Voltage Organic Field-effect Transistors Research Articles

Low Operating Voltage Organic Field-Effect Transistors with Gelatin as a Moisture-Induced Ionic Dielectric Layer: The Issues of High Carrier Mobility.

We have developed low-voltage (<2 V) flexible organic field-effect transistors (OFETs) with high carrier mobility using gelatin as a moisture-induced ionic gate dielectric system. Ionic concentration in the gelatin layer depends on the relative humidity condition during the measurement. The capacitance of the dielectric layer used for the calculation of field-effect carrier mobility for the OFETs crucially depends on the frequency at which the capacitance was measured. The results of frequency-dependent gate capacitance together with the anomalous bias-stress effect have been used to determine the exact frequency at which the carrier mobility should be calculated. The observed carrier mobility of the devices is 0.33 cm2/Vs with the capacitance measured at frequency 20 mHz. It can be overestimated to 14 cm2/Vs with the capacitance measured at 100 kHz. The devices can be used as highly sensitive humidity sensors. About three orders of magnitude variation in device current have been observed on the changes in relative humidity (RH) levels from 10 to 80%. The devices show a fast response with a response and recovery times of ∼100 and ∼110 ms, respectively. The devices are flexible up to a 5 mm bending radius.

Organic FETs using biodegradable almond gum as gate dielectric: A promising way towards green electronics

Abstract Green electronics is an emerging field of research which aims to manufacture devices in an environmentally friendly and sustainable way. Usually, the involved electronic materials are naturally occurring and non-toxic. Also, they can be processed using simple, low energy deposition processes and fabrication techniques. In this work, we present low voltage organic field-effect transistors (OFETs) using almond gum (AG) as the gate dielectric. AG is a natural, biodegradable insulator material that can be directly collected from almond trees and used without any further purification. Moreover, AG possesses interesting properties such as water solubility, ease of processing, good insulation, low leakage current, good film quality, and high capacitance making it a promising dielectric for OFET devices. Bottom gate/bottom contact p-channel OFETs have been processed on glass substrates with poly (3,6-di (2-thien-5-yl)-2,5-di (2-octyldodecyl)-pyrrolo [3,4-c] pyrrole-1,4-dione)thieno [3,2-b] thiophene) (DPPTTT) polymethyl methacrylate (PMMA) blend as the active layer and gold as the source and drain electrodes. The transistors operate at low voltage (VGS ≤ 3 V), with threshold voltages Vth as low as −0.8 V, saturated field effect mobilities μsat above 0.75 cm2 V−1 s−1, subthreshold swings SS around 270 mV/dec and ON/OFF current ratio equal to 103. The combined favourable properties of both almond gum and low voltage operated OFET devices have a high potential to pave a way towards using naturally occurring, biodegradable electronic materials in future disposable sensors or throwaway, low-end electronics.

Scalable Processing of Low Voltage Organic Field Effect Transistors With a Facile Soft-Contact Coating Approach

A soft-contact coating (SCC) method, using a rotatable steel sheet as the meniscus guide, is developed for depositing organic semiconductors (OSCs). The method is used to fabricate bottom-gate bottom-contact organic-field effect transistors (OFETs) using blended solution of small molecule OSC and polymer binder. During the coating process, the meniscus guide maintains very weak contact force to the substrate surface, avoiding damages to the pre-fabricated electrodes. Even at a fast coating speed of 20 mm/s, well controlled crystallization can be achieved to form low sub-gap density of the states (DOS) channels for low voltage OFETs of excellent uniformity. The fabricated low voltage OFETs on plastic substrate in full solution processes exhibit high mobility reaching $1.5 ~{\text {cm}}^{{2}}\cdot \,\,\text{V}^{\text {-1}}\cdot \,\,\text{s}^{\text {-1}}$ , and a small threshold voltage dispersion less than 250 mV for measured 159 devices.

