Multidentate Ligand Passivation: Advanced Strategies to Reduce Non-Radiative Recombination in Perovskite Optoelectronics

Thomas Carter Dec 02, 2025 275

This article comprehensively reviews the cutting-edge strategy of using multidentate ligands to suppress non-radiative recombination in perovskite materials, a critical challenge limiting the performance and stability of solar cells and...

Multidentate Ligand Passivation: Advanced Strategies to Reduce Non-Radiative Recombination in Perovskite Optoelectronics

Abstract

This article comprehensively reviews the cutting-edge strategy of using multidentate ligands to suppress non-radiative recombination in perovskite materials, a critical challenge limiting the performance and stability of solar cells and light-emitting diodes. We explore the foundational principles of defect formation and passivation mechanisms, detail the design and application of innovative molecular structures from recent research, address key troubleshooting and optimization challenges in implementation, and provide a comparative validation of performance outcomes. Synthesizing the latest experimental and theoretical advances, this work serves as a strategic guide for researchers and scientists developing next-generation, high-efficiency perovskite-based optoelectronic devices.

Understanding Non-Radiative Recombination and the Multidentate Passivation Principle

Frequently Asked Questions (FAQs)

Q1: What is non-radiative recombination, and why is it a critical issue in perovskite solar cells?

A: Non-radiative recombination is a process where photo-generated charge carriers (electrons and holes) recombine without emitting light, losing their energy as heat instead. This process is primarily caused by defects within the bulk crystal and at the surfaces and grain boundaries of the perovskite film [1] [2]. These defects, such as undercoordinated Pb²⁺ ions or halide vacancies, act as traps for charge carriers [1]. The impact is severe: it significantly reduces the open-circuit voltage (Voc) and the overall power conversion efficiency (PCE) of the solar cell by creating non-radiative energy loss pathways [3] [4].

Q2: What are the primary origins of non-radiative recombination in perovskite films?

A: The main origins can be categorized as follows:

  • Surface and Grain Boundary Defects: The termination of the crystal lattice leads to undercoordinated ions (like Pb²⁺). These are highly energetic sites that readily trap charge carriers [1] [5].
  • Bulk Defects in the Crystal Lattice: Point defects such as iodine vacancies (VI) and anti-site defects (IPb) have low formation energies in perovskites, making them common non-radiative recombination centers [1].
  • Uncontrollable Crystallization: Fast crystallization processes, especially in Sn-Pb mixed perovskites, can lead to a Sn-rich and I-deficient surface. The accumulated Sn²⁺ is easily oxidized to Sn⁴⁺, creating deep-level defects and severe recombination [6].
  • Non-Ideal Stoichiometry: Deviations from the perfect chemical ratio on the film surface, particularly iodine deficiency, create a high density of vacancy defects that promote recombination [6].

Q3: My devices show a large deficit in open-circuit voltage (Voc). How can I quantify the non-radiative losses?

A: You can quantify the non-radiative voltage loss (ΔVoc,nrad) by measuring the photoluminescence quantum yield (PLQY) of your perovskite film or solar cell device. The relationship is given by the following formula, where k is the Boltzmann constant, T is the temperature, and PLQY is the measured value [3]:

A lower PLQY indicates higher non-radiative losses and a larger Voc deficit. For a more direct measurement on completed devices, electroluminescence external quantum efficiency (EQELED) can be used with a similar equation [3].

Q4: During my experiment on multidentate ligand passivation, the ligand solution does not properly interact with the perovskite surface. What could be wrong?

A: This is typically an issue with the binding affinity or conformation of the ligand. Consider these troubleshooting steps:

  • Verify Binding Sites: Conventional ligands bind through a single active site, which can create a resistive barrier and may not effectively passivate all defects. Multidentate ligands (≥3 binding sites) are needed for stronger, more stable binding and deeper trap passivation [1]. Ensure your ligand has multiple functional groups (e.g., Cl, Se, amine groups) that can coordinate simultaneously with multiple undercoordinated sites on the perovskite lattice [1].
  • Check Ligand Solubility and Solvent: The ligand must be soluble in a solvent that does not dissolve the underlying perovskite layer. Test the solvent on a spare perovskite film first [6].
  • Optimize Concentration and Processing: The concentration of the ligand solution is critical. Too low may result in incomplete coverage, while too high might form an insulating layer that hampers charge transport. Refer to the experimental protocol in Section 2.2 for guidance on optimization [6].

Q5: My passivated devices have good voltage but poor fill factor (FF). What might be causing this?

A: A poor fill factor after passivation often indicates a problem with charge transport rather than passivation itself. A common cause is that the passivating ligand, while effective at defect suppression, has formed a dense, insulating layer on the perovskite surface [1]. This layer creates a barrier for charge extraction at the interface. To resolve this, shift to multi-site-binding ligands that provide strong passivation while allowing for efficient charge transfer across the interface due to their specific molecular orbital coupling [1].

Q6: The stability of my passivated devices is not improving as expected. What factors should I investigate?

A: Device stability is influenced by both intrinsic and extrinsic stressors. If stability is lacking, investigate:

  • Incomplete Passivation: Residual defects can initiate degradation. Use techniques like KPFM to check for uniform surface potential, indicating effective defect suppression [6].
  • Ligand Stability: Labile ligands can detach during aging or device operation, losing their passivating effect. Use ligands that form strong chemical bonds (e.g., multidentate coordination) rather than weak physical adsorption [5].
  • Environmental Stressors: Stability must be tested against combined stressors like light, heat, and humidity. Follow standardized ISOS protocols (e.g., ISOS-L for light soaking, ISOS-D-2 for dark storage at 85°C) to ensure meaningful and comparable results [7].

Experimental Protocols & Methodologies

Protocol: Multi-anchoring Ligand Passivation for FAPbI₃-based Perovskites

This protocol is adapted from a study using an antimony chloride-N,N-dimethyl selenourea complex (Sb(SU)₂Cl₃) to achieve high-performance, air-processed PSCs [1].

1. Synthesis of Sb(SU)₂Cl₃ Complex

  • Reagents: Antimony chloride (SbCl₃), N,N-dimethylselenourea (SU), and anhydrous dichloromethane (DCM).
  • Procedure: Dissolve SbCl₃ and SU in a molar ratio of 1:2 in DCM. Stir the reaction mixture for 6-12 hours at room temperature in a nitrogen-filled glovebox. Recover the complex by evaporating the solvent [1].

2. Perovskite Film Formation and Passivation

  • Precursor Solution: Prepare your standard FAPbI₃ perovskite precursor solution.
  • Ligand Solution: Disspose the synthesized Sb(SU)₂Cl₃ complex in a polar solvent (e.g., isopropyl alcohol) at an optimized concentration (e.g., 0.5-1.0 mg/mL).
  • Deposition: Deposit the perovskite precursor solution onto your substrate and proceed with your standard annealing routine to form the crystalline film.
  • Passivation Treatment: After the perovskite film has cooled, spin-coat the Sb(SU)₂Cl₃ ligand solution onto the film at 3000-4000 rpm for 30 seconds. Perform a brief thermal annealing step (e.g., 100°C for 5 minutes) to remove residual solvent and promote binding [1].

3. Validation and Characterization

  • Fourier Transform Infrared (FTIR) Spectroscopy: Confirm the formation of the complex and its interaction with the perovskite by identifying characteristic shifts in N-H stretching (~3300, ~3200 cm⁻¹) and C-Se stretching (1000-800 cm⁻¹) bands [1].
  • X-ray Photoelectron Spectroscopy (XPS): Analyze the surface composition to verify a more stoichiometric ratio (e.g., I/(Pb+Sn) closer to 3:1) and a reduction in oxidation states (e.g., Sn⁴+ to Sn²⁺) [6].
  • Kelvin Probe Force Microscopy (KPFM): Map the surface potential to ensure a uniform distribution, indicating homogeneous passivation and reduced defect density [6].

Protocol: Surface Reconstruction for Sn-Pb Mixed Perovskites

This protocol uses a two-step chemical polishing and passivation strategy to achieve high-quality surfaces for narrow-bandgap perovskites [6].

1. Chemical Polishing with 1,4-Butanediamine (BDA)

  • Reagent Solution: Prepare a solution of BDA in isopropyl alcohol (IPA) at a concentration of 0.1 mg mL⁻¹.
  • Procedure: After annealing the Sn-Pb mixed perovskite film (e.g., FA₀.₇MA₀.₃Pb₀.₇Sn₀.₃I₃), spin-coat the BDA/IPA solution onto the film at 4000 rpm for 30 seconds. Then, anneal the film at 70°C for 1 minute. This step selectively removes the Sn-rich, I-deficient top layer, exposing a well-crystallized subsurface [6].

2. Defect Passivation with Ethylenediammonium Diiodide (EDAI₂)

  • Reagent Solution: Prepare an EDAI₂ solution in IPA or butanol.
  • Procedure: Immediately after the BDA polishing step, spin-coat the EDAI₂ solution onto the film. Follow with a final annealing step at 100°C for 5 minutes. The EDAI₂ fills organic cation and halide vacancies, further suppressing non-radiative recombination [6].

Protocol: Quantifying Non-Radiative Recombination Losses

1. Photoluminescence Quantum Yield (PLQY) Measurement

  • Setup: Use an integrating sphere and a calibrated spectrometer with an excitation source (e.g., a 640 nm laser).
  • Procedure: Measure the total emitted photons from the sample versus the excitation photons absorbed. Calculate the absolute PLQY.
  • Analysis: Use the PLQY value in the formula from FAQ #3 to determine ΔVoc,nrad [3].

2. Carrier Lifetime Measurement via Transient Techniques

  • Time-Resolved Photoluminescence (TRPL): Useful for studying film-level carrier dynamics and recombination rates.
  • Transient Photovoltage (TPV) / Transient Photocurrent (TPC): Essential for understanding carrier dynamics and recombination mechanisms within a complete working device under various bias lights [8]. These techniques help resolve discrepancies between film and device-level lifetimes.

Table 1: Performance Metrics of PSCs with Advanced Passivation Strategies

Passivation Strategy Perovskite Type Champion PCE (%) Open-Circuit Voltage (Voc) Stability (T80, unencapsulated) Reference
Sb(SU)₂Cl₃ Multi-site Ligand FAPbI₃ (air-processed) 25.03% Not Specified 23,325 h (dark storage) [1]
5,004 h (85°C)
2D/3D Composite (PEA)₂PbI₄ CsPbIBr₂ (all-inorganic) 10.13% Not Specified 500 h (ambient storage) [2]
BDA-EDAI₂ Surface Reconstruction Sn-Pb Mixed (Eg: 1.25 eV) 23.32% Not Specified >550 h (MPP tracking, encapsulated) [6]

Table 2: Key Material Properties Before and After Surface Passivation

Characterization Metric Control Film Passivated Film Measurement Technique Significance
Surface I/(Pb+Sn) Ratio Significantly < 3 Close to 3 (ideal) XPS [6] Reduces halide vacancy defects
Sn⁴+ / Total Sn (%) 27.3% 19.1% XPS [6] Suppresses Sn-related deep traps
Surface Potential Uniformity Large variation (ΔCPD = 45 mV) Homogeneous (ΔCPD = 29 mV) KPFM [6] Indicates uniform defect passivation
Carrier Lifetime Lower (e.g., nanoseconds) Higher (e.g., microseconds) TRPL, TPV [8] Direct evidence of suppressed recombination

Visualizing Passivation Mechanisms and Workflows

cluster_perovskite Perovskite Surface with Defects cluster_ligand Multidentate Ligand (e.g., Sb(SU)₂Cl₃) cluster_result Passivated Surface Pb Undercoordinated Pb²⁺ Ligand Ligand with Multiple Binding Sites (Se, Cl, N-H) Pb->Ligand Coordination Bond Vi Iodine Vacancy (V_I) Vi->Ligand Vacancy Filling Sn Oxidized Sn⁴⁺ Sn->Ligand Defect Suppression Passivated Stoichiometric Surface Reduced Defects Improved Charge Extraction Ligand->Passivated Multi-site Binding

Diagram 1: Multi-site Ligand Passivation Mechanism. Multidentate ligands simultaneously coordinate with multiple defect types on the perovskite surface, leading to a more stable and electronically passivated interface.

Start Start: Perovskite Film Fabrication Step1 Chemical Polishing (BDA/IPA) Removes Sn-rich/I-poor layer Start->Step1 Step2 Thermal Annealing (70°C, 1 min) Step1->Step2 Step3 Defect Passivation (EDAI₂) Fills V_A and V_I vacancies Step2->Step3 Step4 Thermal Annealing (100°C, 5 min) Step3->Step4 Char Characterization (XPS, KPFM, PL) Step4->Char End High-Quality Passivated Film Char->End

Diagram 2: Surface Reconstruction Workflow. A sequential two-step process for Sn-Pb mixed perovskites involving chemical polishing followed by specific defect passivation.

The Scientist's Toolkit: Key Research Reagents & Materials

Table 3: Essential Reagents for Multidentate Ligand Passivation Research

Reagent / Material Function / Role Example & Key Feature
Multidentate Ligand Complex Primary passivator; binds multiple defect sites simultaneously for robust passivation. Sb(SU)₂Cl₃: Binds via 2 Se and 2 Cl atoms to four adjacent Pb²⁺ sites [1].
Chemical Polishing Agent Modifies surface stoichiometry; removes defective top layer to expose better crystal. 1,4-Butanediamine (BDA): Interacts with Pb²⁺/Sn²⁺ and FA⁺ to dissolve Sn-rich surface [6].
Ammonium Salt Passivator Passivates organic cation (VA) and halide (VI) vacancies. Ethylenediammonium Diiodide (EDAI₂): Provides ammonium and iodine to fill common vacancies [6].
Solvents for Processing Carrier for passivation agents; must not dissolve underlying perovskite. Isopropyl Alcohol (IPA) / Butanol: Common for post-treatment of perovskite films [6].
Reference Passivation Ligand Single-site binder for control experiments; highlights multi-site advantage. Alkyl Ammonium Salts: Bind via a single -NH₃⁺ group; can form insulating layers [1].

Frequently Asked Questions (FAQs)

1. What is the primary advantage of using a multidentate ligand over a conventional single-site ligand? The primary advantage is significantly enhanced stability and more effective defect passivation. Conventional single-site ligands bind through only one active site, which can create a resistive barrier due to dense packing and offers limited stability [1]. In contrast, multidentate ligands bind simultaneously to multiple sites on the perovskite surface (e.g., via two Se and two Cl atoms) [1]. This multi-anchoring provides a stronger, more stable interaction that suppresses defect formation, improves moisture resistance, and enhances charge transport across interfaces without introducing significant barriers [1] [9].

2. My perovskite films still show high defect densities despite using passivating ligands. What might be going wrong? This common issue often arises from two factors: the binding strength of the ligand and the number of coordination sites. Single-site ligands have lower adsorption energy and can only passivate one type of defect or undercoordinated ion [1]. Multidentate ligands like Sb(SU)₂Cl₃ exhibit higher adsorption energies (exceeding 3 eV, much higher than the ≈1.5 eV of conventional ligands like oleic acid/oleylamine) [9] and can coordinate with multiple adjacent undercoordinated Pb²⁺ ions simultaneously, leading to more comprehensive defect suppression and increased defect formation energies [1].

3. How do multidentate ligands contribute to the mechanical stability of perovskite nanocrystal films? Multidentate ligands containing aromatic groups, such as tribenzylamine (TBA), enhance mechanical stability by facilitating strong π-π interactions with adjacent charge transport layers, which often contain conjugated organic units [9]. This replaces the weak van der Waals forces from the alkyl chains of traditional ligands, significantly improving interlayer and intralayer adhesion forces and suppressing crack formation during mechanical stress like folding [9].

4. Can you provide an example of a successful multidentate ligand and its performance? A notable example is the antimony chloride-N,N-dimethyl selenourea complex, Sb(SU)₂Cl₃. When used in fully air-processed perovskite solar cells, this multidentate passivator helped achieve a champion power conversion efficiency (PCE) of 25.03% and exceptional stability, with unencapsulated devices projecting a T₈₀ lifetime of 23,325 hours during dark storage [1]. Another is the TBA-SAPP (tribenzylamine carried sodium acid pyrophosphate) ligand for perovskite nanocrystal LEDs, which enabled a record-high external quantum efficiency (EQE) of 16.2% for pure-red flexible devices and robust mechanical properties [9].

Troubleshooting Guide

Problem Possible Cause Solution
Insufficient defect passivation Ligand binds only through a single active site, leading to weak adsorption and incomplete surface coverage [1]. Employ a multi-anchoring ligand (e.g., Sb(SU)₂Cl₃) designed for quadruple-site binding (e.g., 2Se–2Cl) to undercoordinated Pb²⁺ ions for stronger, more stable passivation [1].
Poor charge transport Dense packing of insulating single-site ligands creates a resistive barrier at the interface [1]. Use a multidentate ligand that promotes efficient charge transport. Sb(SU)₂Cl₃, for instance, has an elevated HOMO energy level that supports efficient hole transport [1].
Low mechanical stability in flexible devices Weak van der Waals forces from traditional aliphatic ligands (e.g., OA, OAm) result in poor interlayer adhesion [9]. Implement multibranched aromatic ligands (e.g., TBA-SAPP). The aromatic rings enhance adhesion to adjacent organic transport layers via π-π interactions [9].
Low environmental stability (moisture) Ineffective surface coverage and lack of hydrophobic properties in the ligand [1]. Select a multifunctional multidentate ligand that provides strong surface binding and introduces hydrophobic groups (e.g., methyl groups in Sb(SU)₂Cl₃) to repel moisture [1].

Table 1: Performance Comparison of Single-Site vs. Multidentate Ligands

Ligand Type Example Material Key Performance Metric Result Reference
Single-site binding Conventional ammonium ligands Power Conversion Efficiency (PCE) Creates resistive barrier, limits performance [1]. [1]
Multi-site binding (Dual) Dual-site-binding ligands Defect Passivation / Charge Transport Improved over single-site, but still limited [1]. [1]
Multi-site binding (Quadruple) Sb(SU)₂Cl₃ on PSCs Champion PCE 25.03% (fully air-processed) [1]. [1]
Extrapolated T80 Lifetime (Dark) 23,325 hours [1]. [1]
T80 Lifetime (85°C) 5,004 hours [1]. [1]
Multi-site binding TBA-SAPP on PNCs for LEDs External Quantum Efficiency (EQE) 16.2% (pure-red, flexible) [9]. [9]
Photoluminescence Quantum Yield (PLQY) 96.1% [9]. [9]
Folding Stability (1 mm radius) 5,000 cycles [9]. [9]

Table 2: Adhesion Force Measurements of Different Ligands (via AFM)

Ligand Type Adhesion Force with TPBi (nN) Adhesion Force with Poly-TPD (nN) Reference
C-PNCs (OA/OAm) 0.20 1.17 [9]
TBA-PNCs 0.25 1.73 [9]
TS-PNCs (TBA-SAPP) 0.34 2.13 [9]

Detailed Experimental Protocols

Protocol 1: Synthesis of Sb(SU)₂Cl₃ Multidentate Ligand Complex

Objective: To synthesize the antimony chloride-N,N-dimethyl selenourea (Sb(SU)₂Cl₃) complex for use as a multi-site passivator.

Methodology:

  • React antimony chloride with N,N-dimethylselenourea (SU) in a dichloromethane solvent [1].
  • Follow established synthetic procedures to obtain the Sb(SU)₂Cl₃ complex, which is soluble in polar solvents [1].
  • Characterization: Confirm successful complex formation using:
    • Fourier Transform Infrared (FTIR) Spectroscopy: Look for key peaks:
      • N–H stretching vibrations at ~3300 cm⁻¹ and ~3200 cm⁻¹.
      • N–H bending at ~1650 cm⁻¹.
      • C–Se stretching in the 1000–800 cm⁻¹ range.
      • A characteristic Se–Sb vibrational band between 350–300 cm⁻¹ [1].
    • X-ray Diffraction (XRD): Verify crystallinity, with prominent peaks expected at 15° and 30° [1].
    • Ultraviolet-Visible (UV-Vis) Spectroscopy: Compare the absorption of the solid-state film to the solution. A red-shift of ~10 nm and increased intensity in the film indicates enhanced intermolecular coupling [1].

Protocol 2: Ligand Exchange on Perovskite Nanocrystals (PNCs) with TBA-SAPP

Objective: To replace native oleic acid (OA) and oleylamine (OAm) ligands with TBA-SAPP ligands on PNCs to enhance optoelectronic and mechanical properties.

Methodology:

  • Synthesize control PNCs (C-PNCs) using a standard thermal injection method with mixed Br/I precursors and traditional OA/OAm ligands [9].
  • Perform a ligand exchange process by introducing the TBA-SAPP (TS) complex to the C-PNC dispersion. The detailed procedure is available in the methods section of the associated literature [9].
  • Characterization:
    • Atomic Force Microscopy (AFM): Quantify the adhesion force between the ligand-exchanged PNC film and adjacent charge transport layers (e.g., TPBi, poly-TPD) using tips coated with these materials [9].
    • Photoluminescence Quantum Yield (PLQY): Measure the PLQY of the resulting PNC film to confirm defect reduction and improved emission efficiency. A successful exchange should yield a PLQY above 95% [9].
    • Scanning Electron Microscopy (SEM): Image the film morphology after subjecting it to mechanical folding tests (e.g., 5000 cycles at a 1 mm radius) to assess enhanced mechanical robustness [9].

The Scientist's Toolkit: Essential Research Reagents

Reagent / Material Function in Multidentate Ligand Research
N,N-dimethylselenourea (SU) A key precursor ligand that provides selenium coordination sites for binding to undercoordinated metal ions (e.g., Pb²⁺) on the perovskite surface [1].
Antimony Chloride (SbCl₃) Reacts with SU to form the multidentate Sb(SU)₂Cl₃ complex, providing chloride ions as additional binding sites [1].
Tribenzylamine (TBA) Acts as an organic cation carrier for multidentate anions (e.g., SAPP). Its multi-branched aromatic structure enhances solubility in non-polar solvents and improves mechanical adhesion via π-π interactions [9].
Sodium Acid Pyrophosphate (SAPP) A multidentate X-type Lewis soft base anion that effectively passivates Lewis soft acid sites (e.g., uncoordinated Pb²⁺) on the PNC surface [9].
Sb(SU)₂Cl₃ Complex A premier multi-anchoring passivator that binds via four sites (2Se and 2Cl) to the perovskite lattice, significantly suppressing defects and improving stability [1].
TBA-SAPP (TS) Complex A multifunctional ligand that combines the mechanical adhesion benefits of TBA with the superior defect passivation of the multidentate SAPP anion for use in flexible optoelectronic devices [9].

Mechanism and Workflow Visualizations

multidentate_mechanism SingleSite Single-Site Ligand WeakBinding Weak Adsorption (Low E_ads) SingleSite->WeakBinding  leads to MultiSite Multi-Dentate Ligand (e.g., Sb(SU)₂Cl₃) StrongBinding Strong, Multi-Anchoring Binding (High E_ads >3 eV) MultiSite->StrongBinding  leads to Problems • Incomplete Passivation • Resistive Barrier • Poor Stability WeakBinding->Problems  causes Benefits • Deep Trap Passivation • Low Interface Resistance • Enhanced Stability StrongBinding->Benefits  enables

Diagram 1: Mechanism of action comparing single-site and multidentate ligands.

Diagram 2: Experimental workflow for developing multidentate ligands.

In the pursuit of high-performance perovskite optoelectronics, mitigating defect-induced non-radiative recombination is paramount. Undercoordinated Pb²⁺ and I⁻ ions at surfaces and grain boundaries act as dominant non-radiative recombination centers, severely limiting device performance and stability. Conventional passivation strategies often employ ligands with single active sites, which can create resistive barriers and offer limited stability. This technical guide explores advanced multidentate ligand passivation strategies, where molecules with multiple functional groups simultaneously coordinate with several defect sites. This approach provides stronger, more stable binding, enhances charge transport, and significantly reduces non-radiative recombination, paving the way for more efficient and durable devices.

Troubleshooting Guides & FAQs

Common Experimental Challenges & Solutions

FAQ 1: Why does my passivation treatment lead to a drop in fill factor (FF) and increased series resistance?

  • Potential Cause: Dense packing of insulating organic ligands on the perovskite surface can create a resistive barrier to charge extraction.
  • Solution: Implement multi-site-binding ligands (≥3 sites). The increased number of binding sites reduces the density of ligand packing required for effective passivation. For example, the antimony chloride-N,N-dimethyl selenourea complex Sb(SU)₂Cl₃ binds four adjacent sites on the perovskite lattice via two Se and two Cl atoms. This provides robust defect suppression while maintaining efficient charge transport, enabling a champion power conversion efficiency (PCE) of 25.03% in fully air-processed solar cells [10].

FAQ 2: How can I effectively passivate multiple different defect types (e.g., both Pb²⁺ and I⁻ vacancies) simultaneously?

  • Potential Cause: A single passivation molecule may only target one specific type of defect.
  • Solution: Use a combination of passivators or a molecule with multiple functional groups that can address different defects. A proven strategy is a surface reconstruction method using:
    • 1,4-Butanediamine (BDA): Acts as a chemical polishing agent to reduce Sn⁴⁺ and I⁻ vacancy (Vᵢ) related defects on Sn–Pb mixed perovskite surfaces. It interacts with Pb²⁺/Sn²⁺ via Lewis acid-base coordination and with FA⁺ cations via hydrogen bonding [6].
    • Ethylenediammonium diiodide (EDAI₂): Provides passivation for organic cation vacancies (Vₐ) and I⁻ vacancies (Vᵢ) [6].
    • This combined treatment creates a close-to-ideal stoichiometric surface, minimizing non-radiative recombination.

FAQ 3: My passivation layer is unstable. How can I improve its adhesion and longevity?

  • Potential Cause: Single-site binding offers a relatively weak and potentially reversible interaction with the perovskite surface.
  • Solution: Employ ligands that form multiple coordination bonds and hydrogen bonds. The stability of the Sb(SU)₂Cl₃ complex, for instance, comes from its quadruple-site binding (via 2Se and 2Cl) coupled with an extended hydrogen-bonding network through three N-H...Cl bonds. This multi-point attachment leads to exceptionally stable devices, with unencapsulated cells retaining 98.98% of their initial PCE after 1,584 hours of storage in air [10].

FAQ 4: What is a method to simultaneously passivate defects and suppress harmful ion migration?

  • Potential Cause: Some passivators only mask surface defects without preventing the underlying issue of ion migration, which degrades device performance over time.
  • Solution: Adopt a bilayer passivation strategy. A highly effective interface design involves:
    • An ultrathin ALD-AlOx layer (~1 nm): Deposited directly on the perovskite, this layer conformally passivates surface defects and acts as a robust barrier against ion migration [11].
    • A PDAI₂ (propane-1,3-diammonium iodide) layer: Applied on top of AlOx, this layer provides chemical passivation and facilitates beneficial n-doping of the subsequent C60 electron transport layer, enhancing charge extraction [11].
    • This combination has propelled perovskite/silicon tandem solar cells to an efficiency of 31.6% [11].

Table 1: Performance Metrics of Defect Passivation Strategies

Passivation Strategy Device Type Key Performance Improvement Stability Data
Sb(SU)₂Cl₃ (Multi-site ligand) [10] Fully air-processed PSC Champion PCE: 25.03% T₈₀ (dark): 23,325 hT₈₀ (85°C): 5,004 hT₈₀ (operational): 5,209 h
BDA + EDAI₂ (Surface reconstruction) [6] Sn–Pb mixed PSC (1.25 eV) Champion PCE: 23.32% -
AlOx/PDAI₂ (Bilayer passivation) [11] Perovskite/Silicon Tandem SC Champion PCE: 31.6% (certified 30.8%) 95% PCE retention after 1,000 h MPPT

Table 2: Defect Formation Energy Increase Post-Passivation (Theoretical Calculation) The following data, derived from DFT calculations, shows how effective passivation can make it energetically more costly for defects to form, thus suppressing their concentration [10].

