Surface Passivated Halide Perovskite: Advanced Strategies For Defect Mitigation And Enhanced Optoelectronic Performance

Fundamental Defect Chemistry And Surface Characteristics Of Halide Perovskite Materials

The performance bottleneck in halide perovskite devices originates predominantly from surface and grain boundary defects inherent to the ionic crystal structure. Metal halide perovskites, typically represented by the formula ABX₃ (where A = Cs⁺, CH₃NH₃⁺, or HC(NH₂)₂⁺; B = Pb²⁺ or Sn²⁺; X = Cl⁻, Br⁻, or I⁻), exhibit a soft ionic lattice prone to forming under-coordinated sites during solution processing and thermal annealing 1. These defects manifest as under-coordinated halide anions (X⁻), uncoordinated metal cations (Pb²⁺ or Sn²⁺), and halide vacancies (V_X), which create deep trap states within the electronic bandgap 26.

Without passivation treatment, under-coordinated halide anions and defect sites function as hole-traps, leading to significant charge accumulation at the perovskite/charge-transport-layer heterojunction 26. This charge accumulation accelerates non-radiative recombination, reduces carrier lifetime (typically from >1 μs to <100 ns in defective films), and establishes disadvantageous charge density profiles that inhibit efficient charge extraction under operational conditions 611. For instance, in mixed-halide perovskites (e.g., CsPb(Br_xCl₁₋ₓ)₃), chlorine vacancies create particularly deep trap states that irreversibly capture charge carriers and suppress radiative recombination channels, limiting blue LED external quantum efficiency (EQE) to below 5% for emission at 460–480 nm 14.

The surface of halide perovskites constitutes an ionic cocktail of positive and negative charges, rendering it a strongly evolving interface subject to processing details and ambient conditions 10. The volatile nature of organic (methylammonium, formamidinium) and halide species renders the surface lead-rich with uncoordinated Pb²⁺ ions 10. This Pb-rich surface is expected to electrostatically repel positively charged passivating molecules, complicating the design of effective passivation strategies 10. Moreover, degradation of perovskite films generally initiates from defective surfaces and grain boundaries due to higher reactivity of defect sites, making them most vulnerable to attack by moisture and oxygen 15.

Key defect-related parameters quantified in recent studies include:

• Trap density: Unpassivated films exhibit trap densities of 10¹⁶–10¹⁷ cm⁻³, reducible to 10¹⁵ cm⁻³ or lower upon effective passivation 910
• Carrier recombination lifetime: Passivation extends lifetimes from ~100 ns to >1 μs, as measured by time-resolved photoluminescence (TRPL) 9
• Open-circuit voltage (V_oc) deficit: Defect passivation reduces V_oc loss from ~0.4 V to <0.35 V relative to the Shockley-Queisser limit for 1.55 eV bandgap perovskites 9

Understanding this defect chemistry is essential for designing targeted passivation strategies that address both cationic and anionic defects, as well as structural imperfections at surfaces and grain boundaries.

Molecular Passivation Strategies For Surface Passivated Halide Perovskite

Organic Ligand-Based Passivation Approaches

Organic molecules with Lewis base or Lewis acid functionalities have emerged as versatile passivating agents for halide perovskites. The passivation mechanism relies on coordinate bonding between functional groups and under-coordinated surface ions, effectively screening electrostatic charges and neutralizing trap states 2611.

Halogen bond donors and sulfur-containing molecules: Iodopentafluorobenzene (a halogen bond donor) and thiophene (sulfur-containing organic molecule) have been employed in supramolecular assemblies to passivate under-coordinated halide anions 2611. These passivating agents bind to and screen the electrostatic charge from under-coordinated halide ions and defect sites, resolving charge accumulation issues at heterojunctions 611. Solar cells treated with such agents demonstrate PCE increases from 13% to over 16%, with the enhancement attributed to reduced hole-trapping and improved charge extraction 26.

Nitrogen-containing Lewis bases: Pyridine and related nitrogen-donor molecules passivate under-coordinated Pb²⁺ sites via Lewis base-acid interactions 2611. The lone pair electrons on nitrogen coordinate with Pb²⁺, neutralizing positive surface charges and reducing trap-mediated recombination 6. This approach is particularly effective for lead-rich surfaces resulting from volatile organic cation loss during processing 10.

