Bistability in doped organic thin film transistors

Organic thin film transitors (TFTs) with the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid), PEDOT:PSS, as the active layer and cross-linked, layer-by-layer assembled poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) multilayers as the gate dielectric layer were investigated. A combination of spectroscopic data and device performance characteristics was used to study the behavior of these TFT devices under a variety of controlled environmental test conditions. It was shown that depletion and recovery of the device can be induced to occur by a means that is consistent with the electrochemical oxidation and reduction of water contained in the film. In addition to acting as a reactant, moisture also acts as a plasticizer to control the mobility of other species contained in the film and thereby permits bistable operation of these devices. Raman spectroscopy was used to show that the observed device switching behavior is due to a change in the PEDOT doping level.     

Enabling gate dielectric design for all solution-processed, high-performance, flexible organic thin-film transistors

A novel solution-processed, compositionally and structurally stable dual-layer gate dielectric composed of a UV-cured poly(4-vinyl phenol)-co-poly(methyl methacrylate) bottom layer and a thermally cross-linked poly(methyl silsesquioxane) top layer for organic thin-film transistors is described. This gate dielectric design, coupled with compatible solution-processable semiconductor and conductor materials, has enabled fabrication of all solution-processed, high-performance organic thin-film transistors on flexible substrates. High field-effect mobility and current on/off ratio, together with other desirable transistor properties, are demonstrated.     

Cross‐linked Polymer‐Blend Gate Dielectrics through Thermal Click Chemistry

New cross‐linking reagents were synthesized and mixed with polystyrene (PS) in solution to form a blend. Thin‐films were spin‐coated from the blend and then cross‐linked by thermal activation at relatively low temperature (100 °C) to form cross‐linked gate dielectrics. This new method is compatible with plastic substrates in flexible electronics. The azide and alkyne cross‐linking reagents are kinetically stable at room temperature, so any premature cross‐linking is avoided during processing. This method also significantly improved the dielectric performances of PS thin films. Solution‐processed top‐gate organic field‐effect transistor devices with indacenodithiophene–benzothiadiazole copolymer as semiconductor layer and the cross‐linked PS blend as dielectric layer showed improved performances with lower gate leakages and higher operation stabilities than devices with neat PS film as dielectric layer.     

An Electrochemical Transducer Based on a Pentacene Double‐Gate Thin‐Film Transistor

We report an electrochemical transducer based on an organic double‐gate transistor. The bottom‐gate is given by a p‐doped silicon substrate, which is covered by 300 nm thermal oxide. A 20 nm pentacene film acts as the semiconducting layer, and a 50 nm tetratetracontane (TTC) alkane film is used as a top‐gate dielectric. An aqueous ionic solution acts as top‐gate. We record the transistor transfer characteristics by variation of the electrolyte potential via a Ag/AgCl electrode for various bottom‐gate settings. A change of the electrolyte potential results in a change of the transistor current and the characteristic behaviour of the device is in good agreement with the expected behaviour of a double‐gate transistor. The top‐gate capacitance of the alkane layer is as high as 2.6×10^?8 F cm^?2 determined by impedance measurements, indicating that TTC is a good choice as an organic top‐gate dielectric. The suitability of this transducer configuration for sensing in aqueous media is demonstrated by the detection of hexanoic acid and stearic acid molecules adsorbing to the alkane interface, respectively. We show that the transducer easily achieves a concentration sensitivity in the range of 100 nM.     

A self-assembled nano optical switch and transistor based on a rigid conjugated polymer, thioacetyl-end-functionalized poly(para-phenylene ethynylene)

A nanometer-scale optical switch and transistor were fabricated with thioacetyl-end-functionalized poly(para-phenylene ethynylene)s and Au nanogap electrodes by self-assembly. With photoirradiation, the switch can be switched on/off quickly with a switching ratio as high as 1000. Moreover, the device works well as a p-type transistor. With an increase in gate bias, strong conductance oscillation was observed in this self-assembled transistor (under low temperature 147 K), which is very likely due to single-electron charging oscillations arising from electron tunneling through the nanometer-scale transistor.     

Low‐Voltage All‐Polymer Field‐Effect Transistor Fabricated Using an Inkjet Printing Technique

Summary: An all‐polymer field‐effect transistor (FET) fabricated using an inkjet printing technique is presented in this paper. Poly(3,4‐ethylenedioxythiophene) works as the source/drain/gate electrode material because of its good conductivity. Polypyrrole acts as the semiconducting layer. Poly(vinyl pyrrolidone) K60, an insulating polymer with a dielectric constant of 60, operates as the dielectric layer. All the polymers are diluted with deionized water, and can be printed with a piezoelectric inkjet printing system. The device functions at a depletion mode with low operation voltage. It has a field‐effect mobility of 0.1 cm² · V⁻¹ · s⁻¹, an on/off ratio of 2.9 × 10³, and a subthreshold slope of 2.81 V · decade⁻¹.