Defect Type Change in Formation Energy after Sb(SU)₂Cl₃ Treatment
Iodine Vacancy (Vᵢ) Significantly Increased
Lead Vacancy (V_Pb) Significantly Increased
Anti-site Defect (I_Pb) Significantly Increased

Detailed Experimental Protocols

This protocol outlines the synthesis of a multi-site passivator and its application to a perovskite film.

1. Synthesis of Sb(SU)₂Cl₃ Complex

  • Reagents: Antimony chloride (SbCl₃), N,N-dimethylselenourea (SU), anhydrous dichloromethane (DCM).
  • Procedure:
    • Dissolve antimony chloride and N,N-dimethylselenourea in a molar ratio of 1:2 in anhydrous DCM.
    • Stir the reaction mixture at room temperature for several hours under an inert atmosphere.
    • Recover the synthesized complex, which is soluble in polar solvents.

2. Application as Passivator

  • Solution Preparation: Dissolve the synthesized Sb(SU)₂Cl₃ complex in a suitable polar solvent (e.g., ethyl acetate) to create a treatment solution.
  • Deposition: Spin-coat the passivation solution directly onto the synthesized perovskite film.
  • Characterization Validation:
    • Fourier Transform Infrared (FTIR) Spectroscopy: Confirm the formation of the complex and the presence of N-H stretching vibrations (~3300 cm⁻¹, ~3200 cm⁻¹) and C-Se stretching (1000-800 cm⁻¹) [10].
    • X-ray Diffraction (XRD): Verify the crystalline phase of the complex, with prominent peaks at ~15° and 30° [10].

This protocol describes a two-step surface treatment for Sn-Pb mixed perovskite films.

1. Chemical Polishing with BDA

  • Solution Preparation: Dilute 1,4-Butanediamine (BDA) in isopropyl alcohol (IPA) to a concentration of 0.1 mg mL⁻¹.
  • Treatment: Spin-coat the BDA/IPA solution onto the surface of the cooled perovskite film.
    • Mechanism: BDA acts as a Lewis base, coordinating with undercoordinated Pb²⁺ and Sn²⁺ ions. It also engages in hydrogen bonding with organic cations (e.g., FA⁺), effectively "polishing" away the Sn-rich and I-deficient surface layer [6].

2. Passivation with EDAI₂

  • Solution Preparation: Prepare a solution of Ethylenediammonium diiodide (EDAI₂) in a suitable solvent (e.g., isopropyl alcohol).
  • Treatment: Spin-coat the EDAI₂ solution directly onto the BDA-treated perovskite film.
    • Mechanism: The ammonium cations in EDAI₂ passivate organic cation vacancies (Vₐ), while the iodide anions fill I⁻ vacancies (Vᵢ) [6].

3. Validation via XPS and KPFM

  • X-ray Photoelectron Spectroscopy (XPS): Use to confirm the surface stoichiometry has shifted towards the ideal ratio (I/(Pb+Sn) ≈ 3) and that the Sn⁴+ percentage is significantly reduced [6].
  • Kelvin Probe Force Microscopy (KPFM): Use to demonstrate a more uniform surface potential distribution, indicating a reduction in surface defect states [6].

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Reagents for Multidentate Passivation Research

Reagent / Material Function / Role in Passivation
N,N-dimethylselenourea (SU) [10] Ligand precursor providing Se atoms for coordination with undercoordinated Pb²⁺ ions.
Antimony Chloride (SbCl₃) [10] Central metal ion in the Sb(SU)₂Cl₃ complex, providing a structural framework and Cl atoms for binding.
1,4-Butanediamine (BDA) [6] Chemical polishing agent that coordinates with Pb²⁺/Sn²⁺ and interacts with organic cations to normalize surface stoichiometry.
Ethylenediammonium diiodide (EDAI₂) [6] Bifunctional passivator; its ammonium group passivates cation vacancies, and its iodide anions fill iodine vacancies.
AlOx (via Atomic Layer Deposition) [11] Ultrathin inorganic layer that passivates defects and acts as an ion diffusion barrier.
Propane-1,3-diammonium Iodide (PDAI₂) [11] Organic salt used in bilayer passivation to chemically passivate the interface and promote n-doping of the ETL.

Mechanism Workflow & Signaling Pathways

The following diagram illustrates the multi-site binding mechanism of a ligand like Sb(SU)₂Cl₃ with a perovskite surface, which is central to reducing non-radiative recombination.

G Start Perovskite Surface (Undercoordinated Pb²⁺ & I⁻ ions) A Multi-site Ligand Approach Start->A B Quadruple-Site Binding (2 Se + 2 Cl atoms) A->B C Hydrogen Bonding Network (N-H···Cl bonds) A->C D Suppressed Defect Formation B->D C->D E Reduced Non-Radiative Recombination D->E F Enhanced Device Efficiency & Stability E->F

Diagram 1: Multidentate ligand binding suppresses defects to enhance device performance.

Frequently Asked Questions (FAQs)

FAQ 1: What is the primary advantage of using a multidentate ligand over a conventional single-site ligand for defect passivation? Conventional single-site ligands bind to undercoordinated metal ions (e.g., Pb²⁺ or Sn²⁺) at perovskite surfaces or grain boundaries through only one active site. [1] This often creates a dense, insulating organic layer that imposes a resistive barrier to charge transport. [1] [9] In contrast, multidentate ligands feature multiple donor atoms (e.g., Cl, Se, O, N) that can bind to two, three, or more adjacent sites on the metal ion simultaneously. [1] [12] This multi-site binding results in:

  • Stronger and more stable adsorption onto the perovskite surface, with calculated adsorption energies significantly higher than those of conventional ligands. [9]
  • More effective suppression of defect formation, such as iodine vacancies (Vᵢ) and lead vacancies (V_Pb), by increasing their defect formation energies. [1]
  • Reduced interfacial resistance as the binding mode does not necessitate a densely packed insulating layer, thereby facilitating better charge extraction. [1]

FAQ 2: How do multidentate ligands help in reducing non-radiative recombination? Non-radiative recombination is often triggered by defects that create trap states within the bandgap. [1] Multidentate ligands directly passivate these defect sites, particularly undercoordinated Pb²⁺ ions which act as Lewis soft acids. [9] The multiple Lewis soft base donor atoms in a multidentate ligand (such as Cl⁻ or Se from an antimony complex) efficiently coordinate with these Pb²⁺ sites, forming robust complexes. [1] This coordination:

  • Eliminates trap states, reducing the pathways for non-radiative recombination.
  • Leads to a significant enhancement in photoluminescence quantum yield (PLQY), with reports of near-unity values (96.1%) in passivated perovskite nanocrystal films. [9]
  • Improves the overall crystallinity and film quality of the perovskite, further minimizing defect density. [13] [1]

FAQ 3: My perovskite films show poor environmental stability after ligand treatment. What could be the issue? While ligand passivation targets electronic defects, stability is also influenced by the ligand's molecular structure. Issues may arise if the ligand lacks hydrophobic groups or fails to form a protective network. To enhance stability:

  • Incorporate hydrophobic functional groups: Ligands with hydrophobic moieties, such as aromatic rings (e.g., tribenzylamine) or methyl groups, can shield the perovskite surface from moisture. [1] [9]
  • Utilize ligands that form cross-linking networks: Some multidentate ligands can form extended hydrogen-bonding networks or π-π interactions with adjacent layers, which improves the mechanical robustness and moisture resistance of the film. [1] [9] For example, a Sb(SU)₂Cl₃ complex demonstrated dramatically improved moisture resistance and device stability. [1]

FAQ 4: Why is there a discrepancy between my solid-state crystallography data and solution-state NMR spectra for my metal-ligand complex? This is a common observation in coordination chemistry, especially with fluxional multidentate ligands and Group-1 metal complexes. [12] The solid-state structure, determined by single-crystal X-ray diffraction (SCXRD), represents a single, static snapshot. In solution, the complex is dynamic. Labile dative bonds (e.g., N→M) can rapidly break and reform, causing parts of the ligand to coordinate and decoordinate on the NMR timescale. [12] This dynamic equilibrium can average out the different chemical environments of donor atoms, making the solution-state NMR spectrum appear more symmetric than the solid-state structure. [12] Techniques like Variable-Temperature (VT) NMR are essential to probe these intramolecular coordination changes. [12]

Troubleshooting Guides

Problem: Incomplete Defect Passivation Leading to High Voltage Losses

Symptoms: Low open-circuit voltage (VOC), low fill factor, and dominant non-radiative recombination as observed in photoluminescence quantum yield (PLQY) measurements.

Possible Causes and Solutions:

  • Cause: Weak ligand binding energy.
    • Solution: Design or select ligands with multiple, strongly-coordinating donor atoms. Density Functional Theory (DFT) calculations can predict adsorption energy. Ligands with higher denticity (e.g., tetradentate, hexadentate) generally bind more tightly due to the chelate effect. [14] [12] For instance, a quadruple-site binding mode (2Se–2Cl) showed stronger adsorption and greater charge transfer than single-site binding. [1]
    • Protocol: Perform DFT calculations to model the interaction between the ligand and a model perovskite surface (e.g., PbI₂-terminated FAPbI₃). Compare the adsorption energies (E_ads) and Bader charge transfer for different binding configurations. [1]
  • Cause: Mismatch between ligand binding sites and perovskite lattice geometry.
    • Solution: The spatial arrangement of donor atoms in the ligand should complement the crystal structure of the perovskite. A "square" arrangement of donors (e.g., two Se and two Cl atoms) was shown to match well with the FAPbI₃ lattice, enabling simultaneous coordination to four adjacent Pb²⁺ sites. [1]
    • Protocol: Analyze the crystal structure of your perovskite. Use molecular modeling software to dock the ligand onto the surface and check for geometric compatibility and the ability to bridge multiple metal sites.

Problem: Ligand-Induced Charge Transport Barriers

Symptoms: Increased series resistance, reduced photocurrent, and inefficient charge extraction.

Possible Causes and Solutions:

  • Cause: Formation of an insulating ligand layer.
    • Solution: Use ligands that facilitate charge transport. This can be achieved by incorporating conjugated molecular systems (e.g., aromatic groups) that provide a pathway for charge transfer, or by ensuring the ligand's molecular orbitals (HOMO/LUMO) align favorably with the perovskite's energy levels to support efficient hole or electron transport. [1] [9]
    • Protocol: Perform Ultraviolet Photoelectron Spectroscopy (UPS) to determine the work function and ionization energy of the passivated film. Compare the energy level alignment with the charge transport layers.
  • Cause: Excessive ligand concentration leading to overly thick passivation layers.
    • Solution: Optimize the ligand concentration via a dilution series. The optimal concentration should fully passivate defects without forming a thick, continuous insulating film.
    • Protocol: Prepare a series of solutions with ligand concentrations ranging from 0.1 to 10 mg/mL. Spin-coat them onto the perovskite films and characterize device performance (J-V curves) and film conductivity (e.g., via space-charge-limited current, SCLC, measurements).

Problem: Poor Stability Under Mechanical Stress (for Flexible Devices)

Symptoms: Formation of cracks in the perovskite film after bending or folding.

Possible Causes and Solutions:

  • Cause: Weak interlayer adhesion.
    • Solution: Employ ligands with multi-branched aromatic structures (e.g., tribenzylamine) that can engage in strong π-π interactions with adjacent organic charge transport layers (e.g., TPBi, poly-TPD). This significantly enhances adhesion force compared to traditional aliphatic ligands like oleic acid and oleylamine. [9]
    • Protocol: Quantify adhesion force using Atomic Force Microscopy (AFM) with tips coated with the relevant charge transport layer material. [9] Prepare films with standard and new ligands for a direct comparison.

Quantitative Data on Multidentate Ligand Performance

Table 1: Comparison of Defect Formation Energies with and without Multidentate Ligand Passivation. [1]

Defect Type Formation Energy in Pristine FAPbI₃ (eV) Formation Energy with Sb(SU)₂Cl₃ (eV) Change (eV)
Iodine Vacancy (Vᵢ) 0.82 > 0.82 (significantly increased) Increase
Lead Vacancy (V_Pb) 3.16 > 3.16 (significantly increased) Increase
I-on-Pb Anti-site (I_Pb) 3.88 > 3.88 (significantly increased) Increase

Table 2: Experimental Device Metrics Achieved with Multidentate Ligand Strategies.

Ligand / Complex Material System Key Improvement Reported Metric
Sb(SU)₂Cl₃ [1] FAPbI₃ Solar Cell Power Conversion Efficiency (PCE) 25.03% (fully air-processed)
Operational Stability (T₈₀) 5,209 hours (unencapsulated, 1-sun)
TBA-SAPP (TS) [9] Perovskite Nanocrystal LED External Quantum Efficiency (EQE) 16.2% (pure-red, flexible)
Mechanical Stability Withstood 5,000 folding cycles at 1 mm radius
Isothio-Br [13] FASnI₃ Solar Cell Crystallinity & Microstructure Improved crystallinity and preferential (h00) crystal growth

Experimental Protocols

Protocol 1: Passivating Perovskite Films with a Multidentate Ligand Complex (e.g., Sb(SU)₂Cl₃) [1]

  • Ligand Synthesis: Synthesize the Sb(SU)₂Cl₃ complex by reacting antimony chloride with N,N-dimethylselenourea (SU) in dichloromethane. Purify the product.
  • Solution Preparation: Dissolve the synthesized Sb(SU)₂Cl₃ complex in a polar solvent (e.g., ethyl acetate) to create a stock solution. The concentration should be optimized (e.g., 1-5 mg/mL).
  • Film Deposition: Deposit your perovskite film using your standard method (e.g., two-step sequential deposition for FAPbI₃).
  • Post-Treatment: Immediately after annealing the perovskite film, spin-coat the ligand solution onto the hot film (e.g., 3000-5000 rpm for 30 seconds).
  • Annealing: Thermally anneal the film again at a moderate temperature (e.g., 100°C for 10 minutes) to remove residual solvent and promote strong ligand-perovskite coordination.

Protocol 2: Probing Solution-State Coordination Dynamics via VT-NMR [12]

  • Sample Preparation: Dissolve the Group-1 metal complex (e.g., DETAN-coordinated Li or Na complex) in a deuterated solvent (e.g., toluene-d₈).
  • NMR Acquisition: Acquire ¹H NMR spectra at room temperature to observe the time-averaged symmetry.
  • Variable-Temperature Study: Collect a series of ¹H NMR spectra across a temperature range (e.g., 298 K to 213 K). A wide temperature range is crucial to slow down dynamic processes.
  • Spectral Analysis: Identify signals from specific ligand moieties (e.g., -NCH₂CH₃ groups). Monitor how these signals split or broaden as the temperature decreases, which indicates the decoordination of specific donor atoms and reveals the solution-state coordination mode.

Experimental Workflow and Signaling Pathways

Diagram: Multidentate Ligand Passivation Strategy

G A Perovskite Surface (Undercoordinated Pb²⁺ Defects) B Introduce Multidentate Ligand A->B C Ligand Binds Multiple Sites (Cl, Se, O, N donors) B->C D Strong Coordination Complex Forms C->D X Multi-Site Binding? C->X E Defect Passivation (Vacancy suppression) D->E F Improved Energetic Landscape E->F Y Defects Reduced? E->Y G Enhanced Device Performance (Higher PCE, Stability, PLQY) F->G X->B No X->D Yes Y->B No Y->F Yes

Diagram: Solution-State Ligand Dynamics

G A Solid-State Structure B Solution-State Dynamics A->B Dissolves D Coordination- Decoordination B->D Labile N→M Bonds C VT-NMR Probe C->B Observes E Averaged NMR Signal D->E Fast Exchange @ High Temp

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Reagents for Multidentate Ligand Research.

Reagent / Material Function / Role Example Application
N,N-dimethylselenourea (SU) Precursor for synthesizing ligands with Se donor atoms. Formation of Sb(SU)₂Cl₃ complex for multi-anchoring to Pb²⁺. [1]
Antimony Chloride (SbCl₃) Metal center for constructing a multidentate ligand complex. Reacts with SU to form the Sb(SU)₂Cl₃ passivator. [1]
Tribenzylamine (TBA) Organic cation carrier with multi-branched aromatic rings. Enhances adhesion via π-π interactions in perovskite nanocrystal LEDs. [9]
Sodium Acid Pyrophosphate (SAPP) Multidentate X-type Lewis soft base anion. Passivates undercoordinated Pb²⁺ defects as part of the TBA-SAPP ligand. [9]
Me₆TREN Ligand Tetradentate neutral amine ligand. Used to isolate and stabilize monomeric Group-1 metal complexes for fundamental studies. [12]
DETAN Ligand Hexadentate neutral amine ligand with a macrocycle and sidearms. Provides flexible, "on-demand" coordination sites for Group-1 metals, studied via VT-NMR. [12]

Troubleshooting Guides

FAQ 1: Why does my device's open-circuit voltage (VOC) improve after passivation, but the fill factor (FF) and power conversion efficiency (PCE) decrease?

This common issue arises when the passivation layer itself acts as a resistive barrier, impeding charge extraction at the interface even as it reduces non-radiative recombination.

  • Root Cause: Conventional passivating ligands often bind through only a single active site. When densely packed, these insulating organic layers create a significant charge-extraction barrier, hindering hole and electron transport [1] [15].
  • Diagnosis: Check the conductivity of your passivation layer. Compare current-voltage (I-V) characteristics of devices before and after passivation. A significant increase in series resistance after passivation confirms this problem.
  • Solution: Implement multifunctional passivators that combine defect coordination with conductive moieties. For example:
    • Planar conjugated molecules like 1,8-naphthyridin-2-amine (2-NA) form dense protective layers via noncovalent interactions (π-π stacking, hydrogen bonding) while facilitating charge transport, reducing carrier cooling time from 515 to 240 fs and quadrupling carrier diffusion length [16].
    • Multidentate ligands like the Sb(SU)₂Cl₃ complex bind to multiple adjacent sites, providing stable passivation without dense insulating packing [1].

FAQ 2: How can I enhance the stability of my perovskite solar cells without compromising charge carrier transport?

Stability enhancement often involves surface encapsulation, which can hinder charge transport if not properly designed.

  • Root Cause: Long-chain, insulating ligands used for passivation and stability can disrupt the electronic coupling between perovskite grains or at the charge transport layer interface.
  • Diagnosis: Perform grazing-incidence wide-angle X-ray scattering (GIWAXS) to examine the molecular packing orientation of your passivation layer. A more parallel molecular orientation relative to the substrate facilitates vertical charge transport [15].
  • Solution: Employ binary synergistical post-treatment (BSPT) systems. Blending different ammonium salts (e.g., 4-tert-butyl-benzylammonium iodide (tBBAI) with phenylpropylammonium iodide (PPAI)) can enhance crystallinity, improve molecular packing, and achieve better energy band alignment. This approach has yielded a certified 26.0% PCE while maintaining 81% of initial efficiency after 450 hours of maximum power point tracking [15].

FAQ 3: What strategies can simultaneously passivate defects and improve charge extraction at the perovskite/HTL interface?

The perovskite/hole transport layer (HTL) interface is critical for both recombination losses and charge extraction efficiency.

  • Root Cause: Undercoordinated Pb²⁺ ions at surfaces and grain boundaries act as nonradiative recombination centers, while energy level misalignment impedes hole extraction.
  • Diagnosis: Use X-ray photoelectron spectroscopy (XPS) to examine the Pb:I ratio after passivation. An increased ratio indicates effective filling of iodine vacancies [15].
  • Solution: Utilize symmetrical molecular design with multiple functional groups. Triphenylamine-based molecules like TATA functionalized with carboxyl groups provide a tripartite synergistic mechanism:
    • Optimization of interfacial charge transport through π-π stacking
    • Regulation of the Fermi level to enhance electron extraction
    • Passivation of Pb defects via Pb–O coordination
    • This approach has achieved 25.91% PCE while retaining 90% of initial efficiency after 900 hours of aging [17].

Experimental Protocols

Protocol 1: Multidentate Ligand Passivation for Fully Air-Processed PSCs

This protocol utilizes the Sb(SU)₂Cl₃ complex for multi-site binding passivation, achieving 25.03% efficiency in fully air-processed devices [1].

  • Step 1: Ligand Synthesis

    • React antimony chloride with N,N-dimethylselenourea (SU) in dichloromethane to form Sb(SU)₂Cl₃ complex.
    • Confirm complex formation using Fourier transform infrared (FTIR) spectroscopy (look for N–H stretching at ~3300 cm⁻¹, N–H bending at 1650 cm⁻¹, and C–Se stretching between 1000-800 cm⁻¹).
    • Verify crystallinity through X-ray diffraction (XRD), with prominent peaks at 15° and 30° indicating high structural symmetry.

  • Step 2: Perovskite Film Fabrication and Treatment

    • Fabricate perovskite films using a standard two-step method under ambient conditions.
    • Dissolve Sb(SU)₂Cl₃ in a polar solvent at a concentration of 1.0-1.5 mg/mL.
    • Spin-coat the ligand solution onto the perovskite film at 3000-4000 rpm for 30 seconds.
    • Anneal at 100°C for 10 minutes to facilitate ligand-perovskite interaction.

  • Step 3: Characterization and Validation

    • Perform density functional theory (DFT) calculations to verify the quadruple-site binding configuration (via 2Se–2Cl atoms).
    • Measure defect formation energies - effective passivation should increase formation energies of iodine vacancies (VI), lead vacancies (VPb), and anti-site defects (IPb).
    • Conduct time-resolved photoluminescence to confirm reduced non-radiative recombination.

Protocol 2: Binary Synergistical Post-Treatment (BSPT)

This protocol uses blended organic halide salts to enhance both passivation and charge transport, achieving a certified 26.0% PCE [15].

  • Step 1: Solution Preparation

    • Prepare individual solutions of phenylpropylammonium iodide (PPAI) and 4-tert-butylphenylmethylammonium iodide (tBBAI) in isopropanol (IPA) at 0.5-1.0 mg/mL concentration.
    • Create the binary solution by mixing PPAI and tBBAI at optimal weight ratio (typically 1:1 to 1:2 based on GIWAXS analysis).
    • Stir the blended solution for 2-3 hours at room temperature to ensure complete mixing.

  • Step 2: Film Deposition and Treatment

    • Deposit RbCl-doped FAPbI3 perovskite film using a modified two-step method.
    • Spin-coat the binary solution onto the perovskite surface at 4000-5000 rpm for 30 seconds without further annealing.
    • Repeat the spin-coating process 1-2 times if necessary to ensure complete coverage.

  • Step 3: Structural and Electronic Characterization

    • Perform grazing incident X-ray diffraction (GIXRD) to confirm new diffraction peaks in the small angle region (approximately 4.55° for blended system).
    • Use grazing-incidence wide-angle X-ray scattering (GIWAXS) to verify more ordered molecular packing and parallel orientation relative to substrate.
    • Conduct all-atom molecular dynamics (AA-MD) simulation to analyze radial distribution function (RDF) and confirm enhanced molecular packing between PPAI and tBBAI.

Performance Data Comparison

Table 1: Quantitative Performance Comparison of Different Passivation Strategies

Passivation Strategy PCE (%) VOC (V) FF (%) Stability Retention Key Mechanism
Control (No Passivation) 23.02 [17] - - - Baseline
TATA Triphenylamine 25.91 [17] - - 90% (900 h) [17] Pb–O coordination, π-π stacking
Sb(SU)₂Cl₃ Multidentate 25.03 [1] - - 98.98% (1584 h) [1] Quadruple-site (2Se–2Cl) binding
Binary Post-treatment (tBBAI+PPAI) 26.0 [15] - - 81% (450 h MPP) [15] Enhanced crystallinity, ordered packing
2-NA Planar Conjugated 22.49 [16] - - 93.6% (1008 h) [16] Noncovalent interactions, hot-carrier extraction

Table 2: Molecular Design Features and Their Impact on the Passivation-Transport Trade-off

Molecular Feature Impact on Passivation Impact on Transport Example Materials
Single-site binding Limited passivation efficiency Minimal resistance if sparse Conventional ammonium ligands [1]
Multi-site binding (≥3 sites) Deep trap passivation, suppressed ion migration Reduced resistive barrier Sb(SU)₂Cl₃ [1]
Planar conjugated structure Moderate passivation via noncovalent interactions Excellent charge extraction 2-NA [16]
Symmetrical functional groups Multidentate coordination Enhanced π-π interaction for vertical transport TATA [17]
Binary blended systems Multi-defect passivation Improved energy band alignment tBBAI+PPAI [15]

Signaling Pathways and Molecular Mechanisms

G Perovskite Perovskite Defects Defects Perovskite->Defects Surface GBs SinglePass SinglePass Defects->SinglePass Single-site Binding MultiPass MultiPass Defects->MultiPass Multi-site Binding ChargeTransport ChargeTransport SinglePass->ChargeTransport High Resistance MultiPass->ChargeTransport Efficient Extraction PCE PCE ChargeTransport->PCE

Molecular Passivation Mechanisms

Research Reagent Solutions

Table 3: Essential Research Reagents for Balancing Passivation and Transport

Reagent Chemical Function Role in Trade-off Balance Application Protocol
TATA (4,4',4″-tricarboxytriphenylamine) Pb defect passivation via Pb-O coordination; enhances π-π stacking [17] Simultaneous defect passivation and charge transport enhancement Additive in perovskite precursor (0.5-1.0 mol%)
Sb(SU)₂Cl₃ Complex Multi-site binding via 2Se+2Cl coordination; hydrogen bonding network [1] Quadruple-site binding enables efficient passivation without resistive barrier Surface treatment (1.0-1.5 mg/mL in polar solvent)
tBBAI+PPAI Binary System Mixed organic halide salts with different molecular sizes [15] Enhanced crystallinity and ordered packing improve hole transfer Binary post-treatment (1:1 ratio in IPA, 0.5-1.0 mg/mL)
2-NA (1,8-naphthyridin-2-amine) Planar conjugated molecule; noncovalent interactions [16] Facilitates hot-carrier extraction while passivating undercoordinated Pb²⁺ Interfacial modification between perovskite and HTL
Tribenzylamine (TBA)-SAPP Multidentate X-type Lewis soft base; aromatic cations [9] Enhanced adhesion and defect passivation for flexible devices Ligand exchange in perovskite nanocrystal synthesis

Designing and Implementing Multidentate Ligands for Maximum Efficacy

Frequently Asked Questions: Troubleshooting Ligand Performance

FAQ 1: Why is my multidentate ligand failing to provide effective defect passivation, despite having multiple functional groups?

Ineffective passivation often stems from poor binding strength or steric hindrance that prevents key functional groups from interacting with the target defects.

  • Solution: Prioritize functional groups that form strong, covalent bonds with defect sites. For example, in perovskite systems, phosphonic acid groups can form robust P–O–Pb bonds with undercoordinated Pb²⁺ defects, which are stronger and more durable than coordination bonds alone [18]. Ensure the ligand backbone is rigid enough to pre-organize the functional groups for binding but flexible enough to accommodate the target's geometry.
  • Experimental Protocol:
    • Synthesize your target multidentate ligand.
    • Prepare your material (e.g., a perovskite film) with the intended defects.
    • Apply the ligand solution onto the material surface or incorporate it as an additive during synthesis.
    • Use X-ray Photoelectron Spectroscopy (XPS) to analyze the chemical states of key atoms (e.g., Pb, O). A shift in binding energy confirms successful chemical interaction [18].
    • Perform Fourier-Transform Infrared (FTIR) spectroscopy to identify new vibration signals (e.g., P–O–Pb at ~1076 cm⁻¹), verifying the formation of specific covalent bonds [18].

FAQ 2: My ligand binds well but disrupts charge transport. How can I mitigate this?

This is a common trade-off. The ligand may be creating an insulating layer or improperly aligning energy levels at the interface.

