FAI vs. PEAI Post-Treatment for CsPbI3 Perovskite Quantum Dot Films: A Comparative Study on Surface Ligand Engineering

Abstract

This article provides a comprehensive comparative analysis of two dominant surface ligand exchange strategies—Formamidinium Iodide (FAI) and Phenethylammonium Iodide (PEAI) post-treatment—for CsPbI3 perovskite quantum dot (PQD) films. Aimed at researchers and scientists in photovoltaics and material science, it explores the fundamental chemistry, methodological protocols, and operational challenges associated with each ligand. The scope ranges from foundational principles of surface passivation and phase stabilization to advanced, application-oriented discussions on optimizing photovoltaic and optoelectronic device performance. By systematically validating and contrasting the efficiency, stability, and electronic properties imparted by FAI and PEAI, this study serves as a definitive guide for selecting and implementing ligand exchange protocols to develop high-performance, stable CsPbI3 PQD-based devices.

Understanding Ligand Chemistry and Its Role in CsPbI3 PQD Surface Passivation

CsPbI3 perovskite quantum dots (PQDs) have emerged as a leading material in the field of next-generation photovoltaics and optoelectronics. As all-inorganic nanomaterials, they combine the defect tolerance and excellent optoelectronic properties of perovskites with the quantum confinement effects and solution processability of quantum dots. Their intrinsic optical bandgap of approximately 1.73 eV makes them an ideal candidate for single-junction solar cells and as a top cell in all-perovskite tandem architectures [1]. Furthermore, CsPbI3 PQDs exhibit high photoluminescence quantum yields (PLQY), size-tunable emission, and high charge carrier mobility, making them equally promising for light-emitting diodes (LEDs) and other optoelectronic applications [2] [1].

Despite these advantages, the widespread application of CsPbI3 PQDs is challenged by one critical issue: phase instability. The photoactive cubic perovskite phase (α-CsPbI3) is metastable at room temperature and readily transitions into a non-perovskite, orthorhombic phase (δ-CsPbI3) with a much wider, non-functional bandgap of 2.82 eV [3] [1] [4]. This transition is accelerated by environmental factors such as moisture. Consequently, a central focus of CsPbI3 PQD research is the development of strategies to enhance the stability of the α-phase without compromising its superior electronic properties.

Surface Ligand Engineering: The FAI vs. PEAI Post-Treatment Comparison

A primary strategy to overcome the limitations of CsPbI3 PQDs is surface ligand engineering. The long-chain organic ligands (e.g., oleic acid and oleylamine) used in synthesis ensure colloidal stability but impede charge transport between QDs in a solid film [2] [5]. Replacing them with shorter ligands is essential for device performance. Among various options, Formamidinium Iodide (FAI) and Phenethylammonium Iodide (PEAI) post-treatments have been extensively studied. The table below provides a quantitative comparison of their impacts on PV performance and stability.

Table 1: Performance Comparison of FAI and PEAI Post-Treatments in CsPbI3 PQD Solar Cells

Treatment Parameter	FAI Post-Treatment	PEAI Post-Treatment	PEAI Layer-by-Layer (LBL)
Primary Function	A-site cation halide salt treatment for enhanced QD coupling [3]	Short-chain ligand for surface passivation and stability [2]	In-situ ligand exchange during film deposition [2]
Champion PCE	13.43% [3]	~15% (conventional post-treatment) [5]	14.18% [2]
Open-Circuit Voltage (VOC)	1.20 V [3]	Information missing	1.23 V [2]
Short-Circuit Current (JSC)	14.37 mA/cm² [3]	Information missing	Information missing
Stability	Information missing	>90% initial PCE after 18 days (ambient, unencapsulated with TPPO) [5]	Excellent stability under high humidity (30-50% RH, unencapsulated) [2]
Key Advantages	~60% increase in JSC; doubled film mobility; general method for various AX salts [3]	Enhanced defects passivation; hydrophobic PEA+ improves moisture resistance [2]	Balanced electron/hole transport; enables efficient electroluminescence (130 Cd/m²) [2]

Analysis of Comparative Data

The data reveals a clear trade-off. The FAI post-treatment excels at improving inter-dot electronic coupling and charge carrier mobility, leading to a significant boost in photocurrent (JSC) [3]. It acts as a general strategy to tune QD-QD junctions. In contrast, PEAI-based treatments demonstrate superior surface passivation and environmental stability. The hydrophobic phenyl ring in PEA+ creates a protective layer, enhancing resistance to moisture [2]. The advanced PEAI layer-by-layer (LBL) method, where the ligand exchange occurs during the deposition of each QD layer, represents a significant evolution. This approach ensures more uniform and complete passivation throughout the film bulk, not just the top surface, leading to a higher VOC and enabling the device to function efficiently as both a solar cell and a light-emitting diode [2].

Experimental Protocols for Post-Treatment

To replicate the research findings, detailed methodologies for the two primary post-treatment approaches are provided below.

FAI Post-Treatment Protocol

This protocol is adapted from the work that achieved a certified 13.43% PCE [3].

• QD Film Fabrication: Synthesize CsPbI3 PQDs via the hot-injection method. Deposit the QDs onto a substrate in a layer-by-layer fashion by spin-coating from a non-polar solvent (e.g., octane).
• Initial Ligand Exchange: After each layer deposition, immerse the film into a saturated lead(II) nitrate [Pb(NO₃)₂] solution in methyl acetate (MeOAc) for ~10 seconds to remove the original long-chain ligands partially.
• Film Building: Repeat steps 1 and 2 three to four times to build a thick absorber film (200-400 nm).
• AX Post-Treatment: Immerse the completed QD film into a saturated solution of FAI in ethyl acetate (EtOAc) for approximately 10 seconds.
• Drying: Gently dry the film to complete the process.

PEAI Layer-by-Layer (LBL) Protocol

This protocol is adapted from the work that achieved a 14.18% PCE and electroluminescence [2].

• QD Synthesis and Deposition: Synthesize OA/OAm-capped CsPbI3 PQDs and spin-coate a layer from octane.
• Anionic Ligand Exchange: Treat the freshly deposited layer with methyl acetate (MeOAc) to exchange anionic OA ligands with acetate ions.
• Cationic Ligand Exchange (LBL): Instead of a final post-treatment, immediately after the MeOAc step, treat the film with a solution of PEAI dissolved in a solvent (e.g., ethyl acetate). This step replaces the cationic OAm ligands with PEA+.
• Repetition for Thick Films: Repeat the cycle of spin-coating, MeOAc treatment, and PEAI treatment for a total of 3-5 cycles to build the thick, conductive PQD solid film.
• Final Processing: Proceed with the deposition of subsequent charge transport layers and electrodes to complete the device.

The following workflow diagram illustrates the key differences between these two experimental approaches.

The Scientist's Toolkit: Essential Research Reagents

This table catalogs the key chemical reagents used in the synthesis, processing, and optimization of CsPbI3 PQDs as discussed in the cited literature.

Table 2: Essential Research Reagents for CsPbI3 PQD Studies

Reagent Name	Chemical Function	Role in CsPbI3 PQD Research
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain carboxylic acid and amine	Primary capping ligands during colloidal synthesis to control growth and ensure monodispersity [2] [5] [6].
Formamidinium Iodide (FAI)	A-site cation halide salt (short-chain)	Post-treatment ligand to replace OAm, enhancing inter-dot coupling and charge carrier mobility in the solid film [3].
Phenethylammonium Iodide (PEAI)	Aromatic ammonium halide salt (short-chain)	Surface ligand for passivating defects and improving phase stability; the phenyl group confers hydrophobicity [2] [5].
Methyl Acetate (MeOAc)	Polar aprotic solvent	Washing solvent used in the initial ligand exchange to remove OA and other organics during layer-by-layer deposition [2] [3] [5].
Ethyl Acetate (EtOAc)	Polar aprotic solvent	Solvent for AX salts (like FAI and PEAI) in the final post-treatment or LbL cationic ligand exchange step [2] [3].
Trioctylphosphine Oxide (TOPO)	Lewis base coordinating ligand	Surface passivator for uncoordinated Pb²⁺ sites, suppressing non-radiative recombination and improving PLQY [7] [5].
2-Naphthalene Sulfonic Acid (NSA)	Sulfonic acid ligand	Strong-binding ligand to inhibit Ostwald ripening during synthesis, enabling small, stable, strong-confined QDs for pure-red LEDs [6].

The comparative study of FAI and PEAI post-treatments underscores that surface chemistry is the critical determinant in the performance and stability of CsPbI3 PQD devices. While FAI is highly effective at enhancing charge transport, PEAI, particularly via an LbL strategy, offers a more holistic solution by improving passivation, balancing charge transport, and significantly boosting environmental stability. Future research is likely to focus on hybrid treatment strategies that combine the strengths of different ligands, explore novel covalent ligands (like TPPO [5]), and develop advanced multi-functional ligands that can simultaneously address defect passivation, charge transport, and phase stabilization. Through such sophisticated ligand engineering, the full potential of CsPbI3 PQDs for stable, high-efficiency, and bifunctional optoelectronic devices can be realized.

The Critical Need for Surface Ligand Engineering in PQD Solids

Perovskite quantum dots (PQDs) have emerged as a revolutionary class of semiconductor nanomaterials with exceptional optoelectronic properties, including high photoluminescence quantum yield (PLQY), tunable bandgaps, and superior color purity. However, the transition from high-performing colloidal PQDs to stable, conductive solid films represents a significant bottleneck for commercial applications. The inherent instability of PQD solids stems from their ionic crystal nature and high surface-to-volume ratio, making them susceptible to degradation from environmental factors such as humidity, oxygen, and light exposure [8]. Surface ligand engineering has therefore become an indispensable strategy for modifying the PQD surface chemistry to enhance both stability and charge transport properties in solid-state films.

Ligands are molecules that coordinate to the surface of PQDs during synthesis, serving dual roles of controlling nanocrystal growth and passivating surface defects [8]. In traditional synthesis, long-chain insulating ligands like oleic acid (OA) and oleylamine (OAm) provide excellent colloidal stability but form electrically insulating barriers in solid films, severely limiting inter-dot charge carrier transport [9]. The core challenge lies in replacing these native ligands with shorter conductive alternatives while maintaining structural integrity and defect passivation. This comparative analysis examines the critical role of surface ligand engineering, with particular focus on formamidinium iodide (FAI) versus phenethylammonium iodide (PEAI) post-treatment strategies for CsPbI3 PQD films, to provide researchers with actionable insights for developing high-performance optoelectronic devices.

The Scientific Foundation of PQD Surface Chemistry

Crystal Structure and Instability Mechanisms

The fundamental crystal structure of all-inorganic CsPbI3 PQDs follows the ABX3 perovskite configuration, where Cs+ occupies the A-site, Pb2+ the B-site, and I- the X-site. The [PbI6]4- octahedra form a three-dimensional network, creating a framework that is inherently metastable at room temperature [1]. Phase stability can be predicted using the Goldschmidt tolerance factor (t) and octahedral factor (μ), with CsPbI3 having values (t = 0.89 and μ = 0.47) that border the stability region for the cubic phase [8]. This thermodynamic instability manifests as a rapid transition from the photoactive black phase (α, β, γ) to a non-perovskite yellow phase (δ) under ambient conditions, fundamentally limiting device longevity [1].

The high surface energy of nanocrystals further exacerbates this instability. In PQD solids, the large specific surface area means that surface atoms with unsaturated bonds (dangling bonds) create localized electronic states that act as traps for charge carriers [7]. These surface defects, particularly undercoordinated Pb2+ ions and halide vacancies, dominate non-radiative recombination pathways, reducing PLQY and accelerating degradation [8] [7]. The dynamic binding nature of traditional ligands allows for facile detachment, leaving vulnerable surfaces that facilitate ion migration and phase transformation [8].

The Dual Role of Surface Ligands

Surface ligands play a paradoxical role in PQD technology. During synthesis, they are indispensable for controlling nucleation, growth kinetics, and preventing aggregation through steric hindrance [8]. In application, however, these same ligands become detrimental when they impede charge transport between neighboring QDs in solid films [9]. This creates a fundamental trade-off: excellent colloidal stability versus efficient solid-state charge transport.

Traditional long-chain alkyl ligands like OA and OAm bind to the PQD surface through carboxylate and ammonium groups, respectively [8]. While effective for solution processing, their insulating carbon chains (typically C18) create potential barriers of several hundred meV between dots, severely limiting tunneling probabilities for charge carriers [9]. Furthermore, the binding dynamics of these ligands are highly labile, with constant adsorption-desorption equilibria leading to surface defect exposure over time [8]. Addressing this compromise requires sophisticated ligand engineering strategies that optimize both passivation and conductivity simultaneously.

Table 1: Common Ligand Types and Their Characteristics in PQD Solids

Ligand Type	Binding Mechanism	Advantages	Disadvantages
Long-chain alkyl (OA/OAm)	Carboxylate/Ammonium to surface ions	Excellent colloidal stability, proven synthesis	Poor charge transport, dynamic binding
Short conjugated (2PACz)	Phosphonate/Coordinate bonds	Enhanced charge transport, defect passivation	Limited solubility, complex processing
Halide-based (PEAI)	Ammonium to surface halides	Effective vacancy passivation, stability	Potential over-stabilization affecting structure
Dual-function (PhFACl)	Simultaneous A- and X-site binding	Comprehensive passivation, conductivity	Sophisticated synthesis required

Comparative Analysis of Ligand Engineering Strategies

Ligand Exchange Methodologies and Experimental Protocols

Effective ligand engineering employs either in situ approaches during synthesis or post-synthetic treatment protocols. Post-treatment strategies have gained prominence for their ability to transform as-synthesized PQDs into functional solids without compromising initial nanocrystal quality. A standardized experimental workflow typically involves:

PQD Synthesis and Film Fabrication: CsPbI3 PQDs are synthesized via hot-injection methods, with precise control over reaction temperature (140-180°C), precursor injection volume, and duration to optimize optical properties [7]. The resulting PQDs are precipitated, centrifuged, and redispersed in apolar solvents (e.g., hexane or octane) to form inks. Films are fabricated via layer-by-layer spin-coating, with each layer typically receiving a post-treatment step.

Antisolvent-Mediated Ligand Exchange: During film deposition, antisolvents with appropriate polarity are selected to remove native ligands without damaging the crystal structure. Methyl acetate (MeOAc) has been identified as particularly effective for FAPbI3 PQDs, optimally balancing ligand removal and structural preservation [10]. The antisolvent selectively dissolves and removes insulating OA/OAm ligands while introducing shorter alternative ligands.

Short Ligand Solution Treatment: Solutions of short ligands (e.g., FAI or PEAI) in optimized solvents are applied to the PQD film. Research has demonstrated that protic 2-pentanol, with its appropriate dielectric constant and acidity, maximizes insulating ligand removal while mediating effective binding of short ligands to the PQD surface [9]. This solvent environment facilitates a more complete ligand exchange without introducing halogen vacancy defects.

Thermal Annealing: Mild thermal treatment (typically 70-90°C) follows ligand exchange to enhance binding stability and remove residual solvents, ultimately improving inter-dot coupling and charge transport [10].

The following diagram illustrates this comprehensive experimental workflow:

FAI Versus PEAI Post-Treatment: A Comparative Analysis

Formamidinium iodide (FAI) and phenethylammonium iodide (PEAI) represent two distinct approaches to PQD surface passivation, each with characteristic mechanisms and performance outcomes.

FAI Post-Treatment: FAI functions primarily through A-site cation supplementation, where the formamidinium ions fill vacancies at the A-site of the CsPbI3 perovskite structure. This vacancy filling reduces surface traps and suppresses non-radiative recombination [10]. The small ionic radius of FA+ allows for relatively seamless integration into the surface lattice, maintaining structural continuity. Research on FAPbI3 PQDs treated with benzamidine hydrochloride (structurally related to FAI) demonstrated significantly improved electronic coupling between QDs, yielding solar cell devices with power conversion efficiency (PCE) of 6.4% compared to 4.63% for conventional devices [10]. The enhanced performance stems from both improved surface passivation and reduced inter-dot barriers.

PEAI Post-Treatment: PEAI employs a bulkier ammonium cation that cannot readily incorporate into the three-dimensional perovskite lattice. Instead, the phenethylammonium ions form a protective surface layer, simultaneously passivating halide vacancies through iodide donation and creating a potential barrier that inhibits moisture penetration [7]. This surface stabilization mechanism has demonstrated remarkable effectiveness in enhancing environmental stability, with PEAI-treated CsPbI3 PQDs retaining over 70% of initial PL intensity after 20 days of continuous UV exposure [7]. The aromatic ring in PEAI may additionally facilitate π-π stacking between adjacent ligands, promoting ordered charge transport pathways.

Table 2: Performance Comparison of FAI vs. PEAI Post-Treatment in CsPbI3 PQDs

Parameter	FAI Post-Treatment	PEAI Post-Treatment	Measurement Conditions
PLQY Enhancement	~16-18% [7]	~18% [7]	Relative to untreated PQDs
Device PCE	6.4% (FAPbI3 system) [10]	~16.5% (CsPbI3 system) [9]	Champion solar cell devices
Environmental Stability	Moderate improvement	>70% PL retention after 20 days UV [7]	Ambient conditions with UV exposure
Thermal Stability	Phase stabilization up to 170°C [7]	Enhanced phase stability	Under thermal stress
Charge Transport	Improved electronic coupling [10]	Reduced trap-assisted recombination [7]	Time-resolved PL measurements

The following diagram illustrates the distinct passivation mechanisms of FAI and PEAI ligands:

Advanced Ligand Engineering Strategies and Synergistic Effects

Solvent Engineering for Optimized Ligand Exchange

The solvent environment during ligand exchange plays a decisive role in determining the effectiveness of both FAI and PEAI treatments. Research has demonstrated that protic solvents like 2-pentanol, with tailored dielectric constant and acidity, maximize the removal of pristine insulating ligands while mediating more controllable binding of short ligands to the PQD surface [9]. This solvent-mediated approach minimizes the introduction of halogen vacancy defects that typically accompany aggressive ligand stripping processes.

The optimized solvent system enables a more complete ligand exchange while preserving the structural integrity of the PQDs. In studies utilizing 2-pentanol as the solvent for short ligands, researchers achieved champion CsPbI3 PQD solar cells with remarkable PCE of 16.53%, the highest reported for inorganic PQDSCs [9]. This performance highlights the critical synergy between solvent selection and ligand chemistry in achieving conductive yet well-passivated PQD solids.

Multidentate Ligands and Combinatorial Approaches

Beyond simple ammonium salts, advanced ligand architectures offer additional functionality for PQD surface management. Multidentate ligands featuring multiple binding groups can form more stable coordination complexes with the PQD surface, reducing ligand desorption rates. For instance, trioctylphosphine oxide (TOPO) coordinates strongly to undercoordinated Pb²⁺ sites through its phosphine oxide group, demonstrating 18% PL enhancement in CsPbI3 PQDs [7]. Similarly, l-phenylalanine (L-PHE) provides both carboxylate and amine functional groups for cooperative binding, yielding superior photostability with over 70% PL retention after prolonged UV exposure [7].

Combinatorial approaches using mixed ligand systems have also shown promise. Blending complementary ligands can address multiple degradation pathways simultaneously. For example, incorporating 2-phenyl-4-(1,2,2-triphenylvinyl) quinazoline (2PACz) in PQD films effectively reduced surface defects and suppressed trap-assisted charge recombination, resulting in a 35% increase in charge carrier lifetimes and enabling indoor photovoltaic devices with impressive power conversion efficiency of 41.1% under fluorescent lighting [11].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Research Reagents for PQD Ligand Engineering Studies

Reagent/Chemical	Function in Research	Application Notes
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for all-inorganic PQDs	Requires complete reaction with acids to form cesium oleate
Lead Iodide (PbI₂)	Lead precursor for perovskite framework	Must be thoroughly dried and degassed before use
Oleic Acid (OA)	Standard long-chain carboxylic acid ligand	Critical for controlling growth; excess typically used
Oleylamine (OAm)	Standard long-chain amine ligand	Ratio to OA affects morphology and optical properties
Formamidinium Iodide (FAI)	A-site cation supplement and passivator	Effective for vacancy reduction; small ionic radius
Phenethylammonium Iodide (PEAI)	Surface-passivating ammonium salt	Provides steric protection and halide donation
Methyl Acetate (MeOAc)	Antisolvent for ligand exchange	Optimal polarity for ligand removal without damage
2-Pentanol	Solvent for short ligand solutions	Appropriate dielectric constant and acidity
Trioctylphosphine Oxide (TOPO)	Lewis base ligand for metal site passivation	Strong coordination to undercoordinated Pb²⁺ sites

Surface ligand engineering represents the most critical frontier in advancing PQD solids from laboratory curiosities to commercial technologies. The comparative analysis of FAI and PEAI post-treatment strategies reveals distinct mechanisms and performance advantages: FAI excels at enhancing electronic coupling through lattice integration, while PEAI provides superior environmental protection through surface encapsulation. The optimal choice depends fundamentally on the application priorities—whether maximizing charge transport for photovoltaic efficiency or ensuring operational stability for long-term device functionality.

Future research directions should explore several promising avenues: First, the development of multi-functional ligands with programmable binding groups and auxiliary functions could address multiple degradation pathways simultaneously. Second, advanced characterization techniques operating under operando conditions will provide deeper insights into the dynamic nature of ligand binding during device operation. Finally, machine-learning-assisted ligand design could accelerate the discovery of optimal molecular structures for specific application environments. As these strategies mature, surface-engineered PQD solids will continue to narrow the performance gap with traditional semiconductors, ultimately enabling a new generation of solution-processed, high-efficiency optoelectronic devices.

In the pursuit of high-performance and stable perovskite quantum dot (PQD) solar cells (PQDSCs), surface ligand management is a critical determinant of both device efficiency and operational stability. CsPbI₃ PQDs have emerged as a leading material for photovoltaics due to their ideal bandgap and excellent optoelectronic properties. However, their inherent ionic nature and surface defect states pose significant challenges. This guide provides a comparative analysis of two pivotal surface treatment strategies: Formamidinium Iodide (FAI) and Phenethylammonium Iodide (PEAI) post-treatment. We objectively compare their mechanisms, resultant performance, and stability implications for CsPbI₃ PQD films, equipping researchers with the data and protocols needed to inform their material selection.

