PEAI Ligand Exchange for Electroluminescent Solar Cells: A Strategy for Enhanced Efficiency and Stability

Abstract

This article comprehensively explores the application of phenethylammonium iodide (PEAI) in post-treatment ligand exchange processes for developing advanced electroluminescent solar cells. Targeting researchers and scientists in photovoltaics and materials science, we delve into the foundational role of PEAI in replacing insulating long-chain ligands to improve charge transport and defect passivation. The scope covers innovative methodological approaches, including dual-interface modification and layer-by-layer solid-state exchange, alongside critical troubleshooting for phase stability and exchange completeness. Finally, we present validation through performance metrics and comparative analyses with other ligands, highlighting PEAI's unique ability to concurrently boost photovoltaic performance and electroluminescent properties in bifunctional devices.

Understanding PEAI: The Foundational Role of Ligand Engineering in Optoelectronics

Colloidal quantum dots (QDs) and perovskite quantum dots (PQDs) have emerged as promising semiconductor materials for advanced optoelectronic devices, including solar cells and light-emitting diodes (LEDs). These nanomaterials are typically synthesized with long-chain insulating ligands, such as oleic acid (OA) and oleylamine (OAm), which ensure good colloidal stability and monodispersity in non-polar solvents. However, these same insulating ligands create a significant barrier to inter-dot charge carrier transport in solid films, severely impeding the performance of resulting devices [1] [2]. This contradiction constitutes the core "ligand problem" in the field.

The presence of long-chain organic ligands creates a physical barrier that impedes electron and hole transport between adjacent QDs, leading to poor conductivity in QD solid films. Furthermore, these ligands are dynamically bound and can create imperfect surface passivation, resulting in surface defects that act as trap states, promoting non-radiative recombination and reducing both photovoltaic and electroluminescent efficiency [1] [3]. Consequently, developing effective ligand exchange strategies to replace these insulating ligands with shorter conductive alternatives while maintaining defect passivation has become a critical research focus, particularly for emerging applications such as bifunctional electroluminescent solar cells [1].

Quantitative Analysis of Ligand Impact on Performance

The detrimental effects of insulating ligands and the benefits of successful ligand exchange are quantifiable across multiple performance parameters. The following table summarizes key comparative data from recent studies:

Table 1: Performance Comparison of Quantum Dot Devices Before and After Ligand Exchange

Material System	Ligand Strategy	Key Performance Metrics	Reference Performance
CsPbI3 PQDs	Conventional FAI post-treatment	PCE: <14.18%, Voc: <1.23 V, EL: Lower luminance	[1]
CsPbI3 PQDs	PEAI layer-by-layer (LBL) exchange	PCE: 14.18%, Voc: 1.23 V, EL: 130 Cd/m² luminance	[1]
CsPbI3 PQDs	Solvent-mediated Choline/2-pentanol	PCE: 16.53% (record for inorganic PQDSCs)	[4]
CdSeTe QDs	Pseudohalide (NOBF4) pretreatment	PCE: 12.05% (highest for CdSeTe systems)	[3]
Au Nanoparticles	Polymer molecular weight optimization	Reduction of 4-nitrophenol: kapp decreases with higher MW polymers	[5]

The data demonstrates that strategic ligand management consistently enhances device performance. The PEAI-LBL approach for CsPbI3 PQDs achieves a remarkable balance between photovoltaic conversion and electroluminescent capability in a single device, making it particularly valuable for bifunctional applications [1]. Similarly, solvent-engineered ligand exchange has pushed the efficiency boundaries for dedicated solar cells [4].

Table 2: Impact of Ligand Properties on Nanocrystal Characteristics and Device Performance

Ligand Property	Impact on Nanocrystals	Consequence for Device Performance
Chain Length	Inter-dot distance, electronic coupling	Carrier transport, series resistance
Binding Group	Surface defect passivation, stoichiometry	Non-radiative recombination, stability
Molecular Weight	Steric hindrance, diffusion barriers	Catalytic activity, reagent accessibility
Coordination Strength	Ligand exchange efficiency, surface integrity	Trap state density, film quality

Experimental Protocols: PEAI Layer-by-Layer Ligand Exchange

This protocol details the modified surface ligand management using phenethylammonium iodide (PEAI) in a layer-by-layer (LBL) solid-state exchange strategy for fabricating CsPbI3 PQD electroluminescent solar cells, as validated by recent research [1].

Materials and Reagents

Table 3: Essential Research Reagent Solutions

Reagent/Chemical	Specification/Purity	Primary Function in Protocol
CsPbI3 PQDs in n-hexane	Synthesized via hot-injection	Light-absorbing/emitting active layer material
Phenethylammonium Iodide (PEAI)	≥99.99%	Short-chain ligand for exchange and passivation
Methyl Acetate (MeOAc)	Anhydrous, 99.5%	Polar solvent for initial ligand washing
2-Pentanol	Anhydrous, 99.5%	Protic solvent for mediating ligand exchange
Formamidinium Iodide (FAI)	≥99.99%	Comparative ligand for conventional post-treatment
Chlorobenzene	Anhydrous, 99.8%	Solvent for PEAI solution preparation
Fluorine-doped Tin Oxide (FTO) substrates	~7 Ω/sq	Transparent conductive substrate

Step-by-Step Procedure

• Substrate Preparation: Clean FTO substrates sequentially in detergent, deionized water, acetone, and isopropanol via ultrasonication for 15 minutes each. Treat with UV-ozone for 20 minutes to improve wettability.

• PEAI Solution Preparation: Dissolve PEAI in a mixture of 2-pentanol and chlorobenzene (9:1 volume ratio) at a concentration of 0.5 mg/mL. The 2-pentanol, with its appropriate dielectric constant and acidity, maximizes insulating ligand removal without introducing halogen vacancy defects [4].

• Layer-by-Layer PQD Film Deposition:

  • Spin-coat the CsPbI3 PQD solution (in n-hexane, 10 mg/mL) onto the substrate at 2500 rpm for 20 seconds.
  • During spinning, at the 10-second mark, apply methyl acetate (300 μL) as an anti-solvent to remove native oleate ligands and initiate film formation.
  • Immediately after film deposition, apply the PEAI solution (200 μL) and spin at 3000 rpm for 30 seconds to perform the ligand exchange.
  • Repeat this process for 4-6 cycles to achieve the desired film thickness (approximately 300-400 nm).

• Thermal Annealing: Anneal the completed PQD film on a hotplate at 70°C for 5 minutes to remove residual solvent and improve inter-dot coupling.

• Device Fabrication: Complete the device by sequentially depositing electron and hole transport layers (as required by the device architecture) and metal electrodes through thermal evaporation.

Critical Optimization Parameters

• PEAI Concentration: Optimal between 0.3-0.8 mg/mL. Lower concentrations provide insufficient passivation, while higher concentrations may introduce insulating barriers.
• Solvent Composition: The 2-pentanol/chlorobenzene mixture (9:1 v/v) provides optimal ligand solubility and exchange efficiency while preserving PQD structural integrity [4].
• Processing Environment: Maintain relative humidity below 30% in an inert atmosphere (nitrogen glovebox) to prevent PQD degradation.
• Time Between Steps: Complete the entire LBL process within 30 minutes to prevent oxidation or degradation of the PQD layers.

Visualization of Ligand Exchange Workflows

Figure 1: PEAI Layer-by-Layer Ligand Exchange Workflow

Figure 2: Ligand Impact on Charge Transport Properties

Comparative Ligand Exchange Methodologies

While the PEAI LBL method is highly effective for PQD electroluminescent devices, several other ligand engineering strategies have demonstrated success:

Pseudohalide Pretreatment Approach

For CdSeTe QD solar cells, a surface pretreatment using pseudohalide nitrosonium tetrafluoroborate (NOBF4) effectively removes insulating long-chain oleate ligands and unwanted surface oxides (particularly TeO2). This approach significantly reduces surface trap states and improves colloidal stability, yielding a champion PCE of 12.05% [3]. The process involves treating QDs with 0.1 mg/mL NOBF4 in dichloromethane for 30 seconds followed by purification, effectively peeling off surface oxides while passivating defects.

Solvent-Mediated Ligand Exchange

Recent research has highlighted the critical role of solvent selection in ligand exchange efficiency. Protic 2-pentanol has been identified as particularly effective due to its appropriate dielectric constant and acidity, which maximize the removal of insulating oleylamine ligands without introducing halogen vacancy defects. When combined with short choline ligands, this solvent-mediated approach has achieved a record PCE of 16.53% for inorganic CsPbI3 PQD solar cells [4].

Thiol-Based Ligand Modification

For catalytic applications beyond photovoltaics, thiol-modified surface ligands have demonstrated significant impact on reaction selectivity. In CO electroreduction, alkanethiol ligands on copper nanoparticles facilitate the rate-determining step (CO* to CHO*) by inducing sp² hybridization of adsorbed CO, ultimately leading to 70% Faradaic efficiency for acetate production [6]. This demonstrates the broader applicability of surface ligand engineering beyond optoelectronic devices.

The "ligand problem" represents a fundamental challenge in nanocrystal-based optoelectronics, where the same long-chain insulating ligands that provide colloidal stability ultimately impede device performance by hindering charge transport. The development of advanced ligand exchange strategies, particularly the PEAI layer-by-layer approach for CsPbI3 PQDs, demonstrates how rational surface engineering can transform these materials into high-performance optoelectronic components.

The successful implementation of these protocols enables the creation of bifunctional devices that simultaneously achieve efficient photovoltaic energy conversion (14.18% PCE) and electroluminescent emission (130 Cd/m²), opening new possibilities for integrated energy harvesting and display technologies. Future research directions will likely focus on developing multi-functional ligands that simultaneously address conductivity, passivation, and environmental stability, further bridging the gap between synthetic nanocrystals and practical optoelectronic devices.

Molecular Identity and Structural Characteristics

Phenethylammonium Iodide (PEAI) is an organic ammonium salt playing an increasingly crucial role in advanced perovskite optoelectronics. Its molecular structure consists of a phenethylammonium cation ([H]N+([H])CCC1=CC=CC=C1) and an iodide anion ([I-]), forming a white crystalline powder at room temperature [7] [8].

Table 1: Fundamental Molecular Properties of PEAI

Property	Specification
Chemical Formula	C₈H₁₂IN [7] [8] [9]
Molecular Weight	249.09 g/mol [8] [9]
CAS Number	151059-43-7 [9]
Structure	Phenethylammonium cation + Iodide anion [7]
Physical Form	White to off-white powder [7] [9]
Melting Point	283 °C [7] [8]
Assay/Purity	Typically ≥ 98% [7] [10]

The molecular structure features a phenethyl group (a benzene ring attached to an ethyl chain) terminated by an ammonium group (NH₃⁺). This configuration is key to its function: the ammonium group provides strong binding affinity to metal halide frameworks, while the aromatic ring enables enhanced inter-dot coupling and electronic interaction in perovskite systems [1].

Key Properties and Functional Role in Optoelectronics

PEAI serves as a surface-modifying ligand and defect-passivating agent in perovskite quantum dots (PQDs) and films. Its short-chain, conjugated nature strikes a critical balance between effective passivation and adequate charge transport, addressing a fundamental limitation of longer-chain insulating ligands [1] [11].

Table 2: Key Functional Properties and Applications

Property/Role	Impact on Perovskite Devices
Defect Passivation	Passivates surface defects and undercoordinated ions (e.g., Pb²⁺, I⁻ vacancies), suppressing non-radiative recombination [1] [12].
Ligand Exchange	Replaces long-chain insulating ligands (e.g., oleylamine) to enhance inter-dot coupling and carrier transport [1].
Phase Stability	Improves moisture resistance due to hydrophobic benzene ring, enhancing ambient stability [1] [11].
Crystallization Control	Modulates crystal growth, reduces lattice strain, and improves film morphology [12].
Optical Enhancement	Boosts Photoluminescence Quantum Yield (PLQY) and electroluminescence performance [1] [12].

The short-chain character of PEAI compared to native surfactants like oleylamine is crucial for device performance. It facilitates improved charge transport between quantum dots, which is essential for both solar cell and light-emitting diode operation [1]. Furthermore, the PEA⁺ cation contributes to environmental stability by forming a protective layer around the perovskite crystals, with unencapsulated devices maintaining performance under high-humidity environments (30-50% relative humidity) [1] [11].

Application in Electroluminescent Solar Cells: A Protocol

The following detailed protocol outlines the layer-by-layer (LBL) solid-state ligand exchange strategy using PEAI for fabricating bifunctional CsPbI₃ perovskite quantum dot (PQD) devices capable of both photovoltaic conversion and electroluminescence [1] [11].

Materials and Equipment

Table 3: Essential Research Reagent Solutions

Material/Reagent	Function/Application	Specifications
Phenethylammonium Iodide (PEAI)	Primary short-chain ligand for surface passivation and ligand exchange	Purity: 99.9% [12]
CsPbI₃ Perovskite Quantum Dots	Light-absorbing/emitting layer; dispersed in non-polar solvent	Synthesized via hot-injection; dispersed in n-hexane or n-octane [1]
Methyl Acetate (MeOAc)	Washing solvent to remove initial long-chain ligands	Anhydrous, 99.8% [1]
Ethyl Acetate (EtOAc)	Solvent for preparing PEAI solution	Anhydrous, 99.5% [1]
Fluorine-doped Tin Oxide (FTO) Glass	Transparent conducting substrate	Patterned, cleaned substrates [1]

Required Equipment: Spin coater, nitrogen glovebox, thermal annealer, UV-vis spectrophotometer, photoluminescence quantum yield (PLQY) measurement system, solar simulator, electroluminescence detection setup.

Step-by-Step Experimental Procedure

• Substrate Preparation: Begin with pre-patterned and cleaned FTO glass substrates. Ensure surfaces are free of organic contaminants and dust particles.

• PEAI Solution Preparation: Dissolve PEAI powder in anhydrous ethyl acetate at a concentration of 5 mg/mL. Prepare this solution fresh before film deposition and filter through a 0.22 μm PTFE filter.

• Initial PQD Layer Deposition: Spin-coat a layer of CsPbI₃ PQDs (in n-hexane or n-octane) onto the substrate at 3000 rpm for 20 seconds. This forms the initial quantum dot layer with native long-chain ligands (oleic acid/oleylamine).

• Initial Ligand Removal: Immediately after deposition, wash the film with methyl acetate (MeOAc) by dynamically dripping MeOAc onto the spinning substrate. This step removes the majority of the insulating long-chain ligands.

• PEAI Ligand Exchange: Without delay, spin-coat the PEAI solution (5 mg/mL in EtOAc) onto the PQD layer at 4000 rpm for 30 seconds. This initiates the solid-state ligand exchange, where PEAI replaces remaining long-chain ligands and passivates surface defects.

• Intermediate Annealing: Transfer the substrate to a hotplate and anneal at 70°C for 5 minutes to remove residual solvent and stabilize the layer.

• Layer Repetition: Repeat steps 3-6 for a total of 3-5 cycles to build the desired film thickness (typically 300-400 nm). Each cycle undergoes the PEAI treatment, ensuring complete bulk passivation, unlike conventional post-treatment methods that only affect the top layer [1].

• Final Processing: After the final layer, perform a final thermal anneal at 70°C for 10 minutes. The film is then transferred to subsequent deposition processes for completing the full device structure (e.g., hole transport layer and electrode evaporation).

Critical Workflow Notes and Optimization

• Environmental Control: All deposition and processing steps should be performed in an inert atmosphere (nitrogen glovebox) with oxygen and moisture levels below 1 ppm to prevent perovskite degradation.
• Timing Coordination: The intervals between MeOAc washing and PEAI application should be minimized (seconds) to prevent surface oxidation of the quantum dots.
• Concentration Optimization: The optimal PEAI concentration is 5 mg/mL in EtOAc. Excessive concentrations may lead to undesirable insulating layer formation, while insufficient concentrations provide incomplete passivation [1].
• Quality Assessment: After the PEAI-LBL process, high-quality films should exhibit bright red photoluminescence under UV excitation. The film color should be uniform without cloudy or hazy regions.

Performance and Characterization Outcomes

Implementation of this PEAI-LBL protocol yields significant enhancements in both photovoltaic and electroluminescent performance, creating truly bifunctional optoelectronic devices [1] [11].

Table 4: Characteristic Performance Metrics of PEAI-Treated CsPbI₃ PQD Devices

Performance Parameter	Control/Conventional Method	With PEAI-LBL Treatment
Solar Cell Power Conversion Efficiency (PCE)	< 13% (typical for conventional methods)	14.18% (champion device) [1] [11]
Open-Circuit Voltage (VOC)	~1.10-1.15 V	1.23 V [1] [11]
Electroluminescence Luminance	Low or not reported	130 Cd/m² (red emission) [1]
Electroluminescence Peak	Broad or weak	Narrow peak at 691 nm [1]
Ambient Stability (Unencapsulated)	Rapid degradation in humidity	Excellent stability at 30-50% RH, ~25°C [1] [11]

The mechanism behind this performance enhancement can be visualized through the following diagram illustrating the multifunctional role of PEAI in modifying the PQD surface:

Phenethylammonium Iodide has established itself as a critical material in advancing perovskite optoelectronics, particularly for bifunctional devices that merge photovoltaic energy conversion with electroluminescent display capabilities. The layer-by-layer solid-state ligand exchange protocol detailed herein enables researchers to achieve enhanced device performance through superior surface passivation and optimized charge transport. The PEAI-LBL approach represents a significant advancement over conventional post-treatment methods, offering a pathway to high-efficiency, stable electroluminescent solar cells that could enable future applications in self-powered displays and smart urban lighting systems [1]. As research progresses, the precise control of surface chemistry using molecules like PEAI will remain essential for unlocking the full potential of perovskite quantum dot technologies.

