Printed electronics promises to fabricate low-cost microelectronics on flexible substrates for applications such as flexible displays and RFID tags. In particular, inkjet printing is a popular technique to pattern organic electronic devices because patterns can be changed dynamically. Despite of its widespread use, non-idealities are commonly observed and only partially solved. Here, we demonstrate a solution to the problem of bulging lines.
After ink has been ejected from the nozzle and deposited on the substrate, ink can flow distorting the pattern. This is mainly caused by surface tension forces and the associated Laplace pressure. Traditionally, lines are printed by linearly placing drops one behind the other. By varying the drop spacing, four different regimes are observed: outward bulging of lines, scalloping, line separation and smooth lines. Unfortunately, this approach only achieves smooth lines under certain conditions and generally not at the start of a line. When a new line is started, drops form bulges that are rounded and less line-shaped to minimize surface energy. This starting bulge grows as drops are added to the end of the line due to an imbalance of pressure within the liquid line. Ink flows from a region of high pressure to a region of low pressure (the bulge). Starting bulges can be observed not only in isolated lines but also X-, T-, and L-shapes when a new segment is started e.g. in interdigitated or serpentine patterns. This reduces yield of devices such as printed transistors or requires large margins of safety.
Here, we demonstrate that by manipulating the order of drops printed, we can avoid bulges at the beginning of inkjet-printed lines. The premise of this approach is to maintain symmetry and a net force of zero on every drop added to the line. By canceling out the high pressure at either end of the line, we have removed the pressure imbalance that leads to line bulging. This is achieved by always connecting equidistant line segments or drops of equal length/volume. The basic building blocks are segments of three drops where the middle drop is printed last. We show that these segments do not become rounded unlike three-drop segments that are printed linearly, which causes bulging. Segments are successively connected without allowing pressure imbalances to form. Crucial parameters are the drop spacing within segments as well as the spacing between segments. We show how the contact angle of the ink on the substrate affects these parameters. Finally, smooth lines can be printed with this novel method. This symmetric approach can be extended to more complex two-dimensional patterns by recursively breaking them down into segments of equal length that are connected in a binary fashion.
In summary, we present a novel approach to the inkjet printing of scaled features that can be employed in printed devices. We successfully avoid undesirable line morphologies by ensuring that the pressure within lines is always balanced.