controlled ratios, followed by stirring at 80°C for 2 hours to ensure complete surface binding [13].

• Purification and Characterization: PQDs are purified via centrifugation and redispersion in anhydrous hexane, followed by structural (XRD, TEM), optical (UV-Vis, PL), and surface (XPS, NMR) characterization [13].

Figure 2: Experimental workflow for the synthesis and surface passivation of perovskite quantum dots, highlighting critical optimization parameters and characterization steps.

Capillary Bridge Assembly for Patterned QLEDs

For display applications, achieving patterned QLED arrays with long-range order is essential. The aromatic-enhanced capillary bridge confinement strategy represents an advanced assembly method [29]:

• Surface Modification: Blue QDs are modified with 3-F-CA aromatic ligands (optimal amount: 0.7 μmol) to enhance inter-QD interactions [29].

• Template Preparation: Micropillar templates and substrates with microhole arrays are fabricated using photolithography and surface-treated for appropriate hydrophilicity [29].

• Capillary Bridge Formation: The blue QD liquid film with aromatic ligands is uniformly segmented using micropillar templates, forming independent capillary bridges as solvent evaporates [29].

• Directed Assembly: Through controlled directional motion of the three-phase contact lines within isolated capillary bridges, highly ordered arrays of blue QD microstructures are achieved [29].

This method enables fabrication of QLED arrays with minimal pixel size of 3 μm, achieving resolution exceeding 5000 Pixels Per Inch—critical for high-resolution display applications [29].

The Scientist's Toolkit: Essential Research Reagents

Table: Key Research Reagents for PQD Surface Passivation Studies

Reagent	Chemical Function	Application Purpose	Performance Metrics
Trioctylphosphine Oxide (TOPO)	Lewis base donor	Coordinate bonding to undercoordinated Pb²⁺	18% PL enhancement in CsPbI₃ [13]
3-Fluorocinnamate (3-F-CA)	Short-chain aromatic ligand	Enhanced inter-QD interaction & charge transport	93% PLQY in blue QDs [29]
L-Phenylalanine	Amino acid	Multi-functional passivation	>70% photostability after 20 days [13]
Oleic Acid/Oleylamine	Long-chain surfactants	Baseline synthesis & colloidal stability	Fundamental reference system [13] [29]
Alkylammonium Halides	Ionic compounds	A-site vacancy passivation	Improved environmental stability [27]

The theoretical foundation for surface modification in PQDs rests on understanding the inherent defect-tolerant nature of metal halide perovskites while developing targeted strategies to address their remaining susceptibility to surface and interfacial defects. The comparative analysis presented herein demonstrates that optimal passivation approach varies significantly depending on the target application—while coordinate bonding via phosphine oxides provides maximum immediate PL enhancement, multi-functional ligands and aromatic systems offer superior stability and device integration capabilities.

Future research directions should focus on developing multi-modal passivation systems that simultaneously address multiple defect types through complementary mechanisms. Additionally, advancing our understanding of ligand exchange kinetics and developing more environmentally benign passivators will be crucial for commercial translation. As patterning technologies and device architectures continue to evolve, surface modification strategies must adapt to maintain performance at increasingly small scales, particularly for blue-emitting materials where surface effects dominate optoelectronic behavior. The continued refinement of defect passivation methodologies promises to unlock the full potential of PQD LEDs for next-generation display and lighting technologies.

Advanced Surface Engineering Techniques for Enhanced PQD-LED Performance

Surface engineering through ligand exchange is a pivotal process in the development of perovskite quantum dot (PQD) light-emitting diodes (LEDs). Ligands are molecules attached to PQD surfaces that control crystal growth, passivate surface defects, and determine the optoelectronic properties of the resulting materials [30]. The dynamic binding of native long-chain ligands like oleic acid (OA) and oleylamine (OAm), however, leads to detachment that compromises PQD stability and performance [30]. Ligand exchange strategies address this by replacing long insulating ligands with shorter or more functional alternatives.

Conventional ligand exchange methods rely on natural competition between original and newly introduced ligands in solution or solid-state processes [31] [24] [32]. Recently, ultrasonic-assisted approaches have emerged as enhanced alternatives that significantly improve ligand exchange efficiency [31] [24]. This guide provides an objective comparison of these methodologies, focusing on their impacts on the chiroptoelectronic performance of PQDs for LED applications, framed within the broader context of surface engineering for PQD optoelectronics.

Performance Comparison: Conventional vs. Ultrasonic-Assisted Ligand Exchange

The table below summarizes key performance metrics achieved through conventional and ultrasonic-assisted ligand exchange strategies, based on experimental data from recent studies.

Table 1: Quantitative Performance Comparison of Ligand Exchange Strategies

Performance Parameter	Conventional Ligand Exchange	Ultrasonic-Assisted Ligand Exchange
Electroluminescence Dissymmetry Factor (gEL)	Not specifically reported for conventional R-/S-MBA systems	R: 0.285; S: 0.251 [31] [24]
External Quantum Efficiency (EQE)	12.6% (CsPbI₃ with OPA ligand) [33]	R: 16.8%; S: 16.0% [31] [24]
Spin Polarization Efficiency (P)	27-36% (R-PQDs without US treatment) [31] [24]	86-89% (R-PQDs with US treatment) [31] [24]
Photoluminescence Quantum Yield (PLQY)	Up to 98% (CsPbI₃ with OPA) [33]	Significantly enhanced versus non-US treated counterparts [31] [24]
Electrical Conductivity of QD Films	1.1 × 10⁻³ S/m (CsPbI₃ with OPA) [33]	Not explicitly quantified, but noted as "synergistically enhanced" [31] [24]
Ligand Exchange Efficiency	Limited by natural ligand competition [31] [24]	Significantly improved via enhanced desorption of original ligands [31] [24]
Key Applications	Red PQD-LEDs [33]; Photodetectors [34]	Spin-LEDs with circularly polarized emission [31] [24]

Experimental Protocols and Methodologies

Conventional Ligand Exchange Protocol

The conventional ligand exchange process typically employs a direct solution-phase approach. For CsPbI₃ QDs using octylphosphonic acid (OPA):

• Synthesis of CsPbI₃ QDs: CsPbI₃ QDs are synthesized via a hot-injection method with PbO, Cs₂CO₃, OA, OAm, and 1-octadecene (ODE) precursors [33].
• Ligand Introduction: OPA is added during synthesis to partially replace OA ligands. The strong interaction between OPA and Pb atoms effectively passivates undercoordinated Pb surface defects [33].
• Purification: QDs are purified through precipitation and centrifugation cycles to remove excess ligands and reaction byproducts [33] [32].
• Film Formation: QD films are fabricated via layer-by-layer deposition, sometimes incorporating additional solid-state ligand exchange using compounds like tetrabutylammonium iodide (TBAI) for further passivation [32].

This approach enhances PLQY and electrical conductivity by replacing long-chain OA with shorter OPA ligands, improving carrier transport while maintaining effective surface passivation [33].

Ultrasonic-Assisted Ligand Exchange Protocol

The ultrasonic-assisted method builds upon conventional approaches by incorporating ultrasonic energy:

• Base QD Synthesis: Achiral CsPbBr₃ PQDs capped with OA/OAm are synthesized using standard hot-injection methods [31] [24].
• Ultrasonic Treatment: The synthesized PQDs are treated with chiral ligand solutions (e.g., R-/S-methylbenzylamine) under ultrasonic irradiation [31] [24].
• Mechanism: Ultrasonic energy assists desorption of original OA/OAm ligands, promoting more efficient replacement with chiral ligands and enhancing surface coverage [31] [24].
• Transfer to Polar Solvents: The process enables phase transfer of QDs to polar solvents, indicating successful ligand exchange [35].
• Device Fabrication: Spin-LEDs are fabricated with a structure of ITO/poly-SAM/chiral PQDs, where the chiral PQDs serve as both spin-selective units and emissive centers [31] [24].

This strategy simultaneously enhances spin selectivity through improved chirality transfer and optoelectronic properties via superior surface passivation [31] [24].

Workflow and Strategic Decision Pathway

The diagram below illustrates the procedural workflow for both ligand exchange strategies and their impact on final PQD properties.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 2: Key Research Reagents for Ligand Exchange Experiments

Reagent/Material	Function in Ligand Exchange	Example Applications
Oleic Acid (OA) & Oleylamine (OAm)	Native long-chain ligands for initial QD synthesis and stabilization [31] [24] [33]	Standard capping ligands in hot-injection synthesis of PQDs [31] [33]
R-/S-Methylbenzylamine (R-/S-MBA)	Chiral ligands for imparting spin selectivity in ultrasonic-assisted exchange [31] [24]	Creating chiral PQDs for circularly polarized LEDs [31] [24]
Octylphosphonic Acid (OPA)	Short-chain ligand for conventional exchange; enhances conductivity and passivation [33]	Efficient red PQD-LEDs with improved charge transport [33]
Tetrabutylammonium Iodide (TBAI)	Halide source for solid-state ligand exchange and surface passivation [32]	Iodide passivation of PbS QD films for solar cells [32]
Zinc Iodide (ZnI₂)	Hybrid ligand component providing Zn²⁺ and I⁻ ions for dual passivation [34]	PbSe CQD photodetectors with reduced trap states [34]
1,2-ethanedithiol (EDT)	Short organic ligand for solution-phase exchange; improves interdot coupling [34]	PbSe CQD films for photodetectors [34]
Lead Halide Salts (e.g., PbI₂)	Inorganic ligands for solution-phase exchange; enhance stability [35]	Phase transfer of PbS CQDs for solar cells [35]

Conventional and ultrasonic-assisted ligand exchange strategies each offer distinct advantages for different research and application goals in PQD-LED development. Conventional methods using short-chain ligands like OPA provide reliable improvements in PLQY and electrical conductivity, making them suitable for standard high-efficiency LED applications [33]. The ultrasonic-assisted approach demonstrates superior performance for specialized applications requiring chiral-induced spin selectivity, achieving unprecedented combination of high gEL and EQE in spin-LEDs [31] [24].

The choice between these strategies depends on the target application: conventional methods suffice for general efficiency improvements, while ultrasonic-assisted approaches enable advanced functionalities like circularly polarized emission. Future research will likely focus on combining elements from both strategies to further optimize PQD surface engineering for next-generation optoelectronic devices.

Surface engineering through ligand chemistry is a pivotal determinant in the performance of perovskite quantum dot (PQD) light-emitting diodes (LEDs). Ligands anchored to the PQD surface govern critical processes including charge transport, defect passivation, environmental stability, and even novel functionalities such as spin control. This guide provides a comparative analysis of three dominant organic ligand systems: the conventional oleic acid/oleylamine (OA/OAm) pair, advanced compact ligands like formamidine thiocyanate (FASCN), and emerging chiral molecules such as R-/S-methylbenzylamine (R-/S-MBA). We objectively evaluate their performance through experimental data, detailing the methodologies that underpin these findings, to inform the selection of ligand strategies for high-performance PQD LEDs.

Performance Comparison of Ligand Systems

The following tables summarize key experimental data and performance metrics for the different ligand systems, highlighting their impact on PQD properties and device performance.

Table 1: Impact of Ligand Systems on PQD Material Properties

Ligand System	Key Function	Binding Energy (eV)	Photoluminescence Quantum Yield (PLQY)	Exciton Binding Energy (meV)	Film Conductivity (S m⁻¹)
Oleic Acid / Oleylamine (OA/OAm)	Synthesis stabilization, steric hindrance [24]	-0.18 (OAm), -0.22 (OA) [36]	Low (Baseline) [36]	39.1 [36]	Baseline [36]
Bidentate Ligand (FASCN)	Defect passivation, full surface coverage [36]	-0.91 [36]	Most notable improvement [36]	76.3 [36]	3.95 × 10⁻⁷ (8x higher than OA/OAm) [36]
Chiral Molecule (R-/S-MBA)	Chirality transfer, spin selectivity [24]	Information Missing	Information Missing	Information Missing	Information Missing

Table 2: LED Device Performance Metrics with Different Ligand Systems

Ligand System	Device Type	External Quantum Efficiency (EQE)	Electroluminescence Dissymmetry Factor (gEL)	Key Achievement
Oleic Acid / Oleylamine (OA/OAm)	NIR LED (Control)	~11.5% [36]	Not Applicable	Baseline performance [36]
Bidentate Ligand (FASCN)	NIR LED (FAPbI₃ QDs)	~23% [36]	Not Applicable	Record efficiency for NIR PQD-LEDs [36]
Chiral Molecule (R-/S-MBA)	Spin LED (CsPbBr₃ QDs)	16.8% (R-LED), 16.0% (S-LED) [24]	0.285 (R-LED), 0.251 (S-LED) [24]	Simultaneously high gEL and EQE [24]

Experimental Protocols for Ligand Implementation

Ligand Exchange on Chiral Perovskite QDs

Objective: To replace native OA/OAm ligands with chiral molecules (e.g., R-/S-MBA) to impart chirality and spin selectivity, while maintaining high photoluminescence quantum yield.

• Synthesis of Achiral PQDs: Synthesize CsPbBr₃ PQDs capped with OA/OAm ligands using the standard hot-injection method [24].
• Ultrasonic-Assisted Ligand Exchange:
  • Prepare a solution containing the chiral ligand (e.g., R-MBA).
  • Introduce the synthesized OA/OAm-capped PQDs into the chiral ligand solution.
  • Subject the mixture to ultrasonic (US) treatment. This step is critical, as it actively assists in the desorption of the original, long-chain OA/OAm ligands, thereby significantly improving the exchange efficiency [24].
• Purification and Film Formation: Purify the resulting chiral PQDs via centrifugation and redispersion. Subsequently, deposit thin films via spin-coating for device fabrication and characterization [24].

Surface Treatment with Bidentate Ligands

Objective: To achieve full surface coverage and suppress interfacial quenching sites in near-infrared (NIR) PQDs using a short, tightly-bound bidentate ligand.

• QD Synthesis and Preparation: Synthesize FAPbI₃ QDs capped with standard OA/OAm ligands [36].
• FASCN Chemical Treatment:
  • The synthesized QDs are treated with a solution of formamidine thiocyanate (FASCN).
  • The liquid characteristics of FASCN allow its use without high-polarity solvents, which could otherwise damage the PQDs. Its short carbon chain (less than 3 atoms) prevents steric hindrance, enabling full surface coverage [36].
  • The sulfur and nitrogen atoms in FASCN's thiocyanate group form a bidentate coordination with uncoordinated Pb²⁺ sites on the QD surface. Density-functional theory (DFT) calculations confirm a binding energy of -0.91 eV, which is fourfold higher than that of OA ligands [36].
• Film Formation and Device Fabrication: The FASCN-treated QD ink is spin-coated to form a solid film. The tight binding of the ligand prevents desorption during this process, eliminating interfacial quenching centers and resulting in films with high conductivity and excellent passivation [36].

Visualization of Ligand Strategies and Performance

The following diagrams illustrate the core concepts, experimental workflows, and performance outcomes of the different ligand engineering strategies.

Ligand Engineering Concepts and Workflow

Diagram Title: Ligand Engineering Pathways for PQD LEDs

Comparative Performance Outcomes

Diagram Title: Performance Outcomes of Ligand Engineering

The Scientist's Toolkit: Key Reagents and Materials

Table 3: Essential Research Reagents for PQD Ligand Engineering

Reagent/Material	Function/Description	Relevance to Ligand System
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain surfactants for colloidal synthesis and initial stabilization of PQDs [24].	Foundation for OA/OAm system; baseline for ligand exchange processes [24] [36].
Chiral Amines (e.g., R-/S-MBA)	Molecules with chiral centers that transfer chirality to the PQD lattice, enabling the CISS effect [24].	Core component for chiral ligand system; imparts spin selectivity for CP-LEDs [24].
Formamidine Thiocyanate (FASCN)	Short, bidentate liquid ligand that passivates defects and increases QD film conductivity [36].	Core component for compact ligand system; enables high-efficiency NIR LEDs [36].
Ultrasonic Probe	Applies ultrasonic energy to a solution, facilitating the desorption of native ligands during exchange [24].	Critical equipment for effective chiral ligand exchange to achieve high coverage [24].
Magnetic Conductive Probe AFM (mCP-AFM)	Characterizes spin selectivity (CISS effect) by measuring current with spin-polarized tips [24].	Key instrumentation for quantifying performance of chiral ligand systems [24].

Surface engineering is a pivotal aspect of enhancing the performance and stability of perovskite quantum dot light-emitting diodes (PQD LEDs). Inorganic passivation layers play a critical role in shielding the sensitive perovskite core from environmental degradation while mitigating intrinsic defects that quench luminescence. Among the various strategies, Metal-Organic Frameworks (MOFs) and inorganic oxide shells have emerged as two leading approaches. This guide provides a objective comparison of these methodologies, evaluating their performance based on recent experimental data and detailing the standard protocols for their application in PQD LED research. The objective is to furnish researchers with a clear, data-driven understanding of each method's advantages and limitations to inform material selection and innovation.

Metal-Organic Frameworks (MOFs) are a class of crystalline, porous materials constructed from metal ions or clusters coordinated with organic linkers. [37] [38] Their application in passivation leverages their unique structural properties. When used for passivation, MOFs can form a protective, porous shell around PQDs, where the pore size can be tuned to selectively block moisture and oxygen while allowing for charge transport. [39] [40] The organic linkers can also be functionalized to interact favorably with the perovskite surface, potentially pacifying surface defects.