Carboxylic acid derivatives: 4-Hydroxypicolinic acid (4HPA) has been demonstrated to passivate surface defects in metal halide perovskite thin films by binding as a ligand to the perovskite surface 1. The carboxyl and hydroxyl groups provide multiple coordination sites for interaction with surface metal cations, forming stable chelate complexes that reduce defect density and enhance film stability 1.

Organic dyes as multifunctional passivators: Low-cost industrial organic pigments have been applied as multifunctional passivation agents via facile spin-coating-annealing processes 3. These organic dye derivatives, when converted to their active dye form through thermal annealing, form coatings that at least partially cover the perovskite surface, passivating defects and enhancing both efficiency and stability of perovskite solar cells (PSCs) 3. The composite materials comprising perovskite films with organic dye coatings have achieved highly efficient and stable device performance 3.

Quaternary Ammonium Halide (QAH) Passivation

Quaternary ammonium halides represent a particularly effective class of dual-defect passivators, capable of simultaneously addressing both cationic and anionic defects through their positive and negative ionic components 9. QAHs, which include B-complex vitamin derivatives, feature quaternary ammonium cations that interact with negatively charged defects (such as Pb-I antisites or under-coordinated halide ions) and halide anions that passivate positively charged defects (such as under-coordinated Pb²⁺ or halide vacancies) 9.

This dual-passivation mechanism advantageously reduces charge trap density and elongates carrier recombination lifetime 9. For perovskite solar cells with a bandgap of 1.55 eV, QAH passivation increases open-circuit voltage to 1.15 V and boosts PCE to 21.0% 9. The universality of QAH passivation has been demonstrated across perovskites with bandgaps ranging from 1.51 eV to 1.72 eV, yielding efficiency improvements of 10–35% 9. Moreover, defect healing through QAH treatment significantly enhances the stability of perovskite films under ambient conditions 9.

Specific QAH derivatives examined include:

• Choline-based zwitterions: These molecules combine positive quaternary ammonium groups with negative functional groups, enabling simultaneous passivation of both under-coordinated Pb²⁺ and halide vacancies 9
• Octylammonium iodide: This long-chain alkylammonium halide forms a thin two-dimensional (2D) perovskite layer at the surface, passivating defects while maintaining charge transport properties 18
• Phenethylammonium iodide (PEAI) and butylammonium iodide (BAI): These larger organic ammonium cations form 2D Ruddlesden-Popper phases (A₁ₖA₂ₙ₊₁B₁ₙX₁₃ₙ₊₁) at grain boundaries and surfaces, providing both electronic passivation and moisture barrier properties 18

Multidentate Ligand-Mediated Chemical Etching

A novel passivation strategy involves selective chemical etching of metal atoms from the perovskite surface using multidentate ligands 7. This approach "cleans" the metal halide perovskite surface through removal of excess metal atoms (particularly lead), thereby inhibiting halide segregation by suppressing halide Frenkel defect formation 7. The multidentate ligands comprise organic compounds or salts containing three or more binding groups capable of chelating surface metal cations 7.

This chemical etching process results in:

• Suppressed halide segregation: Critical for maintaining color-stable emission in mixed-halide perovskites used in tunable LEDs and tandem solar cells 7
• Enhanced electroluminescence EQE: Chemically etched perovskites achieve high EQE values with stable emission wavelengths under operational bias 7
• Reduced Frenkel defect formation: By removing excess surface metal atoms, the driving force for halide vacancy-interstitial pair formation is diminished 7

Representative multidentate ligands include ethylenediaminetetraacetic acid (EDTA) and other polycarboxylic acids with multiple coordination sites 7.

Inorganic Passivation Layers

Potassium halide-rich interlayers: Potassium and halide-rich layers formed at perovskite interfaces provide effective passivation by reducing parasitic non-radiative losses, maintaining high charge mobility, and stabilizing bandgap instability 12. These interlayers also reduce migration of halide ions from the perovskite layer into adjacent charge transport layers or metal electrodes, and vice versa 12. The potassium halide-rich layers can be positioned on upper and/or lower surfaces of the perovskite layer, providing versatile passivation configurations 12.

Fluorine-containing passivation layers: A dense fluoride film (e.g., BaF₂) formed on perovskite nanocrystal surfaces within microporous/mesoporous templates addresses structural instability and surface defects 16. This fluorine-containing passivation layer is synthesized via hydrothermal reaction to enrich F⁻ ions, forming a protective coating that enhances luminescent stability and performance under high temperature and light exposure 16. The fluoride passivation reduces surface defects and maintains luminescence intensity, addressing the ionic nature and soft lattice characteristics that make perovskites prone to structural damage 16.