  • Solution: Implement a synergistic bimolecular interface. Use one molecule for primary defect passivation and a second, strategically chosen molecule to modify the surface energetics. For instance, after passivating with a ligand, applying a second like 2-phenylethylammonium iodide (PEAI) can create a negative surface dipole. This upsifts the Fermi level, creating a more n-type surface that enhances electron extraction and prevents charge accumulation [18].
  • Experimental Protocol:
    • Apply your primary passivation ligand to the material.
    • Sequentially deposit a solution of the secondary dipole-modifying molecule (e.g., PEAI).
    • Use Ultraviolet Photoelectron Spectroscopy (UPS) to measure the work function and valence band maximum. A decrease in work function indicates successful surface dipole formation and beneficial energy level alignment [18].
    • Fabricate a full device and characterize current-voltage (J-V) characteristics to confirm improved charge transport and reduced hysteresis.

FAQ 3: How can I design a ligand for an unpredictable or unknown binding site conformation?

When experimental structures are unavailable or binding sites are variable, leverage computational predictions and focus on ligand-aware design.

  • Solution: Use advanced protein structure prediction models like AlphaFold2 (AF2). Prospectively, docking libraries against AF2 models has successfully identified high-affinity ligands for receptors, even when the model's binding site conformation differed from later experimental structures [19]. Furthermore, tools like LABind can predict binding sites in a "ligand-aware" manner by using ligand SMILES sequences to learn specific binding characteristics, which is particularly useful for unseen ligands [20].
  • Experimental Protocol:
    • Obtain or generate an AF2 model of your target protein.
    • Use a docking program to screen a virtual library of your designed ligands against the predicted binding site.
    • Synthesize and test the top-ranking compounds.
    • Validate binding through assays and, if possible, co-crystallization.

Quantitative Performance of Functional Groups

Table 1: Defect Passivation Efficacy of Key Functional Groups

Functional Group Target Defect / System Key Interaction Quantitative Outcome Citation
-COOH (Carboxylic Acid) Undercoordinated Pb²⁺ in Perovskites Lewis acid-base interaction Improved PCE from 16.76% to 20.64% in solar cells [21]
Pyridyl N Undercoordinated Pb²⁺ in Perovskites Lewis base coordination Improved PCE from 16.76% to 20.64% in solar cells [21]
Phosphonic Acid (-PO(OH)₂) Undercoordinated Pb²⁺ in Perovskites Covalent P–O–Pb bond Reduced Voc loss to 59 mV; PCE of 25.53% [18]
-NH₂ (Ammonium, e.g., PEAI) Perovskite Surface Surface dipole formation Creates n-type surface; enhances electron extraction [18]
Chloride (Cl⁻) Iodide vacancies in Perovskites Anionic passivation PCE increased from 16.9% to 20.2% [22]
Pyridine-Carbonyl Imide Molecular Self-Assembly ADDA H-bonding arrays Forms stable supramolecular architectures [23]

Table 2: Troubleshooting Guide for Functional Group Selection

Problem Symptom Likely Functional Group Issue Strategic Correction
Weak or transient passivation Reliance on weak coordination or H-bonding alone Introduce groups that form covalent bonds (e.g., phosphonic acid) [18]
Impaired charge extraction Insulating ligand layer or poor energy level alignment Add a secondary dipole-forming molecule (e.g., ammonium salts) to adjust surface energetics [18]
Poor solubility or processing Highly polar groups in non-polar solvents Adjust backbone hydrophobicity or use a counterion
Lack of specificity Nonselective binding modes Design a multidentate ligand with a pre-organized geometry for the target [23]

Experimental Workflow for Ligand Evaluation

The following diagram outlines a core methodology for developing and testing multidentate ligands for defect passivation, integrating steps from computational design to experimental validation.

G Start Define Passivation Target CompDesign Computational Ligand Design Start->CompDesign Synth Ligand Synthesis CompDesign->Synth Apply Apply to Material System Synth->Apply Char1 Characterize Binding (XPS, FTIR) Apply->Char1 Char2 Characterize Energetics (UPS, KPFM) Char1->Char2 Char3 Measure Device Performance (PCE, Voc, FF) Char2->Char3 Eval Evaluate Stability Char3->Eval Success Successful Passivation Eval->Success Performance Met Refine Refine Ligand Architecture Eval->Refine Performance Not Met Refine->CompDesign Redesign

Ligand Development and Testing Workflow

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for Ligand Passivation Research

Reagent / Material Function / Application Key Characteristic
6,6′-Dithiodinicotinic Acid (DTDN) Multifunctional passivator for perovskites [21] Provides -COOH, pyridyl N, and S atoms for diverse interactions.
4-Methoxyphenylphosphonic Acid (MPA) Forms covalent P–O–Pb bonds on perovskite surfaces [18] Creates a strong, robust interface layer.
2-Phenylethylammonium Iodide (PEAI) Modifies surface energetics and creates interface dipole [18] Enhances electron extraction when used with a passivation layer.
Tetraphenylphosphonium Chloride (TPPP(Cl)) Coordination complex for passivating Pb²⁺ and I⁻ defects [22] Porphyrin ring structure acts as both electron donor and acceptor.
N-(pyridine-2-carbonyl)pyridine-2-carboxamide Building block for supramolecular chemistry [23] Features an ADDA hydrogen-bonding array for self-assembly.
Propane-1,3-diammonium Iodide (PDAI₂) Dual passivation and n-doping agent in perovskites [11] Improves charge extraction and suppresses hysteresis.

Technical Support Center

Troubleshooting Guides

Q1: My device shows a low open-circuit voltage (Voc). What could be the cause and how can I fix it?

A: A low Voc typically indicates high nonradiative recombination at the perovskite/charge transport layer interface. To address this:

  • Verify Passivation Layer Application:
    • Problem: Incomplete coverage or weak bonding of the covalent passivator (e.g., MPA).
    • Solution: Ensure the 4-methoxyphenylphosphonic acid (MPA) solution is freshly prepared and applied to a clean, dry perovskite surface. Confirm the formation of P–O–Pb covalent bonds via XPS analysis by checking for the characteristic peak shift in the Pb 4f core level [18].
  • Check Energy Level Alignment:
    • Problem: The second molecule (e.g., PEAI) has not created a sufficient surface dipole for optimal electron extraction.
    • Solution: Use Ultraviolet Photoelectron Spectroscopy (UPS) to measure the work function of the modified perovskite surface. A successful modification will show a significant decrease in work function, moving the Fermi level closer to the conduction band [18].
  • Review Processing Sequence:
    • Problem: Applying the bimolecular layers in the incorrect order.
    • Solution: The covalent bond-forming molecule (MPA) must be applied first, followed by the dipole-forming molecule (PEAI). Reversing this sequence will prevent the formation of the necessary strong covalent bonds with the perovskite lattice [18].

Q2: The film morphology appears degraded after applying the bimolecular interlayer. How can I prevent this?

A: Film degradation often stems from solvent incompatibility or excessive coverage.

  • Use Compatible Solvents: Apply the MPA and PEAI from solvents that do not dissolve the underlying perovskite layer (e.g., ethanol or isopropanol). Always test the solvent on a spare perovskite sample first [18].
  • Optimize Concentration and Time: The concentration of the solutions and the spin-coating duration (e.g., 30-60 seconds) for the interlayers must be optimized. Too high a concentration can lead to excessive, insulating layers that hinder charge transport. Perform a concentration gradient experiment to find the optimal values [10] [18].

Q3: The efficiency of my device is unstable under operational conditions. How can I improve stability?

A: Operational instability can be linked to inadequate defect passivation or poor environmental protection.

  • Ensure Robust Passivation: Conventional ligands that bind through a single site can create resistive barriers and offer less stable passivation. Implement multi-site-binding ligands (≥3 sites), which provide more robust defect suppression and better stability. For example, an antimony chloride-N,N-dimethyl selenourea complex (Sb(SU)₂Cl₃) can bind to four adjacent sites on the perovskite, dramatically improving moisture resistance and operational stability [10].
  • Confirm Hydrophobicity: Check the water contact angle of the modified perovskite surface. A successful bimolecular interlayer treatment will result in a more hydrophobic surface, which is crucial for long-term stability [18].

Frequently Asked Questions (FAQs)

Q: Why is a bimolecular approach superior to a single-molecule interlayer? A: A single molecule may not simultaneously provide strong covalent bonding and ideal energy level alignment. A synergistic bimolecular strategy allows you to use one molecule (e.g., MPA) for robust, covalent defect passivation and a second molecule (e.g., PEAI) to fine-tune the surface energetics for optimal charge extraction. This separation of functions often yields better results than a compromise-laden single-molecule design [18].

Q: What is the typical thickness of a synergistic bimolecular interlayer (SBI), and why is it important? A: The SBI should be thin, typically less than 6 nm. This ensures effective defect passivation and energy level modulation without introducing a significant resistive barrier to charge transport [18].

Q: How do I characterize the success of the covalent bonding step? A: X-ray Photoelectron Spectroscopy (XPS) is the primary tool. For MPA, look for a splitting of the Pb 4f core level into two doublets and the emergence of a new O 1s peak near 531.5 eV, which confirms the formation of P–O–Pb bonds [18]. Fourier-transform infrared (FTIR) spectroscopy can also show a P–O–Pb vibration signal near 1076 cm⁻¹ [18].

Q: What are the key performance indicators of a successful SBI modification? A: Successful modification should lead to:

  • A significant reduction in nonradiative recombination, leading to a higher Voc [18].
  • A power conversion efficiency (PCE) of over 25% for inverted (p-i-n) devices [18].
  • Exceptional stability, such as retaining over 95% of initial PCE after 1000 hours of aging at 55±5 °C [18], or a T80 lifetime of over 23,000 hours for unencapsulated cells stored in the dark [10].

Table 1: Performance Metrics of Perovskite Solar Cells with Advanced Interlayers

Interlayer Type Device Architecture Champion PCE (%) Certified PCE (%) Voc Loss (mV) Stability (PCE Retention) Reference
Sb(SU)₂Cl₃ (Multidentate Ligand) n-i-p (Air-Processed) 25.03 - - 98.98% (after 1,584 h in dark, unencapsulated) [10]
MPA/PEAI (Synergistic Bimolecular) p-i-n (Inverted) 25.53 (stabilized) 25.05 59 95% (after >1,000 h at 55±5 °C) [18]

Table 2: Key Reagent Solutions for SBI Formation

Reagent Function / Role Key Characterizations
4-Methoxyphenylphosphonic Acid (MPA) Forms strong covalent P–O–Pb bonds with undercoordinated Pb²⁺ ions on the perovskite surface, diminishing defect density. XPS (Pb 4f, O 1s core level shifts), FTIR (P–O–Pb vibration at ~1076 cm⁻¹) [18].
2-Phenylethylammonium Iodide (PEAI) Creates a negative surface dipole, shifting the work function and constructing a more n-type surface for enhanced electron extraction. UPS (decrease in work function, shift of VBM) [18].
Antimony Chloride-N,N-dimethylselenourea Complex (Sb(SU)₂Cl₃) Acts as a multi-site (quadruple) passivator, binding via two Se and two Cl atoms to suppress defect formation and improve moisture resistance. FTIR, XRD, DFT calculations (ESP map) [10].

Detailed Experimental Protocols

Protocol 1: Applying a Synergistic Bimolecular Interlayer (MPA/PEAI)

This protocol is for the functionalization of a perovskite surface in an inverted (p-i-n) device structure [18].

  • Perovskite Film Preparation: Prepare your perovskite film (e.g., Cs₀.₀₅(FA₀.₉₅MA₀.₀₅)₀.₉₅Pb(I₀.₉₅Br₀.₀₅)₃) using your standard method until you have a smooth, crystalline film.
  • MPA Solution Preparation: Dissolve MPA in anhydrous ethanol to form a solution with an optimized concentration (e.g., 0.1 mg mL⁻¹). The solution should be prepared fresh or stored in a dry, inert environment to prevent hydrolysis.
  • MPA Deposition: Directly after annealing the perovskite film and allowing it to cool, deposit the MPA solution onto the film via dynamic spin-coating (e.g., 4000 rpm for 30 seconds). This forms the foundational covalent bond layer.
  • Annealing: Thermally anneal the film at 100°C for 5 minutes to facilitate the covalent bonding reaction between MPA and the perovskite surface.
  • PEAI Solution Preparation: Dissolve PEAI in anhydrous isopropanol to form a solution with an optimized concentration (e.g., 0.5 mg mL⁻¹).
  • PEAI Deposition: Without delay, deposit the PEAI solution onto the MPA-modified perovskite film via spin-coating (e.g., 4000 rpm for 30 seconds).
  • Final Annealing: Thermally anneal the film again at 100°C for 5 minutes to remove residual solvent and complete the formation of the synergistic bimolecular interlayer.
  • Proceed with Device Fabrication: Continue with the deposition of the subsequent electron transport layer (e.g., PCBM) and other electrode materials.

Protocol 2: Passivation with a Multi-Site Binding Ligand (Sb(SU)₂Cl₃)

This protocol describes the use of a multi-anchoring ligand for defect passivation in fully air-processed devices [10].

  • Synthesis of Sb(SU)₂Cl₃ Complex: React antimony chloride with N,N-dimethylselenourea (SU) in dichloromethane following established procedures to form the Sb(SU)₂Cl₃ complex [10].
  • Solution Preparation: Dissolve the synthesized Sb(SU)₂Cl₃ complex in a polar solvent (e.g., ethyl acetate) to create a treatment solution.
  • Film Treatment: Apply the solution onto the prepared perovskite film, typically during or after the film formation process in a two-step fabrication method.
  • Characterization of Complex Formation: Verify the successful formation of the complex and its interaction with the perovskite using:
    • FTIR Spectroscopy: Look for characteristic bands such as N-H stretching (~3300 cm⁻¹), N-H bending (~1650 cm⁻¹), C-Se stretching (1000-800 cm⁻¹), and the Se-Sb vibration (350-300 cm⁻¹) [10].
    • XRD: Confirm the crystallinity of the complex with prominent peaks at 15° and 30° [10].

The Scientist's Toolkit

Research Reagent Solutions

Item Function / Explanation
4-Methoxyphenylphosphonic Acid (MPA) A phosphonic acid that reacts with the perovskite surface to form robust covalent P–O–Pb bonds, providing durable passivation of surface defects [18].
2-Phenylethylammonium Iodide (PEAI) An ammonium salt that forms a surface dipole layer, modifying the perovskite's work function to improve energy level alignment with the electron transport layer [18].
Antimony Chloride-N,N-dimethylselenourea Complex A multi-site binding ligand that passivates defects more effectively than single-site ligands by coordinating to multiple undercoordinated Pb²⁺ ions simultaneously [10].
PCBM ([60]Phenyl-C61-butyric acid methyl ester) A common fullerene derivative used as the electron transport layer (ETL) in inverted (p-i-n) perovskite solar cells [18].
X-ray Photoelectron Spectrometer (XPS) Used to chemically characterize the perovskite surface and confirm the formation of covalent bonds (e.g., P–O–Pb) via analysis of core level shifts [18].
Ultraviolet Photoelectron Spectrometer (UPS) Used to determine the work function and valence band maximum of the perovskite surface, critical for verifying energy level alignment after interlayer modification [18].
Kelvin Probe Force Microscopy (KPFM) Measures the surface potential and its distribution, providing nanoscale evidence of a more uniform surface potential after successful passivation [18].

Experimental Workflow and Mechanism Diagrams

workflow Start Start: Prepare Perovskite Film Step1 Step 1: Apply MPA Solution (Spin-coat) Start->Step1 Step2 Step 2: Thermal Anneal (100°C, 5 min) Step1->Step2 Step3 Step 3: Apply PEAI Solution (Spin-coat) Step2->Step3 Step4 Step 4: Thermal Anneal (100°C, 5 min) Step3->Step4 End End: Proceed with ETL/Electrode Deposition Step4->End

SBI Application Workflow

mechanism Perovskite Perovskite Surface • Undercoordinated Pb²⁺ defects • High nonradiative recombination MPA MPA Application • Forms covalent P–O–Pb bonds • Suppresses defect density • Upshifts Fermi Level Perovskite->MPA 1. Covalent Bonding PEAI PEAI Application • Creates negative surface dipole • Lowers Work Function • Enhances electron extraction MPA->PEAI 2. Dipole Alignment Result Result: Modified Surface • Reduced recombination • Improved energetics • High VOC and efficiency PEAI->Result

SBI Mechanism of Action

Troubleshooting Guides

FAQ 1: Why is my passivation layer causing poor charge transport despite effective defect reduction?

Issue: A common problem where effective defect passivation leads to the unintended consequence of inhibited charge carrier extraction.

Root Cause: This is frequently caused by the formation of an overly dense or insulating passivation layer. Conventional monodentate ligands (those binding through a single atom) can create a physical barrier that blocks charge transfer pathways, especially when packed tightly on the surface [24] [1].

Solutions:

  • Implement Multidentate Ligands: Switch to hyperbranched polymers or complexes with multiple binding sites. For example, a hyperbranched polysiloxane with maleic acid structure (HPSiM) provides rich carbonyl groups that chelate Pb²⁺ ions strongly without forming a resistive barrier, enabling both high passivation and a champion efficiency of 25.38% [24].
  • Use Multi-Site Binding Molecules: Employ ligands like Sb(SU)₂Cl₃ (antimony chloride-N,N-dimethyl selenourea complex) that bind to four adjacent sites on the perovskite surface via two Se and two Cl atoms. This creates a stable bond without dense packing, allowing for efficient charge transport and achieving a PCE of 25.03% [1].
  • Optimize Binding Configuration: Preferentially select molecules where computational studies (DFT calculations) show strong adsorption energy but minimal disruption to the perovskite lattice's electronic structure [1].

FAQ 2: How can I prevent the degradation of my perovskite layer when using a ZnO electron transport layer?

Issue: The perovskite layer deposited on ZnO ETL degrades promptly, leading to poor device stability.

Root Cause: The amphoteric nature of ZnO allows it to react with the perovskite's organic cations (e.g., MA+, FA+). This deprotonation reaction forms zinc hydroxide and degrades the interface [25].

Solutions:

  • Apply In Situ Nanoparticle Passivation: Synthesize ZnO NPs with an ethanolamine post-treatment (E-ZnO). The ethanolamine acts as a passivating ligand on the ZnO surface before perovskite deposition, slowing the detrimental reaction and improving stability to over one week in ambient conditions without encapsulation [25].
  • Choose Less Acidic Cations: Formamidinium (FA+) is less acidic than methylammonium (MA+), resulting in significantly slower reaction kinetics with ZnO. Using FA-based perovskites like FA₀.₉Cs₀.₁PbI₃ enhances interface stability [25].
  • Interface Engineering with a Sacrificial Layer: Introduce a thin interlayer such as C₆₀ or ZnS between the ZnO and perovskite. This physically separates the materials, preventing direct chemical reaction. This approach has yielded hysteresis-free solar cells with a PCE of 20.7% [25].

FAQ 3: My in-situ passivation during synthesis isn't yielding reproducible film quality. What factors should I control?

Issue: Inconsistent perovskite film quality, including variable grain sizes and pinholing, when using in-situ passivation additives.

Root Cause: Uncontrolled crystallization dynamics. The passivation additive is altering the nucleation and crystal growth rates unpredictably [26] [27].

Solutions:

  • Leverage Additives that Modulate Crystallization: Use multifunctional additives like 3,4,5-trifluorobenzamide (TFBZ). Its fluorine substituents form N–H···F hydrogen bonds with formamidinium iodide (FAI), which actively retards the crystallization rate. This leads to larger grains, fewer pinholes, and more uniform films [26].
  • Control Precursor Chemistry: For quantum dot synthesis, manipulate the chemical environment of the precursor. Introducing Hydroiodic Acid (HI) converts PbI₂ into highly coordinated [PbIₘ]²⁻ species, which optimizes nucleation numbers and growth kinetics for CsPbI₃ QDs with near-unity photoluminescence quantum yield (PLQY) [27].
  • Validate Interaction: Use Fourier Transform Infrared (FTIR) spectroscopy to confirm the additive is interacting with the perovskite as intended. Look for characteristic shifts in key vibrational peaks (e.g., C=O, N-H) that confirm coordination with Pb²⁺ or the organic cation [26].

Experimental Protocols & Data Presentation

Experimental Protocol 1: In-Situ Passivation via Multidentate Polymer Additive

This protocol details the incorporation of a hyperbranched polysiloxane (HPSiM) into the perovskite active layer as a bulk additive for defect passivation [24].

1. Solution Preparation:

  • Perovskite Precursor Solution: Prepare your standard perovskite precursor solution (e.g., FA₁₋ₓCsₓPbI₃) in a suitable solvent system (e.g., DMF/DMSO).
  • Additive Stock Solution: Dissolve the synthesized HPSiM polymer in dimethylformamide (DMF) to create a concentrated stock solution.
  • Final Precursor Solution: Blend the HPSiM stock solution into the perovskite precursor solution with vigorous stirring. The typical concentration of HPSiM in the final solution is 0.5-2.0 mg mL⁻¹. A control solution without HPSiM should be prepared simultaneously.

2. Film Deposition:

  • Spin-coat the final precursor solution onto your prepared substrate (e.g., ITO/SnO₂) using your optimized two-step spin-coating program.
  • During the spin-coating process, at the second step, drip an anti-solvent (e.g., chlorobenzene) to induce crystallization.
  • Anneal the film on a hotplate at 100-150 °C for 10-20 minutes to form the crystalline perovskite film. The HPSiM will be incorporated throughout the bulk and at grain boundaries.

3. Characterization and Validation:

  • FTIR Spectroscopy: Compare the FTIR spectra of pure HPSiM and the HPSiM-perovskite film. A shift in the carbonyl (C=O) stretching peak indicates coordination with undercoordinated Pb²⁺ ions.
  • X-ray Photoelectron Spectroscopy (XPS): Analyze the Pb 4f core levels. A shift to higher binding energy in the modified film confirms the strong chelation interaction between Pb²⁺ and the carbonyl groups of HPSiM.
  • Device Fabrication: Complete the solar cell by depositing the hole transport layer and metal electrodes. Measure the photovoltaic parameters to quantify the improvement in PCE and reduction in hysteresis.

Experimental Protocol 2: Post-Synthesis Passivation of Quantum Dots

This protocol describes an in-situ passivation strategy during the synthesis of CsPbI₃ quantum dots (QDs) using hydroiodic acid (HI) to reduce surface trap states [27].

1. Modified QD Synthesis:

  • Standard Setup: Load PbI₂ and 1-octadecene (ODE) into a three-neck flask. Heat to 90°C under vacuum for 1 hour.
  • HI Introduction: After degassing, under a nitrogen atmosphere, add oleic acid (OA) and oleylamine (OLA). Then, inject the desired volume of HI solution (e.g., 50, 100, or 150 µL) into the PbI₂ precursor. HI converts unreacted PbI₂ into soluble [PbIₘ]²⁻ complexes.
  • Reaction: Heat the solution to 165°C. Swiftly inject preheated Cs-oleate solution. Allow the reaction to proceed for 5-10 seconds before immediately cooling the mixture in an ice-water bath.

2. Purification:

  • Precipitate the crude QD solution by adding methyl acetate (MeOAc) and centrifuging.
  • Discard the supernatant, redisperse the pellet in hexane, and repeat the purification cycle.
  • After final centrifugation, dry the QDs and redisperse in octane at a specific concentration (e.g., 70 mg mL⁻¹) for film deposition.

3. Characterization:

  • Photoluminescence Quantum Yield (PLQY): Measure the PLQY of the QD solution. A significant increase (e.g., towards near-unity) for HI-treated QDs indicates a reduction in non-radiative recombination centers.
  • X-ray Diffraction (XRD): Confirm the enhanced crystallinity and phase purity of the HI-manipulated CsPbI₃ QDs. The peaks should be sharp and correspond to the desired cubic phase.
  • Trap Density Measurement: Use space-charge-limited current (SCLC) measurements on electron-only devices to quantitatively show the reduction in defect density.

Table 1: Performance Comparison of Select Passivation Strategies

Passivation Method Material / Ligand Device Architecture Champion PCE (%) Key Stability Metric
In-Situ (Bulk) Hyperbranched Polysiloxane (HPSiM) [24] Inverted PSC 25.38 91.6% of initial PCE after 1000h aging at 55°C & MPP
In-Situ (Interface) Ethanolamine-treated ZnO (E-ZnO) [25] n-i-p with ZnO ETL >18 Stable for 1 week in ambient (25°C, 50% RH), unencapsulated
Post-Synthesis (Multi-site) Sb(SU)₂Cl₃ Complex [1] Fully air-processed PSC 25.03 T₈₀ lifetime of 23,325h (dark storage)
In-Situ (Additive) 3,4,5-trifluorobenzamide (TFBZ) [26] MA-free WBG PSC 22.78 (1.67-eV) 85% of initial PCE after 2400h in ambient air
In-Situ (QD Synthesis) Hydroiodic Acid (HI) [27] CsPbI₃ QD Solar Cell 15.72 Enhanced storage stability reported

Table 2: The Scientist's Toolkit: Key Research Reagent Solutions

Reagent / Material Function / Role in Passivation Key Mechanism / Interaction
Hyperbranched Polysiloxane (HPSiM) [24] Multidentate chelating bulk additive Multiple carbonyl (C=O) groups chelate with undercoordinated Pb²⁺ across grain boundaries and surfaces.
Sb(SU)₂Cl₃ Complex [1] Multi-site binding surface passivator Binds to four adjacent sites on perovskite via 2 Se and 2 Cl atoms, creating a robust, non-resistive layer.
3,4,5-Trifluorobenzamide (TFBZ) [26] Multifunctional crystallization modulator & passivator -C=O and -NH₂ passivate defects; Fluorine atoms form H-bonds to slow crystallization and enhance hydrophobicity.
Ethanolamine [25] In-situ nanoparticle passivator for ETL Coats ZnO NP surface, acting as a sacrificial base to slow deprotonation of perovskite cations, improving interface stability.
Hydroiodic Acid (HI) [27] In-situ precursor modifier for QD synthesis Converts PbI₂ into [PbIₘ]²⁻, optimizing nucleation/growth and providing iodide ions to fill vacancies during synthesis.
D-π-A Dyes (e.g., TTR) [28] Interfacial passivation additive Electron-donating and withdrawing groups enable in-situ defect passivation at interfaces and improve energy level alignment.

Workflow Visualization

workflow Start Start: Choose Passivation Method InSitu In-Situ Passivation Start->InSitu PostSynth Post-Synthesis Passivation Start->PostSynth InSitu_1 Add passivator to precursor solution InSitu->InSitu_1 PostSynth_1 Synthesize perovskite film or quantum dots PostSynth->PostSynth_1 InSitu_2 Deposit perovskite film (Passivation occurs during crystallization) InSitu_1->InSitu_2 InSitu_3 Characterize bulk & interface properties InSitu_2->InSitu_3 PCE_Test Final Device Fabrication & PCE Measurement InSitu_3->PCE_Test PostSynth_2 Apply passivating ligand solution to surface PostSynth_1->PostSynth_2 PostSynth_3 Annealing or rinsing step to remove excess ligands PostSynth_2->PostSynth_3 PostSynth_4 Characterize surface morphology & defects PostSynth_3->PostSynth_4 PostSynth_4->PCE_Test

Passivation Method Selection Workflow

mechanism A Undercoordinated Pb²⁺ Ion (Surface Defect) B Non-Radiative Recombination (Energy Loss) A->B C Monodentate Ligand (Single Binding Site) B->C D Multidentate Ligand (Multiple Binding Sites) B->D E1 Weak/Unstable Passivation Possible Charge Transport Barrier C->E1 E2 Strong Chelation & Stable Passivation Improved Charge Transport D->E2 F Reduced Non-Radiative Recombination Higher Device Efficiency & Stability E1->F Limited Effect E2->F Effective

Multidentate vs Monodentate Passivation

This technical support document is framed within a broader research thesis focused on reducing non-radiative recombination through multidentate ligand passivation. A significant challenge in perovskite/silicon tandem solar cells has been the inherent trade-off between effective defect passivation and efficient charge transport. Conventional single-site passivation molecules often introduce resistive barriers, while multi-site ligands can overcome this but require precise control over their adsorption orientation on the perovskite surface. This case study examines a breakthrough using the triple-site planar molecule 3-amino-4-chlorobenzenesulfonic acid (3A4Cl-BZS), which enabled a device with an open-circuit voltage exceeding 2 V and a power conversion efficiency (PCE) of 31.42% [29].

