Contact resistance and megahertz operation of aggressively scaled organic transistors

Oxygen-Induced Lattice Strain for High-Performance Organic Transistors with Enhanced Stability

Integrating the merits of low cost, flexibility, and large-area processing, organic semiconductors (OSCs) are promising candidates for the next-generation electronic materials. The mobility and stability are the key figures of merit for its practical application. However, it is greatly challenging to improve the mobility and stability simultaneously owing to the weak interactions and poor electronic coupling between OSCs molecules. Here, an oxygen-induced lattice strain (OILS) strategy is developed to achieve OSCs with both high mobility and high stability. Utilizing the strategy, the maximum mobility of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) organic field-effect transistor (OFET) rises to 15.3 cm2  V-1  s-1 and the contact resistance lowers to 25.5 Ω cm. Remarkably, the thermal stability of DNTT is much improved, and a record saturated power density of ≈3.4 × 104  W cm-2 is obtained. Both the experiments and theoretical calculations demonstrate that the lattice compressive strain induced by oxygen is responsible for their high performance and stability. Furthermore, the universality of the strategy is manifested in both n-type and p-type small OSCs. This work provides a novel strategy to improve both the mobility and the stability of OSCs, paving the way for the practical applications of organic devices.

Amplifying the dielectric constant of shellac by incorporating natural clays for organic field effect transistors (OFETs)

We demonstrate in this work the practical use of uniform mixtures of a bioresin shellac and four natural clays, i.e. montmorillonite, sepiolite, halloysite and vermiculate as dielectrics in organic field effect transistors (OFETs). We present a thorough characterization of their processability and film forming characteristic, surface characterization, elaborate dielectric investigation and the fabrication of field effect transistors with two classic organic semiconductors, i.e. pentacene and fullerene C60. We show that low operating voltage of approximately 4 V is possible for all the OFETs using several combinations of clays and shellac. The capacitance measurements show an improvement of the dielectric constant of shellac by a factor of 2, to values in excess of 7 in the uniform mixtures of sepiolite and montmorillonite with this bioresin.

Alignment of organic conjugated molecules for high-performance device applications

High-performance organic semiconductor materials as the electroactive components of optoelectronic devices have attracted much attention and made them ideal candidates for solution-processable, large-area, and low-cost flexible electronics. Especially, organic field-effect transistors (OFETs) based on conjugated semiconductor materials have experienced stunning progress in device performance. To make these materials economically viable, comprehensive knowledge of charge transport mechanisms is required. The alignment of organic conjugated molecules in the active layer is vital to charge transport properties of devices. The present review highlights the recent progress of processing-structure-transport correlations that allow the precise and uniform alignment of organic conjugated molecules over large areas for multiple electronic applications, including OFETs, organic thermoelectric devices (OTEs), and organic phototransistors (OPTs). Different strategies for regulating crystallinity and macroscopic orientation of conjugated molecules are introduced to correlate the molecular packing, the device performance and charge transport anisotropy in multiple organic electronic devices. This article is protected by copyright. All rights reserved.

Asymmetric Chemical Functionalization of Top-Contact Electrodes: Tuning the Charge Injection for High-Performance MoS2 Field-Effect Transistors and Schottky Diodes

The fabrication of high-performance (opto-)electronic devices based on 2D channel materials requires the optimization of the charge injection at electrode-semiconductor interfaces. While chemical functionalization with chemisorbed self-assembled monolayers has been extensively exploited to adjust the work function of metallic electrodes in bottom-contact devices, such a strategy has not been demonstrated for the top-contact configuration, despite the latter being known to offer enhanced charge-injection characteristics. Here, a novel contact engineering method is developed to functionalize gold electrodes in top-contact field-effect transistors (FETs) via the transfer of chemically pre-modified electrodes. The source and drain Au electrodes of the molybdenum disulfide (MoS₂ ) FETs are functionalized with thiolated molecules possessing different dipole moments. While the modification of the electrodes with electron-donating molecules yields a marked improvement of device performance, the asymmetric functionalization of the source and drain electrodes with different molecules with opposed dipole moment enables the fabrication of a high-performance Schottky diode with a rectification ratio of ≈10³ . This unprecedented strategy to tune the charge injection in top-contact MoS₂ FETs is of general applicability for the fabrication of high-performance (opto-)electronic devices, in which asymmetric charge injection is required, enabling tailoring of the device characteristics on demand.

Influence of Injection Barrier on Vertical Organic Field Effect Transistors

Organic field-effect transistors (OFETs) have shown great potential for applications that require low temperature deposition on large and flexible substrates. To increase their performance, in particular a high transconductance and transit frequency, the transistor channel length has to be scaled into the submicrometer regime, which can be easily achieved in vertical organic field effect transistors (VOFETs). However, despite high performance observed in VOFETs, these transistors usually suffer from short channel effects like weak saturation of the drain current and direct source-drain leakage resulting in large off currents. Here, we study the influence of the injection barrier at the source electrode on the OFF currents, on/off ratio, and transconductance of vertical OFETs. We use two semiconducting materials, 2,6-diphenyl anthracene (DPA), and C₆₀ to vary the injection barrier at the source electrode and are able to show that increasing the Schottky barrier at the source electrode can decrease the direct source/drain leakage by 3 orders of magnitude. However, the increased injection barrier at the source electrode comes at the expense of an increased contact resistance, which in turn will decrease its transconductance and transit frequency. With the help of a 2D drift-diffusion simulation we show that the trade-off between low off currents and high transconductance is inherent to the current VOFET device setup and that new approaches have to be found to design VOFETs that combine good switching properties with high performance.

