A real coating display.

While most in the paint and coatings industry do not associate coating technology with the electronics industry, coatings are critically important. Conformal coatings protect sensitive electronic components. Photoresist coatings are crucial in semiconductor chip manufacturing. More recently, coatings have begun to play a role in the production of organic light emitting diodes (OLEDs). The demand for OLEDs has grown tremendously in the past 12 to 18 months, and OLED displays can now be found in many cell phones, particularly the Galaxy phones sold by Samsung. One of the keys to making OLED technology more widely adopted in lighting and large panel (TV) applications specifically is bringing the cost down. Solution processing, using printing and coating methods, is seen by many as having the potential to do just that.

OLEDs compete with liquid crystal displays (LCDs), which are currently the dominant display technology. OLEDs can be thinner, lighter, and brighter, with more saturated colors and faster response times, wider viewing angles, and higher contrast ratios. However, current vapor deposition processes are capital intensive and inefficient up to 90% of the chemicals may not end up on the target. One possible answer is the use of solution processing of either polymers or small molecules which requires much less costly equipment and involves significantly less waste. The challenge is to be Orr able to produce high quality, highly uniform, large area films and to do so without affecting any of the other layers in the OLED stack. courtesy of Novaled.

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At its most basic, an OLED stack consists of a substrate (usually glass, but also metal [steel] foil and plastic), an anode, an emitting layer (EML), and a cathode incorporated into an encapsulant of some kind to protect it against air and moisture.

Practical OLEDS are more complicated, and include hole-injection (HIL), hole-transport (HTL), hole-blocking (HBL), electron blocking (EBL), electron transport (ETL), and electron injection (EIL) layers to improve performance and extend the useful lifetime. If more than one colored emissive layer is included in the stack (a stacked OLED, for white OLED lighting, for example), then buffer layers are also often incorporated between each emissive layer.

The light is emitted by either fluorescent or phosphorescent materials in the EML. Bottom emitting OLEDS, the most common, have a transparent anode so the light can pass through. Top emitting OLEDS require a transparent cathode. The performance of an OLED depends significantly on how the energy levels of the materials in each layer match up. Maximizing light output is the goal. Unfortunately, because the light from the EML is radiated out in all directions, only about 20% of it reaches the top or the bottom of the stack. Therefore, ensuring that the layers function well together is critical.

The HIL moves the holes from the anode into the stack, while the HTL moves the holes from the stack (if there is no HIL) or the HIL to the EML. On the other side of the stack, the ETL facilitates the transport of electrons toward the emissive layer, while the EIL aids in the movement of the electrons from the cathode to the ETL and serves as a buffer layer between the reactive metal cathode and the EML. The HBL and EBL serve to prevent the electrons and holes from recombining outside of the EML.

Coating comes in if a solution process is used to deposit these various layers. Initially, solution processing was considered only for polymer-based materials, but recently a small molecule-based process has been developed by DuPont.

The most common conductive polymer used in OLEDs, typically for the hole injection layer, is poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), which is sold by Agfa (Orgacon) and Heraeus (Clevios). A new HIL material from Plextronics, sulfonated poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl), is offered as an aqueous ink and is designed to deliver uniform thin films in large-area panel manufacturing, according to vice president of business development Jim Dietz. Most recently, Plextronics introduced a new version of its aqueous-based HIL that provides a 150-200 nm planarized layer for OLED lighting applications. The company is also developing a solvent-based formulated ink for use as an HIL in solution-type phosphorescent emitter systems, such as those provided by Universal Display Corporation.

Sumitomo has been a strong supporter of polymer-based OLEDs (or p-OLEDs). In late 2011, the company said it had begun construction of a plant that will produce p-OLEDS for use in large panel OLED Ns. It expects to ship product sometime in 2012.

What is attracting significant attention is the new solution-based approach developed by DuPont Displays that uses small molecule materials rather than polymers (which were traditionally only seen as being deposited by vapor deposition). Either a slot dye coating or a specially developed nozzle printing process is used to lay down each layer of material.

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Slot die coating is an existing method for preparing thin uniform blanket layers. The coating liquid is forced out from a reservoir through a slot by pressure, and transferred to a moving web. Typically, the slot is much smaller in section than the reservoir, and is oriented perpendicular to the direction of web movement. Slots dies are custom manufactured for a specific application and are designed to create uniform coat weights through metered application. A range of viscosities, flow rates (and thus coat weights), and percent solids are tolerated, the system is fully enclosed, and all of the material is applied to the substrate. A precision pump delivers the coating solution to the slot die. Most importantly, coating results are very reproducible.

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Dupont Displays has optimized its HIL and HTL formulations for blanket coating of these layers via a slot die coating process. For the emissive layers, which must be patterned, the company developed a nozzle printing process that simultaneously prints the red, green, and blue layers. The new printing process was specifically developed for printing OLEDs and provides multiple continuous streams of OLED solutions that are drawn across the substrate at high speed. Excellent uniformity can be achieved by controlling the flow rate (which is held constant) and the speed. Larger panels can be produced by increasing the number of nozzles and optimizing the speed and acceleration of the printing head.

Through extensive custom modeling and analytical approaches, DuPont Displays has been able to address the challenging thickness and uniformity requirements for large OLED displays. Its efforts were rewarded in November 2011, when a leading Asian manufacturer of active matrix OLEDs licensed its solution-based nozzle printing process for the production of large television displays. That company is in the process of commercializing the technology.

In addition to Plextronics and DuPont Displays, Merck KGaA (Darmstadt, Germany) is focusing on printed materials. In addition, Novaled is working on adopting a hybrid approach, with a combination of vapor deposition of small molecules and solution processing of polymers. They are all attracted to the potential advantages of solution processing. DuPont Displays, for example, estimates that its two processes use about 0.6 to 2.0 g of material per g of coated layer (slot die coating) 1.0-1.3 g (nozzle printing), while traditional VD processes can consume 5-10 g per g of coated layer. Overall the company believes that production costs can be reduced by around 30% compared to vapor deposition.

If costs can be lowered, there is significant opportunity in the OLED market. Market research firm DisplaySearch expects that OLED display revenues will grow to nearly $6 billion in 2015, while NanoMarkets anticipates that the market for OLED materials will surpass $5 billion in 2018. Samsung, the leading consumer of OLEDs, is also predicting that 50% of cell phones will have active OLED displays in 2014, and by 2015, OLEDs will be the major TV panel technology. Coatings could play a crucial role in making it all happen.