Oxide Shells, such as those based on silicon oxide (SiO₂), titanium oxide (TiO₂), or zirconium oxide (ZrO₂), provide a dense, continuous inorganic barrier. [41] This coating physically isolates the PQD core from the environment. The passivation mechanism often involves the termination of surface dangling bonds on the perovskite crystal by the oxide material, which reduces surface trap states that lead to non-radiative recombination. [41]

Table 1: Comparative Analysis of Key Passivation Properties

Property	MOF Passivation	Oxide Shell Passivation
Structural Nature	Crystalline, porous framework [37] [38]	Dense, amorphous/crystalline, continuous layer [41]
Typical Thickness	Tunable, from few nm to >100 nm [40]	Typically < 10 nm [41]
Porosity	Tunable micro/mesoporosity (0.5 - 5 nm) [38]	Non-porous or controlled mesoporosity [41]
Primary Passivation Mechanism	Molecular sieving, defect site coordination, spatial confinement [39] [40]	Physical barrier formation, surface dangling bond termination [41]
Synthetic Typical Route	Solvothermal, in-situ growth, electrochemical deposition [38] [42]	Sol-gel, hydrolysis, atomic layer deposition (ALD) [41]

Performance Comparison in Optoelectronic Applications

Experimental data from various studies highlight the performance implications of choosing a MOF or oxide shell passivation strategy. While direct head-to-head comparisons for PQD LEDs are limited in the search results, data from related applications like solar cells and general LEDs provide strong indicators of expected performance.

Photoluminescence Quantum Yield (PLQY) is a critical metric for LED materials, indicating the efficiency of light emission. MOF passivation has been shown to enhance PLQY significantly. For instance, in perovskite-based systems, MOF passivation can lead to a relative increase in PLQY of over 50%, primarily by reducing non-radiative recombination pathways at the PQD surface. [39] Oxide shells, particularly SiO₂, also improve PLQY, but the enhancement can be more variable and depends heavily on the formation of a uniform, pinhole-free layer and the chemical compatibility between the oxide and the perovskite lattice. [43]

Operational Stability under continuous illumination or electrical driving is a key challenge for PQD LEDs. MOF coatings excel in this area due to their robust molecular sieving effect. The tunable pore windows can effectively block larger, degrading molecules like H₂O and O₂ while permitting smaller species involved in charge transport. [40] This has been demonstrated to extend the operational lifetime of devices by several hundred hours compared to unpassivated controls. [39] Oxide shells provide an excellent barrier to gas and moisture, but their rigid, inorganic nature can lead to mechanical stress or lattice mismatch at the PQD interface, which may create new defect sites over time or during device operation. [41]

Charge Transport is a double-edged sword in passivation. A thick or poorly conductive passivation layer can hinder the injection of charges into the QD. MOFs are generally insulative, but their ultra-thin, conformal nature and porous structure can allow for tunneling or hopping-based charge transport. [39] [44] Some MOFs and their derivatives are being engineered for higher conductivity. [44] Oxide shells, being wide-bandgap insulators, almost always introduce a charge transport barrier. This is often managed by keeping the shell extremely thin (a few nm) to allow for quantum tunneling, but this can come at the cost of complete coverage and protection. [41]

Table 2: Summary of Experimental Performance Data

Performance Metric	MOF Passivation	Oxide Shell Passivation	Experimental Context
PLQY Enhancement	>50% relative increase [39]	20-40% relative increase [43]	Perovskite nanocrystal films
Operational Lifetime (T₅₀)	>300h (enhancement of 3x) [39]	~200h (enhancement of 2x) [43]	Under constant illumination
Barrier Property	Excellent molecular sieving [40]	Excellent gas/moisture barrier [41]	Environmental stability testing
Impact on Charge Injection	Moderate (tunneling through thin layers) [39] [44]	Can be significant (requires ultra-thin layers) [41]	EL device efficiency measurement

Detailed Experimental Protocols

MOF Passivation via In-Situ Growth

The in-situ growth method involves the formation of the MOF shell directly on the PQD surface, using the QD as a nucleation site.

Protocol Steps:

• PQD Surface Preparation: A solution of purified PQDs (e.g., CsPbBr₃) in a non-polar solvent (e.g., toluene) is prepared. The PQD surface is often treated with a catalytic amount of a proton scavenger or a Lewis base (e.g., dimethylphenylphosphine) to activate the surface for coordination. [38]
• Precursor Preparation: In a separate vial, the MOF precursors are dissolved. A typical recipe for a ZIF-8 shell involves dissolving zinc acetate dihydrate (Zn(OAc)₂·2H₂O) and the organic linker 2-methylimidazole in a compatible solvent mixture (e.g., DMF and methanol). [38] [42]
• Reaction Initiation: The PQD solution is rapidly injected into the vigorously stirred MOF precursor solution at a controlled temperature (e.g., room temperature or 60°C).
• Shell Growth: The reaction proceeds for a predetermined time (seconds to minutes), during which the MOF crystallizes directly onto the PQD surface. The growth time and precursor concentrations are critical for controlling shell thickness and porosity. [42]
• Purification: The resulting MOF-capped PQDs are purified by centrifugation and washed several times with a solvent (e.g., ethanol) to remove unreacted precursors and loosely bound crystals. The final product is dispersed in a stable solvent for further use. [40]

Oxide Shell Passivation via Sol-Gel Coating

The sol-gel method is a common wet-chemical approach for depositing oxide layers, such as SiO₂.

Protocol Steps:

• PQD Dispersion: PQDs are dispersed in a non-polar solvent (e.g., hexane or octane) in a reaction flask. [41]
• Precursor Hydrolysis: A silicon alkoxide precursor, typically tetraethyl orthosilicate (TEOS), is added to a mixture of a polar solvent (e.g., ethanol) and a catalytic amount of water and base (e.g., ammonium hydroxide, NH₄OH). This mixture is stirred to initiate the hydrolysis of TEOS, forming silicic acid and ethanol. [41]
• Coating Reaction: The hydrolyzed TEOS solution is slowly added dropwise to the stirring PQD dispersion. The SiO₂ precursors condense on the surface of the PQDs, forming an amorphous SiO₂ shell.
• Aging and Drying: The reaction is allowed to proceed for several hours to complete the condensation and aging process, which strengthens the SiO₂ network.
• Purification: The SiO₂-capped PQDs are isolated via centrifugation and redispersed in an appropriate solvent. Thermal annealing at a mild temperature (e.g., 60-80°C) may be performed to further condense the shell and remove residual solvents. [41]

The Scientist's Toolkit: Essential Research Reagents

Successful implementation of these passivation strategies requires a specific set of chemical reagents and materials. The table below lists the essential items for the protocols described above.

Table 3: Essential Reagents for Passivation Experiments

Reagent/Material	Function	Example in MOF Protocol	Example in Oxide Protocol
Perovskite Quantum Dots (PQDs)	Core optoelectronic material	CsPbBr₃, CsPbI₃ nanocrystals	CsPbBr₃, CsPbI₃ nanocrystals
Metal Salt	Provides metal nodes for framework/oxide	Zinc acetate (Zn(OAc)₂) [38]	Titanium isopropoxide (Ti(OPr)₄) [41]
Organic Ligand/Linker	Forms coordination bonds with metal nodes	2-Methylimidazole (for ZIF-8) [38]	-
Oxide Precursor	Hydrolyzes to form the inorganic oxide shell	-	Tetraethyl orthosilicate (TEOS) [41]
Catalyst/Base	Catalyzes hydrolysis/condensation reactions	-	Ammonium hydroxide (NH₄OH) [41]
Polar Solvent	Medium for precursor reaction & hydrolysis	DMF, Methanol [38] [42]	Ethanol [41]
Non-Polar Solvent	Medium for dispersing and stabilizing PQDs	Toluene, Octane [38]	Hexane, Octane [41]

Both MOF and oxide shell strategies offer distinct pathways for the inorganic passivation of PQDs. The choice between them is not a simple declaration of superiority but a strategic decision based on the specific requirements of the PQD LED application.

• MOF Passivation is highly suited for applications where exceptional stability against environmental factors is the primary goal, and where a moderate impact on charge transport is acceptable. Its strength lies in its molecular-level designability, which allows for precise pore engineering and potential for defect pacification beyond simple barrier formation. [39] [40]
• Oxide Shell Passivation offers a straightforward and robust method for creating a dense, inert barrier. It is particularly effective when a thin, conformal layer is needed to minimize interference with charge injection, provided the interface between the oxide and the perovskite can be well-controlled to avoid introducing new defects. [41]

Future research will likely focus on hybrid approaches, combining the virtues of both materials. For instance, MOF-derived metal oxides could offer a path to more structured and porous oxide coatings. [41] Furthermore, advancing synthetic control to achieve uniform, ultra-thin MOF layers will be crucial for minimizing charge transport penalties, making MOFs an even more compelling choice for next-generation, high-efficiency, and stable PQD LEDs.

Compositional engineering of metal halide perovskites, with a typical formula of ABX3 (where A is a monovalent cation, B is a divalent metal cation, and X is a halide anion), represents a critical research frontier for addressing the primary limitation of lead halide perovskite quantum dots (PQDs) – their toxicity and environmental impact [12] [45]. The presence of lead in these otherwise promising optoelectronic materials raises significant concerns for large-scale commercial applications and potential environmental contamination [45] [46]. Consequently, developing lead-free or lead-reduced alternatives without compromising the exceptional optical and electronic properties of lead-based perovskites has become a paramount objective.

This guide systematically compares the performance of various lead reduction and lead-free strategies, focusing on their efficacy in light-emitting diode (LED) applications. We objectively evaluate homovalent and heterovalent substitutions, doping approaches, and their resultant impacts on key performance metrics, including photoluminescence quantum yield (PLQY), external quantum efficiency (EQE), color gamut, and operational stability, providing a structured framework for researcher evaluation.

Performance Comparison of Lead Reduction and Lead-Free Strategies

Homovalent Cation Substitution (Sn²⁺)

Table 1: Performance of Tin (Sn²⁺) as a Lead Substitute

Metric	Tin-Based Perovskite Performance	Comparative Lead-Based Performance
Primary Formulation	CH₃NH₃SnBr₃, CsSnX₃ [45] [46]	CH₃NH₃PbBr₃, CsPbX₃ [46]
Toxicity	Lower toxicity (Sn LD₅₀ > 2000 mg/kg) [46]	High toxicity (Pb LD₅₀ ≈ 35 mg/kg) [46]
PLQY	Up to ~70% (emulsion synthesis); typically much lower [46]	~84% to >95% [45] [46]
Reported LED EQE	Up to ~15% [46]	>23% for green/red; ~12-26% for blue [12] [47] [48]
Key Challenge	Oxidation of Sn²⁺ to Sn⁴⁺, creating high defect density and trap states [45]	Intrinsic instability under environmental stressors [12] [46]

Divalent Cation Doping (Mn²⁺, Zn²⁺, Cd²⁺)

Table 2: Performance of Doping with Divalent Cations

Dopant	Key Effects and Performance	Experimental Outcomes
Mn²⁺	- Partial Pb²⁺ substitution reduces lead content [45] [46]- Enhances stability via stronger Mn-Br bonds (Eb = 2.1 eV) [46]- Retains high PLQY (>90%) [46]	- Doubled stability (T50 > 1000 h) [46]- Maintains high performance while reducing toxicity [46]
Zn²⁺	- Investigated for less toxic light harvesting [45]	- Optical and LED performance enhancements noted [45]
Cd²⁺	- Homovalent substitution on B-site [45]	- Toxicity concerns remain (Cd is also toxic) [45]

Heterovalent & Other Lead-Free Alternatives

Table 3: Other Lead-Free Perovskite Variants

Material System	Composition	Optical Performance	Remarks
Bismuth-Based	Cs₃Bi₂Br₉ [46]	PLQY >80%; broad FWHM (40-60 nm) [46]	Low toxicity; broad emission reduces color purity [46]
Manganese-Based	-	Red emission; millisecond-level triplet lifetime [47]	Long operating lifetime, high efficiency, low voltage [47]
Copper-Based	Cs₃Cu₂I₅, CsCu₂I₃ [49]	Emissions at 444 nm, 580 nm [49]	Emerging lead-free system for white LEDs and communication [49]

Experimental Protocols for Key Compositions

Manganese (Mn²⁺) Doping in CH₃NH₃PbBr₃ PQDs

Objective: To synthesize Mn²⁺-doped CH₃NH₃PbBr₃ PQDs with reduced lead content and enhanced stability.

Synthesis Workflow:

Detailed Methodology:

• Precursor Preparation: Dissolve lead bromide (PbBr₂) and methylammonium bromide (MABr) in dimethylformamide (DMF) with oleic acid (OA) and oleylamine (OAm) as coordinating ligands [46].
• Dopant Addition: Prepare a separate solution of manganese bromide (MnBr₂) in DMF, typically at 10-20 mol% relative to the Pb²⁺ concentration, to achieve partial substitution [46].
• Mixing and Reaction: Combine the precursor solutions under an inert atmosphere (e.g., nitrogen or argon) to prevent oxidation and degradation.
• Reprecipitation: Rapidly inject the mixed solution into a poor solvent (toluene) under vigorous stirring. This induces supersaturation and rapid nucleation, forming doped PQDs via the ligand-assisted reprecipitation (LARP) technique [46] [48].
• Purification: Centrifuge the crude solution to separate the PQDs. Discard the supernatant and redisperse the pellet in a non-polar solvent (e.g., hexane or octane) for further characterization and device fabrication [46].

Synthesis of All-Inorganic Cesium Tin Halide (CsSnX₃) PQDs

Objective: To fabricate lead-free CsSnX₃ PQDs for LED applications.

Synthesis Workflow:

Detailed Methodology:

• Tin Precursor: Dissolve tin halide (SnX₂, where X=Br, I) in 1-octadecene (ODE) with OA and OAm in a three-neck flask [45].
• Degassing and Atmosphere: Degas the mixture at 100-120°C under vacuum for 30-60 minutes to remove oxygen and water. Then, flush the flask with inert gas (N₂ or Ar) [45].
• Hot-Injection: Raise the temperature to the reaction range (150-180°C). Rapidly inject a pre-formed cesium-oleate solution [45].
• Reaction Quenching: Immediately cool the reaction mixture in an ice-water bath after 5-10 seconds to terminate crystal growth and prevent oxidation [45].
• Purification: Purify the CsSnX₃ PQDs by centrifugation with anti-solvents (e.g., ethyl acetate/methanol mixtures) to remove unreacted precursors and excess ligands. Store the final product in an inert atmosphere [45].

The Scientist's Toolkit: Essential Research Reagents

Table 4: Key Reagents for Compositional Engineering in PQD Research

Reagent Category	Specific Examples	Function in Experimentation
Lead Sources	Lead bromide (PbBr₂), Lead iodide (PbI₂)	Standard B-site cation source in precursor solutions [46] [48]
Lead Substitutes	Tin(II) bromide (SnBr₂), Manganese bromide (MnBr₂)	Direct Pb replacement or doping agent to reduce toxicity [45] [46]
Cesium Sources	Cesium carbonate (Cs₂CO₃), Cesium bromide (CsBr)	A-site cation for all-inorganic perovskite compositions [48]
Organic Cations	Methylammonium bromide (MABr), Formamidinium iodide (FAI)	A-site cation for organic-inorganic hybrid perovskites [46]
Ligands	Oleic Acid (OA), Oleylamine (OAm)	Surface capping agents to control growth, stabilize dots, and passivate defects [12] [48]
Solvents	Dimethylformamide (DMF), Dimethyl Sulfoxide (DMSO), 1-Octadecene (ODE), Toluene	Polar solvents dissolve precursors; non-polar solvents induce reprecipitation [48]

Compositional engineering offers a multi-faceted approach to mitigating the toxicity issues of lead-based PQDs. While homovalent substitution with Sn²⁺ provides a direct lead-free alternative, it is currently hampered by oxidation and inferior performance. In contrast, partial substitution through Mn²⁺ doping presents a more immediately viable path, significantly enhancing stability and maintaining high efficiency while reducing lead content. Emerging systems based on Bi³⁺ and Cu⁺ offer additional pathways but often trade color purity or overall device efficiency. The choice of strategy involves a careful balance between toxicity reduction, optical performance, and device stability, with current research indicating that partial lead replacement may offer the most practical near-term solution for high-performance PQD-LEDs.

The pursuit of high-performance and stable perovskite quantum dot light-emitting diodes (PQD-LEDs) has catalyzed the development of advanced surface engineering strategies. Among these, hybrid approaches that integrate the distinct advantages of core-shell nanostructures with two-dimensional (2D) materials represent a frontier in the field. Core-shell structures excel in protecting the sensitive perovskite core and mitigating defect-related non-radiative recombination, while 2D materials, notably graphene oxide (GO), offer exceptional charge transport capabilities and chemical stability. When combined, these materials create a synergistic platform that addresses the intrinsic instability of perovskites without compromising their superior optoelectronic properties. This guide provides a comparative analysis of these hybrid approaches, detailing their performance against conventional alternatives and outlining the experimental protocols essential for their implementation. The objective is to furnish researchers with a clear understanding of the structure-property relationships governing these advanced architectures, thereby accelerating the development of next-generation PQD-LEDs.

Performance Comparison of Surface Engineering Approaches

The selection of a surface engineering strategy profoundly influences the performance and stability of PQD-LEDs. The table below provides a comparative overview of the key approaches discussed in this guide.

Table 1: Performance Comparison of Surface Engineering Strategies for PQD-LEDs

Engineering Approach	Key Advantages	Reported Performance Metrics	Primary Challenges
Core-Shell Structures	Enhanced environmental stability; Effective defect passivation; Suppressed ion migration [12].	PLQY up to 97% after UV exposure [50]; Operational lifetime >10,000 hours for some QD-LEDs [50].	Precise control of shell thickness and uniformity; Potential lattice mismatch.
2D Material Integration (e.g., GO)	High charge carrier mobility; Superior chemical stability; Functional groups for binding [51].	Improved charge transport in composite films; Acts as a protective barrier [51] [52].	Achieving homogeneous dispersion; Managing interface energetics.
Hybrid Core-Shell/2D Composites	Combines stability of core-shell with superior charge transport of 2D materials; Synergistic performance enhancement.	COF-based shell doubled cycling stability in harsh conditions [52]; MOF/GO composites show enhanced adsorption capacity and stability [51].	Complexity of synthesis and integration; Optimization of multiple interfaces.
Ligand Engineering	Defect passivation; modulation of surface energy; Improved charge injection [47] [12].	EQE of >30% for green and red PeLEDs [47]; PLQY over 83% with multifunctional ligands [47].	Ligand stability under operational conditions (e.g., heat, bias).

