Ink-jet printing: The route to production of full-color P-OLED displays M. Bale J. C. Carter C. J. Creighton H. J. Gregory P. H. Lyon P. Ng L. Webb A. Wehrum Abstract — Ink-jet-printing technology for displays has been making rapid progress and is now nearing production. This paper will go through the developments that have been carried out at CDT, Ltd., to achieve this status. The ink-jet head and machine development that underpin the capability of the RGB printing process to manufacture P-OLEDs at high speed with high reliability and with the film- forming properties to achieve the appropriate device performance will be described. The important contribution made by the availability of flexible metrological tools will be discussed. Keywords — Printing, ink jet, OLED, polymer, displays, electroluminescence. 1 Introduction Following the discovery of electroluminescence from conju- gated polymer material by a team at Cambridge University in 1989, 1 Cambridge Display Technology (CDT) has been working closely with partners and licensees to develop its polymer organic LED (P-OLED) technology to commercial viability. P-OLED is a versatile technology that has the potential to be used for lighting, simple alphanumeric dis- plays, and full-color flat-panel displays. The key feature of P-OLED technology is that the conjugated polymers can be processed as solutions, allowing printing techniques to be used to directly pattern RGB mate- rials on a substrate. The last five years have seen an increas- ing activity in the development of ink-jet printing for depositing electronic materials. In particular, there have been demonstrations of ink-jet printing of both hole con- duction (HC) and electroluminescent (EL) layers of OLED devices by more than a dozen display manufacturers. A number of these companies have set up pilot-production facilities and have indicated that mass manufacture will start in 2007–2008 time frame. The interest in ink-jet printing in particular is due its scalability to large substrate sizes. In addition, ink-jet print- ing is capable of meeting throughput requirements of dis- play-manufacturing lines and is easily adaptable to different designs, reducing tooling costs. Ink-jet printing is also materi- als efficient – being an additive process. The use of ink-jet printing for P-OLED has required significant development both of printing equipment, ink-jet heads, ink formulation, and process development. In this paper, the development that has been undertaken to take ink-jet printing of P-OLED displays to manufacturing readiness is outlined. 2 Development activities In the graphic-arts industry, ink-jet printing has become well established both for small-office/home-office applica- tions and for industrial wide-format printing. There are, however, significant differences between the application of ink-jet printing for graphic arts and P-OLED displays. For printing P-OLED displays, it is necessary to achieve a high specification of both the drop placement and drop volume. The definition of reliability is also more stringent since, for displays, each drop counts and averaging techniques used to mask failing nozzles are not always possible to implement. In addition, the cost of the substrates is orders of magnitude higher than paper and card so that the cost of scrap is sig- nificant. Once the ink lands on the substrates the processes that occur are also different. The display substrate is not porous, and significant development work has been under- taken to transform the printed droplet into films that achieve the electro-optical performance required for the display. The development of ink-jet printing for P-OLED displays has therefore included development of the ink-jet printer, the ink-jet head, ink formulation, and surface processes. 3 Printer and print-head development The key requirements for the printer/print-head system are that some specified targets are met for drop position, drop volume, print reliability, and throughput. The specification for drop position is determined by the geometry of the dis- play to be printed, the drop size dispensed by the head, and by the surface energies of ink and surfaces. As an example, displays with a color pixel pitch of between 100 and 150 ppi (corresponding to subpixel dimensions of ~85–55 μm), printed using a 10-pl head (corresponding to a drop diame- An Extended revised version of a paper presented at the 25th International Display Research Conference (EuroDisplay ‘05). The authors are with the Technology Development Centre, Cambridge Display Technology, Cardinal Business Park, Godmanchester, Cambridge- shire, U.K.; telephone +44-1480-387424, fax –387342, e-mail: mbale@cdtltd.co.uk. © Copyright 2006 Society for Information Display 1071-0922/06/1405-0453$1.00 Journal of the SID 14/5, 2006 453