Grain Boundary Engineered Halide Perovskite: Advanced Strategies For Defect Passivation And Performance Enhancement

Fundamental Mechanisms Of Grain Boundary Defects In Halide Perovskite Structures

Grain boundaries in polycrystalline halide perovskite films represent planar defects where adjacent crystalline grains meet, creating regions of structural disorder and elevated defect density. Unlike single-crystal perovskites, which exhibit trap-state densities as low as 10⁹–10¹⁰ cm⁻³, polycrystalline films typically display defect concentrations exceeding 10¹⁶ cm⁻³, predominantly localized at grain boundaries 15. These GB defects arise from several mechanisms: (i) incomplete coordination of surface ions during crystallization, leaving undercoordinated Pb²⁺ or halide vacancies; (ii) lattice mismatch and strain accumulation at grain interfaces; and (iii) preferential segregation of impurities and secondary phases to grain boundaries during film formation 3,12.

The electronic impact of grain boundary defects manifests through multiple pathways. First, undercoordinated Pb²⁺ cations at GBs create deep-level trap states within the bandgap (typically 0.3–0.5 eV from band edges), serving as Shockley-Read-Hall recombination centers that reduce carrier lifetimes from >1 μs in single crystals to <100 ns in polycrystalline films 6. Second, strong Coulomb interactions between ions in adjacent grains across the GB interface induce additional defect states, as charge redistribution at the boundary creates local electric fields that further trap charge carriers 3,12. Third, grain boundaries act as preferential pathways for ion migration under operational conditions (light, bias, heat), accelerating device degradation through halide segregation and phase instability 1.

Experimental characterization via transmission electron microscopy (TEM), Kelvin probe force microscopy (KPFM), and photoluminescence (PL) mapping consistently reveals that grain boundaries exhibit lower PL intensity (by 50–80%) and reduced surface potential compared to grain interiors, confirming their role as non-radiative recombination sites 2,20. Time-resolved PL studies demonstrate that carrier lifetimes at grain boundaries are 5–10× shorter than within grains, with typical GB recombination velocities of 10³–10⁴ cm/s 3. These findings underscore the critical need for grain boundary engineering strategies to passivate defects and restore optoelectronic performance.

Potassium Halide Passivation At Grain Boundary Interfaces Of Halide Perovskite

Potassium halide (KX, where X = I, Br, Cl) passivation has emerged as a highly effective strategy for grain boundary engineering in halide perovskites, leveraging the unique chemical affinity of K⁺ ions for halide-rich environments and their ability to form stable interlayers at GB interfaces 1. When introduced during perovskite film formation—typically via spin-coating of KI or KBr solutions atop precursor films or incorporation into the perovskite precursor solution—potassium halides preferentially segregate to grain boundaries and film surfaces, forming continuous or intermittent K⁺- and halide-rich layers that decorate GB interfaces 1.

Mechanisms And Structural Characteristics Of Potassium Halide Grain Boundary Layers

The passivation mechanism operates through multiple synergistic effects:

• Defect site coordination: K⁺ ions coordinate with undercoordinated halide anions (I⁻, Br⁻) at grain boundaries, filling halide vacancies and neutralizing positively charged defects. Energy-dispersive X-ray spectroscopy (EDS) mapping confirms K and halide enrichment at GBs, with local K concentrations reaching 5–10 at% compared to <1 at% in grain interiors 1.
• Electrostatic screening: The potassium halide interlayer reduces Coulomb interactions between adjacent grains by screening ionic charges at the GB interface, thereby suppressing defect-state formation induced by intergrain charge redistribution 1,3.
• Bandgap stabilization: Potassium halide layers mitigate bandgap instability under illumination by preventing halide ion migration across grain boundaries, a primary degradation pathway in mixed-halide perovskites (e.g., MAPbI₃₋ₓBrₓ) 1.

Structural analysis via cross-sectional TEM reveals that potassium halide layers at grain boundaries are typically 2–5 nm thick and can be either continuous (forming a complete interfacial layer) or intermittent (decorating discrete GB segments) depending on processing conditions 1. X-ray diffraction (XRD) patterns show no additional peaks corresponding to bulk KX phases, indicating that the potassium halide remains as an amorphous or highly dispersed interlayer rather than forming separate crystalline domains 1. Importantly, these interlayers do not significantly impede charge transport: Hall effect measurements demonstrate that hole mobility remains high (>10 cm²/V·s) in KI-passivated films, as the thin interlayer allows efficient tunneling or thermionic emission of carriers across the GB 1.

