Nippon Hoso Kyokai NHK means Japan Broadcasting Corporation. It is Japan's sole nationwide public broadcasting corporation. Its laboratories specialize in research and development in the field of broadcasting and related technologies.
In May 2007, it was announced that NHK Science & Technical Research Laboratories has prototyped a flexible TFT array using C16, a polymeric organic semiconductor material, which has a high hole mobility of 0.5 cm2/vs, though the new development of Merck in the UK of a printable at 3.5 cm2/vs polymer ink is higher. While C16 has similar hole mobility to low-molecular-weight organic semiconductors or amorphous Si, its film can be easily formed by spin coating method. The laboratory made 64 x 64 TFT arrays, and the pitch of arrays is 0.5 mm.
Optimising semiconductor and substrate
C16 has a longer chain structure than low-molecular-weight pentacene oligomer, which is frequently used for organic semiconductors but can not be printed other than through a precursor, which can be a route that degrades performance. PEN (polyethylene naphtalate) resin that has excellent heat resistance and thermal stability was used for a substrate rather than the cheaper PET.
Drive voltage for these arrays to achieve a hole mobility of 0.5cm2/vs is 100 V, says NHK if sufficient current is to be supplied to emit light from OLEDs. In a practical use, however, reduction of a drive voltage is required, notes NHK. Use of a gate insulating film made of Ta2O5, which has also been developed by NHK STRL, gives a hole mobility of 0.15cm2/vs at a drive voltage of 10V. "Our next goals are to make a finer array and a larger panel and having the panel recognized as a TV display," explained a demonstrator from NHK STRL.