Yugo Okada

Patternable Solution‐Crystallized Organic Transistors with High Charge Carrier Mobility

Development of high-performance printed semiconductor devices is highly desired with the expectation for the nextgeneration technologies of “printable electronics” providing simply fabricated, fl exible, large-area, low-cost, and environmentally friendly electronic products such as paper-like fl exible displays. Patterned arrays of printed organic fi eld-effect transistors (OFETs) based on chemically stable solutionprocessed organic semiconductors are regarded as key devices that operate as fundamental switching components in, for example, pixel-controlling active-matrix elements. However, performance of conventional solution-coated noncrystal organic thin-fi lm transistors has yet to be improved for practical use in general electronic circuitry. Here, newly developed arrays of patterned crystalline OFETs of air-stable compound 2,9-didecyl-dinaphtho[2,3-b:2’,3’-f ]thieno[3,2-b]thiophene (C 10 -DNTT) formed from hot solution are presented. A method of oriented growth is introduced to provide the singlecrystalline fi lms of C 10 -DNTT that regulates the crystallizing direction and positions in a single process. The benchmark value, 10 cm 2 V − 1 s − 1 , of the charge mobility is achieved for the present OFETs, far exceeding the performance of former devices and opening a practical way to realize printed and fl exible electronics with suffi cient switching speed. The result is attributed to almost perfect molecular periodicity in the crystal fi lms, which allows effective intermolecular charge transport of the electrons.

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.

Monolithic complementary inverters based on organic single crystals.

In the history of the silicon semiconductor technology, the invention of integrated circuits (ICs) has yielded tremendous profi t in the practical economy. The integration technology has been based on fabricating numerous device components on a same monolithic single-crystal platform, allocating sets of p and n -channel metal-oxide-semiconductor fi eld-effect transistors (MOSFETs) for complementary NOT-logic elements, for example. Recently, organic semiconductors are argued as an emerging candidate for post-silicon semiconductor materials because of their unique advantages in low-cost fabrication processes near room temperature, light weight and mechanical fl exibility. So far, however, their compatibility to the monolithic circuitry integration is not yet examined on a single-component platform. In this communication, we present a novel monolithic complementary inverter on a single-crystal organic semiconductor in order to present viability in developing organic integrated circuits. This study is strongly motivated by remarkable progress in material development, device fabrication techniques and fundamental understanding of the charge transport mechanism, resulting in much improved performances of recent organic fi eld-effect transistors (OFETs). The intrinsic carrier transport mechanism is proven to be band transport in high-mobility organic single-crystal transistors, [ 1–3 ] as in silicon single crystals used for the ICs. Already several groups reported that mobility of organic single crystals is as high as 20 cm 2 /Vs or even higher, [ 4–6 ] so that 100 MHz clock frequency can be predicted with micrometer-channel devices assuming a standard charging model. Furthermore, techniques of solution-based crystallization is being developed very recently, [ 7–9 ] so that even mobility as high as 5 cm 2 /Vs is achieved from solution, [ 10 ] suggesting that low-cost production routes on fl exible panels can serve for mass-producing the platforms of low-price and mediumperformance integrated circuits. At present, fairly good inverter performances have been reported only for those with two different materials of p and n -channel organic-semiconductor compounds. [ 11–14 ] In this

Gradual improvements of charge carrier mobility at ionic liquid/rubrene single crystal interfaces

We report evolution of electric characteristics of an electric double layer field-effect transistor based on the ionic liquid/rubrene single crystal interfaces. In contrast to usual devices, the field effect mobility was found to gradually increase with time for a day, followed by minor long-term fluctuations. Although the details of the evolution were somewhat device dependent, the final values of the mobility turned out to be 3–4 times larger irrespective of the initial values. These observations are explained by the evolution of the flat interface by defect-induced spontaneous dissolution of rubrene molecules at the ionic liquid/rubrene single crystal interfaces, revealed by frequency modulation atomic force microscopy.

Carrier dynamics of rubrene single-crystals revealed by transient broadband terahertz spectroscopy

Carrier dynamics of an organic molecular semiconductor, rubrene, was investigated by optical-pump terahertz-probe spectroscopy from 1 to 15 THz. At 294 K, a Drude-like response due to photogenerated hole carriers is observed below 8 THz. The real part σ1(ω) of the optical conductivity is suppressed below 2 THz, indicating the presence of a localization effect. Such a spectral feature was reproduced by a Drude-Anderson model including the effect of dynamical disorder due to intermolecular vibrations. At 50 K, the spectral weight of σ1(ω) due to photocarriers shifts to lower frequency below 4 THz and the suppression of σ1(ω) is hardly observed, which we associate with a reduction of thermal molecular motions. The overall photocarrier generation and recombination dynamics is also discussed.

Correlation between thermal fluctuation effects and phase coherence factor in carrier transport of single-crystal organic semiconductors

We find that the phase coherence factor derived from Hall effect measurements of single-crystal thin-film field-effect transistors of pentacene, which relates the intrinsic charge transport with the phase coherence, has a strong correlation with the thermal fluctuations of transfer energies between neighboring molecules. This observation also holds true for other organic semiconductors such as tetracene, dianthrathiophene (DAT)-V, and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT). This gives us clues for constructing flexible molecular systems with high carrier mobility.

Enhanced carrier mass near the metal insulator transition in La1−xSrxTiO3 and Y1−xCaxTiO3

Abstract Studies of the low-temperature heat capacity in the titanates La 1− x Sr x TiO 3 and Y 1− x Ca x TiO 3 have showed a significant increase of the coefficient of electronic specific heat, γ, due to the strong electronic correlations on approaching the metal-insulator transition boundary. The Mott transition is characterized by the divergent increase of renormalized electron mass with band filling.

Study on the metal-insulator transition of La1−xSrxTiO3 and Y1−xCaxTiO3 by heat capacity measurement

Abstract Carrier-doping (or band-filling) dependence of electronic states is investigated in La1−xSrxTiO3 and Y1−xCaxTiO3 by heat capacity measurement. Coefficient of electronic specific heat, γ, is enhanced significantly near the metal-insulator phase transition boundary, indicating a divergent increase of the effective electron mass due to the strong electronic correlations on approaching the metal-insulator transition boundary.

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.