Fundamental Mechanisms and Ionic Interactions

The performance of CsPbI₃ PQD films is governed by their surface chemistry. Colloidal synthesis typically employs long-chain insulating ligands like oleic acid (OA) and oleylamine (OAm) to stabilize the QDs. These must be exchanged or removed in solid films to facilitate charge transport without introducing detrimental defects or ionic mobility [12] [2] [9].

• FAI Post-Treatment Mechanism: FAI is widely used in a solid-state ligand exchange process. Its small formamidinium (FA⁺) cation replaces protonated OAm (NH₃⁺) ligands, improving the electronic coupling between adjacent QDs. The primary function of FAI is to passivate surface defects, particularly iodine vacancies, by providing a source of iodide ions. This process enhances film conductivity and reduces non-radiative recombination. However, its small ionic size can lead to over-penetration and potential reconstruction of the perovskite lattice at the surface, sometimes compromising phase stability [2] [1] [9].

• PEAI Post-Treatment Mechanism: PEAI introduces the bulkier, hydrophobic phenethylammonium (PEA⁺) cation. This cation is too large to incorporate into the 3D perovskite lattice of the QD core. Instead, it forms a low-dimensional (2D) perovskite layer or a quasi-2D capping layer on the surface of the 3D CsPbI₃ QDs. This 2D/3D hybrid structure provides superior surface passivation and significantly enhances environmental stability due to the hydrophobic benzyl ring of the PEA⁺ cation. A critical drawback, however, is that the PEA⁺ cations can act as a source of mobile ions under operational stress (light, heat), leading to increased ionic density within the device. This mobile ion accumulation can screen the internal electric field, resulting in substantial short-circuit current density (J_SC) losses and accelerated performance degradation over time [12] [2].

The diagram below illustrates the logical decision pathway for selecting and applying these post-treatments, highlighting their distinct outcomes.

Comparative Performance Data

The distinct mechanisms of FAI and PEAI treatments lead to measurable differences in photovoltaic device performance. The following table summarizes key performance metrics reported in the literature, highlighting the trade-offs between voltage, current, and efficiency.

Table 1: Comparative Photovoltaic Performance of FAI- vs. PEAI-Treated CsPbI₃ PQD Solar Cells

Treatment	Average V_OC (V)	Average J_SC (mA/cm²)	Champion PCE (%)	Key Advantages	Key Disadvantages
FAI Post-Treatment	~1.15 [9]	~25.0 [9]	16.53 [9]	High J_SC, excellent charge transport, proven high efficiency.	Can induce phase instability; limited hydrophobic protection.
PEAI Post-Treatment	~1.23 [2]	~24.2 [12]	14.18 [2]	High V_OC, superior humidity stability, enhanced defect passivation.	Significant J_SC loss; mobile ion-induced degradation under operation.

A critical and often overlooked aspect of PEAI treatment is its impact on operational stability. Recent studies reveal that the initial J_SC loss is not merely due to reduced conductivity but is intrinsically linked to mobile ions. The table below quantifies this instability, showing how performance degrades over time under illumination.

Table 2: Analysis of J_SC Loss and Operational Instability in PEAI-Treated Devices

Performance Parameter	Initial State	After 160h SPO Tracking	Underlying Cause
J_SC Loss	1.3 mA cm⁻² average drop [12]	Further degradation up to 14 mA cm⁻² [12]	Mobile ion density increase screening internal field.
Mobile Ion Density	Increased in fresh devices [12]	Significant accumulation over time [12]	Diffusion of PEA⁺ cations into perovskite bulk under bias/heat.
Mitigation Strategy	---	---	Bilayer passivation (e.g., ABS/PEAI, EDAI₂/PEAI) reduces ion density [12].

Experimental Protocols

To ensure reproducibility, this section outlines standardized protocols for applying FAI and PEAI post-treatments to layer-by-layer (LBL) deposited CsPbI₃ PQD films.

FAI Post-Treatment Protocol

• Reagent Preparation: Prepare a solution of 10 mg/mL Formamidinium Iodide (FAI) in ethyl acetate (EtOAc). Ensure the solvent is anhydrous to prevent premature degradation of the PQDs [2] [9].
• Film Deposition: Spin-coat a layer of native CsPbI₃ PQDs (in hexane or octane) onto the substrate.
• Ligand Exchange: Immediately after spin-coating, dynamically drip the FAI/EtOAc solution onto the film for about 20 seconds during rotation. This step replaces OAm ligands with FA⁺ and passivates iodide vacancies [9].
• Rinsing & Annealing: Spin-dry the film and repeat the LBL process until the desired thickness is achieved. Finally, anneal the complete film on a hotplate at ~70°C for 5-10 minutes to remove residual solvent [9].

PEAI Post-Treatment Protocol

• Reagent Preparation: Prepare a solution of 1.5 - 2.0 mg/mL Phenethylammonium Iodide (PEAI) in a tailored solvent such as 2-pentanol or EtOAc. Solvent choice is critical for effective ligand exchange without damaging the PQDs [12] [2].
• Film Deposition: Spin-coat a layer of native CsPbI₃ PQDs onto the substrate.
• Ligand Exchange & 2D Layer Formation: Dynamically drip the PEAI solution onto the fresh PQD layer for 20-30 seconds. The PEA⁺ cations facilitate the formation of a 2D (PEA)₂PbI₄ layer on the 3D QD surface [2].
• Rinsing & Annealing: Spin-dry the film and repeat the LBL process. The final film is annealed at ~70°C for 5-10 minutes to crystallize the surface layer [2].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for FAI and PEAI Post-Treatment Experiments

Reagent / Material	Function / Role	Justification for Use
Cesium Carbonate (Cs₂CO₃)	Precursor for Cs-oleate synthesis.	High purity (99.9%) is essential for reproducible QD synthesis and optoelectronic quality [13].
Lead Iodide (PbI₂)	Pb²⁺ source for perovskite lattice.	High purity (99.999%) minimizes intrinsic defects and non-radiative recombination [13].
Formamidinium Iodide (FAI)	Short-chain ligand for exchange and passivation.	Replaces OAm ligands and fills I⁻ vacancies, directly boosting J_SC and PCE [9].
Phenethylammonium Iodide (PEAI)	Bulky cation for 2D capping layer formation.	Provides superior surface passivation and hydrophobicity, enhancing V_OC and humidity stability [2].
2-Pentanol	Protic solvent for PEAI solution.	Tailored dielectric constant and acidity maximize insulating ligand removal without creating vacancies [9].
Ethyl Acetate (EtOAc)	Solvent for FAI/PEAI solutions and washing.	Effectively removes oleate ligands and is chemically compatible with the perovskite structure [2] [13].
Methyl Acetate (MeOAc)	Antisolvent for initial QD purification.	Used to precipitate and wash QDs after synthesis, removing excess ligands and solvent [13].

The choice between FAI and PEAI post-treatment represents a strategic trade-off for researchers designing CsPbI₃ PQD films. FAI post-treatment is the pathway to achieving the highest reported efficiencies and short-circuit currents, making it ideal for pushing record PCE values. In contrast, PEAI post-treatment offers a robust route to significantly improved open-circuit voltage and humidity stability, albeit at the cost of inherent J_SC losses and mobile-ion-driven degradation that must be mitigated for long-term operation. Emerging strategies, such as the use of bilayer passivation (e.g., ABS/PEAI), are showing promise in suppressing the mobile ions in PEAI-based devices, pointing toward a future where the benefits of both treatments might be synergistically combined [12]. Researchers must therefore align their choice with their primary objective: pursuing peak efficiency or engineering for enhanced stability.

Surface ligand engineering is a critical frontier in advancing the performance of CsPbI₃ perovskite quantum dot (PQD) solar cells. Among the various strategies explored, post-treatment with organic ammonium halides has emerged as a powerful technique for defect passivation and stability enhancement. This guide provides a comparative analysis of two prominent post-treatment ligands: formamidinium iodide (FAI) and phenethylammonium iodide (PEAI). While both aim to replace insulating native ligands and heal surface defects, their fundamental chemical structures impart distinct properties and functionalities to the final PQD film. PEAI, featuring an aromatic phenethyl group, offers unique advantages in defect passivation, charge transport, and moisture resistance compared to the smaller formamidinium cation. This article objectively compares the performance of these two alternatives, drawing on recent experimental studies to provide researchers with a clear understanding of their respective applications and outcomes.

Structural and Mechanistic Comparison of FAI and PEAI

The efficacy of a surface ligand in PQD technology is governed by its chemical structure, which dictates its binding affinity, steric influence, and electronic interaction with the perovskite crystal. FAI and PEAI represent two structurally distinct classes of ammonium salts used in post-treatment.

• Formamidinium Iodide (FAI): The formamidinium cation (FA⁺) is a relatively small organic cation with the formula [CH(NH₂)₂]⁺. Its compact size allows it to replace cesium ions in the A-site of the perovskite lattice, leading to the formation of a mixed-cation (FA,Cs)PbI₃ surface layer or a hybrid structure. While this can improve compositional stability, prolonged or concentrated FAI treatment risks destabilizing the pure CsPbI₃ phase, inducing unintended compositional changes from CsPbI₃ to FA₁₋ₓCsₓPbI₃ [2]. Furthermore, the FA⁺ cation primarily provides electrostatic passivation of negatively charged defects but lacks additional functional groups for enhanced coordination or hydrophobic protection.

• Phenethylammonium Iodide (PEAI): The phenethylammonium cation (PEA⁺) consists of an ammonium head group (-NH₃⁺) attached to a hydrophobic phenyl ring via a two-carbon alkyl spacer (-CH₂-CH₂-). This structure confers two key advantages:

  • Aromatic Coordination: The electron-rich phenyl ring can engage in π-cation interaction with the perovskite surface, providing a stronger, multi-modal binding that goes beyond simple ionic attraction [2] [14].
  • Hydrophobic Barrier: The bulky, non-polar phenyl ring acts as a molecular shield, imparting excellent moisture resistance to the PQD film [2]. Unlike FA⁺, the PEA⁺ cation is too large to incorporate into the 3D perovskite lattice, preventing internal phase disruption and ensuring the stability of the all-inorganic CsPbI₃ structure [14].

The following diagram illustrates the conceptual differences in how FAI and PEAI post-treatments modify the PQD surface.

Performance Comparison: Experimental Data

The structural differences between FAI and PEAI translate directly into varying photovoltaic performance and device stability. The following table summarizes key metrics from recent studies implementing these post-treatments, including a novel layer-by-layer (LBL) approach for PEAI.

Table 1: Performance comparison of CsPbI₃ PQD solar cells with FAI and PEAI post-treatment.

Post-treatment Method	Power Conversion Efficiency (PCE)	Open-Circuit Voltage (VOC)	Stability Retention	Key Findings	Reference
Conventional FAI Post-treatment	~16.5% (highest reported)	~1.20 V	Not specified	Effective but risks phase instability; treatment is difficult to control.	[9]
PEAI LBL Solid-State Exchange	14.18% (champion)	1.23 V	Excellent stability in high humidity (30-50% RH), unencapsulated.	Enhanced inter-dot coupling, superior defect passivation, balanced carrier transport.	[2]
PEAI-based Post-treatment	15.1%	Not specified	Not specified	Aromatic ammonium halides improve efficiency and moisture resistance.	[15]

The data indicates that while FAI treatments can lead to very high efficiencies, the PEAI LBL strategy achieves a more robust combination of high open-circuit voltage (1.23 V) and exceptional stability under humid conditions without encapsulation [2]. This is attributed to the conjugated PEAI ligand enabling enhanced inter-dot coupling and more effective passivation of surface defects compared to conventional post-treatments.

Experimental Protocols for Post-Treatment

To ensure reproducibility, this section outlines standard experimental protocols for applying FAI and PEAI post-treatments to CsPbI₃ PQD films.

Standard FAI Post-Treatment Protocol

• PQD Film Deposition: CsPbI₃ PQD films are deposited onto the substrate using a layer-by-layer (LBL) spin-coating method. Each layer is typically rinsed with methyl acetate (MeOAc) to remove excess solvents and long-chain ligands [2] [16].
• FAI Solution Preparation: A saturated solution of formamidinium iodide (FAI, ≥99.99% purity) is prepared in ethyl acetate (EtOAc, anhydrous) [2].
• Treatment Process: After depositing the final PQD layer, the FAI/EtOAc solution is dynamically spin-coated onto the film (e.g., at 2000 rpm for 30 seconds) [2].
• Annealing: The treated film is annealed on a hotplate at ~70°C for 5-10 minutes to remove residual solvent [2].

Advanced PEAI Layer-by-Layer (LBL) Protocol

The modified PEAI LBL method integrates the ligand exchange directly into the film-building process, as visualized below.

Detailed Steps:

• PQD Layer Deposition: Spin-coat a layer of CsPbI₃ PQDs (e.g., 85 mg/mL in octane) at 2000 rpm for 25 seconds [2] [16].
• Initial Rinse: Immediately rinse the film with methyl acetate (MeOAc) during spin-coating to remove residual solvent and some native long-chain ligands (OA/OAm) [2].
• PEAI Treatment: Instead of proceeding to the next layer, a solution of phenethylammonium iodide (PEAI) in ethyl acetate (EtOAc) is spin-coated onto the freshly rinsed PQD layer [2]. This step is performed after each PQD layer deposition.
• Cycle Repetition: Steps 1-3 are repeated multiple times (typically 3-5 cycles) to build up the desired film thickness (~400 nm) [16].
• Final Annealing: The complete film is annealed at ~70°C for 5-10 minutes [2].

Key Difference: The core innovation of the PEAI-LBL method is the interleaving of the ligand exchange step after every PQD layer, ensuring more uniform and complete surface modification throughout the entire film thickness, rather than just a final treatment on the top surface [2].

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key reagents and materials for FAI and PEAI post-treatment experiments.

Reagent/Material	Function in Experiment	Specifications & Notes
Cesium Lead Iodide PQDs (CsPbI₃)	The light-absorbing active layer.	Synthesized via hot-injection method; dispersed in non-polar solvents (e.g., octane, hexane) [2] [16].
Phenethylammonium Iodide (PEAI)	Aromatic ammonium salt for surface ligand exchange and passivation.	Purify ≥99.99%; short-chain ligand with phenyl group for enhanced binding and hydrophobicity [2] [14].
Formamidinium Iodide (FAI)	Small ammonium salt for A-site cation exchange and surface passivation.	Purify ≥99.99%; use with caution to avoid unwanted phase transformation [2].
Methyl Acetate (MeOAc)	Washing solvent (antisolvent) for initial ligand removal.	Anhydrous grade (≥99.5%); used in layer-by-layer rinsing to remove OA/OAm ligands [2] [16].
Ethyl Acetate (EtOAc)	Solvent for preparing FAI and PEAI post-treatment solutions.	Anhydrous grade (≥99.8%); moderate polarity suitable for dissolving ammonium salts without damaging PQD film [2].
2-Pentanol (2-PeOH)	Alternative protic solvent for ligand exchange.	Used for its tailored dielectric constant and acidity to mediate more effective ligand exchange [9].

This comparative analysis demonstrates that the choice between FAI and PEAI post-treatment is a trade-off between different material properties and device goals. FAI post-treatment can yield very high efficiencies but requires precise control to mitigate risks of phase instability and is often limited to surface-level modification. In contrast, the PEAI LBL solid-state exchange strategy provides a more holistic solution by ensuring thorough, layer-by-layer defect passivation, which enhances charge transport and significantly boosts environmental stability without sacrificing performance. The fundamental role of the aromatic ammonium group in PEAI—enabling strong, multi-site surface binding and creating a hydrophobic barrier—is the key differentiator. For research aimed at developing efficient and robust CsPbI₃ PQD solar cells with high operational stability, PEAI-based ligand management, particularly the LBL approach, represents a highly promising and effective pathway.

Colloidal perovskite quantum dots (PQDs), particularly all-inorganic CsPbI3, have emerged as leading materials for next-generation optoelectronic devices due to their ideal bandgap, high photoluminescence quantum yield, and excellent defect tolerance [1]. The surface chemistry of these nanocrystals, governed by organic ligands, is paramount in determining their electronic coupling, environmental stability, and ultimate device performance. Long-chain ligands used in synthesis ensure colloidal stability but impede inter-dot charge transport, necessitating post-synthetic ligand exchange to shorter ligands for device integration [2] [17] [1]. This process, however, often introduces surface defects and can destabilize the desired black perovskite phase.

Within this context, two distinct ligand engineering strategies have been developed: ionic substitution, exemplified by formamidinium iodide (FAI) treatment, and steric passivation, exemplified by phenethylammonium iodide (PEAI) management. This guide provides a comparative analysis of these approaches, examining their fundamental bonding mechanisms, experimental outcomes, and implications for CsPbI3 PQD solar cell performance. The study is framed within the broader thesis that the molecular interaction between the ligand and the PQD surface—whether primarily ionic or steric—profoundly influences the optoelectronic properties and structural integrity of the resulting solid film.

Comparative Bonding Mechanisms at the PQD Surface

The bonding models for FAI and PEAI post-treatments are fundamentally different, leading to distinct outcomes in surface passivation and film properties.

Ionic Substitution with FAI

The FAI approach functions primarily through an ionic substitution mechanism. The small formamidinium (FA+) cation replaces the native cesium (Cs+) cation on the PQD surface. This is a ligand-exchange process where the primary interaction is the replacement of one ion for another at the A-site of the perovskite lattice [2]. While this effectively removes long-chain insulating ligands, it carries significant risks. The ionic substitution can penetrate beyond the surface, leading to an unintended component change from CsPbI3 to a mixed-cation FA1-xCsxPbI3 perovskite. This compromises the phase stability of the quantum dot and makes the exchange process difficult to control precisely [2].

Steric Passivation with PEAI

In contrast, the PEAI approach employs a steric passivation mechanism. The larger phenethylammonium (PEA+) cation is not designed to substitute for Cs+ in the lattice. Instead, its bulky aromatic phenyl group creates a steric barrier that prevents it from entering the lattice sites, a process resisted by Cs+ cations in all-inorganic perovskites [2] [17]. The PEA+ ion acts as a multifaceted anchoring ligand. Its ammonium group (-NH3+) can effectively passivate cationic (Cs+) vacancies, while the electron-rich phenyl ring can coordinate with uncoordinated Pb²⁺ sites on the PQD surface as a Lewis base [2] [17]. This results in a robust surface passivation without compromising the crystal lattice integrity.

Table 1: Fundamental Comparison of Bonding Models.

Feature	Ionic Substitution (FAI)	Steric Passivation (PEAI)
Primary Mechanism	Replacement of Cs⁺ with FA⁺ at A-sites	Steric blocking and surface defect passivation
Molecular Interaction	Ionic exchange	Multifaceted anchoring: Lewis base binding & vacancy filling
Impact on Lattice	Can cause lattice strain and phase change	Maintains lattice integrity; can restore tensile strain
Ligand Size	Small cation	Larger, bulky cation with aromatic ring
Process Control	Difficult to control; can be over-treated [2]	More controllable, layer-by-layer process possible [2]

Visualizing the Ligand Exchange Workflows

The experimental workflows for applying these post-treatments also differ, impacting the uniformity and quality of the resulting films. The conventional method involves a post-treatment after all QD layers are deposited, while the advanced PEAI method uses a layer-by-layer approach for more uniform passivation.

Diagram 1: Workflow for Conventional and PEAI-LBL Ligand Exchange.

Experimental Performance and Data Comparison

The different bonding mechanisms of FAI and PEAI lead to measurable differences in the optoelectronic properties and device performance of CsPbI3 PQD solar cells.

Quantitative Performance Metrics

Experimental data demonstrates that the steric passivation model, particularly when applied in a layer-by-layer (LBL) method, yields superior results in photovoltaic efficiency and device stability.

Table 2: Comparative Experimental Data for FAI and PEAI Post-Treated CsPbI3 PQD Solar Cells.

Performance Parameter	FAI Post-Treatment	PEAI-LBL Treatment	Measurement Context
Power Conversion Efficiency (PCE)	~13.6% (Control) [17]	14.18% (Champion) [2]	Photovoltaic current-voltage (J-V) measurement
Open-Circuit Voltage (VOC)	—	1.23 V [2]	Photovoltaic J-V measurement
Stability (PCE Retention)	~8.7% after 15 days [17]	83% after 15 days [17]	Ambient conditions, unencapsulated
Electroluminescence Performance	Lower luminance	130 cd/m² luminance [2]	Device operated as an LED
Surface Passivation Quality	Incomplete subsurface passivation [2]	Enhanced defect passivation [2]	Inferrred from PL lifetime & device VOC

Enhanced Stability via Steric Passivation

The enhanced stability observed in PEAI-treated films is a direct consequence of the steric passivation mechanism. The bulky PEA+ ligand creates a hydrophobic barrier around the PQDs, protecting them from moisture ingress [2]. Furthermore, its strong binding to the surface helps mitigate lattice distortion and maintains the tensile strain necessary to stabilize the black perovskite phase at room temperature [17]. In contrast, the ionic substitution of FAI can destabilize the crystal structure, making it more susceptible to phase degradation.

Essential Research Reagents and Materials

The experimental protocols for comparative studies of FAI and PEAI require a specific set of high-purity reagents. The function of each key material is detailed below.

Table 3: Research Reagent Solutions for PQD Ligand Exchange Studies.

Reagent / Material	Function in Research	Key Characteristics
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for CsPbI3 PQD synthesis [2]	High purity (99.99%), anhydrous
Lead Iodide (PbI₂)	Lead precursor for CsPbI3 PQD synthesis [2]	High purity (99.99%), moisture-sensitive
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain native ligands for synthesis [2] [17]	Technical grade (90%), provide colloidal stability
Formamidinium Iodide (FAI)	Ionic substitution agent for ligand exchange [2]	Short-chain, high purity (99.99%)
Phenethylammonium Iodide (PEAI)	Steric passivation agent for ligand exchange [2]	Short-chain with aromatic group
Methyl Acetate (MeOAc)	Antisolvent for washing/removing OA ligands [2]	Anhydrous, used in LBL process
2-Thiophenemethylammonium Iodide (ThMAI)	Multifaceted anchoring ligand (comparative studies) [17]	Contains electron-rich thiophene ring

The comparative analysis between ionic substitution (FAI) and steric passivation (PEAI) reveals a clear paradigm in surface engineering for CsPbI3 PQDs. While FAI post-treatment serves as a foundational method, its ionic mechanism introduces risks of lattice instability and incomplete passivation. In contrast, the PEAI approach, through steric hindrance and multifaceted anchoring, provides a more robust and controllable pathway. It achieves superior surface defect passivation, enhances charge transport, and critically, restores the tensile strain necessary for phase stability. The experimental data corroborates that the steric passivation model consistently leads to higher photovoltaic efficiency, remarkable electroluminescence, and significantly improved operational stability. For researchers and scientists developing advanced PQD optoelectronics, this comparison underscores that the strategic design of ligands, which prioritize non-invasive yet strong surface binding, is key to unlocking the full potential of these nanomaterials.

