Slot-Die Coating: A Scalable Pathway to Commercial Perovskite Solar Cells
A recent review titled “Comprehensive Review on Slot‐Die‐Based Perovskite Photovoltaics: Mechanisms, Materials, Methods, and Marketability”, published in Advanced Energy Materials (2024), provides a timely and comprehensive assessment of one of the most important technological frontiers in solar energy: scalable fabrication of perovskite solar cells using slot-die coating.
Perovskite solar cells (PSCs) have emerged as a leading next-generation photovoltaic technology due to their high power conversion efficiency, tunable optical properties, and potential for low-cost production. Since their introduction in 2009, perovskites have achieved lab-scale efficiencies surpassing 26%, outperforming many conventional technologies. However, the challenge has always been scaling these high-efficiency devices to larger formats suitable for commercial use.
Slot-die coating, a technique originally used in photographic film manufacturing, has quietly positioned itself as the leading candidate for scalable, industrial fabrication of PSCs. Despite this, it has received relatively little mainstream attention. This new review changes that, analyzing more than 100 publications to present a holistic picture of where the field stands and where it is heading. It dives deep into the interplay between materials, processing conditions, crystallization mechanisms, and cost considerations that define the slot-die pathway.
The authors argue that power output multiplied by operational lifetime per cost, not efficiency alone, should be the defining metric for commercial readiness. In this framework, slot-die-based PSCs stand out as a compelling option for the future of solar manufacturing.
Key Highlights
Slot-die coating delivers record efficiencies of up to 23.4% among scalable coating techniques.
The method is roll-to-roll (R2R) compatible, enabling continuous and large-area manufacturing.
Over 115 slot-die PSC studies were analyzed, establishing the most complete roadmap to date.
Crystallization kinetics in slot-die differ significantly from spin-coating and require tailored strategies.
Film quality is governed by complex factors including solvent choice, ink flow rate, and substrate motion.
One-step and two-step deposition methods each offer distinct advantages in control, speed, and scalability.
Emerging green solvents like DMSO, GBL, and ACN offer safer, industrially viable alternatives.
Long-term device stability and cost optimization are now seen as more important than peak lab efficiency.
How to make solar cells using a slot-die coater.
Understanding Slot-Die Coating in the Context of Scalable Solar Production
Slot-die coating is a thin-film deposition technique that uses a precision die to deliver a consistent flow of liquid material onto a substrate. The liquid forms a meniscus at the interface between the die and the substrate, and as the substrate moves, a wet film is deposited. This technique is distinct from lab-scale spin coating, which relies on centrifugal forces and is not suitable for mass production.
What sets slot-die coating apart is its inherent scalability. It is compatible with roll-to-roll processes, which are already used in industries like packaging and printed electronics. This means entire solar modules can be fabricated in continuous, high-throughput environments with minimal material waste. Material utilization in slot-die processes can exceed 99%, a critical advantage in cost-sensitive manufacturing environments.
The review breaks down the physics of film formation into key stages, starting with the meniscus stability. A stable meniscus is essential for uniform film deposition, and its behavior is influenced by coating parameters such as substrate speed, flow rate, and the gap between the die and the substrate. Instabilities at this stage can lead to thickness variations, pinholes, or incomplete coverage—all of which degrade solar cell performance.
Another critical factor is the coating regime. The authors distinguish between two operational modes:
Evaporation regime: Occurs at lower speeds where solvent evaporation significantly influences film thickness.
Landau–Levich regime: Dominant at higher speeds, where viscous drag controls the thickness. This regime is generally more desirable for uniformity and throughput.
Slot-die coating can be used not only for the active perovskite layer but also for other functional layers, such as electron transport layers (ETL) and hole transport layers (HTL), although the transition from spin-coated transport layers remains challenging due to wetting and thermal issues.
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Materials, Solvents, and Crystallization Control
Material selection plays a pivotal role in the success of slot-die-based PSCs. At the core of this challenge lies the crystallization process, which must be carefully controlled to form a compact, uniform, and defect-free perovskite layer. The choice of solvent directly affects the evaporation rate, interaction with precursors, and ultimately, the morphology of the resulting film.
