See-through OLED displays

Compared to established LCD and plasma technologies displays based on organic light emitting diodes (OLEDs) promise more brilliant images, less energy consumption and lower production costs. Furthermore, the organic layers that make up an OLED can be engineered to be transparent in the visible part of the spectrum. In combination with transparent conductive oxides like Indium-Tin-Oxide (ITO) or Aluminum doped Zinc-Oxide (AZO) as contacts OLEDs may be built entirely transparent. One major issue to be addressed in the fabrication of these devices is the deposition of the top transparent contact without damaging the organic layers. Transparent OLED pixels can be arranged to form entirely transparent OLED displays. For the active matrix addressing of the individual OLED pixels, we use TFTs which are transparent themselves. Rather than silicon, they are based on the wide-bandgap semiconductor Zinc-Tin-Oxide (ZTO) and transmit about 80% of the visible light (400-750 nm). The transistors typically have field-effect mobilities of 13 cm(2)/Vs (an order of magnitude larger than a-Si TFTs) and an on-off ratio of 10(6). The OLED pixel which needs to be driven may be positioned directly on top of the driver circuit. The pixels fabricated accordingly have an overall transmittance > 70% in the visible spectrum. The brightness of the OLED pixels could be varied from 0 to 700 cd/m(2) via the gate bias of the driving TFTs. These devices state the initial building blocks of future, large-area, high-resolution transparent OLED displays. More complex transparent driving circuits, required to compensate eventual device variations will be discussed.