Organic field-effect transistors with ultrathin gate insulator

We have used a commercially available Mylar film coated with a thin (≈60 nm) layer of aluminium and an ultrathin (≈3.5 nm) SiO₂ layer as flexible substrate for the manufacture of bottom-gate organic field-effect transistors (OFETs). We show that the SiO₂ layer has insulating properties with a breakdown voltage of 1.6 V and a capacitance of ≈1 μF/cm². We have manufactured organic field-effect transistors using this substrate, regioregular poly(3-hexylthiophene) (rrP3HT) as a p-type semiconductor, and gold source and drain contacts. This results in OFETs that operate with voltages on the order 1 V.     

N-channel operation of pentacene thin-film transistors with ultrathin polymer gate buffer layer

N-channel operation of pentacene thin-film transistors with ultrathin poly(methyl methacrylate) (PMMA) gate buffer layer and gold source–drain electrode was observed. We prepared pentacene thin-film transistors with an 8-nm thick PMMA buffer layer on SiO₂ gate insulators and obtained electron and hole field-effect mobilities of 5.3 × 10^?2 cm²/(V s) and 0.21 cm²/(V s), respectively, in a vacuum of 0.1 Pa. In spite of using gold electrodes with a high work function, the electron mobility was considerably improved in comparison with previous studies, because the ultrathin PMMA film could decrease electron traps on SiO₂ surfaces, and enhance the electron accumulation by applied gate voltages.     

Low-voltage organic n-channel thin-film transistors based on a core-cyanated perylene tetracarboxylic diimide derivative

Using the small-molecule organic semiconductor bis(1H,1H-perfluorobutyl)-dicyano-perylene tetracarboxylic diimide, C₃F₇CH₂-PTCDI-(CN)₂, and a low-temperature-processed, high-capacitance gate dielectric based on a phosphonic acid self-assembled monolayer, we have manufactured n-channel thin-film transistors on glass substrates. The transistors operate with low voltages (2 V) and have an electron mobility of 0.04 cm²/Vs and an on/off ratio of 10⁵. By combining C₃F₇CH₂-PTCDI-(CN)₂ n-channel transistors with pentacene p-channel transistors, we have also manufactured low-voltage, low-power organic complementary inverters with good static and dynamic performance.     

Electrical characterization of pentacene thin-film transistors with polymeric gate dielectric

Pentacene thin films obtained by thermal evaporation at room temperature have been incorporated as the active layer in bottom-gate thin-film transistors (TFTs). The dielectric was spin-cast polymethyl methacrylate (PMMA) baked at only 170 °C. Crystalline silicon wafers and polyethylenenaphtalate (PEN) polymer foils were used as substrates. These devices were electrically characterised by measuring the output and transfer characteristics at different temperatures. Both the channel conductance and field-effect mobility evidenced similar thermal activation energies around 0.15 eV. These results could indicate that electrical transport is mainly controlled by trapping and thermal release of carriers from localised states.     

The ultraviolet-ozone effects on organic thin-film transistors with double polymeric dielectric layers

Organic thin-film transistors (OTFTs) were fabricated based on the double polymeric insulators consisting of polyvinyl alcohol (PVA) and polystyrene (PS). The using of double dielectric layers exhibited an improved device performance comparing with the conventional devices with single dielectric layer. By exposing the polymer insulating layer (PS) under ultraviolet (UV) light for different times (0, 5, 10 and 15 min), the surface properties of the PS film has been further optimized for the growth of pentacene. The grain size and crystallization of pentacene film grown on PS are apparently improved as the increase of surface energy of PS. As a result, a field-effect mobility of 1.38 cm² V^?1 S^?1 and an on/off current ratio of 10⁴ had been obtained by UV treatment for 10 min. All results indicate the surface energy of dielectric layer is a crucial factor to influence the devices performance that can be well tuned by UV treatment. The polymeric materials pair consisting of PS and PVA is a promised candidate as the dielectric layers for high-performance OTFTs.     