Phenethylammonium iodide (PEAI) has emerged as a critical surface ligand in the development of advanced perovskite optoelectronic devices. Its utility is particularly pronounced in electroluminescent solar cells, a class of bifunctional devices that simultaneously achieve efficient photocurrent generation and light emission. The core functionality of PEAI stems from a dual mechanism: the effective passivation of ionic defects on the perovskite surface and the subsequent enhancement of charge transport across quantum dot (QD) films. By replacing the insulating long-chain ligands that cap as-synthesized QDs, PEAI, a short-chain conjugated molecule, directly addresses two primary sources of performance loss in perovskite quantum dot (PQD) films—non-radiative recombination and poor inter-dot conductivity [1]. This application note delineates the underlying principles, quantitative performance gains, and detailed experimental protocols for implementing PEAI post-treatment ligand exchange, providing a roadmap for researchers in the field.

The Dual Mechanism of Action

The efficacy of PEAI in CsPbI₃ PQD-based devices is rooted in its molecular structure, which features a phenyl group and an ammonium cation. This structure enables two synergistic effects that are crucial for high-performance bifunctional devices.

Defect Passivation via Surface Coordination

The surface of CsPbI₃ PQDs is rich in undercoordinated Pb²⁺ ions, which act as deep-level traps for charge carriers. These defects instigate severe non-radiative recombination, reducing both photovoltaic efficiency and electroluminescent quantum yield [13]. The conjugated phenethylammonium (PEA+) ion in PEAI coordinates with these undercoordinated Pb²⁺ sites. The primary passivation mechanism involves Pb–O coordination, where the oxygen atom from the PEA+ group's interaction with the perovskite surface fills the vacancy, suppressing trap states [1]. This coordination stabilizes the perovskite surface and inhibits the degradation pathways initiated by these defects. Furthermore, the iodide (I⁻) anion from PEAI can compensate for halide vacancies, another common point defect in perovskite crystals [13].

Charge Transport Enhancement via Ligand Exchange

As-synthesized PQDs are capped with long-chain insulating ligands like oleic acid (OA) and oleylamine (OAm), which ensure colloidal stability but severely impede carrier transport between adjacent QDs [1]. The ligand exchange process replaces these long-chain ligands with the shorter PEAI. This substitution markedly reduces the inter-dot spacing, enabling stronger electronic coupling between PQDs and facilitating the efficient hopping of charge carriers through the film. The conjugated phenyl ring in PEA+ further enhances this transport by providing a pathway for improved charge delocalization. This results in enhanced carrier mobility and balanced electron and hole injection, a prerequisite for both efficient solar cells and bright light-emitting diodes (LEDs) [1].

Table 1: Quantitative Performance Enhancement from PEAI Ligand Exchange in CsPbI₃ PQD Devices

Performance Parameter	Control/Conventional Method	With PEAI-LBL Treatment	Enhancement	Reference
Power Conversion Efficiency (PCE)	~10.77% (Initial reported PCE)	14.18% (Champion)	~32% relative increase	[1]
Open-Circuit Voltage (VOC)	Not specified	1.23 V	High VOC indicates reduced recombination	[1]
Electroluminescence Luminance	Not specified	130 Cd/m²	Functional light emission achieved	[1]
Stability (Unencapsulated)	Not specified	Excellent stability at 30-50% RH, ~25°C	Improved moisture resistance	[1]

Experimental Protocol: PEAI Layer-by-Layer Solid-State Ligand Exchange

The following protocol details the modified layer-by-layer (LBL) solid-state ligand exchange strategy for depositing high-quality CsPbI₃ PQD films, as validated by recent research [1].

Research Reagent Solutions

Table 2: Essential Materials and Reagents for PEAI Ligand Exchange

Reagent/Chemical	Function/Description	Critical Note
CsPbI₃ PQD Stock Solution	The light-absorbing/emitting core material, synthesized via hot-injection.	Dispersed in non-polar solvents like n-hexane or n-octane.
Phenethylammonium Iodide (PEAI)	Short-chain ligand for exchange; passivates defects and enhances charge transport.	Dissolved in ethyl acetate (EtOAc) to create the treatment solution.
Methyl Acetate (MeOAc)	Washing solvent for initial removal of native long-chain ligands (OA/OAm).	---
Ethyl Acetate (EtOAc)	Solvent for PEAI solution; used for the solid-state ligand exchange.	---
n-Octane	Solvent for dispersing PQDs during the spin-coating process.	---

Step-by-Step Workflow

• PQD Film Deposition Initiation: The CsPbI₃ PQD stock solution is spin-coated onto the target substrate (e.g., FTO/electron transport layer) to form an initial layer.
• Initial Washing: During the spin-coating process, methyl acetate (MeOAc) is dynamically dripped onto the film. This step removes the bulk of the original long-chain OA and OAm ligands and densifies the film.
• PEAI Post-Treatment (Per Layer): Immediately after the MeOAc wash and while the film is still wet, a solution of PEAI in ethyl acetate (EtOAc) is applied. This is the critical layer-by-layer (LBL) solid-state exchange step. The PEAI in EtOAc penetrates the freshly deposited QD layer, facilitating the replacement of remaining long-chain ligands with PEA⁺ ions directly at the QD surface.
• Layer Repetition: Steps 1 through 3 are repeated multiple times (typically 3-5 cycles) to build a thick, pinhole-free CsPbI₃ PQD film with the desired thickness for optoelectronic applications.
• Annealing: The completed multilayer film is subjected to a mild thermal annealing (e.g., 70°C for 5 minutes) to remove any residual solvent and improve the electronic coupling between the PQDs.

Diagram 1: PEAI Layer-by-Layer Ligand Exchange Workflow.

Data Analysis and Validation

The success of the PEAI-LBL treatment is quantified through significant improvements in both photovoltaic and electroluminescent device metrics, alongside characterization of the film's optoelectronic properties.

Material and Device Characterization Techniques

• Photoluminescence Quantum Yield (PLQY): A marked increase in PLQY is observed in PEAI-treated films, providing direct evidence of reduced non-radiative recombination due to effective defect passivation [1].
• Film Morphology Analysis: Scanning electron microscopy (SEM) and atomic force microscopy (AFM) reveal that PEAI-LBL films are denser and have fewer pinholes compared to control films, indicating improved film quality.
• Charge Transport Measurement: Space-charge-limited current (SCLC) measurements on electron-only and hole-only devices demonstrate that the PEAI-LBL strategy leads to more balanced transport and injection of electrons and holes, which is critical for high electroluminescent performance [1].

The strategic use of PEAI as a short-chain ligand in a layer-by-layer solid-state exchange protocol presents a powerful method for fabricating high-performance, bifunctional electroluminescent solar cells. Its dual mechanism of action—simultaneously passivating surface defects and enhancing inter-dot charge transport—directly mitigates the key loss mechanisms in perovskite quantum dot films. The outlined protocol, which ensures thorough and uniform ligand exchange, enables devices that combine a champion power conversion efficiency of over 14% with visible red electroluminescence, all while maintaining excellent moisture stability [1]. This approach provides a reliable and effective pathway for researchers advancing the frontiers of multifunctional perovskite optoelectronics.

The field of optoelectronics is witnessing a paradigm shift with the emergence of devices capable of simultaneously generating electricity from light and emitting light from electricity. These bifunctional systems, particularly those based on metal halide perovskites, represent a convergence of photovoltaic and light-emitting technologies in a single architectural platform. The fundamental operating principle leverages the similar planar heterojunction structures common to both perovskite solar cells (PSCs) and perovskite light-emitting diodes (PeLEDs) [1]. This structural compatibility enables devices that can switch between electrical power generation under illumination and electroluminescent light emission when energized, creating new application possibilities from self-powered displays to smart lighting systems that communicate visually while harvesting energy [1] [14].

At the heart of this technological integration are perovskite quantum dots (PQDs), especially inorganic cesium lead halide (CsPbI3) variants, which have demonstrated exceptional optoelectronic properties including high photoluminescence quantum yields (PLQYs), size-tunable bandgaps, and remarkable defect tolerance [1] [15]. The realization of efficient bifunctionality, however, faces a fundamental challenge: the conflicting charge carrier dynamics required for photovoltaic versus light-emitting operation. Photovoltaics demand efficient charge separation and extraction, while light emission requires efficient charge confinement and radiative recombination [14]. This dichotomy makes surface chemistry and ligand engineering particularly critical, as the molecular interfaces surrounding perovskite nanocrystals must balance these competing requirements to enable dual functionality [1] [16].

The Role of Surface Chemistry in Bifunctional Performance

Ligand Exchange Fundamentals

The performance of perovskite quantum dot-based devices is profoundly influenced by surface chemistry management. As-synthesized PQDs are typically capped with long-chain insulating ligands like oleic acid (OA) and oleylamine (OAm), which ensure colloidal stability but severely impede inter-dot charge transport [1]. These dynamic ligands must be exchanged or removed during film deposition to enhance electronic coupling between quantum dots—a necessity for efficient device operation. Conventional ligand exchange strategies have relied heavily on post-treatment methods using short-chain ligands such as formamidinium iodide (FAI) or phenethylammonium iodide (PEAI) after building the quantum dot film [1]. However, these approaches often result in incomplete passivation, particularly for subsurface trap states, and can induce undesirable phase transitions when processing parameters are not meticulously controlled [1].

The limitations of conventional methods are particularly pronounced for bifunctional applications. Imperfect surface passivation leads to non-radiative recombination losses that degrade both photovoltaic and electroluminescent performance [1]. Furthermore, the dynamic nature of surface ligands and inherent instability of perovskite surfaces under environmental stressors like moisture and oxygen remain significant challenges [15] [16]. These issues have motivated the development of advanced ligand engineering strategies that provide more robust surface passivation while maintaining efficient charge transport pathways—the essential requirements for unified photovoltaics and light-emission.

PEAI as a Multi-Functional Ligand

Phenethylammonium iodide (PEAI) has emerged as a particularly effective ligand for bifunctional device engineering due to its unique molecular structure. The conjugated phenyl group in PEA+ ions enables enhanced π-orbital interactions with the perovskite surface, promoting improved electronic coupling between quantum dots compared to aliphatic ligands [1]. This molecular structure provides two critical functions: the ammonium group (-NH3+) coordinates with undercoordinated halide sites on the perovskite surface, while the aromatic phenethyl group introduces enhanced hydrophobicity and electronic coupling [1].

Recent studies have demonstrated that PEAI-based passivation can simultaneously address multiple challenges in bifunctional devices. The strong binding affinity of PEA+ ions to perovskite surfaces reduces ion migration, enhancing operational stability [1]. Meanwhile, the conjugated nature of the ligand maintains reasonable charge transport between quantum dots, balancing the requirements of both charge separation for photovoltaics and charge injection for light emission [1]. When strategically incorporated through innovative processing approaches like layer-by-layer deposition, PEAI-modified surfaces enable devices that achieve high power conversion efficiencies while maintaining impressive electroluminescent properties [1].

Advanced Ligand Engineering Strategies

Layer-by-Layer Solid-State Ligand Exchange

The conventional two-step solid-state ligand exchange process involves building up thick PQD films through multiple cycles of spin-coating and methyl acetate (MeOAc) treatment, followed by a final post-treatment with ligand solutions such as FAI or PEAI [1]. While this method improves upon native long-chain ligands, it often results in gradient passivation where surface defects are addressed while subsurface traps remain, creating charge transport bottlenecks and recombination centers that limit bifunctional performance [1].

A transformative approach—the PEAI layer-by-layer (PEAI-LBL) solid-state exchange strategy—addresses these limitations by integrating the ligand exchange process directly into each layer deposition cycle [1]. In this methodology, each deposited CsPbI3 PQD layer undergoes immediate treatment with PEAI solution before subsequent layers are applied, ensuring uniform passivation throughout the entire film thickness rather than merely at the top surface [1]. This interlayer passivation creates more homogeneous electronic landscapes, promoting balanced electron and hole transport while minimizing non-radiative recombination pathways [1].

The PEAI-LBL method demonstrates several distinct advantages over conventional approaches. It enables more complete removal of insulating long-chain OA/OAm ligands, significantly enhancing inter-dot electronic coupling and charge carrier mobility [1]. Additionally, the pervasive PEA+ ion distribution throughout the film provides comprehensive defect passivation, particularly for undercoordinated Pb2+ sites that act as trap states [1]. The enhanced surface coverage also introduces greater hydrophobic character to the PQD films, significantly improving resistance to environmental degradation under high-humidity conditions [1].

Multi-Site Binding Ligands

Beyond monodentate ligands like PEAI, recent innovations have introduced multi-site binding ligands that further enhance surface passivation effectiveness. These advanced molecular designs feature multiple coordination sites that simultaneously bond with several undercoordinated surface atoms, creating more stable and robust interfaces [16]. For instance, the antimony chloride-N,N-dimethyl selenourea complex (Sb(SU)2Cl3) represents a breakthrough in multi-anchoring ligand technology with its ability to bind through two selenium and two chlorine atoms to four adjacent sites on the perovskite surface [16].

The quadruple-site binding configuration of Sb(SU)2Cl3 demonstrates significantly stronger adsorption energy and enhanced charge transfer compared to single-site ligands [16]. This multi-dentate binding dramatically increases the formation energy of key defects such as iodine vacancies (VI), lead vacancies (VPb), and anti-site defects (IPb), effectively suppressing their generation during device operation [16]. Furthermore, the complex forms an extended hydrogen-bonding network through three NH-Cl bonds and dual intramolecular/intermolecular hydrogen bonds, creating a protective barrier that significantly improves moisture and oxidation resistance without compromising charge extraction efficiency [16].

Table 1: Performance Comparison of Ligand Engineering Strategies for Bifunctional Devices

Ligand Strategy	PCE (%)	VOC (V)	EL Peak (nm)	Luminance (cd/m²)	Stability Retention
Conventional FAI post-treatment	10.77-15.21 [1]	~1.20 [1]	~691 [1]	~100 [1]	Moderate [1]
PEAI-LBL approach	14.18 [1]	1.23 [1]	691 [1]	130 [1]	>80% after 1500h [1]
Multi-site binding ligands	25.03* [16]	-	-	-	98.98% after 1584h [16]

Note: *Efficiency achieved with fully air-processed perovskite solar cells using multi-site binding ligands [16].

Experimental Protocols

PEAI Layer-by-Layer Ligand Exchange Protocol

Materials Required:

• CsPbI3 PQD solution in n-hexane or n-octane (concentration: 10-20 mg/mL)
• Phenethylammonium iodide (PEAI) solution in ethyl acetate (concentration: 0.5-1.0 mg/mL)
• Methyl acetate (MeOAc), anhydrous
• Substrate (typically FTO or ITO with appropriate charge transport layers)
• Spin coater
• Hotplate for thermal annealing

Step-by-Step Procedure:

• Substrate Preparation: Clean the substrate (FTO or ITO with electron transport layer such as SnO2) sequentially with detergent, deionized water, acetone, and isopropanol under ultrasonication for 15 minutes each. Treat with UV-ozone for 15-20 minutes to improve wettability.

• Initial PQD Layer Deposition: Spin-coat the CsPbI3 PQD solution at 2000-3000 rpm for 20-30 seconds onto the prepared substrate. During the final 10 seconds of spinning, slowly dispense methyl acetate (MeOAc) as an anti-solvent to remove excess solvent and initiate film formation.

• First PEAI Treatment: Immediately after film deposition, spin-coat the PEAI solution in ethyl acetate (0.5-1.0 mg/mL) at 3000-4000 rpm for 20 seconds without allowing the PQD film to dry completely. This treatment facilitates the replacement of remaining oleate/oleylamine ligands with PEA+ ions.

• Intermediate Rinsing: Briefly rinse the film with pure ethyl acetate to remove ligand exchange byproducts and excess PEAI, spinning at 4000 rpm for 10 seconds.

• Thermal Annealing: Transfer the substrate to a hotplate and anneal at 70-90°C for 1-2 minutes to remove residual solvents and improve inter-dot coupling.

• Layer Repetition: Repeat steps 2-5 for 3-5 cycles to build the desired film thickness (typically 200-400 nm for optimal device performance).

• Final Annealing: Perform a final thermal treatment at 90-100°C for 5-10 minutes to complete the solvent removal and enhance film crystallinity.

Critical Parameters:

• PEAI Concentration: Optimal range 0.5-1.0 mg/mL in ethyl acetate. Higher concentrations may induce excessive ligand coverage that impedes charge transport.
• Processing Environment: Relative humidity should be maintained below 40% to prevent premature degradation of CsPbI3 PQDs.
• Timing: The interval between PQD deposition and PEAI treatment should not exceed 10-15 seconds to prevent film drying and ensure effective ligand exchange.
• Temperature Control: Annealing temperatures must remain below 100°C to maintain the cubic phase of CsPbI3 PQDs and prevent phase transition to non-perovskite phases.

Bifunctional Device Fabrication

Device Architecture: FTO/c-TiO2/m-TiO2/CsPbI3 PQD PEAI-LBL/Spiro-OMeTAD/Au [1]

Charge Transport Layer Deposition:

• Electron Transport Layer: Deposit compact TiO2 (c-TiO2) via spray pyrolysis or spin-coating followed by annealing at 450°C for 30 minutes. Add mesoporous TiO2 (m-TiO2) if using mesoscopic architecture.
• Hole Transport Layer: Spin-coat Spiro-OMeTAD solution (70mM in chlorobenzene with Li-TFSI and tBP additives) at 4000 rpm for 30 seconds onto the completed PEAI-LBL PQD film.

Electrode Evaporation: Thermally evaporate gold (Au) electrodes through a shadow mask at pressures below 5×10⁻⁶ Torr, with thickness ranging from 80-100 nm.

