Shingo Iba

Integration of organic FETs with organic photodiodes for a large area, flexible, and lightweight sheet image scanners

A large-area, flexible, and lightweight sheet image scanner has been successfully manufactured on a plastic film by integrating high-quality organic transistors and organic photodetectors. The effective sensing area of the integrated device is 5/spl times/5 cm/sup 2/; the resolution, 36 dots per inch (dpi); and the total number of sensor cells, 5184. The pentacene transistors with top contact geometry have a channel length of 18 /spl mu/m and mobility of 0.7 cm/sup 2//Vs. Organic photodetectors composed of copper phthalocyanine and 3,4,9,10-perylene-tetracarboxylic-diimide distinguish between black and white parts on paper based on the difference in their reflectivity. Since this new area-type image-capturing device does not require any optics or mechanical scanning devices, the present sheet image scanners are mechanically flexible, lightweight, shock resistant, and potentially inexpensive to manufacture; therefore, they are suitable for human-friendly mobile electronics.

Bending experiment on pentacene field-effect transistors on plastic films

We have fabricated very flexible pentacene field-effect transistors with polyimide gate dielectric layers on plastic films with a mobility of 0.3cm2∕Vs and an on/off ratio of 105, and have measured their electrical properties under various compressive and tensile strains while changing the bending radius of the base plastic films systematically. We have found that the change in source-drain current with bending radius is reproducible and reversible when the bending radius is above 4.6mm, which corresponds to strains of ∼1.4±0.1%. Furthermore, the change in source-drain current does not depend on the direction of strain versus direction of current flow.

High mobility of pentacene field-effect transistors with polyimide gate dielectric layers

Polyimide gate dielectric layers cured at 180 °C have been employed to fabricate high-quality pentacene field-effect transistors on polyethylenenaphthalate-based films. The surface roughness (root-mean square) of gate dielectric layers characterized by atomic force microscopy is only 0.2 nm, while that of the base film is 1 nm. The transistors with pentacene channel layers deposited on 990 nm polyimide gate dielectric layers attain the on/off ratio of 106 and mobility of 0.3 cm2/V s. Furthermore, by decreasing the thickness of polyimide gate dielectric layers down to 540 nm, the mobility is enhanced up to 1 cm2/V s.

Ultraflexible organic field-effect transistors embedded at a neutral strain position

We fabricated ultraflexible pentacene field-effect transistors (FETs) with a mobility of 0.5cm2∕Vs and an on/off ratio of 105, which are functional at the bending radius less than 1mm. The transistors are manufactured on a 13-μm-thick polyimide film and covered by a 13-μm-thick poly-chloro-para-xylylene encapsulation layer so that transistors can be embedded at a neutral position. This sandwiched structure can drastically suppress strain-induced changes in transistor characteristics. Furthermore, the FETs show no significant change after bending cycles of 60 000 times on inward and outward bending stresses.

Control of threshold voltage of organic field-effect transistors with double-gate structures

We fabricated pentacene field-effect transistors with planar-type double-gate structures, where the top- and bottom-gate electrodes can independently apply voltage biases to channel layers. The threshold voltage of organic transistors is changed systematically in a wide range from −16to−43V when the voltage bias of the top-gate electrode is changed from 0to+60V. The mobility in the linear regime is almost constant (0.2cm2∕Vs) at various voltage biases of the top-gate electrode and the on/off ratio is 106.

Suppression of DC bias stress-induced degradation of organic field-effect transistors using postannealing effects

We fabricate pentacene field-effect transistors (FETs) showing a very small degradation in performance under a continuous DC bias stress. Pentacene FETs are manufactured on polyimide films with polyimide gate dielectric layers, and then encapsulated by poly-chloro-para-xylylene passivation layers, resulting in very flexible and heat-resistant devices. When such devices are annealed at 140°C for 12h in a nitrogen environment, the change in their source-drain current is 3±1% even after the application of continuous DC voltage biases of VDS=VGS=−40V for 11h. Furthermore, their mobility is increased by postannealing effects from 0.27cm2∕Vsto0.36cm2∕Vs and their on/off ratio is also increased from 103 to 106.

Pentacene field-effect transistors on plastic films operating at high temperature above 100°C

We have manufactured pentacene field-effect transistors (FETs) on polyimide base films with polyimide gate dielectric layers, and characterized electronic performance and surface morphology with application of heat in the temperature range from 30to210°C. It is found that mobility of pentacene FETs is enhanced from 0.27to0.71cm2∕Vs when measurement temperatures varies from 30to160°C under light-shielding nitrogen environment. To investigate postannealing effects, we have measured transfer curves at 30°C after many heat cycles at various temperatures. Mobility is almost constant even after annealing at 130°C, showing the excellent stability of the present device at high temperatures.

Bending Effect of Organic Field-Effect Transistors with Polyimide Gate Dielectric Layers

We manufactured markedly flexible pentacene field-effect transistors (FETs) on a polyethylenenaphthalate base film with polyimide gate dielectric layers, with a mobility of 0.3 cm2/Vs and an on/off ratio of 105. The electric performance of DC current–voltage characteristics was measured by applying compressive and tensile strains while reducing the bending radius down to 3 mm. It was found that the compressive strain leads to an increase in mobility of 10% induced by the change in strain of up to 1.4±0.1%, although the tensile strain leads to a decrease in mobility of 10%. To elucidate the origin of the enhancement of mobility under the compressive strain, we also investigated the strain dependence of capacitance–voltage characteristics for a pentacene channel layer, and almost no change was observed. Our results suggest that the strains markedly affect the spacing between pentacene molecules rather than the number of induced carriers.

A flexible, lightweight Braille sheet display with plastic actuators driven by an organic field-effect transistor active matrix

A flexible, shock-resistant, and lightweight Braille sheet display has been successfully manufactured on a plastic film by integrating a plastic sheet actuator array with a high-quality organic transistor active matrix. This is the first demonstration, to the best of our knowledge, to integrate plastic MEMS (microelectromechanical systems) actuators with organic transistor active matrices, which opens up new versatile possibilities for flexible, large-area electronic applications including tactile displays

A large-area, flexible, and lightweight sheet image scanner integrated with organic field-effect transistors and organic photodiodes

A large-area, flexible, and lightweight sheet image scanner has been successfully manufactured on a plastic film, for the first time, integrating high-quality organic transistors and organic photodetectors. Since this area-type image-capturing device does not require any optics or any mechanical scanning devices, it is innovatively light to carry, shock-resistant and potentially inexpensive to manufacture.