Comparison of Electronic Parameters of Low Voltage Organic Field-Effect Transistors with Novel Gel Gate Insulators

Comparison of Electronic Parameters of Low Voltage Organic Field-Effect Transistors with Novel Gel Gate Insulators

A plastic electronic circuit based on low voltage, organic thin-film transistors for monitoring the X-Ray checking history of luggage in airports

Abstract A circuit based on low voltage Organic Field-Effect Transistors (OFETs) and conceived for monitoring the security check history of luggage in airport environment is reported. An OFET-based, direct X-Rays detector is used as sensing element and integrated in an organic circuit for digitization and memorization of the X-Ray exposure event. An integrated interface to a commercial RFID chip and antenna is also realized to allow remote readout of the circuit status. The operation mechanism of the circuit is described and a complete explanation of the strategy adopted for circuit design is reported. Simulations of the systems in Cadence® Virtuoso environment are presented: X-Ray response of the sensor is modelled, and the overall functionality of the detection and memorization schema are demonstrated. Fabrication and characterization of the circuit under X-Rays in laboratory environment are described. In particular, the correct functionality of the circuit is demonstrated, as well as its actual capability in driving the commercial RFID tag system. The robustness of the circuit to aging and exposure to X-Rays in operation conditions is finally discussed.

A Facile Photo-cross-linking Method for Polymer Gate Dielectrics and Their Applications in Fully Solution Processed Low Voltage Organic Field-effect Transistors on Plastic Substrate

A simple and effective photochemical method was developed for cross-linking of polymer gate dielectrics. Laborious synthetic processes for functionalizing polymer dielectrics with photo-cross-linkable groups were avoided. The photo-cross-linker, BBP-4, was added into host polymers by simple solution blending process, which was capable of abstracting hydrogen atoms from polymers containing active C―H groups upon exposure to ultraviolet (UV) radiation. The cross-linking can be completed with a relatively long wavelength UV light (365 nm). The approach has been applied to methacrylate and styrenic polymers such as commercial poly(methylmethacrylate) (PMMA), poly(iso-butylmethacrylate) (PiBMA) and poly(4-methylstyrene) (PMS). The cross-linked networks enhanced dielectric properties and solvent resistance of the thin films. The bottom-gate organic field-effect transistors (OFETs) through all solution processes on plastic substrate were fabricated. The OFET devices showed low voltage operation and steep subthreshold swing at relatively small gate dielectric capacitance.

A low temperature solution-processed ormosil film for low-voltage organic field-effect transistors

A high performance, low operating voltage organic field effect transistor (OFET) has been fabricated by utilizing organically modified silicate (ormosil) based film as dielectric. The ormosil film is fabricated by a sol-gel method at low temperature (180 °C) and exhibits a high dielectric (k = 33), a smooth surface (Rq = 0.29 nm) and a low leakage current density (5 × 10−9 A cm−2 at −2 V). The ormosil film contains hydrophobic methyl (CH3) functional groups derived from methyltriethoxysilane (MTES) and these groups produce a surface with hydrophobic character and low surface energy for pentacene film growth. The OFET with the ormosil dielectric exhibits excellent performs with high mobility (0.80 cm2 V−1 s−1), low operating voltage (−1.5 V), low threshold voltage (−0.25 V) and low sub-threshold swing (192 mV dec−1). The result demonstrates that the ormosil film can be used as a high performance dielectric for OFETs, and provides a promising way for low power and low cost organic electronics.

Charge-transfer complex modified bottom electrodes for high performance low voltage organic field-effect transistors and circuits

Abstract By using charge transfer complex silver and 2,3-dichloro-5,6-dicyano-p-benzoquinone (AgDDQ) modified silver as the bottom contact source/drain electrodes, high performance organic transistors and complementary inverter circuits using dinaphtho[2,3-b:2′,3’-f]thieno[3,2-b]thiophene (DNTT) as P-type organic semiconductors and N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) as N-type organic semiconductors were demonstrated. Devices with Ag-DDQ bottom contact electrodes exhibit good compatibility for both P and N-type organic semiconductors, the transistors and inverters exhibit excellent stability after storing in air ambient for more than 40 days. The fabrication process is compatible with photolithography technology, which is applicable for large area integrated circuits. All these results indicate the potential application of Ag-DDQ modified electrodes in all-organic, flexible, and low-power electronics.