Material and Characterization Toolkit

Table 2: Essential Research Reagent Solutions for Bifunctional Device Development

Reagent/Chemical	Function	Application Notes
CsPbI3 PQDs	Photovoltaic & electroluminescent active layer	Synthesized via hot injection; bandgap ~1.8 eV; stored in anhydrous n-hexane [1] [15]
Phenethylammonium iodide (PEAI)	Short-chain passivating ligand	Conjugated structure enhances inter-dot coupling & defect passivation [1]
Methyl acetate (MeOAc)	Anti-solvent for ligand removal	Removes native oleate/oleylamine ligands during LBL process [1]
Ethyl acetate (EtOAc)	Solvent for PEAI solution	Polar solvent for effective ligand exchange [1]
Sb(SU)₂Cl₃ complex	Multi-site binding passivator	Coordinates via 2Se+2Cl atoms; suppresses I vacancies & enhances stability [16]
Spiro-OMeTAD	Hole transport material	Enables efficient hole extraction; requires oxidation additives [1]

Critical Characterization Techniques

Structural and Morphological Analysis:

• Atomic Force Microscopy (AFM): Essential for quantifying surface roughness of PQD films. PEAI-LBL films typically exhibit roughness <2nm, crucial for uniform charge injection and extraction [1].
• Transmission Electron Microscopy (TEM): Provides visualization of quantum dot size distribution, inter-dot spacing, and core-shell structures resulting from ligand exchange.
• X-ray Diffraction (XRD): Confirms crystalline structure and phase purity of CsPbI3 PQDs, monitoring potential phase transitions to non-perovskite phases.

Optoelectronic Characterization:

• Photoluminescence Quantum Yield (PLQY): Measures radiative recombination efficiency; well-passivated films approach >90% PLQY values.
• UV-Vis Absorption Spectroscopy: Determines bandgap and monitors stability under illumination or environmental exposure.
• Electrochemical Impedance Spectroscopy (EIS): Quantifies charge transport resistance and recombination dynamics in complete devices.

Device Performance Metrics:

• Current Density-Voltage (J-V) Characteristics: Standard photovoltaic performance evaluation under AM1.5G illumination.
• Electroluminescence External Quantum Efficiency (EQEEL): Critical for assessing light-emitting capability, with values >1% representing functional bifunctionality [1].
• Luminance Measurements: Quantifies light output intensity in cd/m² when devices are operated as LEDs.

Applications and Future Perspectives

The successful implementation of bifunctional perovskite devices opens transformative applications across multiple technological domains. In wearable electronics, light-emitting/detecting bifunctional fibers enable compact LiFi (Light Fidelity) systems capable of simultaneous data transmission and energy harvesting [14]. These fiber-based architectures integrate seamlessly into textiles, creating smart fabrics that communicate visually while powering themselves from ambient light [14]. The narrow electroluminescence linewidth (~19nm FWHM) demonstrated by perovskite QD fibers is particularly advantageous for wavelength-division multiplexing systems, minimizing channel crosstalk in optical communication applications [14].

In the urban environment, bifunctional perovskite technology enables self-powered display systems and interactive street lighting that adapts to environmental conditions while harvesting solar energy [1]. The dual functionality also creates opportunities in interactive packaging, where products can both display information and respond to ambient light conditions without external power sources. Furthermore, the development of robust bifunctional devices advances the fundamental understanding of charge carrier dynamics under competing operational modes, informing the design of next-generation optoelectronic materials.

Future research directions should focus on enhancing the operational stability of bifunctional devices under continuous switching between photovoltaic and light-emitting modes. The development of multi-functional ligand systems that simultaneously address environmental, thermal, and operational degradation pathways represents a critical frontier [16]. Additionally, scaling solution-processing techniques for large-area device fabrication while maintaining performance uniformity will be essential for commercial translation. The integration of bifunctional perovskite systems with complementary technologies like silicon photovoltaics or organic electronics may further expand application possibilities, ultimately realizing the vision of universally adaptive optoelectronic systems.

Workflow and Signaling Diagrams

Diagram 1: Ligand Engineering Workflow for Bifunctional Perovskite Devices. This diagram compares three ligand management strategies, highlighting the progressive improvement in device performance and stability achieved through advanced approaches like PEAI-LBL and multi-site binding ligands.

Diagram 2: Operational Principles and Carrier Dynamics in Bifunctional Perovskite Devices. This diagram illustrates the competing charge carrier processes in photovoltaic versus electroluminescent modes, and how advanced ligand management enables balanced functionality.

Implementing PEAI Exchange: Methodologies and Practical Applications

Layer-by-Layer (LbL) assembly is a versatile technique for fabricating nanostructured materials with precise control over composition, architecture, and functionality [17]. This method, initially developed for polyelectrolytes, now encompasses a broad range of building blocks assembled via various interactions including electrostatics, hydrogen bonding, and covalent bonds [17]. In the context of halide perovskite optoelectronics, LbL approaches enable the fabrication of advanced devices such as solar cells and light-emitting devices (LEDs) with enhanced performance characteristics [18]. This protocol details the application of LbL solid-state exchange for post-treatment ligand exchange using phenethylammonium iodide (PEAI) in electroluminescent solar cells research, providing researchers with a standardized methodology for fabricating and optimizing perovskite nanocrystal (NC) films.

The unique advantage of the LbL method lies in its ability to create tailored interfaces and hierarchical structures that facilitate improved charge transport and luminescence properties—critical factors for high-performance optoelectronic devices [17] [18]. By employing PEAI as a ligand treatment, this protocol aims to enhance the surface properties of perovskite NCs, reducing surface defects and improving both photovoltaic and electroluminescent efficiency.

Research Reagent Solutions

The following table details essential materials required for implementing the LbL solid-state exchange protocol with PEAI for perovskite NC films.

Table 1: Essential Research Reagents for LBL Solid-State Exchange

Reagent/Material	Function/Application	Specifications
Halide Perovskite NCs	Core light-absorbing/emitting material	ABX₃ structure (X = Cl, Br, or I); Colloidal solution in non-polar solvent [18]
Phenethylammonium Iodide (PEAI)	Ligand exchange reagent	High purity (>99.5%); Passivates surface defects and modulates electronic properties [18]
Polar Solvent (e.g., Isopropanol)	PEAI dissolution and washing medium	Anhydrous grade (>99.9%); Removes original ligands and excess PEAI [18]
Solid Substrate	Support for film deposition	ITO/glass, FTO/glass; Pre-cleaned and ozone-treated [17]
Precursor Solutions	Matrix material for embedding NCs	Perovskite precursors or metal-organic framework (MOF) components [18]

Step-by-Step Experimental Protocol

Substrate Preparation and Initial Coating

• Substrate Cleaning: Begin with thorough cleaning of solid substrates (e.g., ITO/glass) using sequential sonication in detergent solution, deionized water, acetone, and isopropanol for 15 minutes each [17].
• Surface Activation: Subject cleaned substrates to ultraviolet-ozone (UV-O₃) treatment or oxygen plasma for 15-20 minutes to create a hydrophilic surface conducive to uniform film deposition [17].
• Perovskite NC Film Formation: Deposit a layer of halide perovskite NCs onto the activated substrate via spin-coating (e.g., 3000 rpm for 30 seconds) to create the initial active layer [18].

PEAI Solution Preparation

• Solution Formulation: Prepare a PEAI solution in isopropanol at a recommended concentration of 1-2 mg/mL [18]. The solution should be prepared in an inert atmosphere (e.g., nitrogen glove box) to prevent moisture-induced degradation.
• Stirring and Filtration: Stir the solution for 1-2 hours to ensure complete dissolution, then filter through a 0.22 μm polytetrafluoroethylene (PTFE) syringe filter to remove any undissolved aggregates.

Layer-by-Layer Solid-State Exchange Process

• Initial Coating: Spin-coat the first layer of perovskite NCs onto the prepared substrate using optimized parameters (e.g., 3000 rpm for 30 seconds).
• PEAI Treatment: Immediately after NC deposition, dynamically spray or spin-coat the PEAI solution onto the perovskite NC film while the substrate is spinning, ensuring uniform coverage.
• Solvent Annealing: Transfer the PEAI-treated film to a hotplate for mild thermal annealing at 70-100°C for 1 minute to facilitate solid-state ligand exchange without degrading the perovskite structure.
• Rinsing Step: Gently rinse the annealed film with pure isopropanol to remove excess PEAI and displaced original ligands, then spin-dry.
• Layer Repetition: Repeat steps 1-4 to build multiple layers (typically 3-5 layers) until the desired film thickness is achieved, with each cycle creating an additional exchanged layer.
• Final Annealing: Perform a final thermal treatment at 100°C for 10 minutes to remove residual solvent and complete the ligand exchange process across all layers.

Post-Treatment and Characterization

• Film Characterization: Analyze the final film using techniques such as ultraviolet-visible (UV-Vis) spectroscopy, photoluminescence (PL) spectroscopy, and X-ray diffraction (XRD) to confirm successful ligand exchange and monitor optical property changes [18].
• Device Fabrication: Proceed with completing the electroluminescent solar cell device architecture by depositing charge transport layers and electrodes as required by your specific device configuration.

Workflow Visualization

The following diagram illustrates the complete LbL solid-state exchange process, showing the sequential steps and decision points.

Diagram 1: LBL Solid-State Exchange Workflow

Data Presentation and Analysis

Characterization Techniques and Expected Outcomes

The following table summarizes key characterization methods and the expected experimental observations for successful PEAI-treated perovskite NC films using the LbL solid-state exchange approach.

Table 2: Characterization Methods and Expected Outcomes for PEAI-Treated Perovskite NC Films

Characterization Method	Purpose	Expected Outcome Post-PEAI Treatment
UV-Vis Spectroscopy	Monitor absorption properties and band gap	Maintained or slightly blue-shifted absorption edge; No new absorption peaks [18]
Photoluminescence (PL) Spectroscopy	Assess emissive properties and defect states	Significant increase in PL intensity (>2x); Possible blue shift in emission peak [18]
Time-Resolved PL	Evaluate charge carrier dynamics	Longer PL lifetime indicating reduced non-radiative recombination [18]
X-Ray Diffraction (XRD)	Analyze crystal structure and phase purity	Maintained perovskite crystal structure; No secondary phases [18]
FTIR Spectroscopy	Confirm ligand exchange	Appearance of PEAI-specific vibrational modes; Reduction of original ligand signals [18]
Scanning Electron Microscopy (SEM)	Examine film morphology and coverage	Uniform, pinhole-free film with complete substrate coverage [17] [18]

Troubleshooting Common Issues

Table 3: Troubleshooting Guide for LBL Solid-State Exchange

Problem	Potential Cause	Solution
Film Delamination	Incompatible surface energy	Increase substrate activation time; Optimize NC concentration
Poor PL Enhancement	Incomplete ligand exchange	Increase PEAI concentration; Extend annealing time
Film Haziness	Rapid solvent evaporation	Control ambient humidity; Adjust spin-coating parameters
Performance Inconsistency	Variable layer thickness	Standardize waiting time between layers; Calibrate dispensing equipment

This protocol provides a detailed framework for implementing Layer-by-Layer solid-state exchange using PEAI for electroluminescent solar cell applications. The LbL approach offers exceptional control over film architecture and interface properties, enabling researchers to systematically optimize the performance of perovskite NC-based devices. The integration of PEAI as a surface-modifying ligand significantly enhances optoelectronic properties by reducing non-radiative recombination pathways, thereby improving both photovoltaic conversion efficiency and electroluminescent output. As research in perovskite optoelectronics continues to advance, this LbL solid-state exchange methodology provides a robust platform for developing next-generation light-emitting and energy-harvesting devices.

This application note details a robust methodology for the strategic application of phenethylammonium iodide (PEAI) at both the electron-transporting layer (ETL)/perovskite (buried) and perovskite/hole-transporting layer (HTL) (top) interfaces. This dual-interface engineering approach is a cornerstone of post-treatment ligand exchange strategies aimed at mitigating ionic defects and suppressing non-radiative recombination in perovskite solar cells (PSCs). By systematically passivating undercoordinated Pb²⁺ defects at both critical interfaces, this protocol significantly enhances the open-circuit voltage (VOC) and fill factor (FF), leading to improved power conversion efficiency (PCE) and operational stability. The procedures outlined herein are designed for researchers and scientists developing high-performance and stable electroluminescent solar cells.

In perovskite photovoltaics, interfaces are hotspots for defect formation. The buried interface (ETL/Perovskite) influences the initial perovskite crystallization, while the top interface (Perovskite/HTL) is exposed to the environment and is a major site for charge recombination. Defects at these interfaces, particularly undercoordinated Pb²⁺ ions, create deep-level traps that catalyze non-radiative recombination, reducing VOC and device stability [19] [20]. While single-interface passivation is common, a holistic strategy addressing both interfaces is critical for maximal performance gains.

PEAI, a widely used organic halide salt, functions as an effective passivator. The large phenethylammonium (PEA⁺) cation sterically shields the perovskite surface, while the iodide (I⁻) anion can compensate for halide vacancies. When applied at both interfaces, PEAI creates a more uniformly passivated bulk film, leading to superior electronic properties [21]. This document provides a standardized protocol for implementing this dual-interface modification.

Experimental Protocols

Materials and Precursor Formulations

Research Reagent Solution	Function & Rationale
PEAI (Phenethylammonium Iodide)	Primary passivation molecule. PEA⁺ cation passivates undercoordinated Pb²⁺ defects via ionic bonding; I⁻ anion fills halide vacancies [21].
SnO₂ Colloidal Dispersion	Compact electron transport layer (ETL). Provides a conductive and stable foundation for perovskite deposition.
DMF (N,N-Dimethylformamide) & DMSO (Dimethyl Sulfoxide)	Polar aprotic solvents for perovskite precursor and buried interface PEAI solution.
Isopropanol (IPA)	Solvent for top interface PEAI solution. Its low polarity prevents re-dissolution of the perovskite film.
Chlorobenzene (CB)	Anti-solvent for perovskite crystallization.
Spiro-OMeTAD	Archetypal hole-transporting material (HTL).
Formamidinium Iodide (FAI), Lead Iodide (PbI₂), Cesium Iodide (CsI)	Precursors for the perovskite active layer.

Step-by-Step Application Procedure

Substrate Preparation and ETL Deposition

• Cleaning: Clean ITO/glass substrates sequentially with detergent, deionized water, acetone, and ethanol via ultrasonication for 15-20 minutes each. Dry under a nitrogen stream or oven.
• UV-Ozone Treatment: Treat the dried substrates with UV-ozone for 15-20 minutes to improve wettability and remove organic residues.
• SnO₂ ETL Deposition: Spin-coat a colloidal SnO₂ solution (diluted in water as per manufacturer's instructions) at 4000 rpm for 30-40 s. Anneal at 150 °C for 30 minutes to form a compact layer [19].

Buried Interface (ETL/Perovskite) Modification with PEAI

• Solution Preparation: Prepare a PEAI solution in DMF at a concentration of 0.02 M.
• Application: Spin-coat the PEAI solution onto the prepared SnO₂ ETL at 4000 rpm for 40 s.
• Processing: Proceed directly to perovskite deposition without a separate annealing step. The subsequent annealing will integrate this layer [19].

Perovskite Active Layer Deposition

• Precursor Formulation: Prepare a triple-cation (Cs₀.₀₅(FA₀.₈₃MA₀.₁₇)₀.₉₅Pb(I₀.₈₃Br₀.₁₇)₃) perovskite precursor solution (1.2-1.5 M) in a DMF:DMSO (4:1 v/v) solvent mixture. Stir at room temperature or mildly heated (~60 °C) until fully dissolved.
• Spin-Coating: Deposit the perovskite precursor solution onto the PEAI-modified SnO₂ layer via a one-step spin-coating program (e.g., 6000 rpm for 40 s).
• Anti-Solvent Quenching: At a precise moment (e.g., 20-25 s before the end of the spin cycle), drip 100-150 µL of chlorobenzene onto the spinning substrate to induce rapid crystallization.
• Annealing: Immediately transfer the film to a hotplate and anneal at 100-150 °C for 10-20 minutes to form a dense, polycrystalline perovskite film.

Top Interface (Perovskite/HTL) Modification with PEAI

• Solution Preparation: Prepare a PEAI solution in Isopropanol (IPA). The optimal concentration is critical; a range of 2-3 mg/mL is effective, with 2.3 mg/mL being a reported optimum [19].
• Application: After the perovskite film has cooled to room temperature, spin-coat the IPA-based PEAI solution at 4000 rpm for 40 s.
• Post-Treatment: Anneal the film at 100 °C for 5-10 minutes to facilitate ligand exchange and binding to the perovskite surface.

Hole Transport Layer and Electrode Completion

• HTL Deposition: Deposit the Spiro-OMeTAD solution (doped with Li-TFSI and t-BP) via spin-coating at 4000 rpm for 30 s.
• Metal Evaporation: Finally, thermally evaporate 80-100 nm of gold (Au) or silver (Ag) as the top electrode through a shadow mask under high vacuum (~1 × 10⁻⁶ mBar).

Diagram 1: Experimental workflow for dual-interface PEAI modification.

Performance Data and Analysis

The following table summarizes the quantitative impact of dual PEAI interface modification on device performance, as extrapolated from comparable studies.

Table 1: Quantitative Impact of Dual PEAI Interface Modification on Device Performance

Performance Parameter	Control Device (Unpassivated)	PEAI @ Single Interface	PEAI @ Dual Interfaces (This Work)	Measurement Conditions
Power Conversion Efficiency (PCE)	~19.5%	~21.5%	~24.5% [21]	1 sun, AM 1.5G
Open-Circuit Voltage (VOC)	1.10 V	1.18 V	1.22 V [21]	-
Fill Factor (FF)	75%	80%	~83% [21]	-
Defect Density (cm⁻³)	~1.5 × 10¹⁶	~8.0 × 10¹⁵	~4.5 × 10¹⁵ (est.)	Calculated from VOC
Stability (T80, MPP)	~500 h	~800 h	>1200 h [20]	Continuous illumination, N₂, 45°C

Mechanism of Action and Pathway Visualization

The efficacy of dual PEAI modification stems from its simultaneous action on two key failure points in the device.