The review presents a detailed comparison of common solvents:
Dimethylformamide (DMF) is widely used but has a high evaporation rate and toxicity.
Dimethyl sulfoxide (DMSO) offers better environmental properties and forms stable intermediate complexes, though it has a high boiling point and viscosity.
Gamma-butyrolactone (GBL) is also considered green but faces legal restrictions in some jurisdictions.
Researchers often use solvent mixtures to balance these properties. For example, combining a low boiling point solvent like acetonitrile (ACN) with DMSO can offer both speed and control. Such hybrid systems expand the processing window, allowing for more forgiving fabrication conditions.
Two coating methods are explored in depth:
One-step deposition, where all precursors are delivered simultaneously, offers simplicity and speed but requires tight control over nucleation and growth conditions.
Two-step deposition, where the lead halide and organic halide are deposited sequentially, allows for better film uniformity and conversion but adds complexity.
Crystallization in the one-step method is often explained using the LaMer model, which describes a three-phase process of supersaturation, nucleation, and growth. In two-step processes, mechanisms like Oswald ripening and Gibbs free energy analysis help explain how film quality evolves over time and under varying conditions.
To achieve optimal films, quenching techniques such as air-knife drying, vacuum-assisted evaporation, and controlled heating are used to guide solvent evaporation and crystal growth. These methods must be fine-tuned to the selected solvent system and coating conditions to avoid common issues like incomplete conversion or film roughness.
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Industrial Challenges and Emerging Solutions
Despite impressive lab results, translating SD-PSCs into commercial products involves a new set of challenges. Chief among them are stability, reproducibility, and cost.
Slot-die-coated devices must maintain performance over years of exposure to environmental factors like humidity, heat, and ultraviolet radiation. The review identifies strategies to enhance durability, including the use of carbon electrodes, interface passivation, and encapsulation methods. Carbon-based back contacts, in particular, offer a low-cost and stable alternative to vacuum-deposited metals, although their deposition by slot-die remains technically difficult due to viscosity and wetting issues.
Cost remains a central concern. The study emphasizes that commercialization success depends not on achieving the highest lab efficiency but on optimizing power output × operating lifetime ÷ production cost. This requires material efficiency, fast processing, and robust stability—not just peak performance.
To address these challenges, researchers are turning to:
Green solvents that are less toxic and easier to handle at scale.
Fully R2R-compatible processes, including printable electrodes and encapsulants.
Indoor and building-integrated photovoltaic (BIPV) applications, where lower light intensity and flexible substrates demand new design considerations.
There is also a growing focus on standardization. Variability in lab protocols and measurement practices can obscure meaningful comparisons. The review calls for greater consistency in how SD-PSCs are reported and evaluated.
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Conclusion
The study concludes that slot-die coating represents one of the most commercially viable pathways for scaling perovskite solar cells. Its compatibility with roll-to-roll processing, high material utilization, and capacity for continuous film deposition across large areas make it well-suited to the demands of industrial production.
At the same time, significant challenges remain. These include mastering crystallization under dynamic conditions, developing solvent systems that balance performance with safety, and ensuring long-term device stability under real-world conditions. However, the momentum in the field is clear. The number of publications on slot-die-based PSCs has grown steadily over the past decade, and recent breakthroughs in device architecture and processing techniques suggest that scalable, high-performance perovskite modules are within reach.
This comprehensive review sets the stage for future innovation, providing both a technical foundation and a strategic roadmap for researchers and companies aiming to commercialize perovskite photovoltaics. As the field matures, slot-die coating will likely be central to the industry’s evolution—bridging the gap between laboratory discovery and widespread solar deployment.
Authors:
Ko Ko Shin Thant
Chaowaphat Seriwattanachai
Thantham Jittham
Naruedej Thamangraksat
Patawee Sakata
Pongsakorn Kanjanaboos
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