Slot-die coating

Typical components Slot-die coating equipment is available in a variety of configurations and form factors. However, the vast majority of slot-die processes are driven by a similar set of common core components. These include: • A fluid reservoir to store the main supply of coating fluid for the system • A pump to drive the coating fluid through the system • A slot-die to distribute the coating fluid across the desired coating width before coating onto the substrate • A substrate mounting system to support the substrate in a controlled manner as it moves through the system • A coating motion system to drive the relative speed of the slot-die and substrate in a controlled manner during coating Depending on the complexity of the coating apparatus, a slot-die coating system may include additional modules for e.g. precise positioning of the slot-die over the substrate, particulate filtering of the coating solution, pre-treatment of the substrate (e.g. cleaning and surface energy modification), and post-processing steps (e.g. drying, curing, calendering, printing, slitting, etc.). Industrial coating systems Slot-die coating was originally developed for industrial use and remains primarily applied in production-scale settings. Combinations of these systems such roll-to-sheet lines are also possible. Both industrial roll-to-roll and sheet-to-sheet systems typically feature slot-dies in the range of 300 to 1000 mm in coating width, though slot-dies up to 4000 mm wide have been reported. Commercial slot-die systems are claimed to operate at speeds up to several hundred square meters per minute, Such large-scale coating systems can be driven by a variety of industrial pumping solutions including gear pumps, progressive cavity pumps, pressure pots, and diaphragm pumps depending on process requirements. Roll-to-roll lines To handle flexible substrates, roll-to-roll lines typically use a series of rollers to continually drive the substrate through the various stations of the process line. The bare substrate originates at an "unwind" roll at the start of the line and is collected at a "rewind" roll at the end. Hence, the substrate is often referred to as a "web" as it winds its way through the process line from start to finish. When a substrate roll has been fully processed, it is collected from the rewind roll, allowing for a new, bare substrate roll to be mounted onto the unwind roller to begin the process again. Because the slot-die coating process can be readily scaled between large and small areas by adjusting the size of the slot-die and throughput speed, processes developed on lab-scale tools are considered to be reasonably scalable to industrial roll-to-roll and sheet-to-sheet coating lines. This has led to significant interest in slot-die coating as a method of scaling new thin film materials and devices, particularly in the sphere of thin film solar cell research for e.g. perovskite and organic photovoltaics. Common coating modalities Slot-die hardware can be applied in several distinct coating modalities, depending on the requirements of a given process. These include: • Proximity coating, in which the substrate is supported by a hard surface (e.g. a precision backing roll or moving support bed) and the slot-die is held at a relatively small coating gap (typically 25 μm to several mm away from the substrate, depending on the wet thickness of the coated layer). • Curtain coating, in which the substrate is supported by a hard surface (e.g. a precision backing roll or moving support bed) and the slot-die is held at a much larger coating gap, enabling much higher coating speeds as long as a suitable Weber number is achieved. • Tensioned web over slot-die coating, in which the substrate web is suspended between two idle rollers placed on opposite sides of the slot-die. The web is then pressed against the lips of the slot-die such that the slot-die itself applies tension to the web. When fluid is pumped through the slot-die onto the substrate, the fluid lubricates the slot-die-substrate interface, preventing the slot-die from scratching the substrate during coating. Furthermore, the concepts governing proximity coating are relevant in understanding the behavior of other coating modalities. Proximity coating is therefore considered to be the default configuration for the purposes of this introductory article, though curtain coating and tensioned web over slot die configurations remain highly relevant in industrial manufacturing. == Key process parameters ==

Film thickness control Slot-die coating is a non-contact coating method, in which the slot-die is typically held over the substrate at a height several times higher than the target wet film thickness. Excessive pumping or insufficient coating speeds result in defect spilling of the coating liquid outside of the desired coating area, while coating too quickly or pumping insufficiently results in defect breakup of the meniscus. The pump rate and coating speed can therefore be adjusted to directly compensate for these defects, though changing these parameters also affects wet film thickness via the pre-metered coating mechanism. Implicit in this relationship is the effect of the slot-die height parameter, as this affects the distance over which the meniscus must be stretched while remaining stable during coating. Raising the slot-die higher can thus counteract spilling defects by stretching the meniscus further, while lowering the slot-die can counteract streaking and breakup defects by reducing the gap that the meniscus must breach. Other helpful coating window plots to consider include the relationship between fluid capillary number and slot-die height, as well as the relationship between pressure across the meniscus and slot-die height. Surface energy effects and drying effects are examples of common downstream effects with a significant influence on final film morphology. Sub-optimal matching of surface energy between the substrate and coating fluid can cause dewetting of the liquid film after it has been applied to the substrate, resulting in pinholes or beading of the coated layer. Sub-optimal drying processes are also often noted to influence film morphology, resulting in increased thickness at the edge of a film caused by the coffee ring effect. Surface energy and downstream processing must therefore be carefully optimized to maintain the integrity of the slot-die coated layer as it moves through the system, until the final thin film product can be collected. External effects Slot-die coating is a highly mechanical process in which uniformity of motion and high hardware tolerances are critical to achieving uniform coatings. Mechanical imperfections such as jittery motion in the pump and coating motion systems, poor parallelism between the slot-die and substrate, and external vibrations in the environment can all lead to undesired variations in film thickness and quality. Slot-die coating apparatus and its environment must therefore be suitably specified to meet the needs of a given process and avoid hardware- and environment-derived defects in the coated film. == Applications ==

Industrial applications Slot-die coating was originally developed for the commercial production of photographic films and papers. flexible packaging, transdermal and oral pharmaceutical patches, LCD panels, lithium-ion batteries and more. Research applications With growing interest in the potential of nanomaterials and functional thin film devices, slot-die coating has become increasingly applied in the sphere of materials research. This is primarily attributed to the flexibility, predictability and high repeatability of the process, as well as its scalability and origin as a proven industrial technique. Slot-die coating has been most notably employed in research related to flexible, printed, and organic electronics, but remains relevant in any field where scalable thin film production is required. Examples of research enabled by slot-die coating include: • Thin film solar cells, to produce electron transport layers, hole transport layers, photoactive layers, and passivating layers in perovskite, organic, quantum dot and multi-junction photovoltaic devices • Solid state and next-gen batteries, to produce electrodes, solid electrolytes, ion selective membranes, protective coatings, and interface modification coatings • Fuel cells and water electrolysis, to produce electrolytes and electrode catalyst coatings • Flexible touch-sensitive surfaces, to produce transparent conductive films • OLED devices, to produce electron transport layers, hole transport layers, and electroactive layers • Biobased and biodegradable packaging, to produce multilayer barrier foils from sustainable materials == References ==