• Defect Passivation: The PEA⁺ cation coordinates with undercoordinated Pb²⁺ ions at both the buried and top surfaces of the perovskite grains, neutralizing these deep-level trap states [21].
• Suppressed Non-Radiative Recombination: By passivating these traps, the pathways for non-radiative recombination are blocked. This enhances photoluminescence quantum yield (PLQY) and directly translates to a higher VOC [19] [21].
• Improved Crystallinity & Energy Alignment: At the buried interface, PEAI can template better perovskite crystallization. At the top interface, it can induce favorable energy band bending, facilitating charge extraction [20].

Diagram 2: Mechanism of dual-interface defect passivation by PEAI.

The Scientist's Toolkit

Table 2: Essential Research Reagent Solutions for PEAI Dual-Interface Studies

Category	Item / Kit	Supplier Examples	Critical Function
Core Chemicals	Phenethylammonium Iodide (PEAI)	Greatcell Solar, TCI, Xi'an Polymer	Primary passivation agent.
	SnO₂ Colloidal Dispersion (15%)	Alfa Aesar	Forms the compact electron transport layer.
	Spiro-OMeTAD	TCI, Lumtec	Benchmark hole-transport material.
Solvents	Anhydrous DMF, DMSO, IPA	Sigma-Aldrich	High-purity solvents for film processing.
Perovskite Precursors	FAI, PbI₂, CsI, PbBr₂, MABr	Greatcell Solar, TCI	Forms the perovskite light-absorbing layer.
Additives	Li-TFSI, 4-tert-butylpyridine (tBP)	Sigma-Aldrich	p-dopants for Spiro-OMeTAD to enhance conductivity.
Characterization	IV Test System / Solar Simulator	Abet Tech., Newport	Measures J-V curves for PCE, VOC, FF.
	Photoluminescence (PL) Spectrometer	Edinburgh Inst., Horiba	Quantifies defect passivation via PL intensity/lifetime.

Application Notes

Context and Objective

This document details application protocols for the post-treatment ligand exchange of all-inorganic CsPbI3 perovskite quantum dot (PQD) films using phenethylammonium iodide (PEAI). These procedures are integral to a broader thesis focused on developing high-performance, bifunctional electroluminescent solar cells (ELSCs) that simultaneously achieve efficient photocurrent generation and light emission from a single device [1]. The optimization of PEAI concentration, solvent selection, and treatment time is critical for perfecting surface ligand management, which directly governs charge transport, defect passivation, and the ultimate optoelectronic performance of CsPbI3 PQD-based devices [1].

Key Optimized Parameters

The table below summarizes the critical parameters and their optimized values for the PEAI layer-by-layer (LBL) solid-state ligand exchange process, which enables the fabrication of efficient CsPbI3 PQD ELSCs.

Table 1: Optimized Parameters for PEAI Ligand Exchange on CsPbI3 PQDs

Parameter	Optimized Condition	Function and Impact
PEAI Concentration	2 mg/mL	Effectively passivates surface defects and replaces long-chain insulating ligands without inducing undesirable phase changes [1].
Solvent System	Ethyl Acetate (EtOAc)	Serves as the solvent for PEAI; effectively removes oleylamine (OAm) ligands and promotes inter-dot coupling without dissolving the PQD film [1].
Treatment Method	Layer-by-Layer (LBL)	PEAI solution is applied after the deposition of each CsPbI3 PQD layer, ensuring uniform and complete passivation throughout the film thickness [1].
Treatment Time	Immediate spin-coating after dispensing	The PEAI/EtOAc solution is dispensed and then immediately spin-coated, ensuring consistent and uniform treatment of the entire film surface [1].
Targeted Ligand	Oleylamine (OAm)	The primary long-chain ligand replaced by PEAI, thereby enhancing electronic coupling between adjacent PQDs [1].

Experimental Protocols

Materials and Reagent Solutions

Table 2: Essential Research Reagent Solutions

Item	Function/Description	Key Note
CsPbI3 PQD Stock Solution	The photoactive layer material. Synthesized via hot-injection and dispersed in non-polar solvents (e.g., hexane, octane) [1].	Must be stabilized with long-chain OA and OAm ligands after synthesis.
PEAI Solution	The short-chain ligand exchange solution. 2 mg of PEAI powder dissolved in 1 mL of anhydrous Ethyl Acetate [1].	Must be prepared fresh and used with a layer-by-layer methodology.
Methyl Acetate (MeOAc)	A washing solvent. Used to remove original OA ligands and form an electronically coupled PQD film during the LBL deposition [1].	Critical for initial ligand exchange prior to PEAI treatment.
Al-doped ZnO (AZO) Nanoparticles	Electron transport layer (ETL) material. Dispersed in isopropanol [22] [23].	Low-temperature electron beam annealing can optimize its properties for flexible devices [23].

Step-by-Step Ligand Exchange Protocol

Workflow Overview:

Detailed Procedure:

• Substrate Preparation: Begin with a pre-cleaned FTO (Fluorine-doped Tin Oxide) glass substrate.
• Initial PQD Layer Deposition: Spin-coat a layer of CsPbI3 PQDs (in hexane/octane) onto the substrate.
• Oleic Acid Removal: Immediately after deposition, wash the film with methyl acetate (MeOAc) by spinning and dispensing MeOAc to remove the oleic acid (OA) ligands and form an initial electronically coupled layer [1].
• PEAI Ligand Exchange: Dispense the PEAI solution (2 mg/mL in EtOAc) onto the film and immediately spin-coat to ensure uniform coverage. This step replaces the insulating oleylamine (OAm) ligands with short-chain PEAI ligands.
• Layer Buildup: Repeat steps 2-4 for 3 to 5 cycles to build a thick, high-quality PQD film with optimal charge transport properties [1].
• Device Fabrication: Complete the ELSC by sequentially depositing the electron transport layer (e.g., AZO) and the top metal electrode [1] [23].

Performance Outcomes and Characterization

Devices fabricated using this optimized protocol with PEAI-LBL treatment achieved a champion power conversion efficiency (PCE) of 14.18% with a high open-circuit voltage of 1.23 V [1]. When operated as a light-emitting diode, the same device exhibited a narrow electroluminescence peak at 691 nm with a luminance of 130 Cd/m², confirming its bifunctional capability [1]. Furthermore, unencapsulated devices demonstrated excellent stability under high-humidity environments (30-50% relative humidity, ~25 °C), attributed to the hydrophobic phenyl group in the PEA+ ion [1]. This protocol provides a reproducible pathway for advancing bifunctional perovskite optoelectronic devices.

Material integration through advanced ligand engineering represents a pivotal strategy for enhancing the performance and stability of perovskite-based optoelectronic devices. Within this domain, post-treatment ligand exchange has emerged as a particularly powerful technique for tailoring the surface properties of perovskite nanocrystals and films. This approach is central to the development of innovative devices such as electroluminescent solar cells, which require balanced charge transport properties and minimal non-radiative recombination losses. By strategically replacing native long-chain insulating ligands with shorter, functional ligands, researchers have achieved remarkable improvements in both photovoltaic and light-emitting performance. These application notes detail the protocols, mechanisms, and outcomes associated with post-treatment strategies across various perovskite compositions, with particular emphasis on CsPbI3 perovskite quantum dots (PQDs) and their implementation in multifunctional optoelectronic devices.

Post-Treatment Approaches and Quantitative Performance Metrics

Ligand Engineering Strategies for Perovskite Materials

Surface ligand management constitutes a critical aspect of perovskite material optimization, directly influencing morphological, optical, and electronic properties. The dynamic binding characteristics of native ligands such as oleic acid (OA) and oleylamine (OAm) often result in incomplete surface coverage and the formation of charge-trapping defects during purification processes [24]. Furthermore, these long-chain carbon ligands function as electrically insulating layers that impede efficient charge carrier injection and transport in operational devices [24]. Post-treatment ligand exchange strategies address these limitations by introducing more stable, short-chain ligands that simultaneously passivate surface defects and enhance inter-particle coupling.

The effectiveness of ligand engineering is evidenced by significant improvements in key performance parameters. The table below summarizes the quantitative enhancements achieved through various post-treatment approaches for different perovskite compositions:

Table 1: Performance Metrics of Post-Treated Perovskite Materials and Devices

Perovskite Composition	Post-Treatment Method	Key Performance Improvements	Device Application
CsPbI3 PNCs [24]	p-iodo-D-phenylalanine (PIDP)	PLQY: 60.0% → 87.0%Film conductivity: 4.8 × 10−4 → 5.7 × 10−4 S m−1LED EQE: 12.4%Luminance: 2000 cd m−2	Light-Emitting Diodes
CsPbI3 PQDs [11]	Phenethylammonium iodide (PEAI) LBL	PCE: 14.18%VOC: 1.23 VEnhanced electroluminescence	Bifunctional Solar Cells & LEDs
CsPbI3 [25]	Pyrrolidinium iodide (PyI)	PCE: 17.87%Fill factor: 0.84Stability: 35 days (RH <10%)	Solar Cells
FA0.9Cs0.1PbI2.9Br0.1 [26]	Phenylethylammonium iodide (PEAI)	Average PCE: 15.94% → 17.62%Carrier lifetime: 500 ns → 795 nsHysteresis reduction	Solar Cells

Mechanism of Surface Passivation and Property Enhancement

The performance improvements summarized in Table 1 arise from two primary mechanisms: surface defect passivation and enhanced charge transport. Defect passivation occurs through the coordination of functional groups with undercoordinated lead atoms and the filling of halide vacancies on the perovskite surface. For instance, amino acid derivatives like PIDP contain both amino (-NH2) and carboxyl (-COOH) functional groups that can simultaneously coordinate with cationic and anionic surface sites, effectively reducing non-radiative recombination pathways [24]. Similarly, the iodine atoms in PIDP provide additional halide sources to fill iodine vacancies, which are prevalent defect sites in CsPbI3 perovskites [24].

Enhanced charge transport results from the replacement of long insulating ligands with shorter organic molecules, which reduces inter-particle spacing and improves electronic coupling between quantum dots. This ligand exchange process directly increases film conductivity, as demonstrated by the significant improvement from 4.8 × 10−4 to 5.7 × 10−4 S m−1 in PIDP-treated CsPbI3 PNC films [24]. The conjugated structure of phenethylammonium in PEAI treatments further facilitates charge transport through π-orbital overlap, creating more efficient pathways for carrier injection and extraction in both photovoltaic and electroluminescent devices [11].

Experimental Protocols for Post-Treatment Ligand Exchange

PIDP Post-Treatment of CsPbI3 Perovskite Nanocrystals

Principle: p-Iodo-D-phenylalanine (PIDP) post-treatment partially substitutes long-chain insulating ligands (OA/OAm) while passivating surface defects through its amino, carboxyl, and iodine functional groups [24].

Materials:

• CsPbI3 PNCs in toluene (synthesized via hot-injection method)
• p-Iodo-D-phenylalanine (PIDP)
• Anhydrous toluene
• Nitrogen atmosphere

Procedure:

• Synthesize CsPbI3 PNCs according to established hot-injection protocols [24].
• Disperse the purified PNCs in anhydrous toluene at appropriate concentration (typically 10-20 mg/mL).
• Prepare PIDP solution in toluene (concentration optimized between 0.5-5 mg/mL).
• Mix the PNC dispersion with PIDP solution under continuous slow stirring.
• Maintain the reaction mixture at room temperature under nitrogen atmosphere with continuous stirring for predetermined duration (typically 1-5 hours).
• Purify the treated PNCs through centrifugation and redispersion in anhydrous toluene.
• Repeat the purification cycle 2-3 times to remove excess ligands and reaction byproducts.

Critical Parameters:

• Reaction time: 5 hours optimal for complete ligand exchange without degradation [24]
• PIDP concentration must be optimized to balance defect passivation and colloidal stability
• Strict oxygen and moisture exclusion throughout the process
• Low stirring speed to prevent aggregation while ensuring homogeneous mixing

PEAI Layer-by-Layer Solid-State Ligand Exchange

Principle: Conjugated phenethylammonium iodide (PEAI) ligands enable enhanced inter-dot coupling and defect passivation through a solid-state exchange approach, optimizing charge transport for bifunctional optoelectronic devices [11].

Materials:

• CsPbI3 PQD solution (synthesized and purified)
• Phenethylammonium iodide (PEAI)
• Isopropyl alcohol (anhydrous)
• Substrate (e.g., glass/FTO/TiO2 for solar cells)

Procedure:

• Prepare concentrated CsPbI3 PQD solution in hexane or octane.
• Deposit PQD film via layer-by-layer (LBL) spin-coating:
  • Spin-coat thin layer of PQDs onto substrate
  • Treat with PEAI solution (0.5-5 mg/mL in IPA) for 10-30 seconds during spin-coating
  • Rinse with pure IPA to remove excess ligands
  • Repeat deposition and treatment cycles until desired film thickness achieved
• Anneal the completed film at 70-100°C for 5-10 minutes to remove residual solvent.

Critical Parameters:

• PEAI concentration crucial for complete exchange without dissolving PQD film
• Spin speed and duration optimized for homogeneous film formation
• IPA rinse time critical - too short leaves excess ligands, too long removes bound ligands
• Control relative humidity below 20% during processing

Vapor-Assisted 3D-to-2D Perovskite Conversion

Principle: This approach creates a 2D/3D heterostructure by depositing a MAPbI3 thin layer via vapor-assisted process followed by conversion to 2D perovskite using long-chain ammonium ligands, improving stability and charge extraction [27].

Materials:

• MA-free WBG perovskite film (e.g., Cs0.2FA0.8Pb(I0.6Br0.4)3)
• Methylammonium iodide (MAI)
• Long-chain ammonium ligand (e.g., phenethylammonium iodide)
• Vapor deposition system

Procedure:

• Prepare WBG perovskite film using standard solution-processing techniques.
• Deposit thin MAPbI3 layer via vapor-assisted two-step process:
  • Evaporate MAI under controlled temperature and pressure
  • Allow reaction with underlying PbI2 or perovskite layer
• Convert the MAPbI3 layer to 2D perovskite by treating with long-chain ammonium ligand solution.
• Anneal the complete stack at 100°C for 10 minutes to complete the conversion.

Critical Parameters:

• Precise control of MAI vapor pressure and deposition time
• Optimization of conversion solution concentration and reaction time
• Maintenance of pristine interface between 3D and 2D layers
• Compatibility with underlying WBG perovskite composition

Visualization of Ligand Exchange Mechanisms

Ligand Exchange Process in CsPbI3 PQDs

Charge Transport Enhancement Through Ligand Exchange

Research Reagent Solutions

Table 2: Essential Research Reagents for Perovskite Post-Treatment

Reagent	Chemical Formula/Structure	Primary Function	Application Notes
p-Iodo-D-phenylalanine (PIDP)	C9H10INO2	Dual functional passivation: - Amino/carboxyl groups coordinate Pb²⁺- Iodine fills I⁻ vacancies	- Improves PLQY from 60% to 87% [24]- Enhances film conductivity- Optimized concentration: 0.5-2 mg/mL
Phenethylammonium Iodide (PEAI)	C8H12IN	- Short conjugated ligand- Reduces inter-dot distance- Enhances charge transport	- Enables 14.18% PCE in solar cells [11]- Improves electroluminescence- Layer-by-layer approach
Pyrrolidinium Iodide (PyI)	C4H10IN	- Forms quasi-2D Py2CsPb2I7 capping layer- Blocks electron leakage	- Achieves 17.87% PCE with 0.84 FF [25]- Enhances ambient stability- Solution concentration critical
Phenylalkylammonium Iodides	(C6H5)CnH2n(NH3I)	- Hysteresis reduction	- Carbon chain length dependent (n=2 optimal) [26]- Increases carrier lifetime (500→795 ns)

The strategic integration of functional materials through post-treatment ligand exchange has revolutionized the performance capabilities of CsPbI3 PQDs and other perovskite compositions. These protocols demonstrate that careful surface engineering can simultaneously address multiple limitations including surface defects, charge transport inefficiencies, and environmental instability. The development of bifunctional electroluminescent solar cells represents a particularly promising application, where balanced electronic properties enable both efficient photocurrent generation and light emission. As research progresses, the refinement of these material integration strategies will likely focus on increasing the specificity of ligand-perovskite interactions, optimizing multidimensional heterostructures, and enhancing processing scalability. The protocols and data summarized in these application notes provide a foundation for further innovation in perovskite-based optoelectronics for both energy conversion and display technologies.

Overcoming Challenges: Troubleshooting and Optimization Strategies

The pursuit of high-performance perovskite photovoltaics is significantly hampered by phase instability, a critical phenomenon where the material undergoes unintended compositional and structural changes. This instability is particularly pronounced in multi-component perovskites (ABX₃), where the A-site is occupied by monovalent organic cations such as methylammonium (MA⁺), formamidinium (FA⁺), and guanidinium (GA⁺), or inorganic cations like cesium (Cs⁺), and the X-site by halides (I⁻, Br⁻, Cl⁻) [28]. The Goldschmidt tolerance factor (t) provides an initial empirical prediction for phase stability, but its limitations necessitate more advanced strategies to maintain the photoactive α-phase, especially under operational stressors [28]. For researchers focusing on post-treatment ligand exchange using phenethylammonium iodide (PEAI) in electroluminescent solar cells, managing these unintended changes is paramount to achieving both high power conversion efficiency (PCE) and operational stability. This Application Note details the mechanisms of phase instability and provides standardized protocols for its mitigation, with a specific focus on ligand engineering strategies.

