Flexible Organic Thin-film Transistors Research Articles

Flexible high-performance organic thin film transistors and PMOS inverters: Trap controlled grain boundaries and contact resistance effect in different channel length devices

Printed high-performance organic thin film transistors (OTFTs) are fabricated on flexible polyethylene naphthalate (PEN) substrate for low-voltage operation. Gate, source and drain electrodes are inkjet-printed using conductive silver ink, while chemical vapor deposition grown parylene is used as dielectric layer. We used a blend of 2,7-dihexyl-dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT-C6) and polystyrene (PS), for active p-type organic semiconducting material, and deposited by a dispenser system to achieve 0.66 cm2/Vs mobility at VGS = −6 V. For a set of 30 OTFT devices, the uniformity in device characteristics is illustrated by plotting the histograms for on current, mobility, and threshold voltage (VTh). Mobility decrease for small channel length due to greater contribution of contact resistance, while negative VTh shift occurs because of the generation of hole traps at dielectric-semiconductor interface attributed to moisture present. Electron traps on grain boundaries of the active semiconducting layer causes positive shift in VTh. By using these high mobility OTFT devices, two different low-voltage operation PMOS (P-channel metal-oxide-semiconductor) inverters are fabricated for which high signal gain value of 24.86 is achieved at VDD = −5 V.

Low-voltage driven flexible organic thin-film transistor humidity sensors

Abstract Relative humidity (RH) is broadly involved in various areas, including industry, agriculture, medical engineering, as well as environments. Thus, development of a high-performance, low-energy consumption, flexible, and miniaturized RH sensing device is quite essential to meet the modern requirements. However, it is still a grand challenge to endow a RH sensor with all the above strengths. Herein, according to the electrical double layer mechanism, a RH-responsive poly (acrylic acid) is employed as a sensing layer and simultaneously a dielectric layer for fabrication of flexible organic thin-film transistor (OTFT) RH sensors, which could remarkably reduce the driven voltage of the OTFT device to 2 V. The sensitivity of such-designed sensors can be manipulated by tuning the driven voltage. Notably, all the fabricated OTFT sensing devices show reversibly linear response to RH and a fast response of around 4 s. Due to the solution-processing abilities and flexibility of the OTFT device, it can be miniaturized and integrated into multiple functional devices, e.g., soft medical devices.

Sub-thermionic, ultra-high-gain organic transistors and circuits

The development of organic thin-film transistors (OTFTs) with low power consumption and high gain will advance many flexible electronics. Here, by combining solution-processed monolayer organic crystal, ferroelectric HfZrOx gating and van der Waals fabrication, we realize flexible OTFTs that simultaneously deliver high transconductance and sub-60 mV/dec switching, under one-volt operating voltage. The overall optimization of transconductance, subthreshold swing and output resistance leads to transistor intrinsic gain and amplifier voltage gain over 5.3 × 104 and 1.1 × 104, respectively, which outperform existing technologies using organics, oxides and low-dimensional nanomaterials. We further demonstrate battery-powered, integrated wearable electrocardiogram (ECG) and pulse sensors that can amplify human physiological signal by 900 times with high fidelity. The sensors are capable of detecting weak ECG waves (undetectable even by clinical equipment) and diagnosing arrhythmia and atrial fibrillation. Our sub-thermionic OTFT is promising for battery/wireless powered yet performance demanding applications such as electronic skins and radio-frequency identification tags, among many others.

Flexible organic thin-film transistor immunosensor printed on a one-micron-thick film

Flexible and printed biosensor devices can be used in wearable and disposable sensing systems for the daily management of health conditions. Organic thin-film transistors (OTFTs) are promising candidates for constructing such systems. Moreover, the integration of organic electronic materials and biosensors is of extreme interest owing to their mechanical and chemical features. To this end, the molecular recognition chemistry-based design for the interface between sensor devices and analyte solution is crucial to obtain accurate and reproducible sensing signals of targets, though little consideration has been given to this standpoint in the field of device engineering. Here, we report a printed OTFT on a 1 μm-thick film functionalized with a sensing material. Importantly, the fabricated device quantitatively responds to the addition of a protein immunological marker. These results provide guidelines for the development of effective healthcare tools.

