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.
