Halide Perovskite Thick Film: Advanced Fabrication Methods, Structural Engineering, And Applications In Optoelectronics

Molecular Composition And Structural Characteristics Of Halide Perovskite Thick Films

Halide perovskite thick films are distinguished by their ABX₃ crystal structure, where A represents an organic cation (methylammonium CH₃NH₃⁺, formamidinium CH(NH₂)₂⁺, or inorganic cesium Cs⁺), B denotes a divalent metal cation (Pb²⁺, Sn²⁺, Ge²⁺, Mn²⁺, or Bi³⁺), and X corresponds to halide anions (I⁻, Br⁻, Cl⁻, or mixed halides) 259. The thick-film morphology, defined as films with thicknesses ranging from 5 μm to over 100 μm, fundamentally alters the optoelectronic properties compared to conventional spin-coated thin films (typically 300–800 nm) 17.

The structural integrity of halide perovskite thick films critically depends on grain size, crystallographic orientation, and phase purity. High-quality thick films exhibit average grain sizes exceeding 30 μm with substantial crystal orientation characterized by Lotgering factors of 0.6–1.0, indicating preferential alignment along specific crystallographic planes 9. For instance, CsPbBr_mI₃₋_m thick films prepared via spray-coating demonstrate nanosheet structures with {00l} orientation and Lotgering factors approaching 80–100%, which significantly enhance charge carrier mobility and reduce non-radiative recombination 1.

Phase composition in thick films can be engineered to include pure three-dimensional (3D) perovskite phases, two-dimensional (2D) layered perovskites with formula (RNH₃)₂(MA)_n₋₁M_nX₃_n₊₁ (where R represents long-chain alkyl groups and n = 1–50), or mixed 2D/3D heterostructures 1112. The 2D phases exhibit quantum confinement effects with tunable bandgaps spanning 1.5–2.3 eV depending on the layer number n, while maintaining superior moisture stability due to hydrophobic organic spacer layers 11. Photoluminescence spectra of 2D perovskite thick films show narrow emission peaks at 550–650 nm (green) or 700–800 nm (red) with full-width-at-half-maximum (FWHM) values of 20–65 nm, indicating high color purity for display applications 11.

The crystallinity level in optimized thick films reaches 90–95%, as confirmed by X-ray diffraction (XRD) analysis showing sharp, intense diffraction peaks with minimal amorphous background 9. This high crystallinity correlates directly with extended carrier diffusion lengths on the order of 1–10 μm in thick films, compared to 100–500 nm in polycrystalline thin films 8. Trap state densities in high-quality thick films are reduced to 10¹⁵–10¹⁶ cm⁻³, approximately one order of magnitude lower than conventional thin films, enabling longer carrier lifetimes exceeding 1 μs 79.

Precursors And Synthesis Routes For Halide Perovskite Thick Films

Precursor Chemistry And Solution Engineering

The fabrication of halide perovskite thick films begins with precise precursor solution formulation. Typical precursor systems comprise metal halides (PbI₂, PbBr₂, SnI₂) and organic/inorganic halide salts (MAI, FAI, CsI) dissolved in polar aprotic solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or γ-butyrolactone (GBL) 2512. The precursor concentration critically determines final film thickness, with molarity ranging from 0.05 M for ultrathin layers to 1.5 M for thick-film deposition 11.

Advanced precursor engineering incorporates metal halide-adduct complexes to control crystallization kinetics and minimize volume changes during film formation 12. For example, PbI₂·DMSO or PbI₂·thiourea adducts enable intermolecular exchange reactions with organic halides (MAI, FAI) to form dense, smooth perovskite films with thickness exceeding 10 μm without substrate thermal damage 12. This adduct-mediated approach maintains film quality even at low processing temperatures (60–100°C), critical for flexible substrate compatibility 12.

Ionic liquid-based precursor systems represent an environmentally benign alternative to volatile organic compound (VOC) solvents 9. Precursor solutions using ionic liquids such as methylammonium formate or formamidinium acetate enable ambient-condition processing while achieving grain sizes exceeding 30 μm and ordering parameters above 0.6 9. These systems eliminate the need for high-boiling-point solvents and redundant solvent-removal steps, reducing processing time from hours to minutes 9.

Sequential Deposition And Two-Step Conversion Methods

Two-step sequential deposition methods offer superior control over thick-film morphology and composition 121517. The process involves: (1) depositing a metal halide precursor layer (PbI₂, SnI₂) on a substrate via spin-coating, blade-coating, or thermal evaporation; (2) converting the precursor layer to perovskite through reaction with organic halide solutions (MAI, FAI in isopropanol) or gaseous methylamine 1517. This approach enables independent optimization of each layer, achieving full-coverage, pinhole-free films with thicknesses up to 50 μm 15.