Nanocluster-Based Ultralow-Temperature Driven Oxide Gate Dielectrics for High-Performance Organic Electronic Devices.

The development of novel dielectric materials with reliable dielectric properties and low-temperature processibility is crucial to manufacturing flexible and high-performance organic thin-film transistors (OTFTs) for next-generation roll-to-roll organic electronics. Here, we investigate the solution-based fabrication of high-k aluminum oxide (Al2O3) thin films for high-performance OTFTs. Nanocluster-based Al2O3 films fabricated by highly energetic photochemical activation, which allows low-temperature processing, are compared to the conventional nitrate-based Al2O3 films. A wide array of spectroscopic and surface analyses show that ultralow-temperature photochemical activation (<60 °C) induces the decomposition of chemical impurities and causes the densification of the metal-oxide film, resulting in a highly dense high-k Al2O3 dielectric layer from Al-13 nanocluster-based solutions. The fabricated nanocluster-based Al2O3 films exhibit a low leakage current density (<10−7 A/cm2) at 2 MV/cm and high dielectric breakdown strength (>6 MV/cm). Using this dielectric layer, precisely aligned microrod-shaped 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C8-BTBT) single-crystal OTFTs were fabricated via solvent vapor annealing and photochemical patterning of the sacrificial layer.

The effect of air exposure on device performance of flexible C8-BTBT organic thin-film transistors with hygroscopic insulators

Organic thin film transistors (OTFTs) are normally sensitive to ambient conditions and show performance degradation in air. On the contrary, the performance of flexible 2,7-dioctyl[1] benzothieno [3,2-b][1]benzothiophene (C8-BTBT) OTFTs using cross-linked polymer layer, poly(4-vinyl-phenol)-4,4′-(hexafluoroisopropylidene) diphthalic anhydride (PVP-HDA), as the dielectric layer can be improved in air conditions with 40% relative humidity. Under soaking in air with 40% relative humidity, the electrical behavior, surface morphology, and contact angle of the flexible C8-BTBT OTFTs using PVP-HAD as dielectric layer with three different thicknesses were investigated. It is found that, when the devices with 375 nm-thick PVP-HDA films are placed in 40% relative humidity air conditions for 6 h, the corrected average mobility (μ) can increase from 3.2 to 5.1 cm2 V−1 s−1. Furthermore, the average threshold voltage (Vth) changes from −12.4 to −9.3 V while keeping a constant ratio of Ion/Ioff = 104. These results indicate that the flexible C8-BTBT OTFTs with PVP-HDA dielectric layer exhibit interesting application prospects.

Highly smooth and conductive silver film with metallo-organic decomposition ink for all-solution-processed flexible organic thin-film transistors

A facile silver metallo-organic decomposition (MOD) ink and its two-step sintering process was developed to fabricate highly conductive and smooth silver film for the flexible organic electronics. Using simple and feasible two-step sintering processes and double-layer processes, the conductivity of silver film with the facile MOD ink was greatly improved from 2.85 × 103 S cm−1 to 1.50 × 105 S cm−1, indicating about 53 times increase and about 24% bulk silver conductivity, and the surface RMS roughness was about 6 nm by atomic force microscope, nearly identical to the thermal-evaporated silver film. The XRD showed that synthesized silver thin films were composed of pure silver and well crystallized to the face-centered cubic structure, similar to metallic silver. And the full XPS spectra showed the presence of the same elements and almost identical peaks of solution-processed silver, as compared to the thermal silver surface elemental data. Besides, the silver film also demonstrated great mechanical stability and adhesion on PET substrate. At last, using silver gate/source/drain electrodes generated from the MOD ink, the all-solution-processed organic thin-film transistors (OTFTs) were demonstrated. The optimized OTFTs exhibited a high field mobility of 0.36 cm2 V−1 s−1 with threshold voltage of 0.35 V and on/off current ratio of 1 × 105.