Frequently Asked Questions (FAQs)

Q1: What is the primary functional advantage of a triple-site planar molecule like 3A4Cl-BZS over conventional single-site passivators? A triple-site planar molecule provides multiple, simultaneous anchoring points to the perovskite surface. This leads to:

  • Superior Defect Passivation: The different functional groups (-SO₃H, -NH₂, -Cl) can coordinate with various types of surface defects (e.g., undercoordinated Pb²⁺ and I⁻ vacancies) concurrently, leading to more comprehensive passivation and significantly reduced non-radiative recombination [29].
  • Mitigated Resistive Barrier: The planar structure and specific functional groups promote a parallel adsorption configuration on the perovskite surface. This minimizes the distance between the active passivation sites and the defects, boosting passivation efficacy while minimizing the introduction of a series resistance that hampers charge extraction [29].

Q2: How does the adsorption orientation of a multi-site ligand influence device performance? The adsorption orientation is critical. A vertical orientation can create a thick, insulating layer that impedes charge transport. In contrast, the parallel adsorption achieved with 3A4Cl-BZS offers two key benefits:

  • Enhanced Passivation: Shortens the path for interaction between the molecule's functional groups and surface defects.
  • Improved Charge Transfer: Allows for closer contact and better electronic coupling with the perovskite layer, facilitating efficient electron and hole transfer across the interface and resulting in higher photovoltage and fill factor [29].

Q3: What specific defects do the functional groups in 3A4Cl-BZS target? The molecule is engineered so that each functional group addresses a specific defect:

  • Sulfonic Acid Group (-SO₃H): Strongly coordinates with and passivates undercoordinated Pb²⁺ ions [29].
  • Amino Group (-NH₂): Interacts with and passivates I⁻ vacancies via hydrogen bonding [29].
  • Chlorine Group (-Cl): Promotes the favorable parallel adsorption on the perovskite surface and can also contribute to defect passivation [29].

Q4: What are the key characterization techniques to confirm successful multidentate passivation? Researchers should employ a combination of techniques:

  • Theoretical Calculations: Density Functional Theory (DFT) to model adsorption configurations and calculate binding energies [29] [1].
  • Spectroscopy: Fourier-Transform Infrared (FTIR) spectroscopy to confirm chemical bonding and interaction between the ligand and perovskite [1].
  • Photophysical Measurements: Photoluminescence (PL) Quantum Yield and Time-Resolved PL (TRPL) to quantify the reduction in non-radiative recombination and increased carrier lifetimes [29].
  • Electrical Characterization: Current-Voltage (I-V) measurements to demonstrate reduced voltage deficit and higher efficiency.

Troubleshooting Guide

Problem Observed Potential Root Cause Recommended Solution
Low Open-Circuit Voltage (Voc) Incomplete defect passivation; ligand not binding optimally to all target sites. Optimize the concentration and processing solvent of the ligand solution. Verify parallel adsorption via DFT calculations [29].
Reduced Fill Factor (FF) Ligand introduction creates a resistive barrier to charge transport. Ensure the ligand adsorption orientation is parallel, not vertical. Check the energy level alignment at the interface [29].
Poor Operational Stability Weak bonding between ligand and perovskite surface; ligand desorption under stress. Employ a multi-site ligand with strong, stable coordination bonds (e.g., chelating ligands). Incorporate hydrophobic groups to improve moisture resistance [1] [30].
Inconsistent Performance Between Batches Uncontrolled molecular orientation during film processing. Standardize the processing conditions (annealing temperature, solvent, and ambient). Use ligands that self-assemble into the desired configuration [29].

Research Reagent Solutions

The following table details key reagents used in advanced multidentate passivation strategies as discussed in the cited literature.

Reagent Name Function / Role in Experiment Key Technical Notes
3A4Cl-BZS (3-amino-4-chlorobenzenesulfonic acid) Triple-site planar modifier for interface engineering. Passivates undercoordinated Pb²⁺ and I⁻ vacancies, enabling parallel adsorption [29]. Functional groups: -SO₃H (for Pb²⁺), -NH₂ (for I⁻ vacancies), -Cl (for planar orientation). Used in perovskite/silicon tandem cells [29].
Sb(SU)₂Cl₃ (Antimony chloride-N,N-dimethyl selenourea complex) Multi-anchoring ligand for bulk and surface passivation in air-processed devices. Suppresses defect formation and enhances moisture resistance [1]. Binds via two Se and two Cl atoms; forms hydrogen-bonding network. Achieved 25.03% PCE in fully air-processed PSCs [1].
EDTA (Ethylene diamine tetraacetic acid) Multidentate molecule for "surface surgery treatment" of perovskite quantum dots (PQDs). Passivates defects and crosslinks QDs [31]. Chelates suspended Pb²⁺ ions and occupies I⁻ vacancies. Also improves electronic coupling between PQDs [31].
Theophylline Multi-functionalized molecule for inverted all-inorganic PSCs. Interacts with uncoordinated Pb²⁺ via C=O group and forms H-bonds with I⁻ ions [30]. Contains C=O and NH₂ groups. Improves phase stability and energy level alignment at the interface [30].

Experimental Protocols & Workflows

Protocol 1: Surface Passivation with a Triple-Site Planar Molecule

This protocol outlines the procedure for modifying a perovskite surface using 3A4Cl-BZS, as referenced in the core case study [29].

Objective: To deposit a monolayer of 3A4Cl-BZS on a perovskite film to passivate interface defects and achieve a parallel adsorption orientation.

Materials:

  • Pre-fabricated perovskite thin film.
  • 3A4Cl-BZS powder.
  • Anhydrous Isopropanol (IPA).
  • Spin coater.
  • Nitrogen glovebox.

Step-by-Step Methodology:

  • Solution Preparation: Dissolve 3A4Cl-BZS powder in anhydrous IPA to create a solution with a concentration of 0.5 - 1.0 mg/mL. Stir thoroughly until fully dissolved.
  • Film Deposition: Place the pre-fabricated perovskite film on the spin coater inside a nitrogen glovebox.
  • Ligand Application: Dynamically spin-coat the 3A4Cl-BZS/IPA solution onto the perovskite film at 4000 rpm for 30 seconds.
  • Post-Treatment Annealing: Immediately after spin-coating, transfer the film to a hotplate and anneal at 100°C for 5 minutes to remove residual solvent and promote molecular ordering and adhesion.

Critical Notes:

  • The entire process should be conducted in an inert atmosphere to prevent degradation of the perovskite layer.
  • The concentration of the ligand solution is critical; too high a concentration may lead to multilayer formation and increased series resistance.

G Start Start: Prepare Perovskite Film S1 Prepare 3A4Cl-BZS/IPA Solution Start->S1 S2 Spin-coat Solution onto Perovskite Film S1->S2 S3 Annealing at 100°C for 5 minutes S2->S3 S4 Characterize Modified Surface (FTIR, PL, DFT) S3->S4 End End: Passivated Film Ready S4->End

Protocol 2: Evaluating Passivation Efficacy via Photophysical Characterization

This protocol describes how to quantify the success of the passivation treatment by measuring improvements in material properties.

Objective: To characterize the reduction in defect density and non-radiative recombination in the passivated perovskite film.

Materials:

  • Control (unpassivated) perovskite film.
  • Passivated perovskite film (from Protocol 1).
  • Photoluminescence (PL) Spectrometer.
  • UV-Vis Spectrophotometer.
  • Time-Resolved PL (TRPL) setup.

Step-by-Step Methodology:

  • Steady-State PL: Measure the PL intensity of both the control and passivated films under identical excitation conditions. A significant increase in intensity for the passivated film indicates suppressed non-radiative pathways.
  • Time-Resolved PL (TRPL): Record the PL decay curves for both films. Fit the data to a multi-exponential decay model. A longer average carrier lifetime (τavg) in the passivated film confirms reduced trap-assisted recombination.
  • UV-Vis Absorption: Perform absorption measurements to ensure the treatment did not adversely affect the perovskite's bandgap or create unwanted absorption features.

Data Analysis:

  • Calculate the PL Quantum Yield (PLQY) improvement.
  • Extract and compare the carrier lifetimes (τ₁, τ₂, τavg) from TRPL data.

G Start Start: Passivated & Control Films P1 Steady-State PL Measurement Start->P1 Dec1 Higher PL Intensity? P1->Dec1 P2 TRPL Measurement Dec1->P2 Yes End Passivation Successful Dec1->End No Dec2 Longer Carrier Lifetime? P2->Dec2 P3 UV-Vis Absorption Measurement Dec2->P3 Yes Dec2->End No P3->End

Protocol 3: Fabricating and Testing the Perovskite/Silicon Tandem Solar Cell

This protocol summarizes the final device integration and testing steps to validate the performance improvement.

Objective: To integrate the passivated perovskite top cell with a silicon bottom cell and measure the tandem device performance.

Materials:

  • Passivated perovskite top cell.
  • Planar silicon heterojunction bottom cell.
  • Sputtering or evaporation system for transparent electrodes.
  • Solar simulator (Class AAA).
  • Quantum Efficiency (QE) measurement system.

Step-by-Step Methodology:

  • Sub-cell Integration: Mechanically stack or monolithically integrate the passivated perovskite top cell with the silicon bottom cell. Use a recombination layer or tunnel junction as needed.
  • Top Electrode Deposition: Deposit a transparent conductive oxide (e.g., ITO) via sputtering to form the top electrode.
  • Current-Voltage (I-V) Testing: Characterize the tandem device under a standard solar simulator (AM 1.5G) to obtain the PCE, Voc, Jsc, and FF.
  • Quantum Efficiency (QE) Measurement: Perform EQE (External Quantum Efficiency) measurements on the tandem device to verify the current matching between the top and bottom sub-cells.

Key Performance Metrics:

  • Target Voc > 2.0 V [29].
  • Target PCE > 31.0 % [29].

Core Concepts: Passivation for Performance

FAQ: What is a dual-site symmetric passivator and why is it used?

A: A dual-site symmetric passivator is a molecule designed with an identical functional group on opposite sides of a symmetrical core structure. This architecture allows a single molecule to bind simultaneously to two undercoordinated lead (Pb²⁺) ion defects on the perovskite surface. In inorganic perovskites like CsPbI₂Br, these defects are a primary source of non-radiative recombination, which drastically reduces solar cell efficiency and stability. The symmetrical, dual-site binding provides more uniform and stable defect passivation compared to single-site or asymmetric molecules [32].

FAQ: How does this approach reduce non-radiative recombination?

A: Non-radiative recombination occurs when charge carriers (electrons and holes) are trapped by defects and release their energy as heat instead of light or electricity. Undercoordinated Pb²⁺ sites are a common trap state. Dual-site passivators like PID directly coordinate with these Pb²⁺ ions using lone pair electrons from their carbonyl (C=O) oxygen atoms. This binding neutralizes the trap states, preventing them from capturing charge carriers. This process directly suppresses non-radiative recombination pathways, leading to a higher open-circuit voltage (VOC) and overall power conversion efficiency (PCE) [32] [33].

Troubleshooting Guides

Issue: Inconsistent Film Morphology and Grain Size

Problem: Perovskite films exhibit small, uneven grains with numerous pinholes after introducing the passivator.

  • Potential Cause 1: Incorrect concentration of passivator in the precursor solution.
    • Solution: Titrate the concentration of the passivator. A concentration that is too low will not effectively modulate crystallization, while one that is too high can disrupt crystal growth. Follow optimized protocols, such as using 3 mg mL⁻¹ in the anti-solvent [34].
  • Potential Cause 2: Incompatible solvent system or overly rapid crystallization kinetics.
    • Solution: Ensure the passivator is fully soluble in the chosen solvent (e.g., Dimethylformamide, Dimethyl sulfoxide) or anti-solvent (e.g., Chlorobenzene). The introduction of the passivator should slow down nucleation, allowing for the growth of larger, more uniform grains [24].

Issue: Insufficient Defect Passivation

Problem: Device performance shows minimal improvement in VOC and fill factor, indicating persistent non-radiative recombination.

  • Potential Cause 1: Weak binding affinity between the passivator and the perovskite surface.
    • Solution: Select passivator molecules with functional groups proven to have strong Lewis base character, such as carbonyls (C=O) or cyano-groups (C≡N). Quantum chemical calculations can predict adsorption energy prior to synthesis [32] [34].
  • Potential Cause 2: Asymmetric molecular structure leading to uneven passivation.
    • Solution: Design or select molecules with a symmetrical structure, such as PID. Symmetry enables balanced, dual-site chelation with undercoordinated Pb²⁺, which provides stronger and more uniform defect coverage compared to single-site binders like BID [32].

Issue: Poor Long-Term Stability

Problem: Devices degrade rapidly despite initial performance improvement.

  • Potential Cause: Passivator molecules are volatile or desorb from the perovskite surface over time.
    • Solution: Employ multidentate, hyperbranched polymers or molecules with high thermal stability. The multiple binding sites in a hyperbranched structure (e.g., HPSiM polymer) create a stronger "chelation" effect, anchoring the passivator more firmly to the surface and enhancing long-term stability [24].

Experimental Protocols & Data

Synthesis of a Model Dual-Site Passivator (PID)

The following protocol is adapted from the synthesis of 2,2′-(1,3-phenylene)bis(1H-isoindole-1,3(2H)-dione) (PID) [32].

Principle: A condensation reaction between phthalic anhydride and a diamine linker.

Materials:

  • Phthalic anhydride
  • m-phenylenediamine
  • Acetic acid (glacial) as solvent and catalyst
  • Deionized water
  • Standard laboratory equipment: round-bottom flask, condenser, magnetic stirrer, hot plate, Buchner funnel.

Step-by-Step Procedure:

  • Reaction: In a 100 mL round-bottom flask, dissolve phthalic anhydride (X mmol) and m-phenylenediamine (Y mmol, in a 2:1 molar ratio) in 30 mL of glacial acetic acid.
  • Reflux: Attach a condenser and heat the mixture to reflux (~118 °C) with continuous stirring for 4-6 hours.
  • Precipitation: After the reaction time, allow the mixture to cool to room temperature. Slowly pour the reaction mixture into 150 mL of cold deionized water with vigorous stirring to precipitate the crude product.
  • Isolation: Collect the solid precipitate by vacuum filtration using a Buchner funnel.
  • Purification: Wash the solid thoroughly with cold water and recrystallize from a suitable solvent (e.g., ethanol or DMF/ethanol mixture) to obtain pure PID as a crystalline solid.
  • Characterization: Confirm the chemical structure using ¹H Nuclear Magnetic Resonance (NMR) spectroscopy [32].

Protocol for Passivating CsPbI₂Br Perovskite Films

This method details the incorporation of PID during film fabrication [32].

Materials:

  • Cesium iodide (CsI), Lead(II) iodide (PbI₂), Lead(II) bromide (PbBr₂)
  • Dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO) as solvents
  • Synthesized PID passivator
  • Chlorobenzene (CB) as anti-solvent

Step-by-Step Procedure:

  • Precursor Solution Preparation: Dissolve the CsPbI₂Br precursor salts (e.g., CsI, PbI₂, PbBr₂) in a mixed solvent of DMF and DMSO to a specific concentration (e.g., 1.2 M). Stir at room temperature until fully dissolved.
  • Passivator Addition: Prepare a separate solution of the PID passivator in DMSO (e.g., 0.5-1.5 mg mL⁻¹) and add it to the main perovskite precursor solution. Alternatively, dissolve PID directly in the anti-solvent (e.g., CB at 3 mg mL⁻¹) for anti-solvent engineering.
  • Film Deposition: Spin-coat the precursor solution onto the substrate.
  • Anti-Solvent Quenching: During the last 5-10 seconds of the spin-coating process, rapidly drop-cast the PID-containing chlorobenzene solution (e.g., 130 µL) onto the spinning film.
  • Annealing: Immediately transfer the film to a hotplate. Use a two-step annealing process: first at a low temperature (e.g., 55°C for 50 seconds) to control solvent evaporation, followed by a high-temperature anneal (e.g., 255°C for 50 seconds) to form the crystalline, black-phase CsPbI₂Br perovskite.

Quantitative Performance Data

The tables below summarize key experimental data from studies on dual-site and multidentate passivation, providing benchmarks for expected outcomes.

Table 1: Performance Comparison of Passivated Inorganic PSCs

Passivation Strategy Power Conversion Efficiency (PCE) Open-Circuit Voltage (VOC) Stability Retention Citation
Dual-site Symmetric Passivator (PID) on CsPbI₂Br 14.39% (Champion) 1.308 V 88% after 75 days (unencapsulated) [32]
Unpassivated Control (CsPbI₂Br) ~13.44% (Baseline) ~1.10 V (Baseline) Not Specified [32]
Hyperbranched Polymer (HPSiM) in inverted PSCs 25.38% (Champion) Not Specified 91.6% after 1000h MPPT @ 55°C [24]
Lewis Base Small Molecule (6TIC-4F) on CsPbIₓBr₃₋ₓ 16.1% (Champion) 1.16 V (after passivation) Improved photostability [34]

Table 2: Optoelectronic and Morphological Improvements with PID Passivation

Characterization Metric Unpassivated CsPbI₂Br Film PID-Passivated CsPbI₂Br Film Significance
Average Grain Size ~110 nm ~200 nm Larger grains reduce grain boundary defects [32].
Trap State Density Higher Significantly Reduced Directly suppresses non-radiative recombination [32].
Charge Carrier Lifetime Shorter Prolonged Indicates reduced charge recombination losses [32].

Visualization of Mechanisms and Workflows

Molecular Passivation Mechanism

G PerovskiteSurface Perovskite Surface (With Undercoordinated Pb²⁺ Defects) SymmetricPassivator Dual-Site Symmetric Passivator (e.g., PID Molecule) PerovskiteSurface->SymmetricPassivator Binds via Dual C=O Groups NonRadiativeRecombination High Non-Radiative Recombination PerovskiteSurface->NonRadiativeRecombination Leads to PassivatedSurface Passivated Surface (Reduced Trap States) SymmetricPassivator->PassivatedSurface Forms ImprovedPerformance High V<sub>OC</sub> & Efficiency PassivatedSurface->ImprovedPerformance Results in

Diagram Title: How Symmetric Passivators Suppress Recombination

Experimental Workflow for Film Preparation

G Step1 1. Synthesize/Select Passivator (e.g., PID) Step2 2. Prepare Perovskite Precursor Solution Step1->Step2 Step3 3. Add Passivator to Precursor or Anti-Solvent Step2->Step3 Step4 4. Spin-Coat Film & Anti-Solvent Quench Step3->Step4 Step5 5. Two-Step Annealing (Low Temp → High Temp) Step4->Step5 Step6 6. Characterize Film & Device (PXRD, SEM, J-V, PL) Step5->Step6

Diagram Title: Passivated Perovskite Film Fabrication Steps

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Materials for Dual-Site Passivation Experiments

Reagent / Material Function / Role Key Characteristics & Notes
Phthalic Anhydride Core building block for synthesizing PID-like passivators. Provides the carbonyl (C=O) functional groups for Pb²⁺ coordination [32].
m-Phenylenediamine Aromatic linker for symmetric passivator synthesis. Creates the symmetrical, dual-site structure in molecules like PID [32].
Cesium Salts (CsI, CsBr) 'A-site' cation precursor in all-inorganic perovskites. Provides thermal stability versus organic cations (e.g., MA⁺, FA⁺) [35].
Lead Halides (PbI₂, PbBr₂) 'B-site' and 'X-site' precursors in the perovskite structure. Source of undercoordinated Pb²⁺ defects that require passivation [32] [34].
Dimethylformamide (DMF) Primary solvent for perovskite precursors. High dissolving power for perovskite salts. Must be anhydrous for reproducible results.
Chlorobenzene (CB) Anti-solvent for film crystallization. Used to dissolve and deliver the passivator during the spin-coating process [32] [34].
Hyperbranched Polymers (e.g., HPSiM) Multidentate polymeric passivators. Offer multiple, strong binding sites and enhanced stability for bulk defect passivation [24].

Ligand Exchange Strategies for Perovskite Nanocrystals and Quantum Dots

FAQs: Addressing Common Experimental Challenges

FAQ 1: Why does ligand exchange often lead to a severe drop in the photoluminescence quantum yield (PLQY) of my perovskite nanocrystal (PNC) film, and how can I prevent this?

A significant PLQY drop typically indicates the introduction of surface defects, such as halide vacancies, during the ligand exchange process. This occurs when the new ligands fail to adequately coordinate with the surface metal ions, leaving unpassivated sites that act as non-radiative recombination centers.

  • Solution: Employ multidentate ligands that have a stronger binding affinity to the PNC surface. For example:
    • Using Formamidine thiocyanate (FASCN), a bidentate liquid ligand, provides a binding energy fourfold higher than that of original oleate ligands, ensuring effective passivation and a notable improvement in PLQY [36].
    • Treating PNCs with ethylene diamine tetraacetic acid (EDTA), a multidentate ligand, can effectively passivate surface defects by occupying I- vacancies and peeling away suspended Pb²⁺ ions, leading to suppressed non-radiative recombination [37].

FAQ 2: The conductivity of my PNC solid film remains poor after ligand exchange. What is the underlying issue?

Poor conductivity often results from insufficient removal of long, insulating native ligands (like oleic acid and oleylamine) and/or a failure to achieve full surface coverage with the new short ligands. This creates large interparticle distances and poor electronic coupling.

  • Solution: Implement a ligand exchange strategy that maximizes the removal of insulating ligands while ensuring the PNC surface remains passivated.
    • A solvent-mediated approach using a protic solvent like 2-pentanol can effectively strip long-chain oleylamine ligands without introducing halogen vacancy defects, significantly improving charge transport [38].
    • Short-chain bidentate ligands like succinic acid (SA) or FASCN reduce interparticle spacing and can increase film conductivity by up to eightfold compared to control films [39] [36].

FAQ 3: How can I improve the environmental stability (against heat and moisture) of PNC films after ligand exchange?

Instability arises from labile ligand binding and incomplete surface coverage, which exposes the ionic perovskite surface to degradation. Ligand desorption creates pathways for water and oxygen ingress.

  • Solution: Utilize ligands that offer tight, multidentate binding to achieve a fully passivated and compact surface.
    • Films treated with FASCN show excellent thermal stability with no observable emission shift after continuous heating at 100°C, whereas control films exhibit significant shifts. They also withstand high humidity (>99%) for over 30 minutes without corrosion [36].
    • Multidentate ligands like EDTA can crosslink adjacent PNCs, creating a more robust and stable network [37].

FAQ 4: My ligand-exchanged PNC films suffer from low open-circuit voltage (VOC) and fill factor (FF) in solar cell devices. How can I address this?

Low VOC and FF are frequently caused by non-radiative recombination at the interface between the PNC film and the charge transport layer, often due to surface defects induced by an uncontrolled crystallization process and stoichiometric imbalance on the PNC surface.

  • Solution: Adopt a surface reconstruction strategy that combines chemical polishing and passivation.
    • A combination of 1,4-butanediamine (BDA) as a polishing agent and ethylenediammonium diiodide (EDAI₂) as a passivator can eliminate Sn-related defects and passivate organic cation and halide vacancies on Sn–Pb mixed perovskite films. This strategy minimizes non-radiative energy loss, leading to increased VOC and FF, and has achieved a certified power conversion efficiency of 28.49% in all-perovskite tandem solar cells [6].

Experimental Protocols for Key Ligand Exchange Strategies

Objective: To replace long-chain insulating ligands (OAm/OA) with short-chain MPA to enhance charge transport in doctor-bladed CsPbBr₃ PNC films.

Materials:

  • CsPbBr₃ PNC film deposited on a substrate via doctor blading.
  • Ligand Solution: 0.1% v/v of 3-mercaptopropionic acid (MPA) in anhydrous ethanol.
  • Wash Solvent: Anhydrous ethanol.

Procedure:

  • Film Preparation: Deposit a thin film of CsPbBr₃ PNCs using the doctor blading technique.
  • Ligand Exchange: Immerse the freshly prepared PNC film into the MPA/ethanol solution for 60 seconds. Gently agitate the container to ensure uniform contact.
  • Washing: Immediately remove the film and rinse it thoroughly with a stream of pure anhydrous ethanol to remove the displaced ligands and excess MPA.
  • Drying: Dry the film under a gentle stream of nitrogen gas.

Troubleshooting Tip: The immersion time is critical. Too short a time results in incomplete exchange, while too long can damage the film. Optimize the time (between 30-90 seconds) based on initial film thickness and quality.

Objective: To passivate surface defects and crosslink PQDs using a multidentate ligand for improved performance in solar cells.

Materials:

  • Purified CsPbI₃ PQDs in toluene (~10 mg/mL).
  • Ligand Solution: EDTA in a suitable solvent (e.g., DMF or DMSO) at a concentration of 5 mg/mL.
  • Non-Solvent: Toluene or ethyl acetate for precipitation.

Procedure:

  • PQD Preparation: Synthesize and purify CsPbI₃ PQDs capped with oleate ligands using standard methods.
  • Mixing: Add the EDTA solution dropwise to the PQD solution under vigorous stirring. The typical volume ratio of EDTA solution to PQD solution is 1:5.
  • Reaction: Continue stirring the mixture for 30 minutes at room temperature to allow EDTA to bind to the PQD surface.
  • Purification: Precipitate the EDTA-treated PQDs by adding an excess of non-solvent (toluene/ethyl acetate) and centrifuging at 8000 rpm for 5 minutes.
  • Redispersion: Discard the supernatant and redisperse the PQD pellet in a fresh anhydrous solvent (e.g., octane) for film deposition.

Troubleshooting Tip: Ensure the EDTA is fully dissolved before adding it to the PQD solution. Incomplete dissolution can lead to heterogeneous passivation and the formation of aggregates.

Data Presentation: Performance of Ligand Strategies

Table 1: Comparative Performance of Different Ligand Exchange Strategies in Photovoltaic Devices

Ligand Strategy Material System Key Performance Metric Reported Value Control Value Reference
BDA + EDAI₂ (Surface Reconstruction) Sn-Pb Mixed Perovskite (Tandem Solar Cell) Certified Power Conversion Efficiency (PCE) 28.49% Not Specified [6]
EDTA (Multidentate Passivation) CsPbI₃ PQD Solar Cell Power Conversion Efficiency (PCE) 15.25% ~10% (typical for oleate-capped) [37]
MPA (Solid-State Exchange) CsPbBr₃ PNC Photodetector Charge Carrier Mobility 5 × 10⁻³ cm²/Vs Lower (unspecified) [40]
FASCN (Bidentate Liquid Ligand) FAPbI₃ QD NIR-LED External Quantum Efficiency (EQE) ~23% ~11.5% (control) [36]

Table 2: Optical and Electronic Properties Achieved with Advanced Ligands

Ligand Binding Energy (eV) Photoluminescence Quantum Yield (PLQY) Exciton Binding Energy (meV) Function / Key Advantage
FASCN -0.91 [36] Most notable improvement vs. control [36] 76.3 [36] Tight binding, high conductivity, full surface coverage
Oleate (OA) -0.22 [36] Baseline 39.1 (control) [36] Standard long-chain ligand (reference)
Succinic Acid (SA) Stronger than OA [39] Significant improvement [39] Not Specified Bidentate binding, improved water compatibility
Metal Salts (e.g., In³⁺) Not Specified 97% (Red CdSe/ZnS NCs) [41] Not Specified Generates intensely luminescent all-inorganic NCs

Workflow and Relationship Diagrams

Ligand Exchange Workflow

Start Start: As-Synthesized PNCs with Long Insulating Ligands A Select Ligand & Solvent Start->A B Perform Exchange (Solid-state or Solution) A->B C Purify & Wash B->C D Characterize Film (PLQY, Mobility, Stability) C->D E Successful? D->E E->A No Optimize F End: Functional PNC Film for Device Integration E->F Yes

Ligand Function & Recombination

Ligand Multidentate Ligand SubGoal1 Strong Binding & Full Coverage Ligand->SubGoal1 SubGoal2 Defect Passivation (Fills Halide Vacancies) Ligand->SubGoal2 SubGoal3 Reduced Interparticle Distance Ligand->SubGoal3 FinalGoal Suppressed Non-Radiative Recombination SubGoal1->FinalGoal SubGoal2->FinalGoal SubGoal3->FinalGoal

The Scientist's Toolkit: Essential Research Reagents

Table 3: Key Reagents for Ligand Exchange Experiments

Reagent / Material Function / Role in Experiment Key Characteristic
3-Mercaptopropionic Acid (MPA) Short-chain, bifunctional linker for solid-state ligand exchange. The thiol (-SH) group coordinates to Pb²⁺, while the carboxyl group can interact with other surfaces. Improves charge transport via reduced interparticle spacing [40].
Ethylenediammonium Diiodide (EDAI₂) A passivator used in conjunction with a polishing agent to eliminate organic cation and halide vacancy defects on perovskite film surfaces. Effective at passivating VA and VI related defects [6].
Formamidine Thiocyanate (FASCN) A bidentate, liquid ligand used for post-synthetic treatment. The SCN⁻ anion coordinates strongly to Pb²⁺ sites. High binding energy, liquid at room temperature, enables full surface coverage [36].
Ethylenediaminetetraacetic Acid (EDTA) A multidentate ligand used to resurface PQDs. It chelates and removes suspended Pb²⁺ ions and passivates I⁻ vacancies. Can crosslink PQDs to improve electronic coupling [37].
1,4-Butanediamine (BDA) Acts as a chemical polishing agent for Sn-Pb mixed perovskite films. Interacts with perovskite via Lewis acid-base coordination and hydrogen bonding. Reduces Sn⁴⁺-related surface defects and modulates surface stoichiometry [6].
2-Pentanol A protic solvent tailored for mediating ligand exchange on PQD solid films. Appropriate dielectric constant and acidity to remove insulating ligands without introducing defects [38].