Schematic of the all‐polymer FET synthesized here.     

Attaching organic semiconductors to gate oxides: in situ assembly of monolayer field effect transistors

This study unveils a new tetracene derivative that forms dense, upright monolayers on the surface of aluminum oxide. These monolayers spontaneously self-organize into the active layer in nanoscale field-effect transistor devices when aluminum oxide is used as the dielectric layer. This method gives high yields of working devices that have source-drain distances that are less than 60 nm, thereby providing a method to electrically probe the monolayer assemblies formed from approximately 10 zeptomoles of material (approximately 104 molecules). Moreover, this study delineates a new avenue for research in thin-film organic transistors where the active molecules are linked to the dielectric surface to form a monolayer transistor.     

二氧化硅栅绝缘层的制备与表面修饰

研究了有机薄膜晶体管的二氧化硅栅绝缘层的性质。二氧化硅绝缘层的制备采用热生长法，氧化气氛是O₂(g)+H₂O(g)，工艺为干氧-湿氧-干氧的氧化过程。制得的绝缘层漏电流在10^-9 A左右。以该二氧化硅作为有机薄膜晶体管的栅绝缘层，并五苯作为有源层制作了有机薄膜晶体管器件。实验表明采用十八烷基三氯硅烷(OTS)进行表面修饰的器件具有OTS/SiO₂双绝缘层结构，可以有效地降低SiO₂栅绝缘层的表面能并改善表面的平整度。修饰后器件的场效应迁移率提高了1.5倍、漏电流从10^-9 A降到10^-10 A、阈值电压降低了5 V、开关电流比从10⁴增加到10⁵。结果显示具有OTS/SiO₂双绝缘层的器件结构能有效改进有机薄膜晶体管的性能。     

Device model for poly(o‐methoxyaniline) field‐effect transistor

We present a detailed study of the electric mechanism of a thin poly(o‐methoxyaniline) (POMA) field‐effect transistor. The device was prepared using Al‐Si/SiO₂/(interdigitated gold lines array)/POMA structure as the gate electrode, insulating layer, source‐drain electrodes, and active layer, respectively. A model is presented for the electrical characteristics of such a device that encompasses the disordered properties of the POMA, the source‐drain electrical‐field dependence of hole mobility, and the carrier and mobility gradients in directions perpendicular to the polymer–oxide interface. The fittings of source‐drain current versus source‐drain voltage, having as parameters the gate voltage, is in good agreement with the experimental data, and the dependence of both the carrier saturation velocity and of the carrier mobility with the gate voltage are obtained. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 74–78, 2005     

电极界面缓冲层对P3HT:PC61BM太阳能电池热稳定性的影响

基于溶液法加工制备的聚合物太阳能电池的高温热稳定性是决定器件能否兼容后续高温热封装工艺, 如热压封装、高温原子层沉积(ALD)等的一个关键. 本文分别利用聚(3, 4-乙烯二氧噻吩)-聚苯乙烯磺酸(PEDOT:PSS)和MoO₃作为阳极缓冲层, 以及ZnO和LiF 作为阴极缓冲层, 制备了结构为氧化铟锡(ITO)/阳极缓冲层/3-己基取代聚噻吩:(6, 6)-苯基C₆₁-丁酸甲酯(P3HT:PC₆₁BM)/阴极缓冲层/Al 的太阳能电池, 系统地比较研究了不同界面缓冲材料对器件光电转换性能及稳定性的影响, 特别是在高温煺火条件下器件的性能稳定性差异. 结果表明, 聚合物太阳能电池的热稳定性同器件的结构以及所用的缓冲层材料有密切的相关性. 其中, 利用MoO₃及ZnO分别作为阳极与阴极界面修饰层的P3HT:PC₆₁BM器件在120-150 ℃的温度范围内能够较好地保持器件的光电转换性能. 这一结果为后续需要高温封装工艺的器件提供了有意义的结构优化指导. 此外, 研究结果还表明利用ZnO作为阴极缓冲层能够改善器件的长时间稳定性.     