Slot-die coating of sol–gel-based organic–inorganic nanohybrid dielectric layers for flexible and large-area organic thin film transistors

The slot-die coating method has enormous potential owing to the industrial applicability of its high throughput capability to the large-area fabrication of solution-processed films. Herein, we demonstrated a slot-die-coated sol–gel-based organic–inorganic (O–I) nanohybrid dielectric film for organic thin film transistors (OTFTs) with highly uniform properties, such as thickness, morphology, and electrical insulating characteristics. Through the modification of the solution properties and coating speed conditions, the slot-die coating process was used to successfully fabricate defect-free dielectric films with uniform morphology, ranging from below 50 to 800 nm. Additionally, from the OTFT behavior, we identified the clockwise hysteresis by the insulator bulk for the deposition of thick O–I nanohybrid dielectrics. Based on these analyses, we can produce flexible large-area OTFT arrays with optimum slot-die-coated dielectric layers, showing an average field-effect mobility of 0.675 cm2 V−1 s−1 with a low standard deviation of 0.052017 cm2 V−1 s−1. These results suggest that this printing technology has considerable potential in the manufacturing of flexible organic electronic devices that require industrial reliability and large-area processing.

Flexible inkjet-printed dual-gate organic thin film transistors and PMOS inverters: Noise margin control by top gate

Low operating voltage dual-gate p-type organic thin film transistors (OTFTs) and PMOS inverters have been fabricated on 125 μm-thick flexible polyethylene naphthalate (PEN) substrates by solution-processed techniques. Inkjet-printed silver electrodes are used for gate, source, and drain contacts while the active organic semiconducting material is printed by a dispenser system over parylene dielectric. Bottom channel, top channel, and dual-gate OTFTs separately display average charge carrier mobilities of 0.03, 0.11, and 0.37 cm 2 V −1 s −1 , respectively. Top-gate is used to independently tune the threshold voltage for bottom-gate OTFT and vice versa. Two PMOS inverters are fabricated with high gain and low supply voltage operation. Ideal symmetric voltage transfer characteristics with high noise margin are achieved by controlling the threshold voltage of the driver OTFT by top-gate bias for low supply voltage, V DD = −2.5 V. • Flexible low voltage dual-gate OTFTs are fabricated by inkjet printing technique. • Dual-gate OTFTs are characterized in bottom-, top-, and dual-gate mode. • V Th of bottom channel OTFT is linearly control by top gate and vice versa. • DLL and ZVLL, PMOS inverters are compared in terms of gain and noise margin. • Ideal PMOS inverters are achieved by controlling V Th of bottom OTFT by top gate.

Stimuli-responsive polymer as gate dielectric for organic transistor sensors

Abstract Temperature is a characteristic often correlated with environmental and health issues. This paper presents an organic thin film transistor (OTFT) based temperature sensor having a detection range of 30–45 °C, which, therefore, encompasses the human body temperature. The OTFT sensor featured thermosensitive poly(N-isopropylacrylamide) (PNIPAM) and pentacene as the gate dielectric and semiconductor, respectively. The PNIPAM film possessed a dielectric constant of 4.2 with very low leakage current density. The OTFT exhibited high electrical performance, with a hole mobility (μ) of 0.90 ± 0.04 cm2 V−1 s−1, a threshold voltage (Vth) of −15.4 ± 1.16 V, and an on/off ratio of 104. Significant changes in the drain current and the values of Vth and μ occurred when the temperature of the device was varied within the range 30–45 °C at an interval of 0.5 °C. The operating principle for this temperature sensor was based on the structural transformation of the PNIPAM dielectric and the enhanced charge transport of the pentacene semiconductor upon varying the temperature. Flexible OTFTs fabricated on polyethylene terephthalate substrate displayed hole mobilities as high as 0.39 ± 0.01 cm2 V−1 s−1, values of Vth of −18.6 ± 0.45, and on/off ratios of 102, and were workable for over 100 bending cycles.