A critical innovation involves using dual alkyl ammonium halides in the conversion step to enhance film uniformity and phase purity 15. Solutions containing both short-chain (methylammonium) and long-chain (butylammonium, octylammonium) organic halides facilitate controlled interdiffusion and grain growth, producing highly crystalline thick films with surface roughness below 10 nm 15. Thermal treatment at 100–150°C for 10–60 minutes completes the conversion, with annealing atmosphere (nitrogen, dry air) influencing final stoichiometry and defect density 1517.

Vacuum-assisted sequential deposition combines the benefits of vapor-phase and solution-phase processing 17. A first metal halide layer is deposited via thermal evaporation, followed by exposure to organic halide vapor or solution to induce in-situ conversion to perovskite 17. This hybrid approach achieves exceptional thickness uniformity (±5% across 10 cm² substrates) and eliminates solvent-related defects, yielding thick films with carrier mobilities exceeding 50 cm²/V·s 17.

Spray-Coating And Scalable Deposition Techniques

Spray-coating has emerged as a scalable method for halide perovskite thick film fabrication, particularly for large-area applications such as X-ray detectors and solar modules 1. The process involves atomizing precursor solutions into fine droplets (10–50 μm diameter) and depositing them onto heated substrates (100–200°C) 111. Multiple spray passes (5–20 cycles) build up film thickness incrementally, with each layer undergoing partial crystallization before the next deposition 1.

Key process parameters include:

• Substrate temperature: 100–200°C to promote rapid solvent evaporation and nucleation 111
• Spray rate: 0.5–2 mL/min to balance deposition speed and film uniformity 1
• Nozzle-to-substrate distance: 10–20 cm to optimize droplet impact energy 1
• Carrier gas pressure: 0.5–2 bar (nitrogen or dry air) to control droplet size distribution 1

Surfactant-assisted spray-coating incorporates amphiphilic molecules (oleic acid, oleylamine) and ligands (octylamine, dodecylamine) into precursor solutions to stabilize nanosheet morphologies and enhance film adhesion 1. CsPbBr_mI₃₋_m thick films prepared with 5–10 mol% surfactant exhibit nanosheet structures with individual sheet thicknesses of 50–200 nm, stacked to total film thicknesses of 20–100 μm 1. These films demonstrate radioluminescence intensities 3–5 times higher than conventional thick films due to reduced light scattering and enhanced X-ray absorption 1.

Infrared (IR)-assisted curing during spray-coating accelerates crystallization and improves phase purity 11. IR irradiation (wavelength 1–3 μm, power density 0.5–2 W/cm²) applied immediately after each spray pass induces rapid heating (heating rate 50–100°C/s) and promotes formation of single 2D phase structures or controlled 2D/3D mixed phases 11. This technique reduces total processing time to under 60 seconds for films up to 10 μm thick while maintaining FWHM values of 20–40 nm in photoluminescence spectra 11.

Room-Temperature Crystallization And Amine-Mediated Synthesis

Room-temperature (RT) crystallization methods eliminate thermal annealing requirements, enabling processing on heat-sensitive substrates such as flexible polymers and textiles 8. The RT isothermal crystallization approach disperses perovskite precursors in solutions containing volatile solvents (ethanol, tetrahydrofuran) and amine additives (methylamine, ethylamine) 8. Rapid solvent evaporation at 15–25°C induces supersaturation and triggers quick transition to tetragonal β-phase perovskite crystals with ultrahigh preferred orientation 8.

The mechanism involves amine molecules coordinating with metal halide species to form intermediate complexes that lower the activation energy for perovskite nucleation 8. Upon solvent evaporation (timescale ≤60 seconds), amine ligands desorb, allowing direct crystallization into the perovskite phase without requiring thermal energy input 8. Films prepared via this method exhibit densely packed hexagonal grains with average sizes of 1–5 μm and achieve power conversion efficiencies of 23.07% in photovoltaic devices, comparable to thermally annealed films 8.

Deuterium oxide (D₂O) incorporation into precursor solutions represents another RT processing strategy 13. Mixing 5–20 vol% D₂O with halide perovskite solutions (MAPbI₃, FAPbI₃) results in formation of organic cations with carbon-deuterium (C-D) bonds, which exhibit stronger bonding and reduced vibrational coupling compared to carbon-hydrogen (C-H) bonds 13. This isotopic substitution enhances film stability under ambient conditions and extends carrier lifetimes by 20–40% due to suppressed non-radiative recombination via reduced electron-phonon coupling 13.