Correlation between the static and dynamic responses of organic single-crystal field-effect transistors

Transistors, the most important logic elements, are maintained under dynamic influence during circuit operations. Practically, circuit design protocols and frequency responsibility should stem from a perfect agreement between the static and dynamic properties. However, despite remarkable improvements in mobility for organic semiconductors, the correlation between the device performances achieved under static and dynamic circumstances is controversial. Particularly in the case of organic semiconductors, it remains unclear whether parasitic elements that relate to their unique molecular aggregates may violate the radiofrequency circuit model. Thus, we herein report the manufacture of micrometre-scale transistor arrays composed of solution-processed organic semiconductors, which achieve near very high-frequency band operations. Systematic investigations into the device geometrical factors revealed that the radiofrequency circuit model established on a solid-state continuous medium is extendable to organic single-crystal field-effect transistors. The validity of this radiofrequency circuit model allows a reliable prediction of the performances of organic radiofrequency devices.

Flexible low-voltage high-frequency organic thin-film transistors

The primary driver for the development of organic thin-film transistors (TFTs) over the past few decades has been the prospect of electronics applications on unconventional substrates requiring low-temperature processing. A key requirement for many such applications is high-frequency switching or amplification at the low operating voltages provided by lithium-ion batteries (~3 V). To date, however, most organic-TFT technologies show limited dynamic performance unless high operating voltages are applied to mitigate high contact resistances and large parasitic capacitances. Here, we present flexible low-voltage organic TFTs with record static and dynamic performance, including contact resistance as small as 10 Ω·cm, on/off current ratios as large as 1010, subthreshold swing as small as 59 mV/decade, signal delays below 80 ns in inverters and ring oscillators, and transit frequencies as high as 21 MHz, all while using an inverted coplanar TFT structure that can be readily adapted to industry-standard lithographic techniques.

Organic-based inverters: basic concepts, materials, novel architectures and applications

The review article covers the materials and techniques employed to fabricate organic-based inverter circuits and highlights their novel architectures, ground-breaking performances and potential applications.

Understanding molecular surface doping of large bandgap organic semiconductors and overcoming the contact/access resistance in organic field-effect transistors

The contact resistance (Rc) and the effective carrier mobility (μeff) are considered as the important indicators of the performance of organic field-effect transistors (OFETs). Conventionally, the contact resistance is regarded as the interface effect between the metal electrodes and the organic semiconductors, while the carrier mobility is correlated to the crystallinity and π-π stacking of the organic molecules. In the staggered OFETs, Rc is actually closely correlated to μeff through the channel sheet resistance. Besides, the accuracy of the carrier mobility directly extracted from the non-ideal transfer curves with significant contact effect is always questionable. Herein, a diffusion-lead surface doping approach is employed to improve the contact resistance and mobility issues simultaneously. By suppressing the trap states in the sublimated 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) organic semiconductor with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), we observed a 3-fold increase in the carrier mobility from 0.5 to 1.6 cm2 V-1 s-1, and the Rc also drops remarkably from 25.7 kΩ cm to 5.2 kΩ cm. Moreover, the threshold voltage (VTH), subthreshold swing (SS) and the bias stability of the OFETs are also significantly improved. Based on the detailed characterization of the C8-BTBT film upon surface doping, including X-ray diffraction (XRD) for the film crystallinity, Kelvin probe force microscopy (KPFM) for the surface potential, trap state investigation by density of states (DOS) measurement and electrical circuit modeling for partial doping analysis, we confirmed that the spontaneous charge transfer process due to the diffusion of the F4-TCNQ dopants in the C8-BTBT matrix can lead to an effective trap filling. This technique and findings can be potentially developed into a general approach for the improvement of different performance parameters of OFETs.

Nanoscale Ion-Doped Polymer Transistors

Organic transistors with submicron dimensions have been shown to deviate from the expected behavior due to a variety of so-called "short-channel" effects, resulting in nonlinear output characteristics and a lack of current saturation, considerably limiting their use. Using an electrochemically doped polymer in which ions are dynamically injected and removed from the bulk of the semiconductor, we show that devices with nanoscale channel lengths down to 50 nm exhibit output curves with well-defined linear and saturation regimes. Additionally, they show very large on-currents on par with their microscale counterparts, large on-to-off ratios of 10⁸, and record-high width-normalized transconductances above 10 S m^-1. We believe this work paves the way for the fabrication of high-gain, high-current polymer integrated circuits such as sensor arrays operating at voltages below |1 V| and prepared using simple solution-processing methods.

Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. In devices fabricated in the inverted (bottom-gate) device architecture, staggered (top-contact) organic TFTs have usually shown or are predicted to show lower contact resistance than coplanar (bottom-contact) organic TFTs. However, through comparison of organic TFTs with different gate-dielectric thicknesses based on the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, we show the potential for bottom-contact TFTs to have lower contact resistance than top-contact TFTs, provided the gate dielectric is sufficiently thin and an interface layer such as pentafluorobenzenethiol is used to treat the surface of the source and drain contacts. We demonstrate bottom-contact TFTs fabricated on flexible plastic substrates with record-low contact resistance (29 Ωcm), record subthreshold swing (62 mV/decade), and signal-propagation delays in 11-stage unipolar ring oscillators as short as 138 ns per stage, all at operating voltages of about 3 V.

