Lateral Dye Distribution With Ink-Jet Dye Doping of Polymer Organic Light Emitting Diodes
- PDF / 2,606,348 Bytes
- 6 Pages / 396 x 630 pts Page_size
- 78 Downloads / 191 Views
ABSTRACT In this work we investigate the lateral dye distribution resulting from the dye doping of a thin polymer film by ink-jet printing (UIP) for the integration of color organic light emitting diodes (OLED's). The dye is found to segregate into distinct outer rings following rapid droplet evaporation, while slower evaporation rates are found to significantly reduce (or eliminate) this effect. The dye segregation phenomena are found to depend critically on the mechanisms of droplet evaporation. Good dye uniformity was obtained using a low vapor pressure solvent, and integrated, 250 micron red, green, and blue polymer organic light emitting diodes (OLED's) were fabricated with this technique. These devices had good color uniformity over most of the device area and similar electrical properties to comparable spin-coated devices without UP.
I. INTRODUCTION Polymer OLEDs are a promising technology for flat panel displays [1]. These devices typically consist of a multi-layer sandwich of a transparent substrate, a transparent anode (in our work, Indium Tin Oxide), a thin film of an organic polymer blend (in our work, the hole-transporting polymer Poly(N-vinyl carbazole) (PVK) doped with an emissive dye), and a reflecting cathode. (The device structure and principle of operation + Ceod.---are illustrated in Fig. 1.) When current is r driven through the device, holes from the ITOanode and electrons from the cathode S a combine in the organic film to form excitons, which emit light as they decay. e,,ucdp,,,, It has been shown that by doping the Fig. 1. Basic OLED structure and operation. organic active layer with a small amount of dye one can tune the emission wavelength [2]. Currently, spin coating is the standard method for depositing a polymer blend film, which produces a uniform layer of polymer. However, this does not allow one to integrate multiple colors onto a single substrate, because the film is the same everywhere. It has been proposed previously [3,4] to locally dope an initially undoped PVK film by depositing droplets of a dye solution onto the film surface and allowing the droplet to evaporate. This task is ideally suited to IJP. This basic procedure is outlined schematically in Fig. 2. To integrate red, green, and blue devices onto a single substrate, solutions of red, green, and blue dyes are locally printed onto the same substrate. Our
211 Mat. Res. Soc. Symp. Proc. Vol. 624 ©2000 Materials Research Society
Dye M
1.
w--
"
Glass
~---.
no
Glassf
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. UIP DROPLET FORMATION Our experimental apparatus consists of a piezo-electric type ink jet printer (supplied by MicroDrop GmbH) with a glass print head