Theoretical study of the source-drain current and gate leakage current to understand the graphene field-effect transistors

We designed acene molecules attached to two semi-infinite metallic electrodes to explore the source-drain current of graphene and the gate leakage current of the gate dielectric material in the field-effect transistors (FETs) device using the first-principles density functional theory combined with the non-equilibrium Green's function formalism. In the acene-based molecular junctions, we modify the connection position of the thiol group at one side, forming different electron transport routes. The electron transport routes besides the shortest one are defined as the cross channels. The simulation results indicate that electron transport through the cross channels is as efficient as that through the shortest one, since the conductance is weakly dependent on the distance. Thus, it is possible to connect the graphene with multiple leads, leading the graphene as a channel utilized in the graphene-based FETs in the mesoscopic system. When the conjugation of the cross channel is blocked, the junction conductance decreases dramatically. The differential conductance of the BA-1 is nearly 7 (54.57 μS) times as large as that of the BA-4 (7.35 μS) at zero bias. Therefore, the blocked graphene can be employed as the gate dielectric material in the top-gated graphene FET device, since the leakage current is small. The graphene-based field-effect transistors fabricated with a single layer of graphene as the channel and the blocked graphene as the gate dielectric material represent one way to overcome the problem of miniaturization which faces the new generation of transistors.     

Spatial control of p-n junction in an organic light-emitting electrochemical transistor

Low-voltage-operating organic electrochemical light-emitting cells (LECs) and transistors (OECTs) can be realized in robust device architectures, thus enabling easy manufacturing of light sources using printing tools. In an LEC, the p-n junction, located within the organic semiconductor channel, constitutes the active light-emitting element. It is established and fixated through electrochemical p- and n-doping, which are governed by charge injection from the anode and cathode, respectively. In an OECT, the electrochemical doping level along the organic semiconducting channel is controlled via the gate electrode. Here we report the merger of these two devices: the light-emitting electrochemical transistor, in which the location of the emitting p-n junction and the current level between the anode and cathode are modulated via a gate electrode. Light emission occurs at 4 V, and the emission zone can be repeatedly moved back and forth within an interelectrode gap of 500 μm by application of a 4 V gate bias. In transistor operation, the estimated on/off ratio ranges from 10 to 100 with a gate threshold voltage of -2.3 V and transconductance value between 1.4 and 3 μS. This device structure opens for new experiments tunable light sources and LECs with added electronic functionality.     

Vibrational spectroscopy reveals electrostatic and electrochemical doping in organic thin film transistors gated with a polymer electrolyte dielectric

We apply attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to directly probe active layers in organic thin film transistors (OTFTs). The OTFT studied uses the n-type organic semiconductor N-N'-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) and a polymer electrolyte gate dielectric made from poly(ethylene oxide) and LiClO4. FTIR spectroscopy of the device shows signatures of anionic PTCDI-C8 species and broad polaron bands when the organic semiconductor layer is doped under positive gate bias (VG). There are two distinctive doping regions: a reversible and electrostatic doping region for VG 2 V. On the basis of intensity loss of vibrational peaks attributed to neutral PTCDI-C8, we obtain a charge carrier density of 2.9 x 10(14)/cm2 at VG=2 V; this charge injection density corresponds to the conversion of slightly more than one monolayer of PTCDI-C8 molecules into anions. At higher gate bias voltage, electrochemical doping involving the intercalation of Li+ into the organic semiconductor film can convert all PTCDI-C8 molecules in a 30-nm film into anionic species. For comparison, when a conventional gate dielectric (polystyrene) is used, the maximum charge carrier density achievable at VG=200 V is approximately 4.5 x 10(13)/cm2, which corresponds to the conversion of 18% of a monolayer of PTCDI-C8 molecules into anions.     

Voltage-induced infrared spectra from polymer field-effect transistors

Charge-induced infrared absorption spectra from the metal-insulator-semiconductor diodes fabricated with aluminum oxide, poly(p-xylylene), and SiO₂ as gate dielectric and regioregular poly(3-octylthiophene) as organic semiconductor have been measured in situ with reflection or transmission configurations by the FT-IR difference-spectrum method. The observed bands have been attributed to the carriers injected into the polymer layers under the application of minus gate bias. The wavenumber of the band around 1300 cm⁻¹ depends on the gate voltage, indicating that the structure of the carriers depends on the carrier concentration. There exist upper limits in the concentrations of the injected carriers. In situ infrared absorption measurements provide the information about the injected carriers, which affect the properties and the functions of polymer field-effect devices.     

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)(Pi BMA) 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.     

Simple synthesis of palladium nanoparticles, β-phase formation, and the control of chain and dipole orientations in palladium-doped poly(vinylidene fluoride) thin films

Palladium nanoparticles (Pd-NP's) are prepared by a simple one-step procedure when poly(vinylidene fluoride) (PVDF) is used as a polymer stabilizer. High-quality Pd-NP-doped PVDF thin films are fabricated where the heat-controlled spin-coating technique is adopted. The effect of Pd-NP's on the crystal modifications and lamellae orientation in PVDF films is investigated using Fourier transform infrared-grazing incidence reflection absorption spectroscopy. The electroactive β phase and edge-on crystalline lamellae are found to be formed preferentially in Pd-NP-doped PVDF films. As a result, Pd-NP-doped PVDF ultrathin films gave a very good discernible contrast between the written and erased data bits, which suggests that they can be used as a scanning-probe-microscopy-based ferroelectric memory device or a ferroelectric gate field-effect transistor memory device in the future.     

