Electronic products, which have permeated every nook and cranny of society and our lives, are currently made using silicon semiconductor devices. However, today's semiconductor devices, cut from silicon wafers using a process such as photolithography, require large-scale, energy-intensive industrial facilities. Therefore, miniaturization and mass production of standardized devices is necessary. On the other hand, organic semiconductor materials, which can be made into an ink solution, have the potential to greatly transform the semiconductor device manufacturing process (Figure 1). For instance, the application of inkjet printing makes it possible to offer electronic devices that accommodate diverse needs in minute detail. This development is expected to result in a new electronics.
Figure 1:Fabrication process comparison
Ricoh has pioneered practical application of the first genuine organic photoconductors (OPCs) and mounted them on copiers and printers that lead the industry in high sensitivity and durability. Building on the foundation of the organic semiconductor material technologies nurtured in OPC material development, we are actively engaged in the research and development of electronic devices, beginning with organic thin-film transistors. Taking materials technology as the starting point and making the most of one of Ricoh's strengths, the capability to comprehensively engage in research and development of device application technologies and inkjet printing and other process technologies to create product functions, we are focusing on advanced R&D to create new products such as e-paper and displays (Figure 2).
Figure 2: Thin-film transistors (TFTs) for display devices
To apply organic semiconductors to a printing process, it is necessary to produce ink with high solvent resolvability and viscosity suited to the printing process. Also, in the production of thin-film transistors (TFT), uniform film-forming characteristics are required because it is necessary to fabricate thin film free of minute voids as small as several dozen nanometers. Accordingly, Ricoh judged that an amorphous polymer had the potential to become the semiconductor material with the highest utility and proceeded with development on that basis. Ordinarily, amorphous polymer utility tends to entail a trade-off with charge carrier mobility. However, pursuing a molecular structure suited to carrier mobility, for which Ricoh's experience in development of OPC materials was utilized, we were able to design a semiconductor polymer maximizing solubility, film deposition characteristics, and mobility. This made it possible to simply fabricate TFTs using spin coating and inkjet printing and to increase FET hole mobility to approximately 0.01cm2/Vs, the level required for practical applications (Figure 3).
Figure 3: Relation between carrier mobility and solubility of organic semiconductors
"The Stilbene Polymers for Organic Field-Effect Transistors." , 2003 MRS Fall meeting (December, 2003)
"SYNTHESIS OF ARYLENE-VINYLENE POLYMERS CONTAINING TRIARYLAMINES FOR ORGANIC FIELD-EFFECT TRANSISTORS" , Sixth International Symposium on Functional pi-Electron Systems (June, 2004)