TUCSON, Ariz. A new family of organic polymers demonstrated at the University of Arizona can "paint" computer displays and photocell arrays onto most any surface, and could enable self-contained "computerless" niche devices that are cheaper and more flexible than what's available today.
The approach loads polymer-based electro-optic ink into inkjet cartridges then uses a conventional inkjet printer to deposit photonic circuits onto flexible surfaces.
"We are still exploring all the different circuit elements we can make with these organic molecules," said Ghassan Jabbour, a University of Arizona researcher who leads the group that demonstrated the method.
By adjusting the formulas mixed from the inkjet cartridges while they spray, Jabbour plans to emulate resistors and similar components found in traditional circuits. An image, for instance, can be hardwired into the variable conductivity of the wires that are painted onto an otherwise fixed grid of organic LEDs, thereby embedding the image in the array rather than requiring a separate memory from which to read it out.
"You have much more flexibility than when using traditional circuit components," said Jabbour, "because you can mix together different compounds for truly continuous changes. For instance, we can mix different molecules from different inkjet cartridges during printing for truly continuous changes in conductivity." He was assisted on the project by a student, Yuka Yoshioka.
So far, Jabbour's group has shown that it can paint complete systems for displaying images via organic LED "portraits" and for collecting energy from inkjet-printed solar-cell arrays.
Using simple software algorithms on a computer to which the inkjet printer is attached, the group demonstrated a versatile painting technology that could create nearly any size photonic circuit. Dispensing extremely small droplets of different organic molecules, the high-speed nozzles of the computer-controlled print heads fired circuits onto an electrically conductive polymer. The resulting inkjet-printed portraits and solar arrays were entirely self-contained; the operation needed just an attached battery to power the display.
"We keep coming up with new processes using inkjet techniques," Jabbour said. "Besides varying the conductivity of wires, we can also mix chemistry with the inkjets to control where we want a lot of electrons and where we don't want electrons. We just program the printer to interpret color commands in terms of the chemical reaction we want. To the printer, it is just printing the same way as if it were putting regular images on paper."
Since the inkjet's molecules are transparent and print on transparent plastic, silicon or glass substrates solar cells can be unobtrusively painted on convenient areas like windows. Likewise, until a portrait is powered up it is similarly transparent. Both the solar cells and the organic LEDs can emit/sense the infrared spectrum, making inexpensive and difficult-to-detect security devices. Infrared solar cells, for instance, could sense intruders and infrared portraits could convey sensitive information to "night-goggled" security personnel.
Jabbour also said his group's approach piqued the interest of other researchers working on MEMS-based technologies, like the handheld "chem-lab on-a-chip" that is being funded by the Defense Advanced Research Projects Agency. The technique's ability to vary the electrical conductivity of wires could be used to make microfluidic channels with built-in ion filters, Jabbour said. "There are so many applications for this technology, that we can't do them all," he said.