High-performance flexible organic thin-film transistor nonvolatile memory based on molecular floating-gate and pn-heterojunction channel layer

Flexible floating-gate structural organic thin-film transistor (FG-OTFT) nonvolatile memories (NVMs) are demonstrated based on an integrated molecular floating-gate/tunneling (I-FG/T) layer and a pn-heterojunction channel layer. Semiconducting polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) nanoparticles and insulating polymer polystyrene are used to build the I-FG/T layers by spin-coating their solution. The dependence of the memory performances on the structure of I-FG/T layers is researched. For achieving a large charge storage capacity, the pn-heterojunction channel, consisting of 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene and F16CuPc, is fabricated to provide both electrons and holes for injecting and trapping in the floating gate by overwriting the stored charges with an opposite polarity at the programming and erasing voltages, respectively. As an optimal result, a high performance flexible FG-OTFT NVM is achieved, with a large memory window of 21.6 V on average, a highly stable charge storage retention capability up to 10 years, and a highly reliable programming/erasing switching endurance over 200 cycles. The FG-OTFT NVM also exhibits an excellent mechanical bending durability with the memory performances maintaining well over 6000 bending cycles at a bending radius of 5.9 mm.

Signal amplification with patterned conducting polymer electrodes based organic thin film transistor circuit

Organic thin film transistors (OTFTs) fabricated with non-destructive patterning techniques are the building blocks in realizing flexible electronic interface for sensing applications. In this study, a single stage differential amplifier is demonstrated with flexible OTFTs. The OTFTs were fabricated with thermally evaporated Pentacene as the semiconducting layer. The electrodes of the OTFTs were realized using a conductive polymer composite, poyaniline:polystyrene sulphonic acid (PANi-PSS), patterned by a modified Parylene lift-off process. Electrical characteristics of bottom contact TFTs with polymeric PANi-PSS electrodes is superior to those with conventional metal electrodes due to a lower charge injection barrier and resultant lower contact resistance. The measurement of the transistor characteristics and the amplifier response reported has been carried out with a wafer probing test jig set up. The differential amplifier realized with p-type Pentacene transistors with mobility, 0.5 cm2/Vs, exhibited a voltage gain of 10 dB in a frequency bandwidth of 1 kHz.

Room-temperature solution-processed, ZrOx-based hybrid gate dielectrics for low-voltage organic thin-film transistors on plastic substrates

For advanced electronic device applications including printable, flexible organic electronics and bioelectronics, it is crucial to develop novel dielectric materials with high dielectric strength and low-temperature processability compatible with plastic substrates for producing flexible and low-voltage organic thin-film transistors (OTFTs). Herein, we report the facile fabrication of rationally designed, high-capacitance hybrid gate dielectric films for low-voltage OTFTs on plastic substrates via a room-temperature solution process. To create room-temperature-processed gate dielectrics with high dielectric strength, we designed organic-inorganic hybrid gate dielectric films by including ZrOx networks as inorganic high-k oxide materials for obtaining large capacitance and 1,6-hexanediol diacrylate as a bifunctional UV cross-linking agent and a flexible organic component for increasing the density and flexibility of films and ensuring room-temperature processability. The UV cross-linked ZrOx-based hybrid composite films with a 15 nm thickness showed excellent dielectric properties with a leakage current density of ∼10−6 A/cm2 and a capacitance of 640 nF/cm2. The film surfaces were modified with hydrophobic self-assembled monolayers to match the surface energy of organic semiconductors for enhancing the OTFT performance. The utility of the hybrid dielectrics for flexible OTFTs was demonstrated by realizing pentacene TFTs on plastic substrates with a low operating voltage at –2.5 V, a high on/off current ratio of 105, a threshold voltage as low as –0.7 V, and a hole mobility up to 0.47 cm2 V−1 s−1, which are comparable to those performed on Si substrates.

A universal strategy to improve the mechanical stability of flexible organic thin film transistors

In flexible electronic systems, mechanical deformation is a key factor that impacts the mechanical stability of flexible devices.

Densely cross-linked polysiloxane dielectric for organic thin-film transistors with enhanced electrical stability

A solution-processable organic–inorganic hybrid material composed of a polysiloxane urethane acrylate composite (PSUAC) was developed through a dual cross-linking mechanism and satisfies all the requirements for use as a gate dielectric for flexible organic thin-film transistors (OTFTs).