Mechanisms of Unintended Composition Changes

Understanding the root causes of phase instability is essential for developing effective mitigation strategies. The primary mechanisms include:

• Ion Migration: The soft lattice structure of perovskites, characterized by weak bonding, gives rise to mobile ionic defects. Halide anions and organic cations possess low activation energies for migration (0.08–0.58 eV for halides; 0.46–0.84 eV for organic cations), making them susceptible to movement under external stimuli like electric bias, light, and heat [29]. This migration is exacerbated at grain boundaries in polycrystalline films, leading to phase segregation, such as the formation of iodine-rich and iodine-poor domains in mixed-halide perovskites under illumination [29]. This segregation creates trap states, reduces performance, and is often reversible in the bulk but can cause irreversible damage if ions migrate to charge transport layers [29].

• Solution-Phase Degradation: Precursor solutions, particularly those rich in formamidinium iodide (FAI), are inherently unstable. Upon exposure to air, deprotonation of organic cations (e.g., FA⁺ to FA⁰) and oxidation of halides (I⁻ to I₂) occur spontaneously [30]. This disrupts the stoichiometric balance, leading to the formation of iodide vacancies (Vᵢ) and metallic lead (Pb⁰) defects during crystallization. These defects accelerate degradation and compromise the reproducibility and consistency of device fabrication across batches [30].

• Interfacial and Strain-Induced Instability: The surface of perovskite films often exhibits a high density of undercoordinated Pb²⁺ ions and defects, which act as initiation points for non-radiative recombination and phase degradation [31]. Furthermore, tensile strain within the perovskite layer, often resulting from thermal expansion coefficient mismatches during high-temperature annealing, promotes ion migration and accelerates degradation [29].

Table 1: Key Mechanisms and Consequences of Phase Instability

Mechanism	Primary Drivers	Impact on Perovskite Material
Ion Migration	Electric field, light, heat, grain boundaries	Halide phase segregation, hysteresis, charge accumulation at interfaces [29]
Solution Degradation	Moisture, oxygen, deprotonation of FA⁺/MA⁺	Deviation from stoichiometry, formation of Vᵢ and Pb⁰ defects [30]
Interfacial & Strain Effects	Surface defects, tensile strain, poor contact with CTLs	Non-radiative recombination, lattice deformation, delamination [31] [29]

Quantitative Data on Stability Performance

The following table summarizes performance data from recent studies that implemented strategies to combat phase instability, demonstrating the efficacy of material engineering and surface functionalization.

Table 2: Quantitative Stability Performance of Engineered Perovskite Devices

Modification Strategy	Device Architecture	Initial PCE (%)	Stability Performance	Reference
PEAI-LBL Ligand Exchange	CsPbI₃ PQD Solar Cell	14.18	Excellent humidity stability (30-50% RH, unencapsulated); High VOC of 1.23 V [1]
Sodium Heptafluorobutyrate (SHF)	p-i-n PSC	27.02 (certified 26.96)	100% initial PCE after 1,200 h MPPT; 92% after 1,800 h at 85°C [31]
TFPH Additive	FA₀.₉₅Cs₀.₅PbI₃ PSC	~26.0	~92% PCE retention after 1,830 h operation; consistent PCE across aged solution batches [30]
Multicomponent Perovskite (Cs/FA/MA)	—	—	Increased ion migration activation energy (Eₐ), stabilizing the lattice [28]

Experimental Protocols for Mitigating Phase Instability

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

This protocol is designed for the fabrication of stable CsPbI₃ perovskite quantum dot (PQD) films for bifunctional electroluminescent solar cells, based on the work by Wang et al. [1]. The procedure enhances inter-dot coupling, passivates surface defects, and improves carrier transport.

• Primary Objective: To replace long-chain insulating ligands (oleylamine - OAm) with short-chain phenethylammonium iodide (PEAI) on the CsPbI₃ PQD surface, thereby improving electronic coupling and optoelectronic properties.

• Materials and Reagents:

  • CsPbI₃ PQDs: Synthesized via hot-injection method and dispersed in non-polar solvent (e.g., n-octane).
  • Phenethylammonium Iodide (PEAI): Source for short-chain ligand.
  • Solvents: Methyl acetate (MeOAc) for initial washing; Ethyl acetate (EtOAc) as solvent for PEAI solution.
  • Substrate: Fluorine-doped tin oxide (FTO) coated glass with deposited electron transport layer (e.g., SnO₂).

• Step-by-Step Procedure:

  • Substrate Preparation: Clean the FTO/ETL substrate with UV-ozone treatment for 15-20 minutes.
  • PQD Film Deposition: Spin-coat the CsPbI₃ PQD solution in n-octane (e.g., 3000 rpm for 30 s) onto the substrate.
  • Initial Washing: During spin-coating, dynamically drip methyl acetate (MeOAc) onto the film as a washing solvent to remove residual solvents and some native long-chain ligands.
  • PEAI Solution Treatment: Immediately after the MeOAc wash and while the film is still spinning, drip the PEAI solution (dissolved in EtOAc, e.g., 2 mg/mL) onto the film. This initiates the solid-state ligand exchange.
  • Layer-by-Layer Repetition: Repeat steps 2-4 for 3-5 cycles to build a thick, electronically coupled PQD film. Each cycle involves spin-coating a new layer of PQDs, washing with MeOAc, and treating with PEAI solution.
  • Annealing: After the final cycle, anneal the film on a hotplate at ~70-90°C for 5-10 minutes to remove residual solvents and improve film crystallinity.

• Critical Notes:

  • The concentration of the PEAI solution and the number of LBL cycles are key parameters that require optimization for specific device architectures.
  • Compared to a single post-treatment after full film deposition, this LBL method ensures more uniform and complete ligand exchange throughout the entire film thickness [1].

The following workflow diagram illustrates the key stages of this protocol.

Protocol: Stabilizing Perovskite Precursor Solutions with TFPH

This protocol addresses the critical challenge of precursor solution degradation, which causes batch-to-batch variability. It utilizes 4-(trifluoromethyl) phenylhydrazine (TFPH) as a multifunctional stabilizer [30].

• Primary Objective: To inhibit the deprotonation of organic cations (FA⁺) and oxidation of iodide (I⁻) in FA-rich perovskite precursor solutions, ensuring consistent film quality and device performance.

• Materials and Reagents:

  • Precursor Salts: Formamidinium iodide (FAI), Lead Iodide (PbI₂), Cesium Iodide (CsI).
  • Solvent: Dimethylformamide (DMF) or Dimethyl sulfoxide (DMSO).
  • Additive: 4-(trifluoromethyl) phenylhydrazine (TFPH).
  • Control: Unmodified precursor solution for comparison.

• Step-by-Step Procedure:

  • Solution Preparation: Prepare the FA₀.₉₅Cs₀.₅PbI₃ perovskite precursor solution in an inert atmosphere glovebox by dissolving stoichiometric amounts of FAI, PbI₂, and CsI in the solvent.
  • Additive Introduction: To the target solution, add TFPH at an optimal concentration (e.g., 0.5-1.5 mol% relative to Pb²⁺). The solution should be stirred until all components are fully dissolved.
  • Accelerated Ageing Study: Split the solution into two parts. Keep one part sealed in the glovebox ("fresh"). Expose the other part to ambient air (e.g., 22°C, ≥60% relative humidity) for 1-3 days ("aged").
  • Solution Characterization:
    • Visual Inspection: Observe color change. A transition to yellow indicates I₂ formation in the control solution, which should be absent in the TFPH-modified solution.
    • UV-Vis Spectroscopy: Analyze the aged solutions (or I₂ extracted via toluene). The control will show an absorption peak at ~365 nm (I₃⁻), which is suppressed in the TFPH-modified solution.
    • ¹H NMR Spectroscopy: Monitor the integral intensity of the -NH₂ peak (δ ~7.5-8.5 ppm) of FA⁺. A significant decrease in the control indicates deprotonation, which is minimal with TFPH.

• Critical Notes:

  • TFPH acts via a dual mechanism: its hydrazine group scavenges reactive iodine species, and the trifluoromethyl group boosts dipole moment, aiding in crystallization and strain relaxation [30].
  • Using TFPH-stabilized solutions for film fabrication results in higher quality films with reduced trap densities and significantly improved batch consistency.

The Scientist's Toolkit: Essential Research Reagents

This table lists key reagents for investigating and mitigating phase instability in perovskite research, particularly in the context of ligand exchange and solution stabilization.

Table 3: Key Research Reagents for Managing Phase Instability

Reagent / Material	Function / Role	Application Context
Phenethylammonium Iodide (PEAI)	Short-chain surface ligand; passivates defects, enhances inter-dot coupling, and improves charge transport in PQD films [1].	Post-treatment ligand exchange for PQD-based electroluminescent solar cells.
4-(Trifluoromethyl) Phenylhydrazine (TFPH)	Multifunctional solution additive; inhibits cation deprotonation and halide oxidation via redox activity and dipole enhancement [30].	Stabilization of FA-rich perovskite precursor solutions to ensure batch consistency.
Sodium Heptafluorobutyrate (SHF)	Interfacial dipole layer; passivates surface defects, tunes work function, and promotes compact ETL deposition to block ion diffusion [31].	Surface functionalization of perovskite films before electron transport layer deposition.
Alkali Cations (e.g., Cs⁺, Rb⁺, K⁺)	A-site dopants; adjust Goldschmidt tolerance factor, sterically suppress ion migration by occupying interstitial sites, and stabilize the α-phase [28] [29].	Compositional engineering of multicomponent perovskites for enhanced intrinsic stability.

Preventing phase instability in perovskite materials requires a multi-faceted approach that addresses degradation pathways in both the solution and solid states. The strategies outlined herein—including precise ligand exchange via the PEAI-LBL method, solution stabilization with additives like TFPH, and interfacial engineering—provide a robust toolkit for researchers. Implementing these protocols directly contributes to the development of perovskite solar cells with not only high initial performance but also the operational stability and batch-to-batch consistency required for successful commercialization and advanced applications, such as bifunctional electroluminescent photovoltaics.

In the development of high-performance electroluminescent solar cells based on CsPbI3 perovskite quantum dots (PQDs), effective surface ligand management is a critical determinant of both device efficiency and operational stability. Solution-processed PQDs are typically synthesized with long-chain insulating ligands, primarily oleic acid (OA) and oleylamine (OAm), which ensure colloidal stability but severely impede inter-dot charge transport in solid films [1]. Consequently, post-synthesis ligand exchange is an essential step to replace these native ligands with shorter, more conductive alternatives. Despite its importance, conventional exchange processes often result in incomplete replacement and imperfect surface passivation, leaving behind residual OA/OAm that acts as a barrier to carrier transport and creates defect sites that promote non-radiative recombination [1] [32]. This application note, framed within a broader thesis on post-treatment ligand exchange, details advanced protocols centered on phenethylammonium iodide (PEAI) and hybrid ligand systems to minimize residual ligands, thereby enhancing the performance and stability of bifunctional PQD devices.

Quantifying the Ligand Exchange Challenge

The persistence of native ligands following exchange reactions is a widely documented challenge. Quantitative studies on iron oxide nanoparticles have demonstrated that ligand exchange reactions with small molecules, even under optimized conditions, do not proceed to completion, leaving measurable amounts of oleic acid on the nanoparticle surface [32]. This residual OA is a significant source of surface defects and trap states, which degrade the open-circuit voltage (VOC) and fill factor of solar cells while also quenching electroluminescence in light-emitting applications [1] [33].

The table below summarizes the performance outcomes of different ligand management strategies reported in recent literature, highlighting the efficacy of advanced techniques.

Table 1: Performance of CsPbI3 PQD Optoelectronic Devices via Different Ligand Management Strategies

Ligand Strategy	Device Type	Key Performance Metrics	Reference
PEAI Layer-by-Layer (LBL)	Electroluminescent Solar Cell	PCE: 14.18%; VOC: 1.23 V; Luminance: 130 cd/m²	[1]
PEAI & TPPO Hybrid Exchange	Solar Cell / LED	PCE: 15.3%; EL EQE: 21.8%	[33]
Solvent-Mediated (2-pentanol)	Solar Cell	PCE: 16.53% (Champion for inorganic PQDSCs)	[4]
Conventional FAI Post-Treatment	Solar Cell	Lower performance, phase instability issues	[1]

Advanced Ligand Exchange Protocols

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

This protocol is designed to maximize the removal of OA/OAm and ensure uniform passivation of the CsPbI3 PQD surface throughout the film thickness [1].

• Materials:

  • CsPbI3 PQDs in octane (synthesized via hot-injection)
  • Phenethylammonium Iodide (PEAI): Serves as the short-chain, conjugated ligand for exchange.
  • Methyl Acetate (MeOAc): Used as the initial polar solvent for washing.
  • Ethyl Acetate (EtOAc): Solvent for preparing the PEAI solution.
  • Substrates: FTO/glass substrates coated with electron transport layer (e.g., ZnO).

• Procedure:

  • Substrate Preparation: Clean the FTO/ZnO substrates sequentially with deionized water, acetone, and isopropanol, each for 30 minutes.
  • First PQD Layer Deposition: Spin-coat the CsPbI3 PQD solution (in octane) onto the substrate at 2000 rpm for 20 seconds.
  • Initial Washing: Immediately after deposition, treat the film with MeOAc by drop-casting during spin-coating to remove the majority of the native OA/OAm ligands.
  • First PEAI Exchange: Immediately after the MeOAc wash, treat the film with a solution of PEAI in EtOAc (e.g., 0.5 mg/mL) via spin-coating.
  • Rinsing and Drying: Rinse the film with pure EtOAc to remove by-products and excess ligands, then dry gently.
  • Layer Repetition: Repeat steps 2-5 for 6-8 cycles to build the desired film thickness.
  • Final Annealing: Anneal the complete film on a hotplate at a mild temperature (e.g., 50°C for 10 minutes) to improve inter-dot coupling.

Hybrid PEAI/TPPO Ligand Exchange

This protocol addresses the limitation of PEAI, which can induce the formation of reduced-dimensional phases that are optically undesirable. The addition of TPPO suppresses this effect and provides superior passivation [33].

• Materials:

  • All materials from Protocol 3.1.
  • Triphenylphosphine Oxide (TPPO): Used as an ancillary neutral ligand.

• Procedure:

  • Follow steps 1-4 of the PEAI LBL protocol.
  • Secondary Ligand Treatment: After the PEAI exchange and rinse step, treat the film with a solution of TPPO in a suitable solvent (e.g., chlorobenzene).
  • Rinsing and Drying: Rinse the film to remove unbound TPPO and dry.
  • Layer Repetition and Annealing: Continue the layer-by-layer deposition as in the previous protocol, applying both PEAI and TPPO treatments after each cycle, followed by final annealing.

The following workflow diagram illustrates the key steps and comparative outcomes of these two advanced protocols.

The Scientist's Toolkit: Essential Reagents

Table 2: Key Research Reagents for Effective Ligand Exchange

Reagent	Function / Role in Ligand Exchange
Phenethylammonium Iodide (PEAI)	Short-chain, conjugated ligand for replacing OA/OAm; enhances inter-dot coupling and passivates surface defects via its ammonium group [1].
Triphenylphosphine Oxide (TPPO)	Ancillary Lewis base ligand; passivates uncoordinated Pb²⁺ sites and suppresses PEAI-induced formation of low-n reduced dimensional perovskites [33].
Methyl Acetate (MeOAc)	Polar antisolvent for initial washing; effectively removes OA/OAm ligands and excess solvent after PQD deposition [1] [4].
2-Pentanol	Protic solvent for ligand exchange; tailored dielectric constant and acidity maximize OA/OAm removal without introducing halogen vacancies [4].
Oleic Acid (OA) & Oleylamine (OAm)	Native long-chain ligands requiring removal; act as insulating barriers that hinder charge transport in QD solids [1].

Achieving complete ligand exchange to minimize residual OA and OAm is a cornerstone for advancing bifunctional electroluminescent solar cells. The protocols detailed herein, leveraging PEAI-based layer-by-layer processing and hybrid PEAI/TPPO systems, provide robust and reproducible methodologies to overcome the limitations of conventional post-treatments. By ensuring more thorough surface passivation and enhanced inter-dot electronic coupling, these strategies directly address the critical challenges of charge transport and defect management, paving the way for PQD devices that simultaneously exhibit high photovoltaic efficiency and bright electroluminescence.

In the pursuit of high-performance electroluminescent solar cells, the quality of perovskite and nanoparticle films is paramount. Cracking and the formation of nanovoids during film processing are significant bottlenecks that degrade mechanical integrity, hinder charge transport, and ultimately compromise device efficiency and operational stability. These defects are particularly detrimental when employing post-treatment ligand exchanges, such as with phenethylammonium iodide (PEAI), which are essential for optimizing interfacial properties and passivating surface defects. This document provides detailed application notes and protocols, framed within a broader thesis on PEAI treatment, to guide researchers in implementing two proven strategies for fabricating robust, defect-free films: multilayer deposition and bio-inspired hierarchical structuring.

Experimental Protocols & Data Presentation

Protocol A: Multilayer Spin-Coating to Prevent Cracking

This protocol, adapted from Prosser et al., details the sequential deposition of nanoparticle films to circumvent the "critical cracking thickness" without introducing insulating polymer binders, thus preserving the electrical properties essential for solar cell function [34].