Detailed Experimental Protocols

Synthesis of Core-Shell Perovskite Quantum Dots

The "hot-injection" method is a standard and highly controllable synthesis technique for high-quality core-shell PQDs [50].

• Primary Reagents: Lead(II) acetate (Pb(OAc)₂), Cesium carbonate (Cs₂CO₃), Oleic acid (OA), Oleylamine (OAm), 1-Octadecene (ODE), and shell precursor (e.g., metal halide for inorganic shell).
• Procedure:
  • Cs-precursor Synthesis: Load Cs₂CO₃, OA, and ODE into a flask. Heat and stir under vacuum at 120°C until the Cs₂CO₃ is completely dissolved, then maintain under an inert N₂ atmosphere.
  • Pb-precursor Synthesis: In a separate three-neck flask, mix Pb(OAc)₂, OA, OAm, and ODE. Dry the mixture under vacuum for 1 hour at 90°C to remove residual water and oxygen.
  • Core Formation: Rapidly raise the temperature of the Pb-precursor solution to 180°C under N₂. Swiftly inject the Cs-precursor solution into this hot reaction mixture. The reaction proceeds for 5-60 seconds to form CsPbX₃ (X=Br, I, Cl) core QDs.
  • Shell Growth: Cool the reaction mixture to a lower temperature (e.g., 100°C). Slowly add the shell precursor solution via a syringe pump to enable controlled, layer-by-layer shell growth.
  • Purification: After the reaction, cool the solution to room temperature and centrifuge the crude solution with an anti-solvent (e.g., acetone or ethyl acetate) to precipitate the QDs. Re-disperse the purified core-shell PQDs in a non-polar solvent like hexane or toluene for storage and further use.

Fabrication of 2D Material-PQD Composite Films

Integrating 2D materials like graphene oxide (GO) with PQDs can be achieved through a solution-processing method [51].

• Primary Reagents: Synthesized PQDs, Graphene Oxide (GO) dispersion, relevant solvents (toluene, isopropanol).
• Procedure:
  • GO Dispersion Preparation: Prepare a stable, homogeneous dispersion of GO in isopropanol (e.g., 1 mg/mL) using prolonged bath sonication.
  • Composite Ink Formulation: Mix the purified PQD solution (in hexane/toluene) with the GO dispersion in a predetermined volume ratio. Subject the mixture to mild sonication and vigorous stirring to ensure a uniform composite ink. The ratio can be adjusted to optimize the charge transport and emissive properties of the final film.
  • Film Deposition: Deposit the composite ink onto the target substrate (e.g., ITO-coated glass with a prepared hole-transport layer) using a suitable technique such as spin-coating or inkjet printing.
  • Film Formation: The film is typically annealed at a low temperature (e.g., 70°C for 10 minutes) to remove residual solvents and facilitate interfacial contact, resulting in a stable PQD/GO composite film.

Device Fabrication and Characterization of PQD-LEDs

The standard device architecture for a PQD-LED is a multilayer stack [12].

• Device Structure: The typical structure is Glass/ITO (anode)/Hole Injection Layer (HIL)/Hole Transport Layer (HTL)/PQD Emissive Layer/Electron Transport Layer (ETL)/Cathode (e.g., Al, Ag).
• Fabrication Workflow:
  • Substrate Cleaning: Clean the ITO-coated glass substrates sequentially with detergent, deionized water, acetone, and isopropanol under sonication, followed by UV-ozone treatment.
  • Charge Transport Layer Deposition: Deposit the HIL (e.g., PEDOT:PSS) and HTL (e.g., Poly-TPD) onto the ITO substrate via spin-coating, with each layer annealed at its specified temperature.
  • Emissive Layer Deposition: Spin-coat the prepared PQD or PQD-composite ink onto the HTL to form a thin, uniform emissive layer.
  • Electron Transport Layer and Cathode Deposition: Deposit the ETL (e.g., TPBi) by thermal evaporation. Finally, deposit the metal cathode (e.g., LiF/Al) through a shadow mask via thermal evaporation under high vacuum.
• Key Characterization Techniques:
  • Electroluminescence (EL): Measures the light output from the device under operation.
  • External Quantum Efficiency (EQE): The ratio of the number of photons emitted from the device to the number of electrons injected, a critical metric for LED performance. Values for green and red PQD-LEDs have exceeded 23% and can reach over 30% [47] [12].
  • Current Density-Voltage-Luminance (J-V-L): Characterizes the electrical and light-output properties of the device.
  • Operational Lifetime (T₅₀): The time taken for the luminance to decay to 50% of its initial value at a constant current density.

Structural and Functional Pathways

The following diagram illustrates the logical relationship between the hybrid structural design, its functional mechanisms, and the resulting performance enhancements in PQD-LEDs.

The Scientist's Toolkit: Essential Research Reagents

Successful research in hybrid PQD-LEDs relies on a suite of specialized reagents and materials. The table below details the core components and their functions.

Table 2: Essential Research Reagents for Hybrid PQD-LED Development

Category/Reagent	Function/Purpose	Key Considerations
Perovskite Precursors
Cesium Carbonate (Cs₂CO₃)	Cesium (A-site) cation source for all-inorganic PQDs [50].	Requires reaction with acid (e.g., OA) to form an active precursor.
Lead Halides (e.g., PbBr₂)	Lead (B-site) and halide (X-site) source for the perovskite lattice [12].	High purity is critical to minimize defects.
Ligands & Surface Agents
Oleic Acid (OA) & Oleylamine (OAm)	Common surface capping ligands to control growth and stabilize colloids [47] [12].	Dynamic binding can lead to desorption; protonation/deprotonation affects stability [47].
Polyethylenimine (PEI)	Polymer for surface passivation and interface modification [52].	Can introduce amine groups that strongly interact with the perovskite surface.
2D Materials
Graphene Oxide (GO)	Provides functional groups for composite formation; enhances charge transport and stability [51].	Degree of oxidation and sheet size impact electronic properties and dispersion.
Charge Transport Materials
PEDOT:PSS	Common Hole Injection Layer (HIL) material [12].	Acidity can impact device longevity.
TPBi	Common Electron Transport Layer (ETL) material [12].	Deposited via thermal evaporation.
Solvents
1-Octadecene (ODE)	High-boiling-point non-coordinating solvent for hot-injection synthesis [50].	Must be purified to remove peroxides and water.
Toluene & Hexane	Non-polar solvents for dispersing and processing ligand-capped PQDs [12].	Anhydrous grades are recommended for optimal performance.

The integration of core-shell architectures with 2D materials represents a powerful hybrid strategy to overcome the perennial challenges of stability and efficiency in PQD-LEDs. As the comparative data and experimental protocols outlined in this guide demonstrate, this approach leverages the distinct advantages of each component: the core-shell structure provides robust protection and defect passivation for the perovskite emitter, while the 2D material facilitates efficient charge transport and adds another layer of environmental shielding. The resulting synergy leads to devices with high external quantum efficiency, superior color purity, and significantly extended operational lifetimes. For researchers, the future development of this field hinges on refining the synthesis for better uniformity and scalability, deepening the understanding of the complex interfaces within these hybrid systems, and continuing the exploration of new, non-toxic material combinations. The hybrid core-shell/2D material approach offers a compelling and versatile pathway toward the commercial realization of high-performance perovskite-based displays and lighting.

Addressing Stability and Toxicity Challenges in PQD-LED Development

The rapid advancement of perovskite quantum dot light-emitting diodes (PQD LEDs) has positioned them as a leading technology for next-generation displays and solid-state lighting. These materials exhibit exceptional optical properties, including tunable emission wavelengths, high photoluminescence quantum yield (PLQY), and narrow emission bandwidths [53] [43]. However, their commercial deployment faces a significant hurdle: environmental degradation from moisture, oxygen, and UV radiation [53] [54]. This review provides a comparative analysis of surface engineering strategies designed to enhance PQD LED stability, offering researchers a detailed examination of experimental approaches, performance data, and key reagents essential for advancing this critical research field.

Comparative Analysis of Protection Strategies

Various strategies have been developed to shield perovskite quantum dots from environmental stressors. The table below objectively compares the primary protection approaches, their implementation methods, and their relative effectiveness against key degradation factors.

Table 1: Comparative Analysis of Protection Strategies for PQD LEDs

Strategy	Implementation Methods	Moisture Resistance	Oxygen Resistance	UV Resistance	Key Advantages	Key Limitations
Inorganic Shell Encapsulation	Growth of silica, alumina, or metal oxide shells around PQDs via sol-gel processes [53].	High	High	Medium	Excellent barrier properties; high chemical stability [53].	Can introduce lattice strain; may reduce PLQY if shell growth is imperfect [53].
Surface Ligand Engineering	Exchange of long-chain insulating ligands (e.g., oleic acid) with shorter or cross-linkable ligands [53].	Medium	Medium	Low	Improves charge transport; passivates surface defects [53].	Limited long-term stability; ligands can desorb over time [53].
Polymer Matrix Encapsulation	Embedding PQDs in polymer matrices (e.g., epoxy, PMMA) using spin-coating or inkjet printing [53] [55].	Medium-High	Medium-High	Medium	Good mechanical flexibility; scalable processing [55].	Potential for incomplete coverage; polymer permeability to gases over time [53].
Composite/Structured Barriers	Use of MXene composites or atomic layer deposition (ALD) of Al₂O₃ for electrodes and barriers [55].	High	High	High	Superior charge transport & heat dissipation; excellent barrier properties [55].	More complex and costly fabrication processes [55].

Experimental Data and Performance Metrics

Quantitative data from accelerated aging tests under controlled environmental conditions provide critical insights into the efficacy of different protection strategies. The following table summarizes key performance metrics reported in recent studies.

Table 2: Quantitative Performance Metrics of Protected PQD LEDs

Protection Scheme	Experimental Conditions	Key Performance Metrics	Reference/System
ALD Al₂O₃ Passivation	Applied to AlGaN-based UVC micro-LEDs; devices subjected to operational stress testing [56].	Suppressed sidewall leakage currents (below 100 fA at -5 V); reduced EQE droop from 67.5% to 17.9% in smaller devices [56].	AlGaN UV Micro-LEDs [56]
MXene Composite Electrode	AgNPs/AgNWs/MXene/PEDOT:PSS composite used as an electrode in flexible PeLEDs [55].	Significant improvement in heat dissipation; stable device operation at 5 V for large-area flexible devices [55].	Flexible PeLEDs [55]
Polymer/Elastomer Substrate	Perovskite layer deposited on pre-stretched elastomeric adhesive (VHB 4905) [55].	Stable luminance and current efficiency under 0% to 20% strain over multiple cycles; 75% brightness retention after 1000 stretch-release cycles [55].	Flexible QD PeLEDs [55]
Inorganic Shell + Polymer	PQDs embedded in a composite of inorganic shells and polymer matrices for WLEDs [43].	Improved operational stability for white light emission; broader emission spectra for enhanced color rendition [43].	Perovskite WLEDs [43]

Detailed Experimental Protocols

To ensure reproducibility, this section outlines standardized protocols for key experiments cited in the comparison tables.

Protocol for Atomic Layer Deposition (ALD) Passivation

This protocol is critical for mitigating non-radiative recombination at PQD surfaces and protecting against atmospheric gases [56].

• Sample Pre-treatment: Following dry etching of the LED mesa, treat the sample with Tetramethylammonium Hydroxide (TMAH) to remove surface debris and smooth the sidewalls [56].
• ALD Chamber Setup: Place the pre-treated samples in the ALD chamber. Use Trimethylaluminum (TMA) as the aluminum precursor and deionized water as the oxygen source.
• Deposition Cycle:
  • Pulse: Introduce the TMA precursor into the chamber for a set duration (e.g., 0.1 seconds).
  • Purge: Flush the chamber with an inert gas (e.g., nitrogen or argon) to remove excess precursor and reaction by-products.
  • Pulse: Introduce the H₂O reactant pulse for a set duration.
  • Purge: Flush the chamber again with inert gas.
• Cycle Repetition: Repeat the deposition cycle 100-200 times to achieve a conformal Al₂O₃ film with a thickness of 10-20 nm.
• Post-deposition Annealing: Anneal the coated samples at a moderate temperature (e.g., 200°C) in an inert atmosphere to improve film quality and reduce defects.

Protocol for Accelerated Aging Test

This standardized protocol evaluates the long-term stability of encapsulated PQD LEDs under combined environmental stress [53] [54].

• Device Preparation: Encapsulate the PQD LED devices using the chosen strategy (e.g., polymer resin, thin-film barrier).
• Environmental Chamber Conditioning: Place devices in an environmental test chamber. Set the temperature to 85°C and the relative humidity to 85% (85/85 test). For oxygen-specific testing, control the oxygen concentration (e.g., 21% to 35%) within the chamber [57].
• Operational Stress: Continuously operate the devices at a constant current density, typically set above the peak EQE point (e.g., 500 A cm⁻² for micro-LEDs) to simulate high-load conditions [56].
• In-situ Monitoring: At regular intervals, measure key optoelectronic parameters:
  • Luminance (L) and External Quantum Efficiency (EQE) using an integrating sphere and spectrometer.
  • Electroluminescence (EL) spectra to track color shifts.
  • Current-Voltage (I-V) characteristics to monitor electrical degradation.
• Data Analysis: Plot the normalized luminance and EQE as a function of time. The lifetime (T₅₀) is defined as the time taken for the luminance to decay to 50% of its initial value.

Ligand Exchange Procedure

This procedure enhances the charge transport and initial stability of PQDs by replacing native long-chain ligands [53].

• PQD Solution Preparation: Synthesize PQDs (e.g., CsPbBr₃) using a standard hot-injection or ligand-assisted reprecipitation (LARP) method, resulting in a solution with native oleic acid and oleylamine ligands.
• Precipitation and Purification: Add a polar anti-solvent (e.g., methyl acetate) to the crude solution to precipitate the PQDs. Centrifuge the mixture and discard the supernatant.
• Ligand Solution Preparation: Prepare a solution of the new short-chain ligand (e.g., didodecyldimethylammonium bromide - DDAB) in an anhydrous solvent.
• Exchange Reaction: Re-disperse the purified PQD pellet in the ligand exchange solution. Stir the mixture for a predetermined time (e.g., 1-2 hours) to allow the ligand exchange to occur.
• Purification: Precipitate and wash the PQDs again with anti-solvent to remove excess ligands and reaction by-products. Repeat the centrifugation and re-dispersion cycle 2-3 times.

Degradation Mechanisms and Protection Pathways

The following diagram illustrates the primary environmental degradation pathways for PQD LEDs and the corresponding protective mechanisms offered by surface engineering strategies.

Diagram 1: Environmental Degradation and Protection Pathways for PQD LEDs.

Experimental Workflow for Stability Assessment

The diagram below outlines a standardized experimental workflow for synthesizing, passivating, and characterizing the environmental stability of PQD LEDs.

Diagram 2: Workflow for PQD LED Stability Testing.

The Scientist's Toolkit: Essential Research Reagents

This section details key materials and reagents used in the featured experiments for the surface engineering and stability testing of PQD LEDs.

Table 3: Essential Reagents for PQD LED Stability Research

Reagent/Material	Function/Application	Specific Example / Rationale
Trimethylaluminum (TMA)	Precursor for Atomic Layer Deposition (ALD) of Al₂O₃ passivation layers [56].	Creates a conformal, pinhole-free inorganic barrier on device surfaces to block moisture and oxygen ingress [56].
Tetramethylammonium Hydroxide (TMAH)	Chemical treatment for LED sidewalls [56].	Used pre-ALD to smooth etched sidewalls and reduce surface defects that act as non-radiative recombination centers [56].
Didodecyldimethylammonium Bromide (DDAB)	Short-chain ligand for surface ligand exchange [53].	Replaces native long-chain ligands (e.g., oleic acid) to improve charge transport while maintaining colloidal stability and passivating surface defects [53].
Silver Nanowires (AgNWs)	Conductive component for flexible transparent electrodes [55].	Form percolation networks in composite electrodes (e.g., with MXene) offering high conductivity, flexibility, and improved stability versus ITO [55].
Polydimethylsiloxane (PDMS)	Flexible elastomeric substrate for device fabrication [55].	Provides excellent mechanical flexibility and stability, enabling the creation of stretchable and bendable LED devices [55].
Poly(methyl methacrylate) (PMMA)	Polymer matrix for encapsulation and color conversion layers [53] [58].	Embeds and protects PQDs from the environment in a robust, transparent polymer host, facilitating application in displays and lighting [53] [58].
MXene (e.g., Ti₃C₂Tₓ)	2D conductive material for composite electrodes [55].	Enhances electrode conductivity, improves mechanical properties, and offers superior heat dissipation in flexible LEDs, boosting operational stability [55].

Metal halide perovskites (MHPs) have emerged as a transformative class of materials for optoelectronic devices, including perovskite quantum dot light-emitting diodes (PQD LEDs), due to their exceptional properties such as wide color tunability, high color purity, narrow emission bandwidths, and high photoluminescence quantum yields (PLQYs) [59] [50]. However, the presence of toxic lead (Pb) in their most common formulations raises serious environmental and health concerns, potentially limiting their commercial viability and societal acceptance [50] [60]. Consequently, developing effective strategies to reduce or eliminate lead content has become a critical research frontier. This guide objectively compares two prominent surface engineering approaches for lead reduction: manganese doping (Mn-doping) and tin (Sn) substitution. Mn-doping reduces lead reliance by incorporating non-toxic manganese into the perovskite structure, while tin substitution involves replacing lead entirely with tin to create lead-free perovskites. Framed within the broader thesis of comparative surface engineering for PQD LEDs, this analysis evaluates these strategies based on optoelectronic performance, structural stability, and experimental feasibility, providing researchers with data-driven insights for material selection and device development.

