Rubrene Single-crystal Field-effect Transistors Research Articles

Flexible Organic Single-Crystal Field-Effect Transistor for Ultra-Sensitivity Strain Sensing

To date, the functional application of the flexible organic single-crystal devices in strain sensors has not been studied. In this letter, the excellent flexibility of the beltlike rubrene single crystals enables the rubrene single-crystal field-effect transistors to be bent inward and outward, and their electrical properties under compressive and tensile strain are demonstrated. The current and mobility of the device show the nearly linear changes under the tensile and compressive strain of <0.4%. The dynamic response of the strain presents good repeatability. The calculated sensitivity under tensile strain is two orders of magnitude higher than the previously reported organic thin-film-based strain sensor. It is demonstrated that the rubrene single-crystal device can effectively detect the movement of a human finger. These results exhibit the promising potential of our transistors in artificial intelligence and healthcare systems.

Controlling Chain Conformations of High-k Fluoropolymer Dielectrics to Enhance Charge Mobilities in Rubrene Single-Crystal Field-Effect Transistors.

A novel photopatternable high-k fluoropolymer, poly(vinylidene fluoride-bromotrifluoroethylene) P(VDF-BTFE), with a dielectric constant (k) between 8 and 11 is demonstrated in thin-film transistors. Crosslinking P(VDF-BTFE) reduces energetic disorder at the dielectric-semiconductor interface by controlling the chain conformations of P(VDF-BTFE), thereby leading to approximately a threefold enhancement in the charge mobility of rubrene single-crystal field-effect transistors.

Charge modulation infrared spectroscopy of rubrene single-crystal field-effect transistors

Polarized absorption spectra of hole carriers in rubrene single crystal field-effect transistors were measured in the infrared region (725–8000 cm−1) by charge modulation spectroscopy. The absorptions, including the superimposed oscillatory components due to multiple reflections within thin crystals, monotonically increased with decreasing frequency. The spectra and their polarization dependences were well reproduced by the analysis based on the Drude model, in which the absorptions due to holes in rubrene and electrons in the gate electrodes (silicon), and multiple reflections were fully considered. The results support the band transport of hole carriers in rubrene.

Electronic transport within a quasi-two-dimensional model for rubrene single-crystal field effect transistors

Spectral and transport properties of the quasi two-dimensional adiabatic Su-Schrieffer-Heeger model are studied adjusting the parameters in order to model rubrene single-crystal field effect transistors with small but finite density of injected charge carriers. We show that, with increasing temperature $T$, the chemical potential moves into the tail of the density of states corresponding to localized states, but this is not enough to drive the system into an insulating state. The mobility along different crystallographic directions is calculated including vertex corrections which give rise to a transport lifetime one order of magnitude smaller than spectral lifetime of the states involved in the transport mechanism. With increasing temperature, the transport properties reach the Ioffe-Regel limit which is ascribed to less and less appreciable contribution of itinerant states to the conduction process. The model provides features of the mobility in close agreement with experiments: right order of magnitude, scaling as a power law $T^{-\gamma}$, with $\gamma$ close or larger than two, and correct anisotropy ratio between different in-plane directions. Due to a realistic high dimensional model, the results are not biased by uncontrolled approximations.

Charge transport and Hall effect in rubrene single-crystal transistors under high pressure

Four-terminal conductivity and Hall coefficient are measured in high-mobility rubrene single-crystal field-effect transistors as functions of gate electric field and external hydrostatic pressure up to 900 MPa. Estimating charge density directly from the measured Hall coefficient regardless of contraction in the gate dielectric layer, genuine pressure dependence of the mobility is extracted. It turned out that enhanced intermolecular charge transfer causes an increase in the mobility of the bandlike charge carriers by 20% GPa up to 600 MPa, which is typically one order of magnitude larger than that in inorganic semiconductors. We also found negative pressure coefficient for the mobility above 600 MPa, suggesting that the external hydrostatic force causes additional molecular displacement due to a specific molecular shape.

