LIGHT-EMITTING DIODES: OLED pixel array is transparent

If windows and other transparent media could be made to display information, designers would have great latitude to create self-contained head-up displays for vehicles, advertisements appearing in the window glass of boutiques, or interactive art displays.

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If windows and other transparent media could be made to display information, designers would have great latitude to create self-contained head-up displays for vehicles, advertisements appearing in the window glass of boutiques, or interactive art displays. Until now, however, most attempts to do this failed because of a lack of compatibility between the various manufacturing steps. Materials must be transparent, sufficiently conductive, and capable of light emission on demand. Transparent flexible plastics are one approach; however, they are sensitive to any manufacturing processes that raise the temperature of the plastic.

Now, a group at the Technische Universität, Institut für Hochfrequenztechnik (Braunschweig, Germany) has taken an important step toward creating a practical transparent display. The scientists fabricated an array of organic light-emitting diodes (OLEDs)-including their driving circuits-on a thin glass sheet, the whole being greater than 70% transparent in the visible but able to emit green light from the pixels when energized (see figure).

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The seal of the Technical University of Braunschweig is seen through a transparent OLED device containing 10 pixels, one of them electrically connected in the power-off state (left) and switched on (right).
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The electroluminescent area consists of a bilayer of hole- and electron-transport hosts of organic compounds 4,4´,4´´-tris(N-carbazolyl)triphenylamine and 1,3,5-tris(phenyl-2-benzimidazolyl)benzene, doped with facial tris(2-phenylpyridine)iridium. Such so-called double-emission layers equipped with a metal electrode had previously been fabricated and shown to be highly efficient; however, those devices were opaque.

In the new OLED device, thin-film wide-bandgap semiconducting layers of transparent zinc tin oxide (ZTO) were deposited on glass that was coated with indium tin oxide (ITO) as the material for the transistor gates; oxygen-plasma-assisted pulsed-excimer-laser deposition was used to create the ZTO layers. The overall process temperature could be kept below 150°C, making the process compatible with plastics manufacturing.

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The OLED transmits throughout the visible spectrum.
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For source and drain contacts, zinc oxide doped with aluminum oxide was deposited and structured by standard photolithography using a transparent photoresist (SU-8). Transparent OLEDs with a pixel size of 1.5 x 1.5 mm were created, along with layered electrical circuits. With ITO also used as the anode, the entire system-15 layers, including additional electron-injection-enhancing layers and the substrate-showed an average transparency of 70% throughout the visible.

The luminescence was measured as a function of operating voltage. At a gate voltage of 5 V, a luminescence of about 700 cd/m2 was measured. The leakage current through the gate remained at 160 nA. For an active-matrix OLED display, three to four transistors per pixel are usually required; in addition, the gate electrode must be patterned-all possible with this technology.

Uwe Brinkmann

REFERENCE

1. P. Görrn et al., Advanced Materials 2006, 18, 738.

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