Protocols for FAI and PEAI Post-Treatment: From Synthesis to Film Fabrication

Colloidal cesium lead iodide (CsPbI3) perovskite quantum dots (PQDs) have emerged as a leading semiconductor material for next-generation optoelectronic devices, including solar cells and light-emitting diodes (LEDs). Their appeal lies in an ideal bandgap (~1.73 eV), high photoluminescence quantum yield (PLQY), and exceptional defect tolerance [18] [1]. The hot-injection method is the cornerstone synthesis technique for producing high-quality, monodisperse CsPbI3 PQDs with precise size and shape control. However, the insulating nature of long-chain ligands used in synthesis and the resulting surface defects necessitate post-synthetic ligand exchange to unlock the material's full electronic potential [2] [18].

This article provides a comparative analysis of two dominant post-treatment strategies: formamidinium iodide (FAI) and phenethylammonium iodide (PEAI). We objectively evaluate their performance in optimizing CsPbI3 PQD films for device applications, presenting a structured comparison of experimental data, detailed protocols, and mechanistic insights.

The Hot-Injection Synthesis Method

The hot-injection method provides a robust foundation for producing high-purity, crystalline CsPbI3 PQDs. The following workflow outlines the core synthesis procedure, which serves as the starting point for subsequent post-treatment comparisons.

Synthesis Protocol [1] [19] [20]:

• Cesium-Oleate Precursor: Cesium carbonate (Cs₂CO₃, 0.407 g) is dissolved in 1-octadecene (ODE, 15 mL) with oleic acid (OA, 1.25 mL) in a 50 mL 3-neck flask. The mixture is dried under vacuum for 1 hour at 120 °C and then heated to 150 °C under a N₂ atmosphere until all Cs₂CO₃ reacts, forming a clear solution.
• Lead-Halide Precursor: In a separate 100 mL 3-neck flask, lead iodide (PbI₂, 0.173 g) is mixed with ODE (15 mL), OA (1.5 mL), and oleylamine (OAm, 1.5 mL). The flask is dried under vacuum for 1 hour at 120 °C.
• Hot-Injection and Reaction: The PbI₂ solution is heated to 150–180 °C under N₂. The prepared Cs-oleate precursor (1.5 mL) is swiftly injected into the reaction flask. After 5–10 seconds, the reaction is quenched by immersing the flask in an ice-water bath.
• Purification: The crude solution is centrifuged (8,000 rpm, 10 minutes) to separate the PQDs. The supernatant is discarded, and the pellet is re-dispersed in an anhydrous non-polar solvent like n-hexane or n-octane for storage and further processing.

Post-Treatment Strategies: FAI vs. PEAI

The initial CsPbI3 PQDs are capped with long-chain OA and OAm ligands, which impede charge transport between adjacent QDs in solid films. Post-treatment with short-chain ligands is essential to remove these insulators and passivate surface defects. FAI and PEAI represent two principal approaches to this challenge, each with distinct mechanisms and outcomes.

Table 1: Comparative Analysis of FAI and PEAI Post-Treatments

Feature	FAI (Formamidinium Iodide) Post-Treatment	PEAI (Phenethylammonium Iodide) Post-Treatment
Chemical Structure	Small cation (FA⁺)	Larger, conjugated cation (PEA⁺) with phenyl group
Primary Function	Ligand exchange, partial A-site cation substitution	Ligand exchange, surface passivation, enhanced coupling
Mechanism	Replacement of OAm ligands; can induce phase change to FA₁₋ₓCsₓPbI₃ [2]	LBL treatment passivates defects per layer; phenyl group enhances inter-dot coupling [2]
Impact on Film Properties	Can compromise phase stability; difficult to control [2]	Improved defect passivation throughout film; balanced carrier transport [2]
Optimal PCE Reported	~10.77% (early reports) [2]	14.18% (champion device) [2]
Open-Circuit Voltage (VOC)	Lower relative to PEAI-LBL	1.23 V (high) [2]
Stability (Unencapsulated)	Moderate, sensitive to treatment time [2]	Excellent, retains performance under high humidity (30-50% RH) [2]
Electroluminescence Performance	Not specifically highlighted	Enhanced luminance of 130 Cd/m² [2]

Experimental Protocols for Post-Treatment

FAI Post-Treatment Protocol [2]: A solution of FAI in ethyl acetate (EtOAc) is prepared. The CsPbI3 PQD film, deposited via layer-by-layer (LBL) spin-coating and methyl acetate (MeOAc) washing, is incubated with the FAI solution for a controlled duration. This is typically followed by spinning and rinsing to remove by-products.

PEAI Layer-by-Layer (LBL) Post-Treatment Protocol [2]: A solution of PEAI in EtOAc is used. Crucially, the PEAI treatment is integrated into the film deposition process after the deposition of each CsPbI3 PQD layer. After spin-coating one layer of PQDs and washing with MeOAc, the PEAI solution is applied to the wet film before the next PQD layer is deposited. This cycle repeats to build a thick, optimally passivated film.

The following diagram illustrates the key mechanistic differences between the conventional FAI method and the innovative PEAI-LBL approach.

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagents for CsPbI3 PQD Synthesis and Post-Treatment

Reagent	Function	Role in the Process
Cesium Carbonate (Cs₂CO₃)	Cesium (Cs⁺) precursor	Provides the 'A'-site cation for the ABX₃ perovskite structure [2] [7].
Lead Iodide (PbI₂)	Lead (Pb²⁺) and Iodide (I⁻) precursor	Forms the 'B'-site and part of the 'X'-site in the perovskite lattice [2] [7].
1-Octadecene (ODE)	Non-coordinating solvent	High-boiling-point solvent that serves as the reaction medium [2] [19].
Oleic Acid (OA) & Oleylamine (OAm)	Capping ligands	Control nanocrystal growth during synthesis and stabilize the colloidal solution [2] [1] [19].
Methyl Acetate (MeOAc)	Washing solvent	Polar solvent used to remove excess OA/OAm and precipitate PQDs during LBL film deposition [2].
Phenethylammonium Iodide (PEAI)	Short-chain ligand	Replaces long-chain ligands, passivates surface defects, and enhances inter-dot coupling in solid films [2].
Formamidinium Iodide (FAI)	Short-chain ligand / cation source	Replaces OAm ligands; can incorporate into the A-site of the perovskite lattice [2].
Ethyl Acetate (EtOAc)	Solvent for post-treatment	Dissolves short-chain ligand salts (FAI, PEAI) for the solid-state ligand exchange process [2].

The choice of post-treatment ligand is a critical determinant in the performance of CsPbI3 PQD-based devices. While FAI post-treatment serves as a foundational method, the data demonstrates that the PEAI layer-by-layer (LBL) strategy offers a superior and more reliable pathway for constructing high-performance optoelectronic devices. The conjugated PEA⁺ cation provides excellent surface defect passivation, promotes stronger inter-dot electronic coupling, and enables more balanced charge transport. This results in devices with higher power conversion efficiencies, significantly improved operational stability, and notable electroluminescent performance. For researchers aiming to optimize CsPbI3 PQD films, the PEAI-LBL method represents the current state-of-the-art in surface ligand management.

The pursuit of high-performance inorganic perovskite quantum dot solar cells (PQDSCs) has positioned CsPbI3 as a leading photovoltaic material due to its ideal bandgap and outstanding optoelectronic properties. A critical step in the fabrication of efficient PQDSCs is the solid-state ligand exchange procedure, which replaces the long-chain insulating ligands used in synthesis with shorter, more conductive ligands. This process is essential for transforming colloidal quantum dot solutions into conductive solid films capable of efficient charge transport. Among the various strategies, post-treatment with formamidinium iodide (FAI) and phenethylammonium iodide (PEAI) has emerged as a predominant technique. This guide provides a detailed, objective comparison of the FAI and PEAI post-treatment procedures, analyzing experimental data on concentration, solvent selection, and processing timing to evaluate their respective impacts on device performance and stability.

Comparative Analysis: FAI vs. PEAI Post-Treatment

Quantitative Data Comparison

The following table summarizes the standard experimental parameters and resulting performance metrics for FAI and PEAI post-treatment procedures as reported in the literature.

Table 1: Standardized Parameters and Performance Outcomes for FAI and PEAI Post-Treatments

Parameter	FAI Post-Treatment	PEAI Post-Treatment
Standard Concentration	Saturated solution in EtOAc [2]	2 mg/mL in EtOAc (for LBL); also used as saturated solution [2] [5]
Standard Solvent	Ethyl Acetate (EtOAc) [2]	Ethyl Acetate (EtOAc) [2] [5]
Treatment Timing	Difficult to control, sensitive to duration [2]	Applied during layer-by-layer (LBL) assembly [2]
Primary Function	Replacement of oleylamine (OAm) ligands [2]	Replacement of OAm ligands; enhanced defect passivation [2]
Reported PCE (Champion)	Up to 16.6% (in optimized devices with other ligands) [2] [15]	14.18% (champion device using PEAI-LBL) [2]
Open-Circuit Voltage (Voc)	--	1.23 V (with PEAI-LBL) [2]
Key Advantage	Established method for creating conductive films [15]	Improved moisture stability, balanced carrier transport, and electroluminescent capability [2]
Key Limitation	Can induce undesirable phase transition to FA1-xCsxPbI3, compromising phase stability [2]	--

Detailed Experimental Protocols

Standard FAI Post-Treatment Protocol

The FAI post-treatment is typically applied as a final step after the deposition of the CsPbI3 perovskite quantum dot (PQD) absorber layer via layer-by-layer (LBL) spin-coating.

• Solution Preparation: A saturated solution of formamidinium iodide (FAI, 99.99% purity) is prepared in anhydrous ethyl acetate (EtOAc) [2]. The exact saturation concentration may vary depending on the ambient temperature and solvent batch.
• Application Procedure: The FAI/EtOAc solution is dynamically spin-coated onto the completed PQD film. The film is typically left to rest for a few seconds to allow the exchange to occur before spinning off the excess solution [2].
• Critical Consideration - Timing: The treatment time is a sensitive parameter. Prolonged exposure to the FAI solution can lead to undesirable chemical changes, transforming the surface of the CsPbI3 QDs into a hybrid FA1-xCsxPbI3 perovskite. This compromises the intrinsic phase stability of the all-inorganic perovskite [2].

PEAI Layer-by-Layer (LBL) Post-Treatment Protocol

The PEAI treatment can be implemented as a conventional final post-treatment or, more innovatively, integrated into the LBL deposition process.

• Solution Preparation: A solution of phenethylammonium iodide (PEAI) in ethyl acetate is prepared. A concentration of 2 mg/mL has been used in the reported LBL procedure [2]. Saturated solutions are also commonly employed [5].
• Application Procedure (Conventional): Similar to the FAI method, the PEAI solution is spin-coated onto the finished PQD film.
• Application Procedure (LBL Strategy): In the modified LBL approach, a layer of CsPbI3 PQDs is spin-coated and washed with methyl acetate (MeOAc) to exchange oleic acid (OA) ligands for acetate. Before depositing the next PQD layer, the film is treated with the PEAI/EtOAc solution. This cycle is repeated for each layer, ensuring more uniform ligand exchange and defect passivation throughout the entire film thickness [2].
• Outcome: This method promotes more effective removal of long-chain OAm ligands, enhances inter-dot coupling, and provides superior passivation of surface defects compared to conventional post-treatment [2].

Underlying Mechanisms and Workflow

Ligand Exchange Process

The following diagram illustrates the procedural workflow for the layer-by-layer deposition and post-treatment of CsPbI3 PQD films, highlighting the critical steps.

Diagram 1: Workflow for LBL PQD Film Fabrication and Post-Treatment. This chart outlines the standard layer-by-layer (LBL) deposition process for CsPbI3 PQD solid films, indicating the two primary post-treatment pathways: FAI as a final treatment and PEAI integrated within the LBL cycle.

The primary mechanism of both FAI and PEAI post-treatment is the replacement of the dynamically bound, long-chain oleylamine (OAm) ligands on the PQD surface with shorter cationic ammonium ligands (FA+ or PEA+). This exchange is mediated by a polar solvent, typically EtOAc, which dissolves the ionic salts and helps remove the displaced long-chain ligands [2] [5]. The use of short ligands reduces the inter-dot spacing, thereby enhancing electronic coupling and charge carrier mobility between adjacent QDs. Furthermore, the ammonium groups in both FAI and PEAI can passivate undercoordinated sites on the perovskite surface, reducing non-radiative recombination losses [2].

The Scientist's Toolkit: Key Research Reagents

Table 2: Essential Materials for PQD Post-Treatment Experiments

Reagent/Material	Standard Function in Procedure	Exemplary Purity/Details
Cesium Lead Iodide PQDs (CsPbI3)	The photoactive absorber material, synthesized with native long-chain ligands (OA/OAm).	Synthesized via hot-injection method [16].
Formamidinium Iodide (FAI)	Short-chain ligand for replacing OAm and passivating surface defects.	99.99% (MaterWin New Materials) [2].
Phenethylammonium Iodide (PEAI)	Short-chain ligand with phenyl group for enhanced passivation and stability.	Commercially available (e.g., GreatcellSolar) [21].
Ethyl Acetate (EtOAc)	Polar solvent for dissolving FAI/PEAI and facilitating ligand exchange.	99.5% anhydrous [2].
Methyl Acetate (MeOAc)	Washing solvent for initial ligand exchange (OA to acetate).	Anhydrous, 99.99% [16].
n-Octane	Non-polar solvent for dispersing PQDs to form the processing ink.	Anhydrous [16] [22].

The choice between FAI and PEAI post-treatment represents a critical trade-off in the design of CsPbI3 PQD solar cells. The standard FAI procedure, using a saturated solution in EtOAc, is a well-established method for creating conductive films and has contributed to some of the highest reported power conversion efficiencies. However, its sensitivity to treatment time and potential to destabilize the all-inorganic perovskite phase are non-trivial drawbacks. In contrast, the PEAI post-treatment, particularly when deployed in a layer-by-layer strategy, offers a more robust path to stable devices. It provides enhanced defect passivation, improved moisture resistance, and enables balanced charge transport without compromising the inorganic structure of the CsPbI3 QDs. The decision ultimately hinges on the research priority: FAI may be favored for pushing the boundaries of peak PCE in controlled environments, while PEAI presents a more promising route toward achieving commercially viable stability without sacrificing significant performance.

The surface chemistry of CsPbI3 perovskite quantum dots (PQDs) is a critical determinant in the performance of resulting optoelectronic devices. Surface ligand engineering, essential for transitioning from colloidal stability to efficient charge transport in solid films, often employs short-chain ammonium salts. Among these, phenethylammonium iodide (PEAI) has emerged as a particularly effective agent for surface passivation and structural stabilization. Research has converged on two primary application methodologies: the conventional single-step post-treatment and the more advanced layer-by-layer (LBL) solid-state exchange. Single-step post-treatment, typically applied after constructing a multi-layered PQD film, often fails to address subsurface defects, leaving trap states that impair performance and stability [2]. In contrast, the emerging LBL strategy integrates the ligand exchange process within the film deposition cycle, aiming for more uniform and thorough passivation. This guide provides a comparative analysis of these two approaches, detailing their experimental protocols, resultant device performance, and underlying mechanisms to inform research and development in perovskite optoelectronics.

Comparative Performance Analysis

The choice between LBL and single-step PEAI application has a profound impact on the optoelectronic properties of CsPbI3 PQD films and the performance of fabricated devices. The table below summarizes key quantitative comparisons derived from experimental studies.

Table 1: Performance Comparison of PEAI Application Strategies

Performance Metric	Layer-by-Layer (LBL) Strategy	Single-Step Post-Treatment
Champion Device PCE	14.18% [2]	~6% [23] [13]
Open-Circuit Voltage (Voc)	1.23 V [2]	Typically lower (specific data not provided in search results)
Electroluminescence Performance	130 Cd/m² luminance [2]	Not specifically quantified for single-step
Ambient Stability (Unencapsulated)	"Excellent stability" under high humidity (30-50% RH) [2]	~80% of initial PCE after 1 month at 20% RH [15]
Defect Passivation	Enhanced passivation throughout the film [2]	Incomplete passivation, especially in subsurface layers [2]
Charge Transport	Balanced electron and hole transport [2]	Limited by residual long-chain ligands and traps [5]

The superior performance of the LBL strategy is attributed to its more comprehensive approach to surface management. While single-step post-treatment primarily passivates the top layer of the film, the LBL process ensures that each layer of QDs undergoes ligand exchange, leading to a more uniform and thoroughly passivated solid. This enhances inter-dot coupling, reduces the trap-state density throughout the entire film thickness, and fosters a more balanced charge transport landscape, which is crucial for both high-efficiency photocurrent generation and electroluminescence [2]. The single-step method, in contrast, often leaves underlying trap states unsatisfactorily passivated, which acts as non-radiative recombination centers and limits both PV and EL performance [2] [5].

Experimental Protocols

Layer-by-Layer (LBL) Solid-State Ligand Exchange

The LBL strategy is an iterative process that couples film deposition with immediate ligand exchange for each layer [2].

• PQD Synthesis & Precursor Preparation: CsPbI3 PQDs are synthesized via the standard hot-injection method using oleic acid (OA) and oleylamine (OLA) as long-chain capping ligands [2] [21]. The PEAI ligand solution is prepared by dissolving PEAI salts in a suitable solvent, typically ethyl acetate (EtOAc) [2].
• Substrate Priming: The substrate (e.g., FTO/TiO2) is pre-wetted with n-octane to create a uniform surface for PQD deposition [2].
• Layer Deposition and Exchange Cycle: This cycle is repeated 3-5 times to build the desired film thickness.
  • Spin-Coating: A layer of OA/OLA-capped CsPbI3 PQDs in n-octane is spin-coated onto the substrate [2].
  • Ligand Exchange: Immediately after deposition, the film is treated by dynamically spin-coating the PEAI solution in EtOAc. This step replaces the insulating OLA ligands with short-chain PEA⁺ cations [2].
  • Washing: The film is rinsed with pure EtOAc to remove the by-products of the ligand exchange reaction and excess ligands [2].
• Film Completion: After the final cycle, the resulting PEAI-LBL CsPbI3 PQD film is ready for the deposition of subsequent charge-transport layers and electrodes [2].

Single-Step Post-Treatment

This conventional method involves building the entire PQD film first, followed by a final, bulk ligand exchange.

• Film Construction: A thick film of OA/OLA-capped CsPbI3 PQDs is constructed on the substrate. This is typically done via a multi-cycle, layer-by-layer deposition using methyl acetate (MeOAc) to remove OA ligands and consolidate the film, but without introducing short-chain ammonium ligands at this stage [5] [15].
• Final Post-Treatment: The as-deposited multi-layer PQD solid is treated with a solution of PEAI in EtOAc via spin-coating or soaking [2] [5]. This single step aims to replace the remaining OLA ligands with PEA⁺ across the entire film.
• Washing and Drying: The film is rinsed with clean EtOAc to remove reaction residues and then dried [5].

The following workflow diagram illustrates the key procedural differences between these two methods.

Underlying Mechanisms and Characterization

The performance differences between the two strategies are rooted in their fundamental interactions with the PQD solid.

• Depth and Uniformity of Passivation: The LBL strategy ensures that ligand exchange occurs at every interface within the film. With each cycle, the PEA⁺ ions can access the surface of the newly deposited QDs before the next layer is added, leading to more uniform defect passivation throughout the film bulk [2]. In contrast, single-step post-treatment struggles with diffusion limitations, often resulting in excellent passivation on the top surface but leaving subsurface trap states unaddressed [2]. These unpassivated defects act as non-radiative recombination centers, reducing both photovoltaic efficiency and electroluminescent yield.

• Impact on Film Morphology and Charge Transport: The sequential nature of the LBL process promotes the creation of a denser, more electronically coupled QD solid. The repeated removal of long-chain insulating ligands and their replacement with short-chain PEAI during stacking significantly reduces the inter-dot distance, enhancing wavefunction overlap between adjacent QDs [2] [5]. This leads to superior charge carrier transport properties. Furthermore, the conjugated phenyl ring in the PEA⁺ ion can facilitate charge injection and transport compared to purely aliphatic ligands, contributing to the balanced electron and hole transport observed in LBL films [2].

• Phase and Environmental Stability: CsPbI3 is metastable in its photoactive cubic phase (α-phase) and tends to transform into a non-perovskite yellow phase (δ-phase). Effective surface passivation is key to preventing this transition. The comprehensive surface coverage achieved by the LBL method better shields the PQDs from environmental factors like moisture [2]. The hydrophobic phenyl group in PEAI provides a protective shell, and its consistent application via LBL creates a more effective moisture barrier throughout the film, thereby significantly enhancing ambient stability [2].

The Scientist's Toolkit: Essential Research Reagents

The following table details key chemicals and materials required for experimental work on PEAI-based ligand exchange in CsPbI3 PQD solar cells.

