Complementary-like inverters based on an ambipolar solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative

We report on high-mobility top-gate organic field-effect transistors (OFETs) and complementary-like inverters fabricated with a solution-processed molecular bis(naphthalene diimide)-dithienopyrrole derivative as the channel semiconductor and a CYTOP/Al₂O₃ bilayer as the gate dielectric. The OFETs showed ambipolar behavior with average electron and hole mobility values of 1.2 and 0.01 cm² V^?1 s^?1, respectively. Complementary-like inverters fabricated with two ambipolar OFETs showed hysteresis-free voltage transfer characteristics with negligible variations of switching threshold voltages and yielded very high DC gain values of more than 90 V/V (up to 122 V/V) at a supply voltage of 25 V.     

Parylene based bilayer flexible gate dielectric layer for top-gated organic field-effect transistors

In this paper, we report on a bilayer insulating film based on parylene-c for gate dielectric layers in top-gate/bottom-contact inkjet-printed organic field-effect transistors (OFETs) with indacenodithiophene-co-benzothiadiazole (IDTBT) and poly([N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bitthiophene)) (P(NDI2OD-T2)) as with p- and n-channel semiconductors. The thin parylene-c film (t = 210 nm) show large gate leakage density (2.52 nA/cm² at 25 V) and low breakdown voltage (2.2 MV/cm). In addition, a degraded field-effect mobility (μ) was observed in printed IDTBT and P(NDI2OD-T2) OFETs with the parylene-c single-layered dielectric. X-ray photoelectron spectroscopy (XPS) analysis reveals that the degradation of μ is due to unwanted chemical interaction between parylene-c and the conjugated polymer surface during the parylene-c deposition process. By inserting 50-nm thick poly(methyl-methacrylate) (PMMA) and polystyrene (PS) layer in-between the parylene-c and conjugated polymer film, highly improved gate leakage density and breakdown voltage are achieved. The printed IDTBT and P(NDI2OD-T2) OFETs with a bilayer dielectric compose of parylene-c and PMMA and PS show significantly improved hole and electron μ of 0.47 cm²/Vs and 0.13 cm²/Vs, respectively, and better operation stability. In addition, we demonstrate inkjet-printed polymer complementary inverter with a high voltage gain of 25.7 by applying a PS/parylene-c bilayer dielectric.     

Free radical fast photo-cured gate dielectric for top-gate polymer field effect transistors

We demonstrated top-gate organic field effect transistors (OFETs) made with free radical photo-cured polymer gate dielectrics and poly(3-hexylthiophene). We introduced a new approach of cross linking dielectric polymers in OFETs by using acrylate monomers cured with UV irradiation directly on the semiconductor. Three different blends were formulated: one self-initiating acrylate oligomer and two epoxy acrylate monomers mixed with 4-phenylbenzophenone as photo initiator and N-methyldiethanolamine as amine synergist. Thin films of these blends were cured in air within one minute. The curing process was monitored with FT-IR spectroscopy and the effect of a wetting agent was studied by measuring the CV characteristics of metal–insulator-semiconductor (MIS) structures made with these formulations. OFETs made with the demonstrated formulations showed high on/off ratios (10⁵–10⁶) and low sub-threshold slopes (0.44–1.42 V/dec).     

Effects of semiconductor/dielectric interfacial properties on the electrical performance of top-gate organic transistors

We investigated the effects of varying the properties of the interface between a semiconductor P3HT layer and a dielectric Cytop™ layer on the performances of the resulting transistor devices by comparing the mobilities of devices prepared with bottom gate/bottom contact or top gate/bottom contact architectures. The reduced channel roughness that arose from the thermal annealing step dramatically enhanced the field-effect mobility, yielding the highest mobility yet obtained for a top-gate transistor: 0.12 cm²/V s. High-performance OFETs may be fabricated by controlling the channel roughness and the properties of the interface between the semiconductor and the gate dielectric.     

并五苯钝化层增强C60有机场效应晶体管性能的研究

We investigated the properties of C₆₀-based organic field-enect transistors（OFETs）（?） a pentacene passivation layer inserted between the C₆₀ active layer and the gate dielectric.After modification of the pentacene passivation layer,the performance of the devices was considerably improved compared to C₆₀-based OFETs with only a PMMA dielectric.The peak field-effect mobility was up to 1.01 cm²/（V·s） and the on/off ratio shifted to 10⁴.This result indicates that using a pentacene passivation layer is an effective way to improve the performance of N-type OFETs.     

