Junto Tsurumi

High‐Performance Solution‐Processable N‐Shaped Organic Semiconducting Materials with Stabilized Crystal Phase

N-shaped organic semiconductors are synthesized via four steps from a readily available starting material. Such semiconductors exhibit preferable ionization potential for p-type operation, thermally stable crystalline phase over 200 °C, and high carrier mobility up to 16 cm(2) V(-1) s(-1) (12.1 cm(2) V(-1) s(-1) on average) with small threshold voltages in solution-crystallized field-effect transistors.

Polarization fatigue of organic ferroelectric capacitors

The polarization of the ferroelectric polymer P(VDF-TrFE) decreases upon prolonged cycling. Understanding of this fatigue behavior is of great technological importance for the implementation of P(VDF-TrFE) in random-access memories. However, the origin of fatigue is still ambiguous. Here we investigate fatigue in thin-film capacitors by systematically varying the frequency and amplitude of the driving waveform. We show that the fatigue is due to delamination of the top electrode. The origin is accumulation of gases, expelled from the capacitor, under the impermeable top electrode. The gases are formed by electron-induced phase decomposition of P(VDF-TrFE), similar as reported for inorganic ferroelectric materials. When the gas barrier is removed and the waveform is adapted, a fatigue-free ferroelectric capacitor based on P(VDF-TrFE) is realized. The capacitor can be cycled for more than 108 times, approaching the programming cycle endurance of its inorganic ferroelectric counterparts.

Transition Between Band and Hopping Transport in Polymer Field‐Effect Transistors

Hall effect and slightly negative temperature dependence of the mobility in polymeric transistors are demonstrated. The semiconductor channel is based on a polycyclopentadithiophene-benzothiadiazole (CDT-BTZ) donor-acceptor copolymer film whose chain direction is oriented by mechanical compression at the surface of an ionic liquid. The mobility is 5.6 cm(2) V(-1) s(-1) at room temperature, and is further improved to 6.7 cm(2) V(-1) s(-1) at 260 K.

Suppressing molecular vibrations in organic semiconductors by inducing strain

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm2 V−1 s−1 by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices.

Wafer-scale, layer-controlled organic single crystals for high-speed circuit operation

A wafer-scale, 2D organic single-crystalline semiconductor revolutionizes near-field communication. Two-dimensional (2D) layered semiconductors are a novel class of functional materials that are an ideal platform for electronic applications, where the whole electronic states are directly modified by external stimuli adjacent to their electronic channels. Scale-up of the areal coverage while maintaining homogeneous single crystals has been the relevant challenge. We demonstrate that wafer-size single crystals composed of an organic semiconductor bimolecular layer with an excellent mobility of 10 cm2 V−1 s−1 can be successfully formed via a simple one-shot solution process. The well-controlled process to achieve organic single crystals composed of minimum molecular units realizes unprecedented low contact resistance and results in high-speed transistor operation of 20 MHz, which is twice as high as the common frequency used in near-field wireless communication. The capability of the solution process for scale-up coverage of high-mobility organic semiconductors opens up the way for novel 2D nanomaterials to realize products with large-scale integrated circuits on film-based devices.

Boron-Stabilized Planar Neutral π-Radicals with Well-Balanced Ambipolar Charge-Transport Properties

Organic neutral π-monoradicals are promising semiconductors with balanced ambipolar carrier-transport abilities, which arise from virtually identical spatial distribution of their singly occupied and unoccupied molecular orbitals, SOMO(α) and SOMO(β), respectively. Herein, we disclose a boron-stabilized triphenylmethyl radical that shows outstanding thermal stability and resistance toward atmospheric conditions due to the substantial spin delocalization. The radical is used to fabricate organic Mott-insulator transistors that operate at room temperature, wherein the radical exhibits well-balanced ambipolar carrier transport properties.

Chemical potential shift in organic field-effect transistors identified by soft X-ray operando nano-spectroscopy

A chemical potential shift in an organic field effect transistor (OFET) during operation has been revealed by soft X-ray operando nano-spectroscopy analysis performed using a three-dimensional nanoscale electron-spectroscopy chemical analysis system. OFETs were fabricated using ultrathin (3 ML or 12 nm) single-crystalline C10-DNBDT-NW films on SiO2 (200 nm)/Si substrates with a backgate electrode and top source/drain Au electrodes, and C 1s line profiles under biasing at the backgate and drain electrodes were measured. When applying −30 V to the backgate, there is C 1s core level shift of 0.1 eV; this shift can be attributed to a chemical potential shift corresponding to band bending by the field effect, resulting in p-type doping.

Molecular doping in organic semiconductors: fully solution-processed, vacuum-free doping with metal–organic complexes in an orthogonal solvent

Chemical doping in π-conjugated organic semiconductors, which involves a redox reaction between a host π-conjugated material and a dopant, is achieved by either co-evaporation, co-dissolved solution, or exposure to a dopant gas. Here, we demonstrate a new route for molecular doping; a thiophene-based semiconducting polymer film can be doped with dopants dispersed in an orthogonal solvent. An increase in conductivity is demonstrated as a result of adopting a strong acceptor dopant, a metal–organic complex, to achieve an efficient charge transfer because the introduced dopant is likely to reside within the polymer lamellae throughout the entire bulk of the organic semiconductor film. Comprehensive magnetotransport and spectroscopic studies confirm that band-like transport is realized in such dopant-implanted conducting polymers. The present method can shed light on molecular doping in materials science because any molecular dopants that are non-evaporative and insoluble can be used with this method.

Heteroacene-based organic single crystal transistors under high pressure

Carrier transport properties of heteroacene-based organic field effect transistors are investigated under the application of hydrostatic pressure. In contrast to monotonic and moderate increase in carrier mobility for inorganic semiconductors, present organic devices exhibit anomalous and giant pressure dependent mobility. These performances are revealed by the combination with x-ray structural analysis; it is suggested that electronic properties of hetero elements and molecular rearrangement in accordance with pressurization play key roles for the realization of such pressure responses.