Radical Transparency

Close-up of capacitor, showing two layers of gold separated by 100nm of polyimide. The group has also developed a high-K printable material. Close-up of capacitor, showing two layers of gold separated by 100nm of polyimide. The group has also developed a high-K printable material. Someday, you might read the morning’s news headlines on the back of your cereal box. That’s the latest possibility demonstrated by the EECS Organic Electronics Group, which has already made headlines for developing cheap, printable chemical sensors and RFID tags that eliminate the need for barcode scanning.

The group has recently been experimenting with zinc oxide, a familiar ingredient in sunblock and diaper cream that has the special properties of working as a semiconductor while also being 93 percent transparent. The researchers already have a palette of inks that can deposit conducting, semiconducting and insulating materials—the building blocks of all solid-state electronics—on a variety of surfaces.

Back in the 1960s, printed circuit boards (PCBs) improved upon wire-wrap circuits by replacing the tangles of wires that connected electronic components with neatly printed patterns of conductive metal. Printed electronics takes the approach further by printing the components themselves along with the connections between them. And adding zinc oxide ink to the mix now makes it possible to print resistors, transistors and other components that light will shine through. This is a clear advantage for all modern displays, in which each pixel draws power from its own transistor.

Explains Professor Vivek Subramanian, who heads the group, “The transistor blocks 10 percent of the light coming through each pixel, reducing the screen’s brightness. A transparent transistor won’t block the light, and you can also increase its size so that it covers most or all of the pixel, which lets you drive the display much faster.”

Coupled with transparent conductors made from indium tin oxide or other mixtures of zinc oxide, the transparent semiconductors also permit the addition of circuitry in the display to make it run more efficiently. “The signal doesn’t need to come from the edges of the display and lose strength on the way in,” says Subramanian. “Now you can connect inputs and boost their strength anywhere.”

Vivek SubramanianVivek SubramanianWhile a transparent semiconductor can improve the brightness, clarity and speed of traditional glass screens, Subramanian’s group is more interested in matching the performance of existing screens with plastic. On many laptops, cell phones and other devices, the screen is the most fragile and expensive part; not only is a plastic substrate cheaper and more durable than glass, it’s also flexible, which means it could be mechanically printed and handled like paper or cardboard—giving a whole new meaning to the “print screen” button at the top of your keyboard.

The special inks that the group uses contain nanoparticles of zinc oxide or other compounds with the desired electrical properties. When the inks are printed onto a hot surface, the particles fuse together, creating a continuous electrical pathway or insulator. The plastic needs to be 100 degrees Celsius, a temperature that’s comfortably below the 150 degrees Celsius level at which it would deform. The surface is printed multiple times, which builds the circuitry layer by layer, so the process resembles four-color printing, but the alignment needs to be even more precise. In terms of miniaturization, the process can’t match etching silicon chips. (No one is printing heavy-duty microprocessors.) But it works well to create simpler circuits over relatively large areas.

“Our students have made their own inks, built their own substrates and have even modified inkjet and gravure printers for printing electronics,” says Subramanian. “The inkjets are a great research tool, but traditional gravure printers, which use etched cylinders, are better for manufacturing. They’re very high speed, and the students have figured out how to precisely align the layers.”

Most significantly, the cost of printing the displays and other components scales up to make them suitable for inexpensive products or even packaging. Disposable printed displays could be used for advertising signage, animated assembly instructions for packaged furniture, video clip players enclosed in magazines, camcorders in cereal boxes, and scrollable display tags for legal fine print. Now that the Organic Electronics Group has added printable displays to their RFID tags and chemical sensors, the milk carton of tomorrow might check itself out at the register, indicate its freshness and then show you how to make a soufflé.