Table 2: Essential Research Reagents for PEAI Ligand Exchange

Reagent/Material	Function/Role	Key Considerations
Phenethylammonium Iodide (PEAI)	Short-chain cationic ligand for surface passivation; replaces OLA [2] [5].	Source of PEA⁺ ion; conjugated phenyl group enhances inter-dot coupling and stability [2].
Oleic Acid (OA) / Oleylamine (OLA)	Long-chain native ligands for colloidal PQD synthesis [2] [21].	Provide initial steric stabilization; must be partially removed for conductive films [5].
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for synthesizing Cs-oleate [2] [21].	High purity (99.99%) is critical for optimal device performance [23] [21].
Lead Iodide (PbI₂)	Lead and iodide precursor for PQD synthesis [2] [21].	High purity (99.99%) required to minimize impurities [23] [21].
1-Octadecene (ODE)	Non-polar solvent for high-temperature synthesis of PQDs [2] [21].	Serves as a reaction medium; must be purified and stored properly.
Ethyl Acetate (EtOAc)	Polar solvent for PEAI ligand solution and washing [2] [5].	Removes ligand exchange by-products; can damage PQD surface if overly aggressive [5].
Methyl Acetate (MeOAc)	Polar solvent for initial removal of OA ligands [5].	Used in film consolidation before cationic ligand exchange [5].
n-Octane / n-Hexane	Non-polar solvents for dispersing and spin-coating OA/OLA-capped PQDs [2] [21].	Good solubility for capped PQDs; low polarity preserves QD surface.

The comparative analysis unequivocally demonstrates that the layer-by-layer (LBL) application of PEAI presents a superior strategy over the conventional single-step post-treatment for developing high-performance CsPbI3 PQD optoelectronic devices. The LBL method's principal advantage lies in its ability to achieve comprehensive and uniform surface passivation throughout the PQD film, which directly translates to enhanced photovoltaic parameters (PCE of 14.18%, Voc of 1.23 V), notable electroluminescent capability (130 Cd/m²), and robust ambient stability. While single-step post-treatment remains a simpler and historically significant protocol, its limitations in passivating subsurface defects and optimizing charge transport hinder its potential for cutting-edge applications. Future research in CsPbI3 PQD surface management should therefore focus on refining LBL protocols, exploring synergistic effects with other covalent ligands or solvent engineering, and adapting these advanced strategies for large-scale, commercial fabrication processes.

The pursuit of high-performance, stable all-inorganic perovskite quantum dot (PQD) solar cells has positioned CsPbI3 as a leading material due to its appropriate optical bandgap and high chemical stability. Central to the fabrication of efficient PQD-based optoelectronic devices is the film processing technique of layer-by-layer (LbL) deposition followed by solid-state ligand exchange. This process is critical for transforming colloidal PQDs capped with long-chain insulating ligands into conductive, robust solid films. The post-treatment of these films, particularly the choice of organic ammonium halides such as formamidinium iodide (FAI) and phenethylammonium iodide (PEAI), profoundly influences the final film's optoelectronic properties and device performance. This guide provides a comparative analysis of these post-treatment strategies within the context of CsPbI3 PQD film research, presenting objective experimental data and detailed methodologies to inform researchers and scientists in the field.

Comparative Analysis of FAI and PEAI Post-Treatment

The post-treatment of CsPbI3 PQD films is a critical step for defect passivation and performance enhancement. The following sections compare the two primary strategies, FAI and PEAI post-treatment, based on recent experimental findings.

PEAI-Based Post-Treatment Strategies

Phenethylammonium iodide (PEAI) is widely used as an ionic short-chain ligand to replace native long-chain ligands like oleylamine (OLA) during the solid-state ligand exchange. The conventional process involves a two-step method: first replacing anionic oleate ligands with acetate ions using methyl acetate (MeOAc)-based solution, followed by a post-treatment to replace residual cationic OLA ligands with short cationic ammonium ligands like PEAI using an ethyl acetate (EtOAc)-based solution [5]. However, due to its large ionic radius, the PEA cation can act as an organic spacer, leading to the formation of reduced-dimensional perovskites (RDPs) on the CsPbI3 PQD solids. While these begin as high-n RDPs (n > 2), they can undergo undesirable phase transition to low-n RDPs, causing structural and optical degradation [24].

To address PEAI-induced instability, researchers have developed a hybrid-ligand approach using triphenylphosphine oxide (TPPO) as an ancillary ligand dissolved in nonpolar solvent octane during ligand exchange. The TPPO ligand covalently binds to uncoordinated Pb2+ sites via Lewis-base interactions, passivating surface traps and preventing H2O penetration while regulating the rapid diffusion of PEAI. This strategy suppresses the formation of low-n RDPs, enabling CsPbI3 PQD solar cells to achieve a power conversion efficiency (PCE) of 15.3%-15.4% with enhanced device stability, maintaining >90% of initial efficiency after 18 days under ambient conditions [5] [24].

Table 1: Performance Metrics of PEAI-Based CsPbI3 PQD Solar Cells

Post-Treatment Strategy	Power Conversion Efficiency (PCE)	Fill Factor (FF)	Stability Retention	Key Findings
Conventional PEAI Ligand Exchange	~16.53% [9]	N/A	N/A	Tailored solvent (2-pentanol) maximizes insulating ligand removal
PEAI with TPPO in Octane	15.4% [5]	N/A	>90% after 18 days [5]	Passivates uncoordinated Pb2+ sites; reduces non-radiative recombination
Hybrid PEAI/TPPO Strategy	15.3% [24]	N/A	N/A	Suppresses formation of low-n RDPs; improves structural stability

Alternative Post-Treatment: Pyrrolidinium Iodide (PyI)

While FAI-specific data was limited in the search results, another promising post-treatment strategy using pyrrolidinium iodide (PyI) has demonstrated exceptional performance. This treatment modifies the surface and grain boundary of CsPbI3 perovskite films, passivating defects and inducing the formation of a quasi-2D Py2CsPb2I7 capping layer between the perovskite layer and hole transport layer. This quasi-2D structure optimizes interface contact and blocks electron transfer from the CsPbI3 photoactive layer to the hole transport layer. The resulting CsPbI3 perovskite solar cells achieved a remarkable PCE of 17.87% with an ultra-high fill factor of 0.84, along with excellent stability, maintaining initial PCE almost unchanged after 35 days in dry air atmosphere (RH <10%) [25].

Table 2: Performance of PyI Post-Treated CsPbI3 Perovskite Solar Cells

Treatment	PCE (%)	Fill Factor	Stability	Mechanism
PyI Post-treatment	17.87 [25]	0.84 [25]	~100% after 35 days (RH<10%) [25]	Forms quasi-2D Py2CsPb2I7 capping layer; passivates surface and grain boundary defects

Experimental Protocols for PQD Film Processing

Layer-by-Layer Deposition and Ligand Exchange

The standard protocol for creating conductive CsPbI3 PQD solid films involves a meticulous layer-by-layer (LbL) assembly process [5]:

• Substrate Preparation: FTO conductive glass is cleaned through successive sonication in deionized water, ethyl alcohol, and isopropyl alcohol for 20 minutes each.
• TiO2 Blocking Layer Deposition: A TiO2 blocking layer solution (0.15 M) is spin-coated onto the FTO substrate at 2000 rpm for 20 seconds, followed by annealing at 550°C for 30 minutes.
• PQD Film Fabrication: Monodispersed OA/OLA-capped CsPbI3 PQDs are synthesized via hot-injection method. The CsPbI3 perovskite precursor solution (0.6 M) is prepared by dissolving CsI, PbI2, and DMAI with a molar ratio of 1:1.3:1.3 in DMF.
• Initial Ligand Exchange: The anionic OA ligands are replaced with short-chain acetate ions through solid-state ligand exchange using NaOAc solution dissolved in MeOAc. This process is repeated to build the desired film thickness via LbL assembly.
• Cationic Ligand Exchange: The CsPbI3 PQD solids are post-treated with PEAI solution dissolved in EtOAc to replace residual cationic OLA ligands with short-chain PEA cations.
• Ancillary Treatment: For hybrid approaches, a solution of TPPO ligand in nonpolar solvent octane is treated on the ligand-exchanged PQD solids for surface stabilization.

Advanced Solvent Engineering for Ligand Exchange

Recent innovations in solvent engineering have significantly improved ligand exchange efficiency. Protic 2-pentanol has been identified as an optimal solvent for mediator ligand exchange due to its appropriate dielectric constant and acidity. This tailored solvent maximizes the removal of pristine insulating oleylamine ligands from the PQD surface without introducing halogen vacancy defects [9]. The process involves employing tailored short choline ligands and 2-pentanol solvent for post-treatment of PQD solids, which improves charge carrier transport and surface defect passivation, yielding solar cell efficiency of up to 16.53% - the highest among inorganic PQDSCs at the time of publication [9].

Visualization of Experimental Workflows and Mechanisms

PQD Film Fabrication and Ligand Exchange Process

(Caption: Workflow for layer-by-layer deposition and solid-state ligand exchange of CsPbI3 PQD films)

Hybrid Ligand Exchange Mechanism

(Caption: Mechanism comparison of conventional vs. hybrid ligand exchange strategies)

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Research Reagents for CsPbI3 PQD Film Processing

Reagent/Chemical	Function	Application Context
Cesium Iodide (CsI)	PbI2 precursor for CsPbI3 perovskite formation	Primary material for all-inorganic perovskite synthesis
Lead Iodide (PbI2)	Pb2+ source for perovskite crystal structure	Essential component in CsPbI3 precursor solution
Oleic Acid (OA)	Long-chain native ligand for colloidal stabilization	Initial capping ligand in PQD synthesis; replaced during ligand exchange
Oleylamine (OLA)	Long-chain native ligand for colloidal stabilization	Initial capping ligand in PQD synthesis; replaced during ligand exchange
Phenethylammonium Iodide (PEAI)	Ionic short-chain ligand for cationic exchange	Replaces OLA ligands; enhances charge transport but may form RDPs
Triphenylphosphine Oxide (TPPO)	Covalent short-chain ligand for surface passivation	Passivates uncoordinated Pb2+ sites; suppresses non-radiative recombination
2-Pentanol	Protic solvent for mediator ligand exchange	Maximizes insulating ligand removal without introducing halogen vacancies [9]
Octane	Nonpolar solvent for ancillary ligand treatment	Preserves PQD surface components while dissolving TPPO ligands [5]
Methyl Acetate (MeOAc)	Polar solvent for anionic ligand exchange	Dissolves ionic salts for replacing OA ligands with acetate ions [5]
Ethyl Acetate (EtOAc)	Polar solvent for cationic ligand exchange	Dissolves PEAI for replacing OLA ligands with PEA cations [5]
Pyrrolidinium Iodide (PyI)	Organic ammonium halide for surface passivation	Forms quasi-2D capping layer; passivates surface and grain boundary defects [25]

The comparative analysis of post-treatment strategies for CsPbI3 PQD films reveals a complex interplay between ligand chemistry, solvent selection, and final device performance. While PEAI-based treatments effectively replace insulating native ligands, they can introduce instability through reduced-dimensional phase formation. The emerging hybrid approach combining PEAI with TPPO in nonpolar solvents addresses these limitations by providing superior surface passivation and stability. The remarkable results achieved with PyI post-treatment further highlight the potential of alternative organic ammonium halides in optimizing CsPbI3 PQD film quality and device performance. These findings underscore the importance of continued innovation in ligand engineering and solvent selection to advance toward commercially viable, high-performance PQD optoelectronic devices.

The integration of CsPbI3 perovskite quantum dots (PQDs) into optoelectronic devices is fundamentally governed by their surface chemistry. The pristine PQDs synthesized with long-chain insulating ligands like oleic acid (OA) and oleylamine (OAm) exhibit poor charge transport between adjacent dots, severely limiting device performance. [2] [1] Consequently, post-synthetic ligand exchange using short-chain molecules is an essential step for constructing functional films. Within this realm, formamidinium iodide (FAI) and phenethylammonium iodide (PEAI) have emerged as two prominent ligands for modifying CsPbI3 PQD surfaces. [2] [26] This guide provides a objective comparison of FAI and PEAI post-treatment strategies, drawing on recent experimental data to elucidate their distinct impacts on the material properties of PQD films and the resulting performance of solar cells and light-emitting diodes (LEDs). The choice between these ligands involves a trade-off between enhancing charge transport and achieving effective defect passivation, ultimately determining the efficiency and stability of the integrated device.

Performance Comparison: FAI vs. PEAI in Optoelectronic Devices

The following tables summarize key experimental findings from recent studies utilizing FAI and PEAI treatments on CsPbI3 PQD films, highlighting their performance in different device architectures.

Table 1: Performance Comparison in Photovoltaic Devices

Treatment	Device Architecture	PCE (%)	VOC (V)	Key Stability Findings	Citation
PEAI-LBL	CsPbI3 PQD Solar Cell	14.18	1.23	Excellent stability in high-humidity (30-50% RH); Unencapsulated.	[2]
FAI Passivation	CsPbI3 PQD Solar Cell	15.10	~1.23 (est.)	Enhanced stability under 1000 lx LED illumination.	[26]
PEAI Passivation	Flexible PSC (PET substrate)	~16-17	Increased	Retained ~85-90% initial PCE after 700 bending cycles.	[27]
Control (No Passivation)	Flexible PSC (PET substrate)	~14	Lower	Retained only ~70% initial PCE after bending.	[27]

Table 2: Performance in Light-Emitting Applications

Treatment	Device Type	EL Peak (nm)	EQE (%)	Luminance (cd/m²)	Citation
PEAI-LBL	CsPbI3 PQD Electroluminescent Device	691 (Red)	Not Specified	130	[2]
Strongly Confined QDs (Other Ligands)	Pure-Red CsPbI3 QD LED	628	26.04	4,203	[6]

Experimental Protocols for Post-Treatment and Device Fabrication

PEAI Layer-by-Layer (LBL) Solid-State Ligand Exchange

This protocol is adapted from the study achieving 14.18% PCE in a CsPbI3 PQD solar cell. [2]

• 1. PQD Film Deposition: CsPbI3 PQDs, synthesized via the hot-injection method and capped with OA/OAm ligands, are dispersed in a non-polar solvent (e.g., n-octane). The film is deposited on a substrate using a layer-by-layer (LBL) spin-coating process.
• 2. Solid-State Ligand Exchange: After the deposition of each PQD layer, a solution of PEAI in ethyl acetate (EtOAc) is dynamically spin-coated onto the film. This step is repeated for each subsequent layer.
• 3. Washing: After each PEAI treatment, the film is rinsed with methyl acetate (MeOAc) to remove the by-products of the ligand exchange reaction and excess salts.
• 4. Device Completion: Once the desired film thickness is achieved (typically 3-5 cycles), the remaining charge transport layers (e.g., spiro-OMeTAD as HTL) and metal electrodes are deposited to complete the solar cell or LED.

FAI Surface Passivation on Perovskite Films

This protocol is based on the study that boosted PCE to 15.10%. [26]

• 1. Perovskite Film Formation: A Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃ perovskite film is deposited via a solution process.
• 2. FAI Post-Treatment: A solution of FAI in isopropanol (IPA) is dynamically spin-coated onto the fully formed perovskite film. The concentration of FAI is optimized (e.g., 2.5 mg/mL).
• 3. Annealing: The FAI-treated film is annealed at 100°C for 10 minutes to facilitate the integration of FAI into the perovskite surface and grain boundaries.
• 4. Device Completion: The hole transport layer (e.g., spiro-OMeTAD or CuSCN) and electrode are subsequently deposited.

Mechanistic Insights: How FAI and PEAI Modify PQD Films

The divergent outcomes of FAI and PEAI treatments stem from their distinct chemical interactions with the perovskite surface. The diagrams below illustrate these mechanisms.

PEAI: Ligand Exchange and 2D/3D Hybrid Structure Formation

PEAI treatment functions primarily through a conjugated ligand exchange process. [2] The small PEA⁺ cation replaces the insulating long-chain OAm ligands on the PQD surface. This replacement enhances electronic coupling between adjacent QDs, thereby promoting carrier transport across the film. [2] Furthermore, the bulky phenethylammonium group cannot be incorporated into the 3D perovskite lattice. Instead, it assembles on the surface, forming a low-dimensional (e.g., PEA₂PbI₄) perovskite capping layer. [27] This creates a 2D/3D hybrid structure that effectively passivates surface defects and provides a hydrophobic barrier, significantly enhancing environmental stability. [2] [27] This mechanism is particularly beneficial for flexible devices, as the passivated surface and grain boundaries better resist crack initiation under mechanical strain. [27]

FAI: Secondary Recrystallization and Surface Integration

FAI treatment works not by replacing capping ligands, but by mediating a secondary crystallization process on the perovskite surface. [26] When applied in a protic solvent like IPA, FAI can dissolve the surface of the perovskite grains. Upon subsequent annealing, it promotes the regrowth of larger, higher-quality grains with reduced grain boundary (GB) area. [26] The formamidinium (FA⁺) cation can also directly integrate into the perovskite lattice at the surface, passivating under-coordinated Pb²⁺ sites and other defects. [26] This leads to a significant reduction in non-radiative recombination centers, which is crucial for achieving high open-circuit voltage (V_OC) in solar cells. [26]

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for FAI and PEAI Post-Treatment Experiments

Reagent Name	Function in Research	Application Context
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for CsPbI3 PQD synthesis.	Fundamental to both methods. [2] [28]
Lead Iodide (PbI₂)	Lead precursor for CsPbI3 PQD synthesis.	Fundamental to both methods. [2] [28]
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain capping ligands for PQD synthesis and stabilization in solution.	Fundamental to both methods; target of ligand exchange. [2] [6]
Phenethylammonium Iodide (PEAI)	Short-chain ligand for exchange and surface passivation.	Core reagent for PEAI-LBL and PEAI post-treatment studies. [2] [27]
Formamidinium Iodide (FAI)	Organic salt for surface passivation and defect healing.	Core reagent for FAI passivation studies. [26]
Methyl Acetate (MeOAc)	Antisolvent for washing and initial ligand removal.	Used in LBL deposition and washing steps. [2] [28]
Ethyl Acetate (EtOAc)	Solvent for PEAI in LBL ligand exchange.	Common solvent for PEAI treatment. [2]
Isopropanol (IPA)	Eco-friendly antisolvent and solvent for FAI solution.	Common solvent for FAI treatment. [26]
2-Pentanol	Protic solvent for tailored ligand exchange.	Used in advanced solvent engineering for high PCE. [9]

In conclusion, the choice between FAI and PEAI post-treatment is application-dependent. PEAI-based ligand exchange is highly effective for constructing efficient and stable QD-solid films from the bottom up, making it ideal for QD-specific devices like electroluminescent solar cells and flexible photovoltaics where environmental and mechanical stability are paramount. [2] [27] In contrast, FAI passivation excels at healing defects and improving the optoelectronic quality of pre-formed perovskite polycrystalline films, leading to high performance in bulk-heterojunction solar cells, including under indoor lighting conditions. [26]

Future research directions should explore the synergistic effects of combining these ligands in a sequential treatment or developing novel ligand structures that incorporate the advantages of both. Furthermore, scaling these post-treatment protocols for large-area, printable PQD solar cells and LEDs remains a critical challenge for commercialization.

Addressing Challenges and Optimizing Performance in FAI and PEAI Treatments

The pursuit of stable, high-performance perovskite quantum dot (PQD) solar cells has established all-inorganic CsPbI3 as a critically important material due to its ideal bandgap and enhanced phase stability compared to its bulk counterparts. A cornerstone of CsPbI3 PQD film processing is the post-synthesis ligand exchange procedure, where native long-chain insulating ligands are replaced with shorter organic salts to enhance electronic coupling between dots. Among the various salts employed, formamidinium iodide (FAI) and phenethylammonium iodide (PEAI) have emerged as prominent candidates. However, the use of FAI introduces a significant risk: its small ionic radius can lead to uncontrolled cation exchange and the formation of unwanted hybrid FA1-xCsxPbI3 phases, compromising the very phase purity and stability that make CsPbI3 attractive.

This guide provides a objective comparison of FAI and PEAI post-treatment strategies, focusing on their efficacy and limitations in preserving the phase purity of CsPbI3 PQD films. We will summarize quantitative performance data, detail key experimental protocols, and analyze the underlying mechanisms that dictate phase stability, providing researchers with a clear framework for selecting and optimizing ligand exchange processes.

FAI vs. PEAI: A Comparative Analysis of Post-Treatment Ligands

The fundamental difference between FAI and PEAI lies in their interaction with the perovskite lattice. FAI's small formamidinium cation (FA+) can readily incorporate into the A-site of the ABX3 perovskite structure, directly substituting for Cs+ ions. In contrast, the larger, bulky phenethylammonium cation (PEA+) primarily acts as a surface-capping ligand, passivating defects without integrating into the 3D perovskite lattice.

Table 1: Key Characteristics of FAI and PEAI Post-Treatment Ligands.

Feature	FAI (Formamidinium Iodide)	PEAI (Phenethylammonium Iodide)
Ionic Size	Small	Large, bulky
Primary Interaction	Cation exchange & lattice incorporation [2]	Surface coordination & defect passivation [2]
Risk of Hybrid Phase	High; forms FA1-xCsxPbI3 [2]	Low; preserves CsPbI3 phase [2]
Impact on Carrier Transport	Can be favorable but phase-dependent	Enhanced inter-dot coupling and balanced carrier transport [2]
Defect Passivation	Moderate, but difficult to control	Effective for surface defects and undercoordinated Pb²⁺ [29]
Stability Impact	Can induce undesirable phase instability [2]	Improves moisture resistance due to hydrophobic phenyl group [2]

Quantitative Performance Comparison

The distinct chemical behaviors of FAI and PEAI result in measurable differences in device performance and stability. The following table synthesizes experimental data from studies employing these ligands in CsPbI3 PQD solar cells.

Table 2: Comparison of CsPbI3 PQD Solar Cell Performance with FAI vs. PEAI Post-Treatment.

Post-Treatment Method	Power Conversion Efficiency (PCE)	Open-Circuit Voltage (VOC)	Phase Stability Notes	Source
Conventional FAI Post-Treatment	~10-16% (device-dependent)	Variable	Sensitive to treatment time; induces component change to FA1-xCsxPbI3, leading to undesirable phase stability [2]	[2]
PEAI Layer-by-Layer (LBL)	14.18% (champion)	1.23 V	Excellent; unencapsulated devices showed excellent stability under high humidity [2]	[2]
PEAI + OAI Bilayer	24.48% (on larger-area films)	Significantly enhanced	Synergistic defect passivation and improved environmental stability [29]	[29]

Experimental Protocols for Ligand Exchange

Conventional FAI Post-Treatment

The standard FAI post-treatment method is a two-step process. First, CsPbI3 PQD films are fabricated using a layer-by-layer (LBL) deposition process, which involves multiple cycles of spin-coating the QD solution followed by washing with methyl acetate (MeOAc) to remove residual solvents and some native ligands. Subsequently, the as-deposited film is treated with a solution of FAI in ethyl acetate (EtOAc). The critical challenge is that the treatment time is difficult to control precisely. Enhancing the treatment time will induce a component change from CsPbI3 to FA1-xCsxPbI3, thus leading to undesirable phase stability [2]. This transformation introduces a hybrid phase that can be metastable and undermine the thermal and environmental robustness of the film.