Study on the characteristics of metal–organic interface for organic thin-film transistors

In this study, the interfacial characteristics between pentacene and Au layers were investigated with varying of the deposition rate of Au layer from 1.0 to 15.0 Å/s. For the devices with the structure of bottom-Au/pentacene/top-Au, it was observed that the electrical characteristics could be improved by increasing the deposition rate of top-Au, and the highest electrical conductivity value, 1.5 × 10⁻⁶ S/cm, was obtained for the device with the top-Au-deposited at 15.0 Å/s. AES results showed that the integrated atomic content of Au in top-Au layer is substantially increased with the deposition rate of top-Au, but there was no critical difference in the depth profile of Au atoms regardless of the deposition rate of top-Au. And also, we fabricated pentacene-based Schottky diodes and measured the hole injection barrier heights from Au electrode into pentacene layer using Fowler-Nordheim theory. Upon the investigations, it was observed that the hole injection barrier was reduced with increasing the deposition rate of Au electrode and the lowest value of 0.12 eV was obtained for the device with the Au electrode deposited at 15.0 Å/s. As a result, the performance of top-contact OTFT could be improved with increasing the deposition rate of Au electrodes (source and drain).     

Thermal annealing effects on morphology and electrical response in ultrathin film organic transistors

Ultra-thin organic film transistors were annealed in vacuum at constant temperature and for different time intervals. This treatment eliminates unintentional doping and reduces hysteresis in I/V curves, both for pentacene and α-sexithienyl, without substantially improving the charge mobility. Devices that originally exhibited high and non-ohmic contact resistance never matched the characteristics of devices with well grown electrode/semiconductor interfaces, even after annealing. Prolonging the annealing time resulted in film rupture due to either re-crystallization or molecular desorption. This process started before healing of molecular disorder at the relevant interfaces could take place.     

The effects of hydroxyl-free polystyrene buffer layer on electrical performance of pentacene-based thin-film transistors with high-k oxide gate dielectric

We have investigated the effects of hydroxyl-free polystyrene (PS) as buffer layer on pentacene-based low-voltage organic thin-film transistors (OTFTs). The PS buffer layer is formed on the HfO₂ layer evaporated on Si substrate by spin-coating method prior to pentacene deposition, existence of which results in a dramatic increase of field effect mobility from 0.09 to 0.59 cm²/Vs and negligible hysteresis. The improved mobility and hysteresis of the OTFTs can be attributed to the formation of smooth and nonpolar hydroxyl-free PS/HfO₂ gate dielectric surface. The PS insulator buffer layer can also effectively reduce gate leakage current by more than 70%. The results demonstrate that using appropriate polymer buffer layer is favorable to improve the performance of the OTFTs operating at low voltages with high mobility and good electrical stability.     

Effect of postfabrication thermal annealing on the electrical performance of pentacene organic thin-film transistors

We report the results of a systematic investigation of the electrical and physical modifications to pentacene organic thin-film transistors (OTFT) that result from postfabrication thermal annealing. The thermally induced electrical modifications of the performance of the pentacene OTFTs were explored, and the morphology and structure of the pentacene films were investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. We found that postfabrication thermal annealing at 50 °C significantly improved the mobility, from 0.19 to 0.36 cm²/Vs, and increased the on/off ratio to almost twice that of the non-annealed device. We also found that annealing increased the pentacene grain size and enhanced the 0 0 1 peak intensity in the XRD pattern, indicating greater molecular ordering. At postfabrication thermal annealing temperatures of 70 °C and above, the pentacene films lose their crystallinity and the OTFT performance is decreased.     

Influence of organic gate dielectrics on the performance of pentacene thin film transistors

The electronic transport of polycrystalline pentacene thin film transistors is investigated. The thermally evaporated pentacene films prepared on organic dielectrics like benzocyclobutane exhibit mobilities comparable with inorganic dielectrics like thermal oxide and plasma enhanced chemical vapor deposited silicon nitride. To gain insights in the electronic transport behavior of the organic thin film transistors (TFTs) the I/V characteristics were simulated by a one-dimensional density-of-states transistor model. The experimental data can be described by using a broad distribution of acceptor-like states deep in the bandgap and a narrow distribution of donor-like states close to the valance band. The influence of the different dielectrics on the defect distribution and the electronic transport will be discussed.     