A simple and robust approach to reducing contact resistance in organic transistors

Efficient injection of charge carriers from the contacts into the semiconductor layer is crucial for achieving high-performance organic devices. The potential drop necessary to accomplish this process yields a resistance associated with the contacts, namely the contact resistance. A large contact resistance can limit the operation of devices and even lead to inaccuracies in the extraction of the device parameters. Here, we demonstrate a simple and efficient strategy for reducing the contact resistance in organic thin-film transistors by more than an order of magnitude by creating high work function domains at the surface of the injecting electrodes to promote channels of enhanced injection. We find that the method is effective for both organic small molecule and polymer semiconductors, where we achieved a contact resistance as low as 200 Ωcm and device charge carrier mobilities as high as 20 cm²V⁻¹s⁻¹, independent of the applied gate voltage.

Minimizing contact effects in organic semiconductor-based devices is a key step toward the development of a low-cost technology for next-generation electronics. Here, the authors reduce contact resistance in organic devices by engineering electrodes with high work function surface domains.

Stable Logic Operation of Fiber-Based Single-Walled Carbon Nanotube Transistor Circuits Toward Thread-Like CMOS Circuitry

A fiber-based single-walled carbon nanotube (SWCNT) thin-film-transistor (TFT) has been proposed. We designed complementary SWCNT TFT circuit based on SPICE simulations, with device parameters extracted from the fabricated fiber-based SWCNT TFTs, such as threshold voltage, contact resistance, and off-/gate-leakage current. We fabricated the SWCNTs CMOS inverter circuits using the selective passivation and n-doping processes on a fiber substrate. By comparing the simulation and experimental results, we could enhance the circuit’s performance by tuning the threshold voltage between p-type and n-type TFTs, reducing the source/drain contact resistance and off current level, and maintaining a low output capacitance of the TFTs. Importantly, it was found that the voltage gain, output swing range, and frequency response of the fiber-based inverter circuits can be dramatically improved.

Organic 2D Optoelectronic Crystals: Charge Transport, Emerging Functions, and Their Design Perspective

2D organic semiconductor crystals are emerging as a fascinating platform with regard to their applications in organic field-effect transistors (OFETs), attributed to their enhanced charge transport efficiency and their new optoelectronic functions, based on their unique morphological features. Advances in material processing techniques have not only enabled easy fabrication of few-monolayered 2D nanostructures but also facilitated exploration of the interesting properties induced by characteristic 2D morphologies. However, to date, only a limited number of representative organic semiconductors have been utilized in organic 2D optoelectronics. Therefore, in order to further spur this research, an intuitive crystal engineering principle for realizing organic 2D crystals is required. In this regard, here, not only the important implications of applying 2D structures to OFET devices are discussed but also a crystal engineering protocol is provided that first predicts molecular arrangements depending on the molecular factors, which is followed by realizing 2D supramolecular synthon networks for different molecular packing motifs. It is expected that 2D organic semiconductor crystals developed by this approach will pave a promising way toward next-generation organic 2D optoelectronics.

Growth Of Organic Semiconductor Thin Films with Multi-Micron Domain Size and Fabrication of Organic Transistors Using a Stencil Nanosieve

To grow small molecule semiconductor thin films with domain size larger than modern-day device sizes, we evaporate the material through a dense array of small apertures, called a stencil nanosieve. The aperture size of 0.5 μm results in low nucleation density, whereas the aperture-to-aperture distance of 0.5 μm provides sufficient crosstalk between neighboring apertures through the diffusion of adsorbed molecules. By integrating the nanosieve in the channel area of a thin-film transistor mask, we show a route for patterning both the organic semiconductor and the metal contacts of thin-film transistors using one mask only and without mask realignment.

Recent Progress in the Development of Printed Thin-Film Transistors and Circuits with High-Resolution Printing Technology

Printed electronics enable the fabrication of large-scale, low-cost electronic devices and systems, and thus offer significant possibilities in terms of developing new electronics/optics applications in various fields. Almost all electronic applications require information processing using logic circuits. Hence, realizing the high-speed operation of logic circuits is also important for printed devices. This report summarizes recent progress in the development of printed thin-film transistors (TFTs) and integrated circuits in terms of materials, printing technologies, and applications. The first part of this report gives an overview of the development of functional inks such as semiconductors, electrodes, and dielectrics. The second part discusses high-resolution printing technologies and strategies to enable high-resolution patterning. The main focus of this report is on obtaining printed electrodes with high-resolution patterning and the electrical performance of printed TFTs using such printed electrodes. In the final part, some applications of printed electronics are introduced to exemplify their potential.