Overcoming Practical Challenges in Multidentate Ligand Implementation

Troubleshooting Guides

Guide: Correcting Inaccurate Adsorption Energy Predictions

Problem: My computational simulations for predicting the most stable adsorption configuration (parallel vs. vertical) do not match subsequent experimental validation.

  • Potential Cause 1: Inaccuracies in the density functional theory (DFA) exchange-correlation functional.
    • Solution: Employ more advanced, high-accuracy computational frameworks like the autoSKZCAM framework, which leverages correlated wavefunction theory (cWFT) such as CCSD(T). This method has been shown to reproduce experimental adsorption enthalpies (Hads) for diverse systems accurately and can resolve debates on adsorption configurations, unlike some DFAs which may predict the wrong stable configuration or fortuitously match experimental Hads for a metastable state [42].
  • Potential Cause 2: Neglecting many-body correlation effects, spin states, and multiple molecular geometries.
    • Solution: For critical benchmarks, use high-fidelity methods like Diffusion Monte Carlo (DMC) calculations. DMC has proven to provide a significantly different and more accurate landscape of adsorption energetics compared to standard DFT, including correctly identifying the lowest-energy configurations and spin states, which are crucial for predicting catalytic behavior and avoiding pitfalls like CO poisoning [43].

Guide: Achieving Specific Molecular Orientation at a Donor/Acceptor Interface

Problem: Inability to control whether organic donor molecules adopt a face-on (parallel) or edge-on (vertical) orientation relative to the acceptor interface, leading to unpredictable device performance.

  • Potential Cause: Uncontrolled crystallization and film formation processes.
    • Solution: Precisely control the film fabrication process. For the conjugated polymer p-SIDT(FBTTh2)2:
      • To induce a Face-on orientation: Cast the film from chlorobenzene (CB) solvent. This yielded a ratio of 99.5:0.5 face-on to edge-on orientation [44].
      • To induce an Edge-on orientation: Cast the film from chlorobenzene (CB) with a 0.4% v/v additive of diiodooctane (DIO). This yielded a ratio of 94:6 edge-on to face-on orientation [44].
    • Verification: Use Grazing Incidence Wide-Angle X-Ray Scattering (GIWAXS) to quantify the bulk and interfacial molecular orientation by comparing the intensities of in-plane and out-of-plane π-stacking peaks [44].

Guide: Minimizing Non-Radiative Recombination at Interfaces

Problem: High non-radiative recombination losses at the interface between a perovskite (or other active material) and a charge transport layer, leading to reduced open-circuit voltage (VOC) and efficiency.

  • Potential Cause: Surface defects induced during the uncontrollable crystallization process, such as uncoordinated Pb2+/Sn2+/Sn4+ ions and organic cation/halide vacancies [6] [33].
    • Solution: Implement a surface reconstruction strategy.
      • Chemical Polishing: Use 1,4-Butanediamine (BDA) to chemically polish the perovskite surface. BDA interacts strongly with the perovskite via Lewis acid-base coordination with Pb2+/Sn2+ and hydrogen bonding with organic cations (e.g., FA+), effectively reducing Sn4+-related defects and creating a surface with a close-to-ideal stoichiometric ratio [6].
      • Multidentate Passivation: Apply a multidentate passivator, such as a hyperbranched polysiloxane with maleic acid structure (HPSiM). The abundant carbonyl (C=O) groups on its flexible, branched framework enable strong multidentate chelation with Pb2+ ions, effectively passivating defects over a wide range on grain boundaries and surfaces [24].
    • Solution: Leverage orientation-dependent properties. If using an organic donor layer, note that devices with donor molecules in a face-on orientation relative to the acceptor have been shown to have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages (VOC) [44].

Frequently Asked Questions (FAQs)

Q1: Why is controlling adsorption orientation (parallel vs. vertical) so critical for device performance?

  • A1: The orientation of molecules at an interface directly governs key electronic processes. In organic photovoltaics, a face-on (parallel) orientation leads to higher open-circuit voltage (VOC) due to reduced non-radiative recombination, while an edge-on (vertical) orientation results in more efficient charge generation [44]. In catalysis, the adsorption configuration (e.g., tilted vs. parallel) dictates the reaction pathway and activation energy for bond cleavage [42] [45].

Q2: What are the key characterization techniques to verify molecular orientation and surface quality?

  • A2:
    • Grazing Incidence Wide-Angle X-Ray Scattering (GIWAXS): Essential for quantifying molecular orientation in thin films (e.g., face-on vs. edge-on) by analyzing π-stacking peak directions [44].
    • X-ray Photoelectron Spectroscopy (XPS): Used to determine the surface stoichiometry and chemical state of elements (e.g., quantifying the Sn4+/Sn2+ ratio on perovskite surfaces) [6].
    • Kelvin Probe Force Microscopy (KPFM): Measures the contact potential difference (CPD) to map surface potential inhomogeneities, which indicate the presence of defects. A uniform CPD distribution after treatment confirms improved surface quality [6].

Q3: My goal is to maximize charge generation efficiency. Should I aim for a parallel or vertical configuration?

  • A3: Based on studies of organic bilayers, you should prioritize achieving a vertical (edge-on) configuration. Devices with donor molecules edge-on to the acceptor interface have been demonstrated to be more efficient at charge generation, attributed to smaller electronic coupling between charge transfer states and the ground state [44].

Q4: How can I computationally determine the most stable adsorption configuration for a new molecule-surface system?

  • A4:
    • Model the Surface: Use a sufficiently large slab model with adequate vacuum layers to avoid periodic interactions [45].
    • Explore Multiple Configurations: Systematically test various adsorption sites (atop, bridge, hollow), orientations (parallel, vertical, tilted), and for open-shell molecules, different spin states [42] [43].
    • Calculate Adsorption Energy: For each configuration, compute the adsorption energy (Eads) using the formula: E_ads = E_total - E_surface - E_molecule [45]. The most negative Eads typically indicates the most stable configuration.
    • Use Accurate Methods: For reliable results, especially with ionic materials, consider using advanced frameworks like autoSKZCAM (cWFT) instead of standard DFAs to avoid incorrect predictions [42].

The following tables consolidate key quantitative data from research findings to aid in experimental planning and analysis.

Table 1: Orientation-Dependent Photovoltaic Parameters

Molecular Orientation Open-Circuit Voltage (VOC) Charge Generation Efficiency Key Observation
Face-on (Parallel) Higher (e.g., +150 mV) [44] Lower [44] Higher charge transfer state energy; Less non-radiative recombination [44]
Edge-on (Vertical) Lower [44] Higher (e.g., higher Internal Quantum Efficiency) [44] Smaller electronic coupling between CT state and ground state [44]

Table 2: Adsorption Energies and Configurations from Computational Studies

Adsorbate-Surface System Preferred Configuration Adsorption Energy (Eads) Method Key Finding
O2 on Pt/graphene V'-mode (tilted side-on, singlet state) [43] -1.23(2) eV [43] Diffusion Monte Carlo (DMC) DMC corrects DFT, identifies different lowest-energy spin/geometry [43]
CO on Pt/graphene Not Specified -3.37(1) eV [43] Diffusion Monte Carlo (DMC) Large disparity with O2 energy suggests CO poisoning risk [43]
Benzene on α-Fe2O3 (01 1 2) Chemisorbed configurations [45] -0.67 eV to -1.44 eV (dependent on specific site) [45] DFT+U Chemisorption enables C-C bond cleavage; Physisorption on (0001) surface is inactive [45]
NO on MgO(001) Dimer cis-(NO)2 configuration ('dimer Mg') [42] Consistent with experiment [42] autoSKZCAM (cWFT) Resolved debate; identified dimer, not monomer, as most stable [42]

Table 3: Reagent Solutions for Surface Passivation and Orientation Control

Research Reagent Function / Role in Experiment Key Mechanism of Action
1,4-Butanediamine (BDA) Chemical polishing agent [6] Lewis acid-base coordination with Pb2+/Sn2+ and hydrogen bonding with FA+; reduces Sn4+ defects and modulates surface stoichiometry [6]
Hyperbranched Polysiloxane (HPSiM) Multidentate chelating passivator [24] Abundant carbonyl (C=O) groups on a hyperbranched framework enable wide-range, multi-site chelation with Pb2+ ions, passivating defects at grain boundaries and surfaces [24]
Diiodooctane (DIO) Solvent additive for orientation control [44] Promotes self-assembly of specific conjugated polymers (e.g., p-SIDT(FBTTh2)2) into an edge-on (vertical) orientation relative to the substrate [44]
Ethylenediammonium Diiodide (EDAI2) Surface passivator [6] Passivates organic cation (VA) and halide vacancy (VI) related defects on the perovskite surface [6]

Experimental Protocols

Protocol: Surface Reconstruction for Defect Minimization in Sn-Pb Perovskite Films

This protocol is adapted from research demonstrating reduced non-radiative recombination and improved device efficiency [6].

Materials:

  • Prepared narrow-bandgap Sn-Pb mixed perovskite film (e.g., FA0.7MA0.3Pb0.7Sn0.3I3).
  • 1,4-Butanediamine (BDA) in Isopropyl Alcohol (IPA) (0.1 mg mL-1).
  • Ethylenediammonium Diiodide (EDAI2) solution.

Procedure:

  • Film Preparation: Deposit the Sn-Pb mixed perovskite precursor solution onto your substrate using your standard method (e.g., spin-coating).
  • Annealing: Anneal the film to form the crystalline perovskite layer.
  • Chemical Polishing:
    • Immediately after annealing and while the film is still hot, dynamically spin-coat the 0.1 mg mL-1 BDA/IPA solution onto the perovskite surface.
    • The strong interaction of BDA with the perovskite will chemically polish the surface, reducing Sn-richness and Sn4+ content.
  • Surface Passivation:
    • Without delay, dynamically spin-coat the EDAI2 solution onto the BDA-treated surface.
    • EDAI2 will passivate the remaining organic cation and halide vacancies.
  • Post-treatment: Anneal the film again at a low temperature (e.g., 100°C for 5 minutes) to remove residual solvent and complete the surface reconstruction.

Validation:

  • Use XPS to confirm the improved I/(Pb+Sn) stoichiometric ratio and reduced Sn4+ percentage.
  • Use KPFM to observe a more uniform surface potential distribution.

Protocol: Controlling Molecular Orientation in Organic Planar Heterojunctions

This protocol is based on the method used to create well-defined, abrupt interfaces with controlled orientation for the polymer p-SIDT(FBTTh2)2 [44].

Materials:

  • Conjugated polymer (e.g., p-SIDT(FBTTh2)2).
  • Solvent A: Chlorobenzene (CB).
  • Solvent B: Chlorobenzene (CB) with 0.4% v/v Diiodooctane (DIO) additive.
  • Acceptor material (e.g., C60).

Procedure:

  • Solution Preparation:
    • For Face-on orientation: Dissolve the polymer in pure chlorobenzene (CB).
    • For Edge-on orientation: Dissolve the polymer in chlorobenzene with 0.4% v/v DIO additive (CB+DIO).
  • Film Deposition:
    • Spin-coat the respective solution onto a prepared substrate (e.g., ITO/PEDOT:PSS) to form a neat polymer film.
  • Interface Formation:
    • Thermally evaporate a layer of C60 (or your chosen acceptor) onto the polymer film under high vacuum.
  • Complete Device Fabrication: Continue with the deposition of the remaining charge transport layers and electrodes.

Validation:

  • Perform GIWAXS on the neat polymer films (before acceptor deposition) to confirm the dominant orientation (face-on for CB, edge-on for CB+DIO).
  • Use cross-section TEM and detailed GIWAXS analysis of the bilayer to confirm the formation of an abrupt, non-intermixed donor/acceptor interface.

Conceptual Diagrams

Orientation Impact on Device Properties

G cluster_orient Molecular Orientation at Interface cluster_faceon Consequences cluster_edgeon Consequences FaceOn Face-On (Parallel) F1 Higher CT State Energy FaceOn->F1 EdgeOn Edge-On (Vertical) E1 Smaller CT/GS Electronic Coupling EdgeOn->E1 F2 Less Non-Radiative Recombination F1->F2 F3 Higher Open-Circuit Voltage (VOC) F2->F3 E2 More Efficient Charge Generation E1->E2 E3 Higher Short-Circuit Current (JSC) E2->E3

Surface Passivation Workflow

G Start Perovskite Film with Defects Step1 Step 1: Chemical Polishing Apply BDA/IPA Solution Start->Step1 Mech1 Mechanism: Lewis acid-base coordination & H-bonding Step1->Mech1 Step2 Step 2: Multidentate Passivation Apply HPSiM Polymer Mech2 Mechanism: Multidentate chelation with Pb2+ Step2->Mech2 Result Result: High-Quality Surface - Reduced Sn4+ Defects - Passivated VA/VI Defects - Minimal Non-Radiative Loss Mech1->Step2 Mech2->Result

Adsorption Configuration Determination

G cluster_comp Computational Workflow cluster_exp Experimental Techniques Comp Computational Prediction C1 1. Model Surface & Adsorbate Comp->C1 Exp Experimental Validation E1 GIWAXS (Orientation) Exp->E1 C2 2. Test Multiple: - Sites (atop, bridge...) - Orientations (||, ⟂, tilted) - Spin States C1->C2 C3 3. Calculate Eads for each Eads = E_total - E_surface - E_molecule C2->C3 C4 4. Identify most stable configuration (most negative Eads) C3->C4 C4->E1 C5 Use cWFT/DMC for accuracy beyond standard DFT C5->C3 E2 XPS / UPS (Stoichiometry / Energy Levels) E1->E2 E3 KPFM (Surface Potential) E2->E3 E4 TPD / FTIR (Adsorption Strength) E3->E4

Technical Support Center

Troubleshooting Guides

Troubleshooting Guide 1: Managing Conductivity and Stability in Lead Halide Perovskite Nanocrystals (NCs)

Problem Statement: Researchers often encounter a trade-off where effective surface passivation by long-chain insulating ligands leads to poor charge transport, hindering device performance [46].

  • Issue: Poor charge transport in NC films due to insulating native ligands.
  • Symptoms: High-performance in single NCs but significant voltage deficits or fill factor losses in fabricated solar cells.
  • Root Cause: Conventional ligands like long-chain alkylamines and carboxylic acids impart colloidal stability but create insulating barriers between NCs [46].

Solution: Implement a ligand engineering strategy to replace native ligands with shorter or conjugated alternatives.

Strategy Ligand Examples Mechanism Key Outcome
Short-Chain Ligands Butylamine [46] Reduces carbon chain length to decrease interparticle distance Improved charge carrier mobility
Conjugated Ligands Not specified in results Provides a pathway for electronic coupling between NCs Enhanced interparticle conductivity
Multidentate Anchors TPPO [47], H2FBP [48] Strong binding via multiple donor sites (e.g., P=O, -COOH) Reduces ligand density required for passivation Superior defect passivation and stability without sacrificing conductivity

Preventive Measures:

  • For NC inks, prioritize ligands that balance dispersibility and conductivity.
  • For solid-state films, employ post-synthetic ligand exchange to replace long-chain ligands after film formation.
Troubleshooting Guide 2: Addressing Voltage Deficits and Non-Radiative Recombination

Problem Statement: Unpassivated surface defects, such as undercoordinated Pb²⁺ ions and halide vacancies, act as traps that cause non-radiative recombination, leading to significant open-circuit voltage (VOC) deficits [47] [49].

  • Issue: Non-radiative recombination at interfacial defects.
  • Symptoms: Lower-than-theoretical VOC, reduced power conversion efficiency (PCE), and shorter charge carrier lifetimes.
  • Root Cause: Incomplete surface coordination during perovskite crystallization [49].

Solution: Employ molecular passivation agents with functional groups that coordinate strongly with surface defects.

Passivation Agent Functional Group Target Defect Quantified Improvement
TPPO [47] P=O Uncoordinated Pb²⁺ Champion PCE: 26.01%, VOC: 1.23 V (VOC deficit: 0.32 V)
N-Bromosuccinimide (NBS) [49] C=O, Br Uncoordinated Pb²⁺, Halide vacancies PCE: 18.24% (11% relative increase), VOC gain: 40 mV
H2FBP [48] -COOH Uncoordinated Pb²⁺, Grain boundaries PCE: 24.90%

Preventive Measures:

  • Introduce passivation molecules during the film fabrication process or as a post-treatment.
  • For multidentate ligands like H2FBP, ensure the molecular structure allows for effective anchoring without causing excessive strain [48].

Frequently Asked Questions (FAQs)

FAQ 1: What are the most effective functional groups for passivating common defects in perovskite materials?

Functional groups that act as Lewis bases are highly effective. The table below lists proven groups and their binding targets.

Functional Group Example Ligand Primary Defect Passivated Evidence of Efficacy
Phosphine Oxide (P=O) TPPO [47] Uncoordinated Pb²⁺ XPS-confirmed binding energy shift; 90% PCE retention after 1200h operation [47].
Carbonyl (C=O) NBS [49] Uncoordinated Pb²⁺ 11% relative PCE increase; 40 mV VOC gain [49].
Carboxyl (-COOH) H2FBP [48] Uncoordinated Pb²⁺ PCE of 24.90%; 89% PCE retention after 1680h [48].

FAQ 2: How do conjugated ligands help mitigate insulating effects?

Conjugated ligands, with their systems of delocalized π-electrons, facilitate electron wavefunction overlap between adjacent nanocrystals or at the perovskite/charge transport layer interface. This creates a more continuous pathway for charge carriers (electrons and holes) to travel, directly countering the insulating barrier effect created by saturated aliphatic ligands [46].

FAQ 3: Can a ligand address multiple degradation pathways simultaneously?

Yes, advanced ligand design focuses on multifunctionality. A prime example is the bent ligand H2FBP, which demonstrates a "dual stress-defect" engineering approach [48]:

  • Defect Passivation: Its carboxyl groups anchor to the PbI framework, passivating ionic defects.
  • Stress Release: Its unique bent, flexible conformation acts as a "spring," releasing residual thermal and mechanical stress during annealing.
  • Environmental Protection: Its hydrophobic -C(CF3)2- core forms a protective layer, shielding against moisture and oxygen ingress [48].

The Scientist's Toolkit: Research Reagent Solutions

Reagent / Material Primary Function in Ligand Design Key Consideration
Triphenylphosphine Oxide (TPPO) [47] Anchoring ligand for interface passivation. Strong P=O group coordinates undercoordinated Pb²⁺ at perovskite/ETL interface. Compatible with MA-free perovskites and p-i-n device architecture.
N-Bromosuccinimide (NBS) [49] Interfacial passivator for multi-cation perovskites. C=O groups passivate Pb²⁺; Br can fill halide vacancies. A low-cost, common organic reagent.
H2FBP [48] Multifunctional bent ligand for stress and defect management. -COOH groups passivate defects; bent structure releases stress; fluorinated core adds hydrophobicity. Its V-shape is critical for stress-buffering capability.
Short-Chain Alkylamines (e.g., Butylamine) [46] Replaces long-chain ligands (e.g., Oleylamine) in NC systems to reduce interparticle distance. Balances colloidal stability with improved charge transport in films.
Multidentate Phenanthroline Ligands [50] Provides enhanced (photo)stability for metal complexes in optoelectronics. Additional donor moieties (OR, SR) enable higher coordination modes. Can modify the nature of excited states and their deactivation processes.

Experimental Protocols

Detailed Protocol 1: Surface Passivation with TPPO for Highly Stable Perovskite Solar Cells

This protocol is adapted from the work of Li et al., which achieved a champion PCE of 26.01% with a minimal VOC deficit [47].

Key Principle: Incorporating TPPO at the interface between the perovskite and the electron transport layer (ETL) to passivate uncoordinated Pb²⁺ defects and suppress ion migration.

Materials:

  • Substrate: Glass/ITO/NiOx/SAM/Perovskite
  • Passivation Solution: TPPO in anisole (concentration: 0.5 mg/mL)
  • Electron Transport Layer: PCBM
  • Electrode: Ag

Procedure:

  • Perovskite Film Deposition: Fabricate your methylammonium-free (MA-free) perovskite film atop the SAM/HTL using your standard method (e.g., spin-coating, annealing).
  • TPPO Layer Application: Immediately after the perovskite film is formed and cooled, spin-coat the TPPO in anisole solution directly onto the perovskite surface.
  • Thermal Annealing: Anneal the film at 100°C for 5 minutes to facilitate the anchoring of TPPO molecules onto the perovskite surface.
  • Device Completion: Proceed with the deposition of the PCBM ETL and the Ag electrode as per your standard fabrication process.

Validation and Characterization:

  • X-ray Photoelectron Spectroscopy (XPS): Confirm successful passivation by observing a shift in the Pb 4f core levels to lower binding energies (e.g., from 138.41 eV to 137.75 eV) [47].
  • X-ray Diffraction (XRD): Check for the formation of new complex peaks at 2θ = 4° and 8°, distinct from the perovskite and pure TPPO [47].
  • Device Performance: Measure current density-voltage (J-V) curves to determine PCE, VOC, and voltage deficit.
  • Stability Testing: Perform maximum power point tracking (MPPT) under 1 sun illumination at 65°C to assess long-term operational stability.
Detailed Protocol 2: Interfacial Modification with NBS for Multi-Cation Perovskites

This protocol is based on the research that used NBS to achieve an 11% relative increase in PCE [49].

Key Principle: Using NBS molecules, containing Lewis base carbonyl groups, to passivate surface defects and accelerate charge extraction.

Materials:

  • Substrate: FTO/c-TiO2/mp-TiO2
  • Perovskite Precursor: RbCsMAFA quadruple-cation formulation
  • Treatment Solution: NBS in chlorobenzene (concentration: 1.0 mg/mL)
  • Hole Transport Layer: Spiro-OMeTAD

Procedure:

  • Perovskite Film Deposition: Deposit the RbCsMAFA perovskite film onto the mp-TiO2 layer and complete the annealing process.
  • NBS Solution Application: While the perovskite film is still hot (e.g., ~100°C), dynamically spin-coat the NBS solution in chlorobenzene onto the film.
  • Rinsing: After the spin-coating is complete, rinse the substrate with fresh chlorobenzene to remove any unbound NBS residue.
  • Final Annealing: Anneal the film again at 100°C for 5-10 minutes.
  • Device Completion: Deposit the Spiro-OMeTAD HTL and metal electrode to finish the solar cell.

Validation and Characterization:

  • Photovoltaic Performance: Compare VOC, JSC, FF, and PCE of NBS-treated devices against untreated control devices.
  • Impedance Spectroscopy: Analyze for reduced interfacial losses and improved charge transport.
  • Time-Resolved Photoluminescence (TRPL): Measure carrier lifetimes to confirm suppression of non-radiative recombination.

Concept and Workflow Diagrams

Ligand Passivation Mechanism

G A Perovskite Surface Defects B Uncoordinated Pb²⁺ A->B C Halide Vacancies A->C H Reduced Non-Radiative Recombination B->H I Suppressed Ion Migration C->I D Ligand Functional Groups E Lewis Base (e.g., P=O, C=O) D->E F Coordinate Bond E->F Forms G Ionic Bond / Fill Vacancy E->G Forms F->B G->C J Improved Device Efficiency & Stability H->J I->J

Ligand Selection Workflow

G Start Define Material System & Goal NC Nanocrystal (NC) System? Start->NC A1 Use Short-Chain/Conjugated Ligands NC->A1 Yes TF Thin Film System? NC->TF No A2 Goal: Improve inter-NC charge transport A1->A2 C1 Select Ligand A2->C1 B1 Use Multidentate Anchoring Ligands TF->B1 Yes TF->C1 Unsure B2 Goal: Defect passivation & interface stabilization B1->B2 B2->C1 C2 Consider: TPPO (P=O) NBS (C=O, Br) H2FBP (-COOH) C1->C2 End Proceed to Experimental Implementation C2->End

Troubleshooting Guides

Guide 1: Troubleshooting Ligand Detachment in Solution-Based Passivation

Problem Cause Solution
Few or no ligands remaining on surface after processing Ligands are unstable and detaching from the surface - Ensure the ligand is polydentate (e.g., bidentate, tridentate) to leverage the chelate effect for enhanced stability. [51] [52]- For metal surfaces, use ligands with five-membered chelate rings, which show significantly higher stability than those with four-, six-, or seven-membered rings. [51]- Confirm the ligand has a 5´ phosphate moiety if ligating nucleic acids. [53]
Inconsistent or weak passivation results Non-radiative recombination at the interface due to poor surface coverage or defective ligand binding. - Employ a sequential surface treatment: first use a chemical polishing agent (e.g., 1,4-butanediamine) to create a more uniform, stoichiometric surface, then apply the passivating ligand. [6]- Characterize the surface post-treatment with techniques like XPS and KPFM to verify a uniform surface potential and successful defect passivation. [6]
Ligand detachment during washing or storage steps The ligand-protein/complex interaction is inherently unstable. - Prior to major experiments, computationally assess ligand binding pose stability using Binding Pose Metadynamics (BPMD). Unstable poses under BPMD bias indicate infrequent occupation and minimal contribution to binding affinity. [54]

Guide 2: Troubleshooting Computational Models of Ligand Recognition

Problem Cause Solution
Simulation does not capture correct ligand recognition kinetics The use of long-time-step protocols like Hydrogen Mass Repartitioning (HMR) can artificially accelerate ligand diffusion and slow the recognition process. [55] - For direct simulation of protein-ligand recognition events, use regular MD time steps (1-2 fs) instead of long-time-step HMR to avoid retarding the observed recognition kinetics. [55]
Inaccurate prediction of metal-ligand complex stability Standard force fields may not properly capture the thermodynamics and long timescales of complex formation. [56] - Utilize advanced simulation approaches that combine optimized non-bonded potentials (e.g., 12-6-4 LJ) with enhanced sampling techniques (metadynamics) and Markov State Models to accurately model complex equilibrium and kinetics. [56]

Frequently Asked Questions (FAQs)

General Principles

Q: Why are multidentate ligands preferred for achieving long-term stability against detachment?

A: Multidentate ligands form chelate complexes, which are significantly more stable than complexes with similar monodentate ligands. This "chelate effect" is driven by thermodynamics. When a multidentate ligand binds, fewer molecules are involved in the reaction compared to multiple monodentate ligands, leading to a favorable increase in entropy. [51] [57] Furthermore, the dissociation rates for chelate complexes are often slower, contributing to their superior longevity. [56]

Q: What is the relationship between ligand passivation and non-radiative recombination?