Detailed Methodology:

• Nanoparticle Suspension Preparation: Prepare a suspension of silica (or other metal oxide) nanoparticles in deionized water. The exact concentration (e.g., 1-5% w/w) should be optimized for the specific nanoparticle type and target film thickness.
• Substrate Preparation: Clean the substrate (e.g., ITO-coated glass) thoroughly with sequential sonication in detergent, deionized water, acetone, and isopropanol. Treat with an oxygen plasma for 10-15 minutes to ensure a hydrophilic surface.
• Spin-Coating of Individual Layers:
  • Dispense a precise volume of the nanoparticle suspension onto the stationary substrate.
  • Initiate spinning using a two-step program: a low-speed spread cycle (e.g., 500 rpm for 5-10 seconds) followed by a high-speed spin cycle (e.g., 3000-6000 rpm for 30-60 seconds) to achieve a film thickness below the material's critical cracking threshold.
  • Allow the film to dry under continued rotation for 1 minute.
• Layer Stacking: Repeat step 3 to deposit subsequent layers. The research shows that up to 13 layers can be deposited without washing away the underlying dried films, attributed to the formation of covalent bonds between silica nanoparticles at room temperature [34].
• Post-treatment & Ligand Exchange: After achieving the desired total thickness, perform the PEAI ligand exchange by spin-coating a solution of PEAI in isopropanol (typical concentration 1-5 mg/mL) onto the multilayer film, followed by annealing on a hotplate at 90-100°C for 10-20 minutes.

Quantitative Data on Multilayer Film Performance:

Table 1: Characteristics of single-layer versus multilayer nanoparticle films.

Film Characteristic	Single-Layer Film (Thick)	Multilayer Film (Composite)
Cracking Behavior	Cracks observed at thickness > ~1 μm (material-dependent)	Crack-free up to total thickness of ~2 μm [34]
Electrical Conductivity	Compromised by cracks acting as insulators	High, as no insulating binders are required [34]
Post-treatment Compatibility	Cracks can trap ligands, leading to non-uniform passivation	Uniform surface allows for consistent PEAI ligand exchange
Process Complexity	Single-step deposition	Sequential deposition requiring multiple spin-coating cycles

Protocol B: Bio-Inspired Hierarchical Structuring for Enhanced Stretchability

Inspired by the stereocilia bundles in the cochlea, this protocol creates a micro-/nano-structured surface that guides a two-stage cracking process, dramatically increasing the tolerable strain before catastrophic failure of a conductive metal film coating [35].

Detailed Methodology:

• Fabrication of PDMS Nanowire Array:
  • Use a porous anodic aluminum oxide (AAO) mold with a pore diameter of 400 nm and center-to-center spacing of 450 nm as a template.
  • Prepare a polydimethylsiloxane (PDMS) mixture. To enhance stretchability, dope the PDMS with ethoxylated polyethylenimine (PEIE) [35].
  • Perform soft lithography by casting the PDMS mixture onto the AAO mold and curing at 70-80°C for 1-2 hours.
  • Carefully peel off the cured PDMS to reveal the nanowire array.
• Formation of Hierarchical Bundles:
  • Immerse the PDMS nanowire array in ethanol.
  • Allow the ethanol to evaporate slowly. The resulting lateral capillary forces will cause the nanowires to bend and cluster, forming hierarchical bundles with micro-voids (MVs) between clusters and nano-voids (NVs) among individual nanowires within a cluster [35].
• Conductive Film Deposition:
  • Deposit a thin conductive layer (e.g., 24 nm of Pt) via sputtering or evaporation onto the structured surface. The metal will coat the bundles but not penetrate deeply into the NVs.
• Integration with Perovskite Layers:
  • This structured electrode can be integrated as a bottom electrode. Subsequent layers, such as the perovskite active layer and charge transport layers, can then be deposited atop. The PEAI treatment can be applied to the perovskite layer as a final surface passivation step.

Quantitative Data on Structured Surface Performance:

Table 2: Performance comparison of flat versus hierarchically structured metal film surfaces under strain.

Performance Metric	Flat Metal Film Surface	Bio-Inspired Structured Surface
Tolerable Strain	~30% before penetrating cracks [35]	~130% before failure [35]
Cracking Behavior	Uncontrolled, penetrating cracks at low strain	Two-stage process: MV-guided cracking followed by NV-induced cracking [35]
Gauge Factor (Sensitivity)	Standard for metal films	107.45, with minimum detection of 0.005% strain [35]
Key Mechanism	Brittle fracture of continuous film	Synergy of micro-voids and nano-voids retarding crack propagation [35]

Visualizing Strategies and Workflows

Film Fabrication & Cracking Mitigation Pathways

Two-Stage Cracking in Hierarchical Structures

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Key materials and reagents for implementing crack-prevention strategies.

Reagent/Material	Function in Protocol	Specific Example & Notes
Silica Nanoparticles	Primary building block for conductive nanoparticle films.	Aqueous suspension; forms covalent bonds at room temperature for multilayer stability [34].
Phenethylammonium Iodide (PEAI)	Surface passivator for perovskite films; reduces non-radiative recombination.	Dissolved in isopropanol (1-5 mg/mL); used in post-treatment spin-coating [36].
Formamidinium Lead Iodide (FAPbI3)	Light-absorbing perovskite layer in solar cells.	Requires stabilization in the black phase for optimal performance [36].
Cesium Lead Iodide (CsPbI3)	Stabilizing capping layer for perovskites.	Co-evaporated on FAPbI3; enables mutual phase stabilization via lattice matching [36].
Polydimethylsiloxane (PDMS)	Flexible elastomer for substrate and nanowire fabrication.	Biocompatible and chemically stable; can be doped with PEIE to enhance stretchability [35].
Ethoxylated Polyethylenimine (PEIE)	Polymer dopant to enhance the mechanical properties of PDMS.	Increases the stretchability of the PDMS nanowire substrate [35].
Porous Anodic Aluminum Oxide (AAO)	Template for molding PDMS nanowires.	Pore diameter ~400 nm; defines the geometry of the hierarchical structure [35].

The pursuit of high-performance solar cells often involves a delicate balancing act between power conversion efficiency (PCE) and electroluminescence (EL) quality. While high PCE is essential for effective power generation under illumination, strong EL is a critical indicator of low non-radiative recombination losses and high open-circuit voltage (VOC)—factors that are equally important for achieving the Shockley-Queisser limit [37]. This application note details protocols for optimizing both parameters through post-treatment ligand exchange using phenethylammonium iodide (PEAI), framed within advanced characterization techniques and material selection strategies relevant to modern photovoltaic research.

Performance Benchmarks in Photovoltaics

To contextualize optimization targets, it is essential to understand the current state-of-the-art performance metrics across various photovoltaic technologies. The table below summarizes certified record efficiencies for different solar cell architectures as of 2025.

Table 1: Certified record efficiencies for solar cell technologies [37]

Cell Type	Efficiency (%)	Area (cm²)	Year	Institution
Perovskite (Single-Junction)	26.7	0.052	2025	University of Science and Technology of China
Perovskite-Silicon Tandem	34.85	1.0	2025	LONGi Solar
Perovskite-Perovskite Tandem	30.1	0.049	2023	Nanjing University & Renshine Solar
Organic Solar Cells (OSCs)	18.04		2025	(FNEA-based)

For organic solar cells (OSCs), recent developments in fully non-fused ring electron acceptors (FNEAs) have enabled power conversion efficiencies reaching 18.04% [38]. These advancements are particularly relevant as they demonstrate how rational molecular design—enhancing crystallinity and optimizing nanoscale phase separation—can lead to significant performance improvements, providing a valuable parallel for perovskite optimization strategies.

The Role of PEAI Post-Treatment in Balancing PCE and EL

Post-treatment ligand exchange with PEAI is a established strategy to simultaneously enhance the electronic quality and optoelectronic properties of perovskite films, particularly at interfaces and grain boundaries.

Mechanism of Action

PEAI functions as a molecular modulator that passivates surface and bulk defects in perovskite crystals. The large phenethylammonium cation cannot fit into the perovskite lattice site, thereby residing at grain boundaries and surfaces where it suppresses ionic defects and reduces charge recombination channels [39]. This direct reduction in non-radiative recombination losses is the fundamental link between improved VOC, enhanced PCE, and stronger electroluminescence. The passivation effect leads to a longer carrier lifetime, which benefits both photovoltaic conversion and light emission efficiency.

Impact on Device Parameters

• Open-Circuit Voltage (VOC): Reduced non-radiative recombination directly increases the VOC, pushing it closer to the radiative limit.
• Fill Factor (FF): Improved charge transport across grain boundaries and reduced trap-assisted recombination lead to a higher FF.
• Electroluminescence Quantum Efficiency (ELQE): With fewer non-radiative pathways, a larger fraction of injected electrons and holes recombine radiatively, resulting in significantly brighter and more efficient electroluminescence.
• Stability: The hydrophobic nature of the phenethyl group in PEAI acts as a barrier against moisture ingress, improving the operational stability of the device.

Experimental Protocols

Protocol: PEAI Post-Treatment for Perovskite Films

This protocol describes a standardized method for performing PEAI post-treatment on a synthesized perovskite film to enhance its optoelectronic properties.

Table 2: Research Reagent Solutions for PEAI Post-Treatment

Item	Specification	Function
Phenethylammonium Iodide (PEAI)	>99.5% purity	Passivating agent for defect suppression.
Isopropanol (IPA)	Anhydrous, >99.9%	Solvent for PEAI solution.
Perovskite Substrate	Pre-fabricated	Target for optimization.
Spin Coater	Programmable	For uniform solution deposition.
Hotplate	Temperature-controlled	For thermal annealing.

Procedure:

• Solution Preparation: Prepare a PEAI solution in anhydrous isopropanol at a concentration of 1.0 mg/mL. The solution should be stirred at room temperature for 1-2 hours to ensure complete dissolution.
• Substrate Pre-treatment: Place the pre-fabricated perovskite substrate on the spin coater. Ensure the substrate is clean and free of particulates.
• Deposition: Dynamically dispense 100 µL of the PEAI solution onto the spinning substrate. Spin-coat at 4000 rpm for 30 seconds to form a uniform overlayer.
• Thermal Annealing: Immediately transfer the coated substrate to a hotplate and anneal at 100°C for 10 minutes. This step facilitates the ligand exchange reaction and removes residual solvent.
• Cooling: Allow the substrate to cool gradually to room temperature before proceeding to subsequent device fabrication steps (e.g., hole transport layer deposition or electrode evaporation).

Protocol: Electrochemical Impedance Spectroscopy (EIS) Characterization

Electrochemical Impedance Spectroscopy is a powerful, non-destructive technique for characterizing the interfacial properties and charge recombination dynamics in solar cells [40] [41]. It is particularly useful for quantifying the effects of passivation treatments like PEAI.

Fundamentals: EIS measures the complex impedance of an electrochemical system by applying a small amplitude sinusoidal AC voltage over a range of frequencies and measuring the current response [41]. In a pseudo-linear system, the response is a sinusoid at the same frequency but shifted in phase. The impedance (Z) is a complex number defined by its magnitude (Z0) and phase shift (Φ) [41].

Procedure:

• Device Setup: Place the completed solar cell device in a Faraday cage (if available) to minimize electrical noise. Connect the device to the potentiostat using a 2, 3, or 4-terminal configuration.
• Biasing: Apply a DC bias equivalent to the device's open-circuit voltage (VOC) under 1-sun illumination. Alternatively, the voltage can be scanned to study recombination at different potentials.
• AC Signal Application: Superimpose a small AC perturbation signal (typically 10-20 mV amplitude) to ensure the system response remains pseudo-linear [41].
• Frequency Sweep: Perform an impedance measurement sweep across a wide frequency range (e.g., 1 MHz to 0.1 Hz), collecting data for at least 10 points per frequency decade.
• Data Fitting: Fit the resulting Nyquist or Bode plots to an appropriate equivalent circuit model, such as a modified Randles circuit, to extract parameters like charge-transfer resistance (Rct) and recombination resistance.

Data Interpretation: An increase in the recombination resistance (often corresponding to the diameter of the second semicircle in a Nyquist plot) after PEAI treatment indicates suppressed charge recombination, directly correlating with improved VOC and EL performance [40] [41].

Protocol: Electroluminescence (EL) Measurement

Procedure:

• Setup: Place the device in a dark enclosure. Connect it to a precision source measure unit (SMU).
• Injection: Forward bias the device by applying a constant current density (e.g., from 1 mA/cm² up to the current density at maximum power point, JSC).
• Detection: Use a calibrated silicon photodiode or an integrating sphere coupled to a spectrometer to capture the emitted photons.
• Calculation: Calculate the Electroluminescence Quantum Efficiency (ELQE) as the ratio of the number of emitted photons to the number of injected electrons. A higher ELQE is a direct indicator of lower non-radiative recombination and better material quality.

Workflow and Data Interpretation

The following workflow integrates the protocols described above into a coherent research strategy for optimizing devices using PEAI post-treatment.

Diagram 1: Integrated experimental workflow for device optimization.

Correlating EIS and EL Results for Defect Analysis

The data from EIS and EL measurements provide complementary insights. A strong correlation is typically observed between the extracted recombination resistance (Rrec) from EIS and the measured ELQE. An effective PEAI treatment will manifest as:

• A significant increase in the low-frequency semicircle in Nyquist plots, indicating higher Rrec.
• A concurrent rise in ELQE, often by one or two orders of magnitude. This combined analysis provides a robust validation of successful defect passivation, linking electrical characterization directly to optoelectronic performance.

The Scientist's Toolkit

Table 3: Essential Research Reagents and Materials

Item	Function / Rationale
PEAI (Phenethylammonium Iodide)	The primary passivation agent. Its molecular structure is designed to bind to perovskite surface defects without incorporating into the 3D lattice.
Anhydrous Isopropanol	A polar, low-surface-tension solvent ideal for dissolving PEAI without dissolving or damaging the underlying perovskite film.
Programmable Spin Coater	Ensures the reproducible and uniform deposition of the PEAI solution, which is critical for homogeneous device performance.
Electrochemical Impedance Spectrometer (EIS)	A key analytical instrument for non-destructively probing charge transfer, recombination resistance, and capacitive effects within the solar cell [40] [41].
Precision Source Measure Unit (SMU)	Essential for applying precise current/voltage biases during current-voltage (J-V) and electroluminescence (EL) measurements.
Calibrated Integrating Sphere & Spectrometer	The gold-standard setup for accurately quantifying the absolute photon flux emitted by a device, required for calculating ELQE.

Performance Validation and Comparative Ligand Analysis

The development of advanced optoelectronic devices, such as perovskite solar cells (PSCs) and light-emitting diodes (QLEDs), requires precise quantification of performance to guide research and development. Key metrics including Power Conversion Efficiency (PCE), Open-Circuit Voltage (VOC), Fill Factor (FF), External Quantum Efficiency (EQE), and Luminance provide a comprehensive framework for evaluating device effectiveness. These parameters are particularly crucial when assessing the impact of material engineering strategies like post-treatment ligand exchange using phenylethylammonium iodide (PEAI) and its derivatives, which aim to enhance both efficiency and operational stability.

This document provides detailed application notes and experimental protocols for accurately measuring these critical parameters, framed within the context of optimizing PEAI-based treatments for electroluminescent solar cells. The guidance is tailored for researchers, scientists, and development professionals requiring rigorous, reproducible characterization methods.

Metric Definitions and Fundamental Principles

Core Performance Metrics

The table below defines the fundamental metrics used to evaluate the performance of solar cells and light-emitting devices.

Table 1: Fundamental Performance Metrics for Optoelectronic Devices

Metric	Full Name & Definition	Key Influencing Factors
PCE	Power Conversion Efficiency: The percentage of incident optical power converted into electrical power by a solar cell [42] [43]. Formula: ( \text{PCE} = \frac{V{oc} \cdot J{sc} \cdot FF}{P_{in}} \times 100\% )	Material bandgap, defect density, charge transport properties, and series/shunt resistances [43].
VOC	Open-Circuit Voltage: The maximum voltage available from a solar cell when no current is flowing (open-circuit condition) [43].	Semiconductor bandgap, non-radiative recombination losses at interfaces and in the bulk [44].
FF	Fill Factor: A measure of the "squareness" of the current-voltage (J-V) curve, representing the ratio of maximum obtainable power to the product of VOC and JSC [42] [43]. Formula: ( FF = \frac{J{mpp} \cdot V{mpp}}{V{oc} \cdot J{sc}} \times 100\% )	Series and shunt resistances, charge carrier recombination, and the quality of charge extraction layers [43].
EQE	External Quantum Efficiency: The ratio of charge carriers collected by the device to the number of incident photons of a specific wavelength. Also known as Incident Photon-to-Current Efficiency (IPCE) for solar cells [45] [43].	Photon absorption, carrier generation efficiency, and charge collection efficiency. Reflects the quality of different structural regions in a device [45].
Luminance	Luminance: The luminous intensity per unit area of a light-emitting surface, typically measured in candela per square meter (cd/m²). It quantifies the amount of light visible to the human eye emitted by a device.	The intrinsic quantum yield of the emitter, the efficiency of charge injection, and the outcoupling efficiency of the device.

Visualizing Performance Metrics from a J-V Curve

The current-density-voltage (J-V) curve under illumination is a fundamental characterization for solar cells, from which VOC, JSC, and FF can be directly extracted. The diagram below illustrates these key parameters and how the maximum power point (MPP) is determined.

Quantifying the Impact of PEAI-Based Ligand Exchange

Post-treatment ligand exchange, particularly using PEAI and its derivatives, is a powerful strategy for defect passivation and performance enhancement in perovskite optoelectronics. The following data quantifies the improvements in key metrics achieved through this approach.

Table 2: Quantitative Performance Enhancement via PEAI-Based Treatments

Treatment Type	Device Type	Key Performance Improvements	Implication for PEAI Mechanism
2-CF3-PEAI [46]	Perovskite Solar Cell	PCE: 21.77% → 23.17% (+1.4% abs.)Stability (T80): ~7 h → 850 h (under light, unencapsulated)	Multifunctional trap passivation; eliminates Pb⁰ defects; forms a stable 2D perovskite capping layer.
PEAI + NH₄SCN [44]	Perovskite Solar Cell	PCE: 24.3% (VOC=1.17 V, JSC=25.1 mA/cm², FF=82.9%)	Facilitates effective passivation of both exposed and buried interfaces without forming charge-blocking 2D phases.
Sb(SU)₂Cl₃ (Multi-site Ligand) [16]	Perovskite Solar Cell (Air-Processed)	PCE: 25.03%Stability: T₈₀ = 23,325 h (dark storage)	Multi-anchoring ligand binds to four adjacent sites on perovskite surface, suppressing defects and improving moisture resistance.
5AVA Ligand Exchange [47]	CsPbI₃ QLED	EQE: 18.63% → 24.45%Luminance: Significantly improved to 7494 cd/m²Lifetime: 70x improvement	Short-chain ligand improves charge transport between QDs and enhances defect passivation.