Performance Enhancements In Optoelectronic Devices

Devices incorporating potassium halide grain boundary passivation exhibit substantial performance improvements:

• Photovoltaic cells: KI-passivated MAPbI₃ solar cells achieve PCEs of 20.5–21.2%, representing a 1.5–2% absolute increase over control devices (PCE ~19%), with open-circuit voltage (Voc) gains of 50–80 mV attributed to reduced non-radiative recombination at GBs 1. External quantum efficiency (EQE) measurements show enhanced photocurrent collection across the entire absorption spectrum, particularly at longer wavelengths (>700 nm) where carrier diffusion lengths are critical 1.
• Light-emitting diodes: Perovskite LEDs with KBr-passivated grain boundaries demonstrate PLQY increases from 45% to 78%, with corresponding external quantum efficiencies (EQE) rising from 8% to 15% 1. Time-resolved PL reveals that average carrier lifetimes increase from 120 ns to 380 ns upon KBr treatment, confirming effective GB defect passivation 1.
• Stability improvements: Potassium halide passivation enhances operational stability by suppressing ion migration at grain boundaries. Devices retain >90% of initial PCE after 1000 hours of continuous illumination (1-sun, 60°C), compared to <70% retention for untreated controls 1. Accelerated aging tests under 85°C/85% relative humidity conditions show that KI-passivated films maintain structural integrity for >500 hours, whereas control films degrade within 200 hours 1.

Lewis Base Passivation Strategies For Grain Boundary Defect Suppression

Lewis base molecules—electron-pair donors such as thiourea, pyridine, trioctylphosphine oxide (TOPO), and polymer-based ligands—provide an alternative grain boundary passivation approach by forming coordinate covalent bonds with undercoordinated Pb²⁺ cations at defect sites 2,6. This strategy is particularly effective for passivating deep-level traps associated with Pb-I antisite defects and Pb-dimer formations at grain boundaries 2.

Chemical Bonding And Defect Passivation Mechanisms

Lewis base passivation operates through direct chemical coordination:

• Pb²⁺ coordination: Lewis bases donate electron pairs to vacant orbitals of undercoordinated Pb²⁺ ions at grain boundaries, forming stable Pb-N or Pb-O coordinate bonds (bond energies ~150–250 kJ/mol) that eliminate deep trap states 2,6. Fourier-transform infrared spectroscopy (FTIR) confirms the formation of Pb-ligand bonds through characteristic shifts in N-H or P=O stretching frequencies 2.
• Grain boundary crystallization control: Incorporation of Lewis bases during film formation influences perovskite crystallization kinetics, promoting larger grain sizes (500 nm–2 μm vs. 200–500 nm for controls) and reducing overall GB density 2. Atomic force microscopy (AFM) shows that Lewis base-treated films exhibit smoother surfaces (RMS roughness <10 nm) with fewer pinholes 2.
• Selective GB accumulation: Similar to potassium halides, Lewis base molecules are expelled from growing perovskite grains during crystallization and accumulate at grain boundaries, where they passivate defects without disrupting the bulk crystal structure 2,12. Cross-sectional energy-filtered TEM (EFTEM) mapping reveals nitrogen or phosphorus enrichment at GBs in thiourea- or TOPO-treated films, respectively 2.

Performance Metrics And Application-Specific Benefits

Lewis base passivation delivers measurable performance enhancements across multiple device platforms:

• Photovoltaic applications: Thiourea-passivated FAPbI₃ solar cells achieve PCEs of 22.1%, with Voc values reaching 1.15 V (compared to 1.08 V for controls), corresponding to a voltage deficit of only 0.38 V relative to the bandgap 2. Fill factors (FF) improve from 76% to 81% due to reduced series resistance and enhanced charge extraction 2.
• Light-emitting devices: Perovskite nanocrystal LEDs with polymer-based Lewis base passivation (e.g., polyethylenimine) exhibit PLQY >90% and EQE >20%, with emission linewidths (FWHM) narrowing from 25 nm to 18 nm due to reduced energetic disorder at grain boundaries 6. Operational lifetimes (T₅₀, time to 50% initial brightness) exceed 100 hours at 100 cd/m², a 5× improvement over unpassivated devices 6.
• Photodetector performance: Lewis base-passivated perovskite photodetectors demonstrate dark current densities <10⁻⁹ A/cm² and detectivities (D*) exceeding 10¹³ Jones across the visible spectrum, enabled by suppressed GB-mediated leakage currents 2.