Speed up Ferroelectric Organic Transistor Memories by Using Two-Dimensional Molecular Crystalline Semiconductors

Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted intensive attention because of their promising potential in nonvolatile memory devices. The quick switching between binary states is a significant fundamental feature in evaluating Fe-OFET memories. Here, we employ 2D molecular crystals via a solution-based process as the conducting channels in transistor devices, in which ferroelectric polymer acts as the gate dielectric. A high carrier mobility of up to 5.6 cm² V^-1 s^-1 and a high on/off ratio of 10⁶ are obtained. In addition, the efficient charge injection by virtue of the ultrathin 2D molecular crystals is beneficial in achieving rapid operations in the Fe-OFETs; devices exhibit short switching time of ∼2.9 and ∼3.0 ms from the on- to the off-state and from the off- to the on-state, respectively. Consequently, the presented strategy is capable of speeding up Fe-OFET memory devices by using solution-processed 2D molecular crystals.

Injection-modulated polarity conversion by charge carrier density control via a self-assembled monolayer for all-solution-processed organic field-effect transistors

We demonstrated modulation of charge carrier densities in all-solution-processed organic field-effect transistors (OFETs) by modifying the injection properties with self-assembled monolayers (SAMs). The all-solution-processed OFETs based on an n-type polymer with inkjet-printed Ag electrodes were fabricated as a test platform, and the injection properties were modified by the SAMs. Two types of SAMs with different dipole direction, thiophenol (TP) and pentafluorobenzene thiol (PFBT) were employed, modifying the work function of the inkjet-printed Ag (4.9 eV) to 4.66 eV and 5.24 eV with TP and PFBT treatments, respectively. The charge carrier densities were controlled by the SAM treatment in both dominant and non-dominant carrier-channel regimes. This work demonstrates that control of the charge carrier densities can be efficiently achieved by modifying the injection property with SAM treatment; thus, this approach can achieve polarity conversion of the OFETs.

Organic Power Electronics: Transistor Operation in the kA/cm2 Regime

In spite of interesting features as flexibility, organic thin-film transistors have commercially lagged behind due to the low mobilities of organic semiconductors associated with hopping transport. Furthermore, organic transistors usually have much larger channel lengths than their inorganic counterparts since high-resolution structuring is not available in low-cost production schemes. Here, we present an organic permeable-base transistor (OPBT) which, despite extremely simple processing without any high-resolution structuring, achieve a performance beyond what has so far been possible using organic semiconductors. With current densities above 1 kA cm⁻² and switching speeds towards 100 MHz, they open the field of organic power electronics. Finding the physical limits and an effective mobility of only 0.06 cm² V⁻¹ s⁻¹, this OPBT device architecture has much more potential if new materials optimized for its geometry will be developed.

A Vertical Organic Transistor Architecture for Fast Nonvolatile Memory

A new device architecture for fast organic transistor memory is developed, based on a vertical organic transistor configuration incorporating high-performance ambipolar conjugated polymers and unipolar small molecules as the transport layers, to achieve reliable and fast programming and erasing of the threshold voltage shift in less than 200 ns.

Direct-written polymer field-effect transistors operating at 20 MHz

Printed polymer electronics has held for long the promise of revolutionizing technology by delivering distributed, flexible, lightweight and cost-effective applications for wearables, healthcare, diagnostic, automation and portable devices. While impressive progresses have been registered in terms of organic semiconductors mobility, field-effect transistors (FETs), the basic building block of any circuit, are still showing limited speed of operation, thus limiting their real applicability. So far, attempts with organic FETs to achieve the tens of MHz regime, a threshold for many applications comprising the driving of high resolution displays, have relied on the adoption of sophisticated lithographic techniques and/or complex architectures, undermining the whole concept. In this work we demonstrate polymer FETs which can operate up to 20 MHz and are fabricated by means only of scalable printing techniques and direct-writing methods with a completely mask-less procedure. This is achieved by combining a fs-laser process for the sintering of high resolution metal electrodes, thus easily achieving micron-scale channels with reduced parasitism down to 0.19 pF mm-1, and a large area coating technique of a high mobility polymer semiconductor, according to a simple and scalable process flow.

Achieving high mobility, low-voltage operating organic field-effect transistor nonvolatile memory by an ultraviolet-ozone treating ferroelectric terpolymer

Poly(vinylidene fluoride–trifluoroethylene) has been widely used as a dielectric of the ferroelectric organic field-effect transistor (FE-OFET) nonvolatile memory (NVM). Some critical issues, including low mobility and high operation voltage, existed in these FE-OFET NVMs, should be resolved before considering to their commercial application. In this paper, we demonstrated low-voltage operating FE-OFET NVMs based on a ferroelectric terpolymer poly(vinylidene-fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] owed to its low coercive field. By applying an ultraviolet-ozone (UVO) treatment to modify the surface of P(VDF-TrFE-CTFE) films, the growth model of the pentacene film was changed, which improved the pentacene grain size and the interface morphology of the pentacene/P(VDF-TrFE-CTFE). Thus, the mobility of the FE-OFET was significantly improved. As a result, a high performance FE-OFET NVM, with a high mobility of 0.8 cm²V⁻¹s⁻¹, large memory window of 15.4~19.2, good memory on/off ratio of 10³, the reliable memory endurance over 100 cycles and stable memory retention ability, was achieved at a low operation voltage of ±15 V.