Metal-organic framework (MOF) glass composites: Amorphous zeolitic imidazolate frameworks (aZIFs) exhibit specific interfacial interactions with metal halide perovskites that stabilize metastable and optically active polymorphic phases while passivating both Lewis acid and base surface defects 4. The structural flexibility of MOF glass enables effective passivation for a variety of metal halide perovskites, including lead, tin, germanium, and double perovskites (e.g., Ag-Bi halide double perovskites) 4. The interfacial interactions advantageously passivate under-coordinated metal or halide sites and negatively charged metal-halide anti-sites at the perovskite surface 4.

Two-dimensional (2D) materials: Graphene and graphene oxide, possessing excellent barrier/anti-diffusion performance along with superior optical, electrical, and mechanical properties, have been applied as passivation layers for halide perovskite optoelectronic devices 13. These 2D material passivation layers are configured to capture one type of charge from the electron-hole pair generated in the photoactive layer, simultaneously providing moisture protection and preventing egress of organic ions when exposed to heat 13. Unlike bulk encapsulation techniques that only protect against moisture, 2D material passivation directly addresses both environmental and thermal stability challenges 13.

Performance Enhancements Achieved Through Surface Passivation Of Halide Perovskite

Solar Cell Efficiency And Stability Improvements

Surface passivation strategies have yielded substantial improvements in perovskite solar cell performance metrics:

Power conversion efficiency (PCE) gains: Passivation with organic molecules, QAHs, and multidentate ligands has increased PCE from baseline values of 13–18% to record values exceeding 21–25.2% 2689. The best-performing PSCs to date (25.2% PCE) have been achieved using quaternary ammonium iodide passivators derived from primary amines 8. These efficiency gains result from reduced trap-mediated recombination, enhanced charge extraction, and improved fill factors (FF) 9.

Open-circuit voltage (V_oc) enhancement: Dual-defect passivation with QAHs increases V_oc for 1.55 eV bandgap perovskites to 1.15 V, approaching the theoretical Shockley-Queisser limit 9. This represents a reduction in V_oc deficit from ~0.4 V to <0.35 V, directly attributable to suppressed non-radiative recombination at passivated surfaces 9.

Operational and environmental stability: Passivated perovskite films demonstrate dramatically enhanced stability under ambient conditions 2611. For example, CH₃NH₃SnI₃ perovskite films, which are extremely sensitive to humidity and oxygen and degrade within seconds under ambient conditions when unpassivated, remain stable for hours, days, and even weeks following molecular passivation 2611. This stability enhancement is critical for tin and germanium-based perovskites, which are typically unstable in air 2611.

Moisture and oxygen resistance: Passivation with hydrophobic organic molecules and 2D materials provides physical barriers against moisture ingress while chemically neutralizing reactive surface sites 1315. Conversion of perovskite surfaces to insoluble, wide-bandgap lead oxysalts through controlled oxidation represents an alternative passivation strategy that enhances moisture resistance 15.

Light-Emitting Diode (LED) Performance

Photoluminescence quantum yield (PLQY) enhancement: Surface passivation of perovskite quantum dots (QDs) with inorganic-organic hybrid ion pairs (e.g., sulfur-based or halide-based ion pairs) achieves PLQY values of 70% or higher 5. Passivated perovskite QDs (e.g., CsPbCl₃, CsPbBr₃, CsPbCl₃₋ₓBrₓ) exhibit amplified spontaneous emission through one-photon or two-photon excitation, enabling applications in low-threshold lasers and high-brightness LEDs 5.

External quantum efficiency (EQE) improvements: Green and red perovskite LEDs have achieved EQEs exceeding 20% through effective surface passivation 14. However, blue perovskite LEDs still lag with EQEs below 5% for emission at 460–480 nm (Rec. 2020 standard: 467 nm) and below 11% for sky-blue emission at 480–490 nm 14. Passivation of chlorine vacancies in mixed-halide (Cl/Br) perovskites using organic pseudohalogens (e.g., organic cations associated with pseudohalogen moieties) addresses the deep trap states responsible for this performance gap 14.

Color stability in mixed-halide LEDs: Chemical etching with multidentate ligands suppresses halide segregation in mixed-halide perovskites, maintaining color-stable emission under operational bias 7. This is essential for display applications requiring precise color coordinates (e.g., Rec. 2020 color gamut) 14.

Photodetector And Radiation Detector Applications