Polymer light emitting diodes made by ink jet printing

Since its discovery in 1989, 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 ink jetted economically even onto relatively large substrates. Ink jet technology for polymer light emitting diodes (PLEDs) and full color displays, however, is not yet an established display production technology. A detailed understanding of the underlying principles and the interaction of print head, ink and substrate is a prerequisite for the development of the technology. This paper presents performance data of ink jetted PLEDs and compares them with the respective spin coated devices. These devices have been prepared on a substrate with a simple, low resolution photo resist structure. As the target application is the manufacture of full color displays, the light emitting polymers investigated in this study were red, green and blue. The results show that a careful choice of the printing parameters and environment results in devices with current-voltage, brightness-voltage and emission characteristics comparable to spin coating samples, which have been prepared as reference. The film homogeneity of the functional layers turned out to be of utmost importance and can be influenced by various jetting parameters.