LIGHT-EMITTING DIODES: OLED pixel array is transparent

June 1, 2006

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).

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).

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.

The OLED transmits throughout the visible spectrum.

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.