Simone I. E. Vulto

21.1: Ink Jet Printing of Passive‐Matrix Polymer Light Emitting Displays

The requirement and characteristics for red, green and blue (RGB) electroluminescent polymers[1,2] suitable for fabricating monochrome and full-color passive-matrix polymer display[3] will be discussed. The controlled deposition of the light-emitting polymers is accomplished through the use of high-resolution ink jet printing[4,5], where the materials are first tested in devices prepared using spincoating techniques. The specifications for polymer materials, as well as the display design which must be taken into account to make a high-resolution passive-matrix display with low power consumption are presented.

High efficiency polymer LEDs: triplets and novel devices

We present results and a discussion of highly efficient polymer Light-Emitting Diodes (polymer LEDs, PLEDs). The external quantum efficiency in current standard devices reaches up to 2-4% only. We have explored two routes to enhance this value. In the first route, PEDOT/PSS is replaced with a novel anode or hole injection layer. The efficiency with some Light Emitting Polymers (LEP) is improved significantly, resulting in an efficacy of 35 cd/A for a yellow emitting poly-(para-phenylene-vinylene) and 20 cd/A for a blue emitting poly-(spirobifluorene). We attribute the major improvement compared to standard devices, where about 10 and 5 cd/A are obtained, respectively, to a combination of improved exciton formation efficiency and light out-coupling efficiency, and to less quenching of the radiative decay under actual device operating conditions. In the second route, we developed a new host polymer with high triplet energy such that transition metal-based green-emitting phosphorescent dyes can be used without significant back transfer of triplet excitons to the polymer host. First results using this system showed about 25 cd/A using a soluble green Ir-based emitter. Importantly, all data are obtained in a standard two-layer device of a hole transport/injection layer and the LEP.

Technology and materials for full-color polymer light-emitting displays

The status of the development of full-color polymer light emitting diodes will be presented. The focus of current materials research is on state-of-the-art red, green, and blue light-emitting polymers (LEP) with high efficiency, optimum color points, low driving voltages and long lifetimes in devices. A general overview of the progress of red, green and blue LEP lifetimes and efficiencies will be given and compared to requirements for both full-color passive and active matrix-displays for mobile display applications. Further, the status of ink-jet printing of LEPs for the industrialization of full-color displays will be discussed, and a comparison of the performance of spin coated and inkjet printed devices will be presented. In addition, two material-related topics studied recently will be discussed; namely, the lifetime of blue light-emitting devices correlated to processing, anodes, cathodes and the blue polymers themselves, and second, the consequences of pulsed-driving schemes on efficiency and lifetime.

Passive-matrix polymer light-emitting displays

Organic light-emitting device research focuses on the use of small-molecule and polymer materials to make organic electroluminescent displays, with both passive- and active-matrix technologies. This paper will focus on the characteristics of red, green, and blue electroluminescent polymers suitable for fabricating monochrome and full-color passive-matrix displays. The stability of polymer OLEDs, and the use of ink-jet printing for direct high-resolution patterning of the light-emitting polymers will also be discussed. It will be shown that the performance of light-emitting polymers is at the brink of being acceptable for practical applications.

Status of red, green and blue light emitting polymers for passive matrix displays

An overview of the requirements for full color passive matrix displays and their implications for the light emitting materials will be presented. Using the performance of light emitting polymers tested in Philips devices the status of the light emitting polymers is reviewed. It will be shown that the performance of light emitting polymers is at the edge of being acceptable for practical applications. Red and green light emitting polymers can already be used for certain monochrome applications. However, for the high-resolution displays used in mobile telecom applications the efficacy for red and the lifetime for green are still somewhat low. Optimization routes for further improvement in terms of efficacies and lifetimes for red and green are identified. The peformance of blue light emitting polymers has rapidly improved over the last year, but the lifetime is still too short for full color applications. Improvement routes for the blue light emitting polymers and its device structure are outlined.