A: In materials like perovskites, non-radiative recombination often occurs at surface defects. Ligands passivate these defects by coordinating with unsaturated bonds (e.g., on Sn or Pb ions). Ineffective passivation, often due to ligand instability or detachment, leaves these defects active, leading to significant energy loss. Effective multidentate ligand passivation minimizes these defects, thereby reducing non-radiative recombination and improving device performance. [6]

Experimental & Characterization Questions

Q: How can I experimentally verify that my ligand is stably bound to the surface?

A: Several techniques can confirm stable ligand binding:

  • X-ray Photoelectron Spectroscopy (XPS): Can detect changes in surface composition and chemical states. A successful, stable passivation will show a surface stoichiometry closer to the ideal bulk ratio and a reduction in oxidative states (e.g., Sn⁴⁺ to Sn²⁺). [6]
  • Kelvin Probe Force Microscopy (KPFM): Measures the surface potential. A uniform surface potential distribution after treatment indicates homogeneous defect passivation and reduced charge-carrier trapping. [6]
  • FTIR and NMR Spectroscopy: Can probe specific chemical interactions between the ligand and the surface, such as Lewis acid-base coordination. [6]

Q: My ligation reaction is inefficient. What are the primary causes?

A: For enzymatic ligation, common causes and solutions are:

  • Missing 5´ Phosphate: Ensure at least one DNA fragment has a 5´ phosphate moiety. [53]
  • Suboptimal Molar Ratios: Systematically vary the molar ratio of vector to insert (e.g., from 1:1 to 1:10). [53]
  • Degraded Reagents: Use fresh reaction buffer, as critical components like ATP can degrade after multiple freeze-thaw cycles. [53]

Computational Questions

Q: How can I computationally validate the stability of a ligand binding pose from a crystal structure?

A: Binding Pose Metadynamics (BPMD) is an enhanced sampling method designed for this. It forces the ligand to explore its local energy landscape. A ligand pose that is stable under this bias is considered to be in a stable energy minimum and is likely correct. In contrast, poses that rapidly fluctuate or dissociate are considered unstable and may be incorrectly modeled in the crystal structure. [54]

Q: Can molecular dynamics reliably simulate the formation of metal-ligand complexes?

A: Yes, with advanced methods. Standard MD is challenging due to the long timescales of complex formation. However, combining finely-tuned force fields (e.g., 12-6-4 Lennard-Jones potentials) with enhanced sampling techniques like metadynamics and Markov State Models has been shown to successfully simulate the equilibrium, kinetics, and mechanisms of metal complex formation in solution, providing results that agree well with experimental stability constants. [56]

Metal Ion Monodentate Ligand Complex Formation Constant (Kf) Multidentate Ligand Complex Formation Constant (Kf)
Ni²⁺ [Ni(NH₃)₆]²⁺ ( 4 \times 10^8 ) [Ni(en)₃]²⁺ ( 2 \times 10^{18} )
Ni²⁺ [Ni(H₂O)₆]²⁺ + 3 tn → [Ni(tn)₃]²⁺ ( 1.86 \times 10^{12} ) [Ni(H₂O)₆]²⁺ + 3 en → [Ni(en)₃]²⁺ ( 6.76 \times 10^{17} )
Performance Parameter Control Device With BDA Polishing & EDAI₂ Passivation
Power Conversion Efficiency (PCE) (1.25 eV bandgap) Not Specified 23.32%
Certified Tandem Cell PCE Not Specified 28.49%
Sn⁴+ Percentage on Surface (by XPS) 27.3% 19.1%
Operational Stability (MPPT retention) Not Specified 79.7% after 550 hours

Experimental Protocols

This protocol details a two-step method to minimize ligand detachment and non-radiative recombination.

1. Principle: A chemical polishing step with 1,4-butanediamine (BDA) removes the Sn-rich, I-deficient top layer, exposing a more stoichiometric surface. Subsequent passivation with ethylenediammonium diiodide (EDAI₂) coordinates with the refreshed surface, effectively neutralizing organic cation and halide vacancy defects.

2. Reagents:

  • Sn-Pb mixed perovskite film (e.g., FA₀.₇MA₀.₃Pb₀.₇Sn₀.₃I₃)
  • 1,4-Butanediamine (BDA) in isopropyl alcohol (IPA) (0.1 mg mL⁻¹)
  • Ethylenediammonium diiodide (EDAI₂) solution

3. Procedure:

  • Film Preparation: Fabricate the Sn-Pb mixed narrow-bandgap perovskite film using your standard method.
  • Chemical Polishing: Treat the surface of the perovskite film by spin-coating or dripping the 0.1 mg mL⁻¹ BDA/IPA solution. Allow the solution to interact with the surface for a short, optimized period (e.g., 30-60 seconds), then spin dry. This step dissolves and removes the defective surface layer.
  • Surface Passivation: Immediately after polishing, apply the EDAI₂ solution to the film. EDAI₂, a bidentate ligand, will coordinate with the newly exposed Pb²⁺ and Sn²⁺ ions, forming a stable passivation layer.
  • Characterization: Use XPS to verify the reduction of Sn⁴+ and achievement of a near-stoichiometric I/(Pb+Sn) ratio. Use KPFM to confirm a uniform surface potential.

1. Principle: Binding Pose Metadynamics uses an enhanced sampling algorithm to bias ligand coordinates, testing its stability within the binding site. A stable pose remains bound, while an unstable one rapidly dissociates or moves significantly.

2. System Setup:

  • Software: A molecular dynamics package capable of running metadynamics (e.g., PLUMED with GROMACS).
  • Initial Structure: Obtain the protein-ligand complex from the PDB.
  • System Preparation: Use a tool like the Protein Preparation Wizard in Maestro to add hydrogen atoms and missing residues, and assign proper protonation states.
  • Collective Variables (CVs): Define CVs that describe the position and orientation of the ligand relative to the protein binding site.

3. Procedure:

  • Equilibration: Run a short standard MD simulation to equilibrate the solvated system.
  • Metadynamics Simulation: Launch the BPMD simulation, applying a history-dependent bias potential to the defined CVs. Multiple short replicates (e.g., 10 ns each) are often performed.
  • Analysis: Calculate the ligand root-mean-square deviation (RMSD) over time from the starting crystal pose. A common metric is the "pose stability score," which quantifies the fraction of simulation time the ligand remains near its initial pose. Stable poses will have low RMSD and high stability scores.

Signaling Pathways and Workflows

G Start Defective Surface (Sn-rich, I-deficient) A BDA Chemical Polishing Start->A B Stoichiometric Surface (Reduced Sn⁴⁺) A->B Removes defective layer C EDAI₂ Passivation (Multidentate Ligand) B->C Provides binding sites End Stable Passivation (Reduced Non-Radiative Recombination) C->End Chelation

Surface Reconstruction Workflow

G cluster_0 Thermodynamic & Kinetic Origins ML Monodentate Ligands CE Chelate Effect ML->CE Weaker Complex MDL Multidentate Ligands MDL->CE Stronger Complex T1 Favorable Entropy (Fewer molecules) CE->T1 T2 Lower Enthalpy (Stable chelate rings) CE->T2 T3 Slower Dissociation Rates CE->T3

Chelate Effect Mechanism

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Reagents for Ligand Passivation and Stability Research

Item Function / Application
Ethylenediammonium Diiodide (EDAI₂) A bidentate ligand used for passivating organic cation (VA) and halide vacancy (VI) defects on perovskite surfaces, forming a stable complex that reduces non-radiative recombination. [6]
1,4-Butanediamine (BDA) A chemical polishing agent used to pre-treat perovskite surfaces, removing Sn-rich/I-deficient layers to create a more uniform, stoichiometric surface for subsequent ligand binding. [6]
Binding Pose Metadynamics (BPMD) A computational protocol (not a reagent) used to validate the stability of ligand binding poses from crystal structures, helping to identify incorrectly modeled ligands before experimental resource investment. [54]
12-6-4 Lennard-Jones Potential An advanced force field parameter set for molecular dynamics simulations that more accurately models interactions with metal ions, enabling realistic simulation of metal-ligand complex formation and stability. [56]

Optimizing Concentration and Processing Conditions for Uniform Surface Coverage

Frequently Asked Questions (FAQs)

FAQ 1: Why is achieving uniform surface coverage so critical in my perovskite solar cell research? Uniform surface coverage is fundamental to minimizing non-radiative recombination, a primary cause of energy loss in perovskite solar cells (PSCs). Inhomogeneous coverage, often caused by agglomerated metal oxide nanoparticles or a defective perovskite surface, creates energy disorder and trap states that promote charge carrier recombination. Optimizing surface coverage through multidentate ligands and surface polishing leads to higher-quality films, improved charge extraction, and significantly enhanced device performance and stability [58] [6].

FAQ 2: What are the common signs of non-uniform surface coverage in my experiments? Common experimental indicators of poor surface coverage include:

  • Low Open-Circuit Voltage (VOC) and Fill Factor (FF): Resulting from energy level misalignment and interfacial recombination at coverage defects [58] [6].
  • Inconsistent Data: Poor reproducibility between device batches due to variable film morphology and defect density [6].
  • Low Power Conversion Efficiency (PCE): Direct consequence of inefficient charge extraction and high non-radiative losses [58] [6].

FAQ 3: How does a multidentate ligand improve surface coverage compared to a conventional ligand? Conventional passivating ligands typically bind to a surface through only a single active site. When densely packed, this can create an insulating barrier that impedes charge transport. In contrast, multidentate ligands feature multiple binding sites (e.g., carboxylic acids, chlorine, selenium atoms) that can anchor simultaneously to several adjacent sites on a substrate or perovskite surface. This multi-point contact provides a stronger, more stable binding that suppresses ligand density-related resistance, improves electron extraction, and ensures more uniform surface modification [58] [1].

Troubleshooting Guides

Issue: Agglomeration of Nanoparticles in Electron Transport Layers

Problem: Pre-synthesized SnO₂ nanoparticles in solution suffer from colloidal agglomeration, leading to micro-size aggregates on the substrate. This creates pinholes, energy disorder, and impedes charge transfer [58].

Solution: Utilize a multidentate ligand as a coordinative buffer on the substrate.

  • Recommended Reagent: 1,2-cyclohexanedinitrilotetraacetic acid (CDTA) [58].
  • Mechanism: The multiple carboxylic acid groups in CDTA strongly chelate with undercoordinated Sn⁴⁺ sites on both the ITO substrate and the SnO₂ nanoparticles. This coordination prevents nanoparticle aggregation and facilitates the deposition of a pinhole-free, uniform electron transport layer [58].
  • Experimental Protocol:
    • Prepare an aqueous solution of CDTA at a concentration of 1-3 mmol/L.
    • Spin-coat the CDTA solution onto a cleaned, UV-ozone-treated ITO substrate.
    • Anneal the substrate at 100°C for 5 minutes to form a stable modification layer.
    • Proceed with the deposition of the SnO₂ nanoparticle layer as usual.
Issue: Sn-Rich and Non-Stoichiometric Perovskite Surface

Problem: The top surface of Sn-Pb mixed perovskite films is often Sn-rich and iodine-deficient, leading to oxidation to Sn⁴⁺ and severe non-radiative recombination [6].

Solution: Implement a sequential surface reconstruction strategy involving chemical polishing followed by passivation.

  • Recommended Reagents: 1,4-butanediamine (BDA) as a polishing agent and Ethylenediammonium diiodide (EDAI₂) as a passivator [6].
  • Mechanism: BDA interacts strongly with the perovskite surface via Lewis acid-base coordination and hydrogen bonding, effectively "polishing" away the Sn-rich, non-stoichiometric top layer. Subsequently, EDAI₂ passivates the newly exposed surface, healing organic cation and halide vacancy defects [6].
  • Experimental Protocol:
    • After depositing and annealing the Sn-Pb mixed perovskite film, spin-coat a 0.1 mg mL⁻¹ solution of BDA in isopropyl alcohol (IPA) onto the film.
    • Anneal briefly to facilitate the polishing reaction.
    • Then, spin-coat a solution of EDAI₂ to passivate the freshly polished surface.
Issue: Instability and Inefficiency in Ambiently Processed Perovskite Films

Problem: Devices fabricated fully in air exhibit lower efficiency and stability, often due to insufficient defect passivation and uncontrolled crystallization.

Solution: Incorporate a multi-anchoring ligand complex that provides deep defect passivation and enhances moisture resistance.

  • Recommended Reagent: Antimony chloride-N,N-dimethyl selenourea complex, Sb(SU)₂Cl₃ [1].
  • Mechanism: This complex binds to four adjacent undercoordinated Pb²⁺ sites on the perovskite surface through two Se and two Cl atoms, creating an extremely stable binding that suppresses defect formation. It also forms a hydrophobic network that improves moisture resistance [1].
  • Experimental Protocol:
    • Synthesize the Sb(SU)₂Cl₃ complex by reacting antimony chloride with N,N-dimethylselenourea in dichloromethane [1].
    • Incorporate the complex into the two-step perovskite fabrication process, either as a surface treatment or an additive, to passivate defects during film formation.

Quantitative Data for Experimental Optimization

Table 1: Performance Metrics of Surface Modification Strategies

Modification Strategy Device Type Champion PCE (%) Key Stability Metrics Citation
CDTA on ITO n-i-p PSCs 24.67% 91.3% of initial PCE after 2000 h dark storage; 92.8% after 500 h one-sun illumination. [58]
BDA + EDAI₂ Surface Reconstruction Sn-Pb Mixed PSCs (1.25 eV) 23.32% Certified 28.49% PCE for all-perovskite tandem cell. [6]
Sb(SU)₂Cl₃ Multi-site Passivation Fully Air-Processed PSCs 25.03% T₈₀ lifetime: 23,325 h (dark), 5,004 h (85°C), 5,209 h (operational). [1]

Table 2: Optimized Concentration Ranges for Key Reagents

Reagent Function Optimal Concentration Solvent Citation
CDTA Multidentate substrate modifier 1 - 3 mmol/L Deionized Water [58]
BDA (Polishing Agent) Surface polishing agent 0.1 mg/mL Isopropyl Alcohol (IPA) [6]
Sb(SU)₂Cl₃ Multi-site binding passivator Synthesized complex Dichloromethane / Ethyl Acetate [1]

The Scientist's Toolkit: Key Research Reagents

Table 3: Essential Reagents for Surface Coverage and Passivation Research

Reagent Function/Mechanism Key Application Context
CDTA Multidentate chelating ligand; binds Sn⁴⁺ via four carboxylate groups, improves SnO₂ dispersion and work function of ITO. Modifying the buried ITO/SnO₂ interface in n-i-p PSCs to improve ETL uniformity and electron extraction. [58]
1,4-Butanediamine (BDA) Chemical polishing agent; interacts with perovskite via Lewis acid-base coordination and hydrogen bonding to remove Sn-rich surface layer. Pre-passivation treatment for Sn-Pb mixed perovskite films to achieve a stoichiometric surface. [6]
Ethylenediammonium Diiodide (EDAI₂) Surface passivator; heals organic cation and iodine vacancy defects on the perovskite surface. Secondary treatment after BDA polishing to finalize surface reconstruction and minimize non-radiative recombination. [6]
Sb(SU)₂Cl₃ Complex Multi-site binding passivator; coordinates to four adjacent Pb²⁺ sites via 2Se and 2Cl atoms, enabling deep trap passivation and high moisture resistance. Achieving high-efficiency and stable PSCs fabricated entirely in ambient air using a two-step method. [1]

Conceptual Workflow and Signaling Pathways

The following diagram illustrates the logical workflow for diagnosing and resolving surface coverage issues, connecting the observed problems with the appropriate strategies and reagents discussed in this guide.

G Start Start: Poor Device Performance (Low PCE, VOC, FF) Problem1 Problem: ETL Agglomeration & Poor ITO/ETL Interface Start->Problem1 Problem2 Problem: Sn-Rich, Defective Perovskite Surface Start->Problem2 Problem3 Problem: Instability in Ambient Processing Start->Problem3 Solution1 Solution: Multidentate Ligand (e.g., CDTA) Problem1->Solution1 Solution2 Solution: Chemical Polishing & Passivation (e.g., BDA + EDAI2) Problem2->Solution2 Solution3 Solution: Multi-site Binding Ligand (e.g., Sb(SU)2Cl3) Problem3->Solution3 Outcome1 Outcome: Uniform SnO₂ ETL Improved Energy Alignment Solution1->Outcome1 Outcome2 Outcome: Stoichiometric Surface Reduced Sn4+ Defects Solution2->Outcome2 Outcome3 Outcome: Deep Defect Passivation Enhanced Moisture Resistance Solution3->Outcome3 Final Final Result: Reduced Non-Radiative Recombination & High Device Performance Outcome1->Final Outcome2->Final Outcome3->Final

Diagram 1: Diagnostic and solution workflow for surface coverage issues.

The diagram below visualizes the mechanism of a multi-site binding ligand, which is central to achieving strong and uniform surface passivation.

G Perovskite Perovskite Crystal Surface (Undercoordinated Pb²⁺ Ions) a Perovskite->a Ligand Multidentate Ligand (e.g., Sb(SU)2Cl3) (Multiple Binding Sites) BindingSite1 Binding Site 1 (Se) Ligand->BindingSite1 BindingSite2 Binding Site 2 (Se) Ligand->BindingSite2 BindingSite3 Binding Site 3 (Cl) Ligand->BindingSite3 BindingSite4 Binding Site 4 (Cl) Ligand->BindingSite4 b BindingSite1->b BindingSite2->b BindingSite3->b BindingSite4->b DefectPassivation Outcome: Deep Trap Passivation Stable Interface Low Interfacial Resistance a->Ligand  Multi-site  Coordination b->DefectPassivation

Diagram 2: Multi-site ligand binding mechanism for surface passivation.

Managing Competitive Interactions in Multi-Component Passivation Systems

Troubleshooting Guides and FAQs

Frequently Asked Questions

Q1: What are the primary advantages of using multidentate over monodentate ligands in passivation systems?

Multidentate ligands offer superior passivation effectiveness due to their multiple binding sites, which enable stronger and more stable coordination with surface defects. While conventional monodentate ligands bind through only a single active site—often creating a resistive barrier due to dense packing—multidentate ligands like ethylene diamine tetraacetic acid (EDTA) can chelate suspended Pb²⁺ ions and occupy multiple I⁻ vacancies simultaneously [31]. This multi-site binding provides more comprehensive defect passivation and can substantially suppress non-radiative recombination. Furthermore, multidentate ligands can act as charge bridges to improve electronic coupling between quantum dots, facilitating better charge carrier transport within solid films [31].

Q2: Why might my device efficiency decrease after implementing a multi-component passivation strategy, and how can I troubleshoot this?

A decrease in efficiency often stems from competitive ligand interactions or improper processing conditions. To troubleshoot:

  • Verify binding hierarchy: Use computational modeling (e.g., DFT calculations) to analyze adsorption energies. Ligands with stronger binding affinity (more negative adsorption energy) should be introduced first. For example, the antimony chloride-N,N-dimethyl selenourea complex (Sb(SU)₂Cl₃) shows a clear binding energy hierarchy, with the quadruple-site (2Se–2Cl) configuration being most stable [1].
  • Check processing sequence: Ensure the initial surface polishing step is performed correctly. For Sn–Pb perovskite films, using 1,4-butanediamine (BDA) to polish the surface before introducing passivators like ethylenediammonium diiodide (EDAI₂) is crucial to remove Sn⁴⁺ defects and create a stoichiometric surface [6].
  • Optimize concentrations: Carefully control the molar ratios of all passivation components. Excessive ligand concentration can form insulating barriers that impede charge extraction.

Q3: How can I experimentally validate successful multi-site binding and the formation of a stable passivation layer?

Several characterization techniques can confirm effective multi-site binding:

  • X-ray Photoelectron Spectroscopy (XPS): Detect changes in elemental composition and oxidation states at the surface. A successful treatment will show a reduction in Sn⁴⁺/Pb²⁺ peaks and a shift toward stoichiometric halide/metal ratios [6].
  • Fourier Transform Infrared (FTIR) Spectroscopy: Identify characteristic vibrational shifts that confirm coordination between ligand functional groups and perovskite surface atoms [1].
  • Kelvin Probe Force Microscopy (KPFM): Measure surface potential uniformity. Effective passivation results in a more homogeneous contact potential difference distribution, indicating reduced surface defects [6].
  • Density Functional Theory (DFT) Calculations: Model charge transfer and adsorption energies to confirm the stability of multi-dentate binding configurations [1].

Q4: What strategies can prevent phase segregation or undesirable chemical reactions between different passivation components?

  • Strategic Sequencing: Apply passivation components in a specific order, starting with surface reconstruction agents followed by defect passivators. The BDA-EDAI₂ strategy demonstrates this well: BDA first polishes the surface, then EDAI₂ passivates the reconstructed surface [6].
  • Compatibility Screening: Pre-screen all chemical components for reactivity using techniques like NMR spectroscopy. This can detect unwanted proton transfer or complex formation in solution [6].
  • Solvent Engineering: Use orthogonal solvents for sequential treatments to prevent dissolution or redistribution of previously applied layers.
Troubleshooting Common Experimental Issues

Problem: Inconsistent passivation results between batches

  • Potential Cause: Variations in initial perovskite film quality or ambient conditions during processing.
  • Solution: Standardize precursor stoichiometry and film formation parameters. Implement controlled humidity during fabrication, as moisture can be leveraged to promote intermediate hydrate phases that improve crystallinity [1]. Document environmental conditions (temperature, humidity) for each experiment.

Problem: Reduced open-circuit voltage (VOC) despite passivation treatment

  • Potential Cause: Incomplete passivation of deep-level traps or introduction of interfacial barriers.
  • Solution: Combine surface polishing with multi-site passivation. Incorporate ligands with complementary functionality—for instance, using both halide-donating and metal-coordinating groups to address both cation and anion vacancies simultaneously [6].

Problem: Decreased long-term stability after passivation

  • Potential Cause: Weak binding energy of ligands or vulnerability to moisture ingress.
  • Solution: Select multidentate ligands that form strong coordination bonds with high adsorption energy. The Sb(SU)₂Cl₃ complex demonstrates exceptional stability due to its quadruple-site binding and hydrogen-bonding network, enabling unencapsulated devices to retain 98.98% of initial PCE after 1584 hours in dark storage [1].

Experimental Protocols & Data Presentation

Quantitative Performance Data of Multidentate Passivation Systems

Table 1: Performance metrics of multidentate ligand systems in photovoltaic devices

Ligand System Device Type Power Conversion Efficiency (PCE) Stability Retention Key Defects Addressed
EDTA [31] CsPbI₃ PQDSC 15.25% (from 13.67% baseline) N/R Suspended Pb²⁺, I⁻ vacancies
Sb(SU)₂Cl₃ [1] FAPbI₃ PSC 25.03% (ambient processed) 98.98% after 1584h (dark, unencapsulated) Undercoordinated Pb²⁺
BDA + EDAI₂ [6] Sn–Pb PSC (1.25eV) 23.32% 79.7% after 550h (MPPT) Sn⁴⁺, organic cation vacancies, I⁻ vacancies
TPPP(Cl) [22] MAPbI₃ PSC 20.2% (from 16.9% baseline) 89% after 30 days (ambient, unencapsulated) Undercoordinated Pb²⁺, I⁻ vacancies

Table 2: Binding characteristics and defect formation energy changes for multidentate ligands

Ligand System Binding Sites Adsorption Energy VI Formation Energy Change VPb Formation Energy Change
Sb(SU)₂Cl₃ [1] Quadruple (2Se+2Cl) Most favorable Significantly increased Significantly increased
EDTA [31] Multidentate (N/O) N/R N/R N/R
BDA [6] Dual (diamine) N/R N/R N/R
Detailed Experimental Protocols

Protocol 1: Surface Surgery Treatment with EDTA for PQDs [31]

  • Synthesis of CsPbI₃ PQDs: Prepare CsPbI₃ perovskite quantum dots using the hot-injection method with oleic acid and oleylamine as capping ligands.
  • Film Formation: Deposit PQDs from hexane solution onto substrate using layer-by-layer spin-coating.
  • Ligand Exchange: After each layer deposition, rinse with methyl acetate (MeOAc) anti-solvent to remove original long-chain ligands.
  • EDTA Treatment: Prepare EDTA solution in appropriate solvent. Immerse PQD solid films in EDTA solution for optimized duration.
  • Characterization: Perform XPS to confirm removal of suspended Pb²⁺ and passivation of I⁻ vacancies. Conduct TRPL to verify reduced non-radiative recombination.

Protocol 2: Multi-Site Passivation with Sb(SU)₂Cl₃ Complex [1]

  • Ligand Synthesis: React antimony chloride with N,N-dimethylselenourea in dichloromethane to form Sb(SU)₂Cl₃ complex. Characterize using FTIR, XRD, and UV-vis spectroscopy.
  • Perovskite Film Fabrication: Deposit perovskite films using two-step method in ambient air conditions.
  • Surface Treatment: Apply Sb(SU)₂Cl₃ solution onto perovskite surface via spin-coating.
  • Computational Validation: Perform DFT calculations to model binding configurations (single-site Se, single-site Cl, dual-site Se-Cl, quadruple-site 2Se-2Cl) and calculate adsorption energies.
  • Device Completion: Fabricate full solar cell architecture with appropriate charge transport layers.
  • Stability Testing: Subject unencapsulated devices to dark storage (20-40% RH, 25°C) with periodic performance measurement.

Protocol 3: Surface Reconstruction with BDA and EDAI₂ [6]

  • Chemical Polishing: Prepare 0.1 mg mL⁻¹ BDA in isopropyl alcohol. Treat pre-formed Sn–Pb perovskite films via spin-coating.
  • Surface Analysis: Characterize polished surface using XPS to verify reduction in Sn⁴⁺ content and improved I/(Pb+Sn) ratio.
  • Defect Passivation: Apply EDAI₂ solution in appropriate solvent onto BDA-polished films.
  • Morphological Study: Perform KPFM to confirm uniform surface potential distribution and reduced potential difference between grain boundaries and interiors.
  • Tandem Device Integration: Incorporate passivated Sn–Pb narrow-bandgap perovskite as bottom cell in all-perovskite tandem structure.

Visualization Diagrams

Multi-Component Passivation Mechanism

G PerovskiteSurface Perovskite Surface (Sn-rich, I-deficient) BDA BDA Polishing Agent PerovskiteSurface->BDA 1. Chemical Polishing ReconstructedSurface Reconstructed Surface (Stoichiometric) BDA->ReconstructedSurface Removes Sn⁴⁺ defects Normalizes stoichiometry EDAI2 EDAI₂ Passivator ReconstructedSurface->EDAI2 2. Defect Passivation SbComplex Sb(SU)₂Cl₃ Multi-site Ligand ReconstructedSurface->SbComplex Alternative Path PassivatedSurface Fully Passivated Surface (Low defect density) EDAI2->PassivatedSurface Passivates organic cation and halide vacancies SbComplex->PassivatedSurface Quadruple-site binding (2Se + 2Cl)

Experimental Workflow for Optimization

G SurfaceAnalysis Surface Analysis (XPS, KPFM) DFT Computational Screening (DFT Calculations) SurfaceAnalysis->DFT Identify key defects ComponentSelection Ligand Selection (Multi-site preferred) DFT->ComponentSelection Predict binding affinity SequentialProcessing Sequential Processing (Polish → Passivate) ComponentSelection->SequentialProcessing Design protocol Characterization Performance Characterization (PCE, Stability Testing) SequentialProcessing->Characterization Implement IterativeOptimization Iterative Optimization (Concentration, Sequence) Characterization->IterativeOptimization Analyze results IterativeOptimization->SurfaceAnalysis Refine approach

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential research reagents for multi-component passivation studies

Reagent Function Key Applications Considerations
EDTA (Ethylenediaminetetraacetic acid) [31] Multidentate chelator CsPbI₃ PQD surface resurfacing Removes suspended Pb²⁺, passivates I⁻ vacancies, improves electronic coupling
Sb(SU)₂Cl₃ [1] Multi-site binding complex Ambient-processed FAPbI₃ PSCs Provides quadruple-site binding (2Se+2Cl), forms hydrogen-bonding network
BDA (1,4-Butanediamine) [6] Surface polishing agent Sn–Pb perovskite surface reconstruction Chemically polishes Sn-rich surfaces, reduces Sn⁴⁺ content
EDAI₂ (Ethylenediammonium diiodide) [6] Defect passivator Sn–Pb perovskite following BDA polishing Passivates organic cation and halide vacancies
TPPP(Cl) (Tetraphenylphosphonium chloride) [22] Coordination complex additive MAPbI₃ perovskite bulk passivation Passivates undercoordinated Pb²⁺ and I⁻ defects, improves crystallinity
Nitric Acid [59] [60] Traditional passivation solution Stainless steel passivation Hazardous fumes, requires specialized safety equipment
Citric Acid [59] [60] Environmentally friendly alternative Stainless steel passivation Safer handling, faster processing, minimal toxic fumes

Quantifying Performance Gains and Comparative Analysis of Ligand Strategies

FAQs: Fundamental Concepts and Troubleshooting

Q1: What do the key performance metrics PCE, VOC, FF, and JSC tell me about my solar cell device?

