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Printed electronics for flexible solid-state lighting Figure 10. Cross section of Philips Lumiblade device. Figure 11. Schematic diagram of GE lamination process for OLED fabrication. flexible substrate materials tend to be soft, care must be taken when cutting and excising OLED devices from the web. Electrode surfaces cannot come in contact with each other (e.g. due to edge smearing) since they would generate an electrical short and cause device failure. Mechanical cutting tools or cutting lasers can be used if they don’t leave burrs. Encapsulation of OLED light engines via hermetic barriers is highly advantageous to ensure long product lifetime. Most barrier systems follow a multilayer approach. Laminate films are comprised of thin polymer layers interspersed with thin ceramic layers to provide hermeticity and maintain flexibility. Coating processes also use a multilayer approach with the hermetic film built up with polymer layers and ceramic layers. Numerous companies are developing barrier films that can be applied via printing/coating or film lamination processes. In addition to offering protection against water vapor and oxygen, these encapsulation layers must also offer abrasion resistance and provide physical protection to the light engine. If they can’t supply these attributes within one barrier film, then additional layers need to be applied that meet the product requirements. OLED device surfaces have a higher Figure 12. Example of printed flexible OLED devices. refractive index than air, which coupled with a planar emitting surface leads to low light out-coupling. Techniques are being pursued to increase light out-coupling by modifying the OLED surface with micro lenses, surface roughening and quantum optics 15 . OLED luminaire Connecting power lines to the OLED device requires a mechanical/electrical connection stable over temperature, that is formed at low temperatures or localized high temperatures, and is mechanically compatible with a flexible substrate. The low temperature requirement arises because several of the OLED material systems cannot take high heat levels, so conventional reflow soldering processes are precluded. Localized contact soldering techniques such as thermodes or hear bars, or laser soldering are potential candidates. Conductive adhesives (anisotropic and isotropic may be useful in these circumstances. Mechanical connectors may offer an alternative approach. Luminaires using vapor deposited OLED light engines are available from Osram 16 and Philips 17 , Figure 9. A cross-section schematic of the Philips Lumiblade device 18 is shown in Figure 10. This luminaire is fabricated on a rigid glass substrate with a vapor deposited OLED and a back cover to seal the device. To help reduce oxygen and water vapor effects, a getter is encapsulated with the OLED. The performance of printed OLED luminaires lags that for vapor deposited ones, but they are rapidly improving. A number of companies are working to develop printed OLED devices. For example, GE is developing a R2R process, depicted in Figure 11, for fabricating printed OLEDs 19 . They fabricate an OLED light engine by laminating together “halfdevices” printed on flexible substrates. An example of printed OLED devices fabricated via a printing process using flexible substrates is shown in Figure 12 20 . OLED value chain Most incandescent and fluorescent lamps are commodity products. LED and OLED SSL products will be able to command premium pricing in their initial roll-out, but they will ultimately participate in a commodity market. In this value chain, cost will be a critical factor as they compete against the incumbent technologies. Manufacturing technologies that drive toward low cost solutions will be critical toward ultimate market success. OLED printing technologies, based on commercial processes and equipment sets, can help realize a low manufacturing cost structure. Also in this value chain, new supply chains and distribution channels are developing. Printing companies are learning how to design and manufacture electronic products and test their performance. These are skills that the SMT manufacturers have in abundance, but interestingly are not presently particularly active in this new value chain creation. Many of the assembly equipment sets, test equipment and manufacturing protocols that the SMT industry routinely uses could be transferred to the SSL industry and open up new sources of revenue. conclusion SSL products based on LED technology are beginning to enter the commercial marketplace, while OLED technology is several years behind for broad market entry. The ability to print OLED devices on flexible substrates will provide a powerful impetus to realize low cost OLED luminaires. Numerous printing processes are amenable to OLED production, and in continued on page 23 18 – Global SMT & Packaging Southeast Asia – Winter 2010 www.globalsmtindia.in
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