Solvent-enhanced Dye Diffusion in Polymer This-Films for OLED Application
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Solvent-enhanced Dye Diffusion in Polymer This-Films for OLED Application F. Pschenitzka, K. Long, and J. C. Sturm Department of Electrical Engineering, Center for Photonic and Optoelectronic Materials (POEM), Princeton University, Princeton, NJ 08544 ABSTRACT A method of solvent-enhanced dye diffusion in polymer films for organic light-emitting diode (OLED) application is introduced. After an initial dye transfer from a dye source substrate into the top of the electractive polymer film, the device substrate is then exposed to solvent vapor. Due to solvent absorption by the polymer film, the glass transition temperature of the polymer is significantly decreased, which leads to greatly enhanced diffusion of the dye in the polymer film. Secondary ion mass spectroscopy shows that the temperature for dye diffusion can be decreased by 150 °C. OLEDs with 0.4% external quantum efficiency were demonstrated. The materials used are the polymer poly(9-vinylcarbazole) (PVK) combined with electron transport molecules (PBD), and the dyes coumarin 47, coumarin 6 and Nile red.
INTRODUCTION Polymer based organic light-emitting diodes (OLEDs) have demonstrated remarkable potential for commercial display applications [1]. Efficiencies of OLEDs are now comparable to those based on conventional crystalline semiconductors [2]. In addition, the fabrication process for OLEDs is possibly simpler, promising lower fabrication costs. Usually, the electroactive polymer film is deposited onto the substrate by spin-coating from solution. While spin-coating is a very low-cost process, it forms a uniform polymer layer which can generally only emit one color. For a full color display, however, devices emitting different colors (red, green and blue, RGB) have to be fabricated next to each other on the same substrate. Therefore, the need to pattern the polymer film is evident. Alternative approaches such as a white emitter with color filters or blue emitter with down-converter are inherently less efficient. Several methods have been introduced to overcome this problem: ink-jet printing of polymer solution [3-5], and screen printing of the polymer [6]. Adding a small amount of chromophores (dyes) (usually less than 1% by weight for fluorescent dyes) into the polymer matrix can change emission color of the OLED. Other methods to pattern the color emission from polymers make use of this fact, for example ink-jet printing of a dye-solution on a substrate previously coated with polymer [7], or photobleaching of a dye [8]. In a previous paper [9], we introduced a method to locally transfer dye from a dye source substrate into the polymer layer of the device. The patterning can either be achieved by simply placing a separate physical mask between the dye source substrate and the device substrate during the transfer process, by patterning a thin film as diffusion barrier on top of the dye source, or by patterning the dye source itself [10,11]. Local heating of the device substrate to pattern the dye transfer has also been reported [12].
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