Control of charge mobility in single-crystal rubrene through surface chemistry

The mobility of rubrene single-crystal field-effect transistors has been measured in the presence of hydrogen and oxygen gas. The mobility of as-grown crystals is ∼10 cm 2/V s but falls to 0.01 cm 2/V s during reduction in a hydrogen environment, and returns to nearly its original value in oxygen. Several cycles of surface reduction and oxidation have been done, which indicates that the surface reactions are reversible and the mobility can be changed to any value between that of the oxidized surface (10 cm 2/V s) and that of the reduced surface (0.01 cm 2/V s). Fourier-transform infrared (FTIR) spectroscopy and photoluminescence (PL) spectroscopy have been applied to examine the oxidized and reduced surface of the rubrene single crystal. FTIR, Raman and PL spectroscopies reveal differences between the rubrene surface and bulk depending on the state of surface oxidation. We conclude that high field-effect mobility in rubrene crystals is a result of surface doping by an oxidized layer on the crystal surface and not due to the intrinsic molecular and crystal structure of rubrene.

High-performance organic field-effect transistors with binary ionic liquids

High-performance rubrene single-crystal field-effect transistors are developed with binary ionic liquid electrolytes used for gating. Inclusion of small amount of inorganic salts in the ionic liquids enhances the degree of dissociation for the organic ions and accelerates formation of the electric-double-layers in response to the gate voltage. High carrier mobility of 2.9 cm 2/Vs is achieved in the rubrene single-crystal transistors with the mixture ionic liquid. In addition to the advantage of the low-voltage operation due to concentrated field in ultra-thin electric-double-layers, drastically increased capacitance at above 100 Hz makes the technique of the ionic liquid gating more attractive for fast-switching devices.

Threshold Voltage and Space Charge in Organic Transistors

We investigate rubrene single-crystal field-effect transistors, whose stability and reproducibility are sufficient to measure systematically the shift in threshold voltage as a function of channel length and source-drain voltage. The shift is due to space charge transferred from the contacts and can be modeled quantitatively without free fitting parameters, using Poisson's equation, and by assuming that the density of states in rubrene is that of a conventional inorganic semiconductor. Our results demonstrate the consistency, at the quantitative level, of a variety of recent experiments on rubrene crystals and show how the use of field-effect transistor measurements can enable the determination of microscopic parameters (e.g., the effective mass of charge carriers).

A comparative study of organic single-crystal transistors gated with various ionic-liquid electrolytes

We report on a comparative study of rubrene single-crystal field-effect transistors with various ionic-liquid electrolytes used for gate insulators. A systematic correlation is found that mobility of the field-effect transistors increases with decreasing electrostatic capacitance of the electric double layers, as the result of highly reproducible comparisons among tens of samples with the variation of anions in the purified ionic liquids. By optimizing the gating ionic liquid, the highest mobility of the electrolyte-gated organic transistors elevated up to 9.5 cm2/V s, which is only a fraction of the value of intrinsic material property, demonstrating an excellent field-effect switching operation.

Modification of Semiconductor-Dielectric Interface in Organic Light-emitting Field-effect Transistors

AbstractIn this work, ambipolar rubrene single crystal field-effect transistors (FETs) with PMMA modification layer and Au/Ca as electrodes were fabricated. The electron mobility was studied in these devices. PMMA modification layer on the surface of SiO2 is necessary for electron behavior. We found that the device with PMMA modified insulator and Au-Ca asymmetric metals possessed hole mobility and electron mobility of 1.27 and 0.017 cm−2/Vs, respectively. Furthermore, the shift of light emitting with applied gate voltage was observed in this device.

Field-Induced ESR Spectroscopy on Rubrene Single-Crystal Field-Effect Transistors

AbstractWe investigate the electron spin resonance (ESR) spectroscopy for the field-induced carriers in rubrene single-crystal field-effect transistors (SC-FETs), and compare the results with those on pentacene thin-film transistors (TFTs). We observe Lorentz-type ESR signal in rubrene SC-FETs whose linewidth is narrowed with increasing gate voltage and temperature. It demonstrates that the ESR linewidth is determined by motional narrowing effect as we reported on pentacene TFTs. Based on the observations, we discuss the multiple trap-and-release (MTR) processes in the two systems with and without grain boundaries.

High-performance Organic Field-effect Transistors with Ionic Liquids

We report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 1.0 µF/cm2 even at 0.1 MHz. With high carrier mobility of 9.5 cm2/Vs in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2 V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the ionic-liquid/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.