A critical role of amphiphilic polymers in organic–inorganic hybrid sol–gel derived gate dielectrics for flexible organic thin-film transistors

The sol–gel method is a low-temperature, highly controllable and cost-effective method for producing homogeneous and high-quality ultrafine nanostructured thin films.

Boosting Direct X‐Ray Detection in Organic Thin Films by Small Molecules Tailoring

AbstractThe attention focused on the application of organic electronics for the detection of ionizing radiation is rapidly growing among the international scientific community, due to the great potential of organic technology to enable large‐area conformable sensor panels. However, high‐energy photon absorption is challenging as organic materials are constituted of atoms with low atomic numbers. Here it is reported how, by synthesizing new solution‐processable organic molecules derived from 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) and 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene, with Ge‐substitution in place of the Si atoms to increase the material atomic number, it is possible to boost the X‐ray detection performance of organic thin films on flexible plastic substrates. Bis(triisopropylgermylethynyl)‐pentacene based flexible organic thin film transistors show high electrical performance with higher mobility (0.4 cm2 V−1 s−1) and enhanced X‐ray sensitivity, up to 9.0 × 105 µC Gy−1 cm−3, with respect to TIPS‐pentacene‐based detectors. Moreover, similar results are obtained for 5,11‐bis(triethylgermylethynyl)anthradithiophene devices, confirming that the proposed strategy, that is, increasing the atomic number of organic molecules by chemical tailoring to improve X‐ray sensitivity, can be generalized to organic thin film detectors, combining high X‐ray absorption, mechanical flexibility, and large‐area processing.

Characterization and Compact Modeling of Self-Aligned Short-Channel Organic Transistors

In this paper, we present the characterization and modeling of downscaled flexible organic thin-film transistors fabricated by photolithography and self-alignment of the source and drain contacts to the gate. A SPICE model based on the unified model and extraction method for thin-film transistors (TFTs) is tailored with high accuracy for the subthreshold regime in order to include the channel length dependence of two semiempirical parameters, viz., the effective mobility and the onset voltage. In addition, we demonstrate the modeling of the hump effect, a phenomenon often present in TFTs fabricated by lithographic methods.

Low-Voltage, Flexible, and Self-Encapsulated Ultracompact Organic Thin-Film Transistors Based on Nanomembranes.

Organic thin-film transistors (OTFTs) are an ever-growing subject of research, powering recent technologies such as flexible and wearable electronics. Currently, many studies are being carried out to push forward the state-of-the-art OTFT technology to achieve characteristics that include high carrier mobility, low power consumption, flexibility, and the ability to operate under harsh conditions. Here, we tackle this task by proposing a novel OTFT architecture exploring the so-called rolled-up nanomembrane technology to fabricate low-voltage (<2 V), ultracompact OTFTs. As the OTFT gate electrode, we use strained nanomembranes, which allows all transistor components to be rolled-up and confined into a tubular-shaped tridimensional device structure with reduced footprint (ca. 90% of their planar counterpart), without any lossof electrical performance. Such an innovative architecture endows the OTFTs high mechanical flexibility (bending radius of <30 μm) and robustness-the devices can be reversibly deformed, withstanding more than 500 radial compression/decompression cycles. Additionally, the tubular device design possesses an inherent self-encapsulation characteristic that protects the OTFT active region from degradation by UV-light and hazardous vapors. The reported strategy is also shown to be compatible with different organic semiconductor materials. All of these characteristics contribute to further extending the potentialities of OTFTs, mainly toward rugged electronics.

Air-stable, transparent flexible ambipolar organic thin film transistors based on CuPc-F16CuPc bi-channel structure

We report the fabrication of air-stable, transparent flexible ambipolar organic thin film transistors based on CuPc and F16CuPc bi-channel films deposited under 10−1 Pa, by using graphene gate electrode, fluoropolymer Cytop dielectric layer, and Au source/drain electrodes on polyethylene terephthalate substrates. The devices show outstanding air-stability and high optical transparency of above 75% with electron and hole mobilities up to 0.031 and 0.029 cm2/V·s, respectively. The excellent ambipolar performance, along with the outstanding air-stability and high optical transparency, make CuPc-F16CuPc bi-channel transistors as a promising candidate for organic integrated circuits.