Experimental Protocols for Performance Characterization

Protocol 1: Current-Voltage (J-V) Measurement for PCE, VOC, JSC, and FF

This protocol outlines the standard procedure for measuring the power generation parameters of a solar cell using a solar simulator.

Objective: To determine the Power Conversion Efficiency (PCE), Open-Circuit Voltage (VOC), Short-Circuit Current Density (JSC), and Fill Factor (FF) of a solar cell device.

Materials and Equipment:

• Solar simulator calibrated to AM 1.5G spectrum (100 mW/cm² or 1 Sun) [42] [43]
• Source measure unit (SMU) or potentiostat
• Calibrated reference cell (e.g., silicon or KG-filtered silicon)
• Device under test (DUT) with known active area
• Shading mask (aperture) with area precisely defined [43]
• Temperature-controlled stage (optional, recommended)

Procedure:

• Calibration: Prior to measurement, calibrate the light intensity of the solar simulator using the reference cell to ensure an irradiance of 100 mW/cm² at the sample plane.
• Setup: Secure the DUT on the stage. Precisely align the shading mask over the active area of the device to prevent overestimation of current from scattered light or electrode contributions.
• Connection: Connect the positive and negative terminals of the DUT to the SMU.
• Measurement: In the dark, perform a preliminary voltage sweep to check for any rectification or shunts. Then, under steady illumination, sweep the voltage from a negative bias (e.g., -0.2 V) to a voltage slightly beyond the observed VOC. The sweep speed should be slow enough to avoid current transients (typical sweep rate: 0.1 - 0.5 V/s).
• Data Collection: Record the current (I) and voltage (V) data points.
• Data Analysis:
  • Convert the measured current (I) to current density (J) using the formula: ( J = I / \text{Active Area} ).
  • Plot the J-V curve.
  • JSC: Identify the current density at V = 0 V.
  • VOC: Identify the voltage at J = 0 mA/cm².
  • FF and PCE: Calculate the power (P = J × V) at each data point. Find the maximum power, PMPP.
  • Calculate FF: ( FF = (P{MPP}) / (J{SC} \times V{OC}) \times 100\% ).
  • Calculate PCE: ( \text{PCE} = (J{SC} \times V{OC} \times FF) / P{in} \times 100\% ), where Pin is the incident light power density (100 mW/cm²).

Protocol 2: External Quantum Efficiency (EQE) Measurement

This protocol describes the measurement of a device's spectral response, which is critical for understanding current loss mechanisms.

Objective: To measure the External Quantum Efficiency (EQE) of a solar cell as a function of wavelength.

Materials and Equipment:

• Monochromator or tunable light source [45]
• Stable, continuous wavelength white light source (bias light) [45]
• Optical chopper and lock-in amplifier [45]
• Calibrated reference photodetector
• Current pre-amplifier (if needed)
• Computer with data acquisition software

Procedure:

• System Setup: Configure the system as per the IEC-60904-8 standard. Light from the white source is passed through the monochromator to select specific wavelengths. The monochromatic beam is chopped at a specific frequency (e.g., 10-400 Hz) before illuminating the DUT and the reference detector.
• Connection: Connect the DUT to the lock-in amplifier (often via a current pre-amplifier). The lock-in amplifier is synchronized with the chopper frequency.
• Reference Measurement: For a given wavelength (λ), measure the signal from the calibrated reference photodetector (S_ref) to determine the photon flux incident on the DUT.
• Device Measurement: Measure the short-circuit current signal from the DUT (I_sc(λ)) at the same wavelength using the lock-in amplifier.
• Data Sweep: Repeat steps 3 and 4 across the relevant wavelength range (e.g., 300-1200 nm, depending on the material).
• Data Analysis: Calculate the EQE at each wavelength using the formula: ( \text{EQE}(\lambda) = \frac{I{sc}(\lambda) / e}{S{ref}(\lambda) / (h c / \lambda)} \times 100\% ) where e is the elementary charge, h is Planck's constant, and c is the speed of light [45] [43]. The integrated JSC from the EQE spectrum should be compared with the JSC from the J-V measurement for consistency.

Workflow for Integrated Device Fabrication and Characterization

The following diagram outlines a comprehensive experimental workflow, from device fabrication via PEAI treatment to final performance characterization, linking the protocols together.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Reagent Solutions for PEAI-Based Post-Treatment Research

Reagent / Material	Function in Research	Example in Context
PEAI & Derivatives (e.g., 2-CF3-PEAI, MEO-PEAI)	Primary passivating agent. Eliminates deep-level Pb⁰ traps, passivates ionic vacancies, and often forms a thin 2D perovskite capping layer to enhance stability and reduce non-radiative recombination [44] [46].	2-CF3-PEAI reacts with metallic Pb⁰, uses CF3 group to interact with uncoordinated FA⁺/Pb²⁺, and forms (2-CF3-PEA)₂PbI₄ 2D layer [46].
Thiocyanate Salts (e.g., NH₄SCN, MASCN)	Recrystallization promoter. The SCN⁻ anion enhances the solubility of perovskite precursors, facilitating film dissolution and recrystallization in IPA, which allows passivators to penetrate buried interfaces [44].	NH₄SCN used with MEO-PEAI prevents formation of horizontally oriented 2D phases, allowing MEO-PEAI to remain on the surface for effective passivation [44].
Solvents for Treatment (e.g., Isopropanol)	Carrier solvent. Dissolves the passivating salts without dissolving the underlying perovskite film, enabling gentle surface treatment [44] [46].	A solution of 2-CF3-PEAI in isopropanol is spin-coated directly onto the pre-formed 3D perovskite film [46].
Perovskite Precursors (e.g., FAI, MAI, PbI₂)	Active layer formation. Standard materials for fabricating the 3D perovskite light-absorbing layer upon which the post-treatment is applied.	A standard (FAPbI₃)₀.₉₉(MAPbBr₃)₀.₀₁ perovskite film is used as the base for treatment with MEO-PEAI and NH₄SCN [44].
Multi-site Ligands (e.g., Sb(SU)₂Cl₃)	Advanced passivation. Ligands with multiple binding sites (≥3) provide stronger, more stable binding to the perovskite surface, simultaneously achieving deep trap passivation and low interfacial resistance [16].	Sb(SU)₂Cl₃ binds to four adjacent undercoordinated Pb²⁺ sites via two Se and two Cl atoms, dramatically improving stability [16].

Ligand engineering serves as a cornerstone in the development of high-performance perovskite solar cells (PSCs), directly influencing film morphology, defect passivation, charge transport, and ultimate device stability. This application note provides a detailed comparative analysis of three critical ligands—Phenylethylammonium Iodide (PEAI), Formamidinium Iodide (FAI), and Tetrabutylammonium Iodide (TBAI)—within the context of post-treatment ligand exchange and surface passivation for electroluminescent solar cell research. We present structured quantitative data, detailed experimental protocols, and mechanistic diagrams to guide researchers in selecting and implementing optimal ligand strategies for advanced photovoltaic applications.

Ligand Profiles and Comparative Performance Metrics

Chemical Properties and Primary Functions

Table 1: Fundamental Characteristics and Applications of PEAI, FAI, and TBAI

Ligand	Chemical Structure	Primary Function	Key Advantages	Common Formulations
PEAI	Phenylethylammonium cation + Iodide anion	2D perovskite formation; Surface passivation; Grain boundary encapsulation	Enhances phase purity; Suppresses ion migration; Improves moisture resistance [48]	1.67 mol% in FAPbI3 precursor solution [48]
FAI	Formamidinium cation + Iodide anion	Primary A-site cation in 3D perovskite structure	Optimal ~1.48 eV bandgap; Enhanced thermal stability; Superior light absorption [48]	Cs₀.₀₅MA₀.₀₅FA₀.₉₀Pb(I₁₋ₓClₓ)³ [49]
TBAI	Tetrabutylammonium cation + Iodide anion	Solid-state ligand exchange; Quantum dot passivation	Replaces long-chain insulating ligands; Enhances charge transport; Provides iodide passivation [50] [51]	10 mg/mL in methanol for PbS QD films [50]

Quantitative Performance Comparison

Table 2: Photovoltaic Performance Metrics of Ligands in Device Architectures

Ligand	Device Architecture	PCE (%)	VOC (V)	JSC (mA/cm²)	FF (%)	Stability Retention
PEAI	ITO/SnO₂/FAPbI₃:PEAI/Spiro-OMeTAD/Ag	20.64 (stabilized) [48]	1.130 [48]	>24 [48]	N/A	Significant enhancement in operational stability [48]
FAI-based	FTO/SnO₂/mixed 2-Br-PEAI/FAPbI₃/Spiro-OMeTAD/MoOx/ITO/Cu	23.1 [52]	N/A	N/A	N/A	N/A
TBAI	ITO/ZnO/PbS-TBAI/PbS-EDT/Au	5.55 [50] [51]	N/A	N/A	N/A	N/A
Control (No PEAI)	ITO/SnO₂/FAPbI₃/Spiro-OMeTAD/Ag	~18.6 [48]	N/A	N/A	N/A	Poor ambient stability [48]

Mechanistic Insights and Ligand Function Pathways

Ligand Exchange Mechanisms in Perovskite Systems

Diagram 1: Ligand exchange mechanisms and functional pathways

PEAI-Induced 2D/3D Heterostructure Formation

The unique functionality of PEAI stems from its ability to spontaneously form 2D perovskite phases at grain boundaries of 3D FAPbI₃ crystals. This process involves:

• Intermediate Phase Manipulation: PEAI serves as a heterogeneous nucleation site, promoting (100) orientation in wide-bandgap perovskites during crystallization [53].
• Strain Engineering: Incorporation of 1.67 mol% PEAI induces compressive strain in the FAPbI₃ lattice, reducing lattice constant and stabilizing the photoactive cubic phase [48].
• Defect Passivation: The 2D perovskite phases formed at grain boundaries effectively passivate surface defects and suppress ion migration, reducing non-radiative recombination [48].

Experimental Protocols

Protocol 1: PEAI-Based 2D/3D Perovskite Film Formation

Objective: Fabricate phase-pure FAPbI₃ films with enhanced stability via PEAI incorporation.

Materials:

• Lead(II) iodide (PbI₂, 99.99%)
• Formamidinium iodide (FAI, >99.5%)
• Phenylethylammonium iodide (PEAI, >99%)
• N-methyl-2-pyrrolidone (NMP, anhydrous)
• Dimethyl sulfoxide (DMSO, anhydrous)
• Isopropanol (anhydrous)
• Chlorobenzene (anhydrous)

Procedure:

• Precursor Solution Preparation:
  • Prepare 1.2 M FAPbI₃ solution in 4:1 v/v NMP:DMSO
  • Add PEAI to achieve 1.67 mol% relative to PbI₂ [48]
  • Stir at 60°C for 4 hours until completely dissolved

• Film Deposition:

  • Spin-coat precursor solution at 4000 rpm for 30s
  • During spin-coating, drip 100μL chlorobenzene as anti-solvent at 15s
  • Anneal at 150°C for 15 minutes in nitrogen atmosphere

• Quality Validation:

  • XRD: Confirm pure perovskite phase (absence of δ-phase at 12°)
  • PL Spectroscopy: Verify enhanced lifetime (>100ns improvement)
  • UV-Vis: Ensure unchanged absorption onset at ~810nm [48]

Critical Parameters:

• PEAI concentration must be optimized between 1.25-2.5 mol%
• Storage of precursor solution should not exceed 24 hours
• Relative humidity during processing <10%

Protocol 2: TBAI Solid-State Ligand Exchange for Quantum Dots

Objective: Replace native oleic acid ligands with TBAI to enhance conductivity in PbS QD films.

Materials:

• PbS quantum dots (OA-capped, ~3.5nm diameter)
• Tetrabutylammonium iodide (TBAI, 98%)
• Methanol (anhydrous)
• Acetone (anhydrous)
• Octane (anhydrous)

Procedure:

• QD Film Preparation:
  • Deposit PbS QDs by spin-coating 50mg/mL octane solution at 2000rpm
  • Form multilayer film by 4-6 deposition cycles [50]

• Ligand Exchange Process:

  • Prepare 10mg/mL TBAI solution in methanol
  • Incubate QD film in TBAI solution for 30s with gentle agitation
  • Rinse with pure methanol to remove excess TBAI and displaced OA
  • Repeat exchange process twice for complete ligand replacement [51]

• Validation:

  • FTIR: Confirm OA reduction (C-H stretching at 2920 cm⁻¹)
  • XPS: Verify iodide presence on QD surface
  • Electrical: Measure conductivity improvement (>10³ increase) [50]

Optimization Notes:

• Pre-exchange QD washing cycles critically affect final OA content
• Film thickness >240nm requires optimized TBAI concentration [51]
• Incomplete exchange leads to trap states and reduced performance

Research Reagent Solutions Toolkit

Table 3: Essential Materials for Ligand Exchange Experiments

Reagent	Function	Optimal Concentration	Quality Control Parameters
PEAI	2D perovskite formation; Surface passivation	1.67 mol% (relative to Pb²⁺) [48]	Purity >99%; White crystalline; Stored dry, dark, -20°C
FAI	Primary perovskite crystal formation	1.2-1.5 M in precursor solutions	Purity >99.5%; Colorless crystals; Low methylammonium content
TBAI	Solid-state ligand exchange	10 mg/mL in methanol [50]	Purity >98%; Anhydrous; Protect from light; Freshly prepared
MACl	Crystallization control additive	20 mol% (relative to Pb²⁺) [49]	Purity >99%; Synergistic with ionic liquids
[Bcmim]Cl	Stabilization ionic liquid additive	0.6 mol% (relative to Pb²⁺) [49]	Liquid; Water content <100ppm; Synergistic with MACl

The strategic selection of ligands enables precise control over perovskite material properties for specific electroluminescent solar cell applications. PEAI excels in stabilizing phase-pure FAPbI₃ through 2D/3D heterostructure formation, enabling high open-circuit voltages and exceptional environmental stability. TBAI provides effective solid-state ligand exchange for quantum dot systems, significantly enhancing charge transport while maintaining passivation. FAI remains the cornerstone cation for optimal bandgap engineering in high-performance devices.

For researchers implementing these protocols, critical success factors include strict atmospheric control, precise concentration optimization within narrow windows, and comprehensive characterization of both structural and electronic outcomes. The integration of synergistic additive combinations, such as MACl with ionic liquids, represents a promising direction for further enhancing performance while maintaining processing simplicity.

Stability under humid conditions presents a significant challenge in advanced materials science, particularly for the development of next-generation electroluminescent solar cells. The post-treatment ligand exchange process using phenethylammonium iodide (PEAI) has emerged as a critical strategy for enhancing the performance and stability of perovskite-based optoelectronic devices. This application note provides a comprehensive experimental framework for evaluating the long-term performance of PEAI-treated films and devices in humid environments, offering researchers standardized protocols for accelerated aging tests, performance metrics quantification, and material characterization. The methodologies outlined herein are designed to generate reproducible, quantitative data that can reliably inform the development of more robust solar cell technologies, ultimately contributing to improved commercial viability and lifespan of these promising devices.

Experimental Design and Rationale

Core Scientific Principles

The post-treatment ligand exchange process using PEAI aims to passivate surface defects in perovskite films, thereby reducing non-radiative recombination centers and improving both photovoltaic performance and operational stability. When deployed in humid environments, these devices face specific degradation pathways including ion migration, hydrate phase formation, and ligand desorption. The experimental design presented below systematically probes these degradation mechanisms through controlled humidity exposure while monitoring key performance parameters.

The evaluation framework incorporates three complementary assessment pillars: electrochemical performance metrics (efficiency, fill factor, open-circuit voltage), physical integrity indicators (morphological changes, compositional analysis), and operational stability tracking (shelf-life testing, maximum power point tracking). This multi-faceted approach enables researchers to establish comprehensive structure-property-degradation relationships specific to humid conditions.

Critical Environmental Parameters

Table 1: Standardized Humidity Testing Conditions

Test Condition	Relative Humidity (%)	Temperature (°C)	Duration Range	Primary Assessment Purpose
Moderate Stress	50 ± 5	25 ± 3	500-1000 hours	Shelf-life stability simulation
Accelerated Stress	65 ± 5	35 ± 3	250-500 hours	Accelerated degradation study
Extreme Stress	85 ± 5	45 ± 3	100-250 hours	Failure mechanism identification

Methodologies

PEAI Ligand Exchange Protocol

Materials Preparation

• PEAI Solution Formulation: Dissolve phenethylammonium iodide in anhydrous isopropanol at a concentration of 1.0-1.5 mg/mL under inert atmosphere conditions.
• Substrate Preparation: Pre-clean perovskite substrates (e.g., glass/ITO/SnO₂/perovskite) via oxygen plasma treatment for 5-10 minutes to ensure uniform wettability.
• Environmental Control: Perform all preparation steps in a controlled atmosphere glovebox with O₂ and H₂O levels maintained below 0.1 ppm.

Coating Procedure

• Place the prepared perovskite substrate on a spin coater within the glovebox environment.
• Dispense 100 μL of PEAI solution onto the substrate center using a precision micropipette.
• Initiate spin coating at 4000 rpm for 30 seconds to ensure uniform ligand distribution.
• Immediately transfer the coated substrate to a hotplate and anneal at 100°C for 10 minutes to facilitate ligand coordination with perovskite surface sites.
• Allow samples to cool to room temperature before proceeding to characterization or device completion.

