Organic Single-crystal Transistors Research Articles

Controlled Deposition of a High‐Performance Small‐Molecule Organic Single‐Crystal Transistor Array by Direct Ink‐Jet Printing

Ink-jet printed small-molecule organic single-crystal transistors are realized by using selective surface energy modification, precise control of volume density of ink droplets on spatially patterned areas, and a co-solvent system to control solvent evaporation properties. The single-crystal formation in bottom-contact-structured transistors via direct printing is expected to permit high-density array fabrication in large-area electronics.

Mica, a Potential Two‐Dimensional‐Crystal Gate Insulator for Organic Field‐Effect Transistors

2D mica crystals (with thickness < 100 nm) obtained by mechanical exfoliation are incorporated for the first time into the design of organic thin film field-effect transistor arrays and organic single crystal transistors as a gate insulator. The size of mica crystals could be up to the dimensions of an A4 piece of paper. All devices so fabricated exhibited high mobility and low operating voltage, indicating the high quality of the crystals and the great potential of mica crystals as a flexible, low-cost, transparent insulator for organic electronics.

Patterning solution-processed organic single-crystal transistors with high device performance

We report on the patterning of organic single-crystal transistors with high device performance fabricated via a solution process under ambient conditions. The semiconductor was patterned on substrates via surface selective deposition. Subsequently, solvent-vapor annealing was performed to reorganize the semiconductor into single crystals. The transistors exhibited field-effect mobility (μFET) of up to 3.5 cm2/V s. Good reliability under bias-stress conditions indicates low density of intrinsic defects in crystals and low density of traps at the active interfaces. Furthermore, the Y function method clearly suggests that the variation of μFET of organic crystal transistors was caused by contact resistance. Further improvement of the device with higher μFET with smaller variation can be expected when lower and more uniform contact resistance is achieved.

Green light emission from the edges of organic single-crystal transistors

We have fabricated ambipolar light-emitting field-effect transistors made of 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5) single crystals, which have 35% fluorescent quantum efficiency. The obtained hole and electron mobilities were 2.9×10−1 cm2/V s and 6.7×10−3 cm2/V s, respectively. These are the highest values among AC5 transistors. Importantly, although the light emission from the crystal surface was less than the detection level of the camera, we observed bright and polarized light emission from the edge of the single crystals. This polarized edge emission is attributed to the strong self-assembled light-confining nature and perfectly aligned transition dipole moments, which are advantageous for future laser devices.

High current densities in a highly photoluminescent organic single-crystal light-emitting transistor

We report the improvement of electron transport in 5,5″-bis(biphenylyl)-2,2′:5′,2″-terthiophene (BP3T) single crystals on a field-effect transistor configuration by systematically investigating the effects of device aging under pure nitrogen and optimizing the organic dielectric layer-fabrication process. We reduced the effect of electron traps and achieved extremely high current densities up to 10 kA/cm2, which is one or two orders of magnitude greater than the current densities achieved in previous devices.

Self Contact Organic Transistors

Thin films of various organic semiconductors, such as pentacene, sexithiophene, copper phthalocyanine, and C60, as well as an organic charge-transfer salt (TTF)(TCNQ) [TTF: tetrathiafulvalene; TCNQ: tetracyanoquinodimethane] are laser-irradiated to form conductive films, which are identified by Raman spectroscopy and atomic force microscopy to be carbon. The resulting practically transparent films are as conductive as laser-sintered carbon films and show temperature-independent conductivity. Source and drain electrodes of organic field-effect transistors are patterned by this method; in these “self-contact” transistors, both the active layers and the electrodes are derived from the same organic film. The laser-sintered carbon films are also utilized for organic single-crystal transistors based on rubrene and TCNQ.

Probing stress effects in single crystal organic transistors by scanning Kelvin probe microscopy

We report scanning Kelvin probe microscopy (SKPM) of single crystal difluoro bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) organic transistors. SKPM provides a direct measurement of the intrinsic charge transport in the crystals independent of contact effects and reveals that degradation of device performance occurs over a time period of minutes as the diF-TESADT crystal becomes charged.

Controlled tunnel-coupled ferromagnetic electrodes for spin injection in organic single-crystal transistors

We report on single-crystal rubrene field-effect transistors (FETs) with ferromagnetic Co electrodes, tunnel-coupled to the conduction channel via an Al 2 O 3 tunnel barrier. Magnetic and electronic characterization shows that the Al 2 O 3 film not only protects the Co from undesired oxidation, but also provides a highly controlled tunnel barrier for overcoming the conductivity mismatch problem when injecting spins from a ferromagnetic metal into a semiconductor. Our FETs provide a significant step towards the realization of a device that integrates FET and spin-valve functionality, one of the major goals of spintronics.

High mobility organic single crystal transistors based on soluble triisopropylsilylethynyl anthracene derivatives

A series of new anthracene based semiconductors are designed and synthesized. By substituting appropriate acenes at the 2,6-positions of triisopropylsilylethynyl anthracene (NMR, IR, DSC and TGA spectra and crystallographic information of TIPSAntBT and TIPSAntNa), two different derivatives were prepared. Especially, TIPSAntNa (naphthalene as a side group) showed superior performance when it was used as channel material. A hole mobility as high as 3.7 cm2 V−1 s−1 was obtained from single crystal OFETs. To elucidate the origin of this high performance, we carried out comparative studies to investigate the direct relationship between the molecular-packing parameters and the field-effect mobility in single-crystal OFETs because the performance of such single-crystal OFETs is not affected by defects and grain boundaries. Comparing TIPSAN single crystal OFETs having four different acene derivatives, and applying the concept of molecular overlap ratio along the long/short axis, we could show that the effective π-stacking area dominantly determines the field-effect mobility of π-stacked materials. In the case of TIPSAntNa, a large π-stacking area and a small π-stacking distance enabled the highest field-effect mobility.