Modified PEAI Layer-by-Layer (LBL) Treatment

A modified solid-state ligand exchange strategy was developed to overcome the limitations of FAI. In this protocol, a phenethylammonium iodide (PEAI) solution is applied after the deposition of each individual CsPbI3 PQD layer during the LBL process, rather than as a single post-treatment at the end [2].

Detailed Workflow:

• PQD Film Deposition: A single layer of CsPbI3 PQDs (synthesized via hot-injection with OA/OAm ligands) is spin-coated onto the substrate.
• Initial Washing: The film is washed with MeOAc to remove the non-polar solvent and a portion of the long-chain ligands.
• PEAI Ligand Exchange: Immediately after washing, a solution of PEAI in ethyl acetate (EtOAc) is spin-coated onto the fresh QD layer. The conjugated PEA+ ions replace the remaining long-chain OAm ligands directly.
• Repetition: Steps 1-3 are repeated for 3-5 cycles to build a thick, electronically coupled CsPbI3 PQD film.
• Final Curing: The complete film is annealed at a mild temperature (e.g., 70°C for 5 minutes) to finalize the ligand exchange and film formation [2].

This method offers superior control, as it ensures more complete and uniform removal of insulating ligands and passivation of surface defects throughout the entire film thickness, not just the top layer. The bulky PEA+ cation acts as a short-chain ligand that sterically hinders its own incorporation into the perovskite lattice, thereby preserving the phase purity of the CsPbI3 while simultaneously enhancing electronic coupling and defect passivation.

The Scientist's Toolkit: Key Reagents and Materials

Table 3: Essential Research Reagents for CsPbI3 PQD Ligand Exchange Studies.

Reagent/Material	Function in Experiment	Key Consideration
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for CsPbI3 QD synthesis via hot-injection [2].	Requires high purity (99.99%) for reproducible QD quality.
Lead Iodide (PbI₂)	Lead precursor for CsPbI3 QD synthesis [2].	Must be anhydrous and high-purity (99.99%) to control defect density.
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain native capping ligands for QD growth and colloidal stability [2].	Dynamic binding necessitates effective removal during film processing.
Formamidinium Iodide (FAI)	Short-chain ligand for post-treatment; can enhance carrier transport [2].	Risk of cation exchange and hybrid phase formation requires precise time control [2].
Phenethylammonium Iodide (PEAI)	Short-chain, bulky ligand for surface passivation and LBL exchange [2] [29].	Hydrophobic phenyl group enhances moisture stability; bulky size protects phase purity [2].
Methyl Acetate (MeOAc)	Washing solvent to remove excess solvents and ligands during LBL deposition [2].	Anti-solvent for PQDs; enables purification without dispersion dissolution.
n-Octylammonium Iodide (OAI)	Co-passivator with long alkyl chain for hydrophobic surface layer [29].	Often used with PEAI in bilayer schemes to synergistically improve stability [29].

The choice between FAI and PEAI post-treatment is a critical determinant of phase purity in CsPbI3 PQD films. While FAI can produce high-efficiency devices, its propensity for uncontrolled cation exchange and formation of unstable hybrid FA1-xCsxPbI3 phases presents a significant challenge for achieving long-term operational stability. In contrast, the PEAI LBL treatment strategy offers a more robust and reliable pathway. By enabling enhanced defect passivation, balanced charge transport, and, most importantly, the preservation of the phase-pure CsPbI3 structure, PEAI-based methods provide a superior foundation for developing efficient and stable PQD optoelectronic devices. Future research should continue to explore such surface ligand management strategies and binary passivation systems to further push the performance boundaries of this promising material.

Mitigating Surface Defects and Achieving Uniform Passivation with PEAI

Surface defects in all-inorganic CsPbI3 perovskite quantum dot (PQD) films, particularly undercoordinated Pb2+ ions and halide vacancies, significantly impede device performance by acting as non-radiative recombination centers. Effective surface passivation is critical for enhancing the power conversion efficiency (PCE) and operational stability of perovskite quantum dot solar cells (PQDSCs). This guide provides a comparative analysis of two dominant post-treatment strategies: phenethylammonium iodide (PEAI) and formamidinium iodide (FAI).

Post-treatment via ligand engineering is a cornerstone technique for modifying the surface chemistry of CsPbI3 PQDs. It directly influences defect passivation, charge transport, and environmental stability. While FAI has been widely adopted, emerging research demonstrates that PEAI offers distinct advantages in achieving more uniform and robust passivation.

Performance Comparison: PEAI vs. FAI Post-Treatment

The table below summarizes key performance metrics of CsPbI3 PQD solar cells treated with FAI and PEAI, based on recent experimental studies.

Table 1: Comparative Performance of FAI and PEAI Post-Treatments in CsPbI3 PQD Solar Cells

Post-Treatment Method	Power Conversion Efficiency (PCE)	Open-Circuit Voltage (VOC)	Key Advantages	Stability Performance
FAI (Conventional)	~16.6% (champion record) [2]	Typically <1.23 V [2]	Established protocol, effective initial ligand exchange [2]	Sensitive to humidity; can induce undesirable phase instability [2]
PEAI (Layer-by-Layer)	14.18% (champion device) [2]	1.23 V (high) [2]	Superior defect passivation; enhanced humidity resistance; balanced carrier transport [2]	Excellent stability in high-humidity environments (30-50% RH) [2]
PEAI (Bilayer with OAI)	24.48% (in inverted PSCs) [30]	High	Synergistic defect passivation and hydrophobic protection [30]	Significantly improved environmental stability [30]

FAI post-treatment, often applied after film deposition, effectively replaces insulating oleylamine ligands but has limitations. It can be difficult to control and may inadvertently transform the CsPbI3 surface into a hybrid FA1-xCsxPbI3 perovskite, compromising phase stability [2]. Furthermore, FAI primarily passivates the top layer of the film, leaving sub-surface trap states unaddressed [2].

In contrast, the PEAI layer-by-layer (LBL) solid-state exchange strategy enables a more uniform and comprehensive passivation throughout the PQD film [2]. The conjugated phenethylammonium ligand strongly coordinates with undercoordinated Pb2+ sites, effectively suppressing deep-level traps. This results in higher open-circuit voltages and improved device stability, as evidenced by a champion VOC of 1.23 V [2]. The hydrophobic phenyl ring in PEAI also enhances the film's resistance to moisture.

Experimental Protocols for PEAI Post-Treatment

PEAI Layer-by-Layer (LBL) Solid-State Exchange Strategy

The following protocol details the modified surface ligand management for depositing CsPbI3 PQD films, as illustrated in the workflow below.

Title: PEAI Layer-by-Layer Experimental Workflow

Detailed Methodology [2]:

• Substrate Preparation: Pre-patterned fluorine-doped tin oxide (FTO) substrates are cleaned sequentially with organic solvents.
• PQD Solution Preparation: CsPbI3 PQDs are synthesized via the hot-injection method and dispersed in octane at a concentration of 85 mg/mL.
• Layer-by-Layer Deposition:
  • Spin-coating: The PQD solution is spin-coated onto the substrate at 1000 rpm for 10 s, then 2000 rpm for 25 s.
  • Washing: Methyl acetate (MeOAc) is dispensed onto the film during spinning to remove native long-chain ligands like oleic acid and oleylamine.
  • Ligand Exchange: Immediately after washing, a solution of PEAI in ethyl acetate (EtOAc) is applied to the film for several seconds to conduct the solid-state ligand exchange. This step is crucial for introducing the short-chain PEAI ligand.
  • Drying: The film is dried after each cycle.
• Film Buildup: Steps 3a-3d are repeated multiple times (typically 3-5 cycles) to build a thick, compact PQD film (≈400 nm). The key differentiator from conventional methods is the application of PEAI solution after the deposition of each CsPbI3 PQD layer, rather than a single post-treatment at the end.

Critical Reagents and Materials

The table below lists essential reagents used in the PEAI-LBL process and their functions.

Table 2: Research Reagent Solutions for PEAI Post-Treatment

Reagent/Material	Function/Role in Experiment	Key Property
CsPbI3 PQDs (in octane)	Light-absorbing active layer material	Ideal bandgap (~1.73 eV); synthesized via hot-injection [1]
Phenethylammonium Iodide (PEAI)	Short-chain passivating ligand	Phenyl group for hydrophobicity; ammonium group binds to Pb2+ defects [2]
Methyl Acetate (MeOAc)	Washing solvent	Removes pristine long-chain insulating ligands [2]
Ethyl Acetate (EtOAc)	Solvent for PEAI	Dissolves PEAI for effective layer-by-layer treatment [2]

Underlying Mechanisms of PEAI Passivation

The superior performance of PEAI post-treatment can be attributed to its dual functionality in defect passivation and moisture protection, as shown in the following mechanistic diagram.

Title: Dual Passivation and Protection Mechanism of PEAI

• Defect Passivation: The ammonium group (-NH3+) in PEAI has a strong binding affinity for undercoordinated Pb2+ ions on the CsPbI3 PQD surface [30]. This coordination neutralizes these deep-level trap states, which are primary sources of non-radiative recombination. This leads to a enhancement in VOC and overall device efficiency [2].
• Hydrophobic Protection: The conjugated phenyl ring in the phenethylammonium moiety provides a hydrophobic shell around the PQDs [2]. This shell acts as a moisture-resistant barrier, inhibiting water ingress and significantly improving the environmental stability of the CsPbI3 film under high-humidity conditions [30].
• Enhanced Charge Transport: Replacing long, insulating ligands like oleylamine with shorter PEAI ligands reduces the inter-dot spacing. This dramatically improves dot-to-dot electronic coupling and facilitates more efficient charge carrier transport through the PQD solid film, which is reflected in higher fill factors [2].

FAI post-treatment has been a foundational method for improving CsPbI3 PQD film conductivity. However, evidence confirms that PEAI-based post-treatment, particularly the layer-by-layer approach, offers a more advanced and effective strategy for comprehensive surface defect mitigation.

The key advantages of PEAI include:

• Superior and Uniform Passivation: The PEAI-LBL method ensures thorough defect passivation throughout the film bulk, not just the top surface [2].
• Exceptional Stability: The innate hydrophobicity of PEAI grants CsPbI3 films remarkable resilience against moisture-induced degradation [2] [30].
• Balanced Charge Transport: By effectively removing insulating ligands while passivating defects, PEAI treatment enables high charge carrier mobility and reduced recombination.

For researchers aiming to maximize the performance and durability of CsPbI3 PQD optoelectronic devices, adopting a PEAI-based ligand management strategy represents a significant step forward. The transition from conventional FAI post-treatment to controlled, layered PEAI application is a critical progression in the pursuit of highly efficient and stable perovskite photovoltaics.

Balancing Ligand Removal and Charge Transport in PQD Solids

The development of efficient and stable perovskite quantum dot (PQD) solar cells represents a significant frontier in next-generation photovoltaics. Central to this progress is the management of the PQD surface, a domain where the imperative for sufficient ligand removal to enable charge transport directly conflicts with the need to preserve surface integrity to prevent defect formation and phase instability. This guide objectively compares two dominant post-treatment methodologies within CsPbI3 PQD films: formamidinium iodide (FAI) and phenethylammonium iodide (PEAI). By synthesizing data on photovoltaic performance, material stability, and optoelectronic properties, this analysis provides a framework for researchers to select optimal surface ligand strategies for their specific applications.

Comparative Analysis of FAI and PEAI Post-Treatments

The conventional layer-by-layer (LbL) assembly of CsPbI3 PQD films involves repeated cycles of spin-coating PQD inks and washing with methyl acetate (MeOAc) to replace insulating oleic acid ligands with short-chain acetates. A critical second step involves a post-treatment to address residual oleylamine (OLA) ligands. It is here that the choice between FAI and PEAI leads to divergent material properties and device outcomes [2].

Table 1: Comparison of FAI and PEAI Post-Treatment Characteristics

Characteristic	FAI Post-Treatment	PEAI Post-Treatment (Conventional)	PEAI Layer-by-Layer (LBL)
Chemical Formula	CH(NH₂)₂I	C₆H₅C₂H₄NH₃I	C₆H₅C₂H₄NH₃I
Primary Function	Replace OLA, passivate defects	Replace OLA, passivate defects	Replace OLA during film deposition
Treatment Method	Post-treatment after full film deposition	Post-treatment after full film deposition	Ligand exchange after each deposited layer
Impact on Composition	Can induce partial cation exchange to FA₁₋ₓCsₓPbI₃, compromising phase stability [2]	Resists cation exchange, preserving CsPbI3 composition [2]	Resists cation exchange, preserving CsPbI3 composition [2]
Defect Passivation	Effective on top layer; underlying trap states may remain [2]	Effective on top layer; improved passivation over FAI [2]	Superior, uniform passivation throughout the entire film thickness [2]
Reported PCE	~13-16% (literature range) [1]	~14.18% (champion device) [2]	~14.18% (champion device) [2]
Key Advantage	Established, effective short-chain ligand	Aromatic group for enhanced coupling, stability	Maximized inter-dot coupling and uniform passivation
Key Disadvantage	Difficult to control, can destabilize crystal phase	Standard post-treatment may not address sub-surface defects	More complex, multi-step processing

Performance and Stability Implications

The data reveals a critical trade-off. FAI treatments, while widely used, are difficult to control precisely. Prolonged exposure can induce a component change from CsPbI3 to FA₁₋ₓCsₓPbI3, introducing undesirable phase instability and compromising the long-term integrity of the PQD film [2]. In contrast, the larger PEA⁺ cation resists incorporation into the perovskite lattice, preserving the all-inorganic CsPbI3 composition and its associated benefits [2].

From an optoelectronic perspective, the conjugated phenyl ring of PEAI enhances inter-dot electronic coupling, which facilitates charge transport. Furthermore, devices employing PEAI-based treatments, particularly the LBL method, demonstrate superior stability under high-humidity environments (30-50% relative humidity) without encapsulation, a key advantage for practical deployment [2].

Experimental Protocols for Post-Treatment

To ensure reproducibility, the following detailed methodologies are provided for the key post-treatment processes cited in this comparison.

Synthesis of CsPbI3 PQD Precursors

The foundation of a high-quality film is a well-synthesized PQD ink. The following hot-injection method is standard in the field [2] [5] [31].

• Cs-Oleate Precursor: Combine 3.7 mmol Cs₂CO₃ with 5 mL oleic acid (OA) and 50 mL 1-octadecene (ODE) in a three-neck flask. Degas the mixture at room temperature, then heat to 120°C under a N₂ atmosphere until the Cs₂CO₃ is completely dissolved. Store for later use [31].
• PQD Synthesis: In a separate three-neck flask, load 2 mmol PbI₂, 1 mmol I₂, 1 mL OA, 2 mL oleylamine (OAm), and 50 mL toluene. Dry under vacuum for 5 minutes, then heat to 105°C under N₂ protection until a clear yellow solution is obtained.
• Injection and Termination: Rapidly inject 6 mL of the preheated Cs-oleate precursor (120°C) into the reaction flask. After 5-10 seconds, immediately cool the reaction vessel in an ice-water bath to terminate the reaction [31].
• Purification: Precipitate the PQDs by adding methyl acetate (3:1 volume ratio to the crude solution) and centrifuging at 10,000 × g for 1-3 minutes. Decant the supernatant and re-disperse the pellet in anhydrous n-hexane or n-octane for storage [2] [5].

Layer-by-Layer Film Deposition and Post-Treatment

The construction of a conductive PQD solid film involves a multi-cycle LbL process, with the post-treatment step being the critical variable.

• Substrate Preparation: Clean patterned FTO or ITO glass substrates with sequential sonication in detergent, deionized water, and ethanol. Treat with UV-Ozone for 15-20 minutes before use.
• Film Deposition:
  • Spin-coat the purified PQD solution in n-octane onto the substrate.
  • During the spin-coating process, dynamically drip methyl acetate (MeOAc) to wash the film and initiate the exchange of OA ligands for acetate ions [2] [5].
  • Repeat this spin-coating and MeOAc washing cycle 3-5 times to build the desired film thickness.
• Post-Treatment Strategies:
  • FAI Post-Treatment: Prepare a solution of FAI (e.g., 5 mg/mL) in ethyl acetate (EtOAc). After the final LbL cycle, drip this solution onto the completed PQD film during spinning, then anneal at ~70°C for 5-10 minutes [2].
  • Conventional PEAI Post-Treatment: Prepare a solution of PEAI (e.g., 2 mg/mL) in EtOAc. Apply this to the completed PQD film in the same manner as the FAI treatment [2].
  • PEAI Layer-by-Layer (LBL) Treatment: This advanced method involves a fundamental change in the process flow, as visualized below.

Diagram: Workflow comparison of PEAI-LBL versus standard post-treatment. The key difference is the integration of the PEAI treatment into every deposition cycle, rather than applying it only once at the end.

The Scientist's Toolkit: Essential Research Reagents

Table 2: Key Reagents for PQD Ligand Exchange Studies

Reagent	Function	Key Considerations
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain native ligands for colloidal synthesis and stabilization [17] [5].	Provide initial size control and dispersion but are insulating. Must be partially removed for device operation.
Methyl Acetate (MeOAc)	Polar solvent for solid-state ligand exchange; removes OA and introduces acetate ligands [2] [5].	Effectively removes long-chain ligands but can also strip surface ions, creating defects if not controlled [5].
Formamidinium Iodide (FAI)	Short-chain cationic ligand for post-treatment; passivates surface defects [2].	Risk of cation exchange with Cs⁺, leading to mixed phases and potential instability [2].
Phenethylammonium Iodide (PEAI)	Short-chain aromatic ammonium salt for post-treatment; passivates defects and enhances inter-dot coupling [2].	Hydrophobic phenyl group improves moisture stability. Larger ionic size minimizes lattice incorporation.
2-Pentanol	Protic solvent for ligand exchange. Tailored properties balance ligand solubility with minimal damage to PQD surface [9].	Higher ligand solubility allows for more complete removal of OAm without creating excessive halogen vacancies [9].
Triphenylphosphine Oxide (TPPO)	Covalent Lewis base ligand for surface stabilization [5].	Strongly coordinates with uncoordinated Pb²⁺ sites. Often dissolved in non-polar solvents like octane to prevent PQD dissolution [5].

The choice between FAI and PEAI post-treatments for CsPbI3 PQD solids is not merely a selection of chemicals but a strategic decision that directs the material's electronic and structural destiny. FAI, while effective, introduces a risk of compositional instability that can undermine device longevity. PEAI, particularly when deployed via an LBL methodology, offers a more robust path by ensuring uniform defect passivation, preserving the CsPbI3 phase, and enhancing moisture resistance without sacrificing electronic coupling. For research efforts prioritizing maximum stability and reproducible performance, the evidence strongly supports the adoption of advanced PEAI-based ligand management strategies. Future work will likely focus on refining these solvent-ligand systems further and exploring novel covalent ligands to push the boundaries of PQD device performance and durability.

Strategies for Enhancing Moisture and Oxygen Stability

The instability of CsPbI3 perovskite quantum dots (PQDs) when exposed to moisture and oxygen remains a significant bottleneck hindering their commercial application in photovoltaics and optoelectronics. [1] [15] While CsPbI3 PQDs possess an ideal bandgap (~1.73 eV) and high defect tolerance, their susceptibility to environmental degradation leads to rapid performance decline in solar cells and other devices. [1] Surface chemistry, governed by ligand interactions, plays a pivotal role in determining both the electronic properties and environmental resilience of PQD films. [2] [9] Within ligand engineering strategies, post-treatment methods using formamidinium iodide (FAI) and phenethylammonium iodide (PEAI) have emerged as two prominent approaches for modifying PQD surfaces. [2] This guide provides a comparative analysis of FAI and PEAI post-treatment strategies, evaluating their effectiveness in enhancing moisture and oxygen stability for CsPbI3 PQD films.

Performance Comparison: FAI vs. PEAI Post-Treatment

The selection of post-treatment ligands significantly influences the photovoltaic performance and stability of resulting CsPbI3 PQD solar cells. The following table summarizes key performance metrics reported for devices fabricated using FAI and PEAI-based treatments.

Table 1: Comparative Performance of CsPbI3 PQD Solar Cells with FAI and PEAI Post-Treatments

Treatment Strategy	Power Conversion Efficiency (PCE)	Open-Circuit Voltage (VOC)	Stability Performance	Key References
FAI Post-Treatment	Up to 16.53% [9]	~1.23 V [2]	Phase instability to FA1-xCsxPbI3; Moisture sensitivity [2]	Jia et al., Joule (2022) [9]
PEAI Post-Treatment	14.18% [2]	1.23 V [2]	Excellent humidity stability (30-50% RH); Phase stabilization [2]	Wang et al. (2024) [2]
PEAI Layer-by-Layer (LBL)	14.18% (Champion) [2]	1.23 V [2]	High stability under humidity; Retained performance unencapsulated [2]	Wang et al. (2024) [2]

Beyond initial photovoltaic parameters, the long-term operational stability under environmental stressors is a critical differentiator. The data indicates that while FAI treatments can lead to very high initial efficiencies, PEAI-based strategies, particularly the layer-by-layer approach, offer superior resilience against moisture.

Table 2: Stability Comparison under Environmental Stressors

Stress Factor	FAI Post-Treatment Response	PEAI Post-Treatment Response
Moisture/Humidity	Sensitive; performance degrades [2]	Excellent stability in 30-50% RH unencapsulated [2]
Phase Stability	Can induce phase change to FA1-xCsxPbI3, which is undesirable [2]	Enhances cubic phase stability of CsPbI3 PQDs [2]
Ligand Passivation	Incomplete passivation of subsurface trap states [2]	Enhanced defect passivation via conjugated phenyl group [2]

Experimental Protocols for Post-Treatment

Standard FAI Post-Treatment Protocol

The conventional FAI post-treatment is typically applied as a final step after depositing the complete CsPbI3 PQD film.

