Daisuke Kumaki

Surface-energy-dependent field-effect mobilities up to 1 cm2/V s for polymer thin-film transistor

The field-effect mobility of a liquid-crystalline semiconducting polymer, poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PB16TTT), has depended significantly on the surface energies of self-assembled monolayers (SAMs) formed on insulating layers. Using a SAM with fluoroalkyl groups, with a low surface energy of 13.3 mN/m, the mobility of PB16TTT reached as high as 1u2002cm2/Vu2009s. These results indicate that an edge-on orientation of the polymer chains progresses more favorably on the surfaces with low surface energies via the liquid-crystalline phase.

Reducing the contact resistance of bottom-contact pentacene thin-film transistors by employing a MoOx carrier injection layer

We report on the reduced contact resistance in bottom-contact (BC) pentacene thin-film transistors (TFTs) with a molybdenum oxide (MoOx) carrier injection layer. MoOx layers were placed between the gate insulator and the source-drain (S-D) electrodes instead of the conventional adhesive layer such as Cr or Ti. The performance of the BC pentacene-TFT with the MoOx injection layer was significantly improved at low operating voltages. The contact resistance of the MoOx∕Au S-D electrodes, estimated using the gated-transmission line method, was nearly two orders of magnitude smaller than that of conventional Cr∕Au electrodes at the gate voltage of −10V. The highest performance was obtained with a MoOx injection layer a few nanometers thick, which was comparable to the effective channel thickness of the pentacene-TFT on the gate insulator. This result indicated the importance of the direct connection between the MoOx injection layer and the effective channel to reduce the contact resistance.

Significant improvement of electron mobility in organic thin-film transistors based on thiazolothiazole derivative by employing self-assembled monolayer

n-type organic thin-film transistors based on a thiazolothiazole derivative were fabricated on a SiO2 gate insulator treated with n-alkyl self-assembled monolayers (SAMs), which were composed of various alkyl chain lengths. The field-effect electron mobility increased depending on the alkyl chain length of the SAMs. A long alkyl chain significantly improved the on current and electron mobility. The highest electron mobility of 1.2cm2∕Vs and on/off ratio of 107 were achieved with an alkyl chain longer than that of tetradecyl-trichlorosilane. This result is attributed to the suppression of the influence of electron trap sites on the SiO2 gate insulator by employing the SAM with the long alkyl chain.

Influence of H2O and O2 on threshold voltage shift in organic thin-film transistors: Deprotonation of SiOH on SiO2 gate-insulator surface

The influence of H2O and O2 on the transistor characteristics in p- and n-type organic thin-film transistors (OTFTs) fabricated on the SiO2 gate insulator was investigated. In both p- and n-type OTFTs, the threshold voltage (Vth) shifted drastically to positive direction after exposure to ambient air and dry air, although the field-effect mobilities in saturation regime were almost unchanged before and after the Vth shift. The Vth shifts to the positive direction indicate that negative charges are generated on the SiO2 gate-insulator surface by exposure to ambient air and dry air. The influence of SiO− on the gate-insulator surface and deprotonation processes of SiOH caused by H2O and O2 were discussed as the origin of the significantly positive Vth shift.

High-mobility and air-stable organic thin-film transistors with highly ordered semiconducting polymer films

We report on high crystalline thin films of liquid-crystalline polythiophene derivative, poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PB16TTT) that exhibit terrace structures and molecular steps of its polymer chains by annealing in its liquid-crystalline phase. The crystallinity of the PB16TTT polymer films formed on SiO2 gate insulating layers with smooth self-assembled monolayer was improved by changing the octyltrichlorosilane treatment time for the SiO2, which led to reproducible high field-effect mobilities of the polymer thin-film transistors up to 0.44cm2∕Vs. High stability of the transistor for repeated stressing in ambient air was also demonstrated.

Air stable, high performance pentacene thin-film transistor fabricated on SiO2 gate insulator treated with β-phenethyltrichlorosilane

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.

Synthesis, physical properties and field-effect transistors of novel thiazolothiazole–phenylene co-oligomers

A series of thiazolothiazole–phenylene co-oligomers was synthesized and their properties, particularly as semiconductors for OFETs, were investigated. The naphthyl and biphenyl- substituted derivatives showed p-type semiconducting behavior with hole mobilities ranging from 10−2 to 10−1 cm2 V−1 s−1. The hole mobility and on/off ratio were enhanced to 0.12 cm2 V−1 s−1 and 106 on the HMDS treated substrate. The introduction of phenyl groups as end substituents was found to be favorable for charge transport and air-stability.

6 inch-flexible AM-OLED moving image display

The electrical performance of an organic thin-film transistor (OTFT) based on pentacene was significantly improved through the control of the layer surfaces and device interfaces. A 6-inch flexible organic light-emitting diode (OLED) display consisting of OTFT and OLED devices on a thin plastic substrate was demonstrated. While operating the OTFT backplane at relatively low voltages below 15 V, a good average field-effect mobility of 0.1 cm2/Vs was exhibited. The flexible active-matrix organic light-emitting diode (AM-OLED) panel displayed distinct color moving images at a frame rate of 60 Hz.

High performance n-type FETs based on heterocyclic ring systems with trifluoromethylphenyl groups

High performance n-type FETs have been accomplished by using novel heterocyclic systems with trifluoromethylphenyl groups. To enhance intermolecular interactions, selenophene rings were introduced. Some FET devices showed higher electron mobilities than 0.1 cm2V-1s-1. The mobilities of the selenophene-containing materials were higher than those of the corresponding thiophene analogues. The relationship between the structures and FET characteristics have been investigated. The threshold voltages were reduced by introducing heterocyclic units with higher electron affinity.