Comparative Performance Analysis of Lead Reduction Strategies

The following table summarizes the key performance characteristics of Mn-doping and Tin Substitution strategies, based on current experimental findings.

Table 1: Performance Comparison of Lead Reduction Strategies for Perovskite Materials

Performance Parameter	Mn-Doping Approach	Tin Substitution Approach
Primary Function	Partial Pb replacement; introduces new emission centers via energy transfer [61] [62]	Complete Pb replacement; direct bandgap engineering [62] [60]
Typical System	CsPbCl₃:Mn [61]; Cs₂NaInₓBi₁–ₓCl₆:Mn [62]	CsSnX₃ (X=Cl, Br, I) [60]
Photoluminescence Quantum Yield (PLQY)	Up to 44.6% (in lead-free Cs₂NaIn₀.₇₅Bi₀.₂₅Cl₆:Mn) [62]	Generally lower than lead-based and Mn-doped counterparts; specifics not quantified in results
Emission Tuning Range	Yellow to orange-red (583–614 nm) via In/Bi ratio control [62]	Tunable across visible spectrum, similar to lead-based perovskites [60]
PL Lifetime	Ultralong, in the range of 3–9 ms [62]	Not specified in search results
Key Advantage	Improved stability (photo-, moisture-, thermal-); dual emission [61] [63]	Inherently lead-free, addressing toxicity concern at source [62] [60]
Stability Performance	~20-day operational stability with minimal CRI/CCT fluctuation [61]	Good stability in air reported for specific compositions [62]

Experimental Protocols and Methodologies

Synthesis of Mn-Doped Perovskite Quantum Dots

1. Hot-Injection Method for Mn-doped CsPbCl₃ QDs: This is a widely used colloidal synthesis technique for high-quality nanocrystals [50]. A lead-free variant synthesizes Cs₂NaInₓBi₁–ₓCl₆:Mn NCs using a variable temperature, one-pot hot injection method [62].

• Precursor Preparation: Cesium carbonate (Cs₂CO₃) is reacted with oleic acid (OA) and 1-octadecene (ODE) to form a Cs-oleate precursor. Separately, lead chloride (PbCl₂) and manganese chloride (MnCl₂) are dissolved in ODE with coordinating ligands (OA and oleylamine, OAm) [62].
• Injection and Reaction: The metal halide precursor solution is heated to a specific temperature (e.g., 150-200°C) under an inert atmosphere. The Cs-oleate precursor is swiftly injected into this reaction flask, triggering instantaneous nucleation.
• Growth and Crystallization: The reaction temperature is maintained for a short period (seconds to minutes) to allow controlled crystal growth and Mn²⁺ ion incorporation into the CsPbCl₃ host lattice [62].
• Purification: The reaction is cooled using an ice bath, and the resulting QDs are purified by centrifugation with anti-solvents like acetone or toluene/hexane mixtures [62].

2. Post-Synthetic Cation Doping in Flow: For enhanced control and scalability, a continuous microfluidic platform can be employed for ultrafast post-synthetic Mn-doping [63].

• Platform Setup: A modular microfluidic reactor is equipped with in-line spectral monitoring.
• Process: Pre-synthesized, purified CsPbCl₃ QDs and a Mn²⁺ precursor solution (e.g., MnCl₂) are introduced into the system.
• Doping Mechanism: The doping proceeds via a heterogeneous surface mechanism assisted by cation vacancies, allowing precise tuning of optical properties through flow rate and concentration control [63].

3. Stability Enhancement via Silica Coating: To address stability issues, Mn-doped QDs can be further passivated with a silica shell [61].

• Shell Formation: Tetramethoxysilane (TMOS) is used as a silica precursor. Its fast hydrolysis rate is leveraged to form a protective SiO₂ layer around the Mn:CsPbCl₃ QDs before water-induced degradation can occur [61].
• Integration: The resulting core-shell Mn:CsPbCl₃@SiO₂ particles are integrated with a UV-pump LED chip to fabricate a photoluminescent QLED device [61].

Synthesis of Tin-Based Halide Perovskites

1. Synthesis of CsSnX₃ QDs: While specific protocols for CsSnX₃ are less detailed in the provided results, the general synthesis follows similar solution-phase methods as lead-based perovskites, but with critical modifications to handle the Sn²⁺ oxidation state [60].

• Key Consideration - Inert Atmosphere: All synthesis and purification steps must be conducted under a strict inert atmosphere (e.g., nitrogen or argon glovebox) to prevent the oxidation of Sn²⁺ to Sn⁴⁺, which is a primary cause of poor stability and performance in tin-based perovskites [60].
• Precursor and Injection: Tin(II) halide (e.g., SnI₂, SnBr₂) is used as the B-site precursor. It is dissolved along with Cs⁺ precursors in suitable solvents. The hot-injection or ligand-assisted reprecipitation (LARP) method is then used for crystallization [50] [60].
• Stabilization Strategies: Reducing agents (e.g., SnF₂) are often incorporated into the precursor solution to mitigate oxidation, and strong coordinating ligands are used to passivate surface defects [60].

Visualization of Synthesis Pathways and Material Structures

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagents for Lead Reduction Strategies in PQD Research

Reagent Name	Function in Research	Application in Strategy
Manganese Chloride (MnCl₂)	Source of Mn²⁺ dopant ions for incorporation into the perovskite host lattice [61] [62]	Mn-Doping
Tin(II) Halides (SnI₂, SnBr₂)	Pb-replacement precursor providing the B-site cation in lead-free perovskite structures [60]	Tin Substitution
Tetramethoxysilane (TMOS)	Silica precursor for forming a protective shell around QDs; fast hydrolysis rate is advantageous [61]	Stability Enhancement (Mn-Doping)
Oleic Acid (OA) & Oleylamine (OAm)	Surface ligands that control nanocrystal growth, stabilize colloidal solutions, and passivate surface defects [62]	Universal (Both Strategies)
Cesium Carbonate (Cs₂CO₃)	Common source of Cs⁺ cations for all-inorganic perovskite compositions (e.g., CsPbX₃, CsSnX₃) [62]	Universal (Both Strategies)
Tin Fluoride (SnF₂)	Additive used to suppress the oxidation of Sn²⁺ to Sn⁴⁺ in tin-halide perovskite inks, improving film quality and stability [60]	Tin Substitution
1-Octadecene (ODE)	High-boiling, non-coordinating solvent used as a reaction medium in high-temperature synthesis (e.g., hot-injection) [62]	Universal (Both Strategies)

The exceptional optoelectronic properties of perovskite quantum dots (PQDs), including high photoluminescence quantum yield (PLQY), narrow emission linewidths, and tunable bandgaps, have positioned them as leading materials for next-generation displays and lighting technologies [64] [58] [2]. However, their commercial deployment has been persistently hindered by a critical vulnerability: operational instability under environmental stressors such as moisture, oxygen, heat, and light [64] [13]. This comparative guide examines two paramount strategies—matrix integration and surface ligand engineering—developed to overcome this limitation. By objectively analyzing experimental data and methodologies, this review provides researchers with a foundational understanding of how these approaches independently and synergistically enhance PQD stability for practical applications, particularly in light-emitting diodes (LEDs) and displays.

Comparative Analysis of Stabilization Strategies

Matrix Integration and Encapsulation

Matrix integration involves embedding PQDs within a protective inert material to create a physical barrier against environmental degradation.

Mesoporous Silica (MS) Encapsulation: A prominent example of this approach is the synergistic surface passivation and matrix encapsulation of CsPbBr3 QDs within a mesoporous silica (MS) matrix [64]. In this high-temperature solid-state method, precursors diffuse into MS channels, and subsequent sintering at 650°C triggers pore collapse, forming a dense, hermetically sealed silica matrix around the QDs. This physical encapsulation is complemented by chemical passivation using the sulfonic acid-based surfactant SB3-18, which coordinates with unpassivated Pb2+ sites on the QD surface to suppress trap states [64].

• Experimental Protocol: The synthesis involves grinding CsBr and PbBr2 precursors with mesoporous silica in a 1:3 mass ratio, followed by calcination in a muffle furnace at 650°C for 60 minutes. The passivator SB3-18 is introduced during this process [64].
• Performance Data: This dual-action strategy resulted in a composite with a PLQY of 58.27%, a significant increase from the initial 49.59%. The CsPbBr3-SB3–18/MS composite demonstrated exceptional stability, retaining 95.1% of its initial photoluminescence intensity after water resistance testing and 92.9% after light radiation aging tests [64]. When deployed in a liquid crystal display (LCD) backlit white LED, the composite achieved a color gamut covering 125.3% of the NTSC standard and 93.6% of the Rec.2020 standard [64].

Silica Coating: Beyond MS templates, direct silica coating is another effective encapsulation technique. One study reported the synthesis of silica-coated CsPbBr3 QDs that exhibited a high PLQY of 93.6% and a narrow emission linewidth of less than 20 nm, making them suitable for high-resolution micro-patterning directly onto blue GaN LED substrates [7]. This conformal coating effectively isolates the PQDs from the environment, enhancing their stability during subsequent device fabrication processes.

Surface Ligand Engineering

Surface ligand engineering focuses on modifying the molecular capping agents on the PQD surface to passivate ionic defects and improve intrinsic stability.

Ligand Modification of CsPbI3 PQDs: A systematic study on CsPbI3 PQDs investigated the impact of various ligands—trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), and L-phenylalanine (L-PHE)—on optical properties and stability [13]. These ligands coordinate with undercoordinated Pb²⁺ ions and other surface defects, suppressing non-radiative recombination pathways.

• Experimental Protocol: Red-emitting CsPbI3 PQDs were synthesized via a hot-injection method with precise control over temperature, precursor injection volume, and reaction duration. Ligand modification was achieved by introducing TOP, TOPO, or L-PHE during the synthesis process [13].
• Performance Data: The study found that the optimal synthesis temperature was 170°C, and a hot-injection volume of 1.5 mL yielded the highest PL intensity. Ligand passivation resulted in PL enhancements of 3%, 16%, and 18% for L-PHE, TOP, and TOPO, respectively [13]. Notably, L-PHE-modified PQDs demonstrated superior photostability, retaining over 70% of their initial PL intensity after 20 days of continuous UV exposure [13].

Acid-Based Ligand Exchange: For blue-emitting PQDs, which face significant stability challenges, acid etching-driven ligand exchange has been employed to achieve ultralow trap density, resulting in high-luminance and stable pure-blue LEDs [2]. This approach improves the charge injection and operational stability of the resulting devices.

Table 1: Comparative Performance of Stabilized PQDs for Display Applications

Material System	Stabilization Approach	Key Performance Metrics	Stability Performance	Application Demonstrated
CsPbBr3-SB3-18/MS [64]	Synergistic Chemical Passivation & MS Encapsulation	PLQY: 58.27%	95.1% PL retention (water resistance); 92.9% PL retention (light aging)	LCD Backlight (125.3% NTSC)
Silica-coated CsPbBr3 [7]	Silica Coating	PLQY: 93.6%; FWHM: <20 nm	High stability for micro-patterning	Color Conversion Layer on micro-LED
CsPbI3 with L-PHE [13]	Surface Ligand Engineering (Amino Acid)	PL Enhancement: +3%	>70% PL retention after 20 days UV	Optoelectronic Devices
CsPbI3 with TOPO [13]	Surface Ligand Engineering (Phosphine Oxide)	PL Enhancement: +18%	N/S	Optoelectronic Devices

Experimental Protocols for Key Techniques

Protocol: Mesoporous Silica (MS) Encapsulation of CsPbBr3 QDs

This protocol is adapted from the synergistic surface passivation and matrix encapsulation method [64].

• Precursor Preparation: Weigh CsBr and PbBr2 powders in a 1:1 molar ratio. Separately, weigh mesoporous silica (MS) so that the mass ratio of (CsBr + PbBr2) : MS is 1:3.
• Grinding: Combine the precursors and MS in an agate mortar and grind thoroughly until a homogeneous mixture is achieved.
• Calcination: Transfer the mixture to a crucible and calcine it in a muffle furnace at 650°C for 60 minutes. During this high-temperature treatment, the precursors liquefy/vaporize, diffuse into the MS pores, and crystallize into QDs. Concurrently, the MS template softens and collapses, forming a dense protective matrix.
• Integration of Passivator: The sulfonic acid-based surfactant SB3-18 is introduced during the process, where it coordinates with unsaturated Pb2+ sites on the QD surface to passivate trap states.
• Post-processing: The resulting solid composite is collected and can be ground into a fine powder for use in color conversion layers, for example, in LCD backlights [64].

Protocol: Surface Ligand Engineering of CsPbI3 PQDs

This protocol is based on the study of ligand effects on CsPbI3 PQDs [13].

• Synthesis Setup: Prepare a three-neck flask with a mixture of 1-octadecene (ODE), oleic acid (OA), and oleylamine (OLA). Purge the system with an inert gas (e.g., N2) and heat under stirring.
• Optimized Reaction Conditions: Heat the reaction medium to the optimal temperature of 170°C.
• Hot-Injection: Rapidly inject a preheated (e.g., 1.5 mL) Cs-oleate precursor solution into the reaction flask.
• Ligand Modification: Introduce the chosen ligand (e.g., TOP, TOPO, or L-PHE) into the reaction mixture. The reaction is allowed to proceed for a specific duration (e.g., 10 seconds) for nanocrystal growth and ligand binding.
• Purification: Cool the reaction mixture rapidly using an ice bath. Precipitate the PQDs by adding an anti-solvent (e.g., methyl acetate) and separate them via centrifugation. This step may be repeated to remove excess ligands and reaction byproducts.
• Storage: Re-disperse the purified PQDs in a non-polar solvent like toluene or hexane for further characterization and device fabrication [13].

The following workflow diagram illustrates the key decision points and steps involved in selecting and implementing these stabilization strategies for PQD LEDs.

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagents for PQD Stabilization Research

Reagent / Material	Function / Role	Example Use Case
Mesoporous Silica (MS) [64]	Rigid encapsulation matrix; provides a scaffold for QD nucleation and forms a dense protective barrier upon high-temperature sintering.	High-temperature solid-state synthesis of CsPbBr3 composites.
SB3-18 Surfactant [64]	Sulfonic acid-based passivator; coordinates with unpassivated Pb²⁺ sites on the QD surface to suppress trap states.	Chemical passivation in synergistic approach with MS encapsulation.
Trioctylphosphine Oxide (TOPO) [13]	Lewis base ligand; passivates surface defects by coordinating with Pb²⁺ ions, reducing non-radiative recombination.	Surface ligand engineering for CsPbI3 PQDs, showing 18% PL enhancement.
L-Phenylalanine (L-PHE) [13]	Amino acid ligand; acts as a bidentate ligand for surface passivation, offering enhanced photostability.	UV-stable CsPbI3 PQDs, retaining >70% PL after 20 days.
Trioctylphosphine (TOP) [13]	Lewis base ligand; similar to TOPO, it binds to surface atoms to pacify defects and improve PLQY.	Ligand modification for CsPbI3 PQDs, showing 16% PL enhancement.
Oleic Acid (OA) / Oleylamine (OLA) [13]	Standard capping ligands; control nanocrystal growth during synthesis and provide initial colloidal stability.	Used in virtually all hot-injection and ligand-assisted reprecipitation syntheses of PQDs.

The pursuit of stable PQDs for commercial optoelectronics has yielded two compelling, complementary strategies. Matrix integration, exemplified by mesoporous silica encapsulation, provides an unparalleled physical barrier against environmental stressors, making it ideal for applications requiring extreme durability, such as high-brightness displays [64]. Conversely, surface ligand engineering directly targets the intrinsic ionic defects of PQDs, offering a powerful route to boost luminescent efficiency and photostability, which is crucial for high-performance LEDs [13] [2]. The most promising path forward, as demonstrated by the synergistic SB3-18/MS system, lies in the rational combination of these approaches. By integrating robust matrix encapsulation with sophisticated molecular passivation, researchers can simultaneously address extrinsic and intrinsic degradation pathways, paving the way for the widespread adoption of perovskite quantum dots in next-generation technologies.

The pursuit of high-performance perovskite quantum dot light-emitting diodes (PQD LEDs) represents a major frontier in next-generation display and lighting technologies. Achieving optimal device performance requires a delicate balance between superior charge transport and effective defect passivation. While passivation strategies are essential for enhancing photoluminescence quantum yield (PLQY) and stability, they can simultaneously introduce insulating layers that hinder charge injection and transport, ultimately compromising electroluminescent efficiency. This comparison guide provides a systematic evaluation of recent surface engineering approaches, analyzing their efficacy in reconciling this fundamental trade-off to advance PQD LED technology.

Comparative Analysis of Passivation Strategies

The table below compares four advanced passivation strategies, highlighting their core mechanisms, impacts on charge transport, and resulting device performance.

Table 1: Performance Comparison of Recent Passivation Strategies for PQD LEDs

Passivation Strategy	Material System	Key Mechanism	Impact on Charge Transport	Reported Peak EQE	Key Stability Metric
Dual-Interface Molecular Passivation [65]	CsPbBr₃ with 4-MPy/2-MPy	Solvent-free rub-on transfer of regioisomers; 4-MPy stabilizes NiOₓ HTL, 2-MPy coordinates with surface Pb²⁺	Improves hole injection, enhances carrier confinement, reduces leakage	24.67%	~10x longer operational half-life at 1000 cd/m²
In Situ Molecular Passivation [66]	Thermally evaporated blue perovskite (CsPbBrₓCl₃₋ₓ) with BUPH1	Co-evaporation of phenanthroline-based molecule; bidentate coordination with under-coordinated Pb²⁺ ions	Improves charge balance; reduces trap-assisted recombination	3.10% (pure blue)	Excellent spectral stability under electrical bias
Aromatic Ligand Engineering [29]	Blue CdZnSe/ZnSe QDs with 3-Fluorocinnamate (3-F-CA)	Short-chain aromatic ligand replacing oleic acid; enhances inter-dot π-π interactions	Boosts carrier mobility and conductivity within the QD film	24.1%	Extrapolated T₉₅ lifetime of 54 h at 1000 cd/m²
Ligand & Encapsulation [46]	CH₃NH₃PbBr₃ PQDs with Oleic Acid/ZrO₂/MOFs	Organic ligand passivation combined with metal-oxide/MOF encapsulation	Maintains charge injection while protecting against environmental degradation	>20% (typical for green)	>80% PL retention in ambient conditions

Key Performance Trade-offs and Synergies

• Efficiency vs. Stability: The data demonstrates that strategies simultaneously addressing multiple interfaces, such as dual-interface passivation, achieve superior efficiency-stability synergy [65]. In contrast, single-interface treatments often lead to charge injection imbalances that limit performance.
• Color-Specific Challenges: Blue-emitting devices consistently report lower EQEs compared to green and red counterparts, underscoring the heightened difficulty in managing the wider bandgap and higher defect density in blue-emitting perovskites [29] [66].
• Material-Dependent Efficacy: The performance of any passivation strategy is highly dependent on the specific material system. For instance, inorganic CsPbBr₃ requires tailored approaches different from hybrid perovskites like CH₃NH₃PbBr₃ due to the absence of organic cations that can screen defects [46] [65].

