Radical polymers improve the metal-semiconductor interface in organic field-effect transistors

Modifying the organic-metal interface in organic field-effect transistors (OFETs) is a critical means by which to improve device performance; however, to date, all of the interfacial modifying layers utilized in these systems have been closed-shell in nature. Here, we introduce open-shell oxidation-reduction-active (redox-active) macromolecules, namely radical polymers, in order to serve as interfacial modifiers in pentacene-based OFETs. Through careful selection of the chemistry of the specific radical polymer, poly(2,2,6,6-tetramethylpiperidine-1-oxyl methacrylate) (PTMA), the charge transport energy level of the interfacial modifying layer was tuned to provide facile charge injection and extraction between the pentacene active layer and the gold source and drain electrodes of the OFET. The inclusion of this radical polymer interlayer, which was deposited in through straightforward inkjet printing, led to bottom-contact, bottom-gate OFETs with significantly increased mobility and ON/OFF current ratios relative to OFETs without the PTMA interlayer. The underlying mechanism for this improvement in device performance is explained in terms of the charge transport capability at the organic-metal interface and with respect to the pentacene grain growth on the radical polymer. Thus, this effort presents a new, open-shell-based class of materials for interfacial modifying materials, and describes the underlying physics behind the practical operation of these materials.     

Direct investigations of the interface impedance of organic field-effect transistors with self-assembled-monolayer-modified electrodes

It has been demonstrated that the modification of electrodes with self-assembled monolayers (SAMs) reduces the contact resistance and improves the device performances of organic field-effect transistors (OFETs). However, it has been difficult to judge if the contact resistance was reduced by the change in the electronic properties or by the change in the morphology of the metal–organic interface caused by the SAM modification because they have been difficult to be separately assessed. We have directly investigated the local impedance and the potential difference at the electrode–channel interfaces of the OFETs with and without modification of the electrodes by a pentafluorobenzenethiol SAM using frequency-modulation scanning impedance microscopy (FM-SIM). The potential profile measurement and the FM-SIM measurement at the interface showed that the improvement of the field-effect mobility in the SAM-modified OFET was caused by the reduction of the energy level mismatch, namely, the hole injection barrier at the source–channel interface, presumably with the reduction of the hole trap sites at the source–channel interface.     

Non-conventional metallic electrodes for organic field-effect transistors

We study micrometer-sized organic field-effect transistors with either Pd or NiFe metallic electrodes. Neither of these materials is commonly used in organic electronics applications, but they could prove to be particularly advantageous in certain niche applications such as organic spintronics. Using organic semiconductors with different carrier transport characteristics as active layer, namely n-type C60 fullerene and p-type Pentacene, we prove that Pd (NiFe) is a very suitable electrode for p- (n-) type semiconductors. In particular, we characterized devices with channel lengths in the order of the micrometer, a distance which has allowed us to evaluate the electronic behavior in a regime where the interfacial problems become predominant and it is possible to reach elevated longitudinal electric fields. Our experimental results agree well with a simple model based on rigid energy levels.     

Theory of negative resistance of junction field-effect transistors

A general theory of the voltage-controlled negative resistance in a field-effect transistor is presented. The prerequisite to the negative resistance is mathematically determined as a relation among the source-gate voltage, the source-drain voltage, and the pinchoff voltage. It is suggested that there will be a variety of circuit configurations with the positive feedback function applied to the gate of the field-effect transistor. Three basic examples of the practical application of the theory are also given.     

Phospholipid film in electrolyte-gated organic field-effect transistors

A totally innovative electrolyte-gated field effect transistor, embedding a phospholipid film at the interface between the organic semiconductor and the gating solution, is described. The electronic properties of OFETs including a phospholipid film are studied in both pure water and in an electrolyte solution and compared to those of an OFET with the organic semiconductor directly in contact with the gating solution. In addition, to investigate the role of the lipid layers in the charge polarization process and quantify the field-effect mobility, impedance spectroscopy was employed. The results indicate that the integration of the biological film minimizes the penetration of ions into the organic semiconductor thus leading to a capacitive operational mode as opposed to an electrochemical one. The OFETs operate at low voltages with a field-effect mobility in the 10⁻³ cm² V⁻¹ s⁻¹ range and an on/off current ratio of 10³. This achievement opens perspectives to the development of FET biosensors potentially capable to operate in direct contact with physiological fluids.     

