Philips Research is not disclosing the exact semiconducting polymer being used or the voltage scheme but described the development as a "major step toward the realization of low-cost, flexible displays made in plastic."

However, because of the inferior performance of plastic transistors compared with conventional silicon thin-film transistors (TFTs), the refresh rate of the transistors is limited to 100 Hz, which would at present limit the number of pixels that could be driven in an active matrix.

The primary advantage of a polymer-based active matrix compared with conventional amorphous silicon is lower-cost production. Fewer production steps and less-stringent clean room requirements are needed. It is also possible, in principle, to print the switches on plastic foil in a reel-to-reel process for flexible substrates.

Flexibility promise

Although Philips hasn't used reel-to-reel processing for this demonstration, the size of the resulting matrix of switches could be very large, bringing the prospect of high-volume, flexible displays closer.

"This is the first time Philips, and as far as we know anyone, has used polymer electronics for active-matrix display driving," a spokeswoman for Philips Research said.

Researchers used a polymer-dispersed liquid-crystal display (PDLCD) to demonstrate their polymeric transistor patterning. A PDLCD is a reflective display that, unlike most LCD technologies, is not based on polarization effects and so can be used to make a flexible display. If displays based on polarization are flexed, the viewing angle will depend on the point of the display being observed.

In a PDLCD, light is either scattered by nonaligned molecules in liquid-crystal domains or the LC domains are transparent because the molecules are aligned by an electrical field. However, the relation between voltage applied and resulting contrast in such displays does not allow multiplexing, thereby ruling out use of a passive-matrix driving mechanism.

Instead, each pixel has to be addressed individually by a thin-film transistor. The resulting active matrix is conventionally made using amorphous silicon-based TFTs and represents a major part of the cost of the display.

The substrate in this demonstrator is glass, but a plastic substrate will be used in the next prototype, which will try to demonstrate a flexible display.

The charge carrier mobility in polymer materials is less than in silicon, so the electronics are slower, which could set a limitation on display size. Polymer electronics in general are slower than silicon-based electronics, whether amorphous or polycrystalline. The technology would have to be optimized and extended to be able to drive displays beyond the present array size of 64 x 64 pixels, Philips said.