At the recent CES convention in Las Vegas, many new products were unveiled and demonstrated. Both the public and professional communities were captivated by the new OLED technologies. However, as the prices were announced, it soon became clear to the techno-savvy consumers at home that it would be many years before OLEDs were a common sight in the average household. Although the prices seemed daunting, the manufacturing methods used to produce these organic screens have been steadily improving. As a revolutionary advance in display technology, OLED displays are expected to overtake the LCD market in the near future. Thus, many researchers have worked tirelessly to develop a more efficient and quicker method in order to make OLEDs a more accessible technology.
Patternable OLEDs operate through a light or heat activated electroactive layer. This is a polymer that exhibits a physical change when stimulated with an electric field. A popular polymer for this layer is PEDOT-TMA, which becomes a highly efficiently hole injection layer. It is a p-type conducting polymer and a modification of the PEDOT monomer, which is based off the EDOT monomer and can be seen below. PEDOT-TMA is a popular material for this layer because it is noncorrosive and dispersible in organic solvents. This is due to the methacrylate groups at both ends of the polymer chain. These caps limit the length of the polymer, making it more soluble in organic solvents and ideal for usage in OLEDs. These OLEDs are conventionally produced through the use of two main patterning technologies, organic vapor phase deposition (OVPD) and inkjet etching (IJE).
The PEDOT-TMA chain. The two methacrylate caps at the end are clearly visible.
Organic Vapor Phase Deposition
The organic vapor phase deposition manufacturing process works by transporting organic molecules to a cold substrate through the use of a hot inert carrier gas. The final film product can be fine-tuned by altering the gas flow rate and source temperature. Uniform films are produced by lessening the carrier gas pressure, which creates a larger velocity and mean free path of the gas. This leads a decrease in the thickness of the boundary layer. OVPD is also popular for OLED production because the process naturally prevents issues related to contaminants from the walls of the chamber. This is because the walls are naturally warm, which prevents molecules from sticking to and producing a film.
However, maintaining a uniform film thickness and dopant concentrations over the large surface area needed for many applications is a demanding task in the context of vacuum evaporation, which is currently the most prevalent method of depositing organic molecular solids. In addition, there is a relatively low yield with OVPD production, which explains the massive production costs. As a remedy, low pressure organic vapor phase deposition has been proposed as a substitute technique. This process has a notably improves command over doping and offers a potential method to rapid, particle-free, uniform deposition of organics on large-area substrates. Further information detailing the uses and advantages of OVPD can be found here.
Inkjet Etching
The most basic method of producing OLEDs incorporates old technology originating from 1976, when the first inkjet printer was created. Inkjet printing is a cheap and efficient alternative to traditional OLED production and is used by several start-up OLED companies such as Kateeva.
The Kateeva system for inkjet etching.
Kateeva’s system of inkjet printing involves a movable platform capable of holding screen substrates of up to 55” diagonal length. The movable platform is able to transfer a selected substrate under various print heads, which are then able to spray droplets of organic material from different nozzles. These drops of organic material are placed precisely (picoliter scale) on a selected substrate to form the pixels on the screen. Additionally, the inkjet printing system is a versatile procedure, as it can be adapted from large television screens to small handheld devices such as smartphones and tablets.
This process is not as accurate or efficient without the use of a shadow mask on the substrate.
Shadow mask used for consistency and quality of the final display.
A shadow mask is a fine metal stencil drawn on the substrate that directs the material spray into its correct place, ensuring the consistency and quality of the final screen. However usage of the shadow mask, while convenient and economical, also has its downfalls. The shadow mask may lead to contamination of the organic materials of the OLED screen. Because the OLED screen is composed of organic products, it is extremely sensitive to outside particles that may cause a drop in the vibrant image quality. Nevertheless, inkjet printing is still a viable method for producing OLED displays and further improvements will reduce the risks of contamination.
New Developments in the OLED Industry
Recently, researchers have discovered that utilizing a molecule of a certain shape and structure allows anew type of OLED that has the potential to emit as much light as a commercial OLED, but without the need for costly rare metals normally demanded to make the OLEDs effective. Rare earth metals such as iridium or platinum are currently vital to OLED displays because it increases the efficiently of the display to commercial standards. The removal of these expensive metals from the production of OLEDs has the potential to significantly reduce the costs of OLEDs. This improvement in OLED structure is vital to advance the technology and reduce prices as the OLED industry of smartphones, televisions, and solid-state lighting continues to grow.
New developments regarding OLED production also includes metal oxide deposition systems. These have potential to become leading production methods because they are more efficient and consistent in their output than the currently costly production methods. For example, the AKT-55KS, a plasma enhanced chemical vapor deposition (PECVD) system that handles 8.5-Gen substrates, is a good option to consider based on its defect-free nature and its ability to keep out unwanted gases.
AKS-55KS
Two systems that hold potential for widespread use is the new physical vapor deposition (PVD) deposition systems, which comes in 2 models: SKT PiVot 25K which controls Gen-6 substrates, and the SKT PiVot 55K which controls 8.5-gen substrates. The advantages of these systems are derived from the utilization of tubular cathodes of donor material, where the cathodes rotate because of the process of deposition.
Conclusion
Constant innovation is a necessity of a demanding market, especially in an industry that is still attempting to establish itself as a viable consumer market. The production of OLEDs is still in its infancy, as there are several obvious and monumental obstacles that present itself in the first phases of starting such a market. The most pressing issue is the costly and inefficient manner in which these organic LED’s are created, which has prompted researchers to find ways to reduce these expenses. This had led to the introduction of the inkjet printer formatted specifically for the production of OLEds as well as the new deposition systems offer viable alternatives that hold promise for a market that contains endless possibilities. One can only speculate the boundless paths that the OLED industry can take, whether it be into solid-state lighting, a lucrative smartphone market, or both. Consumers can only wait to see what the raw potential of this revolutionary technology can manifest into.
BCA Chemistry 