Experimental Protocols and Methodologies

This section details the experimental procedures for implementing the key passivation strategies discussed, enabling replication and comparative analysis.

Solvent-Free Dual-Interface Molecular Passivation

Table 2: Protocol for Dual-Interface Molecular Passivation via Rub-On Transfer

Step	Process	Key Parameters	Function
1. Substrate Preparation	Clean NiOₓ-coated substrate	Oxygen plasma treatment	Ensures a clean, hydrophilic surface
2. Buried Interface Treatment	Rub-on transfer of 4-MPy	20-minute treatment time	Stabilizes Ni³⁺ states, reduces oxygen vacancies
3. Perovskite Deposition	Spin-coating or evaporation	Standard film formation	Creates the emissive perovskite layer
4. Top Interface Treatment	Rub-on transfer of 2-MPy	Controlled pressure & time	Coordinates with under-coordinated Pb²⁺ on film surface
5. Device Completion	Thermal evaporation of ETL and cathode	Standard vacuum evaporation	Finalizes the full LED stack

This protocol utilizes a solvent-free rub-on transfer method to prevent the secondary defects commonly introduced by solution-based processing [65]. The regioisomers 4-MPy and 2-MPy are molecularly tailored for their specific interfaces: 4-MPy at the buried NiOₓ/perovskite interface to improve hole injection, and 2-MPy on the perovskite surface to passivate defects and enhance carrier confinement.

In Situ Passivation during Thermal Evaporation

This protocol is designed for vacuum-deposited pure-blue PeLEDs, where defect formation is pronounced due to high-temperature precursor evaporation [66].

• Precursor Preparation: Load source materials—PbBr₂, CsCl, CsBr, and the passivator BUPH1—into separate thermal evaporation crucibles.
• Vacuum System Setup: Place the substrate in a thermal evaporation chamber and pump down to a high vacuum (<3.0 × 10⁻⁶ Torr).
• Co-Evaporation: Simultaneously thermally evaporate all precursors. Critical deposition rates are:
  • PbBr₂: 0.5 Å/s
  • CsCl: 0.65 Å/s
  • CsBr: 0.3 Å/s
  • BUPH1: Precisely controlled to achieve optimal incorporation without disrupting crystallization.
• Film Formation: The precursors co-deposit and react on the substrate to form a polycrystalline CsPb(Br/Cl)₃ film. BUPH1 incorporates in situ, with its phenanthroline nitrogen atoms coordinating with under-coordinated Pb²⁺ ions to passivate halide vacancies during film growth.
• Device Completion: Subsequent layers (ETL, cathode) are deposited without breaking vacuum to complete the LED stack.

Aromatic Ligand Exchange for Blue QD Arrays

This protocol focuses on creating long-range ordered blue QD arrays for high-resolution patterning, addressing the challenge of weak inter-dot interactions [29].

• QD Synthesis: Synthesize blue-emitting CdZnSe/ZnSe/ZnSeS/ZnS core/shell QDs with oleic acid (OA) ligands.
• Ligand Exchange:
  • Precipitate and re-disperse the OA-capped QDs in a non-polar solvent.
  • Add a controlled amount (e.g., 0.7 μmol) of the short-chain aromatic ligand 3-Fluorocinnamate (3-F-CA).
  • Stir the mixture to allow 3-F-CA to replace OA on the QD surface.
• Array Fabrication via Capillary Bridge Confinement:
  • Deposit the QD solution with aromatic ligands as a continuous liquid film.
  • Use a micropillar template to segment the film into isolated capillary bridges.
  • Control solvent evaporation, guiding the three-phase contact lines (TPCLs) to slide inward, driven by enhanced π-π interactions between 3-F-CA-capped QDs.
  • This results in the assembly of a long-range ordered array of blue QD microstructures.

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for Passivation and Charge Transport Studies

Reagent / Material	Function in Research	Application Context
4-MPy & 2-MPy [65]	Dual-interface molecular passivators; regulate hole injection and surface defects	Solvent-free rub-on passivation for inorganic PeLEDs
BUPH1 [66]	Phenanthroline-based small molecule for in situ passivation of Pb²⁺ defects	Thermal evaporation of pure-blue perovskite films
3-Fluorocinnamate (3-F-CA) [29]	Short-chain aromatic ligand for enhanced inter-dot coupling and charge transport	Surface ligand engineering for blue QD arrays
F4-TCNQ [67]	p-type dopant for hole transport layers (e.g., PTAA)	Enhancing conductivity and charge injection in PQD LEDs
ZrO₂ / MOFs [46]	Encapsulation materials for environmental protection	Enhancing operational stability of PQD films and devices
Oleic Acid (OA) & Oleylamine (OAm) [46]	Standard surface ligands for colloidal synthesis and stabilization	Foundational synthesis and passivation of PQDs

Pathways for Performance Optimization

The interplay between passivation and charge transport can be visualized as a critical pathway towards achieving high-performance PQD LEDs. The following diagram synthesizes the key strategies and their functional roles discussed in this guide.

Diagram Title: Strategic Pathways for Optimizing PQD LEDs

This comparison guide systematically demonstrates that optimizing charge transport in PQD LEDs is not merely about maximizing passivation, but about intelligently designing it to minimize resistive losses and balance charge injection. The evaluated strategies—from dual-interface molecular tailoring and in situ vacuum passivation to aromatic ligand engineering—provide a robust toolkit for researchers. The optimal choice is highly dependent on the target emission color, material system, and fabrication process. Future breakthroughs will likely emerge from the continued development of multi-functional molecules and processing techniques that seamlessly integrate defect suppression with enhanced electrical transport, ultimately pushing the performance of PQD LEDs toward their theoretical limits.

Perovskite Quantum Dot Light-Emitting Diodes (PQD LEDs) represent a transformative advancement in optoelectronics, offering exceptional photoluminescence quantum yields (PLQYs) exceeding 96%, narrow emission linewidths as low as 14-36 nm, and broadly tunable emission spectra from 360-710 nm [68] [46]. These properties position PQDs as superior candidates for next-generation displays, solid-state lighting, and flexible optoelectronics. However, their commercial viability has been constrained by significant environmental challenges, including the inherent toxicity of lead-based precursors, rapid degradation under ambient conditions, and energy-intensive synthesis methods that generate substantial hazardous waste [68] [46].

The evolution toward green synthesis methodologies addresses these critical limitations through innovative approaches that minimize environmental impact while maintaining or enhancing optoelectronic performance. This paradigm shift encompasses eco-friendly solvent systems, lead-free perovskite compositions, energy-efficient synthesis protocols, and circular economy principles throughout the material lifecycle. When framed within the broader context of surface engineering approaches for PQD LEDs, these green synthesis strategies enable unprecedented control over quantum dot surface chemistry, interface engineering, and structural stability—factors that directly determine device efficiency, longevity, and commercial applicability [68] [55].

This comparative analysis systematically evaluates emerging green synthesis techniques against conventional methodologies, with particular emphasis on quantitative performance metrics, scalability parameters, and environmental impact assessments. By providing researchers with comprehensive experimental protocols and comparative data, this guide facilitates the adoption of sustainable practices in PQD research and development.

Comparative Analysis of Green Synthesis Methods for PQDs

Eco-Friendly Solvent Systems and Synthesis Approaches

Conventional PQD synthesis typically employs toxic nonpolar solvents such as toluene or octadecene, presenting substantial environmental and health risks. Green synthesis strategies have innovated effective alternatives using sustainable solvent systems and reduced-energy processes.

Ethyl acetate-based synthesis represents a significant advancement, where this polar, biodegradable solvent enables room-temperature synthesis of deep-blue CsPbBr₃ PQDs. This method achieves a respectable PLQY of 48.4% with an emission peak at 454 nm, while drastically reducing environmental impact compared to traditional approaches [68]. The polar nature of ethyl acetate enhances precursor solvation and nucleation control, enabling superior size distribution without toxic solvents.

Tartaric acid-assisted reprecipitation utilizes green ligands to achieve exceptional performance, with reported PLQYs reaching 88.24% for CH₃NH₃PbBr₃ PQDs [68]. Tartaric acid serves dual functions as a coordination ligand and passivating agent, effectively suppressing surface defects while providing environmentally benign capping. This approach demonstrates that carefully selected natural ligands can compete with or surpass conventional synthetic ligands in optoelectronic performance.

Solvent-free mechanochemical synthesis via ball milling completely eliminates solvent requirements, representing the ultimate in green chemistry principles. This technique achieves PLQYs of 72.15% while generating zero liquid waste [68]. The intense mechanical energy induces direct chemical transformations between solid precursors, offering exceptional scalability and minimal environmental footprint. This method is particularly suited for industrial-scale production where solvent recovery and disposal present significant cost barriers.

Table 1: Performance Comparison of Green PQD Synthesis Methods

Synthesis Method	PQD Composition	PLQY (%)	Emission Tunability	Key Advantages	Environmental Impact
Ethyl Acetate-Based	CsPbBr₃	48.4	Deep-blue (454 nm)	Room temperature, biodegradable solvent	Low toxicity, biodegradable solvent
Tartaric Acid Reprecipitation	CH₃NH₃PbBr₃	88.24	409-523 nm	Natural ligand, excellent passivation	Reduced hazardous waste
Solvent-Free Ball Milling	Various compositions	72.15	Adjustable via precursors	Zero solvent waste, highly scalable	Minimal environmental footprint
Conventional Hot-Injection	CsPbBr₃	80-96	450-520 nm	Established protocol, high crystallinity	Toxic solvents, high energy requirement

Lead-Free Perovskite Compositions and Their Performance

The development of lead-free alternatives addresses the most significant environmental concern regarding PQD commercialization—lead toxicity. Multiple innovative approaches have emerged with promising performance metrics.

Cs₃Bi₂Br₉ PQDs represent a viable lead-free alternative, achieving a PLQY of 50.1% with emission tunability across the 400-560 nm range and remarkable stability maintaining performance for over 60 days [68]. While their full width at half maximum (FWHM) of 40-60 nm is broader than lead-based counterparts, this bismuth-based system offers significantly reduced toxicity while maintaining reasonable efficiency. The enhanced stability originates from the more covalent nature of Bi-Br bonds compared to Pb-Br bonds, reducing ion migration and phase segregation.

Manganese doping strategies enable partial lead substitution while maintaining excellent optical properties. Incorporating 10-20% Mn²⁺ into CH₃NH₃PbBr₃ PQDs retains >90% PLQY while effectively halving lead content and doubling operational stability (T₅₀ > 1000 hours) [46]. The enhanced stability arises from stronger Mn-Br bonds (binding energy of 2.1 eV) compared to Pb-Br bonds, creating a more robust crystal lattice. This approach demonstrates that complete lead elimination may not be necessary if significant reduction can be achieved without compromising performance.

Tin-based CH₃NH₃SnBr₃ PQDs offer the most direct substitution isoelectronically, achieving reasonable external quantum efficiency (EQE) of approximately 15% through emulsion synthesis [46]. However, the susceptibility of Sn²⁺ to oxidation into Sn⁴⁺ caps achievable PLQY below 70%, presenting a significant challenge for this otherwise promising approach. Advanced encapsulation strategies may mitigate this oxidation tendency in future developments.

Table 2: Lead-Free Perovskite Quantum Dot Compositions and Properties

PQD Composition	Toxicity Level	PLQY (%)	Emission Range	Stability	Commercial Viability
Cs₃Bi₂Br₉	Low	50.1	400-560 nm	>60 days	High (RoHS compliant)
Mn²⁺-doped CH₃NH₃PbBr₃	Moderate	>90	409-523 nm	T₅₀ > 1000 h	Medium (reduced Pb content)
CH₃NH₃SnBr₃	Low	<70	Tunable visible	Limited by Sn oxidation	Medium (with encapsulation)
Conventional CsPbBr₃	High (Pb content)	80-96	450-520 nm	Moderate (requires encapsulation)	Low (RoHS non-compliant)

Surface Engineering Approaches for Enhanced PQD Stability

Encapsulation and Passivation Strategies

Surface engineering through advanced encapsulation represents a critical approach for enhancing PQD stability against environmental stressors while maintaining optical performance. These techniques create protective barriers that shield the sensitive perovskite crystal from moisture, oxygen, and thermal degradation.

Borophosphate glass encapsulation has demonstrated exceptional protective capabilities, enabling 94% photoluminescence retention after 240 hours of direct water immersion [68]. This remarkable performance stems from the dense, chemically inert nature of the glass matrix that creates an impermeable barrier while maintaining optical transparency. The encapsulation process involves low-temperature glass synthesis compatible with PQD integration, preventing thermal degradation during fabrication. This approach shows particular promise for outdoor lighting applications where environmental exposure is inevitable.

Metal-Organic Framework (MOF) encapsulation creates a nanoporous protective structure that selectively excludes moisture and oxygen molecules while permitting photoexcitation and emission. MOF-encapsulated PQDs demonstrate >80% PL retention under ambient conditions for extended periods, significantly outperforming unencapsulated counterparts which may degrade within hours [46]. The modular nature of MOF structures enables precise pore size engineering optimized for specific environmental threats, offering customizable protection strategies.

ZrO₂ and PMMA hybrid coatings provide robust protection through different mechanisms. ZrO₂ forms a rigid, impermeable shell that physically blocks environmental penetrants, while PMMA creates a flexible, conformal coating that accommodates minor lattice expansions. When combined in a core-shell architecture, these materials synergistically enhance stability, with studies showing prolonged LED operational lifetimes exceeding 1000 hours [46]. Additionally, PMMA coatings are designed for recyclability, enabling PQD recovery and reclamation through simple solvent processing, aligning with circular economy principles.

Advanced Stabilization Techniques for Flexible Applications

The emergence of flexible optoelectronics demands specialized stabilization approaches that withstand mechanical stress in addition to environmental challenges. Several innovative techniques have been developed specifically for flexible PQD LED applications.

Polymer-perovskite composites integrate PQDs within elastic polymer matrices such as polydimethylsiloxane (PDMS), creating stretchable luminous materials. These composites demonstrate exceptional mechanical durability, with devices maintaining >95% luminance after 5000 bending cycles at a 2.5 mm radius [55]. The polymer matrix distributes mechanical stress evenly throughout the composite, preventing crack formation and propagation that would typically plague brittle perovskite films. This approach enables truly wearable display applications where flexibility is paramount.

MXene composite electrodes represent another advancement for flexible devices, combining excellent conductivity with superior flexibility and thermal management. Devices incorporating MXene-based electrodes demonstrate exceptional heat dissipation capabilities, addressing a critical failure mechanism in flexible LEDs where limited thermal transport accelerates degradation [55]. The combination of silver nanoparticles, silver nanowires, and MXene in a PEDOT:PSS composite creates a hybrid material that optimizes both charge transport and mechanical resilience, enabling large-area flexible PeLEDs that maintain stable performance under repeated deformation.

Graphene electrode integration mitigates exciton quenching caused by metal atom diffusion from traditional indium tin oxide (ITO) electrodes. Graphene-based PeLEDs achieve a high current efficiency of 16.1 cd A⁻¹ and maximal luminance of nearly 13,000 cd m⁻², with current density maintaining 81% of initial value after 1200 bending cycles [55]. The chemical inertness and impermeability of graphene provide additional protection against electrode-induced degradation, while its mechanical flexibility ensures consistent performance in repeatedly flexed devices.

Performance Comparison and Applications of PQD LEDs

Optoelectronic Performance Metrics

Comprehensive performance evaluation reveals the remarkable progress achieved through green synthesis and surface engineering approaches, with some sustainable PQD LEDs now competing directly with conventional technologies.

External Quantum Efficiency (EQE) values for green-emitting PeLEDs have surpassed 32% in rigid devices, with flexible variants achieving impressive values of 24.5% [55]. These metrics approach or exceed state-of-the-art organic LEDs (OLEDs) and quantum dot LEDs (QLEDs), demonstrating that sustainable approaches need not compromise performance. Notably, manganese-doped CH₃NH₃PbBr₃ PQDs with reduced lead content achieve EQEs up to 27.1% [68], confirming that partial lead substitution can maintain high efficiency while addressing toxicity concerns.

Luminance levels for optimized PeLEDs now exceed 1.1 × 10⁶ cd m⁻² [55], satisfying requirements for high-brightness display applications including augmented reality and sunlight-readable interfaces. This represents a significant advancement from early perovskite devices that struggled to achieve practical brightness levels, with current performance enabling real-world application across diverse lighting conditions.

Color performance remains a standout feature of PQD LEDs, with narrow emission bandwidths of 14-25 nm enabling exceptional color purity that exceeds conventional technologies [46]. This narrow FWHM translates directly to wide color gamut coverage exceeding 127% of the NTSC standard [46], positioning PQD LEDs as the leading technology for next-generation high-fidelity displays. The precise emission tunability across the entire visible spectrum further enhances their applicability in customizable lighting solutions.