• Procedure: CsPbI3 PQD films are built up using a layer-by-layer (LBL) spin-coating process, typically involving 3-5 cycles. Each cycle consists of spin-coating the PQD solution followed by rinsing with methyl acetate (MeOAc) to remove excess solvent and initiate ligand exchange. After constructing the full film, a solution of FAI in ethyl acetate (EtOAc) is spin-coated onto the film as a final post-treatment. [2]
• Rationale: The FAI solution replaces the insulating long-chain oleylamine (OAm) ligands with shorter formamidinium-based ligands, improving inter-dot coupling and charge transport. [2]
• Limitation: This method primarily passivates the top layer of the film, leaving trap states underneath inadequately addressed. Furthermore, prolonged treatment can cause a compositional change from CsPbI3 to FA1-xCsxPbI3, compromising phase stability. [2]

PEAI Layer-by-Layer (LBL) Solid-State Exchange Strategy

The PEAI-LBL method integrates the ligand exchange directly into each layer deposition cycle, offering a more uniform and controlled process.

• Procedure:
  • A single layer of CsPbI3 PQDs is spin-coated onto the substrate.
  • The film is treated with methyl acetate (MeOAc) to remove solvent and initial long-chain ligands.
  • Before depositing the next layer, a solution of phenethylammonium iodide (PEAI) is applied to the current layer. This step is repeated after each subsequent PQD layer is deposited. [2]
• Rationale: This conjugated short-chain ligand (PEA+) with a phenyl group provides enhanced defect passivation and promotes better inter-dot coupling. The LBL application ensures thorough removal of insulating OAm ligands and more complete passivation throughout the entire film bulk, not just the surface. [2]
• Advantage: Leads to balanced electron and hole transport, improved moisture resistance due to the hydrophobic nature of the phenyl group, and enhanced phase stability without undesirable composition changes. [2]

The following workflow diagram illustrates the key differences between these two experimental approaches:

Diagram 1: Experimental workflow for FAI and PEAI LBL post-treatment.

The Scientist's Toolkit: Essential Research Reagents

Successful implementation of FAI and PEAI post-treatment strategies requires a specific set of high-purity materials. The following table lists key reagents and their functions in the experimental protocols.

Table 3: Essential Reagents for CsPbI3 PQD Post-Treatment Experiments

Reagent/Chemical	Function/Role	Key Characteristics & Considerations
Cesium Precursors (e.g., Cs₂CO₃, CsAcetate)	Synthesis of CsPbI3 PQDs via hot-injection method [2] [28]	High purity (99.99%); Determines A-site cation source [2]
Lead Iodide (PbI₂)	Provides Pb²⁺ and I⁻ for perovskite lattice formation [2] [28]	Ultra-dry, high purity (99.999%) to minimize impurities [2]
Formamidinium Iodide (FAI)	Short-chain ligand for post-treatment; replaces OAm [2]	High purity (99.99%); Can induce phase instability if overused [2]
Phenethylammonium Iodide (PEAI)	Conjugated short-chain ligand for LBL or post-treatment [2]	Hydrophobic phenyl group enhances moisture stability [2]
Methyl Acetate (MeOAc)	Antisolvent for rinsing/purification during LBL deposition [2] [28]	Anhydrous grade; Removes excess solvent and ligands [2]
Ethyl Acetate (EtOAc)	Solvent for dissolving FAI/PEAI for post-treatment [2]	Anhydrous grade; Used for ligand salt solutions [2]
Oleylamine (OAm)	Long-chain capping ligand from initial PQD synthesis [2]	Must be partially removed/replaced for charge transport [2]

Stability Enhancement Mechanisms

The divergent stability outcomes from FAI and PEAI treatments can be understood by examining their underlying chemical mechanisms and interactions with the PQD surface.

Defect Passivation and Surface Interaction

• FAI Mechanism: FAI primarily functions by replacing the dynamically bound oleylamine (OAm) ligands on the PQD surface. However, this passivation is often superficial. The treatment mainly addresses the top layer of the film, leaving subsurface trap states unpassivated, which can act as initiation sites for degradation. [2]
• PEAI Mechanism: The phenethylammonium ion (PEA+) in PEAI is a bulkier, conjugated molecule. Its phenyl group enables stronger interaction with the PQD surface, providing more effective defect passivation, particularly for iodine vacancy sites. The layer-by-layer application method ensures this passivation occurs throughout the film volume, not just at the surface. [2]

Moisture and Oxygen Barrier Formation

• Hydrophobicity: The key differentiator for moisture stability is the intrinsic hydrophobicity of the phenethylammonium ligand. The aromatic phenyl group creates a more hydrophobic surface on the PQDs, effectively repelling water molecules from the perovskite lattice. [2] FAI, lacking such a hydrophobic moiety, does not confer this protective characteristic.
• Film Morphology and Packing: The PEAI-LBL process promotes the creation of densely packed PQD films with enhanced inter-dot coupling. [2] This dense morphology reduces the pathways for moisture and oxygen ingress into the film, acting as a more effective barrier compared to the more porous films resulting from standard FAI post-treatment.

The following diagram summarizes the core mechanisms through which PEAI treatment enhances stability, directly addressing moisture and oxygen threats:

Diagram 2: Stability mechanisms of PEAI post-treatment.

The comparative analysis reveals that both FAI and PEAI post-treatments offer distinct pathways for enhancing the performance of CsPbI3 PQD films. FAI post-treatment can yield very high power conversion efficiencies, as evidenced by records exceeding 16%. [9] However, for applications where long-term operational stability under ambient conditions is paramount, PEAI-based strategies, particularly the layer-by-layer solid-state exchange method, present a superior alternative. The conjugated structure of the PEA+ ligand provides more robust defect passivation and its inherent hydrophobicity creates an effective moisture barrier. This results in CsPbI3 PQD films that maintain excellent photovoltaic performance and structural integrity under high-humidity environments, a critical advancement toward the commercialization of perovskite quantum dot solar cells and other optoelectronic devices. [2]

CsPbI3 perovskite quantum dots (PQDs) have emerged as a leading material for next-generation photovoltaics, combining the excellent optoelectronic properties of perovskites with the quantum confinement effects of nanoscale materials. [1] However, their widespread application is hampered by surface defects that lead to non-radiative recombination and phase instability. The quest for optimal defect passivation strategies has brought two ligands to the forefront: formamidine iodide (FAI) and phenethylammonium iodide (PEAI). This guide provides a comparative analysis of FAI and PEAI post-treatment methodologies for CsPbI3 PQD films, examining their respective mechanisms, efficacy, and limitations based on current experimental evidence. We further explore the emerging paradigm of synergistic ligand engineering that combines multiple approaches to overcome individual limitations.

Background: The Defect Challenge in CsPbI3 PQDs

Structural Vulnerabilities and Defect Formation

CsPbI3 PQDs exhibit an ABX3 crystal structure where cesium (Cs+) occupies the A-site, lead (Pb2+) the B-site, and iodide (I-) the X-site. [1] This ionic structure, while advantageous for optoelectronic properties, creates inherent vulnerabilities:

• Surface Defects: Inherent ionic nature leads to weakly bound surface ligands that readily detach, creating uncoordinated Pb2+ sites and iodide vacancies. [32]
• Phase Instability: The photoactive cubic phase (α-CsPbI3) readily transitions to a non-photoactive orthorhombic phase (δ-CsPbI3) at room temperature. [1] [16]
• Ligand Limitations: Native long-chain ligands (oleic acid, oleylamine) provide steric stabilization but inhibit inter-dot charge transport due to their insulating nature. [16] [32]

Table 1: Common Defect Types in CsPbI3 PQDs and Their Impacts

Defect Type	Location	Impact on Device Performance
Iodide vacancies	Surface/Lattice	Non-radiative recombination, phase instability
Uncoordinated Pb2+ sites	Surface	Trap states, reduced PLQY
Ligand vacancies	Surface	Aggregation, reduced colloidal stability
Cs+ vacancies	Surface	Reduced charge transport

The Role of Ligand Engineering in Defect Passivation

Ligand exchange strategies address these challenges by replacing native long-chain ligands with functional molecules that simultaneously passivate defects and enhance charge transport. [16] [32] Effective passivation requires molecules with:

• Strong coordination groups (e.g., -NH2, -C=O, P=O) to bind uncoordinated Pb2+ sites
• Appropriate steric properties to balance passivation and charge transport
• Chemical compatibility with the perovskite lattice
• Environmental stability to resist moisture and heat degradation

Individual Ligand Performance: FAI vs. PEAI

Formamidine Iodide (FAI) Post-Treatment

FAI has been employed as a short-chain ligand for surface passivation through post-synthesis treatment. The formamidinium cation (FA+) coordinates with surface sites while providing iodide ions to fill halide vacancies.

Experimental Protocol: FAI post-treatment typically involves dissolving FAI in ethyl acetate (EtOAc) to create a saturated solution (concentration ~1-5 mg/mL). The CsPbI3 PQD films are then treated with this solution during the layer-by-layer deposition process, with each layer spin-coated at 2000 rpm for 30 seconds followed by FAI solution application and drying. [16]

Performance Characteristics:

• Provides iodide ions to fill vacancies and reduce halide-deficient surfaces
• Formamidinium cations offer moderate coordination with surface sites
• Short chain length improves inter-dot charge transport compared to native ligands
• Limited by relatively weak binding energy compared to phosphonate groups

Phenethylammonium Iodide (PEAI) Post-Treatment

PEAI has gained significant attention for surface passivation due to the phenethylammonium cation's ability to coordinate with surface sites while providing steric protection.

Experimental Protocol: PEAI is typically dissolved in appropriate solvents (often ethyl acetate or methyl acetate) at concentrations ranging from 0.5-2 mg/mL. The solution is applied to CsPbI3 PQD films during the layer-by-layer deposition process, with each layer treated and then dried at moderate temperatures (60-80°C). [33]

Performance Characteristics:

• PEA+ cations strongly coordinate with surface sites due to aromatic ring interaction
• Ammonium group provides strong binding to perovskite surface
• Introduces a potential challenge of forming reduced-dimensional perovskites (RDPs)
• The large ionic radius of PEA+ makes it incompatible with the 3D perovskite framework, acting as an organic spacer [33]

Table 2: Direct Comparison of FAI and PEAI Post-Treatment Performance

Parameter	FAI Post-Treatment	PEAI Post-Treatment
Binding Mechanism	Iodide vacancy filling + FA+ coordination	PEA+ coordination with aromatic interaction
Dimensionality Effects	Maintains 3D structure	Can form reduced-dimensional phases (RDPs) [33]
Charge Transport	Moderate improvement	Good improvement but limited by potential RDP formation
Phase Stability	Improved cubic phase stability	Good but RDPs may undergo phase transition [33]
Reported PCE	Up to 13.91% (in combination with BPA) [16]	Up to 15.3% (in hybrid approach with TPPO) [33]
Stability Retention	91% after 800 h storage, 92% after 200 h illumination [16]	Improved but exact metrics not specified in sources

Synergistic Ligand Engineering Strategies

Hybrid Ligand Systems: PEAI with TPPO

Recent research reveals that PEAI alone can induce the formation of high-n reduced dimensional perovskites (RDPs, n > 2) within CsPbI3-PQD solids, which subsequently undergo undesirable phase transition to low-n RDPs, leading to structural and optical degradation. [33] To address this limitation, a hybrid ligand approach incorporating triphenylphosphine oxide (TPPO) as an ancillary ligand has been developed.

Mechanism of Action:

• TPPO prevents H2O penetration and regulates rapid PEAI diffusion, suppressing low-n RDP formation [33]
• TPPO passivates uncoordinated Pb2+ sites through strong P=O→Pb coordination
• PEAI provides additional surface coverage and iodide ions
• The combination maintains structural integrity while enhancing optoelectronic properties

Performance Outcomes:

• Enables efficient CsPbI3-PQD-based solar cells (PCE of 15.3%) [33]
• Improves device stability by suppressing phase transitions
• Reduces non-radiative recombination through comprehensive defect passivation

Multi-Step Ligand Management: BPA with FAI-like Approaches

While not directly combining FAI and PEAI, stepwise ligand management strategies demonstrate the principle of synergistic approaches. One study implemented a two-step "preparation-film formation" strategy using benzylphosphonic acid (BPA): [16]

• Preparation Phase: Introduction of short-chain BPA with P=O group into crude QD solution for initial passivation and long-chain substitution
• Film Formation Phase: Secondary surface modification by adding BPA into washing solvent to completely remove long chains and protect interface

Performance Outcomes:

• PCE improvement from 11.41% to 13.91% [16]
• Storage stability: 91% original efficiency retained after 800 h
• Operational stability: 92% original efficiency retained after 200 h continuous illumination

Experimental Protocols for Comparative Studies

PQD Synthesis and Film Preparation

CsPbI3 QD Synthesis (Hot Injection Method): [16] [21]

• Prepare Cs-oleate by reacting Cs2CO3 (0.814 g) with oleic acid (2.5 mL) in 1-octadecene (ODE, 40 mL) at 120°C under vacuum, then at 150°C under N2 until clear
• In separate flask, mix PbI2 (0.552 g) with ODE (30 mL), degas at 120°C for 1 h
• Add oleylamine (3 mL) and oleic acid (3 mL) under N2 flow, heat to 165°C
• Quickly inject Cs-oleate (2.4 mL), react for 7 seconds, then ice-water bath quenching
• Purify crude solution with methyl acetate centrifugation

Layer-by-Layer Film Deposition: [16]

• Spin-coat PQD solution (85 mg/mL in octane) at 1000 rpm for 10 s + 2000 rpm for 25 s
• Apply washing solvent (MeOAc with/without ligands) during spin-coating
• Repeat process 4 times to achieve ~400 nm thickness
• Final treatment with ligand solutions (FAI, PEAI, or combinations)

Ligand Exchange and Post-Treatment

FAI Treatment: [16]

• Prepare saturated FAI solution in ethyl acetate
• Apply during layer-by-layer deposition after each layer
• Optimize concentration (~1-5 mg/mL) for complete coverage without excessive residue

PEAI Treatment: [33]

• Dissolve PEAI in methyl acetate or ethyl acetate (0.5-2 mg/mL)
• Apply during film deposition with controlled drying steps
• Monitor for potential RDP formation through absorption measurements

Hybrid PEAI/TPPO Treatment: [33]

• Implement both ligands during exchange process
• Optimize ratio to suppress low-n RDP formation while maintaining passivation
• Control application sequence based on desired interface properties

Characterization and Performance Metrics

Structural and Optical Analysis

Phase Purity Assessment:

• X-ray diffraction (XRD) to monitor cubic phase stability and detect RDP formation [1]
• Absorption spectroscopy to track phase transitions and bandgap changes
• Photoluminescence (PL) spectroscopy to quantify non-radiative recombination

Surface Chemistry Analysis:

• Fourier-transform infrared spectroscopy (FTIR) to confirm ligand binding
• Nuclear magnetic resonance (NMR) to quantify ligand density and composition
• X-ray photoelectron spectroscopy (XPS) to analyze surface elemental composition

Device Performance and Stability Metrics

Table 3: Comprehensive Performance Comparison of Ligand Strategies

Performance Metric	Native Ligands	FAI Treatment	PEAI Treatment	Hybrid PEAI/TPPO
PCE (%)	~11.4 [16]	~13.9 [16]	<15.3 [33]	15.3 [33]
VOC (V)	Data not available	Improved	Improved	Significantly improved
JSC (mA/cm2)	Data not available	Improved	Improved	Significantly improved
Fill Factor (%)	Data not available	Improved	Improved	Significantly improved
Storage Stability	Poor	91% (800 h) [16]	Moderate	Significantly improved
Operational Stability	Poor	92% (200 h) [16]	Moderate	Significantly improved
Phase Stability	Moderate	Good	Limited by RDP formation [33]	Excellent [33]

Research Reagent Solutions

Table 4: Essential Research Reagents for PQD Ligand Studies

Reagent	Function	Application Notes
Cesium carbonate (Cs2CO3)	Cs+ precursor for Cs-oleate synthesis	Must be thoroughly dried and stored anhydrous
Lead iodide (PbI2)	Pb2+ precursor for PQD synthesis	High purity (99.999%) recommended
Oleic acid (OA)	Native capping ligand	Requires degassing at 100°C before use [16]
Oleylamine (OAm)	Native capping ligand	Often used with OA for size control [32]
1-Octadecene (ODE)	Reaction solvent	High boiling point solvent for hot injection
Formamidine iodide (FAI)	Short-chain passivator	Source of iodide and coordination cations
Phenethylammonium iodide (PEAI)	Aromatic passivator	Can induce RDP formation; requires control [33]
Triphenylphosphine oxide (TPPO)	Co-passivator	Suppresses RDP formation from PEAI [33]
Benzylphosphonic acid (BPA)	Short-chain phosphonate passivator	Strong Pb coordination; used in stepwise process [16]
Methyl acetate (MeOAc)	Washing solvent	Medium polarity for ligand exchange

Mechanisms and Pathways: A Visual Analysis

The following diagrams illustrate the key mechanisms and experimental workflows discussed in this review:

This comparative analysis demonstrates that both FAI and PEAI post-treatments offer distinct advantages for CsPbI3 PQD defect passivation, with FAI providing reliable phase stability and PEAI offering strong coordination but risking RDP formation. The most promising path forward emerges from hybrid ligand strategies that combine the strengths of multiple approaches while mitigating their individual limitations. The PEAI/TPPO system exemplifies this synergy, achieving enhanced device performance (15.3% PCE) while suppressing the detrimental phase transitions associated with PEAI alone. [33] Future research should explore novel ligand combinations, including potential FAI/PEAI hybrids with appropriate ancillary ligands, to further optimize the balance between passivation efficacy, charge transport, and long-term stability. The field increasingly recognizes that comprehensive defect management requires multi-faceted approaches rather than reliance on single-ligand solutions.

Direct Performance Comparison: Efficiency, Stability, and Electronic Properties

This guide provides a comparative analysis of photovoltaic performance metrics within the context of post-treatment strategies for CsPbI3 Perovskite Quantum Dot (PQD) films. Power Conversion Efficiency (PCE), Open-Circuit Voltage (VOC), Short-Circuit Current Density (JSC), and Fill Factor (FF) are interdependent parameters that collectively define solar cell performance. Surface engineering, particularly via post-treatment and intermediate-treatment with ammonium salts such as FAI (formamidinium iodide) and PEAI (phenethylammonium iodide), is a pivotal method for enhancing these metrics by modulating film quality, passivating defects, and optimizing energy level alignment. This article objectively compares the influence of different treatment approaches on the resulting photovoltaic parameters of CsPbI3 PQD solar cells, supported by experimental data and detailed methodologies.

The performance of a solar cell is quantified by four primary metrics: Power Conversion Efficiency (PCE), Open-Circuit Voltage (VOC), Short-Circuit Current Density (JSC), and Fill Factor (FF). These parameters are extracted from the current density-voltage (J-V) curve measured under standard illumination (AM1.5G) [34].

• Power Conversion Efficiency (PCE) is the ultimate figure of merit, representing the percentage of incident light power converted into usable electrical power. It is the product of VOC, JSC, and FF, divided by the incident light power.
• Open-Circuit Voltage (VOC) is the maximum voltage available from a solar cell at zero current. It is fundamentally governed by the bandgap of the absorber material and is highly sensitive to trap-assisted non-radiative recombination within the device [34].
• Short-Circuit Current Density (JSC) is the current per unit area when the voltage is zero, indicating the cell's capacity to generate current from sunlight. It depends on the light absorption properties of the active layer and the efficient extraction of photo-generated charge carriers [34].
• Fill Factor (FF) describes the "squareness" of the J-V curve and is defined as the ratio of the maximum power point to the product of VOC and JSC. It is influenced by the series and shunt resistances within the solar cell structure, which are in turn affected by film morphology and defect density [34].

A critical challenge in photovoltaic development is the inherent trade-off between VOC and JSC. Modifying the absorber material to reduce its bandgap can enhance JSC by broadening the light absorption spectrum, but this typically results in a lower VOC [35]. Furthermore, a low VOC can signal the presence of trap states, impurities, or non-photoactive phases in the absorber layer, while a low JSC may point to issues with charge extraction or insufficient light harvesting [34]. Therefore, optimizing solar cell performance involves carefully balancing these parameters through material and device engineering.

Comparative Performance Data of CsPbI3 PQD Solar Cells

The following tables consolidate experimental data from recent studies on CsPbI3 perovskite solar cells, highlighting how different treatment strategies impact key performance metrics.

Table 1: Performance comparison of CsPbI3 solar cells with different post-treatments.

Treatment Strategy	Active Layer	PCE (%)	VOC (V)	JSC (mA cm⁻²)	FF (%)	Citation
Guanidine Hydrobromide (GABr) Post-treatment	CsPbI3	18.02	N/A	N/A	N/A	[36]
1-4FP Additive & Post-treatment	CsPb0.7Sn0.3I3	17.19	N/A	N/A	N/A	[37]
FAI Intermediate-treatment	CsPbI3 (inverted)	15.45	N/A	N/A	N/A	[38]
Control Device	CsPb0.7Sn0.3I3	4.89	N/A	N/A	N/A	[37]
Control Device	CsPbI3 (inverted)	11.39	N/A	N/A	N/A	[38]

Table 2: Performance data illustrating the VOC-JSC trade-off in polymer solar cells, a universal challenge in photovoltaics [35].

Small-Molecule Acceptor	VOC (V)	JSC (mA cm⁻²)	PCE (%)
IDTT-BOA	1.19	Low	2.16
IDTT-BOACl	1.16	Low	3.45
ICCl-IDTT-BOACl	0.99	18.37	10.20

The data in Table 1 demonstrates that surface treatments can dramatically enhance PCE. For instance, the 1-4FP post-treatment strategy for tin-lead perovskite films achieved a record PCE of 17.19%, a significant improvement over the control device at 4.89% [37]. Similarly, FAI intermediate-treatment for inverted CsPbI3 cells boosted PCE from 11.39% to 15.45% [38]. Table 2 explicitly shows the VOC-JSC trade-off, where acceptors with higher VOC (IDTT-BOA, IDTT-BOACl) deliver low JSC and mediocre PCE, while a balanced acceptor (ICCl-IDTT-BOACl) achieves a higher PCE by optimizing both VOC and JSC [35].

Experimental Protocols for Post-Treatment of Perovskite Films

The following are detailed methodologies for post-treatment and intermediate-treatment processes as cited in recent literature.

