Lateral Dye Distribution With Ink-Jet Dye Doping of Polymer Organic Light Emitting Diodes
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Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
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Glass
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DyeSolution firom UP
Glass
PVK Dye Doped
3.
Glass
Fig. 2. Procedure for dye-doping by ink jet printing.
objectives in developing this technique are to (a) produce a uniform dye distribution over the device area and film depth and (b) maintain the initial film morphology (so that the electrical device characteristics are not degraded). In employing UP, this technique should be relatively inexpensive to perform and applicable to large area substrates. II. IJP DROPLET FORMATION Our experimental apparatus consists of a piezo-electric type ink jet printer (supplied by MicroDrop GmbH) with a glass print head (which is therefore resistant solvent damage) and x-y-z print head stage Negative •-• motion. In addition, our system has integrated PiezoElectrical Contacts Pressure Line digital imaging equipment, allowing us to view GlassCapillary -NozzleHousing Cavity(25 oL) droplet ejection from the print head nozzle directly and to view drying droplets (from -Piezoelectric (drawing Transducere above) under high magnification. The print (dirawiog eiot -1-0) ao NozzleTip(50 prm) nozzle consists of a 25 gL capillary cavity 0 Ink Drops surrounded by a piezo-electric sleeves which Substrate can contract and expand the fluid cavity (see Fig. 3. Schematic of Ink Jet Nozzle. Fig. 3). To drive a droplet out of the nozzle, a (The negative pressure line is used to first positive then negative pressure pulse is balance aravitational forces.) applied to the fluid through the voltages applied to the piezo-electric sleeve. The positive pulse drives the fluid down into the nozzle tip (which is 50 ltm in diameter), and if sufficient energy is supplied by this pulse, a droplet (with diameter slightly larger than the nozzle diameter) will be ejected. The essential free parameters for controlling droplet ejection are the piezo voltage and the pulse duration. Several parameters are relevant to understanding droplet formation from an ink jet printer: system geometry (i.e. of capillary cavity and nozzle), properties of the fluid being printed (i.e. viscosity, surface tension, and density), and the relationship between the applied voltage and the resulting pressure pulses. Even each of these parameters is known in detail, a closed-form analysis of the governing Navier-Stokes equations is not possible. However, there is a rapidly growing literature on approximate solutions and numerical simulations of ink-jet flow (e.g. [5-7]), and some important trends are observed. Droplet formation can be divided up into four regimes, based on the applied voltage. At very low voltages, no droplet is ejected, because the applied pressure pulse has insufficient energy. At higher voltages, single, stable droplet ejection is observed. At still higher voltages, satellite droplets are observed along with the main droplet, and this regime is generally less stable than the single droplet regime. Finally, at yet higher voltages, the ejected fluid will not form into a main drop
