Technology Advances
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TECHNOLOGY ADVANCES Printing of Organic-Based Photonics and Electronics Demonstrated Functional organic materials have been gaining ground in many technologies including photonics and electronics, most recently in the development of organic light-emitting devices (OLEDs). Because they form the pixels of displays, these are the basis for a class of ultrathin, lightweight, wide-viewing-angle, low-powerconsumption displays that can be made on any kind of substrate, including plastic foils, promising displays that can be folded or rolled up. The two classes of OLEDs are molecularbased and polymeric-based. Regardless of their class, OLEDs have a simple architecture. They consist of a multilayer structure built on a given substrate. A typical polymeric-based OLED structure, or PLED, is shown in Figure 1. The electrically conducting and light-emitting polymer layer is normally spin-coated from solution on the surface of glass coated with indium–tin oxide (ITO), followed by the deposition of a metal electrode. On application of a forward bias, the electrons are injected from the metal side and the holes from the ITO side. The charges drift under the influence of the electric field and recombine on parts of the polymer to form excited states that may result in light-emission upon relaxation to the ground state. Spin-casting and physical vapor deposition (PVD) have been the most commonly used techniques in the fabrication of OLEDs. However, there is a need for lower-cost and more versatile fabrication techniques. This is particularly required for low-information-content displays including signs and logos as well as general lighting. One added advantage of organic materials is that they lend themselves to simple printing technologies such as screen printing and ink-jet printing. In screen printing, similar to ink-jet printing, the use of materials is minimal because materials are only directed to the printed areas. On the other hand, in spin-coating and vacuum-deposition, no control can be exerted to direct the materials to deposit only on the desired areas without complicated and costly masking systems. Therefore, the ratio of lost materials to that of materials defining the active device area is significant in the latter techniques. Unlike ink-jet printing, screen printing is relatively rapid and more versatile. The elements of screen printing consist of the
Figure 1. A typical structure of a single-layer polymeric-based lightemitting device.
Figure 2. Screen-printed polymer for light-emitting applications.
screen, stencil, squeegee, ink, press bed, and substrate. Although a relatively simple printing technique, the characteristics and quality of the printed image are greatly affected by a combination of variables including the printing speed, the angle and geometry of the squeegee, the distance between the screen and the substrate, and the mesh material. As a fabrication process, screen printTechnology Advances provides upto-date reports of materials developments that show potential to bridge the gap between research in
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