Combined Additive and Post-treatment Strategy for Pb-Sn Perovskites

This protocol outlines the method used to achieve a high-quality CsPb0.7Sn0.3I3 film [37]:

• Perovskite Precursor Solution: Prepare the precursor by dissolving CsI, PbI2, SnI2, and SnF2 in a mixed solvent of DMF and DMSO.
• Additive Incorporation: Add a concentration of 1-(4-fluorophenyl) piperazine (1-4FP) directly into the perovskite precursor solution.
• Film Deposition: Spin-coat the precursor solution onto the substrate to form a wet film.
• Post-treatment: After film deposition, dynamically drip an isopropanol (IPA) solution containing 1-4FP onto the spinning film during the annealing process.
• Film Annealing: Complete the thermal annealing step to crystallize the perovskite film. The interaction between the NH group of 1-4FP and Sn²⁺ retards crystallization, improves film homogeneity, and cross-links grains to prevent Sn oxidation [37].

Surface Post-treatment with Guanidine Hydrobromide (GABr)

This method was employed to reduce energy loss and optimize band alignment in CsPbI3 solar cells [36]:

• Perovskite Film Preparation: Deposit the CsPbI3 perovskite film using the standard one-step solution method.
• Post-treatment Solution: Dissolve GABr in IPA at a specific concentration.
• Treatment Application: Dynamic spin-coating of the GABr-IPA solution onto the pre-formed CsPbI3 film.
• Thermal Annealing: Anneal the treated film at 100°C for 5 minutes to facilitate the passivation reaction. This treatment was found to passivate surface defects, reduce non-radiative recombination, and optimize the energy level alignment at the interface [36].

FAI Intermediate-treatment for Inverted CsPbI3 Solar Cells

This strategy involves treatment during film fabrication, rather than after, to improve ambient stability [38]:

• Perovskite Film Deposition (First Step): Spin-coat the CsPbI3 precursor solution to begin film formation.
• Intermediate-treatment Application: Before the film is fully crystallized, introduce a solution of FAI in IPA via spin-coating.
• Film Crystallization: Proceed with thermal annealing to complete the crystallization process. The strong interaction between the FA cation and the [PbI6]⁴⁻ octahedron during formation improves moisture resistance, accelerates crystallization, and reduces exposure time to ambient humidity, leading to superior film quality [38].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key reagents and materials used in the synthesis and treatment of CsPbI3 PQD films and devices.

Reagent/Material	Function/Application	Citation
Cesium Iodide (CsI)	Precursor for the 'A'-site cation in CsPbI3 perovskite.	[37] [1]
Lead Iodide (PbI₂)	Precursor for the 'B'-site cation and iodide anion in the perovskite lattice.	[37] [1]
Tin(II) Iodide (SnI₂)	Partial replacement for PbI₂ to form low-bandgap Pb-Sn alloyed perovskites.	[37]
Dimethylformamide (DMF) / Dimethyl Sulfoxide (DMSO)	Common solvents for preparing perovskite precursor solutions.	[37]
1-(4-Fluorophenyl)piperazine (1-4FP)	Additive and post-treatment agent for passivation and suppression of Sn²⁺ oxidation.	[37]
Formamidinium Iodide (FAI)	Intermediate-treatment or post-treatment salt for surface passivation and stability enhancement.	[38]
Guanidine Hydrobromide (GABr)	Post-treatment agent for defect passivation and reduction of non-radiative recombination.	[36]
Phenyl-C61-butyric acid methyl ester (PCBM)	Electron transport layer material in conventional and inverted device architectures.	[37] [38]
Isopropanol (IPA)	Common solvent for preparing post-treatment solutions.	[37] [36] [38]

Workflow and Mechanism of Post-Treatment Strategies

The following diagram illustrates the general experimental workflow and the proposed mechanisms of action for post-treatment and intermediate-treatment strategies in perovskite film processing.

Figure 1: Workflow for intermediate and post-treatment of perovskite films.

The mechanism of these treatments involves several key effects on the perovskite material and film morphology, as validated by experimental characterizations:

• Surface Reconstruction and Passivation: Post-treatment molecules like 1-4FP interact with the perovskite surface. The amine group (NH) with lone electron pairs coordinates with undercoordinated Sn²⁺ or Pb²⁺ ions, effectively passivating these surface defects and reducing charge recombination centers [37]. This leads to a significant increase in VOC.

• Suppression of Cation Oxidation: In Sn-containing perovskites, a major degradation pathway is the oxidation of Sn²⁺ to Sn⁴⁺. The cross-linking of neighboring perovskite grains by hydrogen bonds from the post-treatment agent creates a physical barrier that prevents oxygen ingress, thereby enhancing operational stability [37].

• Optimization of Energy Level Alignment: Treatments with molecules like GABr can modify the surface potential and work function of the perovskite layer, leading to more favorable energy level alignment with adjacent charge transport layers (e.g., PCBM) [36]. This reduces the electron transport barrier, facilitating charge extraction and boosting JSC and FF.

• Enhanced Crystallization and Moisture Resistance: Intermediate-treatments, such as with FAI salts, integrate the passivator directly into the crystal formation process. The FA⁺ cation strongly interacts with the [PbI6]⁴⁻ octahedron, which not only passivates defects but also improves the film's resistance to moisture during and after fabrication, addressing a key instability of CsPbI3 perovskites [38].

Phase and Morphological Stability Under Environmental Stressors

The phase and morphological stability of cesium lead iodide perovskite quantum dots (CsPbI3 PQDs) under environmental stressors is a critical determinant of their performance and longevity in optoelectronic devices. Colloidal CsPbI3 PQDs have emerged as promising materials for photovoltaics and light-emitting applications due to their ideal bandgap (~1.73 eV), high photoluminescence quantum yield, and defect-tolerant nature [1] [15]. However, these nanomaterials face significant stability challenges arising from their intrinsic ionic character, dynamic surface chemistry, and susceptibility to phase transitions under thermal, moisture, and optical stresses [39] [13].

Surface ligand engineering through post-treatment strategies has become a fundamental approach for enhancing PQD stability. This comparative analysis examines two prominent ligand systems: formamidinium iodide (FAI) and phenethylammonium iodide (PEAI). These ammonium salts facilitate solid-state ligand exchange processes where organic cations replace native insulating ligands on PQD surfaces, thereby influencing both interfacial chemistry and crystal structure stability [2] [1]. The selection between FAI and PEAI post-treatments involves critical trade-offs between charge transport properties and environmental stability, requiring systematic evaluation under relevant operational stressors.

Comparative Performance Under Environmental Stressors

Photovoltaic Performance and Stability Metrics

Table 1: Comparative performance metrics of FAI and PEAI post-treated CsPbI3 PQD solar cells

Performance Parameter	FAI Post-treatment	PEAI Post-treatment	Measurement Conditions
Champion PCE (%)	15.21% [2]	14.18% [2]	Standard solar illumination
Open-circuit Voltage (Voc)	~1.23 V [1]	1.23 V [2]	-
Electroluminescence Performance	EQEEL: 1.05-3.8% [2]	Luminance: 130 Cd/m² [2]	As LED device
Moisture Stability	Poor humidity tolerance [1]	Excellent stability at 30-50% RH [2]	Unencapsulated, 25°C
Thermal Stability	Phase transition at elevated temperatures [39]	Enhanced thermal tolerance [2]	-

Phase Stability Under Thermal Stress

Table 2: Thermal degradation behavior of CsPbI3 PQDs with different A-site compositions

Thermal Stress Response	Cs-rich PQDs	FA-rich PQDs	Experimental Conditions
Degradation Onset Temperature	~150°C [39]	~150°C [39]	In situ XRD, argon flow
Primary Degradation Pathway	γ-phase to δ-phase transition [39]	Direct decomposition to PbI₂ [39]	Heating to 500°C
Ligand Binding Energy	Weaker ligand binding [39]	Stronger ligand binding [39]	DFT calculations
Electron-Phonon Coupling	Weaker LO phonon coupling [39]	Stronger LO phonon coupling [39]	Photoluminescence studies

Experimental Protocols for Stability Assessment

PQD Synthesis and Film Fabrication

CsPbI3 PQD Synthesis via Hot-Injection Method: Cs-oleate precursor is prepared by reacting Cs₂CO₃ (0.610 g) with oleic acid (2.5 mL) in 1-octadecene (30 mL) at 150°C under nitrogen atmosphere [13]. Separately, PbI₂ (1 g) is dissolved in 1-octadecene (50 mL) with oleylamine (5 mL) and oleic acid (5 mL) at 120°C under vacuum. The Cs-oleate precursor is rapidly injected into the PbI₂ solution at 170°C, immediately forming CsPbI3 PQDs. The reaction is quenched using an ice bath, and PQDs are purified via centrifugation [13].

Layer-by-Layer Film Deposition: PQD films are fabricated through iterative spin-coating cycles. Each layer is deposited by spin-coating PQD solution (in hexane/octane) followed by solvent washing with methyl acetate (MeOAc) or ethyl acetate (EtOAc) to remove excess ligands and promote inter-dot coupling [2]. Typical film thickness of 190 nm is achieved through 3-5 deposition cycles [40].

Post-Treatment Methodologies

FAI Post-Treatment: FAI solution dissolved in ethyl acetate is applied as a final treatment after complete film deposition. Conventional concentration ranges from 1-5 mg/mL, with processing time carefully controlled to prevent component change from CsPbI₃ to FA₁₋ₓCsₓPbI₃, which compromises phase stability [2].

PEAI-LBL Strategy: Phenethylammonium iodide (PEAI) solution is applied after each deposition layer during the layer-by-layer process. This approach enables more uniform ligand distribution throughout the film thickness, addressing trap states that remain unpassivated in conventional post-treatment methods [2].

Stability Testing Protocols

Thermal Stability Assessment: In situ XRD measurements from 30°C to 500°C under argon flow monitor phase transitions and decomposition pathways. Temperature-dependent photoluminescence spectra quantify electron-phonon coupling strength [39].

Moisture Stability Testing: Unencapsulated devices are exposed to controlled humidity environments (30-50% RH at 25°C) with periodic performance evaluation. PEAI-treated films demonstrate enhanced hydrophobic character due to phenyl groups [2].

Photostability Testing: Films undergo continuous-wave excitation at varying power densities to determine photostability thresholds. Advanced testing employs 3D microporous substrates to improve thermal dissipation, achieving approximately 2.5× enhancement in stability threshold compared to conventional planar substrates [41].

Stability Mechanisms and Pathways

Phase Transition Pathways Under Thermal Stress

Diagram 1: Thermal degradation pathways of CsPbI₃ PQDs with different A-site compositions. Cs-rich PQDs undergo phase transition from black γ-phase to yellow δ-phase, while FA-rich PQDs directly decompose to PbI₂ at elevated temperatures. Stronger ligand binding in FA-rich PQDs influences the degradation mechanism [39].

Interfacial Stabilization Mechanisms

Diagram 2: Interfacial stabilization mechanisms of FAI versus PEAI post-treatment strategies. Conventional FAI treatment primarily passivates the top surface, leaving bulk traps unaddressed. The PEAI layer-by-layer approach enables uniform passivation throughout the film and enhanced inter-dot coupling [2].

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key research reagents and materials for CsPbI₃ PQD stability studies

Reagent/Material	Function	Experimental Role	Impact on Stability
Phenethylammonium Iodide (PEAI)	Short-chain ligand	Surface passivation, Inter-dot coupling	Enhanced moisture resistance, Phase stability [2]
Formamidinium Iodide (FAI)	A-site cation source	Ligand exchange, Surface modification	Limited to top-layer passivation [2]
Methyl Acetate (MeOAc)	Polar solvent	Ligand removal, Film consolidation	Controls inter-dot distance [2] [13]
Oleic Acid/Oleylamine	Native ligands	Synthesis stabilization, Colloidal dispersion	Impedes charge transport if not removed [2]
2-Pentanol	Protic solvent	Mediates ligand exchange	Maximizes ligand removal, Reduces defects [9]
Cs-oleate	Cesium precursor	Quantum dot synthesis	Determines A-site composition [13]

The comparative analysis of FAI and PEAI post-treatment strategies reveals fundamentally different approaches to enhancing CsPbI₃ PQD stability under environmental stressors. FAI post-treatment provides reasonable initial performance but demonstrates limitations in passivation depth and humidity resistance. In contrast, the PEAI layer-by-layer approach enables more uniform bulk passivation, significantly improving moisture stability while maintaining competitive photovoltaic performance.

The thermal degradation pathways further highlight the complex interplay between A-site composition, ligand binding energy, and stability mechanisms. Cs-rich PQDs undergo phase transitions, while FA-rich systems directly decompose, with ligand binding strength playing a determining role in degradation kinetics. These insights provide critical guidance for researchers designing stable PQD formulations for specific operational environments.

Future developments in PQD stability will likely focus on hybrid ligand systems that combine the advantages of different chemical approaches, advanced substrate engineering for improved thermal management, and multimodal stability testing under combined stressors. The continued refinement of post-treatment methodologies represents a promising pathway toward commercial viability for perovskite quantum dot optoelectronics.

The performance of CsPbI₃ perovskite quantum dot (PQD) solar cells is fundamentally governed by the charge transport properties within the solid-state PQD film, which are primarily dictated by the degree of inter-dot electronic coupling and the resulting carrier mobility [3] [15]. The pristine PQDs, synthesized with long-chain insulating ligands like oleic acid (OA) and oleylamine (OAm), exhibit poor charge transport due to large inter-dot spacing [2] [15]. Consequently, surface ligand engineering is a critical strategy to enhance inter-dot coupling, and post-treatment with short-chain ligands has emerged as a dominant and effective technique [3] [2] [9]. Among various ligands, formamidinium iodide (FAI) and phenethylammonium iodide (PEAI) have been extensively studied as model systems for tuning the surface chemistry of CsPbI₃ PQDs [3] [2]. This guide provides a comparative analysis of FAI and PEAI post-treatment strategies, objectively evaluating their influence on inter-dot coupling, carrier mobility, and overall photovoltaic performance through a synthesis of experimental data and protocols.

Performance Comparison of FAI and PEAI Post-Treatments

The following tables summarize key photovoltaic parameters and characteristics of CsPbI₃ PQD films treated with FAI and PEAI, based on reported experimental data.

Table 1: Comparison of Photovoltaic Performance Parameters

Treatment	PCE (%)	JSC (mA/cm²)	VOC (V)	Fill Factor	Certification
FAI Post-Treatment	13.43 [3]	14.37 [3]	1.20 [3]	0.78 [3]	Certified [3]
PEAI-LBL Treatment	14.18 [2]	~15.2 (est. from PCE)	1.23 [2]	~0.76 (est. from PCE)	Not specified
Conventional FAI-LBL	10.77 (initial report) [42]	-	-	-	-

Table 2: Comparison of Film and Transport Properties

Property	FAI Post-Treatment	PEAI-LBL Treatment
Primary Effect	Improves electronic coupling between QDs [3].	Enhances inter-dot coupling & provides superior surface defect passivation [2].
Impact on Mobility	Doubles film mobility, enabling increased photocurrent [3] [43].	Promotes balanced transport and injection of electrons and holes [2].
Stability	Susceptible to oxygen-induced degradation [42].	Excellent moisture stability (30-50% RH, ~25°C) due to hydrophobic PEA+ group [2].
Key Advantage	General method, significant JSC improvement [3].	Higher VOC, better stability, and balanced charge transport [2].
Key Challenge	Treatment is difficult to control; can induce phase instability [2].	-

Experimental Protocols for Post-Treatment and Analysis

Core Post-Treatment Methodologies

1. Layer-by-Layer (LbL) Film Deposition and FAI Post-Treatment This protocol is adapted from the work that established a certified 13.43% efficiency [3].

• PQD Film Fabrication: CsPbI₃ QDs are deposited in a layer-by-layer fashion via spin-coating from a non-polar solvent like octane.
• Initial Ligand Exchange: After each QD layer deposition, the film is immersed in a saturated solution of lead(II) nitrate [Pb(NO₃)₂] in methyl acetate (MeOAc). This step partially removes the native long-chain OA/OAm ligands and allows for the deposition of subsequent layers without re-dispersing the existing ones.
• FAI Post-Treatment: After building a thick film (200-400 nm, 3-4 layers), the entire film is immersed in a saturated solution of FAI in ethyl acetate (EtOAc) for approximately 10 seconds [3].
• Function: The FAI treatment replaces the remaining insulating ligands, tunes the electronic coupling at the QD-QD junctions, and enhances charge carrier mobility [3].

2. PEAI Layer-by-Layer (PEAI-LBL) Solid-State Ligand Exchange This protocol is adapted from the work achieving 14.18% PCE [2].

• PQD Film Fabrication: The process begins similarly with the spin-coating of a CsPbI₃ QD layer.
• PEAI Ligand Exchange per Layer: Instead of a final post-treatment, after the deposition of each QD layer and immediately after the MeOAc washing step, the film is treated with a PEAI solution [2].
• Function: This method ensures a more thorough and uniform removal of long-chain OAm ligands and passivation of surface defects throughout the entire film thickness, not just the top layer. The conjugated phenyl group in PEA⁺ also contributes to enhanced inter-dot coupling and defect passivation [2].

Key Characterization Techniques for Charge Transport

• Current Density-Voltage (J-V) Measurements: The primary method for determining photovoltaic performance parameters (PCE, JSC, VOC, FF) under simulated solar illumination (AM 1.5G) [3]. Hysteresis between forward and reverse scans can provide insights into ionic mobility and defect-related issues [3].
• External Quantum Efficiency (EQE): Measures the device's responsiveness to different light wavelengths. A broadband improvement in EQE after AX treatments, without a change in the absorption onset, indicates enhanced charge carrier collection efficiency rather than improved light absorption [3].
• Stabilized Power Output (SPO): The device is held at the maximum power point voltage, and the current output is measured over time. This provides a more realistic efficiency metric than the J-V scan for hysteretic devices like perovskites [3].
• Mobility Characterization: While the search results do not specify the exact technique, the doubling of film mobility cited was likely measured using methods such as space-charge-limited current (SCLC) or field-effect transistor (FET) measurements [3] [43].

The following workflow diagram illustrates the key experimental pathways for fabricating and analyzing CsPbI₃ PQD solar cells.

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents and Materials for CsPbI₃ PQD Solar Cell Research

Reagent/Material	Function in Research	Key References
Cesium Carbonate (Cs₂CO₃)	Precursor for synthesizing Cs-oleate, the source of Cs⁺ ions in QDs.	[1] [13]
Lead Iodide (PbI₂)	Provides Pb²⁺ and I⁻ ions for the perovskite crystal structure.	[1] [13]
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain ligands used in synthesis to control QD growth and provide colloidal stability.	[2] [15] [42]
1-Octadecene (ODE)	A non-polar solvent used as the reaction medium for high-temperature QD synthesis.	[1] [13]
Methyl Acetate (MeOAc)	Solvent for washing films; removes OA ligands and enables LbL deposition.	[3] [2] [42]
Formamidinium Iodide (FAI)	Short-chain A-site cation halide salt for post-treatment; enhances QD coupling and mobility.	[3] [2]
Phenethylammonium Iodide (PEAI)	Short-chain ligand for LbL or post-treatment; improves coupling, passivates defects, and enhances stability.	[2] [42]
SnO₂ Nanoparticles	Commonly used as the electron transport layer (ETL) in the device stack.	[42]
Spiro-OMeTAD	A benchmark hole transport material (HTL) for perovskite solar cells.	[42]

This comparison guide demonstrates that both FAI and PEAI post-treatments are powerful strategies for enhancing the charge transport in CsPbI₃ PQD films by improving inter-dot coupling. The choice between them involves a trade-off: FAI post-treatment offers a robust method for significantly boosting photocurrent and mobility, establishing a strong baseline for high-performance devices [3]. In contrast, the PEAI-LBL approach provides a more advanced and controlled ligand management strategy, leading to superior defect passivation, higher VOC, improved stability, and consequently, a higher champion PCE [2]. Recent research further underscores that the ligand shell, not the perovskite core itself, is often the primary factor in device degradation and performance evolution, highlighting the critical role of surface chemistry [42]. Future developments will likely focus on refining solvent systems for ligand exchange [9] and developing novel ligand structures to further optimize the balance between conductivity, passivation, and stability for commercially viable PQD photovoltaics.

The development of bifunctional optoelectronic devices capable of both efficient light detection (solar cells) and light emission (LEDs) from a single structure represents a frontier in perovskite quantum dot (PQD) research. Central to this advancement is the management of the PQD surface, where ligand engineering plays a critical role in balancing carrier injection for electroluminescence with carrier extraction for photovoltaics. This guide objectively compares two predominant post-treatment ligands—Formamidinium Iodide (FAI) and Phenethylammonium Iodide (PEAI)—for CsPbI3 PQD films, providing a detailed analysis of their performance in bifunctional contexts.

Performance Comparison: FAI vs. PEAI Post-Treatment

The choice of ligand profoundly influences the optoelectronic properties and stability of CsPbI3 PQD films. The table below summarizes key performance metrics from recent studies.

Table 1: Comparative Performance of FAI and PEAI Post-Treated CsPbI3 PQD Bifunctional Devices

Performance Metric	FAI Post-Treatment	PEAI Post-Treatment	Remarks
Champion PCE (PV Mode)	15.21% [2]	14.18% - 14.1% [2] [44]	PEAI treatments show highly consistent, high-performance results.
Open-Circuit Voltage (VOC)	~1.23 V [2]	~1.23 V [2] [44]	Both ligands can achieve high VOC, indicating effective defect passivation.
Electroluminescent Luminance	EQEEL of 3.8% [2]	130 Cd/m² [2]	Metrics differ between studies, making direct comparison difficult.
Ambient Stability (Unencapsulated)	--	>90% PCE retained after 15 days [44]	PEAI's hydrophobic aromatic group confers superior moisture resistance.
Phase & Composition Stability	Can induce hybridization to FA1-xCsxPbI3, risking phase instability [2] [44]	Preserves fully inorganic CsPbI3 composition [44]	PEAI anchors to the surface without integrating into the lattice.