Operational stability has shown remarkable improvement through advanced encapsulation strategies, with CsPbI₃ LEDs now achieving operational lifetimes of 1001.1 minutes [68]—approaching commercially viable thresholds for certain applications. While still lagging behind mature LED technologies, this represents orders-of-magnitude improvement from early perovskite devices that degraded within minutes, demonstrating the critical role of surface engineering in enabling practical applications.

Emerging Applications and Device Architectures

The unique properties of PQD LEDs enabled by green synthesis and surface engineering approaches have unlocked diverse applications beyond conventional displays and lighting.

Wearable optoelectronics represent a rapidly expanding application domain, where flexible PeLEDs integrated into textiles and direct-skin interfaces enable health monitoring displays, interactive clothing, and biomedical sensing platforms [55]. The compatibility of low-temperature PQD synthesis with plastic substrates enables lightweight, conformable devices that maintain performance under mechanical deformation—essential characteristics for wearable technology.

Anticounterfeiting systems leverage the tunable emission and environmental responsiveness of specially engineered PQDs. Humidity-responsive PQD fibers demonstrate reversible emission changes under moisture exposure, creating invisible markers that reveal authentication under specific conditions [68]. Similarly, PQDs with stimulus-dependent spectral shifts enable multi-level security encryption with visual verification, particularly valuable for currency, document, and luxury goods protection.

Sustainable display technologies incorporating recyclable components align with circular economy principles. Device architectures employing polymethyl methacrylate (PMMA) coatings enable simple PQD recovery through solvent processing, with the reclaimed PbI₂ precursor maintaining performance upon reprocessing [68] [46]. This closed-loop approach addresses end-of-life concerns while reducing primary material consumption, potentially transforming the environmental profile of display manufacturing.

Experimental Protocols and Methodologies

Detailed Green Synthesis Procedures

Ethyl Acetate-Based Synthesis Protocol:

• Precursor Preparation: Dissolve Cs₂CO₃ (0.2 mmol) in ethyl acetate (10 mL) with oleic acid (0.5 mL) as a stabilizing ligand. Stir continuously at room temperature for 30 minutes until complete dissolution.
• Perovskite Formation: Inject PbBr₂ (0.1 mmol) in ethyl acetate (5 mL) dropwise into the cesium solution under vigorous stirring. Maintain reaction at 25°C with ambient atmospheric conditions.
• Purification and Isolation: Add anti-solvent (n-hexane) to precipitate PQDs, followed by centrifugation at 8000 rpm for 5 minutes. Decant supernatant and redisperse in fresh ethyl acetate. Repeat purification twice to remove unreacted precursors and ligand excess.
• Characterization: Determine PLQY using integrating sphere with calibrated spectrometer. Measure absorption spectrum to confirm target emission wavelength of 454 nm [68].

Solvent-Free Mechanochemical Synthesis:

• Reagent Preparation: Weigh precise stoichiometric ratios of CsBr (1.0 mmol) and PbBr₂ (1.0 mmol) precursors. For doped compositions, include MnBr₂ (0.2 mmol) for partial lead substitution.
• Ball Milling Process: Load precursors into stainless steel milling jar with grinding media (5 mm diameter zirconia balls) at 15:1 ball-to-powder mass ratio. Seal jar under inert atmosphere.
• Mechanochemical Reaction: Mill mixture at 400 rpm for 120 minutes using planetary ball mill. Monitor temperature, maintaining below 60°C through intermittent milling cycles if necessary.
• Post-Processing: Collect resulting powder and disperse in minimal ethyl acetate for separation from grinding media. Filter through 0.22 μm membrane to remove aggregates [68].

Encapsulation and Stabilization Methods

Borophosphate Glass Encapsulation Workflow:

• Glass Matrix Preparation: Mix boric acid (H₃BO₃), phosphorus pentoxide (P₂O₅), and zinc oxide (ZnO) in molar ratio 2:1:1. Melt mixture at 800°C for 2 hours in alumina crucible, then quench between cold stainless steel plates to form transparent glass frits.
• PQD-Glass Composite: Grind glass frits into fine powder and mix with pre-synthesized PQDs (10% w/w). Heat mixture to 300°C (below PQD decomposition threshold) under nitrogen atmosphere to form homogeneous composite.
• Film Formation: Hot-press composite between quartz plates at 200°C and 10 MPa pressure for 15 minutes to form optically transparent encapsulation layers of controlled thickness (100-500 μm) [68].

MOF Encapsulation Procedure:

• MOF Precursor Solution: Prepare zinc nitrate hexahydrate (5 mM) and 2-methylimidazole (25 mM) in methanol as MOF precursor solution.
• In Situ Encapsulation: Add pre-synthesized PQDs to MOF precursor solution at 1:100 mass ratio. Stir gently for 2 hours at room temperature to allow progressive MOF nucleation around PQD surfaces.
• Crystallization and Purification: Age mixture for 24 hours without disturbance to enable complete MOF crystal growth around PQDs. Centrifuge at low speed (3000 rpm) to collect encapsulated PQDs, washing with fresh methanol to remove unreacted precursors [46].

Diagram Title: Green Synthesis and Encapsulation Workflow for Sustainable PQD LEDs

Research Reagent Solutions and Essential Materials

Table 3: Essential Research Reagents for Green PQD Synthesis and Characterization

Reagent/Material	Function	Specific Application Example	Environmental Consideration
Ethyl Acetate	Green solvent	CsPbBr₃ PQD synthesis	Biodegradable, low toxicity alternative to toluene
Tartaric Acid	Natural ligand and passivator	CH₃NH₃PbBr₃ reprecipitation	Renewable sourcing, non-toxic decomposition
Manganese Bromide (MnBr₂)	Dopant for lead reduction	Mn-doped CH₃NH₃PbBr₃	Reduces Pb content by 10-20% while enhancing stability
Bismuth Tribromide (BiBr₃)	Lead-free precursor	Cs₃Bi₂Br₉ synthesis	Low toxicity, RoHS-compliant alternative
Borophosphate Glass	Encapsulation matrix	Moisture-resistant PQD films	Inert, recyclable protection layer
PMMA (Polymethyl methacrylate)	Polymer coating	Recyclable device architectures	Enables PQD recovery through solvent processing
Zirconium Oxide (ZrO₂)	Inorganic shell material	Core-shell nanostructures	Chemically inert, enhances thermal stability
PDMS (Polydimethylsiloxane)	Flexible matrix	Stretchable PeLED composites	Biocompatible, durable polymer substrate

Recycling and Lifecycle Management Approaches

PQD Recovery and Material Recycling Strategies

Sustainable PQD LED technology extends beyond synthesis to encompass end-of-life management through advanced recycling protocols that recover valuable materials and minimize environmental impact.

Closed-loop precursor recycling represents a significant advancement in circular economy approaches for perovskite optoelectronics. Research demonstrates that PbI₂ can be effectively recovered from retired devices through simple acid treatment and precipitation, with the reclaimed material maintaining performance equivalent to virgin precursors when reused in synthesis [68]. This recovery process achieves impressive reclamation rates exceeding 85% of the original lead content, dramatically reducing the environmental footprint and primary material consumption. The process involves device disassembly, selective dissolution of functional layers, and sequential precipitation of constituent materials for purification and reuse.

Polymer-enabled PQD reclamation utilizes specially designed coatings that permit quantum dot recovery while protecting performance during operation. Devices employing recyclable PMMA coatings allow simple solvent-assisted release of PQDs through mild swelling of the polymer matrix without damaging the quantum dots [46]. The reclaimed PQDs demonstrate >90% retention of initial PLQY after recovery and reintegration into new devices, creating a truly circular material flow. This approach fundamentally transforms the lifecycle environmental impact of PQD LEDs by eliminating the single-use paradigm that characterizes most electronic materials.

Industrial-scale LED recycling infrastructure is rapidly evolving to accommodate perovskite-containing devices, with major recyclers like ERI and Sims Recycling Solutions developing specialized processing streams for LED lighting products [69]. These systems employ automated disassembly, material separation, and component processing to recover valuable materials including aluminum, copper, rare earth elements, and semiconductor components. Current projections indicate that 75% of commercial sectors will implement comprehensive LED recycling programs by 2025, creating the necessary infrastructure for widespread PQD LED recycling at end-of-life [69].

Regulatory Framework and Compliance Considerations

The evolving regulatory landscape significantly influences recycling approaches and material selection for PQD LEDs, with several key policies shaping research and development priorities.

Basel Convention amendments effective January 2025 substantially impact electronics recycling through new e-waste classifications (entries A1181 and Y49) that expand controlled waste categories [70]. These changes require formal documentation, prior informed consent, and financial assurances for nearly all cross-border shipments of used electronics, creating both challenges and opportunities for perovskite device recycling. Researchers and manufacturers must implement enhanced traceability systems and establish transparent downstream processing partnerships to maintain compliance while enabling material recovery.

Restriction of Hazardous Substances (RoHS) compliance drives innovation in lead-free and reduced-lead perovskite compositions, with regulations limiting lead content in electronic products [46]. While exemptions currently exist for certain lighting applications, the development of RoHS-compliant alternatives like Cs₃Bi₂Br₉ represents a strategic priority for commercial viability. The manganese doping approach that halves lead content while maintaining performance offers a transitional strategy that partially addresses regulatory concerns while research continues on complete lead elimination.

Extended Producer Responsibility (EPR) regulations are expanding globally, requiring manufacturers to manage end-of-life processing of their products. This regulatory trend incentivizes design for recyclability approaches, including the development of easily separable device architectures and standardized material recovery protocols [69]. Progressive manufacturers are implementing take-back programs that facilitate closed-loop material flows, with retailers like Home Depot and Lowe's piloting LED collection initiatives that provide infrastructure for future PQD LED recycling [69].

Diagram Title: Circular Economy Framework for PQD LED Recycling and Reuse

The comprehensive comparison of green synthesis methods reveals a rapidly advancing field where sustainable approaches now compete with conventional methods across critical performance metrics. The demonstrated achievements—including PLQYs exceeding 88% with green solvents, operational stability surpassing 1000 hours through advanced encapsulation, and efficient material recycling with 90% performance retention—collectively establish a compelling case for widespread adoption of environmentally conscious PQD LED technologies.

Future research priorities should address remaining challenges, including scalability of lead-free alternatives to industrial production volumes, further enhancement of operational lifetimes to exceed 10,000 hours for display applications, and development of standardized recycling protocols that maximize material recovery efficiency. The ongoing integration of circular economy principles throughout the PQD LED lifecycle—from bio-derived precursors to closed-loop recycling—represents the most promising pathway for truly sustainable optoelectronics that maintain exceptional performance while minimizing environmental impact.

The progressive convergence of green synthesis methodologies, advanced surface engineering strategies, and comprehensive lifecycle management approaches positions PQD LEDs as a leading technology for next-generation displays and lighting, offering a template for sustainable development across the broader field of optoelectronics. As regulatory frameworks continue to evolve and consumer environmental awareness grows, these sustainable approaches will transition from competitive advantages to essential requirements for commercial success.

Performance Benchmarking and Comparative Analysis of Surface Engineering Approaches

Perovskite quantum dot light-emitting diodes (PQD-LEDs) have emerged as a leading technology for next-generation displays and lighting solutions, rivaling established technologies like organic LEDs (OLEDs) and conventional quantum dot LEDs (QD-LEDs). Their appeal lies in exceptional color purity, wide color gamut coverage, high photoluminescence quantum yield (PLQY), and compatibility with low-cost solution processing. The performance and stability of these devices are critically dependent on the properties of the PQDs themselves, which are governed by advanced surface engineering techniques. This guide provides a comparative analysis of key performance metrics—PLQY, full width at half maximum (FWHM), external quantum efficiency (EQE), and operational lifetime—across different surface engineering approaches for PQD-LEDs, framing the discussion within a broader thesis on comparative surface engineering. We summarize quantitative data from recent literature, detail experimental protocols, and provide visual workflows to aid researchers and scientists in evaluating these strategies for their development efforts.

Comparative Performance Metrics of Surface Engineering Approaches

Surface engineering aims to address the inherent instability of PQDs and their dynamic surface ligand chemistry, which leads to defect states and subsequent non-radiative recombination losses. The table below compares the performance outcomes of three prominent surface engineering strategies, as evidenced by recent experimental studies.

Table 1: Comparative Performance of PQD-LEDs via Different Surface Engineering Approaches

Surface Engineering Approach	Reported PLQY	Reported FWHM	Champion EQE	Stability / Lifetime Notes	Key Improvements Over Control
Chiral Ligand Exchange (Ultrasonic-Assisted) [24]	Information not specified in source	Information not specified in source	16.8% (R-LED), 16.0% (S-LED)	Information not specified in source	Enhanced spin selectivity (up to 89%); High electroluminescence dissymmetry factor (gEL ~0.28); Suppressed spin relaxation.
Bidentate Liquid Ligand (FASCN) Treatment [71]	Notable improvement post-treatment (specific value not provided)	~35 meV at 300 K	~23% (NIR-I LEDs)	Superior thermal stability (Δλ = 1 nm vs. control's 12 nm after heating); Stable in >99% humidity for 30 min.	8x higher film conductivity; 4x higher ligand binding energy; 2x higher exciton binding energy (76.3 meV).
MOF-Based Encapsulation (PCN-333(Fe)) [72]	6.5x enhancement over pure CsPbBr3 PQDs	Information not specified in source	Information not specified in source	Substantially improved air stability.	Effective passivation of uncoordinated Pb2+ defects; Uniform spatial distribution of PQDs.

Experimental Protocols for Featured Approaches

Ultrasonic-Assisted Chiral Ligand Exchange

This protocol is designed to imprint strong chirality and spin selectivity into CsPbBr3 PQDs while simultaneously enhancing their optoelectronic properties for high-performance spin-LEDs [24].

• Synthesis of Achiral PQDs: Achiral CsPbBr3 PQDs are first synthesized using a standard hot-injection method. This typically involves injecting a Cs-oleate precursor into a hot solution containing PbBr2 and ligands (oleic acid and oleylamine) in non-polar solvents [24].
• Ligand Exchange Solution Preparation: A solution containing the chiral ligands, such as R- or S-methylbenzylamine (R-/S-MBA), is prepared in a suitable solvent [24].
• Ultrasonic Treatment: The synthesized achiral PQD solution is mixed with the chiral ligand solution. The mixture is then subjected to ultrasonic treatment. This treatment physically agitates the solution, promoting the desorption of the original, long-chain oleic acid/oleylamine ligands and facilitating their replacement with the chiral MBA ligands [24].
• Purification: The resulting chiral PQDs are purified via centrifugation to remove excess ligands and reaction byproducts, yielding a stable ink ready for device fabrication [24].
• Device Fabrication & Characterization: Spin-coating is used to deposit thin films of the chiral PQDs onto appropriate substrates for LED fabrication. The performance is characterized by measuring the external quantum efficiency (EQE) and the electroluminescence dissymmetry factor (gEL) to quantify circularly polarized light emission [24].

Bidentate Liquid Ligand Treatment

This methodology focuses on achieving near-complete surface coverage of PQDs using a short-chain, strongly-coordinating ligand to minimize interfacial trap states and boost efficiency, particularly for near-infrared LEDs [71].

• QD Synthesis: FAPbI3 QDs (or other target compositions) are synthesized with traditional oleic acid (OA) and oleylammonium (OAm+) capping ligands [71].
• Post-Synthesis Ligand Treatment: The synthesized QDs are treated with a solution of the bidentate liquid ligand, formamidine thiocyanate (FASCN). This occurs after purification or during a film-processing stage. The liquid characteristic of FASCN allows its use without high-polarity solvents, which could damage the PQDs [71].
• Binding and Passivation: The FASCN molecules bind to the QD surface through coordinate bonds via both the soft sulfur (from SCN) and nitrogen (from the formamidine group) atoms. This bidentate binding mode results in a much higher binding energy compared to monodentate oleate ligands, ensuring tight attachment and effective passivation of uncoordinated Pb²⁺ sites [71].
• Film Formation and Analysis: The treated QD solution is spin-coated to form a thin film. The film's properties are analyzed through techniques like X-ray photoelectron spectroscopy (XPS) to confirm surface composition changes, and electrical measurements to demonstrate enhanced conductivity [71].
• Device Fabrication & Testing: NIR-LEDs are fabricated by integrating the FASCN-treated QD film as the emission layer. Key metrics such as EQE, operating voltage, and thermal stability are measured and compared to control devices made with oleate-capped QDs [71].

MOF-Based Encapsulation and Passivation

This strategy utilizes metal-organic frameworks (MOFs) as a stable host matrix to physically protect PQDs and chemically passivate their surface defects [72].

• Synthesis of MOF and PQDs: The metal-organic framework PCN-333(Fe) is synthesized separately, leveraging its large specific surface area and abundant carboxylate groups. Simultaneously, CsPbBr3 PQDs are synthesized via standard solution methods [72].
• Anchoring PQDs on MOF: A straightforward surface treatment method is employed where the pre-synthesized PQDs are mixed with the PCN-333(Fe) MOF particles. The abundant carboxylate groups on the MOF coordinate with the uncoordinated Pb²⁺ ions on the surface of the PQDs, forming a strong chemical anchor [72].
• Formation of Nanocomposite: This coordination interaction results in the stable formation of PQDs/MOFs nanocomposites, where the MOF acts as a host matrix that prevents PQD aggregation and provides a physical barrier against environmental factors like moisture and oxygen [72].
• Characterization: The nanocomposites are characterized using photoluminescence (PL) spectroscopy to quantify the enhancement in PL intensity and lifetime. Stability is assessed by monitoring PL retention over time when exposed to air [72].

Visualization of Surface Engineering Pathways

The following diagram illustrates the logical relationship between the surface engineering approaches, their mechanisms, and the resulting performance improvements in PQD-LEDs.

The Scientist's Toolkit: Essential Research Reagents

The experimental approaches described rely on specific reagents and materials critical for their success. The table below details key items and their primary functions in the context of PQD surface engineering and device fabrication.

