I. Introduction
Recently, there has been much research and development into the fabrication of devices using organic materials. Organic light-emitting diodes (OLEDs) have already become commercialized for use in smartphones and television displays. In addition, they are expected to be used in the next generation of displays, i.e., flexible displays. Flexible OLED displays are new in concept and exhibit features that liquid crystal displays on glass substrates cannot offer, such as being as thin as paper, lightweight, bendable, and unbreakable even when dropped. Currently, low-temperature polysilicon (LTPS) and oxide semiconductors, which are formed using high-temperature processes, comprise the pixel circuits of flexible OLED displays [1]–[4]. By contrast, organic thin film transistors (OTFTs), which use organic semiconductor (OSC) materials, can be formed using low-temperature processes below 180 °C, enabling the use of plastic film materials other than polyimide (PI). In addition, OSCs are more flexible than inorganic semiconductors, and a reduction in costs can also be expected owing to their solution processability [5], [6]. For the abovementioned reasons, OTFTs may be suitable for use in flexible displays. However, the mobility values of OTFTs should be increased to improve their suitability for pixel circuits in flexible displays, since values obtained in previous reports were insufficient at about 0.15–0.5 [7]–[9]. It has been reported that transistors using dithieno[2,3-d;2’,3’-d’] benzo[1,2-b;4,5-b’]dithiophene (DTBDT-C6) as an OSC in the active layer have higher mobility, although no reports exist in which this approach yielded values high enough for use in flexible displays [10]. In this paper, we discuss the results obtained when coating DTBDT-C6 and polystyrene (PS) blends by inkjet printing to achieve high-resolution OTFTs with high mobility, as well as the use of this technology to develop a color OLED display.