Printing Process Makes Illuminating 'Wallpaper'

University of Umea light-emitting electrochemical cell.A great deal of effort has been devoted to trying to produce low-cost organic light-emitting diodes (OLEDs) via printing processes--inkjet, screen, and gravure. So far, though, making these devices has demanded tightly controlled conditions (including artificial atmospheres).

Now, Andreas Sandström  and collaborators at the Umea University Physics Department in Sweden and the Technical University of Denmark have flipped the switch on light-emitting electrochemical cells (LECs). These devices consist of a mobile-ion active layer sandwiched between a cathode-coated base layer and a semitransparent anode layer.

The LEC is manufactured in a series of simple coating operations, based on a die-slot printing process (think of the way some plastic mayonnaise bottles extrude a ribbon of condiment) that lays down bands of electrically active “ink” on a base membrane of polyethylene terephthalate.  

First comes the bottom cathode layer, a 14-nanometer-thick dusting of zinc oxide particles on indium-tin oxide. Then the active layer is roller-coated on. This is a blend of the light-emitting polymer “Super Yellow” (a poly(phenylene vinylene) derivative) with potassium trifluoromethanesulfonate in poly(ethylene oxide)). Last comes the anode layer, a diluted solution of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate).

Let it dry, wire it up, and ramp up the voltage. The device begins to glow visibly at about 3.7 volts. Brightness increases with voltage, reaching 150 candela per square meter (in the range of the average computer display) at 10 volts.

The researchers are particularly enthusiastic about two points: first, the device is fabricated in open atmosphere. And second, the device operates even if the deposited layers are very uneven; it does not require the tight tolerances that OLEDs demand, and light emission remains highly uniform over the test device’s 300-square-millimeter surface.  Or, as they put it in their paper, “the fabrication yield of the roll-coated LECs is found to be very satisfying, primarily due to the fault-tolerant device geometry with a thick active layer and air-stable materials.” As a bonus, because anode and cathode layers are transparent, the device shines from both its top and bottom surfaces.

While it is tempting to imagine floor-to-ceiling information displays of LEC “wallpaper,” the most probable applications—at least at first—will be commercial signage and attention-getting packaging, Sandström  says. It is “not at all impossible” that development could yield a room-size, pixel-addressable display, but “this is not an area where LECs will excel,” since they are inherently slower than OLEDs (the test device takes about two seconds to switch on, but significant improvements are likely).

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