Structural Engineering And Morphology Control In Halide Perovskite Thick Films

Nanosheet Architecture And Layered Structures

Nanosheet-structured halide perovskite thick films offer unique advantages in optoelectronic applications due to their high surface-to-volume ratio and anisotropic charge transport properties 1. These structures consist of vertically aligned or randomly oriented nanosheets with individual thicknesses of 20–500 nm and lateral dimensions of 1–50 μm, assembled into thick films (10–100 μm total thickness) 1. The nanosheet morphology is achieved through surfactant-mediated growth, where long-chain organic molecules (oleic acid, octylamine) preferentially adsorb on specific crystal facets, directing anisotropic growth 1.

CsPbBr_mI₃₋_m nanosheet thick films prepared via spray-coating with 10 mol% oleylamine exhibit {002} preferential orientation with Lotgering factors of 85–95%, indicating near-perfect vertical alignment 1. This orientation maximizes out-of-plane charge transport while minimizing in-plane carrier recombination, resulting in X-ray detection sensitivities of 2500–4000 μC/Gy·cm² under 50 kVp X-ray irradiation, 2–3 times higher than randomly oriented thick films 1. The nanosheet structure also provides mechanical flexibility, with films maintaining structural integrity under bending radii down to 5 mm 1.

Two-dimensional (2D) layered perovskites with formula (BA)₂(MA)_n₋₁Pb_nI₃_n₊₁ (BA = butylammonium, n = 1–10) enable precise control over quantum well thickness and optoelectronic properties 11. Films with n = 1 (pure 2D phase) exhibit bandgaps of 2.2–2.4 eV and emit blue-green light (peak wavelength 520–550 nm), while n = 5–10 (quasi-2D phases) show bandgaps of 1.6–1.8 eV and red emission (peak wavelength 750–780 nm) 11. Mixed-phase 2D/3D thick films combine the stability of 2D phases with the high carrier mobility of 3D phases, achieving external quantum efficiencies (EQE) exceeding 15% in light-emitting diodes 11.

Grain Boundary Engineering And Defect Passivation

Grain boundaries in polycrystalline halide perovskite thick films act as recombination centers and ion migration pathways, degrading device performance and stability 714. Grain boundary engineering strategies aim to passivate these defects through chemical modification or structural optimization. One approach involves incorporating Lewis base molecules (thiophene, pyridine, 4-hydroxypicolinic acid) that coordinate with undercoordinated Pb²⁺ ions at grain boundaries, neutralizing trap states 14.

4-Hydroxypicolinic acid (4HPA) treatment of MAPbI₃ thick films reduces surface trap density from 5×10¹⁶ cm⁻³ to 8×10¹⁵ cm⁻³, as measured by space-charge-limited current (SCLC) analysis 14. The 4HPA molecules bind to surface Pb²⁺ sites via bidentate coordination through carboxyl and hydroxyl groups, forming stable Pb-O bonds that passivate dangling bonds 14. This treatment increases carrier lifetime from 450 ns to 1200 ns and improves power conversion efficiency in solar cells from 19.2% to 21.8% 14.

Grain size enlargement represents another strategy to minimize grain boundary density 9. Ionic liquid-based processing enables grain growth to 30–80 μm, reducing grain boundary area by 80–90% compared to conventional DMF-processed films (grain size 0.5–2 μm) 9. Large-grain thick films exhibit reduced hysteresis in current-voltage characteristics and improved long-term stability, maintaining 95% of initial efficiency after 1000 hours of continuous illumination under 1-sun conditions 9.

Epitaxial Growth And Single-Crystal Thick Films

Epitaxial single-crystal halide perovskite thick films represent the ultimate structural quality, eliminating grain boundaries entirely and achieving intrinsic material properties 7. These films are grown on lattice-matched substrates (mica, sapphire, SrTiO₃) using space-confined methods such as capillary-bridge-assisted crystallization or chemical vapor deposition 7. The substrate provides a template for oriented nucleation, and slow crystallization (growth rate 0.1–1 μm/hour) allows atomic-scale ordering 7.

MAPbI₃ single-crystal thick films (thickness 5–50 μm) grown on mica substrates via inverse temperature crystallization exhibit carrier mobilities of 100–200 cm²/V·s