Solution processing of hybrid perovskites, particularly methylammonium lead iodide (CH3NH3PbI3), has emerged as a critical technique for fabricating high-performance thin-film solar cells. The methods employed include spin-coating, blade-coating, and antisolvent techniques, each influencing film morphology, crystallization kinetics, and defect passivation. These processes are central to achieving high power conversion efficiencies in perovskite solar cells (PSCs).

Spin-coating is the most widely used method for lab-scale deposition of perovskite thin films. The process involves depositing a precursor solution, typically a mixture of methylammonium iodide (MAI) and lead iodide (PbI2) in a polar aprotic solvent like dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), onto a substrate. The substrate is then rotated at high speeds (typically 2000-6000 rpm) to spread the solution uniformly via centrifugal force. The rapid evaporation of the solvent during spinning initiates nucleation and crystallization. A critical factor is the precursor stoichiometry, where a 1:1 molar ratio of MAI to PbI2 is commonly used to ensure complete conversion to the perovskite phase. The spin-coating process is sensitive to environmental conditions, particularly humidity, which can induce premature crystallization and pinhole formation. To mitigate this, processing in inert atmospheres or gloveboxes is preferred.

Blade-coating, also known as doctor-blading, is a scalable alternative to spin-coating, better suited for large-area deposition. In this method, the precursor solution is dispensed onto the substrate, and a blade spreads it into a thin film by moving at a controlled speed (typically 5-100 mm/s). The gap between the blade and substrate determines the wet film thickness, usually set between 50-200 µm. Unlike spin-coating, blade-coating allows for slower solvent evaporation, which can be adjusted by substrate temperature (50-100°C) and ambient conditions. This slower drying process often leads to larger grain sizes and fewer defects due to improved crystallization kinetics. Blade-coated films also exhibit better uniformity over large areas, making the technique attractive for industrial-scale production. However, optimizing the ink viscosity and solvent system is crucial to avoid Marangoni flow effects, which can cause thickness variations.

Antisolvent techniques are frequently combined with spin-coating or blade-coating to enhance film quality. An antisolvent, such as toluene, chlorobenzene, or diethyl ether, is dripped onto the wet film during the spinning or blading process. The antisolvent selectively dissolves the precursor solvents (DMF/DMSO) without dissolving the perovskite precursors, inducing rapid supersaturation and homogeneous nucleation. This method leads to denser films with improved coverage and reduced pinholes. The timing of antisolvent application is critical—typically within 5-20 seconds after coating initiation—to ensure optimal intermediate phase formation before perovskite crystallization. Delayed application can result in uncontrolled crystallization and increased defect density.

Crystallization kinetics play a pivotal role in determining the final film morphology and optoelectronic properties. The perovskite formation occurs in two primary stages: nucleation and grain growth. Nucleation is governed by the supersaturation level, which is influenced by solvent evaporation rate and antisolvent treatment. Faster solvent removal leads to higher supersaturation, promoting a higher nucleation density but smaller grains. Slower drying, as in blade-coating, allows for Oswald ripening, where larger grains grow at the expense of smaller ones, improving charge transport. Thermal annealing (90-150°C for 10-60 minutes) is often employed post-deposition to enhance crystallinity and remove residual solvents. The annealing temperature must be carefully controlled—excessive heat can degrade the perovskite or induce unwanted phase segregation.

Defect passivation is essential for minimizing non-radiative recombination and improving solar cell performance. Hybrid perovskites are prone to various defects, including undercoordinated Pb²⁺ ions, halide vacancies, and grain boundaries. Passivation strategies include additive engineering, where molecules like thiourea, potassium iodide, or zwitterions are introduced into the precursor solution. These additives coordinate with undercoordinated Pb²⁺ or fill halide vacancies, reducing trap states. Post-deposition treatments with long-chain alkylammonium halides (e.g., phenethylammonium iodide) are also effective in passivating grain boundaries. Another approach involves incorporating excess MAI in the precursor solution, which compensates for iodine vacancies but must be carefully balanced to avoid detrimental phase impurities.

In solar cell applications, the quality of the perovskite layer directly impacts device efficiency and stability. Spin-coated films with antisolvent treatment have achieved power conversion efficiencies exceeding 22% in lab-scale devices. Blade-coated devices, while slightly lower in efficiency (18-20%), demonstrate better scalability and reproducibility. Key performance metrics such as open-circuit voltage (Voc) and fill factor (FF) are strongly correlated with film morphology and defect density. Films with larger grains and fewer defects exhibit higher Voc due to reduced recombination losses. Long-term stability remains a challenge, with moisture, heat, and light-induced degradation being primary concerns. Encapsulation and interfacial engineering with hydrophobic charge transport layers are commonly employed to enhance device lifetime.

The choice of solvent system also affects film formation. Mixed solvents like DMF:DMSO (4:1 v/v) are often used to improve precursor solubility and film uniformity. DMSO coordinates with Pb²⁺, slowing crystallization and improving grain growth. Alternatively, greener solvents like gamma-butyrolactone (GBL) or acetonitrile have been explored to reduce toxicity, though they require adjustments in processing parameters.

In summary, solution processing of hybrid perovskite thin films involves a delicate balance of deposition technique, solvent engineering, crystallization control, and defect management. Spin-coating remains the benchmark for high-efficiency devices, while blade-coating offers a path toward commercialization. Antisolvent techniques and additive engineering further refine film quality, enabling the continued advancement of perovskite solar cells. Future developments will likely focus on optimizing these processes for even greater efficiency, stability, and scalability.