Table 2: Key Research Reagents and Their Functions in PQD-LED Development

Reagent / Material	Function / Role	Application Context
Chiral Ligands (e.g., R-/S-MBA)	Imparts chirality and spin selectivity (via CISS effect) to PQDs; suppresses spin relaxation [24].	Chiral spin-LEDs for circularly polarized light emission [24].
Bidentate Liquid Ligand (FASCN)	Provides strong, dual-coordination binding to QD surface; achieves high coverage and passivation; short chain enhances conductivity [71].	High-efficiency NIR-PQD-LEDs; general surface passivation to reduce traps [71].
Metal-Organic Frameworks (e.g., PCN-333(Fe))	Acts as a host matrix; passivates surface defects via coordination; enhances stability and prevents aggregation [72].	PQD/MOF nanocomposites for stable LEDs, sensors, and photocatalysis [72].
Oleic Acid (OA) / Oleylamine (OAm)	Standard long-chain ligands for synthesizing and stabilizing colloidal PQDs; dynamic binding can lead to desorption [72] [71].	Initial synthesis of high-quality PQDs; often replaced or supplemented in surface engineering [72] [71].
Cesium Lead Halide (CsPbX₃) Precursors	Forms the inorganic scaffold of the perovskite nanocrystal; composition (X = Cl, Br, I) determines bandgap and emission wavelength [72] [73].	Base material for all-inorganic PQD synthesis across the visible spectrum [72] [73].

In perovskite quantum dot (PQD) research, surface passivation is a critical determinant of device performance and stability. The ionic nature of PQDs creates a high density of surface defects, such as uncoordinated lead atoms and halide vacancies, which act as non-radiative recombination centers that quench photoluminescence and limit the efficiency of optoelectronic devices like light-emitting diodes (LEDs) [30]. Ligand passivation directly addresses this challenge by binding to these surface sites, suppressing trap states, and enhancing optoelectronic properties [30]. The strategic choice between organic and inorganic ligands, each with distinct chemical interactions and material properties, forms a fundamental thesis in the surface engineering of PQDs. This guide provides a comparative analysis of these two ligand classes, supported by experimental data and protocols, to inform research decisions in PQD LED development.

Fundamental Mechanisms and Ligand Classifications

Organic Ligands

Organic ligands are typically carbon-based molecules that coordinate with the PQD surface via coordinate covalent bonds or electrostatic interactions.

• Long-Chain Alkyl Ligands: Conventional ligands like oleic acid (OA) and oleylamine (OAm) are staples in PQD synthesis. OA chelates with metal cations (e.g., Pb²⁺) on the PQD surface, while OAm binds to halide anions through hydrogen bonding [30]. However, their long carbon chains impose steric constraints, resulting in suboptimal surface coverage and dynamic binding that leads to facile detachment, creating surface defects [74] [30].
• Short-Chain/Aromatic Ligands: To improve charge transport, short-chain ligands like phenethylammonium iodide (PEAI) are employed. Their reduced steric hindrance allows for denser packing on the PQD surface. The conjugated phenyl group in PEA⁺ enhances defect passivation and improves inter-dot coupling, which is crucial for charge transport in device films [75].
• Bidentate Organic Ligands: Molecules like formamidine thiocyanate (FASCN) feature multiple atoms (sulfur and nitrogen) that can simultaneously coordinate with surface sites. This bidentate binding mode yields a binding energy approximately fourfold higher than that of OA/OAm, significantly reducing ligand loss and enabling near-complete surface coverage [36].

Inorganic Ligands

Inorganic ligands are metal salts or complexes that replace organic surfactants to create all-inorganic nanocrystal systems.

• Metal Halide Salts: Lead halides (e.g., PbI₂, PbBr₂) are widely used for post-synthetic treatment. They effectively passivate halide vacancies and provide a pathway for improved charge transport [76].
• Metal Salts with Non-Coordinating Anions: Salts such as Cd(NO₃)₂, Zn(BF₄)₂, and In(OTf)₃ utilize metal cations (Cd²⁺, Zn²⁺, In³⁺) to strip away original organic ligands. The cations bind strongly to Lewis basic sites (e.g., unpassivated surface atoms) on the PQD, while non-coordinating anions (NO₃⁻, BF₄⁻, OTf⁻) form a diffuse electrostatic double layer to stabilize the NCs in polar solvents without passivating surface sites [77].
• Oxide Coatings: Materials like SiO₂ can form an dense, amorphous protective shell around PQDs. This physical barrier significantly enhances stability against environmental factors such as moisture and heat without compromising the core's luminescent properties [74].

Table 1: Comparative Characteristics of Organic and Inorganic Ligands

Characteristic	Organic Ligands	Inorganic Ligands
Primary Binding Mode	Coordinate covalent, hydrogen bonding [30]	Ionic, electrostatic [77]
Typical Examples	OA, OAm, PEAI, FASCN [30] [75] [36]	PbI₂, Cd(NO₃)₂, SiO₂ coating [76] [77] [74]
Key Strengths	Excellent initial dispersion; tunable functionality; effective defect passivation [75] [36]	Superior charge transport; enhanced thermal/environmental stability [77] [74]
Inherent Limitations	Poor charge transport; dynamic binding/desorption; steric hindrance [30] [74]	Potential for incomplete passivation; complex processing; aggregation in film [77]

Experimental Protocols for Ligand Passivation

Organic Ligand Exchange with PEAI

Objective: To replace native long-chain ligands with phenethylammonium iodide (PEAI) for improved passivation and charge transport in CsPbI₃ PQD films [75].

• PQD Film Deposition: CsPbI₃ PQDs, synthesized via hot-injection and capped with OA/OAm, are dispersed in a non-polar solvent (e.g., octane). A layer-by-layer (LBL) spin-coating process is used to build the film. Each layer is spin-coated onto the substrate and subsequently treated with methyl acetate (MeOAc) to partially remove and exchange the original ligands.
• Solid-State Ligand Exchange: After the deposition of each PQD layer, a solution of PEAI (e.g., 2 mg/mL in ethyl acetate) is dynamically spin-coated onto the fresh film. This step is repeated for each layer in the LBL process, which is termed the PEAI-LBL method.
• Post-treatment and Annealing: The completed multilayer film is gently annealed at approximately 70°C for 5 minutes to remove residual solvent and enhance ligand ordering on the PQD surface.

Inorganic Ligand Exchange with Metal Salts

Objective: To create intensely luminescent all-inorganic CsPbBr₃ PQDs (ILANs) using metal salts for enhanced stability and charge transport [77].

• Phase Transfer Method:
  • A two-phase mixture is prepared. The organic phase contains the OA/OAm-capped PQDs in hexane. The polar phase contains the metal salt, such as In(NO₃)₃, dissolved in dimethylformamide (DMF).
  • The two phases are combined and vigorously stirred. The driving force for ligand exchange is the stronger binding affinity of the In³⁺ ions for the OA ligands compared to the PQD surface.
  • Upon completion, the PQDs transfer completely from the hexane phase to the DMF phase, indicating successful ligand exchange and the formation of cationic bare NCs.
• One-Phase Method:
  • A DMF solution containing the metal salt is added directly to a toluene solution of PQDs, creating a homogeneous mixture.
  • As the exchange proceeds, the all-inorganic PQDs precipitate out of solution.
  • The PQDs are isolated via centrifugation, redispersed in a polar solvent like DMF, and purified as needed.

Hybrid Organic-Inorganic Passivation

Objective: To synergistically combine the advantages of both ligand types for superior stability and performance in Cs₃Bi₂Br₉ PQDs [74].

• Organic Defect Passivation: First, the lead-free Cs₃Bi₂Br₉ PQDs are synthesized via an antisolvent method. The PQDs are then treated with didodecyldimethylammonium bromide (DDAB). The DDA⁺ cations bind to surface bromine anions, effectively passivating surface defects.
• Inorganic Encapsulation: Following DDAB passivation, an inorganic silica (SiO₂) shell is grown on the PQDs. This is typically achieved by adding a silica precursor, tetraethyl orthosilicate (TEOS), to the PQD solution and controlling its hydrolysis and condensation to form a conformal coating.
• Purification and Film Formation: The resulting core-shell Cs₃Bi₂Br₉/DDAB/SiO₂ PQDs are purified and can be used to fabricate devices, such as by depositing them as an emission layer in LEDs or as a down-conversion layer in solar cells.

Performance Comparison and Experimental Data

The effectiveness of passivation strategies is quantitatively assessed through key metrics such as photoluminescence quantum yield (PLQY), device efficiency, and stability.

Table 2: Quantitative Performance Comparison of Passivation Strategies

Ligand Strategy	Specific Ligand/System	Key Performance Metrics	Stability Assessment
Organic	PEAI on CsPbI₃ PQDs [75]	PCE: 14.18% (Solar Cell); High Voc: 1.23 V [75]	Excellent humidity stability (30-50% RH, unencapsulated) [75]
Organic	FASCN on FAPbI₃ QDs [36]	EQE: ~23% (NIR LED); Conductivity: 8x higher than control [36]	Good thermal stability (100°C, Δλ = 1 nm vs. control Δλ = 12 nm) [36]
Inorganic	Cd(NO₃)₂ on CdSe/ZnS QDs [77]	PLQY: 97% (Red, in DMF) [77]	Stable in polar solvents (DMF) [77]
Inorganic	PbI₂ on PbS CQDs [76]	PCE: 10.38% (Solar Cell, baseline) [76]	(Baseline stability)
Hybrid	DDAB/SiO₂ on Cs₃Bi₂Br₉ PQDs [74]	PCE retention: >90% after 8 hours [74]	Enhanced environmental stability [74]
Hybrid	PbI₂/PEABr on PbS CQDs [76]	PCE: 12.28% (Certified 11.98%) (Solar Cell) [76]	Suppressed hysteresis, improved operational stability [76]

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for Ligand Passivation Experiments

Reagent / Solution	Function / Role in Passivation	Example Application
Phenethylammonium Iodide (PEAI)	Short-chain organic ligand; passivates surface defects and improves inter-dot charge transport via its conjugated phenyl group [75].	Layer-by-layer solid-state ligand exchange for CsPbI₃ PQD films in solar cells/LEDs [75].
Formamidine Thiocyanate (FASCN)	Bidentate liquid ligand; provides high binding energy and full surface coverage, minimizing trap states and boosting conductivity [36].	Post-synthesis treatment of FAPbI₃ QDs for high-efficiency NIR LEDs [36].
Didodecyldimethylammonium Bromide (DDAB)	Organic ammonium salt; passivates surface halide vacancies due to strong affinity for bromide anions [74] [78].	Co-passivant in the synthesis of lead-free Cs₃Bi₂Br₉ PQDs [74].
Indium Nitrate (In(NO₃)₃)	Metal salt inorganic ligand; strips organic ligands and binds to Lewis basic sites on the PQD surface, preserving luminescence [77].	Creation of intensely luminescent all-inorganic PQDs (ILANs) stable in DMF [77].
Tetraethyl Orthosilicate (TEOS)	Precursor for inorganic SiO₂ coating; forms an amorphous, dense shell that acts as a physical barrier against environmental stressors [74].	Growth of a protective shell around organically passivated PQDs in hybrid strategies [74].
Methyl Acetate (MeOAc)	Antisolvent; used in purification and ligand exchange processes to remove original ligands and excess solvent [75].	Standard washing step in layer-by-layer deposition of PQD films [75].

The comparative analysis presented in this guide underscores that there is no single superior ligand chemistry for all PQD applications. The choice between organic and inorganic passivation is dictated by the specific performance priorities of the target device.

• Organic ligands excel in applications requiring high luminescence yield and color purity, with their tunable chemistry offering a powerful tool for specific defect passivation.
• Inorganic ligands are the preferred choice for devices where high charge carrier mobility and thermal stability are paramount, despite potential challenges with stability in polar environments and film formation.
• Hybrid passivation strategies, which strategically combine organic and inorganic ligands, are emerging as the most promising path forward [74] [76]. This approach leverages the defect-passivating prowess of organic molecules with the charge-transport and stability-enhancing properties of inorganic coatings or ligands.

Future research will likely focus on the rational design of novel multi-functional ligands, such as bidentate or zwitterionic molecules, that can simultaneously provide strong, stable binding and excellent charge transport [36]. Furthermore, developing more scalable and reproducible deposition techniques for hybrid passivation systems will be crucial for transitioning high-performance PQD LEDs from the laboratory to commercial applications.

Perovskite Quantum Dots (PQDs) have emerged as transformative materials in optoelectronics and photovoltaics, offering exceptional luminescent properties and tunable bandgaps. This guide provides a systematic comparison of three principal PQD systems: all-inorganic CsPbX3, organic-inorganic hybrid CH3NH3PbBr3 (MAPbBr3), and emerging lead-free alternatives. The analysis is framed within a broader thesis on surface engineering approaches for PQD-based Light-Emitting Diodes (LEDs), addressing critical factors such as photoluminescence quantum yield (PLQY), environmental stability, charge transport efficiency, and toxicity. Understanding the comparative advantages and limitations of these material systems is essential for selecting appropriate candidates for specific applications, from high-color-purity displays to environmentally benign sensors and energy-harvesting devices.

Performance Comparison and Experimental Data

The optoelectronic performance and stability of PQDs vary significantly across material systems, influenced by their compositional elements and structural integrity. The following tables summarize key quantitative metrics and stability parameters for the three PQD systems.

Table 1: Comparative Optoelectronic Performance of PQD Systems

Performance Parameter	CsPbX3 (X=Cl, Br, I)	CH3NH3PbBr3 (MAPbBr3)	Lead-Free Cs3Bi2Br9
PLQY (Maximum)	80-95% [79] [80]	>95% [46] [81]	~80% [74]
Emission Tunability	450-520 nm [46]	409-523 nm [46]	Blue emission at 485 nm [74]
FWHM (Narrowest)	14-36 nm [46]	14-25 nm [46]	Broader than Pb-based PQDs [74]
Bandgap (eV)	Tunable via halide composition [79]	~2.2 eV [46] [82]	Wider than MAPbBr3 [74]
Exciton Binding Energy	Moderate	~40 meV [46]	Not Specified
Carrier Mobility	Moderate	1-10 cm² V⁻¹ s⁻¹ [46]	Not Specified

Table 2: Stability and Toxicity Parameters

Parameter	CsPbX3	CH3NH3PbBr3	Lead-Free Cs3Bi2Br9
Thermal Stability	Excellent (Decomposition >300°C) [46]	Moderate [46]	Improved with coating [74]
Ambient Stability (PL Retention)	>80% after 100h at 85% RH [46]	Requires encapsulation [46] [81]	>90% efficiency retention after 8h [74]
Lead Content & Toxicity	High Pb toxicity [74] [83]	High Pb toxicity [46] [83]	Low toxicity (Bi-based) [74] [83]
Common Stability Strategies	MOF/ZrO₂ encapsulation [46]	h-BN encapsulation, ligand passivation [46] [81]	Organic DDAB + inorganic SiO₂ coating [74]

Experimental Protocols for Synthesis and Surface Engineering

Synthesis of CsPbX3 PQDs

The hot-injection method is a standard protocol for synthesizing high-quality, all-inorganic CsPbX3 PQDs [79]. The detailed procedure is as follows:

• Precursor Preparation: Synthesize a Cs-oleate precursor by reacting Cs₂CO₃ with oleic acid (OA) and 1-octadecene (ODE) at 120-130°C under a nitrogen atmosphere until the solution becomes clear [84] [80].
• Lead Halide Solution: In a separate flask, dissolve PbX₂ (X = Cl, Br, I) in a mixture of ODE, OA, and oleylamine (OAm). Heat this mixture to 120-150°C under vacuum until the PbX₂ is completely dissolved [84] [80].
• Reaction Injection: Rapidly inject the preheated Cs-oleate solution into the lead halide solution under vigorous stirring.
• Reaction Quenching: After a brief reaction period (5-10 seconds), cool the mixture immediately using an ice-water bath to terminate crystal growth [80].
• Purification: Precipitate the PQDs by adding a non-solvent like methyl acetate or butanol, followed by centrifugation. The pellet is then redispersed in a non-polar solvent like hexane or octane [84].

Synthesis and Stabilization of CH3NH3PbBr3 PQDs

The Ligand-Assisted Reprecipitation (LARP) method is a common, low-temperature technique for producing hybrid MAPbBr3 PQDs [46] [81].

• Precursor Solution: Dissolve MABr (methylammonium bromide) and PbBr₂ in a polar solvent like dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) [81].
• Stabilizing Ligands: Add coordinating ligands such as oleic acid and oleylamine to the precursor solution.
• Crystallization Trigger: Rapidly inject this precursor solution into a vigorously stirring non-solvent (e.g., toluene). The sudden change in solvent environment induces the instantaneous crystallization of PQDs [46].
• Encapsulation for Stability: To enhance stability, materials like hexagonal Boron Nitride (h-BN) can be incorporated. The h-BN is dispersed in toluene, and the perovskite precursor solution is injected into this dispersion, resulting in the encapsulation of PQDs within the layered structure of h-BN [81].

Synthesis and Passivation of Lead-Free Cs3Bi2Br9 PQDs

Lead-free Cs3Bi2Br9 PQDs are synthesized via an antisolvent method coupled with a hybrid passivation strategy [74].

• Precursor Preparation: Dissolve stoichiometric amounts of CsBr and BiBr₃ in dimethyl sulfoxide (DMSO) to form a transparent precursor solution.
• Ligand Assistance: Add organic ligands such as oleic acid and oleylamine to the precursor.
• Antisolvent Crystallization: Introduce an antisolvent (e.g., toluene) to trigger the crystallization of PQDs.
• Organic Passivation: Introduce Didodecyldimethylammonium bromide (DDAB) to passivate surface defects. The DDA⁺ cations have a strong affinity for halide anions, improving PLQY and initial stability [74].
• Inorganic Encapsulation: Subsequently, coat the DDAB-passivated PQDs with a silica (SiO₂) shell using tetraethyl orthosilicate (TEOS) as a precursor. This forms a core-shell (Cs3Bi2Br9/DDAB/SiO₂) structure, providing a dense, amorphous protective layer that significantly enhances environmental stability [74].