Fluorinated p-n Type Copolyfluorene as Polymer Electret for Stable Nonvolatile Organic Transistor Memory Device

In this study, a kind of fluorinated copolyfluorene, named poly[(4-(octyloxy)-9,9-diphenylfluorene-2,7-diyl)-alt-(2,3,5,6-tetrafluoro-1,4-phenylene)](PODPF-TFP), is synthesized by facile palladium-based direct aromatization. Compared to the non-fluorinated counterpart, poly[(4-(octyloxy)-9,9-diphenylfluorene-2,7-diyl)-alt-(p-phenylene)](PODPF-P), deeper HOMO/LUMO energy level combined with steric hindrance effect endow PODPF-TFP with excellent spectra and morphology stability. Finally, organic field-effect transistor(OFET) memory devices are fabricated with PODPF-P/PODPFTFP as the dielectric layers, and they both exhibit flash type storage characteristic. Owing to the electronegativity of fluorine atom, the device based on PODPF-TFP exhibits larger memory window and more stable I_(on)/I_(off) ratio during a retention time of 104 s as well as a better aging stability. The present study suggests that fluorinated p-n copolyfluorene electrets could enhance the capabilities of charge trapping and storage, which are promising for OFET memory devices.     

Electrolyte-gated transistors based on conducting polymer nanowire junction arrays

In this study, we describe the electrolyte gating and doping effects of transistors based on conducting polymer nanowire electrode junction arrays in buffered aqueous media. Conducting polymer nanowires including polyaniline, polypyrrole, and poly(ethylenedioxythiophene) were investigated. In the presence of a positive gate bias, the device exhibits a large on/off current ratio of 978 for polyaniline nanowire-based transistors; these values vary according to the acidity of the gate medium. We attribute these efficient electrolyte gating and doping effects to the electrochemically fabricated nanostructures of conducting polymer nanowires. This study demonstrates that two-terminal devices can be easily converted into three-terminal transistors by simply immersing the device into an electrolyte solution along with a gate electrode. Here, the field-induced modulation can be applied for signal amplification to enhance the device performance.     

Organophosphonate self-assembled monolayers for gate dielectric surface modification of pentacene-based organic thin-film transistors: a comparative study

Organic thin-film transistors using pentacene as the semiconductor were fabricated on silicon. A series of phosphonate-based self-assembled monolayers (SAMs) was used as a buffer between the silicon dioxide gate dielectric and the active pentacene channel region. Octadecylphosphonate, (quarterthiophene)phosphonate, and (9-anthracene)phosphonate SAMs were examined. Significant improvements in the sub-threshold slope and threshold voltage were observed for each SAM treatment as compared to control devices fabricated without the buffer. These improvements were related to structural motif relationships between the pentacene semiconductor and the SAM constituents. Measured transistor properties were consistent with a reduction in density of charge trapping states at the semiconductor-dielectric interface that was effected by introduction of the self-assembled monolayer.     

Gate dielectric chemical structure-organic field-effect transistor performance correlations for electron, hole, and ambipolar organic semiconductors

This study describes a general approach for probing semiconductor-dielectric interfacial chemistry effects on organic field-effect transistor performance parameters using bilayer gate dielectrics. Organic semiconductors exhibiting p-/n-type or ambipolar majority charge transport are grown on six different bilayer dielectric structures consisting of various spin-coated polymers/HMDS on 300 nm SiO(2)/p(+)-Si, and are characterized by AFM, SEM, and WAXRD, followed by transistor electrical characterization. In the case of air-sensitive (generally high LUMO energy) n-type semiconductors, dielectric surface modifications induce large variations in the corresponding OTFT performance parameters although the film morphologies and microstructures remain similar. In marked contrast, the device performance of air-stable n-type and p-type semiconductors is not significantly affected by the same dielectric surface modifications. Among the bilayer dielectric structures examined, nonpolar polystyrene coatings on SiO(2) having minimal gate leakage and surface roughness significantly enhance the mobilities of overlying air-sensitive n-type semiconductors to as high as approximately 2 cm(2)/(V s) for alpha,omega-diperfluorohexylcarbonylquaterthiophene polystyrene/SiO(2). Electron trapping due to silanol and carbonyl functionalities at the semiconductor-dielectric interface is identified as the principal origin of the mobility sensitivity to the various surface chemistries in the case of n-type semiconductors having high LUMO energies. Thiophene-based n-type semiconductors exhibiting similar film morphologies and microstructures on various bilayer gate dielectrics therefore provide an incisive means to probe TFT performance parameters versus semiconductor-dielectric interface relationships.