Tomohito Sekine

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.

Profile Control of Inkjet Printed Silver Electrodes and Their Application to Organic Transistors

We report on the cross-sectional profile control of printed electrodes fabricated from silver nanoparticle inks with water-based solvents by inkjet printing. Systematically varying the ambient conditions and time for the drying process corresponded to changes in electrode shape. In general, lower humidity levels resulted in concave electrode profiles due to the coffee-ring effect, while higher humidity levels resulted in convex profiles. Printed capacitors with trapezoidal-shaped lower electrodes showed much better electrical breakdown strength than those with concave-shaped lower electrodes. Solution-processed organic thin-film transistors with trapezoidal gate electrodes operated reproducibly and exhibited good electrical characteristics with very low gate-leakage currents. The methods can be utilized in the fabrication of printed electronic devices with stacked layers, such as thin-film capacitors and transistors.

Strain sensitivity and durability in p-type and n-type organic thin-film transistors with printed silver electrodes.

Mechanical flexibility and compatibility of printing processes are key advantage that organic electronic devices have over conventional inorganic devices. However, one of the major remaining issues for organic devices is insufficient mechanical durability of printed electrodes. Here we have investigated the mechanical durability of both p-type and n-type organic thin-film transistors (TFTs) with ink-jet printed silver electrodes from silver nanoparticle inks. The modified silver nanoparticle inks enabled the strong adhesion to the underlying polymer layer, and the fabricated organic TFTs exhibited excellent reproducibility in the bending cycle tests. The strong channel length dependence on the strain sensitivity was observed in both p-type and n-type organic TFTs. The organic TFTs with a short-channel exhibited higher sensitivity to the bending strain. These results suggest that the flexible organic TFTs with printed silver electrodes have excellent mechanical durability and are useful for bending and strain sensors.

Free-Standing Organic Transistors and Circuits with Sub-Micron Thicknesses

The realization of wearable electronic devices with extremely thin and flexible form factors has been a major technological challenge. While substrates typically limit the thickness of thin-film electronic devices, they are usually necessary for their fabrication and functionality. Here we report on ultra-thin organic transistors and integrated circuits using device components whose substrates that have been removed. The fabricated organic circuits with total device thicknesses down to 350 nm have electrical performance levels close to those fabricated on conventional flexible substrates. Moreover, they exhibit excellent mechanical robustness, whereby their static and dynamic electrical characteristics do not change even under 50% compressive strain. Tests using systematically applied compressive strains reveal that these free-standing organic transistors possess anisotropic mechanical stability, and a strain model for a multilayer stack can be used to describe the strain in this sort of ultra-thin device. These results show the feasibility of ultimate-thin organic electronic devices using free-standing constructions.

Enhanced adhesion mechanisms between printed nano-silver electrodes and underlying polymer layers

We have characterized mechanisms to improve the adhesion between printed electrodes prepared from silver nanoparticle inks and underlying polymer layers. Adhesion strength was significantly improved by sintering the inks above the glass transition temperature of the polymer underlayers, whereby enhanced adhesion was realized through interfacial fusion between the silver electrode layer and the underlying polymer layer. The surface energy of the underlayer was found to be an important factor in the improvement of adhesive strength, in that larger and thicker interfused regions between layers were observed for underlayers that had higher surface energies.

Highly stable flexible printed organic thin-film transistor devices under high strain conditions using semiconducting polymers

We report on the high mechanical stability of flexible printed organic thin-film transistor (OTFT) devices under high bending stress conditions. The mechanical stability of organic TFT devices using a semiconducting polymer, PBTTT-C16, was comparable to that of OTFT devices using a typical small-molecule semiconductor, pentacene. The drain current of small-molecule OTFT devices decreased by nearly 60% at a 1.5% tensile strain in a direction parallel to the transistor channel, whereas that of polymer-based OTFT devices decreased by only 10% at the same tensile strain. Furthermore, polymer-based OTFT devices exhibited no significant changes in their electrical characteristics after 1000 bending cycles. This stability could be attributed to the high density and good uniformity of the semiconducting polymer layer. Our results indicate that the OTFT devices with polymer-based semiconducting materials have excellent operational stability and reliability under severe bending stress conditions.

The effect of mechanical strain on contact resistance in flexible printed organic thin-film transistors

Flexible organic thin-film transistors (OTFTs) are studied for novel devices such as wearable and low-cost electronics. However, a change of characteristics of their devices under tensile strain was recognized. Here, we investigated the effects that applied strain has on flexible printed OTFT devices by concentrating on the contact between printed source/drain electrodes and semiconducting layers. The semiconducting layer contributed only to increases in the contact resistance R c, which depended on the physical stability of an interface between the printed electrodes and semiconducting layers in the OTFT devices under tensile strain. It was found that the small-molecule semiconductor OTFT devices were more sensitive to strain, exhibiting increases in R c.

Fully printed and flexible ferroelectric capacitors based on a ferroelectric polymer for pressure detection

We report on the fabrication and demonstration of fully printed ferroelectric capacitors using poly(vinylidene fluoridetrifluoroethylene) [P(VDF–TrFE)]. The printed ferroelectric capacitors were primarily fabricated by ink-jet printing on a thin plastic film substrate. The annealing process for the P(VDF–TrFE) layer was optimized from the viewpoints of surface morphology and crystallinity. A good ferroelectric polarization–electric field loop and piezoelectricity in the P(VDF–TrFE) were achieved for the printed ferroelectric capacitors. We have succeeded in the detection of a weak pressure of 150 mbar using the printed ferroelectric capacitor, which is an indication of a potential application to health-care biosensors. These results were realized by the optimization of the annealing temperature for the P(VDF–TrFE) layer.

Ultrathin flexible memory devices based on organic ferroelectric transistors

Here, we demonstrate ultrathin, flexible nonvolatile memory devices with excellent durability under compressive strain. Ferroelectric-gate field-effect transistors (FeFETs) employing organic semiconductor and polymer ferroelectric layers are fabricated on a 1-µm-thick plastic film substrate. The FeFETs are characterized by measuring their transfer characteristics, programming time, and data retention time. The data retention time is almost unchanged even when a 50% compressive strain is applied to the devices. To clarify the origin of the excellent durability of the devices against compressive strain, an intermediate plane is calculated. From the calculation result, the intermediate plane is placed close to the channel region of the FeFETs. The high flexibility of the ferroelectric polymer and ultrathin device structure contributes to achieving a bending radius of 0.8 µm without the degradation of memory characteristics.

Enhanced memory characteristics in organic ferroelectric field-effect transistors through thermal annealing

We report on the memory characteristics of organic ferroelectric field-effect transistors (FeFETs) using spin-coated poly(vinylidene difluoride/trifluoroethylene) (P(VDF/TrFE)) as a gate insulating layer. By thermal annealing the P(VDF/TrFE) layer at temperatures above its melting point, we could significantly improve the on/off current ratio to over 104. Considerable changes in the surface morphology and x-ray diffraction patterns were also observed in the P(VDF/TrFE) layer as a result of the annealing process. The enhanced memory effect is attributed to large polarization effects caused by rearranged ferroelectric polymer chains and improved crystallinity in the organic semiconductor layer of the FeFET devices.