Simple Process Devised to Make Thin-Film Display Material

Shouheng Sun (Brown University)

Researchers at Brown University in Providence, Rhode Island and Advanced Technology Materials Inc. in Danbury, Connecticut developed a simpler and less expensive process for producing thin films of indium tin oxide used in touch-screen displays and solar panels. The team led by Brown chemistry professor Shouheng Sun (pictured right) published its findings online in a recent issue of the Journal of the American Chemical Society; paid subscription required.

Sun’s team used a chemical solution to produce a conductive indium tin oxide film with a thickness of 146 nanometers — 1 nanometer equals 1 billionth of a meter. The film allows 93 percent of light to pass through, a transparency performance comparable to the glass plates on which the films are deposited.

The researchers also produced the indium tin oxide film on a flexible polyimide substrate, a flexible polymer plastic. Polyimides are used in a wide range of industries, but particularly in consumer electronic display screens and solar photovoltaic cells. In addition, Sun’s research team showed they could vary the the thickness and tin content to find the best combination of transparency and resistance — between 5 and 10 percent appear to be the best solutions.

To produce the films, the team synthesizes nanoscale indium tin oxide crystals in a solution. The researchers then drip the solution on a glass substrate followed by rapid spinning, a process called spin casting. to make a flat and smooth film. The next step in the process is to anneal (bake) the coated plates for various lengths of time; the optimum baking time turns out to be six hours.

The most productive raw materials for making the nanoscale crystals are indium acetylacetonate and tin bis(acetylacetonate)dichloride. The team keeps the synthesized indium tin oxide nanocrystals within a narrow size range, about 11 nanometers in diameter. That size consistency enables the crystals to arrange themselves consistently in the thin films, neither bunched together nor too far apart. The result is a dense but evenly distributed array of crystals, which promotes conductivity.

The next step for the researchers is to refine the process to produce films with conductivity comparable to current processes. They also aim to further drive down electrical resistance and reduce the length of time the films need to anneal. In addition, the team wants to engineer the process to lay down fine patterns of their films, rather than continuous sheets, using inkjet or roll-to-roll printing.

Brown’s partner in the project, Advanced Technology Materials Inc. (ATMI), provided funding and engineering assistance. The company develops high performance materials for for semiconductor, display, and life science industries.

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