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Organic thin-film transistors (OTFTs)—with an active layer based on the organic semiconducting anthracene oligomers, 2,2′-bianthracene (2A), 2,6-trianthracene (3A), and their dihexyl derivatives (DH-2A and DH-3A)—were fabricated. It was found that thin films of anthracene oligomers deposited by thermal evaporation had lamellar structures with a high degree of molecular ordering. Consequently, the OTFTs based on anthracene oligomers showed high field-effect mobilities. The mobility of the OTFTs was increased by the oligomerization and the substitution of alkyl groups. DH-2A on a SiO2/Si substrate showed the highest mobility of 0.13 cm2/V s. It was also found that the electrical characteristics of the OTFTs were improved by using a Ta2O5 gate insulator.

The characteristics of pentacene-based organic thin-film transistors (OTFTs) on plastic substrates with anodized gate insulators using different electrolyte solutions were investigated. The characteristics were significantly improved by using ammonium borate as the solution. The results show a good current-saturation characteristic, field-effect mobility of 0.51 cm2/Vs and a current on/off ratio of 105 at a low drain voltage of 3 V. To investigate the reason for the improvements, the structural and electrical characteristics of OTFTs and insulators were examined using various methods such as X-ray diffraction, atomic force microscope, capacitance-voltage measurement and secondary ion mass spectroscopy. From the results of these experiments, it was found that the orientation of pentacene was improved and the concentration of impurities in the insulator decreased in the case of ammonium borate. We also examined low-voltage driving of liquid crystal devices by the OTFT, targeting its application for flexible displays. Improvement of characteristics by the solution was also confirmed in the driving.

Organic thin-film transistors (OTFTs) which use Ta2O5 as the gate insulators were fabricated on plastic substrates. The gate insulators were synthesized by anodizing the gate electrodes fabricated as stacked structures of aluminum (Al) and tantalum (Ta) at room temperature. The stacked structure suppressed the stress at the interface between the substrate and metal electrodes so that cracks on Ta could be avoided. The organic semiconductor pentacene was used as the active layer on the gate insulator. The OTFTs showed the best performance at an operating voltage of 5 V and a good field-effect mobility of 0.36 cm2/Vs on plastic substrates. The structural order of the pentacene in the film and the temperature dependence of the leakage current in the insulator were also studied to improve the device characteristics.

Organic thin film transistors (OTFTs) that operate at low voltage were fabricated using a tantalum oxide (Ta2O5) gate insulator on plastic substrates. The Ta2O5 insulator was prepared by anodizing a Ta gate electrode. The high dielectric constant of the Ta2O5 enabled the OTFTs to operate at a lower voltage than those of previous devices. The OTFTs exhibited a high field-effect mobility of 0.35 - 0.51 cm2/Vs, and a current on/off ratio of 105-106 at gate voltages of less than 5 V. The threshold voltage of -1.1 V and the subthreshold slope of 0.2 V/decade were the best among those reported to date. We also demonstrated operation of a phosphorescent organic light-emitting device (OLED) with the OTFT operating at a low voltage.

A pentacene thin-film transistor (TFT) was fabricated on a SiO2 gate insulator treated with β-phenethyltrichlorosilane (β-PTS). Employing β-PTS for the surface treatment of SiO2, large grains were present in the initial stage of pentacene crystal growth. The field effect mobility was as high as 1.5cm2∕Vs and the on/off ratio was over 106. The surface treatment dramatically improved the stability in air of the pentacene-TFT’s electrical characteristics. A field effect mobility of over 1cm2∕Vs and on/off ratio of over 105 were maintained after scanning the gate voltage 2000 times in air. This result indicates that the surface treatment with β-PTS not only improved TFT performance but also significantly suppressed the device’s degradation.

A fast-response flexible ferroelectric liquid crystal (FLC) display panel driven by organic TFT (OTFT) was successfully developed. The fabricated OTFT with Ta2O5 gate insulator showed a mobility of 0.3 cm2/Vs and a current on/off ratio of 107 with a low operating drain voltage of −5 V. We obtained fast responses of less than 1 msec and contrast ratio of 14:1 with gray-scale display capability from a 16×16 pixel FLC panel.

We report on n-channel organic thin-film transistors (OTFTs) based on the novel n-type organic semiconductor, perfluoropentacene. The transistor exhibits excellent electrical characteristics, with a high electron mobility of 0.22 cm2/(V s) and a good current on/off ratio of 105. The electron mobility is comparable to the hole mobility of a pentacene OTFT. By combining the n-type perfluoropentacene and the p-type pentacene, we have fabricated ambipolar OTFTs and complementary inverter circuits. The OTFTs with heterostructures of the p- and n-type organic semiconductors can operate as an ambipolar device with high electron and hole mobilities of 0.042 and 0.041 cm2/(V s). The complementary inverter using an n-channel perfluoropentacene OTFT and a p-channel pentacene OTFT exhibits excellent transfer characteristics with a voltage gain of 45. A complementary inverter using the ambipolar OTFTs is also demonstrated.

Organic thin-film transistor (OTFT)-driven 5×5 polymer-dispersed liquid crystal (PDLC) display cells on a flexible plastic substrate have been developed. It is necessary to increase the maximum usable gate voltage of OTFT to obtain a high contrast ratio. We propose a stacked gate insulator that consists of polyvinylphenol and anodized Ta2O5 for decreasing the gate leakage current and increase the maximum voltage. The OTFT with the insulator showed a field-effect mobility of 0.4 cm2/(V s), a current on/off ratio of 105, a low threshold voltage of 1.1 V, and a subthreshold slope of 0.2 V/decade. Leakage current was successively decreased up to a gate voltage of 15 V, maintaining a low-voltage operation of OTFT. Double passivation layers using polyvinylalcohol and photosensitive acrylic material are also proposed to prevent the degradation of OTFT by liquid crystal. The bending characteristics of OTFT on plastic substrates were also measured for various radiuses of curvature. The OTFT can operate at a radius of curvature exceeding 20 mm. On the fabricated display cells, we confirmed a good display operation with a contrast ratio of 10:1 with a low driving voltage of 12–13 V.