High mobility and low operation voltage organic field effect transistors by using polymer-gel dielectric and molecular doping

In this work, we present a method to increase the performance in solution processed organic field effect transistors (OFET) by using gel as dielectric and molecular doping to the active organic semiconductor. In order to compare the performance improvement, Poly (methylmethacrylate) (PMMA) and Poly (3-hexylthiophene-2,5-diyl) P3HT material system were used as a reference. Propylene carbonate (PC) is introduced into PMMA to form the gel for using as gate dielectric. The mobility increases from 5.72×10−3 to 0.26cm2Vs–1 and operation voltage decreases from −60 to −0.8 with gel dielectric. Then, the molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced into P3HT via co-solution. The mobility increases up to 1.1cm2Vs–1 and the threshold voltage downs to −0.09V with doping. The increase in performance is discussed in terms of better charge inducing by high dielectric properties of gel and trap filling due to the increased carrier density in active semiconductor by molecular doping.

Combining inkjet printing and chemical vapor deposition for fabricating low voltage, organic field-effect transistors on flexible substrates

A highly reliable fabrication process of low voltage organic field-effect transistors on plastic substrates by means of large area techniques is here reported. The proposed approach is based on the combination of two large-area techniques, inkjet printing and chemical vapor deposition for the fabrication of a thin dielectric layer. A printing process has been employed also for the fabrication of source and drain electrodes and for the organic active layer. The thorough optimization of the fabrication process allows obtaining transistors operated at voltages below 5V. A morphological characterization is provided in order to demonstrate the effectiveness of the fabrication process and to quantify the impact of the device optimization in its final feasibility. Moreover, an analysis of a statistically relevant set of devices is reported to demonstrate the reproducibility of device performances. Bias and mechanical stress tests are provided to demonstrate the stability of the device performances during continuous polarization and for significant mechanical deformations of the substrate.

Ultra-conformable Organic Field-Effect Transistors and circuits for epidermal electronic applications

In this work we report the development of electronic circuits based on low voltage Organic Field-Effect Transistors (OFETs), entirely fabricated on polymer nanosheets acting as sub-micrometric substrates. The overall thickness of the proposed circuits (including the substrate, a 400 nm-thick Parylene C nanosheet) is only 600 nm, thus making them highly flexible, ultra-conformable and light-weighted. A complete characterization of the fabricated devices is reported. Mechanical performances of the nanosheets are thoroughly discussed. Full swing complementary inverters fabricated on same substrate show low noise margins and gains up to 10. Thanks to a carefully designed self-aligned structure, these devices are characterized by a very good frequency response, with a cut-off frequency usually ranging around 100 kHz. The ultra-conformability of such nanosheets allows their transfer and adhesion on complex target surfaces, such as the human skin without a significant change in their electrical performances, representing a step forward to the realization of conformable electronics particularly suited for personal monitoring systems for healthcare and sport.

Amorphous Strontium Titanate Film as Gate Dielectric for Higher Performance and Low Voltage Operation of Transparent and Flexible Organic Field Effect Transistor.

We report that the pervoskite material, strontium titanate (STO) can be used as a gate dielectric layer of flexible and low voltage organic field effect transistor (OFET). The crystallinity, dielectric constant, and surface morphology of STO films can be controlled by the engineering of the growth condition. Under optimized growth condition, amorphous films of STO show a much better gate dielectric compared to other gate dielectrics used to date, with very small leakage current density for flexible and low voltage (<5 V) OFETs. The amorphous STO film decreases the interface trap density at organic/dielectric interface substantially. Pentacene transistors with amorphous STO gate dielectric show high mobility of 2 cm(2)/(V s), on/off ratio of 10(6), subthreshold swing of 0.3 V/dec and low interface trap density. Similarly excellent performance has been obtained in copper phthalocyanine (CuPc) based OFETs with on/off ratio ∼10(5) and carrier mobility ∼5.9 × 10(-2) cm(2)/(V s). Moreover, the operating voltage (∼5 V) has been reduced by more than one order of magnitude. It has been demonstrated that the low processing temperature of amorphous STO makes it the most suitable gate dielectric for flexible and transparent organic devices to operate under low voltage.