Assembly of Nanoscale Organic Single‐Crystal Cross‐Wire Circuits

Organic single-crystal transistors and circuits can be assembled by nanomechanical manipulation of nanowires of CuPc, F16CuPc, and SnO2:Sb. The crossed bar devices have low operational voltage, high mobility and are stable in air. They can be combined into circuits, providing varied functions including inverters and NOR and NAND logic gates, opening new opportunities for organic nanoelectronics and highly sophisticated integrated logic devices.

Tuning of threshold voltage by interfacial carrier doping in organic single crystal ambipolar light-emitting transistors and their bright electroluminescence

Organic light-emitting field-effect transistors, based on a p-bis[(p-styryl)styryl] benzene (P5V4) single crystal, that possess high mobilities of over 0.1 cm2/V s for both electrons and holes were fabricated. For a small charge injection barrier and successive formation of high exciton density in the carrier recombination zone, the hole accumulation threshold voltage was significantly reduced by interfacial hole doping based on electron transfer from P5V4 molecules to a molybdenum oxide layer. The threshold voltage for hole accumulation was drastically decreased from −80±3 to 2±3 V, leading to dual charge injection and accumulation of a very high current density of J&amp;gt;100 A cm−2 with intense edge electroluminescence.

Comparison of the Mobility–Carrier Density Relation in Polymer and Single‐Crystal Organic Transistors Employing Vacuum and Liquid Gate Dielectrics

Comparison of the Mobility–Carrier Density Relation in Polymer and Single‐Crystal Organic Transistors Employing Vacuum and Liquid Gate Dielectrics

Subthreshold regime in rubrene single-crystal organic transistors

Organic field-effect transistors were fabricated with vapor-grown rubrene single crystals in a staggered top-contact configuration. The devices were electrically characterized by measuring the transfer curves at low drain voltage. In parallel to these measurements, a model is developed to account for the subthreshold regime of the transistors. The model is based on the multiple trapping and thermal release concept, which assumes that charge transport is limited by a single level of shallow traps located close to the transport band edge. It is shown that the threshold voltage no longer establishes at the transition between the depletion and accumulation regimes. Instead, the threshold corresponds to the point at which traps are filled. This results in a subthreshold current that varies linearly with gate voltage. Moreover, the subthreshold current at low drain voltages increases with drain voltage. These finding are in good agreement with the experimental data.

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.

Electronic functionalization of solid-to-liquid interfaces between organic semiconductors and ionic liquids: Realization of very high performance organic single-crystal transistors

High-performance electronic function of current amplification is realized with the use of solid-to-liquid interfaces between organic semiconductors and ionic liquid. To hold in place the ionic liquid of 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide known for low viscosity and high ionic conductivity, an elastomeric well structure is fabricated with polydimethylsiloxane on which organic single crystals of rubrene are electrostatically attached. As the result of rapid formation of electric double layers in the ionic liquid interfacing, the high-mobility organic semiconductor crystals’ fast-switching transistor function is demonstrated with the application of gate voltage, realizing the highest sheet transconductance, namely, amplifying performance, ever achieved.

Highly Efficient Patterning of Organic Single‐Crystal Transistors from the Solution Phase

Highly Efficient Patterning of Organic Single‐Crystal Transistors from the Solution Phase

Organic single-crystal transistors with secondary gates on source and drain electrodes

Rubrene and tetracyanoquinodimethane single-crystal transistors are fabricated incorporating secondary gates (split gates) on source and drain electrodes to reduce the interfacial barriers at the metal/semiconductor contacts. Separating the effect of the injection barriers, the intrinsic carrier transport in the semiconductor channels is extracted for the p-type rubrene crystal transistors and the n-type tetracyanoquinodimethane crystal transistors. The transconductance of the tetracyanoquinodimethane devices is drastically improved by activating the split-gate electrodes, indicating significant injection barriers in the n-type transistors. The result demonstrates that the technique is useful to improve transistor performance when it is restricted by the injection barriers.

High-mobility organic single crystal transistors with submicrometer channels

We demonstrate high-performance electric-field effects in submicrometer-channel organic transistors with rubrene single crystals. Platinum source and drain electrodes are embedded in silicon dioxide gate insulators to reduce thickness of the dielectrics and to minimize the short-channel effect. The miniaturized devices exhibit typical output characteristics with Ohmic linear region, well-defined current saturation, and on-off ratio of 106. Mobility values are in the range of 0.1–0.3cm2∕Vs, which is comparable to those of the best submicrometer organic transistors. Anisotropy in the mobility is detected, indicating that bandlike transport is responsible for the high transistor performance of the short-channel devices.

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.

Retraction: Organic Single-Crystal Transistors with Secondary Gates on Source and Drain Electrodes

Retraction: Organic Single-Crystal Transistors with Secondary Gates on Source and Drain Electrodes