Air stable C60 based n-type organic field effect transistor using a perfluoropolymer insulator

Air stable n-type organic field effect transistors (OFETs) based on C₆₀ are realized using a perfluoropolymer as the gate dielectric layer. The devices showed the field-effect mobility of 0.049 cm²/V s in ambient air. Replacing the gate dielectric material by SiO₂ resulted in no transistor action in ambient air. Perfluorinated gate dielectric layer reduces interface traps significantly for the n-type semiconductor even in air.     

Polymer dielectric layer functionality in organic field-effect transistor based ammonia gas sensor

Ammonia (NH₃) gas sensors based on pentacene organic field-effect transistors (OFETs) are fabricated using polymers as the dielectric. Compared with those incorporating poly(vinyl alcohol), poly(4-vinylphenol) or poly(methyl methacrylate) dielectric, a low detect limitation of 1 ppm and enhanced recovery property are obtained for OFETs with polystyrene (PS) as gate dielectric. By analyzing the morphologies of pentacene and electrical characteristics of the OFETs under various concentrations of NH₃, the variations of the sensing properties of different dielectrics based OFET-sensors are proved to be mainly caused by the diversities of dielectric/pentacene interfacial properties. Furthermore, low surface trap density and the absence of polar groups in PS dielectric are ascribed to be responsible for the high performance of NH₃ sensors.     

Polymer Dielectrics and Orthogonal Solvent Effects for High‐Performance Inkjet‐Printed Top‐Gated P‐Channel Polymer Field‐Effect Transistors

We investigated the effects of a gate dielectric and its solvent on the characteristics of top‐gated organic field‐effect transistors (OFETs). Despite the rough top surface of the inkjet‐printed active features, the charge transport in an OFET is still favorable, with no significant degradation in performance. Moreover, the characteristics of the OFETs showed a strong dependency on the gate dielectrics used and its orthogonal solvents. Poly(3‐hexylthiophene) OFETs with a poly(methyl methacrylate) dielectric showed typical p‐type OFET characteristics. The selection of gate dielectric and solvent is very important to achieve high‐performance organic electronic circuits.     

Polymer nanocomposite dielectric based on P(VDF-TrFE)/PMMA/BaTiO3 for TIPs-pentacene OFETs

TIPs-pentacene OFETs were fabricated on a plastic substrate using polymer nanocomposite dielectric. The blend polymer P(VDF-TrFE)/PMMA (30 wt%) was used as polymer matrix and BaTiO₃ nanoparticles modified by 3-glycidoxypropyltrimethoxysilane (GPTMS) were dispersed as ceramic fillers. The effects of different loadings of BaTiO₃ on the surface morphology and electrical properties of dielectric films were investigated. The formulation of screen-printable dielectric ink of P(VDF-TrFE)/PMMA/BaTiO₃/Silica (SII) was achieved by adding fumed silica as the viscosity modifier. TIPs-pentacene OFETs using SII as the gate dielectric features a mobility of 0.01 cm²/V s, and having a threshold voltage of −6 V. This screen-printable dielectric ink is promising for low operating-voltage fully-printed OFETs.     

Top-gate organic field-effect transistors fabricated on paper with high operational stability

We report on top-gate organic field-effect transistors (OFETs) fabricated on specialty paper, PowerCoat™ HD 230 from Arjowiggins Creative Papers coated with a buffer layer composed of a polyvinyl alcohol (PVA) and polyvinylpyrrolidine (PVP) blend. OFETs operate at low voltages and display average carrier mobility values of 1.7 ± 1.1 × 10⁻¹ cm²/Vs, average threshold voltage values of −1.4 ± 0.2 V, and average on/off current ratio of 10⁵. OFETs also display excellent operational stability demonstrated by stable 1000 scans of the transfer characteristics and by stable on-currents displaying less than 6% change during a DC bias stress test at V_DS = V_GS = −10 V for 1 h. Furthermore, OFETs on paper display a decrease of only 7% in their on-state current during a bending test. The performance of these OFETs on paper is comparable to that displayed by top-gate OFETs with the same geometry fabricated on glass substrates.     

Device characteristics of perylene-based transistors and inverters prepared with hydroxyl-free polymer-modified gate dielectrics and thermal post-treatment

Organic field-effect transistors (OFETs) and complementary inverters were produced on the basis of n-type N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) using the neutral cluster beam deposition (NCBD) method. Significant improvements in surface morphology and crystallinity were observed after the surface modification of SiO₂ gate dielectric layers with hydroxyl-free polymer insulators such as polymethylmethacrylate (PMMA) and cyclic olefin copolymer (COC), and thermal post-treatment. Electric characteristics and operational stability of PTCDI-C8-based OFETs were also clearly enhanced after the surface modification, and in particular, the thermally post-treated OFETs with COC-modified SiO₂ gate dielectrics exhibited a high room-temperature mobility of 0.68 cm²/V s. Two structural types (generic vs. encapsulated) of inverters in top-contact configuration were fabricated by integration of the p-type pentacene and n-type PTCDI-C8 OFETs using COC-modified SiO₂ gate dielectrics. Due to the increased electron mobilities and good coupling between p- and n-type OFETs, hysteresis-free, fast-switching inverters were realized with high gains of ∼20 in the first and third quadrants of voltage transfer characteristics under ambient conditions. The device characteristics of the encapsulated inverters monitored as a function of the time were well maintained with slight degradation.     

