Polymer Light Emitting Diodes Made by Inkjet Printing

Abstract

Since its discovery in 1989 (1,2), electroluminescence from conjugated polymers has been a field of intensive research. One particular advantage of polymers compared to their small molecule counterparts is their processability from solution. This makes them the ideal active layer to be used for flat panel displays because the different colors can be inkjetted (3) economically even onto relatively large substrates. Inkjet technology for polymer light emitting diodes (PLEDs) and full color displays, however, is not fully developed yet, and a detailed understanding of the underlying principles and the interaction of printhead, ink and substrate has not been achieved thus far. This paper presents performance data of inkjetted PLEDs and compares them with the respective spin-coated devices. The results show the importance of the film homogeneities of the functional layers and how they can be influenced by various jetting parameters. Cross sections of inkjetted drops are vastly different depending on the drying conditions (underlayer, substrate temperature, formulation, surrounding atmosphere). Devices have been prepared on a substrate with a simple, low resolution photoresist structure. These devices show that the PL-image alone is not sufficient to predict the resulting EL image.