Tunable contact resistance in double-gate organic field-effect transistors

A study of the contact resistance (R_sd) in pentacene-based double-gate transistors is presented. In top-contact transistors, as the negative bias of the additional top-gate bias is increased, R_sd decreases by over five orders of magnitude for small bottom-gate voltages. In bottom-contact transistors, R_sd is reduced by about ten times for all bias values, implying improved charge transport in all operating regimes. The different tunability of R_sd in top/bottom-contact transistors is attributed to different charge injection modulation by the coplanar/staggered top gate. Therefore, double-gate architecture offers a novel and effective approach to limit R_sd and its relevant impacts on organic transistor.     

有机场效应晶体管有源层的自保护方法

To isolate the active layer from air, double organic layer organic field-effect transistors have been fabricated, based on a two-step vacuum-deposition process. Pentacene acted as the active layer, and subsequently, CuPc was deposited above the pentacene and served as a protecting layer for the active layer. Due to the same electrical characteristics but different morphologies, the bilayer structure was effective in decreasing the contamination of impurities and gas, and then improved the device stability in air.     

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 cm²/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.     

Active layer self-protection process for organic field-effect transistors

y in air.     

Gate-bias assisted charge injection in organic field-effect transistors

The charge injection barriers in organic field-effect transistors (OFETs) seem to be far less critical as compared to organic light-emitting diodes (OLEDs). Counter intuitively, we show that the origin is image-force lowering of the barrier due to the gate bias at the source contact, although the corresponding gate field is perpendicular to the channel current. In coplanar OFETs, injection barriers up to 1 eV can be surmounted by increasing the gate bias, enabling extraction of bulk transport parameters in this regime. For staggered transistors, however, the injection is gate-assisted only until the gate bias is screened by the accumulation channel opposite to the source contact. The gate-assisted injection is supported by two-dimensional numerical charge transport simulations that reproduce the gate-bias dependence of the contact resistance and the typical S-shaped output curves as observed for OFETs with high injection barriers.     

A lithographic process for integrated organic field-effect transistors

This paper reports a photolithographic process for fabricating organic field-effect transistors which provides two layers of metal with arbitrary via placement, and optionally allows for subtractive lithographic patterning of the transistor active layer. The demonstrated pentacene transistors have a field-effect mobility of 0.1/spl plusmn/0.05 cm/sup 2//(V/spl middot/s). Parylene-C is used both as the gate dielectric and an encapsulation layer which allows for subtractive lithographic patterning. Also demonstrated is a PMOS inverter without level shifting circuitry and level-restoring V/sub High/ and V/sub Low/. This work demonstrates a high definition, multilayer, integrated photolithographic process which creates organic field effect transistors suitable for use in integrated circuit applications such as a display backplanes.     

High-mobility tetrathiafulvalene organic field-effect transistors from solution processing

We report on mobilities up to 3.6 cm²/V s in organic field-effect transistors (OFETs) with solution-processed dithiophene- and dibenzo-tetrathiafulvalene (DT- and DB-TTF) single crystals as active materials. In the devices, the channel length varies from 100 μm down to sub 100 nm, and the SiO₂ thickness is either 100 nm, 50 nm, or 20 nm. The devices exhibit excellent operation characteristics with an on/off-ratio exceeding 10⁶. Temperature dependent measurements between 50 and 400 K reveal a thermally activated transport with increased activation above 200 K. The mobility exhibits exponential activation with two distinct exponents.     

液晶栅介质场效应晶体管及突触行为模拟

采用液晶E7作为栅介质,聚异靛蓝噻吩乙烯噻吩(PII-TVT)作为半导体,利用光刻/蚀刻技术制备了漏极-源极-栅极(D-S-G)共面的有机场效应晶体管器件,并测试了晶体管性能,对液晶作为栅介质应用于有机场效应晶体管进行研究。实验结果表明,器件表现出比较特别的晶体管性能,开关比达到10~3。通过光学显微镜观察发现,施加栅极电压后液晶发生形变,表明栅极电压对电极上的液晶分子的取向排列有较大影响。在施加脉冲栅压时,沟道电流随着脉冲栅压时间的延长而增强。利用液晶分子在电场下发生极化和迟滞作用,可一定程度上模拟突触的刺激时间依赖性。     

n-Type self-assembled monolayer field-effect transistors for flexible organic electronics

Within this work we present n-type self-assembled monolayer field-effect transistors (SAMFETs) based on a novel perylene bisimide. The molecule spontaneously forms a covalently fixed monolayer on top of an aluminium oxide dielectric via a phosphonic acid anchor group. Detailed studies revealed an amorphous, two-dimensional semiconducting sheet on top of the dielectric. Reliable transistors with electron mobilities on the order of 10^?3 cm²/V s with limited hysteresis were achieved on rigid as well on flexible substrates. Furthermore, a flexible NMOS-bias inverter based on SAMFETs is demonstrated for the first time.     

