Inkjet printed plastic semiconductors: new understanding of jetting limits

16 Nov 2012. ONDL scientists reported in the Journal of Fluid Mechanics this month the discovery of a new characteristic in the formation of fluid droplets during inkjet printing (doi: 10.1017/jfm.2012.440) that has particular relevance for organic semiconductor thin films.

Inkjet printing is most widely known as the inexpensive technique to place ink droplets on paper to form images and text. This has revolutionised publishing in the last decades by making documents available to anyone with an inkjet printer and a computer. Because these droplets can be placed on demand at any desired location on the substrate, inkjet printing has also been investigated over the past few decades as a potential 3D manufacturing tool for plastic and ceramic articles. In the last decade however, the seeds for another perhaps more interesting revolution were planted, when scientists began to explore its use to deposit plastic semiconductor thin films for device applications, such as the arrays of red-, green- and blue-emitting pixels in polymer organic light-emitting displays. The method is low-cost and fully scalable, which saves on both materials and energy, making it a potentially “green” manufacturing technology for sophisticated devices.

However the science of inkjet printing has some way to catch up with these new exciting technological applications. In this report, Dr Loke-Yuen Wong and his colleagues at ONDL show that the jettable fluid space does not simply depend on the Ohnesorge number as previously thought, but in fact varies with the voltage that is applied to the piezo ceramic transducer in the print head. The jettable fluid space is important to determine the range of fluid properties (such as surface tension and viscosity) that can be printed through a particular nozzle. As a consequence, they suggest that the jettable fluid limits are in fact correctly given on a voltage-Ohnesorge diagram, instead of the older Ohnesorge diagram that has been widely used. The upshot is this now makes the a priori selection of print conditions to achieve the desired droplet speed possible, which takes some of the trial-and-error out of printing of the large variety of fluids encountered in plastic semiconductor device fabrication.