Non-Polar Organic Molecule Insertion For Grain Boundary Coulomb Interaction Blocking

A distinct grain boundary engineering strategy involves the incorporation of non-polar organic molecules—such as polycyclic aromatic hydrocarbons (PAHs) including pyrene, perylene, and coronene—that do not chemically bond to the perovskite but instead physically occupy grain boundary regions to block intergrain Coulomb interactions 3,12. This approach addresses a specific GB defect mechanism: the strong electrostatic coupling between ions in adjacent grains across the boundary interface, which induces additional trap states beyond those arising from simple undercoordination 3,12.

Mechanism Of Non-Bonding Grain Boundary Passivation

The passivation mechanism differs fundamentally from chemical coordination strategies:

• Crystallization-driven segregation: Non-polar organic molecules are added to the perovskite precursor solution (typically at 0.5–2 mol% relative to perovskite) and are expelled from growing perovskite grains during crystallization due to their incompatibility with the ionic perovskite lattice 3,12. Molecular dynamics simulations confirm that PAH molecules preferentially accumulate at grain boundaries, where they form 1–3 nm thick organic interlayers 3.
• Coulomb interaction screening: The organic interlayer acts as a dielectric spacer that reduces the electric field strength between adjacent grains, thereby suppressing the formation of interfacial trap states induced by charge redistribution 3,12. Kelvin probe measurements show that the contact potential difference (CPD) between grains decreases from 150–200 mV to <50 mV upon PAH incorporation, indicating reduced intergrain electrostatic coupling 3.
• Preservation of charge transport: Despite the insulating nature of organic molecules, the thin interlayer thickness (1–3 nm) allows efficient charge carrier tunneling, maintaining high mobility (>15 cm²/V·s for holes) 3,12. Temperature-dependent conductivity measurements reveal that charge transport remains band-like rather than hopping-dominated, confirming minimal disruption to electronic properties 3.

Experimental Validation And Device Performance

Devices incorporating non-polar organic molecule GB passivation demonstrate significant improvements:

• Photoluminescence enhancement: Pyrene-treated MAPbI₃ films exhibit 3–5× higher PL intensity compared to controls, with spatially resolved PL mapping showing uniform emission across grain boundaries rather than the typical GB quenching observed in untreated films 3,12. Time-resolved PL reveals carrier lifetime increases from 200 ns to 650 ns, indicating effective suppression of GB-mediated non-radiative recombination 3.
• Solar cell efficiency gains: Perylene-passivated (FAPbI₃)₀.₉₅(MAPbBr₃)₀.₀₅ solar cells achieve PCEs of 21.8%, with Voc improvements of 60–70 mV and FF enhancements from 78% to 82% 3,12. Incident photon-to-current efficiency (IPCE) measurements show reduced losses at grain boundaries, with integrated photocurrent densities reaching 24.5 mA/cm² 3.
• Stability under operational stress: Non-polar organic molecule passivation enhances thermal stability, with devices retaining >85% of initial PCE after 500 hours at 85°C (compared to <60% for controls), attributed to the organic interlayer's ability to suppress ion migration at elevated temperatures 3,12.

Two-Dimensional/Three-Dimensional Heterostructure Formation At Grain Boundaries

The strategic incorporation of two-dimensional (2D) layered perovskites at the grain boundaries of three-dimensional (3D) perovskite films represents an advanced grain boundary engineering approach that combines defect passivation with enhanced moisture and thermal stability 7,10. This heterostructure strategy leverages the hydrophobic nature of long-chain organic cations (e.g., butylammonium, phenethylammonium) in 2D perovskites to create protective barriers at grain boundaries while maintaining the superior optoelectronic properties of the 3D perovskite bulk 7.

Synthesis And Structural Characteristics Of 2D/3D Grain Boundary Heterostructures

The formation of 2D/3D heterostructures at grain boundaries is achieved through several synthetic routes:

• Sequential deposition: A 3D perovskite film (e.g., MAPbI₃, FAPbI₃) is first deposited via spin-coating or blade-coating, followed by treatment with a solution containing long-chain alkylammonium halides (e.g., butylammonium iodide, BAI) 7. The alkylammonium cations preferentially intercalate at grain boundaries and surface regions, reacting with residual PbI₂ or undercoordinated Pb²⁺ to form 2D perovskite phases with the general formula (RNH₃)₂PbI₄ or (RNH₃)₂(MA)ₙ₋₁PbₙI₃ₙ₊₁ (where R = alkyl chain, n = number of inorganic layers) 7,10.
• **Mixed-precursor approach