Photosensitivity of top gate C60 based OFETs: Potential applications for high efficiency organic photodetector

The comparison of light-induced effects in bottom-gate and top-gate organic field effect transistors (OFETs) provide a clear indication, that the nature of interface between the active layer and the gate dielectric plays a major role in the observed light-induced threshold voltage shift. The nature of interface was also analyzed by electron spin resonance (ESR) experiments, which provides a direct evidence for the creation of free radical species when parylene is deposited on the top of the C₆₀ semiconductor layer. The rate of change of light-induced threshold voltage shift strongly depends on the wavelength and intensity of the incident light, and transverse electric field at the interface. The observed effects provide a strong base for the realization of high efficiency organic photodetectors and optical memory devices. The responsivity of organic photodetector was measured up to 1047 A/W.     

Self-assembled monolayers for electrode fabrication and efficient threshold voltage control of organic transistors with amorphous semiconductor layer

In this paper we show that thiolated self-assembled monolayers (SAMs) can be used to anchor source–drain gold electrodes on the substrate, leading to excellent electrical performances of the organic field-effect transistor (OFET) on a par with those using a standard electrode process. Using an amorphous semiconductor and a gate dielectric functionalized with SAMs bearing different dipole moments, we demonstrate that we can tune the threshold voltage alone, while keeping nearly unchanged the other electrical properties (hole carrier mobility, I_on/I_off ratio, subthreshold swing). This differs from previous studies for which SAMs functionalization induced significant changes in all the OFET electrical performances. This result opens doors to design organic circuits using reproducible amorphous semiconductor based OFETs for which only the threshold voltage can be tuned on demand.     

Novel organic inverters with dual-gate pentacene thin-film transistor

We report on the fabrication and characterization of dual-gate pentacene organic thin-film transistors (OTFTs) with plasma-enhanced atomic-layer-deposited (PEALD) 150 nm thick Al₂O₃ as a bottom-gate dielectric and PEALD 200 nm thick Al₂O₃ as a top-gate dielectric. The V_th of dual-gate OTFT has changed systematically with the application of voltage bias to top-gate electrode. When voltage bias from −10 V to 10 V is applied to top gate, V_th changes from 1.95 V to −9.8 V. Two novel types of the zero drive load logic inverter with dual-gate structure have been proposed and fabricated using PEALD Al₂O₃ gate dielectrics. Because the variation of V_th due to chemical degradation and the spatial variation of V_th are inherent in OTFTs, the compensation technology by dual-gate structure can be essential to OTFT applications.     

Organic field-effect transistor based sensors with sensitive gate dielectrics used for low-concentration ammonia detection

A novel organic field-effect transistor (OFET)-sensor concept is presented based on the application of an ion-conducting organic dielectric material, which is chemically adapted to change its electronic properties upon contact with an analyte, thereby generating an electrically detectable response. By employing pH-sensitive, ring-opening metathesis polymerized materials as gate dielectrics in bottom-contact OFETs with a meander-shaped top-gate structure, the concept was successfully realized and evaluated with ammonia (NH₃) as gaseous analyte, easily providing distinct sensor response at concentration levels as low as 100 ppm. In addition to current–voltage OFET-analysis, optical spectroscopy and capacitance measurements were used to rationalize the underlying sensor mechanism, which is mainly attributed to a deprotonation of the pH-sensitive groups of the active-sensing dielectrics by NH₃ and a resulting generation of mobile ions, leading to an increase of the charge carrier density within the OFET channel. The proposed concept provides several advantages over existing OFET-sensor detection principles, including the separation of the sensing mechanism from the charge-transport functionality of the semiconductor, inherent protection of the latter against air exposure and increased selectivity by the application of specific dielectric materials. It therefore offers a great deal of promise in contributing to the development of cheap, integrated, smart and flexible (bio)sensor systems.     