Study of Solid/liquid Interfaces in Organic Field-effect Transistors with Ionic Liquids

AbstractWe report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 1.0 μF/cm2 even at 0.1 MHz. With high carrier mobility of 9.5 cm2/Vs in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2 V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the ionic-liquid/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.

Fabrication of high-mobility organic single-crystal field-effect transistors with amorphous fluoropolymer gate insulators

High-mobility rubrene single-crystal field-effect transistors are built on highly water- and oil-repellent fluoropolymer gate insulators. Roughness is introduced at the surface once to provide good adhesion to metal films and photoresist polymers for stable electrodes. Before constructing interfaces to crystals, smoothness of the fluoropolymer surface is recovered by annealing at a moderate temperature to maximize carrier mobility. Mobility values estimated in the saturation region reproducibly exceeded 15 cm 2/V s for all the 10 devices, reaching 30 cm 2/V s for the best two devices. The results demonstrate that the water-repellency and smoothness of the dielectric polymers are favorable for the excellent transistor performance.

High-mobility, low-power, and fast-switching organic field-effect transistors with ionic liquids

We report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid (IL) electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 1μF∕cm2 even at 0.1MHz. With high carrier mobility of 1.2cm2∕Vs in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the IL/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.

Current saturation and Coulomb interactions in organic single-crystal transistors

Electronic transport through rubrene single-crystal field-effect transistors (FETs) is investigated experimentally in the high carrier density regime (n≃0.1 carrier molecule−1). In this regime, we find that the current does not increase linearly with the density of charge carriers, and tends to saturate. At the same time, the activation energy for transport unexpectedly increases with increasing n. We perform a theoretical analysis in terms of a well-defined microscopic model for interacting Fröhlich polarons, which quantitatively accounts for our experimental observations. This work is particularly significant for our understanding of electronic transport through organic FETs.

High-mobility Organic Single-crystal Transistors with Amorphous Fluoropolymer Gate Insulators

AbstractHigh-mobility rubrene single-crystal field-effect transistors are built on highly water- and oil-repellent fluoropolymer gate insulators. Roughness is intentionally introduced at the surface once to provide good adhesion to metal films and photoresist polymers for stable bottom electrodes. Before constructing interfaces with rubrene crystals, smoothness of the fluoropolymer surface is recovered by annealing at a moderate temperature to maximize mobility of the carriers induced near the interfaces. The estimated mobilities in the saturation region reproducibly exceeded 15 cm2/Vs for all the ten devices fabricated in this method and reach 30 cm2/Vs for the best two samples among them. The results demonstrate that the water-repellency and smoothness of the dielectric polymers are favorable in the excellent transistor performance.

Low-power and Fast-switching Organic Field-effect Transistors with Ionic Liquids

ABSTRACTWe report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 10 μF/cm2 even at 0.1 MHz. With high carrier mobility of 1.2 cm2/Vs in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2 V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the ionic-liquid/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.

Bias-dependent contact resistance in rubrene single-crystal field-effect transistors

The authors report a systematic study of the bias-dependent contact resistance in rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt electrodes. They show that the reproducibility in the values of contact resistance strongly depends on the metal, ranging from a factor of 2 for Ni to more than three orders of magnitude for Au. Surprisingly, field-effect transistors with Ni, Co, and Cu contacts exhibit an unexpected reproducibility of the bias-dependent differential conductance of the contacts once this has been normalized to the value measured at zero bias. This reproducibility may enable the study of microscopic carrier injection processes into organic semiconductors.

Rubrene single crystal field-effect transistor with epitaxial BaTiO3 high-k gate insulator

High quality BaTiO3 thin-film epitaxially grown on a Nb-doped SrTiO3 (BTO/Nb-STO) substrate by a laser ablation technique is employed as a high-k gate insulator for a field-effect transistor of a rubrene single crystal in order to search for the possibility of high carrier accumulation. The high dielectric constant ϵ of 280esu for the prepared BaTiO3 thin-film accumulates 0.1holes∕rubrene-molecule, which is 2.5 times as high as the maximum carrier number of 0.04holes∕rubrene-molecule attained in the case of SiO2. This is the highest carrier number so far obtained in organic field-effect transistors (FETs). Other important parameters of rubrene single crystal FETs on BTO/Nb-STO are described in comparison with those on SiO2/doped-Si.