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In a touchscreen display or a solar panel, any conductive
overlay had better be clear. Engineers employ transparent thin films of indium
tin oxide (ITO) for the job, but a high-tech material’s properties are only
half its resume. They must also be as cheap and easy to manufacture as
possible. In a new study, researchers from Brown University and ATMI Inc.
report the best-ever transparency and conductivity performance for an ITO made
using a chemical solution, which is potentially the facile, low-cost method
manufacturers want.
“Our technology is already at the performance level for
application in resistive touch screens,” said Jonghun Lee, a Brown chemistry
graduate student and lead author of the paper posted online by the Journal of the American
Chemical Society.
The group made conductive ITO films 146 billionths of a
meter thick that allowed 93% of light to pass through, a transparency
comparable to the glass plates they were deposited on. The team also made their
films on top of bendable polyimide, showing that it could potentially be useful
for making flexible display technologies.
In several experiments they showed that by varying the
thickness and the tin content (between 5% and 10% was best) they could vary the
transparency and resistance to find the best combination.
“By controlling the concentration of the nanocrystal solution,
we could control the thickness of the film from 30 nm to 140 nm,” Lee said.
The solution
To make the films, the team synthesized nanoscale ITO crystals in a solution.
Then they made a flat and smooth film of them by dripping the solution onto a
glass plate followed by rapid spinning, a process called spin casting. From
there they baked, or annealed, the coated plates for several hours (the best
length of time turned out to be six hours) and then tested their transparency
and conductivity.
Spin casting is simple as high-tech manufacturing
processes go, but finding the chemistry that allows spin casting to produce a
high-performance ITO thin film has proven elusive. A key achievement described
in the new paper, was finding the materials needed to make the nanoscale ITO
crystals in the first place, said Shouheng Sun, professor of chemistry at Brown
and the study’s corresponding author.
The best chemicals turned out to be indium
acetylacetonate and tin bis(acetylacetonate)dichloride. They synthesized ITO
nanocrystals that had a narrow range of sizes, about 11 billionths of a meter
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.
“If the particle clumps, then you cannot get uniform
assembly and you can’t get good conductivity,” Sun said.
This discovery was critical for achieving the high-level
performance detailed in the paper, but the team knows it still needs to build
on that progress—for instance, to match the conductivity performance of films
made by a process called sputtering.
“The next step is to improve conductivity to a magnitude
commensurate with sputtered ITO while realizing the reduced cost and process
efficiency benefits expected of a solution-based ITO deposition method,” said
Melissa Petruska, senior scientist at ATMI and co-author of the paper.
In new experiments, therefore, 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.
Source: Brown University