Critical Analysis of Performance Trade-offs

• Efficiency vs. Stability: While FAI-based devices have achieved a marginally higher record PCE, this can come at the cost of long-term phase stability. The FAI treatment is sensitive to reaction time and can incorporate organic FA+ cations into the perovskite lattice, potentially forming a hybrid composition (FA1-xCsxPbI3) that may be less stable than the fully inorganic structure [2] [44]. PEAI treatment excels in preserving the fully inorganic nature of CsPbI3 QDs, ensuring robust phase stability without sacrificing performance [44].
• The Hydrophobicity Advantage: A key differentiator for bifunctional device stability is the ligand's inherent properties. The phenethyl group in PEAI is highly hydrophobic. When incorporated onto the PQD surface, it creates a protective layer that significantly enhances moisture resistance, as evidenced by devices retaining over 90% of their initial PCE after 15 days in ambient conditions [44]. Conventional FAI treatment removes the native hydrophobic ligands, creating a path for moisture penetration and accelerated degradation [44].

Experimental Protocols for Performance Evaluation

To ensure the reproducibility of the data presented in Table 1, this section outlines the core experimental methodologies employed in the cited research.

CsPbI3 PQD Synthesis

The foundation of a high-performance device is high-quality quantum dots. The standard protocol is the hot-injection method [2] [13]:

• Cs-oleate Preparation: Cesium carbonate (Cs2CO3) is dissolved in a mixture of 1-octadecene (ODE) and oleic acid (OA) at 150 °C under an inert atmosphere [13].
• Reaction Injection: A separate lead precursor solution is prepared by dissolving lead iodide (PbI2) in ODE with OA and oleylamine (OAm). This solution is heated to 170 °C, at which point the preheated Cs-oleate is swiftly injected [13].
• Purification: The reaction is quenched in an ice bath. The resulting CsPbI3 QDs are purified by centrifugation with a non-polar solvent like hexane or octane to remove unreacted precursors and excess ligands [2].

Layer-by-Layer Film Deposition and Ligand Exchange

Creating thick, conductive PQD films requires a multi-step deposition process.

• Substrate Preparation: Fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) substrates are typically cleaned and coated with electron transport layers (e.g., TiO2, SnO2) [2].
• Film Fabrication: The CsPbI3 QD ink (in octane or hexane) is spin-coated onto the substrate.
• Ligand Exchange Process:
  • For FAI-based films: The as-deposited QD film is treated by spin-coating a solution of FAI in ethyl acetate (EtOAc). This replaces the long-chain OAm ligands with shorter FA+ cations [2] [44].
  • For PEAI-based films (PEAI-LBL): A layer-by-layer solid-state exchange strategy is used. After spinning each layer of QDs, the film is treated with a solution of PEAI in methyl acetate (MeOAc) or EtOAc. This process is repeated 3-5 times to build the desired film thickness, ensuring complete and uniform ligand exchange from the bottom to the top layer [2].
• Final Processing: After the final ligand exchange, the films are annealed at a mild temperature (e.g., 70°C for 5-10 minutes). Subsequent hole transport layers (e.g., Spiro-OMeTAD, PTAA) and metal electrodes (e.g., Au, Ag) are deposited to complete the device [2].

Device Characterization Protocols

• Photovoltaic (PV) Characterization: Current-density voltage (J-V) curves are measured under standard AM 1.5G solar illumination (100 mW/cm²) to extract PCE, VOC, short-circuit current (JSC), and fill factor (FF) [2].
• Electroluminescent (EL) Characterization: Devices are biased in forward direction in the dark. Luminance is measured using a photometer or spectrometer, while the External Quantum Efficiency of Electroluminescence (EQEEL) is calculated from the emitted photon flux and the injected current [2].
• Stability Testing: Unencapsulated devices are stored in ambient conditions (e.g., 30-50% relative humidity, ~25 °C). The normalized PCE is tracked over time to assess operational stability [2] [44].

The following workflow diagram synthesizes the key experimental and analysis steps involved in this comparative evaluation.

The Scientist's Toolkit: Essential Research Reagents

The experimental protocols rely on a specific set of chemical reagents, each with a defined function in the synthesis and treatment processes.

Table 2: Key Research Reagents for CsPbI3 PQD Post-Treatment Studies

Reagent Category	Specific Examples	Function & Rationale
Precursor Salts	Cesium Carbonate (Cs2CO3), Lead Iodide (PbI2)	High-purity (>99.99%) sources of Cs+ and Pb2+ cations for QD synthesis, ensuring minimal impurity-induced defects.
Long-Chain Ligands	Oleic Acid (OA), Oleylamine (OAm)	Used during synthesis to control QD growth and provide colloidal stability in non-polar solvents.
Solvents	1-Octadecene (ODE), n-Hexane, n-Octane	High-boiling point, non-polar solvents for synthesis (ODE) and dispersion/purification (Hexane, Octane).
Short-Chain Ligands	Formamidinium Iodide (FAI), Phenethylammonium Iodide (PEAI)	The core post-treatment agents. Replace long-chain ligands to enhance inter-dot coupling and charge transport.
Wash Solvents	Methyl Acetate (MeOAc), Ethyl Acetate (EtOAc)	Polar solvents used during film processing to remove excess long-chain ligands and facilitate solid-state ligand exchange.

The pursuit of high-performance bifunctional devices creates a complex set of requirements for CsPbI3 PQD films. While FAI post-treatment can yield exceptional photovoltaic efficiency, its tendency to alter the perovskite composition and its lack of a hydrophobic moiety present challenges for long-term, stable operation.

In contrast, PEAI-based post-treatment, particularly via a layer-by-layer (LBL) strategy, emerges as a more robust solution. It successfully balances efficient charge transport with outstanding defect passivation. Crucially, the hydrophobic nature of the PEA+ cation and its ability to passivate the film without integrating into the lattice provide a decisive advantage for stabilizing the optically active black phase of CsPbI3 against moisture. For researchers targeting practical and durable bifunctional electroluminescent solar cells, PEAI-LBL management represents a more reliable and promising ligand engineering pathway.

Long-Term Operational Stability and Degradation Pathways

All-inorganic cesium lead iodide (CsPbI3) perovskite quantum dots (PQDs) have emerged as a leading material for next-generation photovoltaics and red-light-emitting devices due to their ideal bandgap (~1.73 eV), high photoluminescence quantum yield (PLQY), and superior defect tolerance compared to their organic-inorganic counterparts [1]. However, the commercialization of CsPbI3 PQD-based technologies faces a significant challenge: their inherent instability under operational environmental conditions. The weak binding of traditional long-chain ligands and susceptibility to phase transition from the optically active black cubic phase (α-phase) to a non-perovskite yellow orthorhombic phase (δ-phase) remains a critical bottleneck [45].

To address these instability issues, post-treatment strategies involving short-chain ligands have become a cornerstone of CsPbI3 PQD research. Among these, formamidinium iodide (FAI) and phenethylammonium iodide (PEAI) have shown considerable promise. This comparison guide objectively analyzes the performance of FAI versus PEAI post-treatment methodologies for CsPbI3 PQD films, focusing explicitly on long-term operational stability and elucidated degradation pathways. Framed within a broader thesis on comparative surface ligand engineering, this guide provides researchers with critical experimental data and protocols to inform material selection for stable optoelectronic devices.

Experimental Protocols: Ligand Exchange and Stability Assessment

A clear understanding of the experimental methodologies is crucial for interpreting comparative data. The following sections detail the standard protocols for film fabrication and stability testing.

Layer-by-Layer (LBL) Solid-State Ligand Exchange

This procedure is commonly used to deposit high-quality CsPbI3 PQD films, with the post-treatment ligand integrated either as a final step or during the layering process.

• Materials Synthesis: CsPbI3 PQDs are typically synthesized via the hot-injection method, using precursors such as Cs2CO3, PbI2, and ligands including oleic acid (OA) and oleylamine (OAm) in solvents like 1-octadecene (ODE) [2] [45]. The resulting PQDs are purified and dispersed in a non-polar solvent like hexane.
• Film Fabrication with FAI Post-Treatment: The CsPbI3 PQD film is built up using a layer-by-layer (LBL) spin-coating process. A layer of PQDs is deposited onto a substrate, followed by washing with a non-solvent like methyl acetate (MeOAc) to remove excess ligands and solvent. This cycle is repeated 3-5 times to achieve the desired film thickness. Subsequently, the film undergoes a final post-treatment by spin-coating a solution of FAI in ethyl acetate (EtOAc) [2].
• Film Fabrication with PEAI-LBL Treatment: In the modified approach, the ligand exchange is integrated into each layer. After the deposition and MeOAc washing of each individual PQD layer, the film is immediately treated with a solution of PEAI in EtOAc. This layer-by-layer solid-state exchange ensures that the conjugated phenethylammonium ligand encapsulates the PQDs throughout the entire film stack, rather than仅仅on the surface [2].

Stability and Degradation Testing Protocols

To evaluate long-term operational stability, researchers subject films and devices to controlled stress conditions and employ a suite of characterization techniques.

• Ambient Stability Testing: Unencapsulated films or devices are stored in ambient air with controlled relative humidity (e.g., 30-50%) and temperature (e.g., ~25 °C). Their performance is monitored over time [2].
• Phase Stability Analysis: X-ray diffraction (XRD) is used to track the crystal structure of the films, monitoring for the appearance of peaks corresponding to the non-perovskite yellow δ-phase [45].
• Optical Properties Tracking: UV-visible absorption and photoluminescence (PL) spectroscopy measure changes in the bandgap and emission intensity. A drop in PL intensity indicates an increase in non-radiative recombination centers due to defect formation or degradation [2] [45].
• Chemical State Analysis: X-ray photoelectron spectroscopy (XPS) can detect changes in the surface chemistry and the formation of oxidation products [46].
• Morphological Stability: Transmission electron microscopy (TEM) visualizes changes in the size, distribution, and structural integrity of the PQDs after aging [47].

The workflow for film fabrication and stability assessment is summarized in the diagram below.

Performance Comparison: FAI vs. PEAI Post-Treatment

The following tables consolidate key experimental data from recent studies, providing a direct comparison of the photovoltaic performance and stability outcomes achieved with FAI and PEAI post-treatments.

Table 1: Photovoltaic Performance of CsPbI3 PQD Solar Cells

Performance Parameter	FAI Post-Treatment	PEAI-LBL Treatment	Measurement Conditions
Champion PCE (%)	10.77 - 15.21% [2] [1]	Up to 14.18% [2]	Simulated AM 1.5G illumination
Open-Circuit Voltage (V)	Typically < 1.20 V	1.23 V [2]	Simulated AM 1.5G illumination
Electroluminescence Performance	EQEEL: 1.05% - 3.8% [2]	Luminance: 130 Cd/m² [2]	In LED operation mode
Defect Passivation Efficacy	Incomplete, especially in underlying layers [2]	Enhanced surface and bulk passivation [2]	Inferred from PLQY and Voc

Table 2: Long-Term Stability Comparison

Stability Metric	FAI Post-Treatment	PEAI-LBL Treatment	Test Conditions
Ambient Stability	Rapid phase transition reported	>86% PL intensity retention after 20 days [45]	Unencapsulated, 30-50% RH, ~25°C [2]
Phase Stability	Sensitive to treatment time; can induce unwanted mixed phases [2]	Stabilized cubic phase [2]	XRD measurement over time
Ligand Binding Strength	Moderate	Strong, via conjugated phenyl group [2]	FTIR, XPS
Resistance to Ligand Loss	Prone to loss during processing	Enhanced retention due to short-chain and strong binding [2]

Degradation Pathways and Stabilization Mechanisms

Understanding the failure modes of CsPbI3 PQD films and how ligands mitigate them is essential for material design.

Common Degradation Pathways

The primary degradation routes for CsPbI3 PQDs involve both physical transformation and chemical decomposition.

• Phase Transition: The transition from the black cubic α-phase to the yellow orthorhombic δ-phase is a major instability, driven by moisture and the inherent thermodynamic instability of the α-phase at room temperature [45].
• Ligand Desorption and Surface Defect Formation: The dynamic binding of traditional long-chain ligands (OA/OAm) makes them prone to desorb during film processing and operation. This exposes under-coordinated Pb²⁺ sites, creating surface traps that act as non-radiative recombination centers, quench PL, and reduce device efficiency [2] [47] [45].
• Oxidation and Iodide-Mediated Cyclic Degradation: While more extensively studied in tin-based perovskites, a cyclic degradation pathway involving oxidation is highly relevant. Sn/ PbI₂, a product of oxidation, can react with moisture to form HI, which is then oxidized by oxygen to produce I₂. Iodine is a highly aggressive oxidizer that can further attack the perovskite, establishing a self-sustaining degradation loop [46]. This mechanism underscores the critical role of surface passivation in blocking the initiation of oxidation.

Stabilization Mechanisms of PEAI and FAI

The divergent stability outcomes of FAI and PEAI treatments can be traced to their distinct molecular structures and interaction modes with the PQD surface.

• PEAI's Multi-Faceted Protection: The PEAI-LBL strategy confers stability through several synergistic mechanisms as shown in the pathway below.

  • Enhanced Ligand Binding: The conjugated phenyl group in PEA⁺ interacts strongly with the Pb-I lattice, leading to more robust and stable binding compared to FAI [2].
  • Improved Hydrophobicity: The phenethylammonium group provides a higher hydrophobic character, effectively shielding the moisture-sensitive perovskite lattice from ambient humidity [2].
  • Bulk Passivation: By treating each layer during deposition, PEAI-LBL ensures defect passivation throughout the film bulk, not just the top surface. This leads to more balanced charge transport and injection, reducing localized joule heating and degradation [2].

• FAI's Limitations: The FAI post-treatment, while effective at boosting initial performance, primarily passivates only the top surface of the film. The underlying layers may retain poorly passivated defects. Furthermore, excessive FAI treatment time can induce a component change from CsPbI3 to a less stable FA₁₋ₓCsₓPbI3 mixed perovskite, compromising long-term phase stability [2].

The Scientist's Toolkit: Essential Research Reagents

This table lists key materials and their functions for researchers aiming to replicate or build upon these ligand exchange studies.

Table 3: Essential Reagents for CsPbI3 PQD Ligand Exchange Studies

Reagent / Material	Function in Research	Key Considerations
Cesium Carbonate (Cs₂CO₃)	Cesium precursor for CsPbI3 PQD synthesis [45]	High purity (99.99%) required for optimal performance.
Lead Iodide (PbI₂)	Lead precursor for CsPbI3 PQD synthesis [45]	Must be stored in a dry, dark environment.
Oleic Acid (OA) / Oleylamine (OAm)	Long-chain capping ligands for nucleation and growth control [2] [45]	Dynamic binding necessitates post-synthetic exchange.
1-Octadecene (ODE)	Non-polar solvent for high-temperature synthesis [45]	Requires degassing to remove oxygen and water.
Formamidinium Iodide (FAI)	Short-chain ligand for post-treatment passivation [2]	Concentration and treatment time critical to avoid phase impurities.
Phenethylammonium Iodide (PEAI)	Short-chain, conjugated ligand for LBL exchange [2]	Provides enhanced binding and hydrophobicity.
Methyl Acetate (MeOAc)	Washing solvent to remove excess ligands and solvent [2]	Anti-solvent for PQDs; used in LBL process.
Ethyl Acetate (EtOAc)	Solvent for preparing FAI/PEAI post-treatment solutions [2]	Anhydrous grade recommended.

Conclusion

This comparative study elucidates that the choice between FAI and PEAI post-treatment is not a simple binary selection but a strategic decision based on target device performance. FAI treatment, while effective for initial conductivity, can lead to phase instability due to ionic integration. In contrast, PEAI treatment, particularly in a layer-by-layer approach, offers more robust surface passivation, superior moisture resistance, and enables balanced charge transport for high-performance bifunctional devices. The key takeaway is that PEAI provides a more controlled and stable ligand management strategy for demanding applications. Future research should focus on developing novel multi-dentate ligands that combine the benefits of both, exploring lead-free alternatives for reduced toxicity, and establishing standardized accelerated aging protocols to predict long-term operational stability, thereby paving the way for the commercial viability of CsPbI3 PQD-based optoelectronics.

References

• 1. CsPbI 3 perovskite quantum dot solar cells - RSC Publishing [https://pubs.rsc.org/en/content/articlehtml/2022/ma/d1ma01075a]
• 2. Modified surface ligand management of CsPbI 3 perovskite ... [https://www.sciencedirect.com/science/article/abs/pii/S1566119924000570]
• 3. Enhanced mobility CsPbI3 quantum dot arrays for record ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC5659658/]
• 4. Enhanced Phase and Reduced Stability for... Bandgap [https://link.springer.com/article/10.1134/S0036024424701279]
• 5. Stabilized Perovskite Quantum Dot Solids via Nonpolar Solvent... [https://pmc.ncbi.nlm.nih.gov/articles/PMC10427392/]
• 6. Stable and efficient CsPbI3 quantum-dot light-emitting ... [https://www.nature.com/articles/s41467-024-50022-8]
• 7. Effect of Surface Ligand Modification on the Optical ... [https://www.sciencedirect.com/science/article/abs/pii/S2468023025023211]
• 8. Ligand Engineering of Inorganic Lead Halide Perovskite ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11280295/]
• 9. Tailoring solvent-mediated ligand exchange for CsPbI3 ... [https://www.sciencedirect.com/science/article/pii/S2542435122002343]
• 10. Ligand exchange engineering of FAPbI3 perovskite quantum ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC9756204/]
• 11. Researchers use ligand-passivation engineering to ... [https://www.perovskite-info.com/researchers-use-ligand-passivation-engineering-achieve-high-performance-indoor]
• 12. Mitigating Mobile‐Ion‐Induced Instabilities and ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12306393/]
• 13. Photo-Induced Black Phase Stabilization of CsPbI 3 QDs ... [https://www.mdpi.com/2079-4991/10/8/1586]
• 14. Organic Ammonium Halide Modulators as Effective Strategy ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC8132166/]
• 15. Perovskite Quantum Dots in Solar Cells - PMC [https://pmc.ncbi.nlm.nih.gov/articles/PMC8895128/]
• 16. Stepwise−Process−Controlled Ligand Management ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC10708130/]
• 17. Multifaceted anchoring ligands for uniform orientation and ... [https://www.sciencedirect.com/science/article/abs/pii/S1385894724058017]
• 18. CsPbI₃ Quantum Dots for Photovoltaic Applications ... [https://www.sciencedirect.com/science/article/abs/pii/S0925838825053897]
• 19. Perovskite Quantum Dots for Emerging Displays [https://pmc.ncbi.nlm.nih.gov/articles/PMC9268187/]
• 20. Mn and Br co-substitution CsPbI 3 perovskite quantum dots ... [https://www.sciencedirect.com/science/article/abs/pii/S1876107025001920]
• 21. High-Performance Perovskite Quantum Dot Solar Cells ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC9576836/]
• 22. Manipulating solvent fluidic dynamics for large-area ... [https://www.nature.com/articles/s41467-024-45488-5]
• 23. Photo-Induced Black Phase Stabilization of CsPbI3 QDs ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC7466586/]
• 24. Synergistic Hybrid-Ligand Passivation of Perovskite Quantum Dots... [https://pubmed.ncbi.nlm.nih.gov/39887773/]
• 25. JRM | Free Full-Text | Treating CsPbI₃ Perovskite with... [https://www.techscience.com/jrm/v11n8/53380/html]
• 26. Ambient-atmosphere fabricated perovskite solar cells with ... [https://www.sciencedirect.com/science/article/abs/pii/S0169433224012558]
• 27. Enhanced Efficiency and Mechanical Stability in Flexible ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC12299172/]
• 28. Room-temperature stable CsPbI3 perovskite quantum dots ... [https://www.sciencedirect.com/science/article/abs/pii/S0925838823017620]
• 29. | Bilayer interfacial engineering with Researching /OAI for... PEAI [https://www.researching.cn/articles/OJfb630058110214f3]
• 30. Bilayer interfacial engineering with PEAI/OAI for synergistic ... [https://www.jos.ac.cn/article/doi/10.1088/1674-4926/25030046]
• 31. Highly Stable CsPbI 3 Perovskite Quantum Dots Enabled ... [https://www.mdpi.com/2076-3417/13/13/7529]
• 32. How to improve the structural stabilities of halide perovskite ... [https://nanoconvergencejournal.springeropen.com/articles/10.1186/s40580-024-00412-x]
• 33. Synergistic Hybrid-Ligand Passivation of Perovskite ... [https://pubmed.ncbi.nlm.nih.gov/39887773]
• 34. Analyzing and Improving Solar Cell Metrics: FF, Voc, and Jsc [https://www.ossila.com/pages/solar-cells-low-device-metrics]
• 35. Balancing the Voc-Jsc trade-off in polymer solar cells ... [https://www.sciencedirect.com/science/article/abs/pii/S1566119922000180]
• 36. Decreasing energy loss and optimizing band alignment for ... [https://pubs.rsc.org/en/content/articlelanding/2020/ta/d0ta02488k]
• 37. Critical role of post-treatment induced surface ... [https://www.sciencedirect.com/science/article/abs/pii/S138589472306285X]
• 38. Interface Engineering with Formamidinium Salts for ... [https://pubmed.ncbi.nlm.nih.gov/37883207/]
• 39. Thermal tolerance of perovskite quantum dots dependent ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC10113222/]
• 40. CsPbI3 Perovskite Quantum Dot-Based WORM Memory ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11299139/]
• 41. Research on the Importance of Substrate Morphological ... [https://www.sciencedirect.com/science/article/abs/pii/S0022231325005150]
• 42. Post‐Degradation Recovery of CsPbI3 Quantum Dot Solar ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11840470/]
• 43. Enhanced Mobility CsPbI3 Quantum Dot Arrays for Record ... [https://research-hub.nrel.gov/en/publications/enhanced-mobility-cspbi3-quantum-dot-arrays-for-record-efficiency/]
• 44. Hydrophobic stabilizer-anchored fully inorganic perovskite ... [https://www.sciencedirect.com/science/article/abs/pii/S2211285520305620]
• 45. Enhancing Photoluminescence and Stability of CsPbI 3 ... [https://www.mdpi.com/2073-4352/13/1/45]
• 46. Degradation mechanism of hybrid tin-based perovskite ... [https://www.nature.com/articles/s41467-021-22864-z]
• 47. Stable and efficient CsPbI3 quantum-dot light-emitting ... [https://pmc.ncbi.nlm.nih.gov/articles/PMC11228028/]