High-performance diketopyrrolopyrrole-based organic field-effect transistors for flexible gas sensors

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm²/V s on a glass substrate and 2.98 ± 0.19 cm²/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μ_FET, V_Th and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH₃ gas sensors. As the concentration of NH₃ increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼10⁷ to ∼10⁹ Ω at V_SD = −5 V and V_GS = −5 V.     

Local charge carrier mobility in disordered organic field-effect transistors

In conventional field-effect transistors, the extracted mobility does not take into account the distribution of charge carriers. However, in disordered organic field-effect transistors, the local charge carrier mobility decreases from the semiconductor/insulator interface into the bulk, due to its dependence on the charge carrier density. It is demonstrated that the conventional field-effect mobility is a good approximation for the local mobility of the charge carriers at the interface.     

Small-molecule vacuum processed melamine-C60, organic field-effect transistors

The transfer of benchtop knowledge into large scale industrial production processes represents a challenge in the field of organic electronics. Large scale industrial production of organic electronics is envisioned as roll to roll (R2R) processing which nowadays comprises usually solution-based large area printing steps. The search for a fast and reliable fabrication process able to accommodate the deposition of both insulator and semiconductor layers in a single step is still under way. Here we report on the fabrication of organic field effect transistors comprising only evaporable small molecules. Moreover, both the gate dielectric (melamine) and the semiconductor (C₆₀) are deposited in successive steps without breaking the vacuum in the evaporation chamber. The material characteristics of evaporated melamine thin films as well as their dielectric properties are investigated, suggesting the applicability of vacuum processed melamine for gate dielectric layer in OFETs. The transistor fabrication and its transfer and output characteristics are presented along with observations that lead to the fabrication of stable and virtually hysteresis-free transistors. The extremely low price of precursor materials and the ease of fabrication recommend the evaporation processes as alternative methods for a large scale, R2R production of organic field effect transistors.     

Characterization of ohmic contacts in polymer organic field-effect transistors

It is well known that contact resistance R_c limits the performance of organic field-effect transistors (OFETs) that have high field-effect mobilities (μ_FET ≳ 0.3 cm² V⁻¹ s⁻¹) and short channel lengths (L_ch ≲ 30 μm). The usual transfer-line method (TLM) to analyze R_c calls for extrapolation of total resistance to zero L_ch at constant drain and gate voltages. This requires an unrealistic assumption that R_c does not vary with source−drain current I_sd (nor with channel carrier density σ). Here we describe a self-consistent TLM analysis that instead imposes the condition of constant I_sd and σ. The results explicitly reveal the dependence of R_c on I_sd and σ. We further describe how this R_c(I_sd, σ) surface can be modelled to yield the specific contact resistivity ρ_c of the metal/organic semiconductor (OSC) interface, a key parameter that has so far been neglected in OFETs. We illustrate the application of these analyses to high-performance staggered top-gate bottom-contact poly(2,5-bis(alkyl)-1,4-dioxopyrrolo [3,4-c]pyrrole-3,6-diyl-terthiophene-2,5″-diyl) (DPPT2-T) OFETs fabricated on bottom Au source–drain electrode arrays, with high contact-corrected μ_FET of 0.5 cm² V⁻¹ s⁻¹. We show that when these electrodes are modified to impose weak, and then strong hole-doping of the DPPT2-T interface, R_c diminishes and its dispersion, i.e. dependence on I_sd and σ, weakens. The ultimate ρ_c attained for the strongly hole-doped contact is ca. 1 Ω cm², broadly independent of I_sd and σ, which we propose is a hallmark of a true metal/OSC ohmic contact. For comparison, the bare Au/DPPT2-T contact gives ρ_c of the order of 10 Ω cm² with a marked σ dependence. The lowest ρ_c reached here shortens the current transfer length down to ca. 5 μm, enabling short electrode lengths to be advantageously employed in technology.     

Contactless charge carrier mobility measurement in organic field-effect transistors

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes.     

聚酰亚胺为栅绝缘层的并五苯场效应晶体管

以真空蒸发的有机半导体材料并五苯为有源层,以旋涂的聚酰亚胺作为栅绝缘层,以真空蒸发的Al为栅、源和漏电极,成功制作了顶接触式并五苯有机场效应晶体管(OFET).测试表明,在源漏电压为70 V时,器件的载流子迁移率μ为0.079 cm2/V·s,器件的开关电流比为1.7×104.