Effects of hole and electron trapping on organic field-effect transistor transfer characteristic

Routine organic field-effect transistor measurements are performed at negative and positive gate voltages leading to the occurrence of both hole and electron trapping. Despite this fact, the big majority of studies have focused either on hole trapping or on electron trapping but not on both at the same time. This paper presents the influences of trapped electron concentration n_trap and trapped hole concentration p_trap on the transfer characteristic (TC) features: onset voltage, hysteresis and transconductance, i.e. apparent mobility. Some effects are common to both charge types: (1) hysteresis is due to a combination of lower detrapping rate than sweep rate for n_trap and p_trap, (2) the transconductance is decreased by the super-linear V_G dependence of p_trap and by n_trap detrapping. One effect is opposite to both charge types: p_trap (n_trap) shifts the onset voltage towards negative (positive) value. We consider that the knowledge of trap-induced effects from both charge types is useful for correctly interpreting and understanding TCs.     

Investigation of organic n-type field-effect transistor performance on the polymeric gate dielectrics

We report a copper hexadecafluorophthalocyanine (F₁₆CuPc) based n-type organic field-effect transistor (OFET) with polymeric gate dielectrics with different physical/electrical properties. The gate dielectrics are four types of cross-linked poly(4-vinylphenol) and newly prepared poly(4-phenoxy methyl styrene) and those are characterized based on surface tension, leakage current and capacitance. The performance of F₁₆CuPc OFETs with those gate dielectrics was compared. We found that the composition of the gate dielectrics and the interfacial interaction of F₁₆CuPc with the gate dielectric play a decisive role in the performance of OFETs. The effect of physical/electrical properties, composition and processing condition of the gate dielectrics on the device performance was investigated.     

Characteristics of poly(vinyl acetate) as a gate insulating material in organic thin film transistors

We report the physical characteristics of poly(vinyl acetate) (PVAc) as a polymeric gate insulator in the organic thin film transistors (OTFTs), in which the OTFTs require not only a simple packing process with a relatively low cost compared to conventional inorganic electronics but also their compatibility with flexible substrates. The PVAc gives dielectric constant values in the range of 3.2–8.3, which can be adopted as a good gate insulator for OTFTs. The fabricating condition of OTFTs using the PVAc was improved by both adjusting its viscosity with that of a medium, and controlling the surface morphology and spin coating conditions. All these meet the requirement for a new organic gate-insulator, which can improve the performance of the present OTFTs.     

Fabrication and characterization of the pentacene thin film transistor with a Gd2O3 gate insulator

Pentacene thin film transistors (TFTs) on a high-κ Gd₂O₃ gate insulator layer were fabricated and characterized. The Gd₂O₃ layer was grown by ion beam assisted deposition (IBAD) on a heavily-doped silicon substrate in ultra high vacuum, where the measured dielectric constant of the oxide layer was about 7.4. The maximum field effect mobility and the on/off ratio of the TFTs were 0.1 cm²/Vs and about 10³, respectively. The threshold voltage of the device was dramatically decreased (15.3 V → −3.5 V) as the high-κ Gd₂O₃ layer replaced the SiO₂ layer.     

SiO2陶瓷复合材料高温介电性能研究

在室温至1000℃范围内,采用谐振腔法测试SiO2陶瓷复合材料介电性能随频率、温度的变化规律,根据电介质理论,并结合材料本身的成分及结构特征,讨论了SiO2陶瓷复合材料介电性能变化的物理本质.利用高温XRD分析了SiO2复合材料从室温至1500℃之间的相组成变化,推测出SiO2复合材料介电性能受相变影响的温度为1500℃.     

