High-mobility Organic Transistors Research Articles

Achieving Bright Organic Light-Emitting Field-Effect Transistors with Sustained Efficiency through Hybrid Contact Design.

Organic light-emitting field-effect transistors (OLEFETs) with bilayer structures have been widely studied due to their potential to integrate high-mobility organic transistors and efficient organic light-emitting diodes. However, these devices face a major challenge of imbalance charge transport, leading to a severe efficiency roll-off at high brightness. Here, we propose a solution to this challenge by introducing a transparent organic/inorganic hybrid contact with specially designed electronic structures. Our design aims to steadily accumulate the electrons injected into the emissive polymer, allowing the light-emitting interface to effectively capture more holes even when the hole current increases. Our numerical simulations show that the capture efficiency of these steady electrons will dominate charge recombination and lead to a sustained external quantum efficiency of 0.23% over 3 orders of magnitude of brightness (4 to 7700 cd/m2) and current density (1.2 to 2700 mA/cm2) from -4 to -100 V. The same enhancement is retained even after increasing the external quantum efficiency (EQE) to 0.51%. The high and tunable brightness with stable efficiency offered by hybrid-contact OLEFETs makes them ideal light-emitting devices for various applications. These devices have the potential to revolutionize the field of organic electronics by overcoming the fundamental challenge of imbalance charge transport.

Understanding the influence of contact resistances on short-channel high-mobility organic transistors in linear and saturation regimes

Solution-processed organic field-effect transistors (OFETs) based on 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT) exhibit a high channel field-effect mobilities (μ FET) of 10 cm2 V−1 s−1, while effective μ FET significantly decreases with reducing channel length. Here, we investigate the influence of contact resistances on the effective μ FET of short-channel C12-BTBT FETs operated in the linear and saturation regimes. The numerical calculations using an equivalent circuit involving source and drain contact resistances reveal a large influence of the effective gate-source voltage on the reduction of saturation μ FET in short-channel OFETs. An anomalous trend in the channel-length dependence of linear and saturation μ FET in C12-BTBT FETs is also discussed.

Spatial extent of wave functions of charge carriers in a thienothiophene-based high-mobility molecular semiconductor

Thienothiophene-based molecules are the typical building blocks of high-mobility organic transistors exhibiting band-like transport behaviors. The spatial extent of the charge carrier wave functions is a fundamental quantity for characterizing such transport properties microscopically. Here, we investigate the ESR of hole carriers generated by the iodine doping of thin films of an alkylated thienothiophene derivative. The observed hyperfine-determined linewidth yields the spatial extent of wave functions over 100 molecules. The spatial extent is nearly an order of magnitude more than that in pentacene, which supports the band-like transport and is consistent with the high crystalline order of the present system.

A Pseudo-Regular Alternating Conjugated Copolymer Using an Asymmetric Monomer: A High-Mobility Organic Transistor in Nonchlorinated Solvents.

Pseudo-regular alternating PDPP-TVS copolymers using an asymmetric monomer (thiophene-vinylene-selenophene (TVS)) are synthesized. Unlike regular alternating copolymers, these polymers are highly soluble in nonchlorinated solvents such as tetra-hydrofuran, toluene, xylene, and tetralin. The organic field-effect transistor devices fabricated using these polymers dissolved in nonchlorinated solvents exhibit a high hole mobility up to 8.2 cm(2) V(-1) s(-1).

Materials and Device Properties of High-mobility Organic Transistors

Materials and Device Properties of High-mobility Organic Transistors

Low-temperature carrier dynamics in high-mobility organic transistors of alkylated dinaphtho-thienothiophene as investigated by electron spin resonance

Charge carriers in high-mobility organic thin-film transistors of alkylated dinaphtho-thienothiophene (C10-DNTT) have been directly observed by field-induced electron spin resonance (FI-ESR) down to 4 K. FI-ESR spectra of π-electron hole carriers of C10-DNTT exhibited clear anisotropy, indicating a highly organized end-on molecular orientation at the device interface. The intra-grain and inter-grain carrier motion were probed by the motional narrowing effect of the ESR spectra. The intra-grain motion was clearly observed even at 4 K, showing intrinsically high mobility of C10-DNTT crystallites. On the other hand, significantly low activation energy of ∼10 meV for inter-grain carrier hopping, compared with pristine DNTT, was observed, which shows that the alkyl substitution drastically enhances the carrier mobility of DNTT system.