Cyanoethyl cellulose-based nanocomposite dielectric for low-voltage, solution-processed organic field-effect transistors (OFETs)

Low voltage organic field-effect transistors (OFETs) using solution-processed cyanoethyl cellulose (CEC) and CEC-based nanocomposites as the gate dielectric are demonstrated. Barium strontium titanate (BST) nanoparticles are homogeneously dispersed in CEC to form the high-k (18.0 ± 0.2 at 1 kHz) nanocomposite insulator layer. The optimised p-channel DPPTTT OFETs with BST-CEC nanocomposite as the gate dielectric operate with minimal hysteresis, display field-effect mobilities in excess of 1 cm2 V−1 s−1 at 3 V, possess low subthreshold swings (132 ± 8 mV dec−1), and have on/off ratios greater than 103. Addition of a 40–50 nm layer of cross-linked poly(vinyl phenol) (PVP) on the surface of the nanocomposite layer significantly decreases the gate leakage current (<10−7 A cm−2 at ±3 V) and the threshold voltage (< −0.7 V) enabling operation of the OFETs at 1.5 V. The presented bilayer BST-CEC/PVP dielectrics are a promising alternative for the fabrication of low voltage, solution-processed OFETs that are suitable for use in low power, portable electronics.

A study on threshold voltage stability of low operating voltage organic thin-film transistors

A low operating voltage (<2 V) organic field-effect transistor (OFET) using phenylhexyltrichlorosilane (PTS) self-assembled monolayer (SAM) dielectric and copper phthalocyanine (CuPc) as semiconductor with improved mobility (0.035 cm2 V−1 s−1) and threshold voltage stability was demonstrated. This device showed better performance when compared to an OFET with octyltrichlorosilane (OTS-8) SAM dielectric. The improved mobility was attributed to the 2D growth mode of CuPc on PTS SAM because of surface energy matching between the two, whereas CuPc film on OTS-8 showed a 3D growth mode with larger grain boundary density. The higher threshold voltage stability of OFETs on PTS SAM was attributed to the efficient coverage and screening of trap centres at dielectric/semiconductor interface due to stronger intermolecular linking and formation of closely packed surface by the bulky phenyl end groups. Decrease in grain boundaries offered by 2D growth of CuPc for electron and hole trapping was also found to be another reason for improved threshold voltage stability. The results indicated that the nature of the end group of SAM dielectric, surface chemistry of dielectric and initial growth mode of semiconductors are all responsible for improvement in threshold voltage stability and enhanced performance of OFET.

Study of poly(3-hexylthiophene)/cross-linked poly(vinyl alcohol) as semiconductor/insulator for application in low voltage organic field effect transistors

In this work we study the cross-linked poly(vinyl alcohol)/poly(3-hexylthiophene) interfacial properties of an organic field effect transistor. We use cross-linked poly(vinyl alcohol) prepared with different ammonium dichromate:poly(vinyl alcohol) proportions, ranging from 0% to 35%, as insulator. Using admittance spectroscopy, we show that the interfacial properties change when the ammonium dichromate concentration is altered. The interfacial properties and the better insulation are responsible for the improvement of the device performance in these organic field effect transistors, achieving best performance in the blend with ammonium dichromate:poly(vinyl alcohol) proportion of 0.25:1.

Solution processed photopatternable high-k nanocomposite gate dielectric for low voltage organic field effect transistors

We report an organic field effect transistors (OFETs) with photo-patternable, solution processed nanoparticle composite high-k gate dielectric layer. The dielectric layer consists of Barium Titanate (BT) nanoparticles dispersed in SU-8, which makes it possible to use solution-processable methods to prepare the dielectric layer. The dielectric constant k of the nanoparticle composite films can be tuned over a wide range by varying the concentration of BT particles, which enables lower voltage operation possible with these composite gate dielectric films. OFETs with P3HT as the semiconducting layer have been demonstrated; it was found that the OFETs with the nanocomposite dielectric layer show a significant improvement in the drive current yet retaining the photopatternability, which is an advantage for circuit fabrication. The composite being a high-k enables low voltage operation (~4V) compared to pristine SU-8 as a gate dielectric operating at high voltages (~40V). Working organic transistors and inverters with a high-k nanocomposite dielectric layer (k>13) with considerably lower leakage current have been demonstrated. This method allows low cost preparation of OFETs without the complicated gate dielectric deposition methods.

