Transparent electrodes for use in touchscreens have been made by inkjet printing. Their creators at ETH Zurich in Switzerland say the new electrodes boast higher conductivity and are more transparent than those made of indium tin oxide, the usual material in today’s smartphones and tablets.
Transparent electrodes comprise a barely visible pattern of conductive material, which allows the device to recognise whether and where precisely a finger is touching the screen. Indium tin oxide is fairly transparent, but it’s only moderately conductive. Gold and silver are better conductors. But there’s a problem: as the cross-sectional area of gold and silver wires grows, the conductivity increases, but the grid’s transparency decreases.
The solution is to build metal walls a mere 80 to 500nm wide, which are almost invisible when seen from above. They are two to four times taller than wide (Adv. Funct. Mater., doi: 10.1002/adfm.201503705).
‘We use a non-transparent material like gold and silver and pattern it in a smart way – high walls, large spacing between the walls, fine walls – we barely cover a couple of % of the area and [the electrodes] are almost perfectly transparent,’ explains lead author Patrik Rohner. ‘Compared with other techniques, material use is efficient and cost is low, even with gold.’
The process relies on inks made from metal nanoparticles in a solvent being drawn as ultra-small droplets out of a glass capillary by an electrical field. The solvent evaporates, allowing a 3D structure to be built, drop by drop.
In a touchscreen, the transparent electrode lies above the pixels of the display, so that more transparent electrodes will allow more light from the pixels to reach your eye. ‘The finer the pattern the better the resolution of the touchscreen, but if the circuit is too fine and not conductive anymore, the signal is weak. So you want a very fine grid in x-y ,which still has low resistance,’ Rohner explains.
This is especially important for larger screens, which require better conductivity. The process, unlike the product of indium tin oxide electrodes, does not require a cleanroom environment, so could be more cost-efficient. Another possible application could be solar cells, where greater transparency and conductivity allows for more energy harvesting, say the researchers.
‘The main drawback of this printing method is the low printing speed, at the moment about 30 min for an area of only 100 x 100 µm2,’ comments material scientist Klaus Ellmer at the Helmholtz Centre in Berlin, Germany. ‘The speed can be increased by increasing the drop frequency and by using many nozzles in parallel. However, it is questionable if this method can be scaled up to large areas – up to m2 for solar cell or display applications – with a reasonable speed, ca 1 m2/min.’