Study of contact resistance of high-mobility organic transistors through comparisons

Realization of high-frequency low-cost organic electronics requires high-mobility organic field-effect transistors (OFETs) with short channels, where influence of contact resistance becomes more serious than either lower mobility or longer channel devices. To reduce the contact resistance, we systematically and quantitatively investigate the influence of the lowest unoccupied molecular orbital (LUMO) level of an electron acceptor layer, the active layer thickness, and the side chain of active layer itself on contact resistance of top-contact high-mobility OFETs through a series of comparative analysis. We find that the acceptor of 1,3,4,5,7,8-hexafluoro tetracyano naphtha quinodimethane (F6TNAP) with a deeper LUMO level is efficient for carrier injection and that the bulk resistance plays an important role in such devices. By optimizing the parameters, we get the lowest contact resistance of only 110Ωcm, and thus recorded effective mobility of 8.0cm2/Vs is attained for polycrystalline thin film transistors and still kept as high as 6cm2/Vs at shorter channel lengths.

Observation of field-induced charge carriers in high-mobility organic transistors of a thienothiophene-based small molecule: Electron spin resonance measurements

Electron spin resonance (ESR) measurements are performed on field-induced charge carriers in high-mobility organic transistors of polycrystalline dioctylbenzothieno[2,3-b]benzothiophene (C${}_{8}$-BTBT) films. The angular dependences of the ESR spectra are well fitted by orthorhombic $g$ values with the largest component of ${g}_{Y}$ $=$ 2.0110 nearly parallel to the normal direction of the substrate but tilted by 5\ifmmode^\circ\else\textdegree\fi{}. The results indicate a highly organized end-on alignment of C${}_{8}$-BTBT molecules at the device interface. The ESR linewidth exhibits motional narrowing down to 4 K, providing microscopic evidence for high carrier mobilities within the crystalline domains of C${}_{8}$-BTBT.

The morphology of integrated self-assembled monolayers and their impact on devices – A computational and experimental approach

We report on a combined experimental and computational study on self-assembled monolayers (SAMs). The dielectric properties of SAMs based on n-alkyl phosphonic acids in large area thin film devices (organic field-effect transistors with good hole mobilities up to 0.3 cm 2 V −1 s −1 at −1 V and capacitors) depend on their chain length, but not consistently to theoretical pictures of tunneling through saturated n-alkanes. An unexpected saturation of current density with increasing chain length was obtained in devices, what impact on the leakage current in transistors and current density in capacitors. This is attributed to different self-assembled monolayer morphology ranging from an amorphous state for short alkyl chains to a quasi-crystalline state for longer alkyl chains. Molecular dynamics (MD) simulations provide a deeper insight into the nature of the three-dimensional intermolecular interactions and support the proposed SAM morphology. The change in morphology leads to a reduced effective SAM thickness in devices, which could be described by a Simmons approach quantitatively. The morphological aspect of self-assembled molecules is of enormous importance beyond the applications of SAMs in low-voltage, high mobility organic transistors because of it’s relevance for the hole field of molecular scale electronics. It demonstrates that even small changes in the molecular design can change the molecular interactions and monolayer assembly.

Sub-Å Resolution Electron Density Analysis of the Surface of Organic Rubrene Crystals

The electron density distribution of an organic semiconductor is observed as a function of the depth from the crystal surface or interface. The surface x-ray scattering technique combined with a recently developed analyzing technique, coherent Bragg rod analysis, enables us to observe the electron density profile. The obtained near-surface electron density profile of a single crystal of rubrene, which is known as a high-mobility organic transistor material, shows not only a large positional distribution of the molecules at the surface, but also a sub-A molecular deformation that affects the molecular orbital.

High-mobility double-gate organic single-crystal transistors with organic crystal gate insulators

High-mobility organic transistors are fabricated on both surfaces of approximately 1-μm-thick rubrene crystals, molecularly flat over an area of 10×10μm2. A thin platelet of 9,10-diphenylanthracene single crystal and surface-passivated SiO2 are used for the gate insulators. Because of the minimized densities of hole-trapping levels at the interfaces and in the rubrene crystal, the field-induced carriers do not necessarily reside near the interface but are distributed in the bulk of the semiconductor by adjusting the two gate voltages. Making use of the highly mobile carriers in the inner crystal, the mobility is maximized to ∼43cm2∕Vs.

Very high-mobility organic single-crystal transistors with in-crystal conduction channels

Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers. The interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, so that the inner channel plays a significant part in the transfer performance. With the in-crystal carriers less affected by scattering mechanisms at the interface, the maximum transistor mobility reaches 18cm2∕Vs and the contact-free intrinsic mobility turned out to be 40cm2∕Vs as the result of four-terminal measurement. These are the highest values ever reported for organic transistors.