A UK consortium has developed a process for inkjet printing transparent conductors for displays, aiming to cut costs compared with indium tin oxide (ITO).
The inkjet-able material is antimony tin oxide (ATO). “The reason for choosing antimony is two-fold,” Steve Lipiec, MD of Stoke-on-Trent-based chemicals firm Keeling & Walker told EW. “The material is cheaper and it is easier to get hold of than indium.”
The consortium turned ATO into an ink as it is too inert to be etched in the traditional sputter-pattern-etch process used to deposit ITO, said Lipiec.
“We have a proprietary process for making nano-particulate ATO which can be dispersed into a printing medium, in this case water,” explained Lipiec.
This does introduces a limitation: the substrate has to be wettable to be printed. The consortium is working on non-water inks for difficult surfaces.
Once printed, the ink needs annealing to fuse the particles into a continuous conductor.
Thermal annealing at 700°C gives the best results so far. A sheet resistance of 100O/square compares with 20-50O/square for sputtered ITO, with 90 per cent light transmission comparable to ITO, said Lipiec.
“Transmittance and resistance are interdependent and the inkjet process allows for the thickness of the coating to be adjusted to provide the optimum characteristics, as well as simple and direct printing of features,” said Lipiec.
“We have successfully annealed it on Corning Eagle 2000 LCD glass with no visible damage,” he added.
Thermal annealing down to 400°C is possible, with increased sheet resistance, as is laser annealing.
Eventually it is hoped to cut annealing temperatures while maintaining or reducing sheet resistance, or to remove the annealing step completely so that ATO can be applied to plastics.
Finer particle size, and annealing with a flash of energy at the correct wavelength are under study.
The DTI, which part-funded the programme, has just put further cash into a two-year programme to transfer the process to plastic substrates.
Final printed conductor costs have yet to be established, but as a guide to possible savings, Lipiec said ITO is 90 per cent indium, which costs $1,000/kg, whereas antimony tin oxide is 90 per cent tin at $10/kg. Antimony is under $3/kg.
The consortium also includes Swindon-based industrial ink-jet printer maker Patterning Technologies and Nottingham Trent University, which provided electroluminescent display materials and annealing know-how.