August 7, 2012 — Brown University developed a simpler, cheaper, solution-based manufacturing process for indium tin oxide (ITO) conductive films used in displays and solar cell manufacturing.
Brown researchers, with support from ATMI Inc., reported the best-ever transparency and conductivity performance for an ITO made using a chemical solution.
Figure. Electron microscopy (cross-section, left, and facing view) shows an even distribution of indium titanium oxide nanocrystals essential for a highly conductive, transparent thin film. Credit: Sun Lab/Brown University.
The ITO offers high enough performance for applications in resistive touch screens, said Jonghun Lee, a Brown chemistry graduate student. The films allow 93% of light to pass through, deposited at 146nm thick. Their transparency is comparable to the glass substrate.
The team also made their films on top of bendable polyimide, showing that it could potentially be useful for making flexible display technologies.
To make the films, the team synthesized nanoscale ITO crystals in a solution. Then they made a flat and smooth film by spin casting the solution on a glass plate. Coated plates were then annealed for several hours (ideal anneal time was 6 hours) and then tested their transparency and conductivity.
The materials research was key to enable the simple spin-casting assembly method, said Shouheng Sun, professor of chemistry at Brown. The best chemicals turned out to be indium acetylacetonate and tin bis(acetylacetonate)dichloride. Researchers synthesized ITO nanocrystals that had a narrow range of sizes, about 11nm in diameter. That consistency meant that when the crystals arranged themselves in the thin films, they neither bunched together in clumps, nor stayed too far apart. The result was a dense but evenly distributed array of crystals, which promotes conductivity.
By varying the thickness and the tin content (between 5 and 10%), researchers varied the transparency and resistance for the best results. The key to a smooth, consistent film was the uniform size of ITO nanocrystals. The researchers settled on a diameter of around 11nm. “By controlling the concentration of the nanocrystal solution, we could control the thickness of the film from 30 to 140nm,” Lee said.
The team will now work on matching the conductivity performance of films made by sputtering, while maintaining the cost and process efficiency benefits of solution-based deposition, according to Melissa Petruska, senior scientist at ATMI.
In new experiments, the team plans to further drive down electrical resistance, to reduce the length of time the films need to anneal, and to lay down fine patterns of their films, rather than continuous sheets, using inkjet or roll-to-roll printing.
Results are published in a paper posted online Aug. 1 by the Journal of the American Chemical Society. In addition to Sun and Lee, the other Brown authors are Sunghwan Lee, Guanglai Li and David Paine. Petruska is a co-author on the paper. ATMI provided project funding and engineering assistance for the research.
Learn more at www.brown.edu.