These parameters, derived from the current-density versus voltage (J-V) curve under illumination, are the primary indicators of your solar cell's power generation capability [61] [62].

  • Power Conversion Efficiency (PCE): This is the ultimate measure of performance, calculated as the ratio of maximum electrical power output to incident optical power. It is the product of VOC, JSC, and FF [62].
  • Open-Circuit Voltage (V_OC): This is the maximum voltage available from the device when no current is flowing. It is a measure of the quality of your absorber material and the degree of non-radiative recombination within the device [63] [61].
  • Short-Circuit Current Density (J_SC): This is the maximum current density flowing through the device when the voltage is zero. It reflects the device's ability to absorb photons and collect the resulting charge carriers [61] [62].
  • Fill Factor (FF): Describing the "squareness" of the J-V curve, the FF is a ratio of the maximum power to the product of VOC and JSC. It is influenced by series and shunt resistances within the device [61] [62].

Q2: My device is showing a low V_OC. What are the potential causes and solutions?

A low V_OC typically signals significant energy loss, often due to non-radiative recombination at defect sites [63] [61].

  • Potential Causes:
    • Trap States & Defects: Uncoordinated ions (e.g., Pb²⁺, Sn²⁺/⁴⁺) at surfaces and grain boundaries create trap states that promote charge recombination [24] [64] [6].
    • Impurities or Non-Photoactive Phases: Incomplete conversion of the absorber material can leave behind impurities that act as recombination centers [61].
    • Mismatched Energy Band Alignment: Poor energy level alignment between layers can hinder charge extraction and increase recombination [61].
  • Recommended Solutions:
    • Implement Passivation Strategies: Introduce multidentate ligands (e.g., hyperbranched polysiloxanes, bidentate molecules) that can strongly chelate with multiple undercoordinated metal ions simultaneously, effectively neutralizing defect sites [24] [64] [1].
    • Improve Absorber Layer Crystallinity: Use chemical polishing agents or optimized fabrication processes to produce high-quality, stoichiometric perovskite films with fewer intrinsic defects [6].

Q3: What is non-radiative V_OC loss, and why is its analysis critical for achieving high efficiency?

In an ideal solar cell, all recombination is radiative. However, in real devices, non-radiative recombination via defects leads to significant voltage losses, known as non-radiative V_OC loss [63]. Analysis is critical because:

  • Identifies Loss Mechanisms: It quantifies the extent of voltage loss due to non-radiative pathways, separating it from other fundamental losses [63].
  • Guides Research Strategy: For many new solar cells like perovskites and organics, VOC is far from its theoretical limit, making the reduction of non-radiative VOC loss a more effective path to efficiency gains than optimizing J_SC [63].
  • Quantifies Improvement: Techniques like measuring the electroluminescence quantum efficiency (EL EQE) can quantify non-radiative loss. A higher EL EQE (e.g., >10⁻⁵%) directly correlates with a lower non-radiative VOC loss and a higher actual VOC [63].

Q4: How can I diagnose general hardware or measurement issues affecting my device metrics?

Before attributing poor performance to material issues, rule out these common problems [61] [65]:

  • Verify Your Light Source: Ensure your solar simulator is calibrated correctly, as J_SC is highly sensitive to light intensity [61].
  • Check for Shading and Soiling: Even partial shading can reduce the output of an entire module string. Keep panels clean [65].
  • Inspect Electrical Connections: Loose, corroded, or damaged wiring and connectors can cause significant resistive losses, reducing FF and overall output [65] [66].
  • Confirm Device Area Measurement: For accurate J_SC and PCE, the illuminated active area must be measured precisely [62].

Troubleshooting Guides

Guide 1: Diagnosing and Correcting Low V_OC via Defect Passivation

Objective: To identify and mitigate non-radiative recombination losses leading to low V_OC through advanced passivation techniques.

Background: A substantial V_OC deficit often stems from unpassivated defects at perovskite surfaces and grain boundaries. Multidentate ligands offer a robust solution by forming stable, multi-point bonds with undercoordinated ions [24] [1].

Experimental Protocol:

  • Defect Characterization:

    • Use techniques like X-ray Photoelectron Spectroscopy (XPS) to identify surface composition and oxidation states (e.g., Sn⁴⁺ content) [6].
    • Perform Kelvin Probe Force Microscopy (KPFM) to map surface potential variations, indicating inhomogeneous charge distribution due to defects [6].

  • Passivation Solution Preparation:

    • Select a Multidentate Ligand: Choose ligands with multiple electron-donating functional groups (e.g., carbonyl C=O, chloride Cl⁻). Examples from research include hyperbranched polysiloxane with maleic acid (HPSiM) or the antimony-chloride-selenourea complex Sb(SU)₂Cl₃ [24] [1].
    • Prepare Treatment Solution: Dissolve the ligand in an appropriate solvent (e.g., isopropyl alcohol) at an optimized concentration (e.g., 0.1 mg/mL for BDA polishing agent) [6].

  • Application:

    • Bulk Additive: Incorporate the ligand directly into the perovskite precursor solution [24].
    • Surface Treatment: Spin-coat the passivation solution onto the freshly prepared perovskite film, followed by a mild thermal annealing step to remove solvent [1] [6].

  • Validation:

    • Device Fabrication & J-V Measurement: Complete the solar cell and measure the J-V curve. A successful passivation will show a clear increase in VOC while maintaining or improving JSC and FF [24].
    • EL EQE Measurement: Use a system like the Enlitech REPS to measure the electroluminescence quantum efficiency. A significant increase in EL EQE confirms a reduction in non-radiative recombination losses [63].

The following diagram illustrates the logical relationship between surface defects, their passivation, and the resulting improvement in device performance.

G A Uncoordinated Pb²⁺/Sn²⁺ Ions B Surface Defects & Trap States A->B C Non-Radiative Recombination B->C D High Non-Radiative V_OC Loss C->D I Suppressed Non-Radiative Loss C->I Suppresses E Low Measured V_OC D->E F Apply Multidentate Ligand G Strong Multi-Site Chelation F->G H Defect Passivation G->H H->I Reduces J Higher Measured V_OC I->J

Guide 2: Systematic Performance Failure Analysis

Objective: To provide a step-by-step diagnostic procedure for underperforming solar cell devices.

Background: Performance issues can arise from electronic, optical, or morphological problems. A systematic approach isolates the root cause [61] [66].

Diagnostic Protocol:

  • Inspect the J-V Curve:

    • Low V_OC and Stepped Curve: May indicate shorted bypass diodes or severe mismatch from microcracks [66].
    • Low FF and "S-shaped" Curve: Suggests extraction barriers or severe series resistance from energy level misalignment or poor contacts [61].
    • Low J_SC: Points to poor charge collection or inadequate light absorption [61].

  • Quantify the Performance Drop:

    • Compare current data on a clear day with historical data from similar conditions. A consistent, unexplained drop >10-15% indicates a problem [65].

  • Advanced Characterization:

    • External Quantum Efficiency (EQE) Measurement: Measures the cell's current response as a function of light wavelength. A dip at specific wavelengths can indicate incomplete charge collection or absorption issues [62].
    • Electroluminescence (EL) Imaging: Reveals localized defects, microcracks, and current mismatch across the module that are not visible to the naked eye [66].
    • Infrared (IR) Imaging: Identifies hotspots caused by shading, soiling, or internal shorts [66].

Table 1: Performance Impact of Different Ligand Passivation Strategies in Perovskite Solar Cells

Ligand / Additive Type Example Materials Key Binding Mechanism Reported Champion PCE Key Improvement (vs. Control)
Hyperbranched Polymer HPSiM (Polysiloxane with maleic acid) [24] Multidentate chelation via carbonyl/Si-O groups with Pb²⁺ [24] 25.38% [24] Enhanced V_OC and operational stability [24]
Multi-Site Complex Sb(SU)₂Cl₃ (Antimony chloride-selenourea) [1] Quadruple-site binding via 2Se & 2Cl atoms to Pb²⁺ [1] 25.03% (in fully air-processed devices) [1] Reduced defect density, high V_OC, exceptional stability [1]
Bidentate Ligands Nicotinimidamide, N,N-diethyldithiocarbamate [64] Two-point coordination with uncoordinated metal ions [64] 25.30% [64] Increased VOC, JSC, and thermal stability [64]
Chemical Polishing + Passivation 1,4-butanediamine (BDA) + Ethylenediammonium diiodide (EDAI₂) [6] Surface reconstruction to remove Sn⁴⁺ defects & passivate organic cation/halide vacancies [6] 28.49% (certified tandem cell) [6] Minimized non-radiative loss at interface, increased V_OC and FF [6]

Table 2: Common Device Issues and Correlated Metric Changes

Observed Symptom Potential Physical Cause Most Affected Metric(s)
High non-radiative recombination Unpassivated surface/grain boundary defects (e.g., uncoordinated Pb²⁺, Sn⁴⁺) [63] [6] Low V_OC [63] [61]
Shunted pathways or poor charge extraction Pinholes in film, poor layer connectivity, energy level misalignment [61] Low FF, Low J_SC [61]
Incomplete light absorption or collection Active layer too thin, low mobility, severe bulk recombination [61] Low J_SC [61]
Shorted bypass diodes, microcracks [66] Manufacturing defect, physical damage Low V_OC, Stepped I-V curve [66]
Potential Induced Degradation (PID) [66] Leakage currents due to voltage potential with ground Low VOC and Low JSC [66]

The Scientist's Toolkit: Key Reagents and Materials

Table 3: Essential Research Reagents for Defect Passivation and V_OC Loss Analysis

Reagent / Material Function / Application Key Mechanism
Multidentate Ligands (e.g., HPSiM, Sb(SU)₂Cl₃) [24] [1] Bulk additive or surface treatment for defect passivation. Forms strong, stable chelates with multiple undercoordinated Pb²⁺/Sn²⁺ ions simultaneously, effectively neutralizing trap states [24] [1].
SnX₂ Additives (X = F, Cl, Br, I) [67] Antioxidant added to Sn-based perovskite precursor solutions. Creates a Sn-rich environment, suppresses Sn²⁺ oxidation to Sn⁴⁺, and reduces Sn vacancy concentration [67].
Chemical Polishing Agents (e.g., 1,4-butanediamine - BDA) [6] Surface treatment to reconstruct perovskite film surface. Interacts strongly with perovskite to dissolve defective surface layer, exposing a more stoichiometric and crystalline subsurface, reducing Sn⁴⁺ content [6].
EL EQE Measurement System (e.g., Enlitech REPS) [63] Quantifies electroluminescence quantum efficiency for V_OC loss analysis. Precisely measures extremely low EL signals (down to 10⁻⁵%) to quantitatively calculate non-radiative recombination losses (ΔV₃) [63].
Surface Characterization Tools (XPS, KPFM) [6] Analyzes surface composition, chemical states, and potential. Identifies surface defects (e.g., Sn⁴⁺/(Sn²⁺+Sn⁴⁺) ratio) and maps inhomogeneous charge distribution, validating passivation efficacy [6].

The following diagram outlines a generalized experimental workflow for developing and validating a passivation strategy to improve V_OC.

G Start Start: Device with Low V_OC Step1 Surface/Bulk Defect Characterization (XPS, KPFM) Start->Step1 Step2 Design/Select Multidentate Ligand Step1->Step2 Step3 Apply Passivation (Bulk or Surface) Step2->Step3 Step4 Fabricate Complete Solar Cell Step3->Step4 Step5 J-V and EL EQE Measurement Step4->Step5 Decision V_OC and EL EQE Increased? Step5->Decision Decision->Step2 No End Success: Strategy Validated Reduced Non-Radiative Loss Decision->End Yes

This technical support center addresses the critical role of ligand chemistry in perovskite solar cells (PSCs), focusing on the paradigm shift from conventional single-site ligands to advanced multidentate ligands. Defects at surfaces and grain boundaries of polycrystalline perovskite films, particularly undercoordinated Pb²⁺ ions and halide vacancies, are primary sources of non-radiative recombination, limiting device efficiency and stability [1] [68]. Effective passivation of these defects is essential to minimize trap states and approach the Shockley-Queisser theoretical efficiency limit. This resource provides troubleshooting guides and FAQs to help researchers implement multidentate ligand strategies, which offer superior defect passivation and enhanced device performance compared to single-site ligands.

Quantitative Performance Comparison

The table below summarizes key performance metrics from recent studies, demonstrating the advantages of multidentate ligands.

Table 1: Performance Comparison of Ligand Strategies in Perovskite Solar Cells

Ligand Type Specific Ligand Device Type Champion PCE (%) Stability (T80, unencapsulated) Key Passivation Mechanism
Multidentate Sb(SU)₂Cl₃ Complex Air-processed PSC 25.03 [1] 23,325 h (dark storage); 5,004 h (85°C) [1] Quadruple-site (2Se, 2Cl) binding to undercoordinated Pb²⁺ [1]
Multidentate EDTA CsPbI₃ QD Solar Cell 15.25 [31] Information Not Specified Chelates suspended Pb²⁺; occupies I⁻ vacancies [31]
Multidentate BL Molecule 1.77 eV WBG PSC 19.24 [69] 88% initial PCE after 300 h MPP [69] Coordination with PbX₂; H-bonding with FA⁺ [69]
Single-Site Conventional Ammonium Ligands General PSC Typically lower Limited, not quantified in sources Single active site binding, can form resistive barriers [1]

Experimental Protocols & Methodologies

Protocol 1: Multi-Site Passivation with Sb(SU)₂Cl₃ Complex

This protocol is for fully air-processed, high-stability PSCs [1].

  • Ligand Synthesis: Synthesize the Sb(SU)₂Cl₃ complex by reacting antimony chloride with N,N-dimethylselenourea (SU) in a dichloromethane solvent, following established procedures [1].
  • Precursor Preparation: Incorporate the synthesized Sb(SU)₂Cl₃ complex as a multi-site passivator additive into your perovskite precursor solution.
  • Film Fabrication (Two-Step Method):
    • Deposit the PbI₂ layer.
    • Subsequently react the PbI₂ layer with organic halide salts (e.g., FAI) in ambient air.
    • The moisture-assisted transformation in air promotes intermediate hydrate phases, regulating ion diffusion kinetics and improving crystallinity.
  • Annealing: Perform thermal annealing to convert the intermediate phases into the final perovskite film. The multidentate ligand will be incorporated during crystallization.

Protocol 2: Surface Surgery Treatment (SST) of Quantum Dots with EDTA

This protocol is for resurfacing all-inorganic perovskite quantum dots (PQDs) to improve electronic coupling [31].

  • PQD Synthesis: Synthesize CsPbI₃ PQDs using the standard hot-injection method with oleic acid (OA) and oleylamine (OAm) as initial capping ligands [31].
  • Ligand Exchange (Standard): Deposit PQDs from a non-polar solvent (e.g., hexane) onto a substrate to form a solid film. Rinse with an anti-solvent (e.g., methyl acetate, MeOAc) to remove most original long-chain ligands. This step inherently introduces surface defects.
  • Multidentate Post-Treatment:
    • Prepare a solution of the multidentate ligand EDTA.
    • Immerse the PQD solid film in the EDTA solution.
    • The EDTA chelates and removes suspended Pb²⁺ ions and passivates I⁻ vacancies created during the ligand exchange.
    • EDTA also acts as a "charger bridge," crosslinking adjacent QDs to enhance charge transport across the film.

Protocol 3: Homogenized Crystallization for WBG Perovskites with BL Ligand

This protocol addresses light-induced phase segregation in mixed-halide wide-bandgap perovskites [69].

  • Precursor Doping:
    • Add the multidentate ligand N, N'-methylene-bis(2-amino-5-sulfhydryl-1,3,4-thiadiazole) (BL) directly into the 1.77 eV WBG perovskite precursor solution (e.g., containing mixed I/Br halides).
  • Crystallization Control:
    • The electron-donating groups in BL coordinate with lead halides (Lewis acid-base interaction).
    • The N–H groups in the BL molecule form hydrogen bonds with formamidinium (FA⁺) cations.
    • These interactions slow down the crystallization kinetics, allowing for more homogeneous ion diffusion and reducing internal compressive stress in the final film.
  • Film Formation: Proceed with standard spin-coating and annealing processes. The result is a uniform halide distribution, suppressing light-induced phase segregation.

Troubleshooting Guides

Common Problem: High Resistive Losses After Passivation

  • Observed Symptom: Good open-circuit voltage (VOC) but poor fill factor (FF) and high series resistance.
  • Potential Cause (from [1]): Dense packing of single-site passivating ligands can create an insulating barrier that impedes charge extraction at the interface.
  • Solution:
    • Switch to Multidentate Ligands: Implement a multi-anchoring ligand like Sb(SU)₂Cl₃. Its single molecule binds multiple sites, providing effective passivation without forming a thick, compact insulating layer.
    • Optimize Concentration: Titrate the concentration of your passivator. Even multidentate molecules can cause charge transport issues if used in excess.

Common Problem: Incomplete Defect Passivation

  • Observed Symptom: Low VOC and persistent non-radiative recombination, as indicated by low photoluminescence quantum yield (PLQY).
  • Potential Cause: Single-site ligands may only passivate one specific type of defect (e.g., either Pb²⁺ or I⁻ vacancies) or may not effectively passivate deep-level traps.
  • Solution:
    • Use Multifunctional Passivators: Employ ligands with multiple functional groups that can simultaneously address different defects. For example, EDTA can chelate Pb²⁺ and fill I⁻ vacancies [31].
    • Leverage Synergistic Effects: The Sb(SU)₂Cl₃ complex passivates undercoordinated Pb²⁺ via Se/Cl atoms and can also fill iodine vacancies with its isolated Cl atoms [1].

Common Problem: Poor Stability Under Light or Heat

  • Observed Symptom: Rapid degradation of WBG perovskites under illumination (phase segregation) or general device failure at elevated temperatures (e.g., 85°C).
  • Potential Cause (from [69]): Internal strain and inhomogeneous halide distribution in mixed-halide perovskites accelerate ion migration and decomposition.
  • Solution:
    • Internal Strain Reduction: Introduce multidentate ligands like BL into the WBG perovskite precursor. This delays crystallization, promoting a more homogeneous and relaxed film with reduced residual stress, thereby suppressing ion migration [69].
    • Robust Surface Coordination: The strong, multi-point coordination of ligands like Sb(SU)₂Cl₃ enhances the overall structural stability of the perovskite lattice, leading to exceptional thermal stability [1].

Frequently Asked Questions (FAQs)

Q1: From a mechanistic standpoint, why do multidentate ligands suppress non-radiative recombination more effectively than single-site ligands?

Multidentate ligands provide a more robust and stable solution for defect passivation due to two key mechanisms:

  • Enhanced Adsorption Energy and Stability: As the number of binding sites between the ligand and the perovskite surface increases, the adsorption energy becomes more negative, indicating a stronger and more stable interaction [1]. A single-site ligand can be relatively easily desorbed, leaving defects exposed, whereas a quadruple-site ligand is anchored much more firmly, providing durable passivation.
  • Multi-Defect Passivation: A single multidentate ligand molecule can simultaneously passivate multiple adjacent undercoordinated sites. For example, Sb(SU)₂Cl₃ can coordinate with four neighboring undercoordinated Pb²⁺ ions via its two Se and two Cl atoms within a single lattice unit [1]. A single-site ligand would require four molecules to achieve the same effect, potentially creating a thicker, more resistive layer.

Q2: How do I select an appropriate multidentate ligand for my specific perovskite composition?

Selection should be based on the functional groups of the ligand and the dominant defects in your system. Use this table as a guide:

Table 2: Research Reagent Solutions Guide

Reagent / Ligand Chemical Function Application in PSCs
Sb(SU)₂Cl₃ Complex [1] Se (Lewis base) and Cl atoms coordinate to Pb²⁺; N-H forms H-bonds. Multi-anchoring passivator for high-efficiency, air-processed PSCs. Targets undercoordinated Pb²⁺.
EDTA (Ethylene Diamine Tetraacetic Acid) [31] Multidentate chelator for metal ions; carboxyl groups can passivate vacancies. "Surface surgery" for PQDs. Chelates excess/suspended Pb²⁺ and occupies I⁻ vacancies.
BL Molecule [69] Electron-donating groups coordinate Pb²⁺; N-H groups H-bond with FA⁺. Additive for WBG perovskites to homogenize crystallization and suppress phase segregation.
N,N-dimethylselenourea (SU) [1] Precursor for synthesizing the Sb(SU)₂Cl₃ complex. Used in ligand synthesis, not as a direct passivator.

Q3: My PQD solar cells suffer from poor charge transport after ligand exchange. What can I do?

This is a common issue when long-chain, insulating native ligands (OA/OAm) are removed, leaving a high density of traps and poor inter-dot coupling [31].

  • Recommended Solution: Implement a post-treatment with a multidentate crosslinking ligand like EDTA. This strategy, termed "surface surgery treatment," serves a dual purpose:
    • It passivates the surface defects created during the initial ligand exchange.
    • It acts as a molecular "bridge" between adjacent quantum dots, facilitating electronic coupling and substantially improving charge carrier transport through the PQD solid film, thereby boosting device efficiency [31].

Visualization: Workflow and Mechanism

The following diagram illustrates the generalized experimental workflow for applying multidentate ligands and their superior binding mechanism compared to single-site ligands.

G cluster_strat Ligand Introduction Strategy cluster_mechanism Binding Mechanism at Atomic Level start Start: Perovskite Precursor A Additive Engineering (Add to precursor solution) start->A B Surface Treatment (Post-deposition treatment) start->B C Perovskite Crystallization A->C B->C D Multidentate Ligand Action C->D E Result: High-Quality Film D->E Perov Perovskite Surface (Undercoordinated Pb²⁺, V₍I₎) Multi Multidentate Ligand (e.g., Sb(SU)₂Cl₃) Perov->Multi  Strong, multi-point  coordination Single Single-Site Ligand Perov->Single  Weak, single-point  coordination

Diagram 1: Multidentate ligand workflow and binding mechanism.

In the pursuit of advanced materials for optoelectronics and photovoltaics, reducing non-radiative recombination is a fundamental challenge. Non-radiative recombination pathways, often mediated by trap states, compete with radiative processes, diminishing device performance and efficiency. Multidentate ligand passivation has emerged as a powerful strategy to suppress these losses by coordinating with undercoordinated atoms at surfaces and grain boundaries, thereby eliminating trap states [24] [1]. This technical support center provides a foundational guide for validating the success of such passivation strategies through rigorous photophysical characterization. Accurate measurement of key parameters—including photoluminescence quantum yield (PLQY), photoluminescence (PL) lifetime, and trap state density—is essential for establishing clear structure-property relationships and guiding the rational design of new materials.

The following sections are structured in a question-and-answer format to directly address the specific experimental challenges researchers face. They provide detailed methodologies, troubleshooting guides, and data interpretation frameworks to ensure reliable and reproducible characterization of photophysical properties within the context of multidentate ligand passivation research.

Frequently Asked Questions (FAQs) & Troubleshooting

Photoluminescence Quantum Yield (PLQY) Measurements

FAQ 1: What is PLQY and why is it a critical metric in passivation research?

The photoluminescence quantum yield (ΦPL) is the ratio of the number of photons emitted to the number of photons absorbed by a sample [70]. It is a direct measure of the efficiency of the radiative recombination process. In the context of multidentate ligand passivation, an increase in ΦPL following treatment is a primary indicator of successful suppression of non-radiative recombination pathways. This is because the passivator coordinates with defect sites (e.g., undercoordinated Pb²⁺ in perovskites), preventing them from capturing charge carriers and dissipating energy as heat [24] [1]. A high PLQY, sometimes approaching unity, signifies that nearly all absorbed light is re-emitted radiatively [71].

FAQ 2: My PLQY values are inconsistent between measurements. What are the common pitfalls and how can I avoid them?

Inconsistencies in PLQY often stem from sample preparation, instrument parameters, or data analysis errors. The table below summarizes common issues and their solutions.

Table 1: Troubleshooting Guide for PLQY Measurements

Problem Potential Cause Solution
Low/Inconsistent PLQY Oxygen Quenching: Presence of oxygen, especially for phosphorescent samples with triplet states [70]. Degas Solutions: Purge solutions with an inert gas (e.g., N₂, Ar) before measurement.
Sample Impurities: Impurities act as non-radiative quenchers [70]. Purify Samples: Use recrystallization, chromatography, or other purification techniques.
Inner Filter Effect: High optical density at excitation wavelength leads to non-uniform excitation and re-absorption of emission [70]. Optimize Concentration: Ensure absorbance at excitation wavelength is typically below 0.1 to ensure uniform light penetration.
Inaccurate Reference Values Using Wrong Standard: An inappropriate reference standard invalidates the measurement. Match Standards: Use a standard with a similar emission spectrum and refractive index to your sample.
Signal Instability Photodegradation: The sample decomposes under the excitation light. Minimize Exposure: Use lower excitation power, shorter integration times, or a neutral density filter.

FAQ 3: Could you provide a basic protocol for measuring absolute PLQY using an integrating sphere?

A robust protocol ensures reliable data, which is crucial for validating passivation efficacy.

Experimental Protocol: Measuring Absolute PLQY with an Integrating Sphere

  • Sample Preparation:

    • Prepare a stable, homogeneous solution or a solid film of the material to be tested.
    • For solution measurements, ensure the solvent is of high purity and the sample is fully dissolved.
    • For films, ensure they are uniform and free of pinholes or defects.
    • For oxygen-sensitive compounds (e.g., those involving phosphorescence), carefully degas the solution and seal the cuvette [70].

  • Instrument Setup:

    • Place the empty integrating sphere in the spectrometer and initiate the data acquisition software.
    • Calibrate the system according to the manufacturer's instructions using a calibrated light source if necessary.

  • Data Acquisition:

    • Collect a baseline spectrum with the sphere empty.
    • Place the sample (in a cuvette or as a film) inside the sphere. Do not place it in the direct excitation path.
    • Excite the sample and collect the emission spectrum. This spectrum will contain peaks from both the sample's emission and the scattered excitation light.
    • Move the sample into the direct excitation path within the sphere.
    • Excite the sample again and collect the second emission spectrum.

  • Data Analysis & Calculation:

    • The software typically uses the two spectra to calculate the absolute quantum yield. The calculation is based on the ratio of the integrated intensity of the sample's emission to the integrated intensity of the absorbed excitation light.

Photoluminescence (PL) Lifetime Measurements

FAQ 4: How does PL lifetime provide insights complementary to PLQY?

The PL lifetime (τ) is the average time a molecule remains in the excited state before returning to the ground state [70]. While PLQY gives the efficiency of emission, the lifetime provides information about the kinetics of the excited-state decay. Under the assumption of no photochemical reactions, the measured ΦPL and τ can be used to calculate the radiative (kᵣ) and non-radiative (kₙᵣ) decay rate constants using the relationships [70]: ΦPL = kᵣ / (kᵣ + kₙᵣ) and τ = 1 / (kᵣ + kₙᵣ) Successful multidentate passivation that reduces trap states should lead to a decrease in kₙᵣ, which often results in a lengthening of the observed PL lifetime, even if the PLQY is already high [71].