Parasitic Capacitance Effect on Dynamic Performance of Aerosol-Jet-Printed Sub 2 V Poly(3-hexylthiophene) Electrolyte-Gated Transistors

Printed, low-voltage poly(3-hexylthiophene) (P3HT) electrolyte-gated transistors (EGTs) have favorable quasi-static characteristics, including sub 2 V operation, carrier mobility (μ) of 1 cm²/(V s), ON/OFF current ratio of 10⁶, and static leakage current density of 10^-6 A/cm². Here we study the dynamic performance of P3HT EGTs in which the semiconductor, dielectric, and gate electrode were deposited using aerosol-jet printing; the source and drain electrodes were patterned by conventional microlithography. With a source-to-drain separation of 2.5 μm, the highest theoretical achievable switching frequency is ∼10 MHz, assuming the movement of charge through the semiconductor is the limiting step. However, the measured maximum switching frequency of P3HT EGTs to date is ∼1 kHz, implying that another process is slowing the response. By systematically varying the device geometry, we show that the frequency is limited by the capacitance between the gate and drain (i.e., parasitic capacitance). The traditional scaling of switching time with the square of channel length (L) does not hold for P3HT EGTs. Rather, minimizing the size of the drain electrode increases the maximum switching speed. We achieve 10 kHz for P3HT EGTs with source/drain electrode dimensions of 2.5 μm × 50 μm and channel dimensions of 2.5 μm × 50 μm. Further improvements will require additional shrinkage of electrode dimensions as well as consideration of other factors such as ion gel thickness and carrier mobility.

Doped Organic Transistors

Organic field-effect transistors hold the promise of enabling low-cost and flexible electronics. Following its success in organic optoelectronics, the organic doping technology is also used increasingly in organic field-effect transistors. Doping not only increases device performance, but it also provides a way to fine-control the transistor behavior, to develop new transistor concepts, and even improve the stability of organic transistors. This Review summarizes the latest progress made in the understanding of the doping technology and its application to organic transistors. It presents the most successful doping models and an overview of the wide variety of materials used as dopants. Further, the influence of doping on charge transport in the most relevant polycrystalline organic semiconductors is reviewed, and a concise overview on the influence of doping on transistor behavior and performance is given. In particular, recent progress in the understanding of contact doping and channel doping is summarized.

Switching from weakly to strongly limited injection in self-aligned, nano-patterned organic transistors

Organic thin-film transistors for high frequency applications require large transconductances in combination with minimal parasitic capacitances. Techniques aiming at eliminating parasitic capacitances are prone to produce a mismatch between electrodes, in particular gaps between the gate and the interlayer electrodes. While such mismatches are typically undesirable, we demonstrate that, in fact, device structures with a small single-sided interlayer electrode gap directly probe the detrimental contact resistance arising from the presence of an injection barrier. By employing a self-alignment nanoimprint lithography technique, asymmetric coplanar organic transistors with an intentional gap of varying size (< 0.2 μm) between gate and one interlayer electrode are fabricated. An electrode overlap exceeding 1 μm with the other interlayer has been kept. Gaps, be them source or drain-sided, do not preclude transistor operation. The operation of the device with a source-gate gap reveals a current reduction up to two orders of magnitude compared to a source-sided overlap. Drift-diffusion based simulations reveal that this marked reduction is a consequence of a weakened gate-induced field at the contact which strongly inhibits injection.

Universal diffusion-limited injection and the hook effect in organic thin-film transistors

The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.

Improving Contact Interfaces in Fully Printed Carbon Nanotube Thin-Film Transistors

Single-walled carbon nanotubes (CNTs) printed into thin films have been shown to yield high mobility, thermal conductivity, mechanical flexibility, and chemical stability as semiconducting channels in field-effect, thin-film transistors (TFTs). Printed CNT-TFTs of many varieties have been studied; however, there has been limited effort toward improving overall CNT-TFT performance. In particular, contact resistance plays a dominant role in determining the performance and degree of variability in the TFTs, especially in fully printed devices where the contacts and channel are both printed. In this work, we have systematically investigated the contact resistance and overall performance of fully printed CNT-TFTs employing three different printed contact materials-Ag nanoparticles, Au nanoparticles, and metallic CNTs-each in the following distinct contact geometries: top, bottom, and double. The active channel for each device was printed from the dispersion of high-purity (>99%) semiconducting CNTs, and all printing was carried out using an aerosol jet printer. Hundreds of devices with different channel lengths (from 20 to 500 μm) were fabricated for extracting contact resistance and determining related contact effects. Printed bottom contacts are shown to be advantageous compared to the more common top contacts, regardless of contact material. Further, compared to single (top or bottom) contacts, double contacts offer a significant decrease (>35%) in contact resistance for all types of contact materials, with the metallic CNTs yielding the best overall performance. These findings underscore the impact of printed contact materials and structures when interfacing with CNT thin films, providing key guidance for the further development of printed nanomaterial electronics.

Soft-Etching Copper and Silver Electrodes for Significant Device Performance Improvement toward Facile, Cost-Effective, Bottom-Contacted, Organic Field-Effect Transistors

Poor charge injection and transport at the electrode/semiconductor contacts has been so far a severe performance hurdle for bottom-contact bottom-gate (BCBG) organic field-effect transistors (OFETs). Here, we have developed a simple, economic, and effective method to improve the carrier injection efficiency and obtained high-performance devices with low cost and widely used source/drain (S/D) electrodes (Ag/Cu). Through the simple electrode etching process, the work function of the electrodes is more aligned with the semiconductors, which reduces the energy barrier and facilitates the charge injection. Besides, the formation of the thinned electrode edge with desirable micro/nanostructures not only leads to the enlarged contact side area beneficial for the carrier injection but also is in favor of the molecular self-organization for continuous crystal growth at the contact/active channel interface, which is better for the charge injection and transport. These effects give rise to the great reduction of contact resistance and the amazing improvement of the low-cost bottom-contact configuration OFETs performance.