Al2O3钝化层对a-IGZO薄膜晶体管电性能的增强机制研究

采用空心阴极增强脉冲激光沉积技术(HC-PLD)在室温制备高质量的Al₂O₃薄膜,作为非晶铟镓锌氧(a-IGZO)TFT器件的钝化层,显著增强了Al₂O₃/a-IGZO TFT器件的亚阈值特性,其原因在于空心阴极引入的氧等离子体抑制了Al₂O₃/a-IGZO界面处氧空位的形成。进一步研究发现,针对a-IGZO薄膜的180℃退火处理有利于消除弱结合氧并抑制深能级缺陷,提高载流子迁移率并降低阈值电压漂移;而针对Al₂O₃/a-IGZO TFT器件进行100℃退火处理有助于消除其界面附近的氧空位,降低载流子浓度,改善亚阈值特性。结合2步退火工艺所制备的Al₂O₃/a-IGZO TFT器件迁移率高达22.8 cm²·V^-1·s^-1,亚阈值摆幅为0.6 V·decade^-1,综合电性能优异。     

Controlling the turn-on-voltage in low-voltage Al2O3 organic transistors with mixed self-assembled monolayers

In this paper we present a method of using blends of two silanes with different functional groups to precisely tune the turn-on-voltage to 0 V. In addition, we show how the transistor behaviour of an amorphous polymer low-voltage transistor is affected by modification of the Al₂O₃ dielectric with self-assembled monolayers of molecules with different functional groups. Controlling the turn-on voltage is essential for any practical applications, especially for low-voltage transistors. This method opens new doors to designing stable, low-voltage organic circuitry in a reproducible manner.     

NO2 gas sensing based on ordered ultrathin films of conducting polymer and its nanocomposite

The metal oxides films are widely used for CO, aromatic hydrocarbon and NO₂, gas-sensing applications, but such sensors mostly lack selectivity and operate at high temperatures (300–500 °C). We focused on the use of organized ultrathin films of conducting polymers and their nanocomposites, which recognize selectively the NO₂ gases with a very high sensitivity (≤ppb). Polyhexylthiophene (PHTh), poly(ethylene dioxythiophene) (PEDT), PHTh–PEDT copolymer, sulfonated polyaniline, polyaniline (PANI)–SnO₂, polypyrrole (PPy)–SnO₂, PEDT-SnO₂, PHTh–SnO₂ and copolymer (HTh-EDT)–SnO₂ conducting polymer and nanocomposite thin films were fabricated and used for gas sensing applications. The physical properties (UV and FTIR) such films were investigated before and after the NO₂ gas treatment. Regioregular PHTh, its copolymer P(HTh-PEDT), and their metal oxide nanocomposites films detected NO₂ gas with a high sensitivity.     

The origin of hole injection improvements with MoO3/Al bilayer electrodes in pentacene thin-film transistors

The electronic structures of pentacene/MoO₃/Al and pentacene/Al were studied using in situ photoemission spectroscopy. The secondary electron cutoffs, highest occupied molecular orbitals (HOMOs) and core level changes were measured upon deposition of the pentacene and MoO₃ to study the electronic structures at the interface. We obtained complete energy level diagrams for each interface, showing that the insertion of the MoO₃ layer between pentacene and Al induces the HOMO level shift of pentacene toward lower binding energies compared to that without MoO₃. This results in reduction of the hole injection barrier, which is responsible for the improvements in electronic device performance.     

聚合物AY在硅酸二钙-铝酸钠溶液体系中的吸附和分布

本文通过研究聚合物AY在硅酸二钙-铝酸钠溶液体系中的吸附规律,发现其吸附类型为饱和吸附.达到吸附饱和的时间为40min,饱和吸附量为2.17rng.g-1;聚合物AY在硅酸二钙上的吸附行为属"L"型等温线,附合Lang-muir单分子层吸附模型,为化学吸附为主.同时,考察了聚合物AY在熟料溶出过程中液固分离时的分布走势,结果表明,用含有80mg.L-1～160mg.L-1聚合物AY的铝酸钠溶液溶出熟料时.70%的聚合物AY吸附在固相上带出系统.残留在铝酸钠溶液中的量较少.