FAQ 5: What are the different methods for measuring PL lifetime and when should I use each?

The choice of method depends on the required temporal resolution, sensitivity, and accessibility.

Table 2: Comparison of PL Lifetime Measurement Techniques

Method Principle Temporal Resolution Key Advantages Best For
Time-Correlated Single Photon Counting (TCSPC) Builds a histogram of photon arrival times after a pulsed excitation [72]. High (ps to ns) High sensitivity, excellent for low-light and low-repetition-rate signals, direct lifetime histogram [72]. Standard laboratory measurements of ns-lived excited states.
Frequency-Domain FLIM Measures the phase shift and demodulation of emitted light relative to intensity-modulated excitation [72]. High (ps to ns) Fast acquisition, robust to intensity artifacts, can be implemented with analog electronics for high-speed readout [72]. High-speed, intensity-independent lifetime imaging.
Frame-Straddling Imaging Uses a camera to capture two images: one during the excitation pulse and one capturing the delayed luminescence [73]. Moderate (ms to μs) Accessibility: Uses standard cameras. Mapping: Enables wide-field lifetime imaging of chemical species (e.g., oxygen) [73]. Mapping luminescence lifetimes over a wide area with standard equipment.

FAQ 6: I am getting a poor fit for my lifetime decay curve. What should I check?

A poor fit often indicates an incorrect model or experimental artifacts.

  • Check the Fitting Model: Determine if the decay is single-exponential or multi-exponential. Passivation often simplifies a complex, multi-exponential decay (indicating multiple environments or decay pathways) into a more single-exponential decay as trap states are homogenized.
  • Instrument Response Function (IRF): Ensure your fitting software correctly accounts for the IRF. A poor fit at the very beginning of the decay (the "rise time") often indicates a misaligned IRF.
  • Exclude Scattered Light: Make sure the emission filter effectively blocks the excitation light, as scattered light will appear as an extremely short-lived component and distort the fit.
  • Signal-to-Noise Ratio (SNR): Ensure you have collected enough photon counts for a statistically valid fit. A low SNR will lead to large confidence intervals in the fitted parameters.

Trap State Density Characterization

FAQ 7: How can I directly quantify the density of trap states that are passivated?

Quantifying trap density provides the most direct evidence of a passivator's efficacy. Several electrical and optical methods can be employed.

Table 3: Techniques for Evaluating Trap State Density

Technique Principle Applicability Insight
Transfer Characteristic Analysis (for FETs) Analyzes the subthreshold swing in a transistor's transfer curve (ID-VG) to extract interface trap density (D_it) [74]. Electronic devices like perovskite or organic thin-film transistors (TFTs) [75]. Directly measures electrically active traps at the semiconductor/dielectric interface that affect charge transport.
Frequency-Modulated AFM (fm-AFM) Maps nanoscale dissipation due to charge trapping/de-trapping events at interfaces (e.g., Si-SiO₂) [76]. Nanoscale materials and interfaces. Provides spatial mapping of individual trap locations, offering insight beyond bulk averages [76].
Thermal Admittance Spectroscopy Measures capacitance as a function of frequency and temperature to probe trap energy levels and densities in the bandgap. Semiconductors and solar cell materials. Resolves the energy distribution of trap states (DOS).
Transient Absorption/Photoluminescence Dynamics Analyzes ultrafast carrier trapping (ps-fs timescales) and longer non-radiative recombination [71]. All photoactive materials (e.g., quantum dots, perovskites). Can distinguish between hot-carrier trapping and band-edge carrier recombination, identifying specific loss pathways [71].

FAQ 8: Can neural networks be used to analyze trap density?

Yes, data-driven approaches are emerging as powerful tools. For example, a neural network with a Long Short-Term Memory (LSTM) structure can be trained on simulated or experimental data to learn the complex mapping between a device's transfer characteristics and its underlying interface trap density (Dit) and fixed oxide charge density (Qox) [74]. This method can efficiently evaluate D_it for commercial devices, bypassing the need for manual parameter tuning in traditional simulation methods [74].

The Scientist's Toolkit: Research Reagent Solutions

The following table lists key materials and their functions as identified in recent high-impact passivation studies.

Table 4: Essential Research Reagents for Multidentate Passivation

Research Reagent Chemical Function Role in Passivation Demonstrated Outcome
Hyperbranched Polysiloxane with Maleic Acid (HPSiM) [24] Multidentate chelator with rich carbonyl (C=O) groups. Coordinates with uncoordinated Pb²⁺ ions across a wide range on perovskite grain boundaries/surfaces. Champion perovskite solar cell efficiency of 25.38%; enhanced long-term stability [24].
Antimony Chloride-N,N-dimethyl selenourea Complex (Sb(SU)₂Cl₃) [1] Multi-anchoring ligand with Se and Cl binding sites. Quadruple-site binding (2Se+2Cl) to adjacent undercoordinated Pb²⁺ on perovskite surfaces. PCE of 25.03% in fully air-processed devices; record operational stability [1].
Atomic Hydrogen / Forming Gas (N₂+H₂) [76] Passivating agent for dangling bonds. Passivates donor-like traps at the Si-SiO₂ interface through annealing. Measured reduction in interface trap density via fm-AFM [76].

Conceptual Diagrams and Workflows

Multidentate Passivation Mechanism

This diagram illustrates the core concept of how a multidentate ligand passivates multiple defect sites on a crystal surface, suppressing non-radiative recombination.

G Ligand Ligand Defect1 Undercoordinated␣Site␣1 Ligand->Defect1 Defect2 Undercoordinated␣Site␣2 Ligand->Defect2 Defect3 Undercoordinated␣Site␣3 Ligand->Defect3 NonRadiativeLoss Non-Radiative␣Recombination Defect1->NonRadiativeLoss Defect2->NonRadiativeLoss Defect3->NonRadiativeLoss RadiativeEmission Radiative␣Emission

Diagram 1: Multidentate passivation prevents non-radiative recombination at crystal defects.

Integrated Photophysical Validation Workflow

This workflow charts the experimental journey from sample preparation and passivation to comprehensive photophysical characterization and data analysis.

G A Sample Preparation␣&␣Passivation B UV-Vis Absorption␣Measurement A->B C PLQY Measurement A->C D PL Lifetime␣Measurement A->D E Trap Density␣Characterization A->E F Data Analysis␣&␣Validation B->F C->F D->F E->F

Diagram 2: Integrated workflow for photophysical validation of passivated materials.

Frequently Asked Questions (FAQs)

FAQ 1: What are the key stability metrics, and how should they be reported? The most widely used metric for operational lifetime is T80, defined as the time elapsed until a device retains 80% of its initial performance. For consistent reporting, it is strongly recommended to follow established protocols like the International Summit on OPV Stability (ISOS). These protocols define standardized stress conditions (temperature, humidity, environment, light, electrical load) across three levels of sophistication: Basic (Level 1), Intermediate (Level 2), and Advanced (Level 3) [77] [78]. Reporting should always specify the exact testing conditions and the protocol level used.

FAQ 2: Our unencapsulated control devices degrade too rapidly for meaningful data. How can we improve baseline stability to better evaluate our passivation strategies? Rapid degradation in control devices often stems from pervasive defects at grain boundaries and surfaces, which act as entry points for environmental stressors. Incorporating a multidentate passivator as a bulk additive in the perovskite precursor solution can be highly effective. For example, using a hyperbranched polymer rich in carbonyl groups can chelate with a wide range of undercoordinated Pb²⁺ ions, creating a more robust and intrinsically stable perovskite film that delays the onset of degradation [24]. This provides a more stable baseline for testing advanced encapsulation.

FAQ 3: We observe excellent shelf-life stability but rapid performance loss under operational conditions. What could be causing this? This discrepancy points to field-induced degradation mechanisms. Stability under dark storage (shelf-life) does not account for the combined stresses of light, electrical bias, and heat during operation. Key mechanisms to investigate include:

  • Ion Migration: Light and heat can activate the migration of ions within the perovskite lattice, leading to phase segregation and interface decomposition.
  • Non-Radiative Recombination: Under operation, defects not passivated by your ligand can become active recombination centers, generating heat and localizing energy that accelerates degradation [79].
  • Interfacial Degradation: The charge transport layers may degrade or react with the perovskite layer under bias and illumination. Ensure your multidentate ligands do not create a resistive barrier at these critical interfaces [1].

FAQ 4: How can we correlate accelerated aging tests with real-world operational lifetime? Accelerated aging tests use elevated stress levels (e.g., higher temperature, concentrated light) to predict long-term behavior. A common approach is Arrhenius-type behavior analysis, where the degradation rate is modeled at several elevated temperatures to extrapolate the lifetime at real-world operating temperatures [78]. It is critical to perform these tests on devices that have achieved stabilized performance, as the initial "burn-in" degradation can skew results [78]. Validating these models with real-world outdoor testing data, where available, is the ultimate benchmark [80].

Troubleshooting Guides

Problem: Inconsistent lifetime results between batches of devices.

  • Potential Cause 1: Inconsistent perovskite film quality and defect density. The effectiveness of multidentate passivation is highly dependent on the number and type of defects present.
  • Solution: Strictly control the crystallization process. Multidentate additives like HPSiM can delay nucleation and crystal growth, leading to more uniform, high-quality films [24]. Standardize your annealing protocol and environmental conditions (e.g., humidity) during fabrication.
  • Potential Cause 2: Variation in the distribution of the passivation ligand.
  • Solution: Ensure the ligand is fully soluble in the precursor solvent and that the solution is mixed thoroughly. Hyperbranched polymers can offer higher solubility than linear polymers, promoting more consistent incorporation [24].

Problem: Introducing the passivator improves stability but reduces initial efficiency.

  • Potential Cause: The passivating layer may be acting as an insulating barrier, impeding charge carrier extraction at the interface [1].
  • Solution: Redesign the ligand to facilitate charge transport. This can be achieved by selecting functional groups that not only passivate defects but also create favorable energy level alignment at the perovskite/transport layer interface. For instance, the Sb(SU)₂Cl₃ complex was designed with an elevated HOMO level to support efficient hole transport [1].

Problem: Device performance degrades rapidly under continuous thermal stress at 85°C.

  • Potential Cause: The passivation bonds are thermally unstable, or the perovskite lattice itself is degrading.
  • Solution: Employ ligands that form stronger, multidentate coordination with the perovskite surface. A quadruple-site binding ligand (e.g., via 2Se–2Cl) has been shown to dramatically improve thermal stability, with unencapsulated devices retaining over 80% performance after 5,000 hours at 85°C [1]. Additionally, ensure your encapsulation is sufficient to protect against moisture ingress, which synergistically accelerates thermal degradation.

Stability Benchmarking Data

The following table summarizes operational stability data from recent studies utilizing multidentate passivation strategies, serving as a benchmark for high-performance devices.

Table 1: Benchmarking Stability Performance of PSCs with Multidentate Passivation

Passivation Strategy Device Architecture Test Condition Stability Performance Source
Hyperbranched polysiloxane (HPSiM) Inverted (p-i-n) MPPT tracking, 55°C, N₂ 91.6% of initial PCE retained after 1,000 hours [24]
Antimony chloride-Selenourea complex (Sb(SU)₂Cl₃) n-i-p, Fully air-processed Continuous 1-sun illumination, ~25°C T₈₀ lifetime of ~5,209 hours (unencapsulated) [1]
Antimony chloride-Selenourea complex (Sb(SU)₂Cl₃) n-i-p, Fully air-processed Dark shelf life, 25°C, 20-40% RH T₈₀ lifetime of ~23,325 hours (unencapsulated) [1]
Antimony chloride-Selenourea complex (Sb(SU)₂Cl₃) n-i-p, Fully air-processed Thermal stress, 85°C, dark T₈₀ lifetime of ~5,004 hours (unencapsulated) [1]

Experimental Protocols

Protocol 1: ISOS-L-2 (Intermediate Level Laboratory Weathering) This is a standard protocol for assessing operational stability under simulated light and thermal stress [78].

  • Initial Characterization: Measure the initial current-density voltage (J-V) curves under a calibrated solar simulator (Standard Test Conditions: 1000 W/m², AM 1.5G spectrum, 25°C).
  • Stress Application: Place the device in a environmental chamber with a simulated sunlight source. Maintain a constant temperature of 65°C at the device surface. Control the relative humidity to < 30%.
  • Load Condition: Operate the device at its Maximum Power Point (MPP) using a maximum power point tracker for the duration of the test.
  • Monitoring: At fixed intervals (e.g., every 24-48 hours), temporarily remove the device, allow it to cool to 25°C, and record a full J-V curve to track performance parameters (PCE, JSC, VOC, FF).
  • Data Reporting: Report the normalized PCE as a function of time. The T80 lifetime under these specific conditions should be clearly stated, along with all other test parameters [78].

Protocol 2: Thermal Cycling Test (Basic Level) This protocol tests resilience to thermal fatigue [78].

  • Initial Characterization: As in Protocol 1.
  • Cycling Profile: Place the device in a thermal chamber. Cycle the temperature between -40°C and +85°C.
  • Dwell Time: Maintain the temperature at each extreme for a minimum of 10 minutes.
  • Transition Rate: The temperature transition rate should be controlled, with a recommended rate of < 100°C/hour.
  • Cycles: Perform a minimum of 50 cycles.
  • Final Characterization: After the completed cycles, measure the final J-V characteristics at standard test conditions and calculate the percentage degradation.

Research Reagent Solutions

Table 2: Key Reagents for Multidentate Passivation Research

Reagent / Material Function in Experiment
Hyperbranched Polysiloxane (e.g., HPSiM) Bulk additive for multidentate chelation of Pb²⁺ ions via carbonyl groups; suppresses defect formation at grain boundaries and improves film quality [24].
Multisite Binding Ligands (e.g., Sb(SU)₂Cl₃) Surface passivator that coordinates with multiple undercoordinated Pb²⁺ sites simultaneously (e.g., via 2Se and 2Cl atoms), offering superior stability versus single-site ligands [1].
N,N-dimethylselenourea (SU) Ligand precursor for forming coordination complexes with metal halides (e.g., SbCl₃) to create advanced multidentate passivators [1].
Tetraethyl orthosilicate Reactant used in the one-step polycondensation synthesis of hyperbranched polysiloxane-based passivation agents [24].

Experimental Workflow Visualization

G Start Start Experiment P1 Perovskite Precursor Preparation Start->P1 P2 Add Multidentate Ligand (e.g., HPSiM) P1->P2 P3 Film Deposition & Crystallization P2->P3 P4 Complete Device Fabrication P3->P4 C1 Initial PCE Measurement P4->C1 C2 Stability Test Setup C1->C2 C3 Apply Stressors: Light, Heat, Bias C2->C3 C4 Monitor Performance Over Time C3->C4 C5 Calculate Degradation Rate & T₈₀ C4->C5 End Data Analysis & Reporting C5->End

Diagram 1: Stability testing workflow for passivated perovskite devices.

G Stressors Environmental Stressors Heat, Light, Moisture Defects Active Defect Sites Unpassivated Pb²⁺, Halide Vacancies Stressors->Defects Mechanism Degradation Mechanism Ion Migration, Non-Radiative Recombination Defects->Mechanism Outcome Performance Loss Reduced PCE, FF, Vₒc Mechanism->Outcome Solution Multidentate Ligand Passivation Solution->Defects

Diagram 2: Logical relationship between stressors, defects, and the passivation solution.

FAQs on Passivation Mechanism Characterization

1. How can XPS confirm the formation of a covalent bond between a passivator and the perovskite surface?

XPS confirms covalent bond formation by detecting characteristic shifts in the binding energy of core-level electrons and the appearance of new chemical species. For instance, when a phosphonic acid-based molecule (4-methoxyphenylphosphonic acid, MPA) forms a P–O–Pb bond with the perovskite surface, the Pb 4f core-level spectrum splits into new doublets at higher and lower binding energies compared to the original Pb-I/Br state. Simultaneously, the O 1s spectrum shows a significant increase and the emergence of a new peak around 531.5 eV, consistent with the formation of P–O–Pb bonds and the deprotonation of the phosphonic acid group [18]. These chemical state changes, along with shifts in other elemental peaks (e.g., N 1s, I 3d, P 2p), provide direct evidence of a strong covalent interface rather than a weak coordination bond [18].

2. What FTIR signatures indicate successful multidentate binding to undercoordinated Pb²⁺ or Sn²⁺ ions?

FTIR spectroscopy reveals successful multidentate binding through shifts in the vibrational peaks of the functional groups involved in coordination. For example:

  • For P–O–Pb bonds: A new vibration signal appears near 1076 cm⁻¹, and the P–O(H) vibration peaks show obvious downward shifts [18].
  • For Lewis acid-base coordination: When molecules like 1,4-butanediamine (BDA) interact with Pb²⁺/Sn²⁺, the stretching and bending vibrations of the N-H group (e.g., at 3343, 3284, and 1599 cm⁻¹) show obvious shifts [81].
  • For complex multidentate ligands: In a Sb(SU)₂Cl₃ complex, the presence of a characteristic Se–Sb vibrational band between 350–300 cm⁻¹, along with N-H stretching vibrations, confirms the complex's structure and its potential for multi-site binding [1].

3. Our UPS data shows minimal Fermi level shift after passivation. What could be the reason?

A minimal Fermi level shift suggests that the passivation treatment did not significantly alter the electronic structure of the perovskite surface. Potential reasons include:

  • Insufficient Coverage: The passivation layer may not be continuous or dense enough to induce a measurable change in the overall surface electronic properties.
  • Competing Effects: The passivator might be simultaneously passivating both electron-donating and electron-accepting defects, resulting in a net zero shift of the Fermi level.
  • Ineffective Binding: The molecules may not be interacting strongly enough with the surface to cause charge redistribution. A successful treatment, such as with a bimolecular interlayer, typically causes a clear decrease in work function (e.g., from 4.54 eV to 4.20 eV) and an upshift of the valence band maximum, indicating a more n-type surface [18].

4. How does DFT modeling help distinguish between single-site and multi-site binding configurations?

Density Functional Theory (DFT) calculations provide quantitative insights into the stability and electronic effects of different binding modes:

  • Adsorption Energy: Multi-site binding configurations generally have significantly lower (more negative) adsorption energies compared to single-site binding, indicating a more stable and robust interaction with the perovskite surface [1].
  • Charge Transfer Analysis: Multi-site binding often induces a substantially larger charge transfer at the interface compared to single-site binding, which enhances interfacial bonding and defect passivation [1].
  • Defect Formation Energy: DFT can calculate the energy required to form surface defects. Effective multi-site passivators increase the formation energy of common defects like iodine vacancies (Vᵢ) and lead vacancies (VPb), thereby suppressing their formation [1].

Troubleshooting Common Experimental Issues

Table 1: Troubleshooting XPS, FTIR, and UPS Characterization

Technique Problem Possible Cause Solution
XPS No discernible chemical shift is observed after passivation. 1. Passivator not bonded or only physisorbed.2. Surface contamination blocking interaction.3. X-ray-induced damage degrading the sample. 1. Verify reaction conditions (solvent, concentration).2. Ensure rigorous surface cleaning pre-treatment.3. Use a lower X-ray power and shorter analysis time [82].
FTIR Poor signal-to-noise ratio for thin film samples. 1. Insufficient film thickness.2. Signal dominated by substrate absorption. 1. Use Attenuated Total Reflection (ATR)-FTIR mode to enhance surface signal [82].2. Increase the number of scans to improve signal averaging.
UPS The secondary electron cutoff (SECO) is unclear or broad. 1. Sample charging, especially on insulating substrates.2. Surface contamination. 1. Apply a low negative bias to the sample (e.g., -5 to -10 V) to shift spectra away from detector cutoff.2. Ensure ultra-high vacuum (UHV) conditions and clean surfaces.
General Inconsistency between measured Fermi level alignment and device performance. The measured area may not be representative due to surface potential inhomogeneity. Use complementary techniques like Kelvin Probe Force Microscopy (KPFM) to map surface potential distribution and verify uniformity after passivation [18] [81].

Table 2: Quantitative Data from Passivation Studies Using XPS, FTIR, and UPS

Analysis Technique Key Measurable Parameter Control Sample Passivated Sample Implication
XPS Sn⁴⁺ percentage (Sn 3d) [81] 27.3% 19.1% Reduction in harmful Sn⁴⁺ defects.
XPS I/(Pb+Sn) surface ratio [81] Deviated from 3:1 Close to ideal 3:1 Surface stoichiometry restored.
UPS Work Function (eV) [18] 4.54 eV 4.20 eV More n-type surface, improved electron extraction.
UPS VBM to Fermi Level (eV) [18] 0.87 eV 1.20 eV Fermi level upshifted closer to conduction band.
KPFM Surface Potential Difference, ΔCPD [81] 45 meV 29 meV More uniform surface potential, reduced defect density.
Device Non-radiative Voc loss [18] — 59 mV One of the smallest losses reported, indicating superior passivation.

Detailed Experimental Protocols

Protocol 1: XPS Characterization for Covalent Bond Verification

  • Sample Preparation: Deposit the perovskite film and subsequent passivation layers on a conductive substrate (e.g., ITO/glass). For air-sensitive samples, use an inert atmosphere transfer vessel to avoid air exposure before loading into the XPS chamber [83].
  • Instrument Setup: Use a monochromatic Al Kα X-ray source (1486.6 eV). Set the analyzer pass energy to 50 eV for high-resolution scans and 100 eV for survey scans. To minimize radiation damage, limit the exposure time for a single sample to 1 hour or less [82].
  • Energy Calibration: Calibrate the spectra by setting the adventitious carbon C 1s peak to 285.0 eV [82].
  • Data Acquisition:
    • Acquire a survey spectrum (0-1100 eV) to identify all elements present.
    • Acquire high-resolution spectra for core levels of interest: Pb 4f, O 1s, I 3d, N 1s, Sn 3d, P 2p, and any element specific to the passivator.
  • Data Analysis:
    • Use a Shirley or Tougaard background subtraction.
    • Deconvolute the high-resolution spectra using mixed Gaussian-Lorentzian line shapes.
    • Identify new chemical states (e.g., P–O–Pb) by their distinct binding energies and compare their intensities with and without passivation [18].

Protocol 2: FTIR Spectroscopy for Binding Mode Analysis

  • Sample Preparation: Prepare the passivated perovskite film on an IR-transparent substrate (e.g., Si wafer). For ATR-FTIR, the substrate is not critical as the crystal is in direct contact with the sample.
  • Instrument Setup: Use an ATR-FTIR spectrometer with a diamond or ZnSe crystal. Collect 64 scans at a resolution of 4 cm⁻¹ to ensure a good signal-to-noise ratio [82].
  • Background Measurement: Collect a background spectrum with the clean ATR crystal (or bare substrate in transmission mode).
  • Data Acquisition: Place the sample in firm contact with the ATR crystal and collect the spectrum in the range of 4000-600 cm⁻¹.
  • Data Analysis:
    • Perform baseline correction and atmospheric compensation (for CO₂ and H₂O vapor).
    • Identify key functional group regions (e.g., O-H, N-H, C=O, P=O).
    • Look for peak shifts, broadening, or the appearance of new peaks (e.g., Se-Sb vibration at 350-300 cm⁻¹) compared to the spectrum of the pure passivator and the control perovskite film [1] [81].

Protocol 3: UPS Measurement for Surface Energetics

  • Sample Preparation: Identical to XPS preparation. Cleanliness and inert transfer are critical.
  • Instrument Setup: Use a He I radiation source (21.22 eV). Apply a small negative bias (e.g., -5 V to -10 V) to the sample to clearly observe the secondary electron cutoff (SECO).
  • Data Acquisition:
    • SECO Region: Acquire a spectrum at low kinetic energy (0-20 eV) to determine the work function. WF = hν - (Ecutoff - EFermi), where E_cutoff is the kinetic energy at the SECO.
    • Valence Band Region: Acquire a spectrum near the Fermi edge (high kinetic energy) to determine the valence band maximum (VBM). Plot the spectrum on a logarithmic scale and extrapolate the leading edge to the baseline to find the VBM onset.
  • Data Analysis: Calculate the work function and the position of the VBM relative to the Fermi level. A decrease in work function and an increase in the VBM-to-Fermi level distance indicate a shift towards an n-type surface [18].

Experimental Workflow and Data Interpretation

Workflow for Passivation Analysis

Start Start: Sample Prep (Perovskite + Passivation) FTIR FTIR Analysis Start->FTIR XPS XPS Analysis Start->XPS UPS UPS Analysis Start->UPS Integrate Integrate Results FTIR->Integrate Binding Group Vibration Shifts XPS->Integrate Chemical State & Bond Formation UPS->Integrate Work Function & Band Alignment DFT DFT Calculation DFT->Integrate Theoretical Modeling of Binding Confirm Confirm Mechanism Integrate->Confirm Multi-technique Correlation

Data Correlation for Mechanism Confirmation

Evidence Evidence for Multidentate Passivation FTIR2 FTIR: Multi-group vibration shifts Evidence->FTIR2 XPS2 XPS: New chemical states (e.g., P-O-Pb) Evidence->XPS2 UPS2 UPS: Fermi level shift (n-type) Evidence->UPS2 DFT2 DFT: High adsorption energy & charge transfer Evidence->DFT2 Outcome Reduced Non-Radiative Recombination FTIR2->Outcome Strong Binding XPS2->Outcome Defect Passivation UPS2->Outcome Improved Charge Extraction DFT2->Outcome Stable Configuration

Research Reagent Solutions for Passivation Studies

Table 3: Key Reagents for Multidentate Passivation Research

Reagent / Material Function / Role in Passivation Example Use Case
4-Methoxyphenylphosphonic Acid (MPA) Forms strong covalent P–O–Pb bonds with the perovskite surface, diminishing defect density and upshifting the Fermi level [18]. Synergistic bimolecular interlayer for inverted perovskite solar cells [18].
2-Phenylethylammonium Iodide (PEAI) Creates a negative surface dipole, constructing a more n-type perovskite surface and enhancing electron extraction [18]. Second component in a bimolecular interlayer to optimize energetics [18].
Ethylenediammonium Diiodide (EDAI₂) Passivates organic cation (Vₐ) and Iodide (Vᵢ) vacancy defects through its ammonium and iodide ions [81]. Surface reconstruction of Sn-Pb perovskite films in tandem with a polishing agent [81].
1,4-Butanediamine (BDA) Acts as a chemical polishing agent, reducing Sn⁴⁺-related surface defects and improving surface stoichiometry [81]. Surface reconstruction of Sn-Pb perovskite films [81].
Sb(SU)₂Cl₃ Complex Multidentate ligand that binds to four adjacent undercoordinated Pb²⁺ sites via two Se and two Cl atoms, enabling deep trap passivation [1]. Multi-site passivator for fully air-processed perovskite solar cells [1].
Ethylene Diamine Tetraacetic Acid (EDTA) Multidentate ligand that chelates suspended Pb²⁺ ions and passivates I⁻ vacancies, while crosslinking quantum dots to improve electronic coupling [31]. Surface surgery treatment for perovskite quantum dot solar cells [31].

Conclusion

The strategic application of multidentate ligands represents a paradigm shift in suppressing non-radiative recombination, directly addressing one of the most significant bottlenecks in perovskite optoelectronics. By enabling stronger, more stable binding to surface defects through multiple coordination sites, this approach has proven capable of simultaneously enhancing device efficiency, operational voltage, and long-term stability, as evidenced by perovskite/silicon tandem cells surpassing 31% efficiency and minimal non-radiative voltage losses below 60 mV. Future research directions must focus on the development of dynamically responsive ligands, the exploration of non-toxic alternatives, and the refinement of scalable deposition techniques to bridge the gap between laboratory innovation and widespread commercial deployment. The insights gleaned from this multidisciplinary field are poised to accelerate the advent of a new generation of high-performance, durable renewable energy technologies.

References