Ultra-high gain diffusion-driven organic transistor

Emerging large-area technologies based on organic transistors are enabling the fabrication of low-cost flexible circuits, smart sensors and biomedical devices. High-gain transistors are essential for the development of large-scale circuit integration, high-sensitivity sensors and signal amplification in sensing systems. Unfortunately, organic field-effect transistors show limited gain, usually of the order of tens, because of the large contact resistance and channel-length modulation. Here we show a new organic field-effect transistor architecture with a gain larger than 700. This is the highest gain ever reported for organic field-effect transistors. In the proposed organic field-effect transistor, the charge injection and extraction at the metal–semiconductor contacts are driven by the charge diffusion. The ideal conditions of ohmic contacts with negligible contact resistance and flat current saturation are demonstrated. The approach is general and can be extended to any thin-film technology opening unprecedented opportunities for the development of high-performance flexible electronics.

Organic field-effect transistors offer limited gain due to the large contact resistance and the channel length modulation. Here, Torricelli et al. show a new transistor architecture where the charge injection and extraction are driven by the charge diffusion and a gain larger than 700 is achieved.

Advanced Organic Permeable-Base Transistor with Superior Performance

An optimized vertical organic permeable-base transistor (OPBT) competing with the best organic field-effect transistors in performance, while employing low-cost fabrication techniques, is presented. The OPBT stands out by its excellent power efficiency at the highest frequencies.

Vertical organic transistors

Organic switching devices such as field effect transistors (OFETs) are a key element of future flexible electronic devices. So far, however, a commercial breakthrough has not been achieved because these devices usually lack in switching speed (e.g. for logic applications) and current density (e.g. for display pixel driving). The limited performance is caused by a combination of comparatively low charge carrier mobilities and the large channel length caused by the need for low-cost structuring. Vertical Organic Transistors are a novel technology that has the potential to overcome these limitations of OFETs. Vertical Organic Transistors allow to scale the channel length of organic transistors into the 100 nm regime without cost intensive structuring techniques. Several different approaches have been proposed in literature, which show high output currents, low operation voltages, and comparatively high speed even without sub-μm structuring technologies. In this review, these different approaches are compared and recent progress is highlighted.

Macroscopic and high-throughput printing of aligned nanostructured polymer semiconductors for MHz large-area electronics

High-mobility semiconducting polymers offer the opportunity to develop flexible and large-area electronics for several applications, including wearable, portable and distributed sensors, monitoring and actuating devices. An enabler of this technology is a scalable printing process achieving uniform electrical performances over large area. As opposed to the deposition of highly crystalline films, orientational alignment of polymer chains, albeit commonly achieved by non-scalable/slow bulk alignment schemes, is a more robust approach towards large-area electronics. By combining pre-aggregating solvents for formulating the semiconductor and by adopting a room temperature wired bar-coating technique, here we demonstrate the fast deposition of submonolayers and nanostructured films of a model electron-transporting polymer. Our approach enables directional self-assembling of polymer chains exhibiting large transport anisotropy and a mobility up to 6.4 cm² V⁻¹ s⁻¹, allowing very simple device architectures to operate at 3.3 MHz. Thus, the proposed deposition strategy is exceptionally promising for mass manufacturing of high-performance polymer circuits.

Semiconducting polymers with high mobility are essential for the development of high-frequency flexible electronics, whose fabrications rely on robust printing techniques. Bucella et al. report the fabrication of n-type polymer field-effect transistors, with mobility up to 6.4 cm² V⁻¹s operated at 3.3 MHz, by room temperature bar-coating technique.

Stencil nano lithography based on a nanoscale polymer shadow mask: towards organic nanoelectronics

A stencil lithography technique has been developed to fabricate organic-material-based electronic devices with sub-micron resolution. Suspended polymethylmethacrylate (PMMA) membranes were used as shadow masks for defining organic channels and top electrodes. Arrays of pentacene field effect transistors (FETs) with various channel lengths from 50 μm down to 500 nm were successfully produced from the same batch using this technique. Electrical transport measurements showed that the electrical contacts of all devices were stable and the normalized contact resistances were much lower than previously studied organic FETs. Scaling effects, originating from the bulk space charge current, were investigated by analyzing the channel-length-dependent mobility and hysteresis behaviors. This novel lithography method provides a reliable means for studying the fundamental transport properties of organic materials at the nanoscale as well as enabling potential applications requiring the fabrication of integrated organic nanoelectronic devices.

Flexible low-voltage organic complementary circuits: finding the optimum combination of semiconductors and monolayer gate dielectrics

Low-voltage p-channel and n-channel organic transistors with channel lengths down to 0.5 μm using four small-molecule semiconductors and ultra-thin dielectrics based on two different phosphonic acid monolayers are fabricated on plastic substrates and studied in terms of effective mobility, intrinsic mobility and contact resistance. For the optimum materials combination, flexible complementary circuits have signal delays of 3.1 μs at 5 V.