Ameliorating the bias stress stability of n-type OFETs

The bias stress effect on C₆₀ based n-type OFETs was studied comprehensively. The role of dielectric layers and the active layers on bias stress effect was quantified by choosing three different dielectric layers with three different morphologies of active layers. It was found, that the bias stress induced charges can be trapped in active layer as well as in dielectric layers. The charge trapping in active layers happened ten times faster as compared to the trapping of charges in dielectric layers. It was proved, that the use of appropriate dielectric layers increases the bias stress stability by decreasing bias stress effects up to 55%. It was experimentally proven that the fabrication of electrically stress stable devices is possible by using C₆₀ layers grown at higher substrate temperature or with large grain sizes. The OFETs fabricated with larger grain sizes also fulfil the criterion of Guard bands for modelling of electronic circuits, which is a first step of organic electronics towards the industrialization. On the basis of experimental evidences, the criterion of choosing appropriate dielectric layers for OFETs was also proposed.     

Organic field-effect transistors based on single-crystalline active layer and top-gate insulator consistently fabricated by electrostatic spray deposition

An electrostatic spray deposition (ESD) method was applied to prepare both crystalline domains of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) and insulating films of poly(methyl methacrylate) (PMMA) for fabricating top-gate single-crystal organic field-effect transistors (OFETs). The electrical characteristics of the top-gate device were compared to those of the bottom-gate one (SiO₂ bottom-gate insulator) with the same active layer, and the lower charge-trap density at the interface between the top-gate insulator and single-crystalline active layer was demonstrated. The drain current compression in the output characteristics of the top-gate device, however, occurred due to the large parasitic resistance between the source/drain electrodes and accumulation channel. Reducing the thickness of the single-crystalline active layer resulted in a high charge-carrier mobility of 0.29 cm²/V s (channel length of 5 μm).     

Performance of pentacene-based organic field effect transistors using different polymer gate dielectrics

Pentacene-based organic field effect transistors（OFETs） are fabricated using poly（methyl methacrylate）（PMMA） and polyimide（PI） as gate dielectrics,respectively.The fabricated OFETs exhibit reasonable device characteristics.The field-effect mobility,threshold voltage,and on/off current radio are determined to be 3.214 ×10^-2 cm^2 /Vs,-28 V,and 1 ×10^3 respectively for OFETs with PMMA as gate dielectrics,and 7.306×10^-3cm^2 /Vs,-21 V,and 2 ×10^2 for OFETs with PI.Furthermore,the dielectric properties of gate insulator layer are tested and the dipole effect at the semiconductor/dielectrics interface is also analyzed by a model of energy level diagram.     

Preparation and properties of thin parylene layers as the gate dielectrics for organic field effect transistors

We report on the fabrication and characterization of parylene C thin layers for organic electronic devices passivation and gate dielectric of organic field effect transistors (OFETs) development. The investigated thin parylene layers were deposited from the vapour phase in thickness ranging from 3 to 800 nm at room temperature. The thickness and surface morphology of parylene layers were characterized by ellipsometry and AFM technique. The quality of parylene structures were analysed by X-ray reflectivity and diffraction as well as micro-Raman spectroscopy. The measurements confirmed perfect homogeneity and structural properties of parylene layers. Two types of pentacene OFETs were prepared on the silicone dioxide and parylene surface with bottom contact structures. The results demonstrated that using parylene, as the gate dielectric layer is an effective method to fabricate OFETs with improved electric characteristics.     

Self‐Assembled,Millimeter‐Sized TIPS‐Pentacene Spherulites Grown on Partially Crosslinked Polymer Gate Dielectric

Here, a highly crystalline and self‐assembled 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pentacene) thin films formed by simple spin‐coating for the fabrication of high‐performance solution‐processed organic field‐effect transistors (OFETs) are reported. Rather than using semiconducting organic small‐molecule–insulating polymer blends for an active layer of an organic transistor, TIPS‐Pentacene organic semiconductor is separately self‐assembled on partially crosslinked poly‐4‐vinylphenol:poly(melamine‐co‐formaldehyde) (PVP:PMF) gate dielectric, which results in a vertically segregated semiconductor‐dielectric film with millimeter‐sized spherulite‐crystalline morphology of TIPS‐Pentacene. The structural and electrical properties of TIPS‐Pentacene/PVP:PMF films have been studied using a combination of polarized optical microscopy, atomic force microscopy, 2D‐grazing incidence wide‐angle X‐ray scattering, and secondary ion mass spectrometry. It is finally demonstrated a high‐performance OFETs with a maximum hole mobility of 3.40 cm² V^?1 s^?1 which is, to the best of our knowledge, one of the highest mobility values for TIPS‐Pentacene OFETs fabricated using a conventional solution process. It is expected that this new deposition method would be applicable to other small molecular semiconductor–curable polymer gate dielectric systems for high‐performance organic electronic applications.