Yudai Yoshimura

Fully-printed high-performance organic thin-film transistors and circuitry on one-micron-thick polymer films

Thin, ultra-flexible devices that can be manufactured in a process that covers a large area will be essential to realizing low-cost, wearable electronic applications including foldable displays and medical sensors. The printing technology will be instrumental in fabricating these novel electronic devices and circuits; however, attaining fully printed devices on ultra-flexible films in large areas has typically been a challenge. Here we report on fully printed organic thin-film transistor devices and circuits fabricated on 1-μm-thick parylene-C films with high field-effect mobility (1.0 cm(2) V(-1) s(-1)) and fast operating speeds (about 1 ms) at low operating voltages. The devices were extremely light (2 g m(-2)) and exhibited excellent mechanical stability. The devices remained operational even under 50% compressive strain without significant changes in their performance. These results represent significant progress in the fabrication of fully printed organic thin-film transistor devices and circuits for use in unobtrusive electronic applications such as wearable sensors.

Flip-flop logic circuit based on fully solution-processed organic thin film transistor devices with reduced variations in electrical performance

Organic reset–set (RS) flip-flop logic circuits based on pseudo-CMOS inverters have been fabricated using full solution processing at a relatively low process temperatures of 150 °C or less. The work function for printed silver electrodes was increased from 4.7 to 5.4 eV through surface modification with a self-assembled monolayer (SAM) material. A bottom-gate, bottom-contact organic thin-film transistor (OTFT) device using a solution-processable small-molecular semiconductor material exhibited field-effect mobility of 0.40 cm2 V−1 s−1 in the saturation region and a threshold voltage (VTH) of −2.4 V in ambient air operation conditions. In order to reduce the variations in mobility and VTH, we designed a circuit with six transistors arranged in parallel, in order to average out their electrical characteristics. As a result, we have succeeded in reducing these variations without changing the absolute values of the mobility and VTH. The fabricated RS flip-flop circuits were functioned well and exhibited short delay times of 3.5 ms at a supply voltage of 20 V.

Control of threshold voltage in organic thin-film transistors by modifying gate electrode surface with MoOX aqueous solution and inverter circuit applications

We controlled the threshold voltage of organic thin-film transistors (TFTs) by treating only the gate electrode surface with a MoOX aqueous solution and used them to build inverter circuits. The threshold voltage was changed by varying the concentration of the MoOX aqueous solution. A strong correlation between the work function of the gate electrode and the threshold voltage was observed. The threshold voltage of one of the two organic TFT devices in the inverter circuit was selectively changed by +2.3 V by reducing the concentration of the MoOx solution. We controlled the switching voltage of p-type organic inverter circuits and obtained excellent inverter characteristics. These results indicate that using a MoOx aqueous solution to control the threshold voltage is very useful for integrated circuits applications.

Fluorosurfactant-assisted photolithography for patterning of perfluoropolymers and solution-processed organic semiconductors for printed displays

A photolithographic patterning technique of perfluoropolymers was developed for the micropatterning of solution-processed organic semiconductors. We found that a small amount of fluorosurfactant additives in photoresist solutions enabled preparation of the spin-coated films even on the dewetting surfaces of perfluoropolymers. Subsequent photolithography and dry/wet etching achieved excellent patterning of perfluoropolymers, which provided a high contrast of wettability in microscale on the substrate. The dewetting patterns were applied for isolating solution-processed organic thin-film transistor arrays for a 100 ppi active-matrix display by selective dewetting. The isolation of the semiconducting layers achieved significant improvement in the current on/off ratio with a high mobility of >1 cm2 V−1 s−1.

Flip-Flop Circuits using Fully Solution Processed Pseudo-CMOS Circuits

Organic flip-flop (FF) circuits based on pseudo-CMOS inverters have been fabricated by fully solution-processes. Bottom-gate, bottom-contact organic thin-film transistors using a solution-processable small molecular semiconductor exhibited an average mobility 0.35 cm 2 /Vs. The fabricated FF circuits were successfully operated in ambient air condition and exhibited a short delay time (3.5 ms) at a supply voltage of ±20 V.