High performance flexible electronics for biomedical devices

Plastic electronics is soft, deformable and lightweight and it is suitable for the realization of devices which can form an intimate interface with the body, be implanted or integrated into textile for wearable and biomedical applications. Here, we present flexible electronics based on amorphous oxide semiconductors (a-IGZO) whose performance can achieve MHz frequency even when bent around hair. We developed an assembly technique to integrate complex electronic functionalities into textile while preserving the softness of the garment. All this and further developments can open up new opportunities in health monitoring, biotechnology and telemedicine.

Development of high-performance printed organic field-effect transistors and integrated circuits

In this perspective article, we provide a recent overview of the route to realize high-performance printed organic transistors and integrated circuits.

Boost up carrier mobility for ferroelectric organic transistor memory via buffering interfacial polarization fluctuation

Ferroelectric organic field-effect transistors (Fe-OFETs) have been attractive for a variety of non-volatile memory device applications. One of the critical issues of Fe-OFETs is the improvement of carrier mobility in semiconducting channels. In this article, we propose a novel interfacial buffering method that inserts an ultrathin poly(methyl methacrylate) (PMMA) between ferroelectric polymer and organic semiconductor layers. A high field-effect mobility (μ_FET) up to 4.6 cm² V⁻¹ s⁻¹ is obtained. Subsequently, the programming process in our Fe-OFETs is mainly dominated by the switching between two ferroelectric polarizations rather than by the mobility-determined charge accumulation at the channel. Thus, the “reading” and “programming” speeds are significantly improved. Investigations show that the polarization fluctuation at semiconductor/insulator interfaces, which affect the charge transport in conducting channels, can be suppressed effectively using our method.

High performance organic transistor active-matrix driver developed on paper substrate

The fabrication of electronic circuits on unconventional substrates largely broadens their application areas. For example, green electronics achieved through utilization of biodegradable or recyclable substrates, can mitigate the solid waste problems that arise at the end of their lifespan. Here, we combine screen-printing, high precision laser drilling and thermal evaporation, to fabricate organic field effect transistor (OFET) active-matrix (AM) arrays onto standard printer paper. The devices show a mobility and on/off ratio as high as 0.56 cm(2)V(-1)s(-1) and 10(9) respectively. Small electrode overlap gives rise to a cut-off frequency of 39 kHz, which supports that our AM array is suitable for novel practical applications. We demonstrate an 8 × 8 AM light emitting diode (LED) driver with programmable scanning and information display functions. The AM array structure has excellent potential for scaling up.

High-performance ZnO nanowire transistors with aluminum top-gate electrodes and naturally formed hybrid self-assembled monolayer/AlO(x) gate dielectric

A method for the formation of a low-temperature hybrid gate dielectric for high-performance, top-gate ZnO nanowire transistors is reported. The hybrid gate dielectric consists of a self-assembled monolayer (SAM) and an aluminum oxide layer. The thin aluminum oxide layer forms naturally and spontaneously when the aluminum gate electrode is deposited by thermal evaporation onto the SAM-covered ZnO nanowire, and its formation is facilitated by the poor surface wetting of the aluminum on the hydrophobic SAM. The hybrid gate dielectric shows excellent electrical insulation and can sustain voltages up to 6 V. ZnO nanowire transistors utilizing the hybrid gate dielectric feature a large transconductance of 50 μS and large on-state currents of up to 200 μA at gate-source voltages of 3 V. The large on-state current is sufficient to drive organic light-emitting diodes with an active area of 6.7 mm(2) to a brightness of 445 cd/m(2). Inverters based on ZnO nanowire transistors and thin-film carbon load resistors operate with frequencies up to 30 MHz.

Split-gate organic field-effect transistors for high-speed operation

Split-gate organic field-effect transistors have been developed for high-speed operation. Owing to the combination of reduced contact resistance and minimized parasitic capacitance, the devices have fast switching characteristics. The cutoff frequencies for the vacuum-evaporated devices and the solution-processed devices are 20 and 10 MHz, respectively. A speed of 10 MHz is the fastest device reported so far among solution-processed organic transistors.

High-performance vertical organic transistors

Vertical organic thin-film transistors (VOTFTs) are promising devices to overcome the transconductance and cut-off frequency restrictions of horizontal organic thin-film transistors. The basic physical mechanisms of VOTFT operation, however, are not well understood and VOTFTs often require complex patterning techniques using self-assembly processes which impedes a future large-area production. In this contribution, high-performance vertical organic transistors comprising pentacene for p-type operation and C60 for n-type operation are presented. The static current-voltage behavior as well as the fundamental scaling laws of such transistors are studied, disclosing a remarkable transistor operation with a behavior limited by injection of charge carriers. The transistors are manufactured by photolithography, in contrast to other VOTFT concepts using self-assembled source electrodes. Fluorinated photoresist and solvent compounds allow for photolithographical patterning directly and strongly onto the organic materials, simplifying the fabrication protocol and making VOTFTs a prospective candidate for future high-performance applications of organic transistors.