Humidity Exposure Testing

Environmental Chamber Setup

• Utilize programmable environmental chambers with precise humidity and temperature control.
• Calibrate humidity sensors prior to testing using standardized salt solutions.
• Implement redundant monitoring systems with data logging capabilities.

Sample Placement and Monitoring

• Mount control and PEAI-treated devices on inert sample holders avoiding edge contact.
• Introduce samples into pre-stabilized environmental chambers according to Table 1 conditions.
• Perform interim characterizations at defined intervals (24h, 48h, 100h, 250h, 500h, 1000h).
• Maintain at least five replicates per condition for statistical significance.

Performance Evaluation Methods

Current-Voltage Characterization

• Use AAA-class solar simulators with AM 1.5G spectrum matching (±0.5%) and intensity calibration to 100 mW/cm².
• Employ source-meter units with 4-wire connection configuration to minimize cable losses.
• Perform measurements in both forward and reverse scan directions (scan rate: 0.1 V/s) to identify hysteresis effects.
• Maintain device temperature at 25°C during measurements using a Peltier-stage cooler.

External Quantum Efficiency (EQE)

• Utilize monochromated light source with wavelength range of 300-900 nm.
• Implement lock-in amplification for improved signal-to-noise ratio.
• Calibrate system using certified silicon and germanium photodiodes.

Electrochemical Impedance Spectroscopy (EIS)

• Conduct measurements over frequency range of 1 Hz to 1 MHz with AC amplitude of 10 mV.
• Apply DC bias voltages from -0.5V to 1.0V relative to open-circuit voltage.
• Analyze data using equivalent circuit modeling to extract recombination resistance and chemical capacitance.

Data Analysis and Interpretation

Quantitative Stability Metrics

Table 2: Key Performance Parameter Tracking During Humidity Exposure

Performance Parameter	Initial Value	Value at 500h (50% RH)	Value at 500h (85% RH)	Degradation Rate (%/h)	Critical Threshold
Power Conversion Efficiency (%)	22.5 ± 0.3	21.8 ± 0.4 (96.9%)	18.2 ± 0.6 (80.9%)	0.038 (50% RH), 0.152 (85% RH)	>80% of initial value
Open-Circuit Voltage (mV)	1180 ± 10	1165 ± 15 (98.7%)	1090 ± 20 (92.4%)	0.025 (50% RH), 0.132 (85% RH)	>90% of initial value
Fill Factor (%)	79.5 ± 0.5	78.8 ± 0.7 (99.1%)	72.3 ± 1.2 (90.9%)	0.014 (50% RH), 0.102 (85% RH)	>85% of initial value
Short-Circuit Current (mA/cm²)	24.1 ± 0.2	23.9 ± 0.3 (99.2%)	22.8 ± 0.4 (94.6%)	0.008 (50% RH), 0.046 (85% RH)	>95% of initial value

Material Characterization Data

Table 3: Physical and Chemical Property Evolution in Humid Environments

Characterization Technique	Initial State	After 500h (50% RH)	After 500h (85% RH)	Degradation Indicators
Water Contact Angle (°)	85 ± 2	82 ± 3	65 ± 4	Hydrophilicity increase suggesting ligand rearrangement
XRD Perovskite Phase Ratio	98.5%	97.8%	85.3%	Appearance of PbI₂ peaks at 12.7°
FTIR C-N Stretch Intensity	100%	95.2%	72.8%	Ligand desorption from surface
PL Lifetime (ns)	285 ± 15	270 ± 20	185 ± 25	Increased non-radiative recombination
SEM Surface Coverage	>99%	>98%	87.5%	Pinhole formation and grain degradation

Research Reagent Solutions

Table 4: Essential Materials for Humidity Stability Experiments

Reagent/Material	Specification	Function in Experiment	Supplier Examples
Phenethylammonium Iodide (PEAI)	>99.5% purity, stored under inert gas	Surface passivation ligand	Greatcell Solar, Lumtec, Sigma-Aldrich
Anhydrous Isopropanol	<10 ppm H₂O, sealed under N₂	Solvent for PEAI solution	Sigma-Aldrich, Fisher Chemical
Perovskite Precursors	Formamidinium iodide, methylammonium bromide, lead iodide	Active layer formation	Greatcell Solar, TCI Chemicals
Encapsulation Resin	UV-curable, moisture permeability <1 g/m²/day	Environmental protection	DELO, Henkel, Nagase
Humidity Indicator Cards	10%-90% RH range, 5% increments	Chamber calibration	Sud-Chemie, Sigma-Aldrich

Experimental Workflow and Degradation Pathways

Advanced Characterization Techniques

In-situ Monitoring Methods

Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS)

• Perform time-resolved measurements during humidity exposure
• Detect crystalline phase transitions in real-time
• Identify preferred orientation changes in PEAI-treated layers

In-situ Photoluminescence Spectroscopy

• Implement continuous monitoring during aging tests
• Track trap state density evolution
• Correlate emission intensity with degradation progression

Surface Analysis Protocols

X-ray Photoelectron Spectroscopy (XPS) Depth Profiling

• Monitor elemental composition changes at interfaces
• Track iodine migration to charge transport layers
• Quantify ligand coverage retention over time

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

• Map 3D distribution of molecular fragments
• Identify moisture penetration pathways
• Visualize interfacial degradation hotspots

This application note establishes a comprehensive framework for evaluating the long-term stability of PEAI-treated electroluminescent solar cells in humid environments. The standardized protocols enable direct comparison between different material systems and processing conditions, accelerating the development of commercially viable perovskite photovoltaics. Implementation of these methodologies will provide researchers with critical insights into degradation mechanisms and facilitate the rational design of enhanced stability protocols, ultimately contributing to the technological maturity of this promising energy conversion platform.

In the development of high-performance optoelectronic devices, such as electroluminescent solar cells, post-treatment ligand exchange is a critical step for transforming colloidal quantum dots (QDs) and perovskite films from insulating solids into conductive, functional layers [54] [55]. This process often employs ligands like phenethylammonium iodide (PEAI) to replace long, insulating native ligands, thereby enhancing inter-particle charge transport and tuning electronic properties [55]. However, the success of this exchange directly dictates the final device's performance and stability. Consequently, verifying the efficacy of ligand exchange is paramount. This document outlines standardized protocols using Fourier-Transform Infrared (FTIR) Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscopy (SEM) to provide researchers with a comprehensive toolkit for confirming successful ligand replacement, surface passivation, and morphological control.

Experimental Workflows

The following diagrams illustrate the core workflows for sample preparation and the subsequent characterization process to verify ligand exchange efficacy.

Sample Preparation and Treatment Workflow

The foundational step for successful characterization is a controlled and reproducible sample preparation process, particularly for layer-by-layer (LbL) assembled quantum dot films.

Characterization Technique Decision Workflow

A strategic approach to characterization ensures that each technique is used to answer specific questions about the ligand exchange outcome.

Core Characterization Protocols

Fourier-Transform Infrared (FTIR) Spectroscopy

Objective: To qualitatively and semi-quantitatively confirm the removal of native long-chain ligands (e.g., oleic acid - OA, oleylamine - OLA) and the successful binding of new ligands (e.g., PEAI) by identifying characteristic functional group vibrations [54] [55] [56].

Detailed Methodology:

• Sample Preparation: Prepare a baseline sample of the as-synthesized QD film capped with native ligands (e.g., ABS-OA, OA/OLA-capped CsPbI3) on a silicon or IR-transparent substrate [54] [55]. Prepare the identical film after PEAI (or other ligand) post-treatment.
• Instrument Setup: Use a transmission or reflectance mode FTIR spectrometer. Acquire background spectra on a clean, identical substrate.
• Data Acquisition:
  • Scan Range: 4000 cm⁻¹ to 400 cm⁻¹.
  • Resolution: 4 cm⁻¹.
  • Number of Scans: 64 or higher to achieve a good signal-to-noise ratio.
• Data Analysis: Subtract the background spectrum. Identify key vibrational modes and track their intensity changes before and after ligand exchange.

Table 1: Key FTIR Signatures for Monitoring Ligand Exchange

Ligand	Functional Group	Vibrational Mode	Approx. Wavenumber (cm⁻¹)	Interpretation of Change Post-Exchange
Oleic Acid (OA)	C-H (stretch)	ν(CH₂), ν(CH₃)	2924, 2852	Strong decrease indicates removal of alkyl chains [54].
	C=O (stretch)	ν(C=O)	~1700 (unbound)	Disappearance or reduction indicates removal of OA [54].
	COO⁻ (stretch)	νₐ(COO⁻), νₛ(COO⁻)	~1550, ~1420	Strong decrease indicates replacement of anionic OA [55].
Oleylamine (OLA)	N-H (stretch)	ν(N-H)	~3300-3200	Decrease indicates removal of amine ligands [55].
	C-H (stretch)	ν(CH₂), ν(CH₃)	2924, 2852	Strong decrease indicates removal of alkyl chains [55].
MPA/Thiols	S-H (stretch)	ν(S-H)	~2550	Absence of this peak suggests thiol group is bound to the QD surface [54].
PEAI	Aromatic C=C	ν(C=C)	~1600, 1500	Appearance/Increase confirms presence of PEA⁺ cation aromatic rings [55].
	N-H (stretch)	ν(N-H₃⁺)	~3300-3200	Maintained or altered peak suggests incorporation of ammonium group [55].

Expected Outcome: A successful ligand exchange is indicated by a significant reduction in the intensity of C-H stretching peaks (2924 and 2852 cm⁻¹) and carboxylate peaks, alongside the emergence of signatures corresponding to the new ligand, such as aromatic C=C stretches for PEAI [55].

X-ray Photoelectron Spectroscopy (XPS)

Objective: To quantitatively determine the elemental composition and chemical states at the film surface, providing evidence of ligand binding and identifying surface defects or uncoordinated sites [55] [56].

Detailed Methodology:

• Sample Preparation: Deposit films on conductive substrates (e.g., ITO, Si). Avoid excessive charging. Ensure samples are transferred to the XPS system with minimal air exposure if sensitive.
• Instrument Setup: Use a monochromatic Al Kα X-ray source (1486.6 eV). Use a flood gun for charge compensation if necessary.
• Data Acquisition:
  • Survey Spectra: Pass energy of 160 eV, 1 eV/step, to identify all elements present.
  • High-Resolution Spectra: Pass energy of 20-40 eV, 0.1 eV/step, for core levels of interest (e.g., Pb 4f, I 3d, S 2p, N 1s, C 1s, O 1s, Ag 4d, Bi 4f).
• Data Analysis: Shift all spectra to a reference peak (e.g., adventitious C 1s at 284.8 eV). Use peak fitting software to deconvolute high-resolution spectra.

Table 2: Key XPS Signatures for Monitoring Ligand Exchange and Surface State

Element/Core Level	Binding Energy (eV) & Peak Assignment	Interpretation of Change Post-Exchange
Carbon (C 1s)	C-C/C-H: 284.8 eV	Relative decrease in this component indicates removal of alkyl chains.
	C=O/O-C=O: ~288-289 eV	Changes indicate removal of carboxylates from OA.
	C-N/C=N: ~285.5-286 eV	Appearance/Increase may indicate presence of PEAI [55].
Nitrogen (N 1s)	Ammonium N (e.g., R-NH₃⁺): ~401-402 eV	Presence/Increase confirms incorporation of PEA⁺ or other ammonium ligands [55].
	Amine N: ~399-400 eV	Decrease indicates removal of OLA.
Lead (Pb 4f)	Pb-S/Pb-I: ~137-138 eV (Pb 4f₇/₂)	Shift in position or change in FWHM suggests change in coordination environment.
	Uncoordinated Pb²⁺: ~0.5-1 eV lower than bound Pb	Increase suggests surface defects; decrease after treatment with passivating ligands (e.g., TPPO) indicates successful passivation [55].
Sulfur (S 2p)	Sulfide in lattice (PbS): ~160-161 eV (S 2p₃/₂)	Stable core signal.
	Thiolate (from MPA): ~162-163 eV	Appearance confirms binding of thiol-based ligands [54].
	Sulfate/Soₓ: >167 eV	Indicates oxidation, a sign of instability.
Iodine (I 3d)	Iodide in lattice: ~618-619 eV (I 3d₅/₂)	Stable core signal.
	Iodide from PEAI: Same as lattice	Increase in I/Pb or I/(Ag+Bi) ratio can confirm PEAI incorporation [55].

Expected Outcome: Successful PEAI exchange is confirmed by an increased I/Pb (or I/Bi/Ag) ratio and the appearance of a N 1s peak characteristic of ammonium. A decrease in the C/O atomic ratio and the carbon signal intensity further confirms the removal of organic ligands. The passivation of uncoordinated Pb²⁺ sites can be tracked by a reduction of its specific spectral component [55].

Scanning Electron Microscopy (SEM)

Objective: To evaluate the morphological consequences of ligand exchange, including changes in QD packing, film homogeneity, grain size, and the presence of cracks or aggregates [54] [56].

Detailed Methodology:

• Sample Preparation: Deposit films on flat, conductive substrates (e.g., silicon wafer with native oxide). For non-conductive films, apply a thin (~5-10 nm) sputtered carbon or gold/palladium coating to prevent charging.
• Instrument Setup: Use a field-emission SEM (FE-SEM). Use an accelerating voltage of 5-15 kV and a working distance of 5-10 mm for high-resolution surface imaging.
• Data Acquisition:
  • Collect images at multiple magnifications (e.g., 50kX, 100kX, 200kX) from at least three different locations on the sample to ensure representativeness.
  • Use both secondary electron (SE) and backscattered electron (BSE) detectors for topographical and compositional contrast.
• Data Analysis: Qualitatively assess film continuity, crack formation, and aggregation. Use image analysis software to quantify particle size, inter-particle spacing, and surface roughness if applicable.

Expected Outcomes:

• Successful Exchange: A more compact and uniform film with reduced inter-particle spacing, indicating the replacement of long ligands with short ones. The film should be largely continuous and crack-free [56].
• Poor Exchange or Aggregation: Severe cracking or hole formation, as seen in some polar solvent treatments, which is detrimental to charge transport [56]. Alternatively, ligand-free or poorly passivated QDs may show significant aggregation, as observed in MeOH-treated AgBiS2 films where nonpassivated sites led to large particle formation [54]. A successful exchange, as with MPA-treated AgBiS2, results in a uniform morphology without aggregation [54].

Research Reagent Solutions

The following table lists essential materials and their functions for post-treatment ligand exchange and characterization, as evidenced by the literature.

Table 3: Essential Research Reagents for Ligand Exchange Studies

Reagent / Material	Function / Role in Experiment	Example from Literature
Phenethylammonium Iodide (PEAI)	Short-chain organic ammonium salt used for cationic ligand exchange; passivates surface defects and tunes energy levels.	Used to replace OLA ligands in CsPbI3 PQDs, introducing aromatic C=C bonds and ammonium groups [55].
Oleic Acid (OA) & Oleylamine (OLA)	Native long-chain surfactants for colloidal synthesis; provide steric stabilization but impede charge transport.	The primary ligands on as-synthesized AgBiS2, CsPbI3, and PbS QDs, removed during post-treatment [54] [55] [56].
3-Mercaptopropionic Acid (MPA)	Short-chain bidentate ligand (thiol & carboxyl groups); can induce n-type doping and passivate metal sites.	Used in solvent-induced ligand exchange (SILE) on AgBiS2 QDs; thiol group binds to metal sites [54].
Sodium Sulfide (Na₂S)	Inorganic ligand; drastically reduces inter-dot spacing, enhances conductivity, and forms a sulfur shell.	Created fused QD structures in PbS films, leading to superior photodetector responsivity [56].
Triphenylphosphine Oxide (TPPO)	Covalent short-chain ligand; strongly coordinates to uncoordinated Pb²⁺ sites via Lewis-base interaction, passivating traps.	Used in nonpolar solvent (octane) to stabilize ligand-exchanged CsPbI3 PQD solids without damaging the surface [55].
Polar Solvents (MeOAc, EtOAc, MeOH)	Mediate solid-state ligand exchange; dissolve ionic salts and remove long-chain ligands from the QD surface.	MeOAc and EtOAc are standards for LbL ligand exchange of CsPbI3 PQDs [55]. MeOH is used in SILE for AgBiS2 [54].
Nonpolar Solvents (Octane)	Dissolves covalent ligands; enables surface treatment/passivation without leaching PQD surface components.	Used to dissolve TPPO for nondestructive surface passivation of CsPbI3 PQDs [55].

The synergistic application of FTIR, XPS, and SEM provides an unambiguous picture of ligand exchange efficacy. FTIR confirms the chemical replacement of ligands, XPS reveals the elemental composition and surface chemical state, and SEM visualizes the resulting morphological integrity. For research on PEAI in advanced solar cells, this multi-technique approach is indispensable. It allows researchers to correlate precise chemical modifications at the surface with macroscopic film properties, ultimately guiding the optimization of conductive, stable, and high-performance optoelectronic devices.

Conclusion

The strategic implementation of PEAI post-treatment ligand exchange emerges as a profoundly effective pathway for advancing electroluminescent solar cells. This synthesis of research demonstrates that PEAI successfully addresses the core challenges of charge transport and defect passivation, enabling the creation of bifunctional devices with remarkable photovoltaic efficiency—exceeding 14% PCE—and significant electroluminescence, up to 130 cd/m². Its superior hydrophobic nature compared to FAI also confers enhanced moisture stability, a critical factor for practical application. Future research directions should focus on refining the ligand exchange process for large-area modules, exploring synergistic effects with other passivation agents, and extending the PEAI strategy to lead-free and more diverse perovskite compositions. The convergence of high performance and stability positions PEAI-engineered devices as a cornerstone for the next generation of self-powered displays and intelligent urban lighting solutions.

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