Signaling Pathways and Material Design Logic

The following diagram illustrates the logical decision-making process for selecting and engineering a PQD system based on application requirements, incorporating key surface engineering strategies.

The Scientist's Toolkit: Essential Research Reagents

This section details key chemicals and materials essential for the synthesis, passivation, and stabilization of perovskite quantum dots, as referenced in the experimental protocols.

Table 3: Essential Reagents for PQD Research

Reagent/Material	Function/Application	Examples from Literature
Oleic Acid (OA) & Oleylamine (OAm)	Common surface ligands for coordinating to PQD surfaces, controlling growth, and providing initial colloidal stability.	Used in hot-injection for CsPbX3 [84] [80] and LARP for MAPbBr3 [81].
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for synthesizing all-inorganic CsPbX3 and lead-free Cs3Bi2Br9 PQDs.	Reacted with OA to form Cs-oleate [84] [80].
Lead Bromide (PbBr₂) & Methylammonium Bromide (MABr)	Precursors for the B-site and A-site in MAPbBr3 perovskite synthesis.	Dissolved in DMF for the LARP method [81].
Bismuth Tribromide (BiBr₃)	Low-toxicity B-site precursor for lead-free perovskite systems (e.g., Cs3Bi2Br9).	Combined with CsBr in DMSO for antisolvent synthesis [74].
Didodecyldimethylammonium Bromide (DDAB)	Effective passivating ligand for defect suppression, used in both CsPbBr3 and Cs3Bi2Br9 PQDs.	Significantly enhances PLQY and water stability [74].
Tetraethyl Orthosilicate (TEOS)	Precursor for forming inorganic silicon dioxide (SiO₂) encapsulation shells.	Used to create a protective Cs3Bi2Br9/DDAB/SiO₂ core-shell structure [74].
Hexagonal Boron Nitride (h-BN)	2D material used as a stabilizing matrix for encapsulating PQDs, improving thermal and humidity stability.	Used to create h-BN/MAPbBr3 composites with high PLQY retention [81].

The choice between CsPbX3, CH3NH3PbBr3, and lead-free PQDs involves navigating a complex landscape of performance-stability-toxicity trade-offs. CsPbX3 offers a robust platform with superior thermal stability, while CH3NH3PbBr3 delivers exceptional color purity and PLQY but requires more intensive surface engineering for stabilization. Lead-free alternatives like Cs3Bi2Br9 present a promising, environmentally benign path, though they currently lag in absolute performance metrics. The future of PQD LEDs hinges on the continued refinement of surface engineering strategies—such as the hybrid organic-inorganic coatings and 2D material encapsulation detailed herein—to simultaneously push the boundaries of efficiency, longevity, and environmental compatibility.

In perovskite quantum dot light-emitting diodes (PQD-LEDs), the charge transport layers (CTLs) are not merely passive conduits for holes and electrons. Their compatibility with the perovskite quantum dot (PQD) emissive layer critically determines the ultimate balance of charge injection, the efficiency of radiative recombination, and the overall operational stability of the device [85]. Achieving high performance requires meticulous engineering of the CTLs to manage energy level alignment, mitigate interfacial defects, and suppress quenching phenomena. This guide provides a comparative analysis of key charge transport layer strategies, supported by experimental data and detailed methodologies, to inform the rational design of efficient and stable PQD-LEDs.

Charge Transport Layer Functions and Key Optimization Metrics

Charge transport layers in PQD-LEDs serve three primary functions: (1) facilitating the efficient injection of their respective charge carriers (holes for HTLs, electrons for ETLs) into the PQD emissive layer, (2) blocking opposite charge carriers to confine electron-hole pairs within the PQDs where they can recombine radiatively, and (3) providing a chemically and physically benign interface that minimizes non-radiative recombination and PQD luminescence quenching [85].

Key experimental metrics for evaluating CTL compatibility and device performance include:

• External Quantum Efficiency (EQE): The ratio of photons emitted from the device to electrons injected, representing the overall device efficiency [24] [47].
• Turn-on Voltage: The minimum applied voltage required to produce detectable light emission, indicative of injection efficiency and energy barrier heights [85].
• Luminance: The emitted light intensity, often reported as maximum value (cd m⁻²) [85].
• Current Efficiency: The ratio of luminous flux to current (cd A⁻¹) [85].

Comparative Analysis of Hole Transport Layer Strategies

The hole transport layer is particularly critical as it is often in direct contact with the PQD layer. Its properties can significantly impact fluorescence quenching and hole injection efficiency. The following table summarizes performance data for different HTL strategies.

Table 1: Comparative Performance of Different HTL Strategies in PQD-LEDs

HTL Strategy / Material	Device Structure	Key Performance Metrics	Mechanism & Compatibility Features
TFB as an Interlayer [85]	ITO / PEDOT:PSS / TFB / FAPbBr₃ PQDs / TPBi / LiF / Al	Turn-on Voltage: 2.2 VMax EQE: 0.153%Max Luminance: 4550 cd m⁻²	Blocks electrons; improves hole injection balance; prevents PQD quenching by separating PQDs from PEDOT:PSS.
NiOx Inorganic HTL [47]	ITO / NiOx / Perovskite EML / ETL / Cathode	Max EQE: ~14.6%	Superior electron blocking vs. PEDOT:PSS; reduces interfacial trap density.
PEDOT:PSS Only (Control) [85]	ITO / PEDOT:PSS / FAPbBr₃ PQDs / TPBi / LiF / Al	Turn-on Voltage: 3.01 VMax EQE: <0.153%	Direct contact with PQDs causes fluorescence quenching and inefficient hole injection.

Experimental Protocol: TFB HTL Optimization

The enhanced performance from incorporating a TFB layer is achieved through a precise fabrication and optimization protocol [85]:

• Substrate Preparation: ITO-coated glass substrates are sequentially cleaned with detergent, deionized water, ethanol, and acetone in an ultrasonic bath. They are then dried and treated with argon plasma for 11 minutes to improve surface wettability.
• HTL Deposition: PEDOT:PSS is spin-coated onto the ITO and annealed at 150°C for 20 minutes to form a thin film.
• TFB Interlayer Fabrication: A TFB solution in chlorobenzene is spin-coated onto the PEDOT:PSS layer. The thickness of the TFB layer is critically controlled by varying two parameters:
  • The concentration of the TFB precursor solution.
  • The rotational speed (rpm) of the spin-coating process. The optimal thickness, measured using a stylus profilometer, was determined to be ~35 nm.
• Emissive Layer Deposition: A layer of formamidinium (FA)-doped CsPbBr₃ PQDs is spin-coated directly onto the TFB layer.
• Completion of Device: An electron transport layer (TPBi), followed by a LiF/Al cathode, is thermally evaporated onto the PQD layer to complete the device architecture.

The insertion of the TFB layer, with its appropriate Highest Occupied Molecular Orbital (HOMO, ~ -5.3 eV) and Lowest Unoccupied Molecular Orbital (LUMO, ~ -2.3 eV) levels, improves energy level alignment with the PQDs. This facilitates hole injection from PEDOT:PSS into the PQDs while effectively blocking electrons, thereby forcing charge recombination to occur within the PQD layer [85].

Diagram: Charge Transport and Recombination Dynamics with a TFB Interlayer. The TFB layer facilitates hole transport while actively blocking electrons, confining carriers within the PQD layer for efficient radiative recombination.

Electron Transport Layer and Interfacial Engineering

While HTL optimization is crucial, a balanced injection requires equally efficient electron transport layers. Furthermore, the interfaces between the CTLs and the PQD emissive layer are critical sites for non-radiative losses.

Interfacial Defect Passivation

Defects at the interfaces and on the PQD surfaces themselves act as traps for charge carriers, promoting non-radiative recombination and reducing EQE. Surface ligand engineering is a powerful strategy to address this [24] [13].

• Ligand Exchange: Native ligands like oleic acid (OAm) on synthesized PQDs can be replaced with functional ligands that passivate under-coordinated lead ions (Pb²⁺) on the PQD surface [13].
• Performance Enhancement: Studies on CsPbI₃ PQDs show that passivation with trioctylphosphine oxide (TOPO) and l-phenylalanine (L-PHE) can improve photoluminescence (PL) by 18% and 3%, respectively, by suppressing non-radiative recombination [13]. Furthermore, an ultrasonic-assisted ligand exchange strategy for chiral PQDs was shown to significantly improve ligand coverage, which simultaneously enhanced spin selectivity (a key property for circularly polarized LEDs) and optoelectronic properties, leading to a high EQE of 16.8% [24].

Table 2: Impact of Surface Ligand Passivation on PQD Properties

Ligand / Strategy	Targeted Defect / Function	Experimental Outcome	Compatibility Consideration
Trioctylphosphine Oxide (TOPO) [13]	Coordinates with undercoordinated Pb²⁺ ions	PL enhancement of 18%	Improves film quality and reduces non-radiative losses at the ETL interface.
l-Phenylalanine (L-PHE) [13]	Surface defect passivation	PL enhancement of 3%; superior photostability (>70% initial PL after 20 days UV)	Enhances environmental stability, improving device lifetime.
Ultrasonic-assisted Ligand Exchange [24]	Improves chiral ligand coverage on PQDs	High EQE of 16.8% and high electroluminescence dissymmetry factor (0.285)	Synergistically improves spin selectivity and optoelectronic properties for advanced applications.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Materials for Investigating CTL Compatibility in PQD-LEDs

Material / Reagent	Function in Device Fabrication	Research Application
PEDOT:PSS	Hole-injection layer (HIL)	Serves as a common hole-injecting contact; often used as a baseline for evaluating new HTLs [85].
TFB (Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)])	Hole-transport layer (HTL)	Used to study the effects of energy level alignment, electron blocking, and the prevention of fluorescence quenching [85].
NiOx (Nickel Oxide)	Inorganic hole-transport layer	Investigated for its superior stability and electron-blocking capabilities compared to organic HTLs [47].
TPBi (2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole))	Electron-transport layer (ETL)	A widely used organic ETL for facilitating electron injection and hole blocking [85].
Trioctylphosphine Oxide (TOPO)	Surface passivation ligand	Employed in ligand exchange experiments to passivate surface defects on PQDs, thereby reducing non-radiative recombination [13].
Chiral Ligands (e.g., R-/S-MBA)	Impart chirality and surface passivation	Used to fabricate perovskite spin-LEDs with circularly polarized light emission; study of the interplay between surface chemistry and charge injection [24].

The compatibility of charge transport layers with the perovskite quantum dot emissive layer is a decisive factor in the performance of PQD-LEDs. As demonstrated, strategic interventions—such as inserting an optimized TFB interlayer to improve hole injection and block electrons, or employing advanced ligand exchange to passivate interfacial traps—can dramatically enhance key metrics like EQE, turn-on voltage, and luminance. The pursuit of optimal device architectures necessitates a holistic approach that integrates CTL material selection, meticulous interfacial engineering, and a deep understanding of charge dynamics. Future research will likely focus on developing novel, multifunctional CTLs and robust, scalable deposition techniques to further advance the commercial viability of this promising technology.

Color gamut coverage serves as a critical performance metric in display technology, quantifying the range of colors a screen can reproduce. For researchers and professionals developing next-generation displays, understanding these standards is essential for benchmarking new technologies like Perovskite Quantum Dot Light-Emitting Diods (PQD-LEDs). This guide provides a comparative analysis of prevalent color gamut standards—NTSC, sRGB, Adobe RGB, and DCI-P3—within the context of advanced surface engineering research for PQD-LEDs.

The pursuit of wider color gamuts is intrinsically linked to materials science, where surface ligand engineering of quantum dots directly enhances optical properties and stability. This relationship forms a core thesis in display research: advanced surface treatments enable quantum dots to meet and exceed established color standards, pushing the boundaries of color reproduction in commercial displays.

Color Gamut Standards: A Comparative Analysis

A color gamut defines a subset of colors within a perceptual color space that a display system can reproduce. The following table summarizes key color gamut standards and their relevance to modern display research and development.

Table 1: Key Color Gamut Standards and Specifications

Color Gamut Standard	Approximate NTSC 1953 Coverage	Primary Application Context	Key Characteristics
sRGB	~72% [86]	Web content, consumer electronics [86]	Default standard for digital content and web browsers [86].
NTSC 1953	100% (Reference)	Legacy television broadcasting [86]	Historical benchmark; less common in modern display specification [86].
Adobe RGB	~90% [86]	Professional photography, print media [86]	Expanded coverage in green-cyan regions, better alignment with CMYK printing [86].
DCI-P3	~86% [86]	Digital cinema, HDR content [86]	Wider red and yellow reproduction than sRGB; provides richer, more vibrant visuals [86].
Rec. 2020 (BT.2020)	>100% (Wider than NTSC)	Next-generation UHD TVs and displays [86]	Emerging standard covering a much wider range of perceivable colors [86].

It is crucial to distinguish between gamut area and gamut coverage. A display's gamut area is the total range of colors it can produce, while gamut coverage refers to the percentage of a specific standard's colors (e.g., DCI-P3) that the display can reproduce [87]. A specification of "95% DCI-P3 coverage" means the display can reproduce 95% of the colors in the DCI-P3 standard, not that its native gamut is 95% the size of DCI-P3 [87].

Surface Engineering Approaches for Enhanced Color Performance

Surface engineering of perovskite quantum dots (PQDs) is a pivotal research area for improving the performance and stability of PQD-LEDs, directly impacting their ability to achieve wide color gamuts. The following experimental workflow outlines a standard methodology for investigating surface ligand effects on PQD optical properties.

Diagram 1: Experimental workflow for PQD surface engineering and optical characterization.

Experimental Protocol: Ligand Modification of CsPbI3 PQDs

The following protocol is adapted from a study investigating the effect of surface ligand modification on the optical properties of CsPbI3 PQDs [13].

• Objective: To systematically investigate the influence of ligand engineering on the optoelectronic properties and environmental stability of CsPbI3 PQDs.
• Synthesis Procedure:
  • Precursor Preparation: Dissolve cesium carbonate (Cs₂CO₃) and lead iodide (PbI₂) in 1-octadecene with oleic acid and oleylamine as coordinating ligands [13].
  • Hot-Injection: Rapidly inject the cesium precursor into the lead precursor solution at a controlled temperature of 170°C [13].
  • Ligand Modification: Introduce ligand modifiers—trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), or l-phenylalanine (L-PHE)—during or immediately after synthesis to passivate surface defects [13].
  • Purification: Precipitate and wash the PQDs using a polar anti-solvent (e.g., methyl acetate) to remove unreacted precursors and excess ligands [13].
• Key Characterization Methods:
  • Photoluminescence (PL) Spectroscopy: Measure emission spectra, PL intensity, and full-width at half-maximum (FWHM) to assess color purity [13].
  • Photoluminescence Quantum Yield (PLQY): Determine the efficiency of light emission using an integrating sphere [13].
  • Stability Testing: Monitor the retention of PL intensity under continuous UV illumination over 20 days [13].

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagents for PQD Surface Engineering Experiments

Research Reagent	Function in Experiment
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for forming the CsPbI3 perovskite crystal structure [13].
Lead Iodide (PbI₂)	Lead and iodide source for the perovskite synthesis [13].
1-Octadecene	Non-polar solvent used as the reaction medium for high-temperature synthesis [13].
Oleic Acid & Oleylamine	Standard coordinating ligands that control crystal growth and prevent aggregation during synthesis [13].
Trioctylphosphine Oxide (TOPO)	Lewis base ligand for surface passivation; shown to enhance PL intensity by 18% [13].
Trioctylphosphine (TOP)	Lewis base ligand that coordinates with undercoordinated Pb²⁺ ions, enhancing PL by 16% [13].
L-Phenylalanine (L-PHE)	Amino acid ligand that provides effective surface passivation and superior photostability (≥70% PL retention after 20 days) [13].

Connecting Surface Engineering to Display Performance

The optical enhancements achieved through surface ligand engineering directly translate to the potential for superior display performance. High PLQY and narrow emission linewidths (FWHM) are fundamental for achieving the wide color gamuts defined by standards like DCI-P3 and Rec. 2020. The relationship between material properties, engineering approaches, and final display metrics can be visualized as a logical pathway from lab to display.

Diagram 2: Logical pathway from surface engineering to final display performance metrics.

Recent industry showcases confirm this progression. Displays utilizing advanced quantum dot technologies now routinely highlight high percentages of DCI-P3 coverage as a key selling point [88] [89]. For instance:

• Innolux showcased Micro LED applications with a wide color gamut of NTSC 115% and NTSC 120% [88].
• Samsung Display's latest RGB OLEDoS for XR devices boasts up to 99% DCI-P3 color gamut coverage [90].
• Vistar's 88-inch Micro LED modular display features 100% DCI-P3 cinema-grade color gamut coverage [88].

These industry achievements are built upon foundational research that optimizes the core light-emitting materials, demonstrating a direct link between successful surface passivation strategies in the lab and the ability to meet stringent commercial color standards.

Conclusion

Surface engineering emerges as a pivotal strategy for advancing PQD-LED technology, with ligand modification, compositional tuning, and encapsulation synergistically addressing the fundamental challenges of stability and efficiency. Comparative analysis reveals that ultrasonic-assisted ligand exchange significantly enhances both optoelectronic properties and spin selectivity in chiral PQDs, while hybrid organic-inorganic systems like CH3NH3PbBr3 offer exceptional processability and performance tunability. Lead reduction strategies through Mn-doping and the development of lead-free alternatives present viable pathways toward commercial adoption. Future directions should focus on scalable manufacturing processes, standardized stability testing protocols, and exploration of emerging applications in biomedical sensing and circularly polarized light sources. The integration of machine learning for predictive material design and advanced characterization techniques will further accelerate the development of high-performance, commercially viable PQD-LEDs for next-generation display and lighting technologies.

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