Vacuum-processed polyethylene as a dielectric for low operating voltage organic field effect transistors

We report on the fabrication and performance of vacuum-processed organic field effect transistors utilizing evaporated low-density polyethylene (LD-PE) as a dielectric layer. With C60 as the organic semiconductor, we demonstrate low operating voltage transistors with field effect mobilities in excess of 4cm2/Vs. Devices with pentacene showed a mobility of 0.16cm2/Vs. Devices using tyrian Purple as semiconductor show low-voltage ambipolar operation with equal electron and hole mobilities of ∼0.3cm2/Vs. These devices demonstrate low hysteresis and operational stability over at least several months. Grazing-angle infrared spectroscopy of evaporated thin films shows that the structure of the polyethylene is similar to solution-cast films. We report also on the morphological and dielectric properties of these films. Our experiments demonstrate that polyethylene is a stable dielectric supporting both hole and electron channels.

Low voltage organic field effect transistors with a poly(hexylthiophene)–ZnO nanoparticles composite as channel material

AbstractWe report on solution processable organic field effect transistors prepared using a poly(3‐hexylthiophene)–ZnO nanoparticles composite as channel semiconductor material and cross‐linked polyvinyl alcohol as gate insulator. Our transistors show a field effect mobility of 0.35 ± 0.06 cm2/V s, threshold voltage of –1.30 ± 0.11 V, and Ion/Ioff ratio of (1.0 ± 0.1) × 103. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Toward Cheap Disposable Sensing Devices for Biological Assays

There is a significant demand for small, portable, and inexpensive analytical devices, which can be used in a wide range of sensing applications (e.g., food monitoring, detection of chemical, biological poisoning agents, environmental monitoring, medical diagnostics, military defense, etc.). Sensors based on organic semiconducting polymers, which are suitable for large-area, low-cost, flexible, and eventually single-use throwaway electronics, provide a unique opportunity in that sense. We report on low-operating voltage organic field-effect transistor devices, which can be used as sensors in electrolytes and liquid media, using a biofunctionalized, biocompatible, regioregular poly(3-hexylthiophene) semiconducting layer. Measurements in electrolytes and complex media relevant for cell analysis have shown that the devices can be directly used as ion-sensitive transducers and are suitable for in vitro biosensing applications. With the demonstration of biocompatible semiconducting polymeric devices, we have overcome a substantial hurdle for the realization of low-cost and mass-produced sensors, opening new possibilities of biological sensing using organic devices.

Low-operating voltage organic field-effect transistors with high-k cross-linked cyanoethylated pullulan polymer gate dielectrics

The performance of organic field-effect transistors (OFETs) with high-k cross-linked cyanoethylated pullulan (CLCEP) polymer as a gate dielectric was studied. The optimized film spin-coated from the polymer blend showed high dielectric constant (13.4), high capacitance (53.4 nF cm−2, at 1 MHz), and negligible hysteresis enabling reliable and low-voltage OFET operations (−3V). With optimized cross-linking temperature, a field-effect mobility of 1.32 cm2V−1 s−1, an on/off current ratio (Ion/Ioff) of ∼105 and a threshold voltage (Vth) as low as −1.05 V were obtained. Also a low inverse subthreshold slope (SS) of 89 mV dec−1 was obtained which is close to the theoretical value of 57 mV dec−1 at room temperature. The number of trap states was estimated from threshold voltage shift and SS values, and was confirmed to be related to the OH intensities measured via FT-IR. The morphology and microstructure of the pentacene layer grown on CLCEP dielectrics were also investigated and correlated with OFET device performance.