Toward printed integrated circuits based on unipolar or ambipolar polymer semiconductors

For at least the past ten years printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had already been fabricated by printing techniques at industrial scale, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications despite their great potential. In the case of logic integrated circuits (ICs), which constitute the focus of this Progress Report, the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector. Solution-processable polymeric semiconductors are the best candidates to fulfill the requirements for printed logic ICs on flexible substrates, due to their superior processability, ease of tuning of their rheology parameters, and mechanical properties. One of the strongest limitations has been mainly represented by the low charge carrier mobility (μ) achievable with polymeric, organic field-effect transistors (OFETs). However, recently unprecedented values of μ ∼ 10 cm(2) /Vs have been achieved with solution-processed polymer based OFETs, a value competing with mobilities reported in organic single-crystals and exceeding the performances enabled by amorphous silicon (a-Si). Interestingly these values were achieved thanks to the design and synthesis of donor-acceptor copolymers, showing limited degree of order when processed in thin films and therefore fostering further studies on the reason leading to such improved charge transport properties. Among this class of materials, various polymers can show well balanced electrons and holes mobility, therefore being indicated as ambipolar semiconductors, good environmental stability, and a small band-gap, which simplifies the tuning of charge injection. This opened up the possibility of taking advantage of the superior performances offered by complementary "CMOS-like" logic for the design of digital ICs, easing the scaling down of critical geometrical features, and achieving higher complexity from robust single gates (e.g., inverters) and test circuits (e.g., ring oscillators) to more complete circuits. Here, we review the recent progress in the development of printed ICs based on polymeric semiconductors suitable for large-volume micro- and nano-electronics applications. Particular attention is paid to the strategies proposed in the literature to design and synthesize high mobility polymers and to develop suitable printing tools and techniques to allow for improved patterning capability required for the down-scaling of devices in order to achieve the operation frequencies needed for applications, such as flexible radio-frequency identification (RFID) tags, near-field communication (NFC) devices, ambient electronics, and portable flexible displays.

Flexible low-voltage organic transistors with high thermal stability at 250 °C

Low-operating-voltage flexible organic thin-film transistors with high thermal stability using DPh-DNTT and SAM gate dielectrics are reported. The mobility of the transistors are decreased by 23% after heating to 250 °C for 30 min. Furthermore, flexible organic pseudo-CMOS inverter circuits, which are functional after heating to 200 °C, are demonstrated.

Efficient charge injection in p-type polymer field-effect transistors with low-cost molybdenum electrodes through V2O5 interlayer

Here we report high-performance polymer OFETs with a low-cost Mo source/drain electrode by efficient charge injection through the formation of a thermally deposited V2O5 thin film interlayer. A thermally deposited V2O5 interlayer is formed between a regioregular poly(3-hexylthiophene) (rr-P3HT) or a p-type polymer semiconductor containing dodecyl-substituted thienylenevinylene (TV) and dodecylthiophene (PC12TV12T) and the Mo source/drain electrode. The P3HT or PC12TV12T OFETs with the bare Mo electrode exhibited lower charge carrier mobility than those with Au owing to a large barrier height for hole injection (0.5-1.0 eV). By forming the V2O5 layer, the P3HT or PC12TV12T OFETs with V2O5 on the Mo electrode exhibited charge carrier mobility comparable to that of a pristine Au electrode. Best P3HT or PC12TV12T OFETs with 5 nm thick V2O5 on Mo electrode show the charge carrier mobility of 0.12 and 0.38 cm(2)/(V s), respectively. Ultraviolet photoelectron spectroscopy results exhibited the work-function of the Mo electrode progressively changed from 4.3 to 4.9 eV with an increase in V2O5 thickness from 0 to 5 nm, respectively. Interestingly, the V2O5-deposited Mo exhibits comparable Rc to Au, which mainly results from the decreased barrier height for hole carrier injection from the low-cost metal electrode to the frontier molecular orbital of the p-type polymer semiconductor after the incorporation of the transition metal oxide hole injection layer, such as V2O5. This enables the development of large-area, low-cost electronics with the Mo electrodes and V2O5 interlayer.

Controlled charge transport by polymer blend dielectrics in top-gate organic field-effect transistors for low-voltage-operating complementary circuits

We report here the development of high-performance p- and n-channel organic field-effect transistors (OFETs) and complementary circuits using inkjet-printed semiconducting layers and high-k polymer dielectric blends of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) and poly(methyl methacrylate) (PMMA). Inkjet-printed p-type polymer semiconductors containing alkyl-substituted thienylenevinylene (TV) and dodecylthiophene (PC12TV12T) and n-type poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-dithiophene)} (P(NDI2OD-T2)) OFETs typically show high field-effect mobilities (μ(FET)) of 0.2-0.5 cm²/(V s), and their operation voltage is effectively reduced to below 5 V by the use of P(VDF-TrFE):PMMA blends. The main interesting result is that the OFET characteristics could be tuned by controlling the mixing ratio of P(VDF-TrFE) to PMMA in the blended dielectric. The μ(FET) of the PC12TV12T OFETs gradually improves, whereas the P(NDI2OD-T2) OFET properties become slightly worse as the P(VDF-TrFE) content increases. When the mixing ratio is optimized, well-balanced hole and electron mobilities of more than 0.2 cm²/(V s) and threshold voltages below ±3 V are obtained at a 7:3 ratio of P(VDF-TrFE) to PMMA. Low-voltage-operated (∼2 V) printed complementary inverters are successfully demonstrated using the blended dielectric and exhibit an ideal inverting voltage of nearly half of the supplied bias, high voltage gains of greater than 25, and excellent noise margins of more than 75% of the ideal values.