Mechanically flexible organic materials
Plastic substrates, which are tough and yet light, even those with a large total area, are ideally suited for use in human-friendly electronic devices. Unlike inorganic substances, organic materials can be used to create high-quality thin films on plastic substrates, which have poor heat resistance.

* In this context, an organic material is one with molecules consisting of a carbon (C) skeleton linking together other elements. These materials are similar to resins, such as plastics and vinyl. Conventional transistors are made from silicon (Si), an inorganic substance. While silicon crystals are extremely hard, organic substances are light and flexible, making them ideal for creating flexible, human-friendly devices. Normally organic materials are poor electrical conductor. However, in recent years it has become possible to produce organic compounds that conduct electricity. Examples include conductive polymers.

Organic Transistors Manufactuable at Room Temperature
Organic transistors are transistors with organic semiconductors (Figure 2) in their semiconductor layer (Figure 3). Organic transistors with several tens of nanometers-thick organic semiconductors are called "organic thin-film transistors" (OTFT). As with a normal inorganic transistor, it is possible to switch the current flowing between the source electrode and drain electrode in an organic transistor on or off by varying the voltage at the gate electrode (*). One of the differences between organic and inorganic transistors is the temperature at which they are fabricated. Conventional inorganic transistors require high temperatures (500-1,000ºC), but organic transistors can be made between room temperature and 200ºC. Organic transistors can even be formed even on a plastic substrate, which is vulnerable to heat. Therefore organic transistors enable to realize not only light and thin, but also flexible device elements, allowing them to be used in a variety of unique devices, such as flexible displays and sensors.

Displays play an important role in the interaction between human beings and information. Organic materials are used in the display mechanisms of today's LCD and OLED displays, but their control systems consist of inorganic transistors on glass substrates (silicon TFT). This type of display is difficult to bend. However, a display with organic transistors on a plastic substrate would be completely flexible. Such a display could also be dropped without breaking, rolled up, or folded.

Sony has demonstrated its ability to create flexible, drop-proof displays by using organic transistors on plastic substrates to create drivers for a flexible transmissive LCD display (announced in July 2005, [1]) and a flexible OLED display (announced in May 2007, [2]).

Another important characteristic of organic transistors is the simplicity of the method used to produce them. Inorganic transistors require massive vacuum systems and complex manufacturing processes. However, most organic materials can be dissolved in organic solvents to create "inks" that can be used to create circuits simply by printing them under normal atmospheric conditions. Printing technology can also be readily adapted to the production of flexible substrates and large substrates. With the methods used to manufacture conventional inorganic transistors, only a few percent of the materials are actually used. Printing is far more efficient, and by optimizing the method it is possible to achieve usage rates as high as 90%. For this reason, this manufacturing method is also attractive from an environmental perspective. Sony is currently conducting research into the fabrication of organic transistors through printing.

The organic transistor is a new technology that is still in the early stages of its evolution. Comprehensive development efforts targeting printing processes, device fabrication and design methods are expected to result in improvements in both performance and reliability, paving the way for practical implementation of the technology. We have focused on flexible displays as an example of the types of devices in which this technology will be used. Other applications are likely to include sensors and input devices. By bringing these ideas together, it should be possible to create a variety of unique applications.

References

1. N. Yoneya, N. Hirai, N. Kawashima, M. Noda, K. Nomoto, M. Wada, J. Kasahara, I. Yagi, K. Tsukagoshi, Y. Aoyagi, Digest of Tech. Papers of AM-LCD 05, 25 (2005).
2. I. Yagi, N. Hirai, M. Noda, A. Imaoka, Y. Miyamoto, N. Yoneya, K. Nomoto, J. Kasahara, A. Yumoto, T.Urabe, Society for Information Display 07 Digest , 1753(2007).
   Sony press release
   http://www.sony.co.jp/SonyInfo/News/Press/200705/07-053/